BIOMECHANICS OF MECHANICAL HEART VALVE by ghkgkyyt

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									The best place to find helping hand is at the end of your arm --- Swedish Proverb.


                         BIOMECHANICS OF MECHANICAL HEART VALVE 1
             Benjamín González, Humberto Benítez, Kenneth Rufino, Merisabeth Fernández and Waleska Echevarría 2

Abstract - Heart valves all are prone to disease and                The mitral valve, which lies between the two left chambers
malfunction, and can be replaced by prosthetic heart                of the heart, consists of two triangular-shaped flaps of
valves. The two main types of prosthetic heart valves are           tissue called leaflets. The leaflets of the mitral valve are
mechanical and bioprosthetic. The mechanical valve is               connected to the heart muscle through a ring called the
excellent in terms of durability, but is hindered by its            annulus, which acts like a hinge.
tendency to coagulate the blood. Bioprosthetic valve is
less durable and must be replaced periodically. All                 The mitral valve is anchored to the left ventricle by
valve types must be durable, because the body is an                 tendonlike cords, resembling the strings of a parachute,
extremely hostile environment for a foreign object,                 called chordae tendineae cordis.
including prosthetic heart valves. Today, engineers are
researching new designs of prosthetic heart valves. They            When working properly, heart valves open and close fully.
use the mechanical properties to make an artificial heart           In mitral regurgitation, the mitral valve does not open or
valve design. An artificial mitral valve is an option for           close properly. Some blood from the left ventricle flows
humans with irreparable valve disease.                              backward into the left atrium with each heartbeat.
                                                                    Regurgitation refers to the leakage (backflow) of blood
Key words -- Heart valve, Biocompatibility, Alumina,                through a heart valve.
Titanium, Biomaterial, Polyether urethane, Polyester,
Pyrolitic carbon.

                      INTRODUCTION

Heart valves prevent the backflow of blood, which ensures
the proper direction of blood flow through the circulatory
system. Without these valves, the heart would have to work
much harder to push blood into adjacent chambers. The
heart is composed of 4 valves (Figure 1). The Tricuspid
valve is between the right atrium and right ventricle. The
Pulmonary valve is between the right ventricle and the
pulmonary artery. The Aortic valve is between ventricle
and the aorta and the Mitral valve is between the left
atrium and left ventricle. It opens and closes to control                          Figure 1. Heart Valves [1].
blood flowing into the left side of the heart.
                                                                    Heart Valve Problems [7]
 Heart valves open like a trapdoor. The leaflets of the mitral
valve open when the left atrium contracts, forcing blood            There are numerous complications and diseases of the heart
through the leaflets and into the left ventricle. When the          valves that can prevent the proper flow of blood. Heart
left atrium relaxes between heart contractions, the flaps           valve diseases fall into two categories: Stenosis and
shut to prevent blood, that has just passed into the left           Incompetence. The stenotic heart valve prevents the valve
ventricle, from flowing backward.                                   from opening fully, due to stiffened valve tissue. Hence,
                                                                    there is more work required to push blood through the
1
    This review article was prepared on December 8,                 valve. Whereas, the incompetent valves cause inefficient
    2003 for the course on Mechanics of Materials - I.              blood circulation and cause backflow of blood in the heart,
    Course Instructor: Dr. Megh R. Goyal. Professor in              called as regurgitation.
    Biomedical     Engineering,  General       Engineering
    Department, PO BOX 5984, Mayagüez Puerto Rico                   Treatment Options [22]
    00687-5984.             For      details      contact:
    m_goyal@ece.uprm.edu         or         visit       at:         On a large scale, medication is the best alternative, but in
    http://www.ece.uprm.edu/m~goyal/home.htm                        some cases defective valves have to be replaced with a
                                                                    prosthetic valve in order for the patient to live a normal
2
    The authors are in the alphabetical order.                      life. An enormous amount of research and development
                                                                    has proven to be most beneficial, as prosthetic heart valve
                                                                    technology has saved thousands of lives. Engineers and



December 2003       Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez           1
scientists have done much work to design a valve that can          For a decade and a half, the caged ball valve was the best
withstand millions, if not billions, of cardiac cycles. The        artificial valve design. In the mid-1960s, new classes of
two main prosthetic valve designs include mechanical and           prosthetic valves were designed that used a tilting disc to
bioprosthetic (tissue) Heart Valves.                               better mimic the natural patterns of blood flow. The tilting-
                                                                   disc valves have a polymer disc held in place by two
Mitral Valve Replacement       [22]                                welded struts. The disc floats between the two struts in
                                                                   such a way, as to close when the blood begins to travel
 Valve replacement is done when valve repair is not
                                                                   backward and then reopens when blood begins to travel
possible. Artificial Heart valve is the last solution for
                                                                   forward again. The tilting-disc valves are vastly superior to
people with a damage heart valve caused by any disease as
                                                                   the ball-cage design. The titling-disc valves open at an
regurgitation, etc… In valve replacement surgery, an
                                                                   angle of 60° and close shut completely at a rate of 70
artificial prosthetic valve replaces the damaged mitral
                                                                   times/minute. This tilting pattern provides improved central
valve. The two types of artificial valves are mechanical and
                                                                   flow while still preventing backflow. The tilting-disc valves
tissue. Mechanical valves, which are made of biomaterials,
                                                                   reduce mechanical damage to blood cells. This improved
may last a long time. However the patient with a
                                                                   flow pattern reduced blood clotting and infection.
mechanical valve must use an anticoagulant medication
                                                                   However, the only problem with this design was its
such as warfarin (Coumadin, Panwarfin) for the rest of life
                                                                   tendency for the outlet struts to fracture as a result of
to prevent blood clots from forming on the valve. If a blood
                                                                   fatigue from the repeated ramming of the struts by the disc.
clot forms on the valve, the valve won’t work properly. If a
clot escapes the valve, it could lodge in an artery to the
                                                                   Bileaflet valves were introduced in 1979. The leaflets
brain, blocking blood flow to the brain and causing a
                                                                   swing open completely, parallel to the direction of the
stroke. Tissue valves are made of biological tissue such as a
                                                                   blood flow.      The bileaflet valves were not ideal valves.
pig’s valve. These kinds of valves are called bioprostheses.
                                                                   The bileaflet valve constitutes the majority of modern valve
These may wear out over time and may need to be replaced
                                                                   designs. These valves are distinguished mainly for
in another operation. However the tissue valve can avoid
                                                                   providing the closest approximation to central flow
use of long-term anticoagulation medication.
                                                                   achieved in a natural heart valve.
Mitral valve repair or replacement involves open-heart             Mechanical Heart Valve
surgery. Through an incision in the breastbone (sternum),
the heart is exposed and connected to a heart-lung machine         Prosthetic Heart Valves are fabricated of different
that assumes the breathing and blood circulation during the        biomaterials. Biomaterials are designed to fit the peculiar
procedure. The surgeon then replaces or repairs the valve.         requirements of blood flow through the specific chambers
After the operation, which lasts several hours, the patient        of the heart, with emphasis on producing more central flow
spends one or more days in an intensive care unit, where           and reducing blood clots. Some of these biomaterials are
the general recovery is closely monitored.                         alumina, titanium, carbon, polyester, polyurethane etc…
History and Advances of Artificial Heart Valves [1]
                                                                   The mechanical properties of these biomaterials involve
                                                                   how a material responds to the application of a force. The
The first mechanical prosthetic heart valve was implanted
                                                                   three fundamental types of forces that can be applied are
in 1952. Over the years, 30 different mechanical designs
                                                                   stretching (tension), bending, or twisting. Materials
have originated worldwide. These valves have progressed
                                                                   respond to the forces by deforming (changing shape). An
from simple caged ball valves, to modern bileaflet valves.
                                                                   elastic response is reversible, while an inelastic response is
The caged ball design is one of the early mechanical heart         irreversible. In the elastic region, an elastic modulus relates
valves that use a small ball that is held in place by a welded     the relative deformation a material undergoes to the stress
metal cage. The ball in cage design was modeled after ball         that is applied. The transition between elastic deformation
valves used in industry to avoid backflow. Natural heart           and failure occurs at the yield point (or stress) of the
valves allow blood to flow straight through the center of          material. In designing a component with the material, an
the valve. This property is known as central flow, which           inelastic response is considered failure. Failure can be
keeps the amount of work done by the heart to a minimum.           plastic deformation or ductile failure. It can also be
With non-central flow, the heart must work harder to               breaking, including brittle failure or fracture. Mechanical
compensate for the momentum lost due to the change of              properties of a material in the range of elastic behavior
direction of the fluid. Caged-ball valves completely block         include its elastic modulus under tension and shear stresses,
central flow; therefore the blood requires more energy to          its Poisson’s ratio, its resilience, and its flexural modulus.
flow around the central ball. In addition, the ball may cause      The transition to failure is denoted by the yield stress or
damage to blood cells due to collision. Damaged blood              breaking strength of the material.
cells release blood-clotting ingredients; hence the patients
are required to take lifelong prescriptions of anticoagulants.


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez              2
                    BIOMATERIALS                                  Alumina as biomaterial

The requirements for an artificial heart valve are                Alumina is the most widely and versatile ceramic. Much of
staggering. The valve must be easy to insert. It must last a      the research on this ceramic was done during the 1950s and
long time. It must be able to open and close 35 million           1960s.     Alumina is chemically stable against most
times a year for 20 to 50 years. It must allow high blood         environments except hydrofluoric acid and some molten
flow with minimal turbulence and must not leak. The valve         salts. These traditional ceramics set upon hydration if
also should not cause blood clots and also:                       produced in the special form of re-hydratable alumina
                                                                  cement (more commonly in the form of calcium aluminate
         Collapse to 5 mm when crimped.                           cement). Also alumina is widely used for medical implants
         Top of stent expands to 25 mm.                           like mechanical heart valve. This type of ceramic is also
         Middle of stent expands to 30 mm.                        used in several medical fields as dentistry, orthopedical and
         Bottom of stent expands to 25 mm.                        cardiologist application.
         Deployed height is 25.4 mm.
         Collapse to 5 mm when crimped.                          It is a ceramic, non-metallic, and inorganic compound that
         Barrel shaped.                                           displays great strength and stresses resistance to corrosion
         Top stent expands to 30 mm.                              wear and low density. Alumina is a highly bioinert
         No damage to leaflets.                                   material and resistant to most corrosive environments,
         Length is 12.7 mm - 25.4 mm.                             including the highly dynamic environment of the human
                                                                  body.
1. Alumina (Al2O3) : aluminium oxide [17]
                                                                  Compatibility between Bioceramics and the Human
Alumina (Al2O3) is a bioinert material. Bioinert materials        Environment [17 and 26]
do not chemically react with the local chemicals As a
result, cells can survive next to the material but do not         The major problem on implants designs is the fairly limited
form a union with it. Often fibrous protective cells grow         choice of materials, and consequently, determination of the
near the implant surface to protect local cells from              compatibility of the material of choice with the tissue.
mechanical damage. Bioinert materials were first used for         There are no standard methods of compatibility testing and
prosthetics. These materials can be very strong but have          the number of variables involved is usually much larger
the disadvantage of not bonding to the local cells.               than the typical engineering problem. For example, human
Numerous problems have been encountered in anchoring              blood is 1/3 as salty as seawater, stays at a steady 37ºC, and
the bioinert implants to bone. In early implants, some            contains active enzymes (the immune system).
implants became deformed or displaced, causing serious
damage to the surrounding tissue.                                 Human body is one of the most corrosive environments that
                                                                  inorganic substances can encounter. Furthermore, as the
Alumina is a traditional ceramics that offer many                 various metabolic processes occur in an organism the
advantages compared to other biomaterials. These are              various complex molecules that may enclose a substance
harder and stiffer than steel; more heat and corrosion            continually change in concentration and variety. Lactic acid
resistant than metals or polymers; less dense than most           produced from muscle cells during anaerobic cellular
metals and their alloys; and their raw materials are both         respiration is a prime example. Additionally, the time
plentiful and inexpensive. Design requirements for alumina        factor of the “compatibility reaction” is important; the
as a biomaterial are:                                             implant - tissue interaction is a sequential chain of
                                                                  reactions, characteristic for the material and the patient.
         High fluid resistance.
         Avoid hemorrhage.                                        Beyond the chemical factors, the response of tissue
         Low incident of thromboembolism.                         depends additionally on geometric characteristics of the
         Be economic.                                             implant, e.g. shape, size, surface/volume ratio. These
         High performance.                                        factors will generally determine the state of stress at the
         Avoid stiffening of the leaflets.                        interface and thus could interfere with the interfacial
         Optimal designs.                                         reactions.    Porosity and its size distribution within an
         Good thermal conductivity.                               implant have been shown to affect the interactions. It has
         Ability to open and close 35 million times a year        been generally established, that tissue will grow into pores
         for 20-50 years.                                         larger than ~120 nm.
         Biocompatibility.                                         Most polymers seem to be slowly “digested” by the human
         Avoid blood clots.                                       body and metals are slowly corroded: high concentration of
         Available easily.                                        metallic elements has been detected close to the (metallic)
                                                                  implant surface. Passive oxide film can significantly slow


December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             3
down the reactions with tissue, rendering titanium or             more scratch resistant than metal or polyethylene; so it is
stainless steel virtually neutral. However, polymeric and         most durable than other valve materials.
metallic implants are generally classified as “temporary”
implants, with very low adhesive strength of attachment to        b. Ion release: Since ceramics do not release ions, there are
the tissue.                                                       no long-term unknowns pertaining to systemic effects due
                                                                  to ion release with this hard bearing couple, unlike metal-
Alumina is considered bioinert due to a thin layer of             metal bearing couples.
titanium ions on its surface, although some studies show
that the body can absorb alumina. Various researchers have        c. Friction and wetability: A material that holds
found alarmingly low levels of Al in rats’ nervous systems        lubrication to its surface is considered wet able. Alumina
after the 20-week postoperative period.                           ceramic is a more wet able material than metal. Lubrication
                                                                  helps to reduce friction between components. Alumina
The bioinert ceramics, like ZrO2, Al2O3, SiC, Si3N4 do not        ceramic has improved since the 1974. Third generation
develop strong interfaces, but also do not liberate ions into     materials have nearly twice the strength as the original
the internal environment. This is at the expense of lesser        material because of enhancements in purity, density and
mechanical performance and reliability of ceramics, as            grain size.
compared to metals. A compromise is ceramic-coated
metal, although some additional liabilities are created (e.g.     d. Fracture: This property continues to be the primary
adhesion of the coating; increased processing costs etc.).        concern regarding ceramic components. Improper handling
                                                                  and implantation, poor implant design and material, or
Alumina Mitral Valve [26]                                         mismatched components caused fractures in early ceramic
                                                                  designs. When correctly implanted, the fracture rate has
Ceramic materials are somewhat limited in applicability by        been reported between one-tenth to one-twentieth of a
their mechanical properties, which in many respects are           percent (0.001 - 0.0005) and it is projected that
inferior to those of metals. The principal disadvantage is a      contemporary materials will be even lower. Alumina
disposition to catastrophic fracture in a brittle manner with     should be use, only in compression.
very little energy absorption.
                                                                  e. Strength: Though ceramics are brittle in nature, alumina
The ceramic mitral valve is comprised of a single crystal         ceramic inserts are extremely strong and exceed FDA
alumina disc and titanium valve ring. Alumina consists of         Guidance Document standard for ceramic heads of 46 kN
aluminum and oxygen ions. These ions combine firmly by            or 10,340 pounds burst strength. This exceeds the strength
ionic bond and are arranged in hexagonal closed packed            of the ceramic head as well as the neck of the femoral stem.
structure. The single crystal alumina disk is 1.0 mm thick.       As with any modular interface under load, there is a
Both mechanical and chemical polishing smoothed the               potential for micro motion and associated fretting and/or
surfaces. The valve ring was milled from a single piece of        corrosion. However, the alumina design minimizes the
titanium and was coated with Tin by reactive ion plating          amount of motion at the taper interface, which should
(See appendix IV). Alumina has a good blood                       reduce the corrosion potential.
compatibility, excellent wear resistance, largely inert and
durability.    Alumina mitral valve avoids thrombus               Alumina mechanical properties are summarized as:
formation and thromboembolism.
                                                                       •    Good mechanical strength (Figure 3).
Tensile strength of single alumina is more than three times            •    Good thermal conductivity.
greater that LTI carbon. Alumina has hardness eight times              •    High electrical resistivity.
greater than LTI carbon. Alumina is insoluble in water and             •    High hardness (Figure 2).
has high corrosion.                                                    •    Wear resistant.
Properties of Alumina                                                  •    Good chemical stability.
                                                                       •    Largely inert.
a. Scratch resistance: The extreme hardness of alumina is
                                                                       •    Excellent tribological characteristics.
second only to a diamond. Metal-on-metal articulations can
be scratched causing an abrasive surface. Foreign debris in
the joint may also accelerate implant wear. Alumina is




December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez            4
                                                                  Biomaterials and implant research will continue to
                                                                  concentrate on serving the needs of medical device
                                                                  manufacturers and recipients, as well as medical
                                                                  professionals, and on developing technologies to meet
   TitaniumNitride                                                those needs. Future biomaterials like alumina will
                                                                  incorporate biological factors directly into an implant’s
          Titanium                                                surface to improve biocompatibility and bioactivity. New
                                               Series1            projects will be directed at materials development for
          I arbon
        LT C                                                      improved mechanical integrity, corrosion resistance, and
                                               Series2
          Alumina                                                 biocompatibility. Institute engineers will also apply
                                                                  statistical finite element analysis, stereo imaging strain
                  0    5    10    15    20                        analysis, and composite materials to the biomaterials
                                                                  program. Therefore, alumina is one of these experimental
                                                                  materials for the future.
                                                                  Figure 3. Tensile Strength (MPa) of biomaterials [26].
Figure 2. Hardness of different biomaterials [26].
                                                                            1000

Valve Problems with Alumina                                                  800
                                                                             600
a. It is a hard material so that machining is difficult.                     400
Therefore, some molding process must be developed which                      200
can produce a finished valve with the accurate internal                        0




                                                                                                          Titanium
                                                                                    Alumina




                                                                                                                     Carbon
                                                                                              Stainless
shape required to achieve good homodynamic performance.




                                                                                                                      LTI
                                                                                                Steel
b. The tissue covering requires a porous, textured alumina
surface on which to anchor itself firmly, but the main body
of the conduit valve must be in the most dense form of
alumina, with virtually no porosity, in order to maintain
structural strength. The molding process must therefore           2. Polyester
accommodate variable porosity in some way.
                                                                  What is polyester? [27]
c. Alumina cannot avoid thromboembolism totally.
                                                                  Polyester is a synthetic resin formed by the condensation of
                                                                  polyhydric alcohols with diabasic acids. Polyesters are
Alumina Future                                                    thermosetting plastics used in making sythentic fibres and
                                                                  constructional plastics. It is an extremely resilient fibre that
Research is being done to combine alumina with other              is smooth, crisp and particularly springy. Its shape is
materials for better heart valve implants.          These         determined by heat and it is insensitive to moisture. It is
experiments try to avoid tromboembolism, which is the             lightweight, strong and resistant to creasing, shrinking,
major problem on heart valve implants. Today, there are no        stretching, mildew and abrasion. It is readily washable and
materials to avoid totally thrombosis. Alumina is a               is not damaged by sunlight or weather and is resistant to
material with good mechanical properties, but mechanical          moths and mildew. The following requirements must be
heart valve manufactures didn’t use for this purpose.             satisfied to use polyester as a biomaterial in heart valves
Instead alumina is utilized in dental implants.                   implants:
Lawsuits against medical device manufacturers,                         •    The body’s immune system must not attack the
restructuring of FDA approval procedures, patient                           biomaterial.
expectations, and the health care reform movement are                  •    Compatible with body tissues and fluids
changing the future of the medical device community and                •    Must have strength, flexibility and hardness
shaping the direction of biomaterials research. As a result,           •    Must be nontoxic, nonreactive or biodegradable
new materials and manufacturers will be required to meet               •    The replacement valve must be smooth to prevent
FDA standards. Another important issue not often                            the destruction of blood vessels.
discussed is that implant recipients expect an implant to              •    The valve must also be anchored to the inside of
function and to last forever.                                               the heart.
                                                                       •    Must be an elastomer so it can be flexible during
                                                                            the pumping cycle.


December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez               5
     •    The material must not exhibit mechanical fatigue         well as prosthetic dilatation and failure caused this concept
          over the device’s lifetime.                              to be abandoned in the late 1970s. Surprisingly enough, the
     •    The material’s surface must have an acceptable           ideal values for the porosity and water permeability of a
          low propensity for thrombus formation, as well as        vascular prosthesis are defined poorly.
          the best possible blood compatibility
     •    Must not be prone to calcification.                      In spite of the success of expanded PTFE grafts that remain
     •    The material must be easily formed into complex          patent for many years without any tissue encapsulation, it is
          shapes.                                                  still believed that complete healing of the luminal surface is
                                                                   a critically important requirement for long-term patency.
Structure and Physical Properties of Polyester                     One approach to achieve this was proposed by DeBakey
                                                                   and to improve the anchoring of fibrous tissue, increase
     •    Polyester chains tend to be flexible and are easily      cellular adhesion, and hence promote the formation
          entangled or folded.                                     neointima by the use of the velour design. This involves
     •    Degree of crystallinity is the amount of ordering        weaving or knitting rather than straight fibers to give a
          in a polymer.                                            rougher, randomized, and more open appearance to the
     •    Stretching or extruding a polymer can increase           external and/or internal surface of the graft. External velour
          crystallinity.                                           enables better incorporation of the graft within the host
                                                                   tissue, whereas internal velour encourages the formation of
     •    Degree of crystallinity is also determined by
                                                                   thicker neointima. This may be less important in clinical
          average molecular mass.
                                                                   practice since complete endothelialization is never
     •    Bonds formed between polyester chains make the
                                                                   accomplished in humans. Even so, internal external and
          polyester stiffer.
                                                                   double velour grafts are widely available.
Development of Polyester in Vascular Surgery
                                                                   As a result of the complications such as dilatation
                                                                   associated with the light-weight weft-knitted design,
Vascular prostheses fabricated as polyester textile tubes are
                                                                   manufacturers have taken steps to increase the strength of
most frequently used devices in peripheral vascular surgery
                                                                   grafts by using thicker polyester yarns and tighter, more
for the replacement of large and medium sized vessels.
                                                                   compact woven constructions. The more open woven
Long term results representing a period of follow-up over
                                                                   velour constructions should be anastomosed with a larger
15 – 20 years have shown satisfactory results when Dacron
                                                                   than normal bite or cut with a hot cautery in order to reduce
grafts are installed in the aortic and iliac sites. Technical
                                                                   the risk of fraying at the suture line. The regular woven and
developments to improve the device over the years have
                                                                   low porosity woven design are used widely for the
passed through different generations of concepts. The
                                                                   replacement of the thoracic aorta and for interventions
relative merits of these different designs are still a matter of
                                                                   involving a cardiopulmonary bypass with heparinization.
intensive research.
                                                                   For those surgeons who prefer the ease of handling and
                                                                   suturing of the knitted construction, most major models
Woven or Knitted Design
                                                                   with the more dimensionally stable warp knitted prosthesis
                                                                   have now replaced former weft knitted models, thus
Weslowski was the first to recognize the importance of the         ensuring the same good anchorage of the neointima but
porosity within the graft wall for the healing process of the      avoiding the complications associated with dilatation and
graft. By using a more open textile structure with large           raveling of the textile structure in vivo. The importance of
pores between the polyester fibers it was predicted that           maintaining the initial strength of vascular prostheses at an
cellular elements and fibrous tissue would be able to              acceptable level is now widely accepted.
penetrate the interstices of the graft wall and generate a
well-attached, more completely healed surrounding                  Externally Supported Design
capsule.     Unfortunately,     measurements     of    water
permeability were mistakenly assumed to measure the                The problem of flattening and occlusion of a vascular graft
porosity of the graft wall, and as a result manufacturers          at the point where it crosses a knee or hip joint is well
produced thinner and thinner textile structures using finer        known. External reinforcement of the graft by means of a
and finer polyester yarns with a view to improve the               rigid spiral support has proven to be effective in alleviating
healing performance of the prostheses. The creation of the         this problem and has found merit in the axillofemoral
ultra-light-weight design provided the surgeon with a more         position as well. The performance observed during animal
flexible graft that was easier to handle and suture. But too       trials as well as clinical observations of explanted devices
high water permeability posed difficulties in preclothing          suggest that high levels of friction and fatigue can occur to
the graft so as to achieve hemostasis. Problems of                 the textile structure underneath the rigid external support.
hemorrhage at the time of implantations and complications          This is particularly problematic with those models where
associated with secondary hematomas around the grafts as           the external support is not well attached to the outer surface


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             6
of the prosthesis. As a result, perhaps the most valuable           of polyester have gained in popularity due to the similar
application for this type of design is in the axillofemoral         rates of occlusion and ease of use. Despite these
bypass where compression of the graft may occur, when the           advantages, there is continuing concern over the
patient is lying on the relevant side.                              nevertheless high rates of occlusion, for both native and
                                                                    synthetic grafts which significantly contributes to the
Composite Design                                                    greater than 30% failure rate experienced within one year.
                                                                    As these failures often lead to limb loss and other serious
In order to improve the biocompatibility of porous                  complications, the availability of a non-thrombogenic graft
synthetic grafts, it has been proposed that the polyester           or one with reduced thrombo-genicity would have
textile structure be impregnated or coated with a                   significant clinical impact and could serve to reduce the
crosslinked protein. A number of different proteins have            incidence of thrombosis related complications. Polymeric
been studied, including albumin, collagen, gelatin, elastin,        biomaterial surfaces such as polyester and PTFE are
and chitosan. Observations from comparative in vivo                 intrinsically thrombogenic. Grafts composed of these
studies as well as from our explant retrieval program               materials can be surface modified in order to reduce this
indicate that the healing process is virtually identical with       inherent thrombogenicity. Heparin treatment of the surfaces
the coated and uncoated grafts. The only difference, if any,        of a number of medical devices such as catheters, heart
resides in the rate of the healing process, wich is slowed by       valves, stents and bypass circuitry has successfully been
the addition of the protein coating. Because the prosthesis         used as a means of reducing surface thrombogenicity.
is already nonpermeable to blood and ready to implant the
moment it is removed from is sterile packaging, the need            In 1991, InterVascular developed the concept of adding
for blood transfusion and preoperative manipulation is              unfractionated high molecular weight heparin to the inner
reduced. In addition, since the coating is resorbed slowly, it      lumen of a graft through a stable bonding process. It was
has been proposed that antibiotics and growth promoting             believed that this modification of the graft surface could
factors be added to the protein in order to reduce the risk of      significantly reduce its thrombogenicity and potentially
infection and enhance the healing of the neointima.                 improve graft performance and clinical outcome. Heparin is
                                                                    coupled to the InterGard Heparin surface using tri-
The cellular seeding of vascular prostheses with endothelial        dodecylammonium chloride (TDMAC) which forms an
cells appears to be a very promising technique. The                 insoluble complex with heparin and in turn binds with high
experimental research to date has improved our                      affinity to the polyester flow surface through its long
understanding of the different functions of the endothelial         hydrophobic tails. The heparinized graft is then coated with
cell and its interactions with blood. However, the efficiency       collagen which acts as a barrier to prevent rapid release of
of the cell seeding procedure leaves much to be desired,            the heparin from the graft surface. A series of studies was
and, while the technique has proven useful in a few human           performed to evaluate the safety and efficacy of the
trials, it is not yet ready for routine clinical use. In the long   InterGard heparin coated graft. Animal studies were
term, there it do appear to be beneficial in using this             performed to confirm that no bleeding complications and
technology, particulary in femoropopliteal and distal sites         good healing characteristics were associated with the use of
where the rate of flow is limited and in reducing the               the heparin bonded graft.
incidence of infection.
                                                                    In addition, complete ISO 10993 biocompatibility tests
Another type of surface coating proposed for vascular               were performed to assure that safety and biocompatibility
grafts involves the use of the plasma graft polymerization          requirements were met. Bench studies were conducted to
process. This technique can modify the surface chemistry            evaluate the retention of heparin on the InterGard heparin
and hence the biocompatibility of a synthetic material.             graft in a simulated model of circulation using
Typically, plasma of fluorethylene gas is generated in a            physiological flow rates and pressures. In these studies,
evacuated chamber containing the prosthesis by means of a           heparin levels remained constant for 7 days in the
high-frequency magnetic field. The free radicals so                 InterGard heparin collagen coated graft but declined
produced react rapidly with each other and with any surface         dramatically in the non-collagen coated graft demonstrating
they encounter, depositing a thin layer of a fluorocarbon           that the stable bonding process of ionic coupling to
polymer on the polyester fibers of the vascular graft. The          TDMAC followed by hydrophobic interaction with
flow surface is thus likely to be more hydrophobic and              polyester immobilizes the heparin to the graft. Furthermore,
biocompatible. Preliminary results in animals have so far           the collagen coating helps retain the heparin complex
been promising, but they have not been confirmed in                 preventing its premature release. Additional studies were
humans.                                                             performed which demonstrated that the heparin-collagen
                                                                    coating dramatically reduces the deposition of fibrin (a
Biocompatibility                                                    measure of thrombogenicity) relative to uncoated polyester
   Although saphenous vein remains the material of choice           grafts. These studies coupled with on going promising
for vascular reconstruction, fem-popliteal grafts composed          clinical data continue to support the safety, utility and


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez            7
clinical benefits associated with the InterGard Heparin            lenses, removable dental prostheses, renal dialyzers), and
graft.                                                             transient applications (e. g. cardiopulmonary bypass, over-
                                                                   the-needle catheters, diagnostic and therapeutic catheters).
Polyesters Chemical Properties [20]                                The polymers used most often in these applications are the
                                                                   silicone elastomers, the acrylics, polyvinyl chloride,
                                                                   fluorinated ethylene propylene and polycarbonates.
Polyesters are formed either by a reaction between a dibasic
acid and a dihydroxy alcohol or by the polymerization of a
                                                                   In the past ten years, research work on the artificial heart
hydroxy carboxylic acid. The chemical structure of a
                                                                   has stimulated interest in this new family of polymers, the
polyester is shown in figure 4. Polyesters are naturally clear
                                                                   segmented polyurethane elastomers. Originally developed
and colorless; however they can be colored and made
                                                                   for commercial applications, these polymers exhibit high
according to specifications. Polyesters do not show wear
                                                                   flexure endurance, high strength, and inherent
with exposure to poor weather conditions. They are highly
                                                                   nonthrombogenic characteristics, and are expected to have
resistant to chemical deterioration, withstanding most
                                                                   a positive effect on future medical applications. Segmented
solvents, acids, and salts. They are also resistant to heat
                                                                   polyurethane polymers are widely used as artificial heart,
damage and can be made to be self-extinguishing.
                                                                   vascular grafts, catheter, diaphragm of blood pump,
                                                                   pacemakers wire insulation, heart valves, cardiac-assist
Not to be outdone, DuPont was also at the forefront of
                                                                   devices, components of hemodialysis units, skin grafts and
polyurethane technology in the U.S., receiving patents in
                                                                   blood filters. Since the segmented polyurethane exhibit
1942 covering the reactions of diisocyanates with glycols,
                                                                   high strength, nonthrombogenic characteristics, the most
diamines,    polyesters    and certain other active
                                                                   important applications appear to be in the cardiovascular
hydrogencontaining chemicals. From these humble
                                                                   area. Because of higher hydrolytic resistance and better
beginnings emerged the polyurethanes, the most versatile
                                                                   properties at low temperatures, the structures of
polymers in the biomaterials armamentarium.
                                                                   polyurethanes prepared from lactones can be used as
                                                                   medical, solvent-activated, pressure-sensitive adhesives.

                                                                   Future scope of polyurethane

                                                                   A material used in the leaflet heart valves, mechanical heart
                                                                   valve coatings and total artificial heart is polyether-based
Figure 4. Chemical Structure of polyester [20].                    polyurethane. However, one drawback of this material is
                                                                   the absorption of the proteins and thus, the onset of
3. Polyurethanes [25 and 27]                                       thrombosis and bacterial infection. The right materials have
                                                                   the good mechanical properties of polyurethane while
Polymers are considered some of the most promising class           eliminating the risk of thrombosis and bacterial infection.
of biomaterial. They can be selected according to certain          Unfortunately, scientists have been unable to find a suitable
characteristics such as mechanical resistance, degradability,      substitute with such mechanical properties as well as
permeability, solubility, as well as transparency.                 relative biocompatibility. Therefore, scientists have begun
Polyurethanes are the polymers most widely used in the             searching for possible improvements to polyurethane in an
construction of blood-contacting products and devices.             attempt to increase its biocompatibility.

History of polyurethane                                            One possible solution to the compatibility problem is to
                                                                   synthesize a polymer alloy consisting of polyurethane along
Nineteen eighty-seven marked the 50th anniversary of the           with a phospholipid polymer. A current polymer alloy that
introduction of polyurethanes. Professor Otto Bayer was            has shown promise in combating the onset of thrombosis as
synthesizing polymer fibers to complete with nylon when            well as bacterial infection is 2-methacryloyloxethyl
he developed the first fiber-forming polyurethane in 1937.         phosphorylcholine (PMEH) with segmented polyurethane.
                                                                   Research on this alloy has shown a significant decrease in
Polyurethanes technology                                           the amount of proteins absorbed at the blood-suface
                                                                   interface. In fact, when protein adsorption data was
Current activities of suppliers, designers, manufacturers          recorded, the amount of the protein adsorbed on the 2-
and physicians clearly indicate that devices manufactured          methacryloyloxethyl      phosphorycholine        segmented
from synthetic polymers have become an integral part of            polyurethane tubing was only 17% of that adsorbed by
health-care technology. Initially focused on life-threatening      segmented polyurethane tubing alone. In similar attempt,
situations, their clinical uses now include permanent              scientists synthesized an alloy of polyurethane with the
implantation (e. g. artificial hearts, hip prostheses,             addition of poly (tetramethylammonium) oxide and
intraocular lenses), intermediate applications (e. g. contact      methylene diphenylene diisocyanate along with chain


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez            8
extenders of 3-trinethylammonium-1,2-propanedioliodide            demonstrates an impressive combination of mechanical
(TMPI)     and     3-dimethylamino-1,2-propanedioliodide          properties and biological compatibility. The Polymer
(DMP). This alloy had been previously found to restrict the       Technology Group developed Elasthane in response to
onset of thrombosis. In an experiment conducted to                Dow's decision to limit Pellethane's use in chronically
determine the protein attachment rate constant of three           implanted medical devices. In developing Elasthane, PTG
polyurethane alloys as well as pure polyurethane. This            invested in the same continuous reactor technology to offer
polyurethane, labeled PEU-N, was found to have a higher           the only Pellethane substitute with the same high molecular
attachment constant (.00059 cm/min) than either of the            weight and reduced thermal history as Pellethane. PTG has
other polyurethane alloys including a phospholipids               rigorous quality control and documentation of the
polymer alloy (PEU-G). However, PEU-N did have a lower            manufacturing procedures, formula optimization, and
adhesion constant than pure polyurethane (PEU-B).                 precision feed pumps. Formal validation of Elasthane was
                                                                  accomplished through rigorous short- and long-term testing
Not all polyurethanes are equally effective in their              in conjunction with a major academic institution and a
biocompatibility properties. Polyurethanes comprise a large       medical device company that has since received approval to
family of materials, with urethane linkage being the only         implant the material. A comprehensive FDA Masterfile also
common characteristic. They have been found to vary in            backs Elasthane.
clinical applications. When implanted in the human body,
polyester-based polyurethanes tend to undergo a rapid             Elasthane™ polyether urethane is a thermoplastic
hydrolysis and should be avoided in medical applications.         elastomer formed as the reaction product of a polyol, an
Due to their quick crystallization, polycaprolactone-based        aromatic diisocyanate and a low molecular weight glycol
polyurethanes can be used as medical applications, but only       used as a chain extender. Polytetramethylene oxide
as pressure-sensitive adhesives. Polybutadiene-based              (PTMO) is reacted in the bulk with aromatic isocyanate,
polyurethanes have been investigated, yet no medical              4,4'-methylene bisphenyl diisocyanate (MDI), and chain
application has been found to date. Castor oil-based              extended with 1,4-butanediol.
polyurethanes can be used, but due to their poor tear
resistance, have a very limited use in medical applications
                                                                  Application of Elasthane™ polyether urethane
since they are virtually insensitive to hydrolysis, and
therefore are very stable in the physiological environment.
                                                                  Numerous medical devices and technologies have benefited
                                                                  from the combination of exceptionally smooth surfaces,
                                                                  excellent mechanical properties, stability, and good
4. Polyether urethane
                                                                  biocompatibility of Elasthane™ polyether urethane.
In the preparation of this type of polymers, polyether-based      Pellethane is currently the polyurethane used for the
glycols are used. If they are cured with aromatic diamines        tricuspid semilunar valves. Due to its high molecular
then their structure-property relationships will be very          weight, valves fabricated from Elasthane have shown to
similar to those of polyester urethanes. At high NCO/NH2          reduce the degree of calcification. Furthermore, Elasthane
ratios the excess isocyanate forms biuret branch points.          that has been chemically modified with polyethylene oxide
Thus, an increase in cross-linking causes a reduction in          (P) and sulfonate (SO) SMEs showed lower surface platelet
modulus, elongation, compression set, and tears strength.         adhesion and thrombus formation, suggesting improved
                                                                  blood compatibility.
The secondary reactions occur to a much less extent than
the primary reactions but their importance must not be            Hydrodynamic evaluation of Pellethane valves showed
underestimated. Formation of allophanates or biurets is           minimum pressure drop and very low energy losses
responsible for some of the cross-linking and branching           compared with other commercially available valves. It was
and therefore has an important influence on the properties        also found that in durability tests, prototypes have lasted
of the polyurethane product.                                      for 17 years.
Elasthane™ polyether urethane [19]                                Mechanical Properties of Polyether urethane [10]

Elasthane™ polyether urethane is a high-strength, aromatic        At lower hardness levels, practically all elastomeric
thermoplastic with a chemical structure and properties very       materials, including polyurethane elastomers, merely bend
similar to Pellethane® 2363 polyetherurethane series, which       under impact. As conventional elastomers are compounded
has been used to fabricate a large number of implantable          up to higher hardness they tend to lose elasticity and crack
devices, including pacemaker leads and cardiac prosthesis         under impact. On the other hand, polyurethane elastomers
devices such as artificial hearts, heart valves, intraaortic      when at their highest hardness levels have significantly
balloons, and ventricular assist devices. PTG's Elasthane is      better impact resistance than almost all plastics.
designed for chronically-implanted medical devices and


December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez           9
Such great toughness, combined with the many other                 useful to improve blood compatibility of implantable
outstanding properties associated with the high hardness           polyurethanes, and may also be advantageous as regards
polyurethane leads to many applications in engineering.            fatigue durability of flexing materials in long term
(Appendix I gives properties of different kinds of                 applications.
Polyurethanes).
                                                                   Polyurethane heart valves: Fatigue failure, Calcification
                                                                   and Polyurethane Structure
Biocompatibility of Polyether Urethane
                                                                   Six flexible-leaflet prosthetic heart valves, fabricated from
The blood contacting surface of some leaflets hearts valves
                                                                   a polyether urethane urea (PEUE), underwent long-term
are made of polyether urethane (PEU, n = 22). This
                                                                   fatigue and calcification testing by Dernacca GM,
polyurethane can be resistant to thrombus formation when
                                                                   Guldransen NJ; Wikinson R; and Wheatley DJ. They
processed into an ultra smooth contacting surface.
                                                                   discovered that three valves exceeded 800 million cycles
Elastomeric polyurethanes are inherently thromboresistant.
                                                                   without failure. Three valves failed at 775, 460, and 544
Although blood compatibility and nonthrombogenicity are
                                                                   million cycles, respectively. Calcification was observed
subject to many complex factors, such as polymer surface
                                                                   with and without associated failure in regions of high
composition, device configuration, and blood-flow
                                                                   strain. Comparison with similar valves fabricated from a
characteristics, they tend to perform well in numerous
                                                                   polyether urethane (PEU) suggested that the PUE is likely
device configurations. Their apparent thromboresistance is
                                                                   to fail sooner as a valve leaflet. Localized calcification was
thought to reside in polyurethane’s ability to preferentially
                                                                   developed in PEUE leaflets at the primary failure site of
absorb serum albumin.
                                                                   PEU leaflets, close to the coaptation region of three
                                                                   leaflets. The failure mode in PEU valves had the
When the biomaterial surface comes into contact with
                                                                   appearance of abrasion wear associated with calcification.
blood, a protein layer of fibrin results from the
                                                                   High strains in the same area may render the PEUE leaflets
polymerization of fibrinogen. When bacteria interacts with
                                                                   vulnerable to calcification. Intrinsic calcification of this
the surface of a blood-contacting biomaterials it does so
                                                                   tape, however, is a long-term phenomenon unlikely to
through this adsorbed protein layer. Therefore, bacteria can
                                                                   cause early valve failure. Both polymers performed
easily attach itself to the material surface and cause
                                                                   similarly during static in vitro and in vivo calcification
infection.
                                                                   testing and demonstrated a much lesser degree of
                                                                   calcification than bioprosthetic types of valve materials.
When choosing a material to combat bacterial adhesion, it
                                                                   Polyurethane valves can achieve the durabilities required of
is essential that the material limits protein adsorption.
                                                                   an implantable prosthetic valve, equaling the fatigue life of
Proteins tend to be negatively polarized and thus
                                                                   currently available bioprosthetic valves.
hydrophobic in nature. With this in mind, it is beneficial to
select a material that is similarly polarized, thus likely to
                                                                   Polyurethane heart valve durability: Effects of leaflets
repel the proteins from the biomaterial’s surface, hindering
                                                                   thickness and material
the protein-surface interaction and protein adsorption by
the surface. By disrupting this adsorption of proteins, the
                                                                   The durability of a flexible trileaflet polyurethane valve is
material is less likely to develop a protein layer and less
                                                                   determined by the thickness of its leaflets. Leaflet thickness
likely to promote the development of bacterial growth and
                                                                   is also a major determinant of hydrodynamic function. The
infection.
                                                                   study was conducted by Dernacca GM; Guldransen NJ;
                                                                   Wikinson R; and Wheatley DJ examined valves (n = 31)
Surface modification of polyurethane heart valves:
                                                                   with leaflets made of polyether urethane (PEU, n = 22) or a
effects on fatigue life and calcification
                                                                   polyether urethane urea (PEUE, n = 9), of varying
                                                                   thickness distributions. The valves were subjected to
Polyurethane heart valves can be functionally durable with
                                                                   accelerated fatigue test at 37ºC and failure was monitored.
minimal calcification, in vitro. In vivo, these characteristics
                                                                   Leaflet thickness ranged from 60 to 200µm. PEU leaflet
will depend on the resistance of the polyurethane to
                                                                   thickness bore no relationship to durability, which was less
thrombogenesis and biodegradation. Surface modification
                                                                   than 400 million cycles. PEUE valves, in contrast,
may improve the polyurethane in these respects, but may
                                                                   exceeded 800 million cycles. Durability in PEUE valves
adversely affect calcification and durability. This study
                                                                   was directly related to leaflet thickness ( r = .93, p < 0.001),
investigates the effects of surface modifications of two
                                                                   with good durability achieve with median leaflet
polyurethane heart valves (PEU and PEUE) on vitro fatigue
                                                                   thicknesses of approximately 150 µm. Thus polyurethanes
and calcification behavior. Modifications included heparin,
                                                                   valves can made with good hydrodynamic properties and
taurine or aminosilane. Aminosilane modification of PEUE
                                                                   with sufficient durability to consider potential clinical use.
valves increased durability compared with PEO
modification. Appropiate surface modification may be


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez               10
New polyurethane heart valve prosthesis: Design,                   being used in more than 4 million implants in more than 25
manufacture and evaluation                                         different valve designs for a clinical experience on the
                                                                   order of 18 million patient-years.
In light of the thrombogenicity of mechanical valves and
the limited durability of bioprosthetic valves, alternative        Pyrolytic carbon (PyC) belongs to the family of turbostratic
designs and materials are being considered for prosthetic          carbons, which have a similar structure of graphite, but
heart valves. A new tri-leaflet valve, made entirely from          subtly different. In graphite, the carbon atoms are
polyurethane, has been developed. The valve comprises              covalently bonded in planar hexagonal arrays that are
three thin polyurethane leaflets (approximately 100µm              stacked and held together by weak interlayer bonding. For
thick suspended from the inside of a flexible polyurethane         turbostratic carbons, the stacking sequence is disordered,
frame. The closed leaflet geometry is elliptical in the radial     resulting in wrinkles or distortions within layers. This
direction and hyperbolic in the circumferential direction.         structural distortion provides the superior ductility and
Valve leaflets are formed and integrated with their support        durability of pyrolytic carbon, compared to other carbon
frame I a single dip coating operation. The dipping process        structures such as graphite.
consistently gives rise to tolerably uniform leaflet thickness
distributions. In hydrodynamic test, the polyurethane valve
exhibits pressure gradients similar to those for a
bioprosthetic valve (St Jude Bioimplant), and levels of
regurgitation and leakage are considerably less than those
for either a bileaflet mechanical valve (St Jude Medical) or
the bioprosthetic valve. Six out of six consecutively
manufactured polyurethane valves have exceeded the
equivalent of 10 years function without failure in
accelerated fatigue tests. The only failure to date occurred
after the equivalent of approximately 12 years cycling, and
three valves have reached 527 million cycles
(approximately 13 years equivalent).

5. Pyrolytic Carbon

Background

Dr. Jack Bokros and Dr. Vincent Gott [11] discovered
pyrolytic carbon, the premier material for artificial heart
valves at General Atomics (GA). In 1966, Dr. Bokros was
working on pyrolytic carbon coatings for nuclear fuel
particles for the GA gas-cooled nuclear power reactors. He
stumbled upon its potential for medical uses through what
has been called “a lesson in serendipity”. He read an article
by Dr. Vincent Gott, who has been testing carbon-based
paint as a blood compatible coating for artificial heart           Figure 5. Crystal structure of graphite [23].
components. Bokros contacted Gott who initiated the
collaboration.
                                                                   Mechanical properties [23]
Dr. Gott was searching for a material to use in artificial
heart valves that did not provoke blood clots and had the          The Pyrolytic carbon, with its inherently dense, glassy
mechanical durability to endure for a recipient’s lifetime.        structure and its ability to be highly polished, has become a
Pyrolytic carbon, from GA, met both of his need. GA                popular choice. Furthermore its electrical conductivity is
initiated a development project headed by Dr. Brokros to           useful in allowing it to become electrostatically charged so
add the needed durability to the material. This endeavor           that it can repel the blood cells. This unique material is one
was successful and the biomedical grade of pyrolytic               of the most blood-compatible of all man-made materials, as
carbon was rapidly incorporated into the existing heart            opposed to metals. The human body recognizes implanted
valve designs.                                                     metal as a foreign material, and protects itself from the
                                                                   object by coating it with layers of blood. But, pyrolytic
Today, pyrolytic carbon (Figures 5 and 6) remains a                carbon and other so-called blood-compatible coatings are
popular material available for mechanical heart valves,            unrecognized by the body and are accepted.


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             11
Figure 6. Acoustic emission amplitude versus frequency
for crack extensionsg. This plot shows an emission peak at
90kHz, indicating a normal mode crack extension in a               Figure 7. Fracture stress versus density for unalloyed LTI
pyrolytic carbon test sample [23].                                 pyrolytic carbons [26].

In its processed form, pyrolytic carbon is a microscopically
smooth, hard, black ceramic-like material. Like ceramic, it
is subjected to brittleness.
Fortunately, pyrolytic carbon possesses a mechanical
property that mitigates this fragility in the presence of
flaws, making it inherently difficult to accidentally
introduce cracks of significant size into the material. In
particular, unlike true ceramics, pyrolytic carbon is highly
ductile. Thus, if a sharp, hard object is pressed into
pyrolytic carbon, it can respond by deforming locally to
accommodate the object elastically. When the object is
withdrawn, there may be no residual depression, and little
or no microcracking surrounding the site. It is this intrinsic,
atomic microstructure-derived resistance to externally
imposed crack nucleation that permits such an otherwise
brittle material to be used in the human body.
The mechanical properties of pyrolytic carbon are largely
dependent on the density as shown in Figures 8 and 9. The
increased mechanical properties are directly related to the        Figure 8. Elastic modulus versus density for unalloyed LTI
increased density, which indicates that the properties             pyrolytic carbons [25].
depend mainly on the aggregate structure of the material.

Graphite and glassy carbon have lower mechanical strength          Deposition of pyrolytic carbon coatings for heart valves
than pyrolytic carbon as given in table 1. However, the
average modulus of elasticity is almost the same for all           For heart valves, a silicon-alloyed pyrolytic carbon is used
carbons. The strength and toughness of pyrolytic carbon are        in the form of a thick coating on a polycrystalline graphite
quite high compared to graphite and glassy carbon. This is         substrate. Silicon is added to improve mechanical
due to the smaller number of flaws and unassociated                properties such as stiffness, hardness, and wear resistance,
carbons in the aggregate.                                          without significant loss in biocompatibility. Components
                                                                   are made by co-depositing carbon and silicon carbide on
                                                                   the graphite substrate by a chemical vapor-deposition,
                                                                   fluidized bed process that uses a gaseous mixture of
                                                                   silicon-containing carrier gas with a hydrocarbon.




December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez           12
Table 1. Properties of various types of carbon [25].

                           Type
                      Graphite       Glassy      Pyrolytic
                                                 carbon
 Density, g/ml         1.5 -1.9        1.5          1.5 -2.0

 Elastic                  24           24                 28
 modulus,
 MPa
 Toughness,              138           172                517
 m-N/ cm3                                                (525a)

 Compressive
 strength                6.3           0.6                4.8
  a
      1.0 w /o Si – alloyed pyrolytic carbon, Pyrolite
      (Carbomedics, Austin, Tex).
                                                                     Figure 8. OmnicarbonTH mechanical heart valve [17].
Pyrolytic Carbon Mechanical Valves in the Market
                                                                     Adverse Events potentially associated with the use of
a. OmnicarbonTM
                                                                     mechanical cardiac valves include:
The Omnicarbon mechanical heart valve is manufactured,
                                                                         •    Angina.
marketed and sold, by Medical CV. Blair Mowery,
president and chief executive officer of Medical CV, noted               •    Cardiac arrhythmia.
that the results from at least 10 clinical studies, including            •    Clinically significant transvalvular regurgitation.
over 10,000 patient years of use, have consistently                      •    Disc impingement/ entrapment.
demonstrated one-third to one-half fewer complications                   •    Endocarditis .
with the Omnicarbon valve, such as blood clots and stroke,               •    Heart failure.
compared to other mechanical valves. The Omnicarbon                      •    Hemolysis or hemolytic anemia.
heart valve is a monoleaflet valve; a valve with a single                •    Hemorrhage.
hingeless pivoting disc to employ pyrolytic carbon in both               •    Myocardial infarction.
its housing and disc for improved blood compatibility. Also              •    Nonstructural dysfunction.
Omnicarbon valve does not have fixed pivot recesses that                 •    Perivalvular leak.
are characteristic of bileaflet designs and that are                     •    Stroke.
demonstrated to be the primary location for blood clot                   •    Structural dysfunction.
formation.                                                               •    Thromboembolism.
                                                                         •    Tissue interference with valve function.
The disc is slightly curved and retained within the housing              •    Valve thrombosis.
ring, located 1800 from each other on the other side of the
housing ring. The disc closes on the housing ring at a 120           Precautions and Warnings: In order to avoid harmful
angle relative to the plane of the housing ring, and can             damages to the health of the patient, the following
open to a maximum angle of 800. The disc rotates freely              precautions and warnings must be taken into account:
within the housing ring because there are no fixed hinges
within the housing ring. Because there are no struts
                                                                         •    Do not use the valve if the use-before-date on the
protruding across the flow orifice, the open disc separates
                                                                              package has expired.
the flow channel into two orifices.
                                                                         •    If the disc disengages undetected handling
                                                                              damage or extreme pressure on the disc may
Indications for Use:
                                                                              cause this. Should disengagement occur, do not
                                                                              attempt to re-engage the disc into the valve
The OmnicarbonTM is indicated for the replacement of
                                                                              housing; the valve should not be implanted.
dysfunction, native or prosthetic, aortic or mitral valves.
                                                                         •    If the valve came in contact with blood, do nor
                                                                              attempt to clean and resterilize such a valve for
Contra indications:
The OmnicarbonTM is contraindicated for patients unable to                    use in another person. Foreign protein transfer
tolerate anticoagulation therapy.                                             and/ or residue from cleaning agents may cause a
                                                                              tissue reaction.


December 2003        Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez            13
     •    Passing a catheter, surgical instrument, or             immune system. It must minimize blood clotting and
          pacemaker lead through the OmnicarbonTM valve           damage to the blood with minimum amount of effort.
          may cause serious valvular insufficiency, damage
          the valve, and/ or cause catheter entrapment.
                                                                  ON-X and common safety problems
     •    Over sizing occurs when too large a valve is
          forced into the tissue annulus. This may cause          Thrombosis is a problem that causes blood clots on the
          adjacent tissue to inhibit the free movement and        working surface of a valve, which impairs valve function.
          full    travel      of     the      valve   disc.       As a thrombus gets bigger, it will eventually block the
                                                                  moving parts so that the valve can no longer open and / or
     •    No hard, sharp instruments should come in               close fully. By using extremely smooth ON-X carbon and
          contact with the disc or valve housing ring may         by designing the valve to induce smooth flow and thorough
          cause scratches or other surface imperfections          self-cleaning, ON-X reduces the risk of thrombosis.
          which may result in blood injury, thrombus
          formation    and/     or   structural   damage.         In the case of tissue encroachment, the body’s healing
                                                                  process can also impair valve function. As the body heals
     •    A valve soiled by fingerprints or foreign               around the mechanical heart valve, tissue builds up around
          materials may cause clotting or blood damage.           the valve. This becomes a problem if the tissue grows over
                                                                  the valve and begins to block it or restrict the moving parts.
                                                                  The ON-X valve was designed with leaflet guards and
                                                                  optimized length to ensure that tissue doesn’t interfere with
b. ON-X Carbon [17]                                               valve function.

Dr. Vincent Gott compared the clotting tendencies of              In the blood damage problem, turbulence and rapid
silicon carbide, pyrolytic carbon alloyed with silicon            changes in pressure can affect the blood flow. The longer
carbide and pure pyrolytic carbon. Pure carbon was shown          flared body of the ON-X smoothes flow. The leaflets are
to be least thrombogenic. MCRI overcame the need for              free to open completely to align with the flow and only
silicon carbide by applying new technologies to the               move a short distance to close, which reduces turbulence
pyrolytic carbon manufacturing processes. Without silicon         and buffers. The ON-X valve minimizes damage to blood
carbide, the pure carbon’s surface finish is unmatched in         cells.
purity and smoothness. There was an additional reward in
purifying carbon. ON-X carbon is 50% stronger than                Typical mechanical properties of ON-X Carbon are given
previous carbons. Its added flexural strength is essential to     in table 2.
the manufacturability of the ON-X valve’s sophisticated
design.


How does ON-X work?

Like natural valves, mechanical heart valves are one-way
valves that are opened and closed by the action of the blood
pushing on flaps known as leaflets. The ON-X valve’s
leaflets (Figure 9) are somewhat like double doors that
open and close but never latch. In the case of doors, if the
wind blows from one direction, the doors will be blown
open. If blown from the other direction, the doors will be
close. This analogy is an over simplification as the
demands of the body can be both rigorous and subtle.


Safety and efficiency of ON-X

 Two measures of a good mechanical heart valve are safety
and efficiency. To be safe, a valve must not wear out, break      Figure 9. ON-X mechanical heart valve [17].
or malfunction. It must not be ejected by the body’s


December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             14
Table 2. Typical surface and mechanical properties of On-           Table 3. Biocompatibility Tests & Results [9].
X Carbon [9].
                                                                                    Tests                       Results
                                                                      Cytotoxicity L-929                  non-cytotoxic
            Property                  Units          On-X             Membrane Elution
                                                                      Sensitization ISO                   0% sensitization:
  Wear Resistance               mm3/km, 10-6     <1.23                Kligman                             Grade I
                                                                                                          sensitization
  Coefficient of Friction       ------           0.15                                                     rate, not significant
                                                                      Irritation Saline CSO               negligible irritant
  Young's Modulus               GPa              26                   Acute Systemic                      negative
                                                                      Toxicity Saline CSO
  Flexural Strength             MPa              490                  Rabbit Pyrogen                      non-pyrogenic
                                                                      USP Physical / Chemical             passes USP
  Density                       gm/cm3           1.9                  Screening Tests                     standards
                                                                      Mutagenicity Ames                   non-mutagenic
  Strain to Failure             %                1.6                  Hemolysis Direct Contact            non-hemolytic
                                                                      Rabbit Blood
  Strain Energy                 MPa-mm/mm        7.7                  Complement Activation               non-activating

  Residual Stress               MPa              18.2               The titanium alloy Ti-6Al-4V is used as the carrier
                                                                    structure for replacement heart valves. The titanium is ring
  Fracture Toughness            MPa m1/2         1.67               shaped and supports the moving mechanisms of the
                                                                    replacement valve. It also carries the polyester structure
                                                                    that binds the valve to the tissue.
  Fatigue Threshold             m/cycle          1.11
                                (DK70.3)
                                                                    Medical grade titanium alloys have a significantly higher
  Fatigue Crack Velocity        m/cycle, 10-15   3.98               strength to weight ratio than competing stainless steels.
                                                                    The range of available titanium alloys enables medical
  Critical Surface Tension      dynes(cm)        42                 specialists’ designers to select materials and forms closely
                                                                    tailored to the needs of the application. The full range of
  Surface Roughness             Ra(nm)           33.9               alloys reaches from high ductility commercially pure
                                                                    titanium used where extreme formability is essential, to
  Surface Chemistry             Atomic %         ~85                fully heat treatable alloys with strength above 1300 MPa
  Carbon                                                            (190 ksi). Shape–memory alloys based on titanium further
                                                                    extend the range of useful properties and applications. A
  Surface Chemistry             Atomic %         0                  combination of forging or casting, machining and
  Silicon                                                           fabrication are the process routes used for medical
                                                                    products.
  Surface Chemistry             Atomic %         ~15
  Oxygen                                                            Functional Requirements

                                                                    The following requirements must be satisfy to use titanium
                                                                    as a biomaterial in heart valves implants:
                                                                         •   The body’s immune system must not attack the
6. Titanium (Ti) [24 and 27]                                                 biomaterial.
                                                                         •   Compatible with body tissues and fluids
The high strength, low weight, outstanding corrosion                     •   Must has strength, flexibility and hardness
resistance possessed by titanium and titanium alloys have                •   Must be nontoxic, nonreactive or biodegradable
led to a wide and diversified range of successful                        •   The valve must also be anchored to the inside of
applications which demand high levels of reliable                            the heart.
performance in surgery and medicine. More than 1000                      •   The material must not exhibit mechanical fatigue
tones (2.2 million pounds) of titanium devices of every                      over the device’s lifetime.
description and function are implanted in patients
                                                                         •   The material’s surface must have an acceptable
worldwide every year.
                                                                             low propensity for thrombus formation, as well as
                                                                             the best possible blood compatibility
                                                                         •   Must not be prone to calcification.


December 2003       Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez            15
Titanium Performance in Medical Applications                       Advancements: Titanium

The titanium alloy Ti-6Al-4V is classified as biologically         Material selection for implantable medical devices has
inert biomaterial or bioinert. Titanium is judged to be            improved with the availability of nitinol, or “NiTi”, a
completely inert and immune to corrosion by all body               nickel-titanium alloy that has proved to be biocompatible,
fluids and tissue, and is thus wholly biocompatible. As            durable and non-thrombogenic.
such, it remains essentially unchanged when implanted into
human bodies because of its excellent corrosion resistance.        Researchers at University of California, Los Angeles
The human body is able to recognize bioinert materials as          (UCLA) have designed thin-film NiTi semi-lunar heart
foreign, and tries to isolate them by encasing them in             valve for use in both surgical and non-surgical
fibrous tissues. However, they do not illicit any adverse          (transcatheter) human heart valve replacements:
reactions and are tolerated well by the human body.
Furthermore, they do not induce allergic reactions such as              •    Surgically implantable thin-film NiTi valves are
has been observed on occasion with some stainless steels,                    undergoing in vitro testing to determine their
which have induced nickel hypersensitivity in surrounding                    functionality, durability and corrosive properties;
tissues.
                                                                        •    Designs and prototypes of percutaneously
                                                                             inserted catheter-based thin-film NiTi valves are
The favorable characteristics of titanium including
                                                                             under continuing development.
immunity to corrosion, biocompatibility, strength, low
                                                                        •    Use of thin-film nitinol as a novel material for the
modulus and density. The lower modulus of titanium alloys
                                                                             development of improved human prosthetic heart
compared to steel is a positive factor. Two usefulness
                                                                             valves for surgical implantation and for
parameters of the implantable alloy are the notch
                                                                             percutaneous insertion.
sensitivity. The ratio of tensile strength in the notched
versus un-notched condition and the resistance to crack
propagation, or fracture toughness. Titanium scores well in
                                                                   7. Biomaterials versus stainless steel (Table 4)
both cases. Typical NS/TS ratios for titanium and its alloys
are 1.4 - 1.7 (1.1 is a minimum for an acceptable implant
material).    Fracture toughness of all high strength
                                                                   Alumina is a versatile material with applications in
implantable alloys is above 50 MPam-1/2 with critical crack
                                                                   medicine, because it has good compatibility with human
lengths well above the minimum for detection by standard
                                                                   environment. Compared to stainless steel, mechanical
methods of non-destructive testing. The two most common
                                                                   properties of Alumina are worst in modulus of elasticity,
types of Ti-6Al-4V used for the implants are Ti-6Al-4V
                                                                   shear modulus, thermal expansion coefficient. Alumina is
Grade 5 and Grade 23.
                                                                   better in stress, strain and safety factor. Stainless steel is
                                                                   more resistant but it is not utilized on heart valve implants,
Ti-6Al-4V (Grade 5)
                                                                   because it doen not have good biocompatibility with human
This alpha-beta alloy is the workhorse alloy of the titanium       blood. Alumina is a bioceramic while stainless steel is a
industry. The alloy is fully heat treatable in section sizes up    biometal with different density.
to 15 mm and is used up to approximately 400°C (750°F).
Since it is the most commonly used alloy – over 70% of all         Stainless steel is stiffer than the titanium alloy Ti6Al4V.
alloy grades melted are a sub-grade of Ti6Al4V.                    This is explained by the steel’s higher modulus of elasticity
                                                                   (196 GPa vs. 120 GPa). Steel is more rigid than titanium
The addition of 0.05% palladium (grade 24), 0.1%                   (steel’s higher shear modulus: 80 GPa vs. 44 GPa).
ruthenium (grade 29) and 0.5% nickel (grade 25)                    Stainless steel is more fracture resistant than titanium (steel
significantly increases corrosion resistance in reducing           has more tensile strength: 875 MPa vs. 616 MPa). Titanium
acid, chloride and sour environments, raising the threshold        is more resistant to yielding that stainless steel (Ti has a
temperature for attack to well over 200°C (392°F).                 higher yield stress: 950 MPa vs. 700 MPa). See appendix
                                                                   VI.
Ti-6Al-4V (Grade 23)
The essential difference between Ti6Al4V ELI (grade 23)            Pyrolytic carbon is less stiffer than stainless steel. This is
and Ti6Al4V (grade 5) is the reduction of oxygen content           explained by the steel’s higher modulus of elasticity (196
to 0.13% (maximum) in grade 23. This offers improved               GPa vs. 17-28 GPa). Stainless steel is more resistant than
ductility and fracture toughness, with some reduction in           pyrolytic carbon because of its tensile strength is biggest
strength. Grade 29 also having lowered level of oxygen will        compared to pyrolytic carbon (875 MPa vs. 200 MPa) .
deliver similar levels of mechanical properties to grade 23
according to processing.



December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez              16
                                          Table 4. Mechanical Properties of biomaterials

 Property        Units     Alumina       Titanium      Polyetheruretane          Pyrolitic       Stainless        Polyester
                                                                                 Carbon            Steel

Poisson’s        N/A       0.33         0.33           0.40                 0.3-0.4           0.27-0.30         0.33-0.49
Ratio

Hardness         GPa       20.6         2.24           50                   10                5-8.5             90

Young’s          GPa       392          120            0.16                 17-28             196               1.84
Modulus

Shear            GPa       163          44             1-2                  N/A               75-80             0.744-1.586
Modulus
Tensile          MPa       637          616            49.7                 200               875               48.3
Strength
Compressive      GPa       4900         N/A            50-70                900               N/A               59.6
Stress

Yield Stress     MPa       15.4*103     950            11.9                 100               700               59.3

Ultimate         MPa       119          930            50                   N/A               59.3              46.5
Stress

Coefficient      10-6      6.2          11.9           25                   10                N/A               70
of Thermal       per °C
Expansion
(Linear)



Stainless steel is stiffer than the polyetherurethane. This is         the long-term use of anticoagulant therapy. The prosthetic
explained by the steel’s higher modulus of elasticity (196             heart valve should be surgically implanted with ease and
GPa vs. 0.016 GPa). Stainless steel is more fracture                   not interfere with normal cardiac function and anatomy.
resistant than polyetherurethane (steel has more tensile               The normal function of the prosthetic valve should be
strength: 875 MPa vs.49.7 MPa).                                        quiet, should not damage cellular blood elements or cause
                                                                       denaturing of proteins. Finally, prosthetic valves should be
Stainless steel is stiffer than Polyester. This is explained by        readily available, manufactured with ease and relatively
the steel’s higher modulus of elasticity (196 GPa vs. 1.84             inexpensive.
GPa). Polyester is less fracture resistant than Stainless steel
(stainless steel has more tensile strength: 875MPa vs.                 Cardiovascular surgeons must weigh the advantage of the
48.3MPa). Stainless steel is more resistant to yielding that           durability of mechanical type prostheses without the need
polyester (Stainless steel has a higher yield stress: 700 MPa          for long-term anticoagulant therapy. Therefore, physicians,
vs. 59.3MPa).                                                          biomedical engineers and other inventors have yet to
                                                                       design the “ideal” prosthetic valve substitute.
                          SUMMARY
When designing prosthetic heart valves, there are several              One approach to meet the characteristics of the “ideal”
characteristics of natural heart valves one aims to mimic.             prosthetic valve includes new fixing processes for
These include minimal transvalvular pressure gradients,                bioprosthetic valves that greatly improve durability,
minimal regurgitation fractions, central flow characteristics          decrease the incidence of dystrophic calcification and do
and complete biocompatibility. The materials for the                   not change the relatively nonthrombogenic nature of
prosthesis     should    be    durable,     non-toxic     and          existing bioprostheses. Another approach would be the
nonthrombogenic; ideally, the materials should not require             introduction of a new durable, nonthrombogenic material


December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez               17
from which mechanical type prosthesis could be fashioned.          17. http://www.medtronics.com
Some think a synthetic valve which could be constructed
                                                                   18. http://www.mkt-intl.com/ceramics/aluminaphotos.htm
to copy the durability and higher than acceptable
transvalvular pressure gradients.                                  19. http://www.mst.dk/udgiv/Publications/1999/87-7909-
                                                                        416-3/html/bil03_eng.htm
Currently available prostheses are markedly improved over
some earlier valve substitutes, but the search for the “ideal”     20. http://www.polymertech.com/materials/elasthane.html
prosthetic valve continues. To advance towards this goal, a
                                                                   21. http://www.pyrocarbon.com
major breakthrough in either materials science or collagen
biochemistry must occur.                                           22. http://www.sts.org
                                                                   23. http://www.swri.org/3pubs/ttoday/summer99/valve.htm
                ACKNOWLEDGEMENTS                                   24. http://www.titanium.org
                                                                   25. Park,    J.B.   1984.     Biomaterials   Science      and
We would like to thank Dr. Megh R. Goyal, University of
Puerto Rico, for his advice. The authors also to thank Yi-              Engineering. pp.212-216, 252-256 New York: Plenum
Ren Woo (St. Jude Medical Principal engineer) and Carlos
                                                                        Press, New York.
Rosario, M.D. (Mayaguez Cardiology), Prof. Pablo
Cáceres, at University of Puerto Rico for providing                26. Mitamura Y, Hosooka K, Matsomoto T, Otaki K and
technical information.
                                                                        Sakai K. Development of a fine ceramic Heart valves.
                                                                        Journal of Biomaterials Application. Publisher Sage
                      REFERENCES
                                                                        Publication, London.
1.   http://cape.uwaterloo.ca/che100projects/heart/files/test      27. Sharma, Szycher. 1991. Blood Compatible Materials
     ing.htm                                                            and Devices. Technomic Publishing Company. Inc.
2.   http://titaniuminfogroup.co.uk                                     pp.33, 156-163.
3.   http://www.azom.com
4.   http://www.azom.com/details.asp?ArticleID=105                                         GLOSSARY

5.   http://www.azom.com/details.asp?ArticleID=2103                Alloy: a material that consisting of two or more metals or a
6.   http://www.ceramics.nist.gov/srd/summary/scdaos.htm           metal and non-metal.

7.   http://www.cnn.com/HEALTH/library/DS/00421.html               Anesthesiologists:    a     physician    specializing  in
                                                                   anesthesiology (anesthesia is used during some procedures
8.   http://www.domme.ntu.ac.uk/research/biomec/pap...ri           and surgery).
     alchoice.html
                                                                   Anticoagulant: a drug used to thin the blood, keeps blood
9.   http://www.fda.com                                            from clotting.
10. http://www.gla.ac.uk/departments/cardiacsurgery/biog
                                                                   Aorta: the largest artery in the human body, it carries
     _Berraca.htm                                                  blood from the heart to every part of the body.
11. http://www.icr-heart.com/journal/unalloyed_pyrolytic
                                                                   Aortic valve: the valve between the left ventricle and the
     _carbon_for_i.htm                                             aorta.
12. http://www.lib.umich.edu/dentlib/Dental_tables/toc.html
                                                                   Atria (Atrium): upper-receiving chambers of the heart
13. http://www.library.drexel.edu/research/guides/pdfs/ma
     terialproperties.html                                         Calcification: formation of calcium deposits on the surface
                                                                   of the material.
14. http://www.library.drexel.edu/research/guides/pdfs/ma
     terialproperties.html#bio                                     Cardiac catheterization: a highly specialized non-surgical
                                                                   technique that allows cardiologists to examine coronary
15. http://www.mcritx.com/carbon_properties.htm                    arteries for blockage using thin catheters inserted into the
16. http://www.medhelp.org/forums/cardio/archive/848.html          heart.



December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             18
Cardiologists: a physician specializing in the heart.              Valves: flap-like structures, which control the flow of
                                                                   blood through the heart.
Cardio thoracic Surgeons: heart surgeons.
                                                                   Valve surgery: there are a total of 4 valves in the human
Creep: additional strains develop, when loaded for long            heart, valve surgery repairs or replaces damaged or scarred
period of time is applied.                                         valves.
                                                                   Ventricle: the large lower pumping chamber of the heart.
Diastole: during the cardiac cycle, when the heart relaxes
& allows blood to flow in.
Ductile: capable of being drawn out into a thin wire or
thread.

Hingeless : do not have a movable joint by means of which
it can turn on the frame.
Infarct: dead tissue as a result of obstructed blood flow to
the area.

Minimally invasive surgery: techniques that use small
incisions to gain access to the surgical site.

Mitral valve: the valve that separates the left atrium from
the left ventricle.

Modulus of Elasticity: slope of the straight line from the
stress- strain diagram.

Pericardial valve: tissue valve made from bovine tissue.

Poisson’s Ratio: ratio of the lateral and axial strain,
property of materials.

Polish: to make or become smooth and glossy by rubbing.

Porcine valve: tissue valve made from a pig’s aortic heart
valve.

Pulmonary valve: the valve between the right ventricle
and the pulmonary artery.

Regurgitation: backward flow of blood due to inability of
valve to work properly.

Restenosis: recurrence of the blockage or narrowing of the
artery or valve.

Stenosis: narrowing of artery or heart valves.

Strain: elongation per unit of length.

Stress: intensity of force per unit of area.

Systole: in the cardiac cycle, when the heart contracts
(pumps).

Thrombus: a blood clot.



December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez          19
APPENDIX I: FDA POLICIES OF MECHANICAL
MITRAL VALVE REPLACEMENT. [9]

1. ATS Open Pivot® Bileaflet Heart Valve                          2. Subpart D--Cardiovascular Prosthetic Devices
This is a brief overview of information related to FDA's
                                                                  Sec. 870.3925 Replacement heart valve.
approval to market this product. See the links below to the
Summary of Safety and Effectiveness and product labeling
                                                                  (a) Identification. A replacement heart valve is a device
for more complete information on this product, its
                                                                  intended
indications for use, and the basis for FDA's approval.
                                                                  to perform the function of any of the heart`s natural valves.
                                                                  This device includes valves constructed of prosthetic
Product Name: ATS Open Pivot® Bileaflet Heart Valve
                                                                  materials, biologic valves (e.g., porcine valves), or valves
Manufacturer: ATS Medical, Inc.
                                                                  constructed of a combination of prosthetic and biologic
Address: 3905 Annapolis Lane, Suite 105, Minneapolis,
                                                                  materials.
Minnesota 55447
Approval Date: October 13, 2000
                                                                  (b) Classification. Class III (premarket approval).
Approval Letter:
http://www.fda.gov/cdrh/pdf/p990046a.pdf
                                                                  (c) Date premarket approval application (PMA) or notice of
                                                                  completion of a product development protocol (PDP) is
What is it? The ATS Open Pivot® Bileaflet Heart Valve is
                                                                  required. A PMA or
a mechanical heart valve with two leaflets (flap like
                                                                  a notice of completion of a PDP is required to be filed with
structures) in the shape of a circle, each leaflet a half the
                                                                  the Food and Drug Administration on or before December
circle, surrounded by a ring made of polyester fabric. The
                                                                  9, 1987 for any replacement heart valve that was in
leaflets are made of carbon. The valve is used to replace a
                                                                  commercial distribution before May
patient’s own aortic or mitral valve, or another prosthetic
                                                                  28, 1976, or that has on or before December 9, 1987 been
aortic or mitral valve.
                                                                  found to be substantially equivalent to a replacement heart
                                                                  valve that was in commercial distribution before May 28,
How does it work? The ATS Open Pivot® Bileaflet Heart
                                                                  1976. Any other replacement heart valve shall have an
Valve uses two half discs (bileaflets) that open and close as
                                                                  approved PMA or a declared completed PDP in effect
blood flows through the valve to operate like the patient’s
                                                                  before being placed in commercial distribution.
natural heart valve. (Heart valves control the blood flow
                                                                  [45 FR 7907-7971, Feb. 5, 1980, as amended at 52 FR
through the chambers of the heart.)
                                                                  18163, May 13, 1987; 52 FR 23137, June 17, 1987]
When is it used? The ATS Open Pivot® Bileaflet Heart
                                                                  3. Subpart D--Cardiovascular Prosthetic Devices
Valve is intended to replace diseased, damaged, or
                                                                  Sec. 870.3945 Prosthetic heart valve sizer.
malfunctioning natural or prosthetic aortic or mitral valves.
Heart valves may not always work as well as they should.
                                                                  (a) Identification. A prosthetic heart valve sizer is a device
Disease or other heart valve malfunction may cause the
                                                                  used to measure the size of the natural valve
heart valve tissue to thicken, harden, weaken, or stretch. If
                                                                  opening todetermine the size of the appropriate
the valve fails to open and close properly, it can block or
                                                                  replacement heart valve.
interfere with blood flow causing a decrease in the efficient
                                                                  (b) Classification. Class I (general controls). The device is
flow of blood through the heart. This can reduce a patient’s
                                                                  exempt from the premarket notification procedures in
quality of life.
                                                                  subpart E of part 807
What will it accomplish? A patient who has a diseased,
damaged, or malfunctioning heart valve may feel weak,
tired, or otherwise handicapped. Surgical replacement of
the affected heart valve may be an effective option to
improve the patient’s quality of life.

When should it not be used? The valve should not be used
in patients who are unable to tolerate anticoagulant therapy
or the use of blood-thinning drugs.




December 2003     Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez             20
                       APPENDIX II: OTHER PATENTS FOR COMPANY'S PRODUCTION.

                                                           COMPANY NAME                DEVICE DESCRIPTION /
    APPLICATION              DEVICE TRADE NAME
                                                            CITY, STATE, &                 INDICATIONS
   NUMBER / DATE of
                                                                  ZIP
     APPROVAL

P810002/S056               St. Jude Medical® Mechanical St. Jude Medical, Inc.    Approval for the changes to the country
                           Heart Valve – Mater Series                             of origin of the bovine collagen used
5/3/01                     Coated Aortic Valved Graft, St. Paul, MN               for the Hemashield® graft, which is a
                           Model CAVGJ-514 00                                     component of the device. The device,
Real-Time                                               55117                     as modified, will be marketed under the
                                                                                  trade name St. Jude Medical®
                                                                                  Mechanical Heart Valve – Master Series
                                                                                  Coated Aortic Valved Graft, Model
                                                                                  CAVGJ-514 00 in sizes 19, 21, 23, 25,
                                                                                  27, 29, 31, 33 mm, and is indicated for
                                                                                  the replacement of the aortic valve and
                                                                                  the ascending aorta.

P000037                    On-X® Prosthetic Heart        Medical Carbon Research Approval for the On-X® Prosthetic
                           Valve, Model ONXA             Institute, LLC          Heart Valve, Model ONXA in the aortic
5/30/01                                                                          position including sizes 19, 21, 23, 25,
                                                         Austin, TX              and 27/29 mm. This device is indicated
                                                                                 for replacement of diseased, damaged,
                                                         78754                   or malfunctioning native or prosthetic
                                                                                 heart valves in the aortic position.

P790018/S031               Medtronic Hall™ Prosthetic    Medtronic Heart Valves, Approval for a modification to the
                           Heart Valve (Models A7700     Inc.                    controlled environment area for certain
11/24/97
                           and M7700)                                            manufacturing steps.
                                                         Irvine, CA

                                                         92714




December 2003   Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.        21
                                  APPENDIX III: POLYURETHANE PROPERTIES.


            Trade name            CARDIOMAT          MITRATHANE            PAMPUL-3-             Pur 1025/1
                                       610                 2007               AMEO
                                   KONTRON               MITRAL            BEIERSDORF            ENKA AG
                Producer          Cardiovascular        MEDICAL                AG
                                       Inc.          Internacional Inc.
                                                           Segm.
             Pur - type          Polyetherurethane   Polyetherurethane    Polyetherurethane   Polyesterurethane
                                                            urea

        CONCENTRATION                   15                   25                  10                   15
             (%)

           SOLVENTS               THF/ DIOXAN:             DMAC                DMAC                DMAC
                                       2/1

            VISCOSITY              2010 at 30°C        77500 at 23°C        4200 at 23°C        48500 at 23°C
           ( m Pa s) at °C

        DENSITY ( g/cm^3 )          1.11 + 0.03               ?                  ?                   1.12

           HARDNESS                     80                 65 + 5              75 + 5                 87
            ( Shore A)

            TENSILE                    28.0              39.2 + 5.0          49.0 + 7.0              50.6
           STRENGTH
            ( N/mm^2)

         ELONGATION at                  500               775 + 50            605 + 30               649
           BREAK ( %)




December 2003    Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.   22
   APPENDIX IV: ALUMINA MITRAL VALVE AND THROMBUS FORMATION COMPARED WITH OTHER
                                      MATERIALS.




                                            ALUMINA MITRAL VALVE



                                             THROMBUS FORMATION




December 2003   Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.   23
                APPENDIX V: ADVERSE EVENTS OF DIFFERENT HEART VALVE IMPLANTATION.


                                                Early Postoperative Adverse Events
                                                          n (% or cases)




                           Event                             AVR (125)           MVR (70)                 DVR (37)


 Death, all causes                                           4 (3.2)                  6 (8.6)              2 (5.4)
 Thromboembolism, AII                                        5 (4.0)                  0                      0
 Thromboembolism, TIA                                        2 (1.6)                  0                      0
 Thromboembolism, Nontransient                               3 (2.4)                  0                      0
 Valve Thrombosis                                            0                        1 (1.4)                0
 Anticoagulant – Related Hemorrhage, major                   0                        1 (1.4)                0
 Endocarditis                                                1 (0.8)                  0                      0
 Perivalvular Leak, major                                    1 (0.8)                  0                      1( 2.7)
 Pannus Tissue Interference                                  0                        0                      0
 Hemolytic Anemia                                            0                        0                      0
 Structural Failure                                          0                         0                     0
 Unacceptable Hemodynamics                                   0                         0                     0
 Other Nonstructural Dysfuction                              0                         0                     0
 Reoperation                                                 2 (1.6)                  1 (1.4)                1 (2.7)
 Explantation                                                2 (1.6)                  1 (1.4)                0




Abbreviations; n= number of patients
AVR = aortic valve replacement, MVR = mitral valve replacement, DVR = double valve replacement
TIA = transient ischemic attack




December 2003         Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.   24
                          APPENDIX VI: MECHANICAL PROPERTIES COMPARISON WITH OTHER BIOMATERIALS.




                                 APPENDIX VII: TABLE OF TITANIUM YIELD STRENGTH VERSUS DENSITY.


                                                          Yield Strength vs Density


                                                                                                              Heat treated
                         Ti6Al4V                                                                               Annealed
 Yield Strength (MPa)




                                                                                   Unalloyed Grade 4
                                                                                                              Cold-worked
                        CoNiCrMo
                                                           Wrought annealed
                        F 562
                                                           As-cast
                                                                      Cold-worked
                         316L SS
                                              Annealed
                                           Cold-worked
                            Ta
                                      Annealed
                                 0       20       40       60        80      100      120      140      160       180      200
                                                                      Density (g/cm^3)




December 2003                    Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.   25
                                              APPENDIX VIII: EXERCISES

1. AXIALLY LOADED MEMBERS: A polyester graft supports a tensile load of 48 lb when the heart beats. The inner
and outer diameter of graft are d1 = 0.143 in. and d2 = 0.169 in. respectively, his length is 0.5 in. The elongation due
to the load is 0.018 in. Find the stress and strain.




Solution:

Given:

d1 = 0.143 in

d2 = 0.619 in

L = 0.5 in

P = 48 lb

Calculate the cross-sectional area:

      A = ( π / 4 ) ( d2² - d1² ) = ( π / 4 ) ( 0.169² - 0.143² ) = 0.00637 in. ²

Calculate the stress:

    σ = P / A = 48.0 lb / 0.00637 in. ² = 7,535 psi

Calculate the strain :

    ∈ = δ / L = 0.018 in. / 0.5 in. = 0.036




December 2003    Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.      26
2. AXIALLY LOADED MEMBERS: A stainless steel implant of 2.0 in is put inside a wrist bone. Its temperature rises of 27ºC
to 45ºC. Calculate the thermal strain and temperature-displacement relation of the metal. Assume the thermal expansion
coefficient (α) of the implant is 0.000007/ºC.




                                                                            Bone
                                                       metal
                                                      implant




Solution:

Given:

T1 = 27ºC

T2 = 45ºC

L = 2.0 in

α = 0.000007/ºC

Є T = α(T2 - T1)

= (0.000007/ºC)(45ºC - 27ºC)

= (0.000007/ºC)(18ºC)

= 1.26 x 10-4

δT=LЄT

= (2.0in)(1.26 x 10^-4) = 2.52 x 10-4 inch




December 2003      Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.    27
3. TORSION: A metallic bar made of stainless steel is implanted in the human vertebral column. The bar has a
diameter d=0.025 m, length L=0.3075m and shear modulus of elasticity. The bar is subjected to torque T, acting at
the ends.
     a. If it has a load of 1,000N at .15375 m, calculate the magnitude of the torque.
     b. Using the results of the part a, what is the maximum shear stress in the bar?
     c. What is the angle of twist between the ends?
     d. If the allowable shear stress is 2.037 MPa and the allowable angle of twist is 2.5o rad, what is the maximum
          permissible torque?


                                                      d = 0.025m



                                                  L= 0.3075m

Solution:

Given:

d = 0.025 m
L = 0.3075 m
G = 196*106 Pa
P = 1000 N
‫ح‬allow = 2.037 MPa
             o
  allow = 2.5 rad


    a.    T= Pd
           = ( 1000N)(0.025m) = 25 Nm

    b.    ‫ح‬max = 16T/ πd3
              = (16)(25N.m)/ π(0.025m)3 = 8.1487 MPa

    c.    Ip= πd4/32
          = π (0.025m)4 /32 = 38.3 nm4

          = TL/ GIp
          = (25Nm)(0.3075m)/(196x10^6 Pa)(38.3x10^-9m4) = 1.2º rad

    d. T1= πd3‫ح‬allow/16

         ‫ح‬allow= P/A =1,000N /[(π/4) (0.025m)2] = 2.037 MPa
         T1= π (0.025m)3(2.037x10^6 Pa)/16 = 6.25 Nm

         T2= GIP allow/L
           = [(196x10^9N/m2)(38.4x10^-9m4(2.5º)( π rad/180º)] / 0.3075m = 1068 Nm

         The maximum permissible torque is smaller of T1 and T2.

         T1=6.25 Nm



December 2003    Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.   28
4. COMPRESSION: An open alumina mitral valve has an inner radius of 0.0508 m and outer radius of 0.0635 m. It is loaded by
a compression blood flow equal to 1,750 mm Hg. A) Determine the force to the valve sectional area. B) Find the maximum shear
stress on this mitral valve prosthesis. ***Hint: Use correct units.




                                                                               P = 1,750mmHg




                                                         Free Body Diagram

Solution:

Presion = Force/Area                  Force = Area x Presion

Force = (1,750 mmHg) π(.0635+. 0508)2

Force =22.86 mm Hg-m2
22.86 mm Hg (101.325KPa/ 760 mmHg) = 3.048 KPa

Maximum Shear Stress = (V*Q)/(I*b) = (4*P)/(3*π) =
[4 (3.048 KPa)/ 3π] * [(R22+R2R1+R12)/(R24-R14)]
12.192 [(0.06352 + 0.1143 + 0.05082)/ (0.06354 - 0.05084)] =

Maximum stress = 1.47/9.6x10-6= 153.135 KPa




December 2003    Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.           29
5. TENSION, COMPRESSION AND SHEAR: An artificial mitral valve has a carrier structure made of a titanium Ti6Al4V ring.
The ring has a diameter of 30 mm. This ring is formed into a round bar to be used in the lab for experiments with a tensile test
machine. The bar is stretched to a final length of 190 mm. Find the tensile load applied to the specimen and the dilatation.
           Ti6Al4V Poisson’s Ratio = 0.33
           Ti6Al4V Young Modulus = 120 GPa
           Ti6Al4V Yield Stress = 950 MPa

Titanium ring:




                                                                               1.5 mm




Solution:

a. Initial Length of Bar:
            Circumference =2*pi*(30)
                          = 188.50 mm

b. Strain:
             (Lf – Li) / Li = (190-188.5)/188.5
                            = 0.0079

c. Normal Stress= E*Strain
                     = (120*10^9)*(0.0079)
                     = 948*10^6 Pa

d. Hook’s valid?
                         948*10^6 Pa is less than Yield Stress (950*10^6 Pa)

e. Tensile Load (P) = normal stress*Area
                   P = 948*10^6 Pa*(pi/4)*(0.0015)^2 = 1675.25 N (Tension)
f. Dilatation (e) = Strain/(1-2V)
                 = 0.0079*(1-2(0.33))
                 = 0.002686*100
                 = 0.2686 %




December 2003        Applications of Engineering Mechanics in Medicine, GED at University of Puerto Rico, Mayagüez.          30

								
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