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1 BIOMECHANICS OF HEART VALVES

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					     Conscience is the inner voice that warns us that someone might be looking. -- H. L. Mencken

                                   BIOMECHANICS OF HEART VALVES1
       Dimitrios Psalidas Rodríguez, Josie R. Bohn Sauveterre, Mariano Matos Santiago and Reyinald García Montañez2

                        ABSTRACT                                    years, the field of heart valve replacement became one of
The heart is a vital part of the human anatomy because              the major growth areas in cardiac surgery, and it is
it functions as a pump to circulate blood throughout the            estimated that over 150,000 valves are replaced in the
body. Heart valves allow the heart to pump blood to                 world every year. Moreover, world demand for these
specific locations efficiently. These valves are prone to           devices continues to expand at a rate of 10% to 12% per
disease and malfunction, and can be replaced by                     year.
prosthetic heart valves. The two main types of                                     NATURAL AORTIC VALVE
prosthetic heart valves are mechanical and
bioprosthetic. The mechanical valves are excellent in               An aortic valve consists of three leaflets and three
terms of durability, but are hindered by their tendency             corresponding cavities, called sinuses of Valsalva.
to coagulate the blood. Bioprosthetic valves are less               Apertures of the right and left coronary arteries are present
durable and must be replaced periodically. All valve                in two of the sinuses and, accordingly, the sinuses are
types must be durable, because the body is an extremely             named right coronary sinus, left coronary sinus, and non-
hostile environment for a foreign object, including                 coronary sinus. At the lower margin the sinuses join the
prosthetic heart valves. Today, chemical engineers are              left ventricle, and at the upper margin they become part of
researching new designs of prosthetic heart valves.                 the ascending aorta. The leaflets are attached at the base of
Many engineers believe the future lies within the regime            the valve ring (annulus fibrosus). The noduli arantii are
of tissue engineering.                                              thickenings at the middle of the free edge of the leaflet and
                                                                    are believed to be important in reducing the leakage when
                     KEYWORDS                                       the valve is closed. At the commisures the free edges are
Heart Valve, Titanium, Pyrolitic Carbon, Commisures,                connected to the aortic wall. The contact area between two
Elastomers, EPDM polymer, Alumina, Hemodynamics,                    leaflets, called coaptation area, is formed by the lunulae,
Mitral Valve, Pyrogen, Radiopacity, and Regurgitation.              which are much thinner that the leaflet body. They have no
                                                                    load bearing the function, but provide a safety margin for
                    INTRODUCTION                                    the valve to close without regurgitation.
Heart valves are passive devices that open and close about
105,000 times a day.1 Their2 function is to maintain the
unidirectional flow of blood through the heart.

Pathological changes may result in a restriction of the
opening of a valve (stenosis) or loss of competence,
allowing back flow through the closed valve
(regurgitation). Mitral and aortic valves are most frequently
affected. In all cases, the work load for the heart is
increased and cardiac function may be compromised. For
patients with severely symptomatic valve disease, valve
replacement offers improvement of the cardiovascular
function, long term survival and quality of life.
                                                                                Figure 1. Human Heart Diagram [9].
Artificial heart valves were first used in the late 50’s and        The leaflets are made from a very pliable, spongy material
early 60’s. Following the success of these early implants,          containing fibers. They consist of three layers: fibrosus,
the replacement of diseased valves with prostheses became           the layer at the aortic side that contains a dense network of
rapidly a routine clinical procedure. During the next 30            collagen fibers; spongiosa, the middle layer, still containing
                                                                    some       Collagen        fibers    but      mainly      acid
                                                                    mucopolysaccharides; ventricularis, the layer at the
1
      This review article was prepared on May 14, 2004 for          ventricular side, containing both elastin and collagen fibers.
      the            course on Mechanics of Materials - I.          Collagen fibers are approximately organized in clearly
      Course Instructor: Dr. Megh R. Goyal, Professor in            visible bundles. They seem to originate mainly from the
      Biomedical Engineering, General Engineering                   Leaflet attachment close to the commisures and run
      Department, PO Box 5984, Mayagüez, Puerto Rico                circumferentially. Elastin fibers, on the other hand, are
      00681-5984.          For       details       contact:         randomly oriented and not visible. Collagen and elastin
      m_goyal@ece.uprm.edu           or      visit      at:         fibers have remarkably different mechanical properties.
      http://www.ece.uprm.edu/~m_goyal/home.htm                     Collagen can elongate no more than 2 to 4% from their
                                                                    relaxed length, whereas Elastin can elongate up to 100%
2                                                                   from their original length.
      The authors are in alphabetical order.


     May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez            1
                                                                  particularly beneficial to seek an alternative for the rigid
The specific orientation of collagen fibers and the random        conduit proposed here since the large internal surface area
orientation of elastin fibers determine the material              (approximately 4 500 mm²) is an order of magnitude
properties of the leaflets. In the circumferential direction      greater than the exposed surface area of a typical
the main role is played by the collagen bundles, which are        conventional valve of the same diameter.
very important for leaflet durability. They transfer the load
on the leaflets in the closed state to the sinuses, such that     The homodynamic characteristics of the valve should be
leaflets and sinuses form a unique bearing structure. In the      preserved. This involves undisturbed blood flow, a low
axial direction the leaflets are much more extensible,            pressure drop over the valve, low incidence of hemolysis
indicating the elastin fibers are mainly being strained.          and a gradual valve closure which starts during systole and
Indeed, the leaflets must be quite elastic in the axial           is enabled by the preserve of cavities formed by the sinuses
direction to maintain the sharp curvature at the coaptation       of Valsalva. As a consequence valve prosthesis should have
area without producing high stresses.                             leaflets made of soft material and preserve sinuses of
                                                                  valsalva or replace them. Leakage is kept low in the natural
Lately leaflets have been shown to be curved in only one          valve by the lunulae, which form the cooptation areas and
direction thus being cylindrical. In the cylindrical leaflets,    the noduli arantii. In a synthetic valve the leaflets should
stresses in circumferential direction are higher than in axial    also have the possibility to form sufficient mutual contact
direction. The structure of the leaflet is best suited to         area to prevent too much regurgitation. In a stented valve,
withstand these stresses. The shape of the leaflet is also        where the leaflets are attached at the outside of the stent,
critical for its ability to reverse curvature. A single curved    this implies very narrow stent post with a relatively sharp
surface can reverse curvature more easily and with less           edged at the top of the aortic side. For both valve types the
creasing than a doubly curved surface. Since the leaflets         free edges of the leaflets should be long enough.
must reverse curvature each time the valve opens and
closes, it is advantageous for them to have a cylindrical         The natural leaflets have fiber reinforced structure. They
shape rather than a spherical shape.                              consist of an elastin network with circumferential oriented
                                                                  bundles of, much stiffer, collagen fibers causing a marked
During diastole, aortic pressure is higher than the               anisotropy. Theses fibers run into the aortic wall, to form a
ventricular pressure; the valve is closed. Next, the left         strongly integrated structure. The load of the leaflets in the
ventricular pressure increases causing the commisures to          closed configuration, the pressure difference between the
move outward thereby pulling the leaflets to produce a            aorta and the left ventricle is manly taken up by the collage
stellate orifice. The aortic valve initially opens without any    fibers. The elastin network is thinner at the attached to the
detectable forward flow.          After expansion of the          aortic wall, this part functions as a hinge. The elastin
commisures, the entire leaflets can move to the open              network function manly a very flexible seal giving a high
position without bending in the radial direction. If the base     degree of mobility during opening and closing this
did not expand, the leaflets would become redundant,              separation if thought to be the natural solution of the
function and geometry would become abnormal and high              conflicting demands of strength and leaflet flexibility.
deformations would occur that could lead to calcification.        Therefore valve prosthesis should have fiber reinforced
Next, the forward flow is responsible for the full opening of     leaflets in which the combination of materials mimics the
the leaflets.                                                     natural composite. Moreover, to obtain the same
                                                                  mechanical integration of the leaflets and wall, the fibers of
According to Padula [1965, 683-689.], the aortic valves           the stented valve should run on the outside of the stent from
opens fully very early in systole, but only for a very short      leaflet to leaflet and, for the stenless valve the fibers should
time after which the valve aperture becomes triangular.           run from leaflets into the wall.
According to van Steenhoven, the shape of the fully opened
valve is related to peak flow and stroke volume. The              The leaflets are attached to the aortic wall which acts as a
higher the peak flow the more a circular geometry is              flexible suspension. This determines to large extent the
approximated.                                                     shape of the leaflets during opening and closure: the very
                                                                  unfavorable wrinkling of the leaflets which causes high
                CHOICE OF MATERIAL                                strain and bending stresses is prevented. Moreover a
                                                                  flexible suspension is expected to reduce stress peaks just
                                                                  after valve closure. In conclusion a stenless valve is
The general approach in materials selection for mechanical
                                                                  preferred.
heart valves is to try and produce a surface which is so
smooth that blood cells will roll along them and not
                                                                  A maximum resistance against fatigue is required to
become attached. Hence pyrolitic carbon, with its
                                                                  maximize the lifer span. Thus is translated in choosing
inherently dense, glassy structure and its ability to be
                                                                  durable materials, and the need form minimum stresses or,
highly polished, is a popular choice. Furthermore its
                                                                  at least concentrate stresses in specially incorporated load
electrical conductivity is useful in allowing it to become
                                                                  carrying parts and finally Appling design rules known for
electrostatically charged so that it can repel the blood cells.
                                                                  improving the fatigue behavior of a structure and causing
Nevertheless, it is customary to use chronic anticoagulant
                                                                  safe behavior. The latter means the along with the fiber
therapy with all mechanical valves, even those constructed
                                                                  reinforcement that mimics the collagen fiber structure a
largely from pyrolitic carbon, indicating that this approach
is not necessarily the best one to pursue. It would be


May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                      2
random network of fibers is required that further lowers in       polyethylene fibers (HP-PE, DSM) are applied as
the matrix and acts as a crack arresting medium.                  continuous fibers. Their diameter is 0.06 mm and
                                                                  relatively low Young’s modulus of 30 GPa. Also other
Fiber reinforcements are not a new idea however one               fibers were tested, for instance Lycra fibers, but their
should look for the optimal fiber layout within the limits of     strength to break was too low for our application. Chopped
the manufacturability of theses layouts. Such a layout can        Pe fibers, with a length of about 5 cm and diameter less
only be found if a numerical model of the valve is available      than 0.05 mm were considered. However, nylon fibers were
as a design tool. All important aspects must be incorporated      found to bee too stiff for our application, so chopped PE
in the model.                                                     fibers were chosen.

The design of a complex structure such as a synthetic             Knitted Nylon and polyethylene networks were also tested
leaflet prosthesis, which has to function under very              for reinforcement instead of continuous fibers. Knitted
complex and demanding conditions, is a very difficult task        composites are in general easier to manufacture than wound
which asks for a deliberate approach. Not all problems can        composites. To manufacture a composite with continuous
be solved at once, they have their own ranking. For               fibers the fiber winding method. Various fiber layouts can
example, when the design and manufacturing procedure are          be made and combined. The fist one is unidirectional fiber
such that materials are interchangeable, studying                 layout. Fibers run parallel to the circumferential direction,
biocompatibility and in vivo testing can be postponed until       imitation the distribution of collagen fibers in a natural
a mechanical and hemodyamical optimal design is                   leaflet. Such layout influences mostly the composite
obtained. Typical dimensions are shown in figure 1.               behavior in the circumferential direction (Figure 3).

                    EPDM MATERIAL                                 The second one is a sinusoidal lay out (Figure 4).
                                                                  Depending on the angle between the fibers and the axis of
The synthetic material for the matrix should be                   the cylinder, the composite has different material
biocompatible, flexible and easy to process in any desired        properties. If the angle is 45, for instance the properties will
shape. EPDM rubber can be used as it matches these                be the same in the circumferential direction and more
requirements, but polyurethane or silicon rubber could also       complaint in the axial direction in this example the
be used. EPDM (Ethylene-Propylene-Diene-monomer)                  sinusoidal fiber layout has an angle of circa 70 with respect
rubbers belong to the family of the ethylene-propylene            to the circumferential direction. This kind of layout
rubbers. Copolymerization of ethylene with propylene              prevents propagation of cracks in the leaflets. The effect of
gives an EPM rubber, which needs to be cross linked. For          reinforcement with different layouts on the most important
that a third monomer, diene is added in small amounts. The        mechanical properties of EPDM is shown in Table 1.
diene used in EPDM rubber is mostly dicyclopentadiene
(DCPD). To cross link the rubber peroxide is used.
Activated by temperature, the peroxide molecules break
and form free radicals, which react with the polymer
molecules, creating cross links between them. EPDM
rubber is suitable to make bioprosthesis because it has a
low swell grade in blood, a good biocompatibility, high
chemical stability, and favorable mechanical properties.
Moreover, as the polymer chain of EPDM is completely
saturated, the resistance to degradation from oxygen and
chemicals is excellent. For this reason products made of
EPDM are very durable.

The properties of polyurethane plastic are similar to the         Figure 2. Typical dimensions of a synthetic leaflet
issue that the human body uses in the valve construction.         prosthesis. [10].
So far the EPDM is a material that assimilates the natural
tissue but it is also reinforced by collagen distributed in the
same direction as the circumference of the valve
(unidirectional) the purpose if this is to transfer the stress
the base and walls of the valve were it can hold more stress.
This why fiber reinforcement is the main stress reducing
expedient in the natural valve. By means of the fibers, the
leaflets transfer the load to the aortic walls. Not only fibers
can be used to reduce stresses. A flexible aortic base and
leaflet attachments also seem to improve the performance
and life span of a valve. The goal of this valve is the same      Figure 3. Ethylene-Propylene-Diene-monomer [10].
concept of the original human valve. To accomplish this
polymer is combined with strong, high modulus
reinforcement; the resulting material has superior properties
compared to the matrix material alone. High performance


May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                      3
     a.   Manufacturing of a stented valve                       developed. EPDM rubber was chosen as matrix material,
                                                                 continuous and chopped PE fibers were used as
                                                                 reinforcement. It is possible to reinforce the material with
 Valves are made on a mold. The first mold used to make a        different fiber layouts. The unidirectional layout reinforces
stented valve, was of Teflon, an auto-lubricant material.        the material in the circumferential direction and it should
The mold had a conical shape, designed to enlarge the            be combined with another layout that reinforces the leaflet
cooptation area between the leaflets. A nylon stent was          in the axial direction. A possible solution is applied on the
inserted in a stent shaped cavity.                               mold during dipping. The manufacturing procedure
                                                                 described here can be used to manufacture prototypes with
Using Teflon for the mold ensures easy removal of the            other matrix fiber materials, such as polyurethane or silicon
valve when the vulcanization process is completed. On the        rubber instead of EPDM rubber.
other hand, Teflon is a soft material that gets easily
scratched and damaged; also the mold can become
deformed after a few vulcanization cycles, due to relaxation
of internal stresses. For these reasons, it was replaced by a
stainless steel shaped cavity. To avoid sticking of the
rubber on the steel, the mold was covered with polymeric
coating. Such coating does not influence the manufacturing
procedure of the properties of the rubber. In this way
removing the valve form the mold becomes as easy as in
the case of the Teflon mold. After insetting the stent in the
stent cavity, the mold is ready for valve manufacturing.

A solution of EPDM rubber was prepared in the following
way: 30 gr. K520 were cut in small pieces and dissolved in
400 gr. Xylene. This gave a solution an appropriate
viscosity for dip coating. Next the solution was mixed at a
temperature of 80 C for about 12 hours to get a
                                                                           Figure 4. Sinusoidal Fiber lay out [11].
homogeneous mixture. When the solution had retuned to
room temperature, the cross linker debenzoyl peroxide was
added (1% in weight).After the Xylene has evaporated,
                                                                         Figure 5. Unidirectional Fiber layout. [10].
leaving a dry rubber layer, the dipping procedure is
repeated. When the second rubber layer is also dry, we can
proceed with the winding procedure.

Given a fiber layout, a data file containing the velocities of
the two motors is created. A motor keeps the mold rotating
while the other motor moves the eye that guides the PE
fiber, coming from a spool. The fiber is kept in position on
the mold with a pressing cylinder. After the winding
procedure is completed it is appropriate to press manually
the fibers on the mold, using again some rubber solution
with low viscosity, to remove air bubbles below the fibers.

Next, the dipping step can be repeated, in order to create a
symmetrical structure of fibers between the rubber layers.
Dipping four times and using a rubber solution for
preweting the fiber, we obtain leaflets with thickness of
abut 0.2 mm, which gives a good compromise between
strength and stiffness of the leaflets.                                Table 1. Mechanical Properties of EPDM [10].
When the manufacturing of the valve is completed the                  Sample         E (MPa)       SF (MPa)       EB (%)
mold is put in an oven to vulcanize the rubber. A curing              EPDM          1.5            7              611
time of two hours at 120 ˚C is enough to make the rubber              non-
suitable for our application. The valve is then carefully             reinforced
removed from the mold with the aid of soapsuds. A valve               Sin-long      110            14             16
produced in this way is free in its open configuration. From          Sin-tran      16             13             45
in vitro tests, it became clear that regurgitation of such a
                                                                      Uni-long      93             10             10
valve is higher than biological or polyurethane valves.
                                                                      Uni-tran      1.5            2.9            419
Another way to increase the cooptation area is to make the
leaflets a bit longer that stent posts. Starting from design
criteria, synthetic prosthesis with three leaflets is


May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                  4
If fibers other that PE are used, probably the winding           mold can be then dipped and wounded as described before.
procedure should be adapted to the fiber mechanical              Eventually, the mold is put in. an oven to vulcanize the
properties. For instance, if lira fibers are used, it is         rubber, and then the valve can be removed from it with the
advisable to prestrain the fibers to a certain degree and then   aid of soapsuds.
wind them on the mold, because of their great extensibility.
 The manufacturing procedure for a stentless valve is more       Starting from design criteria, synthetic valve prosthesis
complicated because the irregular shape of the mold does         with three leaflets is developed. The manufacturing method
not allow the use of a pressing cylinder to keep the fiber in    for making a stented and stentless, fiber-reinforced,
place, during the winding procedure. The fiber must be           synthetic heart valve prosthesis is presented. Fiber-
kept in place manually. Despite this difficulty with             reinforcement is believed to be an important step in
manufacturing, stentless valves are believed to have             reducing stress peaks in the leaflet matrix material, which
superior Hemodynamics and probably long term                     are believed to cause indirectly calcification and eventually
performance.                                                     valve failure. EPDM rubber was chosen as matrix material,
                                                                 and continuous and chopped PE fibers were used as
b. Manufacturing of a Stentless valve                            reinforcement.

Manufacturing a stentless valve (Figure 5) is quite more         It is possible to reinforce the material with different fiber
complicated than a stented one. Two manufacturing                layouts. The unidirectional layout reinforces the material in
procedures are developed applying two different molds.           the circumferential direction and it should be combined
The dipping and winding procedures, as described for the         with another layout that reinforces the leaflet in the axial
stented valve, are followed. The winding procedure is            direction. A possible solution is the use of chopped PE
performed to reinforce the leaflets, with the difference that    fibers, which are easily added to the rubber solution and
fibers must be pressed manually on the mold, because of its      applied on the mold during dipping.
irregular shape, which does not allow the use of a pressing
cylinder. The mold is then partially covered with Teflon         The sinusoidal layout reinforces the material in both axial
tape and put in an oven to vulcanize the leaflets. The Teflon    and circumferential direction. The optimal fiber layout, that
tape prevents the covered rubber from vulcanization              gives the highest reduction of stresses, is determined by
                                                                 computer simulations different leaflet shapes and
Next, the mold is removed from the oven and cooled. The          geometries are possible for the stented valve. The
Teflon tape is removed together with part of the rubber.         manufacturing procedure described here can be used to
The leaflets are now covered with Teflon tape. The dipping       manufacture prototypes with other matrix and fiber
procedure is continued to make the aortic root with the          materials, such as Polyurethane or Silicon rubber instead of
sinuses of Valsalva and a piece of the aorta, creating           EPDM rubber.
continuity between aortic base, commisures and aorta, A
winding procedure is also required to reinforce this outside     If fibers other than PE are used, probably the winding
structure, with a sinusoidal fiber layout.                       procedure should be adapted to the fiber mechanical
                                                                 properties. For instance, if Lycra fibers are used, it is
The valve is put in the oven again for the vulcanization.        advisable to prestrain the fibers to a certain degree and then
There after, it is removed from the mold with the help of        wind them on the mold, because of their great extensibility.
soapsuds, and the Teflon tape is taken out, leaving the
leaflets free from the sinus cavities. Two problems inherent     The manufacturing procedure for a stentless valve is more
to this design are:                                              complicated because the irregular shape of the mold does
                                                                 not allow the use of a pressing cylinder to keep the fiber in
      • The leaflets are made in an open configuration, and
      as a consequence, the regurgitation is too high;
      • The attachment between leaflets and sinus cavities,      place, during the winding procedure. The fiber must be
      at the commisures, is too weak as no fibers run from       kept in place manually. Despite this difficulty with
      the leaflets into the sinuses of the valve.

These problems are solved with a new design for the
stentless mold. Mold II consists of two parts. Part I is used
to make the leaflets. They are not completely open, but in
an half closed configuration. Dipping and winding
procedure are the same as before, except that some PE fiber
ends are left free.

Part II of the mold, representing the sinus cavities and a
piece of the aorta, are dipped separately in the rubber
solution in order to create a rubber layer on the outside.
Next, it is mounted on part I and the free fiber ends can
now be pressed on part II, creating in this way a direct
connection of the leaflet fibers with the sinuses. The whole                Figure 5: Stentless heart Valve [11].



May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                   5
manufacturing, stentless valves are believed to have              Product liability concerns eventually lead to Ethicon's
superior Hemodynamics and probably long term                      decision to terminate the manufacture and sale Biomer in
performance.                                                      the late 1980s.

c. Recommendations:                                               The Polymer Technology Group, Inc. (PTG) developed
                                                                  Biospan segmented polyurethane in response to the crisis
          •    EPDM rubber not very tear resistant, if not        created by the withdrawal of Biomer. Although designed as
               reinforced by fibers, and therefore it could       a direct replacement for Biomer, PTG made several
               be replaced with Polyurethane or Silicon           improvements to the manufacturing process of Biospan,
               rubber, which also should be reinforced by         including scaling up the reaction from 5 to 100 gallons,
               fibers.                                            using glass-lined reaction vessels and depth filters that
          •    The nylon stent should be substituted by a         minimize contamination, and implementing rigorous
               PE stent, to avoid creep problems during           quality control and documentation of the manufacturing
               fatigue tests.                                     procedures. As a result, PTG consistently produces Biospan
          •    The pulse duplicator system should be              batches of precise molecular structure. This consistency,
               adapted to test stentless valves. It has been      combined with an extensive FDA Master File, has
               observed that regurgitation values for the         facilitated the approval process for clinical applications of
               stentless valve are rather high, but from high     Biospan. In its first eight years on the market, the total
               speed video recordings it seems that the           number of clinical VAD and artificial heart cases utilizing
               closure of the valve is complete. Analyzing        Biospan, under branded and private label, has surpassed
               the duration of opening and closing, it has        that of Biomer by thousands of cases.
               been seen that this valve stays open too
               long, and this could be the reason for high        For device components that require high strength,
               regurgitation values. Attempts to increase         flexibility, and fatigue resistance, Biospan should be
               the stiffness of the sinuses and the aortic        considered as a candidate biomaterial. Whereas typical
               walls of the valve do not change much the          urethanes lose stability with increasing soft segment
               results.    It is believed that the pulse          concentration, Biospan SPU has excellent physical
               duplicator system should be adapted to test        properties at nominal 65A Shore hardness. In the absence
               stentless valves.                                  of (cobalt) corrosion products, Biospan not only resists
          •    The numerical model should include fluid-          degradation, but actually increases in molecular weight in
               structure interaction, to correctly simulate       vivo in certain applications. Thus, Biospan is the most
               the behavior of a heart valve.                     elastomeric biomaterial available today with a proven
          •    The prototypes have to be tested for fatigue       combination of biocompatibility, biostability, and flex life
               and in vitro.                                      in many critical applications.

                                                                  In addition to the proven Biospan polymer (i.e., the original
                  BIOSPAN MATERIAL                                Biomer replacement), PTG also offers experimental grades
                                                                  and variations of Biospan with a variety of bulk and surface
Biospan segmented polyurethane (SPU) is the critical              properties to satisfy specific requirements. For example,
biomaterial used in the majority of clinical ventricular          Biospan® C has an aliphatic polycarbonate soft segment
assist devices and artificial heart cases. It is one of the       that may impart additional oxidative stability to long-term
most extensively tested biomaterials on the market, backed        implants, such as pacemaker leads, catheters, and stints,
by a comprehensive FDA Master File. Biospan is the most           where metal oxide-induced oxidation (MIO) is a potential
elastomeric biomaterial available that simultaneously             degradation mechanism. Variations of Biospan and
exhibits an impressive combination of physical and                Biospan C are also available with surface properties similar
mechanical     properties     together    with    biological      to silicone, fluorocarbon, polyethylene oxide, or
compatibility.                                                    hydrocarbon polymers. By incorporating PTG's proprietary
                                                                  Surface-Modifying End Groups™, desirable surface
SPUs have been used as biomaterials (e.g., in heart assist        properties may be obtained, including reduced coefficient
devices) since the late 1960s, when it was shown by               of      friction,     improved       abrasion     resistance,
Boretos and Pierce that Lycra spandex, the SPU                    thromboresistance, and control of wettability. PTG has also
manufactured by DuPont (as textile fiber), has flex-fatigue       developed and patented a new generation of Biospan that
resistance superior to silicone rubber. Ethicon, a Johnson        incorporates silicone into the soft segment, thus delivering
and Johnson subsidiary, licensed the technology from              the biocompatibility and stability of silicone to an
DuPont for medical applications. Ethicon introduced a             extremely soft biomaterial with unique bulk properties.
batch-synthesized version of Lycra spandex under the
tradename Biomer SPU. For 20 years, Biomer was used to            Certain Biospan materials are available unconfigured in a
fabricate a variety of experimental blood pumps, such as          dimethylacetamide (DMAc) solution, suitable for
ventricular assist devices and artificial hearts, including the   fabrication processes such as casting, dipping, spinning,
well-known "Jarvik Heart," implanted in Dr. Barney Clark.         and spraying. Alternatively, PTG has the GMP/cleanroom
Although Biomer performed well in most of these early             facilities to fabricate devices and device components from
applications, it was used in relatively few clinical cases.       Biospan, according to your specifications.



May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                   6
           CHEMISTRY OF BIOSPAN SPU                                            Table 2. Properties of Biospan [4].

Biospan SPU is similar in chemistry to Dupont’s Lycra                                   Procedure
                                                                    Properties                             Value           Units
Spandex. This series of solvent-based elastomers is based                               ASTM
on an aromatic polyetherurethaneurea with a soft segment
of polytetramethyleneoxide (PTMO) and a hard segment of             Initial Modulus     D-1708             850             psi
diphenylmethane diisocyanate and mixed diamines. It
contains an additive package consisting of an antioxidant           Tensile
                                                                                        D-1708             6000            psi
and a copolymer of decyl methacrylate and                           Strength
diisopropylaminothyl methacrylate. Surface properties can
be tailored to the specifications with surface modifying end        Ultimate
                                                                                        D-1708             850             %
groups. Properties of Biospan are listed in Table 2.                Elongation

1. Applications of Biospan                                          Hardness            D-2240             70A             -

Numerous medical devices and technologies have benefited            Glass
from the combination of softness, excellent mechanical              Transition          D-3418             -65             ˚C
properties, stability, and good biocompatibility of Biospan         Temperature
segmented polyurethane. Several clinical applications are
                                                                    Water
presented below where Biospan is used with continued                                    D-570              1.5             %
                                                                    Absorption
success:
                                                                    Weight
                                                                                                                           Dalto
a. Vascular Prostheses                                              Average             D-3593             180,000
                                                                                                                           ns
Synthetic vascular grafts are used to replace damaged               Appearance          NA                 Translucent
vessels in the body, or to by-pass blocked arteries and                                                                    -
veins. Large-diameter vascular grafts fabricated from
Biospan have been shown to retain their elastic                     Solution            NA                 < 25
characteristics long after repeated immersion in hot water,         Concentration                                          %
and also exhibit better thromboresistance than materials
such as PET and e-PTFE.
                                                                 Table 3. Properties of polyurethane and Stainless Steel [4].
b. Left Ventricular Assist Devices (LVADs)
                                                                                                                     Stainless
The left ventricle accounts for 80 percent of heart                 Property          Units      Polyurethane
                                                                                                                       Steel
functions, making bypass of the left side of the heart one of
                                                                    Poisson’s
the most common methods of heart assist. Biospan is used                              N/A            0.40            0.27– 0.30
                                                                    ratio
in this application to fabricate blood pump diaphragms
because of its excellent flexure and wear properties. A             Hardness          GPa             50              5 – 8.50
blood sac of a totally implantable LVAD fabricated from             Young’s
Biospan was implanted for 244 days with no                                            GPa            0.16                 196
                                                                    modulus
thromboembolic complications.                                       Shear
                                                                                      GPa            1–2                 75 – 80
                                                                    modulus
c. Total Artificial Hearts (TAH)                                    Tensile
                                                                                      Mpa            49.7                 875
                                                                    strength
Development of a totally implantable artificial heart               Compressive
remains one of the greatest challenges in biomedical                                  GPa           50 – 70                0
                                                                    stress
engineering. Biospan is currently being used as a bladder
                                                                                                                          700
material in a number of different artificial heart programs,        Yield stress      MPa            11.9
showing a good degree of blood compatibility and physical
                                                                    Ultimate
stability. Biospan also displays low lipid absorption, a                              MPa             50                  59.3
                                                                    stress
distinct advantage of over similar devices manufactured
from silicone rubber.                                               Coefficient
                                                                    of thermal        MPa             25                  N/A
2. Processing of Biospan                                            expansion

To prepare films or components from Biospan solution,
PTG recommends the following protocol:




May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                         7
     1.   The only materials that should ever come into
          contact with Biospan solution are glass, stainless         a.       Do NOT use On-X Carbon heart valve if:
          steel, or Teflon.
     2.   Heat the immediate work area to 30-35 °C.                       •     The prosthesis has been dropped, damage, or
     3.   Provide HEPA-filtered air and adequate                                mishandled in any way;
          ventilation.                                                    •     The tamper evident seal is broken;
     4.   Dry the film or component in the temperature                    •     The serial number tag does not match the
          controlled area. The elevated temperature is                          container label;
          required because dimethylacetamide (DMAc) is a                  •     The expiration date has elapsed
          hygroscopic solvent and will pick up moisture at
          room temperature, resulting in a poor quality
          part.                                                      b.       Do NOT resterilize any On-X Prosthetic Heart
     5.   After the bulk of the solvent has evaporated (20-                   Valve:
          60 minutes), dry the part in an oven at 60 °C (140
          °F) to remove the remaining solvent. Please note                •     Once it is removed from its plastic container;
          that if too much solvent remains when the part is               •     More than 3 times – resterilization of a valve
          placed in the oven, bubbles may form. The oven                        which has passed the sterility expiration date is
          time will vary depending on the dimensions of                         permitted, up to this limit, only if the valve has
          the part.                                                             remained in the original unopened container and
     6.   A final water extraction may remove trace                             undamaged;
          DMAc. Perform this water extraction at 60 °C for                •     With any method other than steam sterilization,
          24 hours or longer, depending on sample
                                                                                with the identified resterilization parameters.
          thickness.

          ON-X PROSTHETIC HEART VALVE                                c.       DO NOT pass a catheter, surgical instruments, or
                                                                              transvenous pacing lead through the prosthesis as
The On-X Prosthetic Heart Valve is a bi leaflet mechanical                    this may cause valvular insufficiency, leaflet
heart valve, which consists of an orifice housing with two                    damage,        leaflet     dislodgment,   and/or
leaflets. The orifice inflow area has a flared inlet designed                 catheter/instrument/lead entrapment.
to reduce flow turbulence, and the outflow rim consists of
leaflet guards designed to protect the leaflets while in the
closed position. The leaflets rotate around tabs located             d.       Handle the prosthesis with only MCRI Prosthetic
within the inner circumference of the orifice ring. In the                    Heart Valve Instruments, particularly during
closed position, the each leaflet forms a nominal angle of                    selection of the valve size, other sizers may result in
40º relative to the plane of the orifice. In the open position,               improper valve selection.
the plane of each leaflet forms a nominal angle of 90º
relative to the plane of the orifice. The leaflets have a travel
arc of 50º to the closed position.                                   e.       Avoid damaging the prosthesis through the
                                                                              application of excessive force to the valve orifice or
 On-X Carbon is a pure unalloyed form of pyrolytic carbon.                    leaflet
The leaflets consist of On-X Carbon deposited on a                   f.       Avoid contacting the carbon surfaces of the valve
graphite substrate, which is impregnated with 10 weight%                      with gloved fingers or any metallic or abrasive
tungsten to provide radiopacity.                                              instruments as they may cause damage to the valve
                                                                              surfaces not seen with the unaided eye that may
The sewing cuff is constructed of polytetrafluoroethylene                     lead to accelerated valve structural deterioration,
(PTFE) fabric mounted on the orifice using titanium                           leaflet escape, or serve as a nidus for thrombus
retaining and 5-0 suture material. This form of sewing cuff                   formation.
attachment to the orifice allows for rotation of the sewing
cuff in situ during implantation. Orientation reference            Alternatives Practices and Procedures
marks are provided on the sewing ring for valve
orientation.                                                       Alternative forms of treatment other than the On-X Carbon
                                                                   include medical therapy with drugs or surgical treatments
The On-X Prosthetic Heart Valve is available in aortic sizes       such as annuloplasty or valvuloplasty with or without the
19, 21, 23, 25 and 27/29 mm. Valve sizes 19mm through              use of implantable materials (i.e., sutures and/ or
25mm are designed for supra-annular implantation, while            annuloplasty rings). When the patient requires replacement
the valve size 27/29 mm is designed for intra-annular              of his/her native or previously placed prosthetic valve, the
implantation.                                                      option of choosing a mechanical or biological valve exists.
                                                                   The choice of replacement valve depends upon factors that
The On-X Heart Valve is contraindicated for patients               include the patient’s age, preoperative conditions, cardiac
unable to tolerate anticoagulation therapy.                        anatomy, and ability to tolerate anticoagulation therapy.

1. Warnings and Precaution:



May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                         8
                                  Table 4. Dimensions (millimeters) of On-X Carbon [21].

                                   Tissue                        External
                                                    Orifice                       Profile           Profile
                                   Annulus                       Sewing                                         Internal
                                                    Internal                      Height            Height
                                   (mounting)                    Ring                                           Orifice
                                                    Diameter                      (closed)          (open)
     Model          Size &         Diameter                      Diameter                                       Area
     Designator     Type

                                   A                D            S                H                 H           mm2


     ONXA-19        19 Aortic      19               17.4         23.0             10.8              13.3        228

     ONXA-21        21 Aortic      21               19.4         26.0             11.9              14.7        284


     ONXA-23        23 Aortic      23               21.4         29.0             13.1              16.1        344


     ONXA-25        25 Aortic      25               23.4         32.0             14.2              17.8        411


     ONXA-          27/29
                                   27-29            23.4         34.0             14.2              17.8        411
     27/29          Aortic




                                                                 rotators are available for each size On-X valve. The
   2.  Magnetic        Resonance        imaging     (MRI)        instruments are reusable.
   Compatibility:
                                                                 Sizers and instrument handles have metallic regions
   The On-X Prosthetic Heart Valve has been shown to             that are bendable. Repeated bending of these metallic
   be MRI safe when tested using systems operating with          regions can lead to fatigue and fracture. To avoid
   shielded static magnetic field strengths of 1.5 Tesla or      instrument fracture during use, the steam should be
   less. Note, however, that the effects of a time-varying       inspected for surface cracks are present, the sizer
   magnetic field were not examined. The testing should          and/or instrument handle should be discarded and
   not cause significant MRI image artifacts or                  replaced. Leaflet probes and rotators are flexible, but
   distortion- Should this occur, this phenomenon                are not intented to be bent to a permanently deformed
   produces no harmful effects to the patient.                   state.

   3. Storage                                                    5. Sizer

   The On-X Prosthetic Heart Valve has been qualified            The sizer is used to gauge the resulting tissue annulus
   for a maximum storage life of 5 years from the date of        diameter after the annulus is prepared for implant. The
   manufacture. The storage life of the On-X valve is            sizer has a bendable steam on each end. The sizer is
   recorded on the outer package label. Appropriate              cylindrical for size 19 mm through 25 mm valves and
   inventory control should be maintained so that                conical for size 27/29 mm valves.
   prostheses with earlier expiration dates are
   preferentially implanted and expiration is avoided. To        6. Profile Sizers
   protect the valve, it should be stored in its outer box       The aortic profile sizer models On-X aortic valve
   until used. The storage environment should be clean,          profile. It is used after sizing to assure fit of the aortic
   cool, and dry.                                                valve without obstruction of the coronary arteries.
                                                                 Aortic profile sizer is provided for size 19 mm through
   4. Accessories                                                25 mm aortic valves, where the valve sewing ring is
                                                                 intented to remain supra-annular. The size 27/29 mm
   The On-X prosthetic Heart Valve is designed to be             aortic valve is an intra-annular design, thus no profile
   used only with MCRI On-X instruments. The                     sizer is supplied for this size valve.
   instruments, supplied separately, are provided in kits
   which include sizers, rotators, a universal instrument        7. Instrument Handle
   handle, and a universal leaflet probe. Sizers and



May 2004    Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez                        9
The instrument handle facilitates holding the valve or
the rotator during surgery. The instrument handle is               Table 5. Mechanical Properties On-X Carbon 21].
comprised of a grip, a bendable stem, and tip. The
instrument handle tip is inserted into the valve holder                   Property             Units        Value
while the valve is still in the package inner container.
The tip is inserted into the valve holder by placing it             1. Critical surface
                                                                                             dynes/cm         42
directly into the slot on top of the valve holder. It               Tension
snaps into place after the application of a light                   2. Surface
                                                                                               Nm             33.9
insertion force. Upon snapping into position, the valve             Roughness
and holder are firmly retained by the instrument                    3. Surface
handle. Removal of the valve from the inner container               Chemistry, Carbon           %             -85
is performed after the valve holder is snipped onto the             (Atomic)
instrument handle.                                                  4. Surface
                                                                                                %              0
                                                                    chemistry, Silicon
8. Rotator                                                          5. Surface
The valve rotator is used for reorienting an in situ                                            %             -15
                                                                    Chemistry, Oxygen
valve and may be used to verify leaflet mobility. The                                                         <
rotator consists of a plastic head with a centrally                 6. Wear Resistance       mm³/km
                                                                                                         1.238*10¯ 6
located leaflet mobility probe and an attached handle.              7. Coefficient of
The rotator is properly oriented for insertation into the                                        -            0.15
                                                                    friction
valve when the cross-bar on the head is aligned with
the leaflet pivot axis and the probe is inserted into the           8. Young’s modulus         GPa            26
central orifice between the leaflets.
                                                                    9. Flexural Strength       MPa            490
The rotator may be used with or without the
instrument handle attached. To attach the rotator to the            10. Density              gm/cm3           1.9
instrument handle, insert the instrument handle tip
directly into the slot on the end of the rotator handle.            11. Strain to failure       %             1.6
The rotator snaps into place after the application of a                                       MPa-
light insertion force.                                              12. Strain energy                         7.7
                                                                                             mm/mm
9. Leaflet Probe                                                    13. Residual stress        MPa            18.2
The leaflet probe is a flexible rod with tapered ends.
The leaflet probe may be used to gently move the
leaflets to verify that they open and close freely.              Table 6. Biocompatibility Test and Results [21].
                                                                              Tests
Accessory Cleaning and Sterilization:                                                           Result
Instruments for implantation of the On-X Prosthetic
                                                                     Cytotoxicity L-929          Non-Cytotoxicity
Heart Valve are supplied separately, NON-STERILE,
                                                                     Membrane Elution
and must be cleaned and sterilized prior to use.
Standard hospital surgical instrument cleaning                       Sensitization ISO           0% sensitization:
procedures must be used. Note: the metallic                          Kligman                     Grade 1
instruments are made of titanium or stainless steel.                                             sensitization rate,
The      plastic   instruments      are    made      of                                          not significant
polyphenylsulfone,     polysulfone,     polyetherimide,              Irritation Saline CSO       Negligible irritant
polytetraflueroethylene, polyetheretherketone, or
silicone. Materials used in these instruments can
                                                                     Acute Systemic Toxicity     Negative
withstand standard steam and flash steam sterilization.
                                                                     Saline CSO
                                                                     Rabbit Pyrogen              Non- Pyrogenic

                                                            USP Physical / Chemical              Passes USP
                                                            Screening Tests                      Standards
                                                                     Mutagenicity Ames           Non-mutagenic

                                                                     Hemolysis Direct            Non-Hemolytic
                                                                     Contact Rabbit Blood
                                                                     Complement Activation       Non-Activating




May 2004      Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez           10
               ALUMINA MATERIAL                               improvement, especially in the treatment of children,
                                                              where the needs for refined flow characteristics,
Bioceramics fullfil a unique function as biomedical           extended wear life and complete biocompatibility is
materials. The development of biomaterials and                most acute. Recent work in biomechanics has resulted
manufacturing techniques has broadened the diversity          in a design for a new valve prosthesis mounted in a
of applications within the human body. Bioceramics            conduit so that it can be used outside the heart as part
satisfy needs as diverse as low coefficients of friction      of a surgical procedure to correct gross congenital
for lubricating surfaces in joint prostheses, surfaces on     anomalies in a child’s cardiovascular system -
heart valves that avoid blood clotting, materials that        essentially a replumbing operation. The success of this
stimulate bone growth and those that can harness              design is evidenced by the fact that an acrylic version
radioactive species for therapeutic treatments.               has been adopted for the inlet and outlet valves of a
Alumina is a traditional bioinert ceramic that has been       novel ventricular assist device - a temporary
used for the last thirty years. It is a highly stable oxide   mechanical heart to sustain heart failure victims until a
known for its general chemical inertness and hardness.        transplant becomes available. The most significant
These properties are exploited for implant purposes,          feature of the conduit valve, however, is the fact that it
where it is used as an articulating surface in hip and        is to be made from alumina when it is intended for
knee joints. Its ability to be polished to a high surface     permanent implantation in children. Early examples of
finish makes it an ideal candidate for this wear              the use of ceramics in biomedical implants were based
application, where it operates against materials such as      on the fact that ceramics such as alumina are very
ultra high molecular weight Mitral Valve                      resistant to wear and chemical attack. The
Replacement. Orthopedic applications include:                 Biomechanics Group, however, also documented that
femoral head, bone screws and plates, porous coatings         alumina will grow a fine covering of non-vascular
for femoral stems, porous spacers, knee prosthesis and        tissue when immersed in the blood stream. This
mechanical heart valves.                                      covering is enough to camouflage the prosthesis from
                                                              any further interaction with the blood, but fine enough
The design of mechanical heart valves has undergone           (<0.1mm thick) not to interfere with the mechanical
continuous development over the last fifty years. The         function of the valve. This means that alumina valves
biocompatibility of alumina has motivated its                 will exhibit not just excellent wear properties but also
consideration for use in prosthetic heart valves: for         unequalled biocompatibility, which will obviate the
example, as a tilting disk in a ceramic mechanical            need for continued anticoagulation.
valve. Because durability and reliability are vital
requirements for such biomedical use, it is critical that     Enormous problems remain, however, in producing an
the mechanical properties and anisotropy of sapphire          alumina conduit valve. The two principal ones which
be well documented and understood. However,                   will be addressed in this work are: alumina is so hard a
because the human heart beats some 40 million times           material that machining is out of the question and so
per year and cyclic fatigue resistance is considered to       some molding process must be developed which can
be a limiting property for many replacement cardiac           produce a finished valve with the accurate internal
valve devices, McNaney, Mitamura, and Ritchie,                shape required to achieve good hemodynamic
focused on the sub critical crack-growth properties of        performance. And second, the tissue covering requires
sapphire.                                                     a porous, textured alumina surface on which to anchor
                                                              itself firmly, but the main body of the conduit valve
When a man-made material is placed in the human               must be in the most dense form of alumina, with
body, tissue reacts to the implant in a variety of ways       virtually no porosity, in order to maintain structural
depending on the material type [24]. Therefore, the           strength. The molding process must therefore
mechanism of tissue attachment (if any) depends on            accommodate variable porosity in some way. Studies
the tissue response to the implant surface. In general,       show that single crystal alumina has better blood
materials can be placed into three classes that               compatibility and more mechanical strength than
represent the tissue response they elicit: inert,             porous alumina. Although, porous alumina doesn’t
bioresorbable, and bioactive. Alumina (Al2O3) is a            inhibit crack growth under fatigue, an advantage
material that is nearly chemically inert in the body and      obtained by having small grain size.
exhibit minimal chemical interaction with adjacent
tissue. A fibrous tissue capsule will normally form           The general approach in materials selection for
around alumina implants. Therefore, tissue attachment         mechanical heart valves is to try and produce a surface
of this material can be through tissue growth into            which is so smooth that blood cells will roll along
surface irregularities, by bone cement, or by press           them and not become attached. Hence pyrolitic
fitting into a defect. This morphological fixation is not     carbon, with its inherently dense, glassy structure and
ideal for the long-term stability of permanent implants       its ability to be highly polished, is a popular choice.
and         often       becomes         a       problem.      Furthermore its electrical conductivity is useful in
                                                              allowing it to become electro statically charged so that
Artificial heart valves have progressed greatly since         it can repel the blood cells. Nevertheless, it is
the first few experimental surgical implants.                 customary to use chronic anticoagulant therapy with
Nevertheless there is still much more scope for               all mechanical valves, even those constructed largely



May 2004       Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez         11
from pyrolitic carbon, indicating that this approach is         failed because of excessive fibrin deposits on the back
not necessarily the best one to pursue. It would be             of the Delrin occluded flaps, but successfully
particularly beneficial to seek an alternative for the          established that the fine, firmly anchored, non-
rigid conduit proposed here since the large internal            vascular tissue covering was formed rapidly on the
surface area (approximately 4 500 mm²) is an order of           alumina surface. Also, the reduction in drug intake
magnitude greater than the exposed surface area of a            greatly improves the quality of life for a recipient.
typical conventional valve of the same diameter.
                                                                Alumina has very high wear resistance but compared
Recent success with short term artificial heart valves          to metal, it has low fracture toughness and tensile
has led to much research in the development of new              strength which means it can be used in compression
long life valves. A proposed design for a porous                only. Even though stainless steel is a tougher material,
alumina Mitral valve has been suggested. Because of             due to potential long term release of niquel and
the bioinertness and low wear rates of alumina the              chrome into the body, it is restricted to temporary
material could be expected to last many years in the            devices, making alumina a better choice of material
body. A second advantage of the porous ceramic is               for mechanical heart valves.
that the pores will support the mechanical attachment
of live cells which will protect the blood from                 In conclusion alumina is a good material to use in
mechanical damage by the implant. Experiments on                heart valves because of its hardness, wear resistance
valve cages bearing a sintered porous metal coating             and excellent biocompatibility. Single crystal alumina
had shown that a tissue covering could rapidly be               has better blood compatibility and more mechanical
formed which would prevent any further contact                  strength than porous alumina, making the first a better
between the blood and the underlying surface material           choice. Nevertheless, pyrolitic carbon, with its
[17]. Since this covering stabilized at a thickness of          inherently dense, glassy structure and its ability to be
about 100 microns, and therefore would not interfere            highly polished, is a more popular choice.
with the action of a mechanical valve, this                     Furthermore its electrical conductivity is useful in
phenomenon offers many attractions for any valve                allowing it to become electrostatically charged so that
intended for implant in children since there would be           it can repel the blood cells. Alumina is a more popular
little need to use chronic anticoagulation after the            choice in hip and knee joints rather than in mechanical
recovery period.                                                heart valves.

This experimental evidence backed up the observation
that orthopedic prostheses made from alumina rapidly
became covered in a fine tissue coating. To confirm
this interpretation of the tissue covering phenomenon,
a series of animal trials were carried out on a
prototype twin-flap Mitral valve with the body
manufactured from DERANOX 997, a high grade
alumina with purity of 99.7%. Including chopped
strands of organic flock filler with the alumina powder
induced an additional porosity of 17%. The filler was
burnt away on firing leaving a network of randomly
connected long pores, with the idea of providing even
stronger anchorage against the surface shear stresses
found in valves of that type. Several valves with
varying features were constructed and implanted, the
test animals being specially bred mini-pigs. The valves




                                              Figure 6. Heart Valves [9].



May 2004     Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez           12
                                           Table 7. Material Properties of Alumina [4].


Property                 Condition             Units         960P              975P    995P        96S        ZTA
Al2O3                                            %           96.0              97.5    99.7        96.0       80.0
Bulk Density                  20oC             g/cm3         3.67              3.75    3.96        3.77        4.1
Tensile Strength              20oC              Mpa           205               205     220         nd          -
Flexural                      20oC             >MPa           375               375     410        295        450
(Bending)
Strength
Elastic Modulus               20oC              Gpa          300               355         375      nd         340
Hardness                      20oC            kg/mm2          10                12          14      nd          16
Fracture                      20oC            Mpa*m1         4-5               4-5         4-5      nd           7
                                                 /2
Toughness
                                o
Porosity                      20 C               %            0                 0       0           0          0
                                                 o
Max. working                    -                C           1600              1650    1700         na        1500
temp
Coef. Thermal            25-300 oC             10-6/oC           7.1           7.2         7.8      6.7            -
Expansion

                         Table 8. Properties of Biomaterials for Mechanical Heart Valves.

Properties            Units         Alumina           Titanium         Polyurethane   Pyrolitic   Stainless   Polymer
                                                                                      Carbon      Steel
Poisson’s Ration      N/A           0.33              0.33             0.40           0.3-0.4     0.27-0.30   0.33-0.44

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 Modulus         GPA           163               44               1.2            n/a         75-80       0.744-1.58

Tensile Strength      MPA           637               616              49.7           200         875         48.3

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 Stress       MPA           119               930              50             n/a         59.3        46.5

Coefficient of        10-6          6.2               11.9             25             10          n/a         70
Thermal               per oC
Expansion
(linear)




May 2004         Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez          13
           GENERAL OBSERVATIONS                               been been for this materials for example electro
                                                              charging the valve so it will repel the blood from its
Synthetic valves aim to combine the advantages of             walls. On the other hand biocompatibility is still a
both mechanical and biological valve prostheses, such         factor if the material is not fully compatible problems
as long durability, flexible materials and natural shape,     with blood flow will occur since blood clogs will start
without the disadvantages, such as anticoagulant              forming in the valve the only remedy for this is
therapy. Moreover, they provide more freedom in the           anticoagulants.
design. However, until now synthetic valves, mostly           important points for this materials are smoothness and
made of polyurethane or silicon rubber, have                  biocompatibility. For example valves of alumina or
durability problems due to calcification.                     carbon with a long period of installation have given
                                                              problems of blood flow due to roughness of the valve
Many studies have shown a like between calcification          walls causing regurgitation some techniques have used
and stresses in the tissue. It has been observed that in      for this materials for example electro charging the
the areas of the leaflets where there are the highest         valve so it will repel the blood from its walls. On the
tensile ad bending stresses, calcification mostly             other hand biocompatibility is still a factor if the
occurs. Although the origin of this phenomenon is             material is not fully compatible problems with blood
still unclear, the aim of many researchers is to reduce       flow will occur since blood clogs will start forming in
the stresses in order to reduce calcification, which will     the valve the only remedy for this is anticoagulants.
increase the lifetime of prosthesis.

The fibers carry the load due to pressure difference                             CONCLUSIONS
and transfer it to the aortic walls, for a stentless valve,
or to the stent for stented valve. The same stress-           Polyurethane seems to be a material with hi potential
reducing mechanism is present in the natural valve,           for this kind of applications even do a polyurethane
where collagen fibers are the load bearing structure.         valve requires much more time since it requires
For carbon or alumina worries about stress are not a          special designs due to its material properties. Many
trouble big focus as for polyurethane. The most               people have benefited from prosthetic heart valves
important points for this material are smoothness and         over the past 30 years. Chemical engineers believe
biocompatibility. For example valves of alumina or            that the future of prosthetic valves lies in the regime of
carbon with a long period of installation have given          tissue engineering. This would improve the
problems of blood flow due to roughness of the valve          biocompatibility factor, and increase the life
walls causing regurgitation some techniques have used         expectancy          of       the        heart        valve
being for this materials for example electro charging         flow will occur since blood clogs will start forming in
the valve so it will repel the blood from its walls. On       the valve the only remedy for this is anticoagulants.
the other hand biocompatibility is still a factor if the
material is not fully compatible problems with blood
flow will occur since blood clogs will start forming in       special designs due to its material properties. Many
the valve the only remedy for this is anticoagulants.         people have benefited from prosthetic heart valves
                                                              over the past 30 years. Chemical engineers believe
                   CONCLUSIONS                                that the future of prosthetic valves lies in the regime of
                                                              tissue engineering. This would improve the
Polyurethane seems to be a material with hi potential         biocompatibility factor, and increase the life
for this kind of applications even do a polyurethane          expectancy          of       the        heart        valve
valve requires much more time since it requires
special designs due to its material properties. Many          of tissue engineering. This would improve the
people have benefited from prosthetic heart valves            biocompatibility factor, and increase the life
over the past 30 years. Chemical engineers believe            expectancy       of      the    heart    valve
that the future of prosthetic valves lies in the regime of
tissue engineering. This would improve the
biocompatibility factor, and increase the life
expectancy          of       the        heart        valve

                                                              alva: A maneuver elicited by bearing down for the
Vals for this materials for example electro charging          purpose of decreasing venous blood return to the right
the valve so it will repel the blood from its walls. On       side of the heart. The Valsalva maneuver can
the other hand biocompatibility is still a factor if the      accentuate certain cardiac abnormalities (murmurs) for
material is not fully compatible problems with blood          the purpose of diagnosis.
flow will occur since blood clogs will start forming in
the valve the only remedy for this is anticoagulants.
                                                              Xylene: A color




May 2004       Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez         14
            ACKNOWLEDGEMENTS                               16.      www.fda.gov
We thank: Dr. Megh R. Goyal at University of Puerto
Rico for his guidance; Medical Carbon Research             17.      www.iom3.org
Institute (MCRI) for information on On-X Prosthetic
Heart Valve; and “Material technology at Eidnhoven         18.      MA RC Analysis Research Corporation,
University of technology Po Box 513 5600 MB                         1996.
Eidnhoven, The Netherlands”                                19.      www.mate.tue.nl

                BIBLIOGRAPHY                               20.      www.materials.unsw.edu
1.       Abrahams, M. mechanical behavior of the
         tendon in vitro. Med. BioL Engng 5 (1967),        21.      Medical Carbon Research Institute, LLC
         433-443.                                                   P000037. 5/3/01, Journals Based on On-X
                                                                    Carbon;
2.       www.asd-online.com

3.       ATS Medical, Inc. Heart Valve                     22.      www.mmat.ubc

4.       www.azom.com                                      23.      www.mse.cornell.edu

5.       BELLHOUSE, B. J. Velocity and pressure            24.      www.polymertech.com
         distribution in the aortic valve. J. fluid
         Mech. 37 (1969), 587-6OO                          25.      ROOM, N. D. Fatigue-induced damage in
                                                                    gluteraldehyde preserved heart valve tissue.
6.       Bellhousb, B. J. Biological tissue in heart                J. Thorac. Cardiovasc. Surg, 76 (1978), 202-
         valve replacement. In The fluid mechanics                  205.
         of the aortic valve. Butterworth, Heinemann,
         1972, pp. 23-47.                                  26.      www.swri.edu

7.       Bernacca, G. M.. Mackay, T. G., Wilkinson,                           GLOSSARY
         R., and Wheatley. D. Polyurethane heart
         valve. Biomaterials 16 (199a), 279-285.           Commisures: In nematodes: Connecting bands of
                                                           nerve tissue.
8.       BLACK, M. M., AND DRURY, P. J.
         Mechanical and other problems of artificial       Elastomers: Polymers of silicone having properties
         valves. Current topics in pathology(1994),        similar to those of vulcanised natural rubber, namely
         127-128                                           the ability to be stretched to at least twice their
                                                           original length and to retract very rapidly to
9.                                                         approximately their original length when released.
10.      Carton, R. W., Dainauskas, J., and Clark,
         R.E.     Elastic properties of single elastic     Hemodynamics: "Hemodynamics is concerned with
         fibres. J. Appl. Physiology 17 (1962), 547-       the forces generated by the heart and the motion of
         551.                                              blood through the cardiovascular system. It is once a
                                                           body of theory and an experimental science, to which
11.      University of Technology, The Netherlands,        physicists and mathematicians have contributed
         1997.                                             equally with physicians and physiologists." William
                                                           R. Milnor, Preface of Hemodynamics 1982.
12.      www.cordis.com
                                                           Hemolysis: Hematology - Disruption of the integrity
13.      Deck, J.D., Thubrikar, M.J., Schneider            of the red cell membrane causing release of
         (1988), 7-16.                                     hemoglobin.

14.      DOSSGHE, K, Vanermen, H., AND                     Mitral valve: Valve with two cusps; situated between
         DaeneN, W. Hemodynamic performance of             the left atrium and the left ventricle
         the Prima Edwards Stentless aortic
         xenograft): early results of a multi-center       Mutagenicity: The degree or measure of the ability to
         clinical trial. Thorac, Cardiovasc.-i. Surg,      cause mutation.
         44 (1996), 11-14.
                                                           Pyrogen: Any substance that can cause a rise in body
15.      www.facct.ntu.ac.uk/research/-                    temperature.
         groups/biomech/projects/heartvalve.html




May 2004    Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez    15
Pyrolytic carbon: Carbon that is deposited on a             Xylene: A colorless flammable volatile liquid
heated graphite substrate by vapor phase                    hydrocarbon used as a solvent.
decomposition of gaseous hydrocarbons, usually
methane, at 1800-2300 degrees Celsius.                                                       Valsalva:           A
                                                            maneuver elicited by bearing down for the purpose of
Radiopacity: opacity to X-rays or other radiation.          decreasing venous blood return to the right side of the
                                                            heart. The Valsalva maneuver can accentuate certain
Regurgitation: Backward flow of blood into the heart        cardiac abnormalities (murmurs) for the purpose of
or between the chambers of the heart when a valve is        diagnosis.
incompetent.
                                                            Xylene: A colorless flammable volatile liquid
Valsalva: A maneuver elicited by bearing down for           hydrocarbon used as a solvent.           Valsalva: A
the purpose of decreasing venous blood return to the        maneuver elicited by bearing down for the purpose of
right side of the heart. The Valsalva maneuver can          decreasing venous blood return to the right side of the
accentuate certain cardiac abnormalities (murmurs) for      heart. The Valsalva maneuver can accentuate certain
the purpose of diagnosis.                                   cardiac abnormalities (murmurs) for the purpose of
                                                            diagnosis.
Xylene: A colorless flammable volatile liquid
hydrocarbon used as a solvent.                              Xylene: A colorless flammable volatile liquid
                                                            hydrocarbon used as a solvent.
                                      Valsalva:      A
maneuver elicited by bearing down for the purpose of
decreasing venous blood return to the right side of the
heart. The Valsalva maneuver can accentuate certain
cardiac abnormalities (murmurs) for the purpose of                                     Valsalva:      A maneuver
diagnosis.                                                  elicited by bearing down for the purpose of decreasing
                                                            venous blood return to the right side of the heart. The
                                                            Valsalva maneuver can accentuate certain cardiac
Xylene: A colorless flammable volatile liquid               abnormalities (murmurs) for the purpose of diagnosis.
hydrocarbon used as a solvent.
                                                            Xylene: A colorless flammable volatile liquid
                         Valsalva:       A maneuver         hydrocarbon used as a solvent.
elicited by bearing down for the purpose of decreasing
venous blood return to the right side of the heart. The
Valsalva maneuver can accentuate certain cardiac
abnormalities (murmurs) for the purpose of diagnosis.




Valsalva: A maneuver elicited by bearing down for           Valsalva: A maneuver elicited by bearing down for
the purpose of decreasing venous blood return to the        the purpose of decreasing venous blood return to the
right side of the heart. The Valsalva maneuver can          right side of the heart. The Valsalva maneuver can
accentuate certain cardiac abnormalities (murmurs) for      accentuate certain cardiac abnormalities (murmurs) for
the purpose of diagnosis.                                   the purpose of diagnosis.

Xylene: A colorless flammable volatile liquid
hydrocarbon used as a solvent.




  Valsalva: A maneuver elicited by bearing down for
the purpose of decreasing venous blood return to the
right side of the heart. The Valsalva maneuver can
accentuate certain cardiac abnormalities (murmurs) for
the purpose of diagnosis.

Xylene: A colorless flammable volatile liquid
hydrocarbon used as a solvent.




May 2004     Applications of Engineering Mechanics in Medicine, GED – University Of Puerto Rico, Mayagüez      16

				
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