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					It’s good to be important but, it’s important to be good - - - Anonymous

                                             Andres Delannoy, Edgard Martínez and Victor Torres 2

Abstract      -- The human nervous system is fairly
comprehensive and complex. It is susceptible to various
disorders, injuries and diseases. Today, the treatments
for these disorders are available to a satisfactory level.
Some of these treatments involve the implant of
prostheses into the body: To replace or fix a certain
part, to stimulate sensory perception or to block a
specific perception such as pain. In this paper, we study
properties of biomaterials for neurological implanted
stimulators. The use of platinum, titanium, lithium, and
silver is extensive and key for most of the treatments to
the brain and spinal cord. Properties, such as strain,
stress, modulus of elasticity, strength biocompatibility
resistance and durability are presented.

Key words – Neurological Stimulators, Brain Prostheses,


Medical technologies and devices are vital, integral
components of patient care and have an impact on the use
in a very concrete way. They make contributions to a large
number of health care areas. The medical devices sector is
in constant evolution: Particularly in the health care sector,
society and citizens directly benefit from technological
progress.                                                                         Figure 1. An implant in the spinal cord [18].

Spinal cord injuries are among the most devastating of all
neurological disorders. In most countries paralysis due to
spinal cord injury occurs at an annual rate of 40 persons per
million, with each individual costing society an average of
$2.3 million in medical care and loss of earnings. In
addition to the financial consequences, permanent paralysis
is accompanied by a drastic change in the life style and
independence. Spinal cord injuries are predominantly
prevalent among younger individuals, with more than 55%
of the injuries incurred by people 16-30 years of age. Life
expectancies after injury, though still lower than normal,
continue to increase and range from 37.1 years for high
tetraplegia to 46.2 years for paraplegia [18].

The numbers in brackets refer to references in the bibliography.
    This review article was prepared on May 12, 2005 for the
    Course on Mechanics of Materials -- 1. Course Instructor:
    Dr. Megh R. Goyal . Professor in Biomedical Engineering
    Department, PO Box 5984, Mayaguez, PR, 00681-5984. For
    details contact: or visit at:
                                                                                      Figure 2. Implant in cerebellar [22].
    The authors are in alphabetical order.

May 2005            Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez                 J1
                RESEARCH ADVANCES

Cerebellar Implants

The principle of Artificial Stimulation of the brain started
with the studies of the functional architecture of the
cerebral cortex that were started in the early twentieth
century. Wilder Penfield observes that electrical
stimulation of the Visual Cortex generally evoke the
perception of points of light [21].

The observations of the visuotopic organization of
electrically evoked phosphenes have lead to investigating
the possibility of using electrodes array to provide to blinds
a factional artificial vision. In the late sixties experiments
demonstrated that surface electrodes would not be an
effective mean to produce a useful visual sense in
individuals completely blind.

At the present, Schmidt et al. develop have investigated the
psychophysical percepts produced by intracortical
microstimulation from penetrating electrodes with
electrical currents. It gave a hope of a more resolution in
the artificial vision.

In the future it is planed to perform a new electrode array
that can provoke mare periodical currents stimulation more
closely. If this becomes possible the formation of Artificial           Figure 3. Complete artificial vision system [21].
Vision Devices may become a well develop solution for the
blind persons [28].

Artificial Vision is made possible by connecting a digital
camera directly to the visual cortex in the back of the brain.
To transmit the information to the visual cortex, a digital
camera picks-up a black and white image which is
downloaded to a sub-notebook computer which transmits
the data to the microcontroller Stimulus Generator that
transmit the impulses to the Cortical Electrode Array [21]
as shown in figure 3.

The electrode array is implanted inter-cranially just
between the two visual cortex of the two cerebral lobes (on
the mesial surface of the occipital lobes as shown in figure
4) The surgical procedure is not very complex; it can be
made with local anesthesia. The implant can start working
about one month after the implantation [21; 28].

The electrode array [21] consists of platinum foil ground
plant with hexagonal array holes of 5mm diameters on
3mm centers. At the center of each hole are 1.0 mm
diameters platinum electrodes, connected by a separate
insulated wire to a connector contained in a carbon
                                                                            Figure 4. X-ray of electrode array [21].
percutaneous pedestal. The platinum electrode arrays are
encased in biocompatible plastic (Figure 5).
                                                                  b. Platinum-group metals
a. Materials
                                                                  Platinum electrodes are used in the implant because of its
The information of the exact materials used in this medical       resistance to corrosion and oxidation on such
device is limited, because this device is in process of           environments; and good conductivity. It has an electrical
                                                                  conductivity of 9.4 m/ mm² for a 99.95-99.99% pure
patentation [25].
                                                                  platinum [27; 30].

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J2
          Figure 5. Electrode layout [21].

c. Upgrades for the artificial vision system

In the future, it is planned the implantation of 256
additional electrodes to improve the resolution of the
patient vision. Also a more powerful computer with
software will be used to magnify the image 8 times bigger.
This will improve the vision from 20/1200 to 20/400 [25].

Scientists are also exploring three-dimensional arrays of
electrodes to provide a more resolution of the image to the
patient. The development of these electrodes arrays is a          Figure 7. Example of another 3-D array silicon insulation
difficult and complex task (Figure 6). These 3-D electrode        [19].
arrays are of silicon materials that allow the creation
smaller and more elaborated or complex electrodes as
shown in figures 7 and 8 [19].

With the communication technology, the wireless medical
devises will be more useful [29]. These devices are being
developed and advances are being made. The challenging
task consists so that the electrodes will survive the
implantation process. This means that the implant has to be
strong enough to penetrate the cerebral cortex without
being destroyed. The smaller the electrode is made, the
weaker it becomes [28].

There is no actual evidence that by electrical stimulation of
the visual cortex can be generated colored phosphenes.
Actually the scientists hope that some day will be possible
to make an actual connection between machine and the
human brain.

                                                                 Figure 8. The final goal is to develop electrodes capable of
                                                                 stimulate close to a neurological level [19].

                                                                 2. Spinal Cord Stimulators

                                                                 Spinal cord stimulators (neurostimulators) provide relief
                                                                 from chronic pain. A compact electronic device is
                                                                 surgically implanted in the wall of the lower abdomen and
                                                                 connected by wire to a strip of electrodes placed next to the
                                                                 back of the spinal cord. The electronic equipment supplies
                                                                 low-voltage electrical pulses at the electrodes, which alter
                                                                 the pain signals in the spinal cord to provide partial or
                                                                 complete pain relief. The stimulators are programmable, so
                                                                 that the signal can be adjusted for optimal pain relief after
                                                                 implantation (Figures 9 and 10).
            Figure 6. 3-D array electrode [19].

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez             J3
The implantable spinal fusion stimulator is designed for an
adjunct therapy to a spinal fusion procedure. The use of this
implant provides a faster consolidation of the bone grafts.
To date, this implantable device has been utilized to
increase the probability of bone fusion in more than 70,000

Magnetic resonance (MR) imaging can pose several
problems for a patient with an implantable spinal fusion
stimulator, including movement or dislodgement of the
stimulator by magnetic field interactions: Damage to the
electronic circuit of the stimulator by exposure to the
electromagnetic fields during operation of the MR system;
heating of the device and adjacent tissue by RF energy
absorption [10].

There are two types of neurostimulation systems [1;2]: One
that is completely internal (surgically implanted) and one       Figure 10. CAT (left) and MRI (right) scans of a spinal
with both internal and external components. The                  stimulator in place [2].
neurostimulation system is typically implanted in a two
stage procedure. Stage one involves implantation of a lead
for trial screening, and stage two involves implantation of      The goal of neurostimulation is to reduce the pain rather
the complete neurostimulation system.                            than eliminating pain. It has been shown that in carefully
                                                                 selected chronic patients, neurostimulation may improve
                                                                 pain relief, increase activity levels, and reduce the use of
Each neurostimulation system consists of:                        narcotic medications. Neurostimulation controls pain by
                                                                 delivering low voltage electrical stimulation to the spinal
 • A power source which generates the electrical                 cord or targeted peripheral nerve to block the sensation of
   stimulation.                                                  pain.

 • An extension wire which conducts electrical                   To understand how spinal cord stimulation works, it is
   stimulation from the power source to the lead.                necessary to understand the components of a spinal cord
                                                                 stimulation system. The system consists of three
 • One or two leads which deliver electrical stimulation         components designed to work together: leads,
   to the spinal cord or target peripheral nerve.                generator/receiver and a programmer/transmitter [4;5].

                                                                 A relative short surgical procedure is required to place the
                                                                 leads in the space above the spinal column and to place a
                                                                 generator/receiver under the skin. When the power source
                                                                 is turned on, electrodes on the lead stimulate the nerve
                                                                 fibers associated with the painful areas. The procedure is
                                                                 performed on an out patient basis in two stages: stage one
                                                                 is a trial stimulation and stage two is a implantation of the
                                                                 permanent device [5].

                                                                 The trial stimulation helps to determine if the procedure
                                                                 will be successful. It will indicate if stimulation is correct
                                                                 for the type, location and severity of the pain. It will also
                                                                 evaluate the effectiveness of various stimulation settings of
                                                                 the device. The insertion of trial leads is performed under
                                                                 light sedation. The lead is then attached to an external
                                                                 power supply and stimulation settings are programmed.
                                                                 After the trial procedure, the patient will be sent home with
                                                                 instructions on how to use the trial stimulator and care for
                                                                 the incision site. After 3-5 days, the patient will return to
                                                                 the doctor’s office to discuss permanently implanting the
                                                                 receiver or removing the trial leads.

                                                                 Leads are very thin cables or wires. One end of the cable
                                                                 has an electrical connector, which is connected to the
                                                                 implanted generator/receiver, and the other end of the cable
 Figure 9. X-ray of a person with a spinal stimulator [2].

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J4
has metal electrodes. These electrodes deliver the electrical     finally move their legs by them self (artificially). The
impulses to the nerve fibers. Before determining if spinal        project implant electrodes to the paraplegic, allowing to
cord stimulation is an option, the condition is thoroughly        electrically stimulated the muscles. This permits that the
evaluated and assessed. A psychologist may assess the             legs of the paraplegic moves artificially. The device is
mental condition of the patient. A neurosurgeon or                implanted in the near the spinal cord, connecting it with the
physician will evaluate his current medication regime and         leg muscle.
physical condition. The physician will want to review all
previous treatments including medication, physical therapy,       This project can help to the persons that had a complete
injection and surgeries [3].                                      trauma in the spinal cord between the forth and eleventh
                                                                  dorsal veritable. Still have strong muscles that can be
The typical candidate may have one or more failed spinal          stimulated and contract them selves [26].
surgeries. According to Elliot Krames M.D. there are six
criteria that must be met before having the operation             The system consist of a device Neuromedics and an IBM
[1966]:                                                           integrated circuit of one square centimeter, implanted under
                                                                  the abdomen skin. From the chip, it is connected 8 wires
     • Do not have psychological problems.                        that connect the chip whit the electrodes that stimulated
                                                                  two nerves and five groups of muscles. This allows the
     • Conservative therapies have failed.                        system to control 10 muscles of the 50 that it has the entire
                                                                  member [26].
     • A trial stimulation was successful.
                                                                  The figure 11 shows a spinal cord stimulation system
     • The source of pain has been verified by a doctor.          consisting of a pulse generator implanted under the skin of
                                                                  the abdomen and lead that carries electric current from the
     • Will not benefit from additional surgery.                  generator to electrodes placed over the spinal nerves.

     • Not seriously dependent on pain medication or               PAST, PRESENT AND FUTURE SCOPE [6;10;11;35 ]
       other drugs.
                                                                  The first brain implants were surgically inserted in 1974 in
                                                                  the state of Ohio, USA and also in Stockholm, Sweden.
If the pain is caused by a correctable condition, then this       Brain electrodes were inserted into the skulls of babies in
must be fixed first. Also, if the person has a cardiac            1946 without the knowledge of their parents. In the 1950s
pacemaker, he or she cannot have a stimulator.                    and 60s, electrical implants were inserted into the brains of
                                                                  animals and humans, especially in the U.S., during research
3. Neuromuscular Stimulators                                      into behavior modification, and brain and body functioning.
                                                                  Mind control (MC) methods were used in attempts to
A family of miniature implantable neuromuscular
                                                                  change human behavior and attitudes. Influencing brain
stimulators has been developed using surface-mounted
                                                                  functions became an important goal of military and
Philips 4000-series integrated circuits. The electronic
                                                                  intelligence services [35].
components are mounted on printed circuits (platinum/gold
on aluminum) and the electrical connections are made by
reflow soldering. The plastic integrated-circuit packages,
ceramic resistors and metal interconnections are protected
from the body fluids by a coating of biocompatible silicone
rubber. This simple technology provides reliable function
for at least 4 months under implanted conditions. The
circuits have in common a single lithium cell power-supply
(3.2 V) and an optical sensor which can be used to detect
light flashes through the skin after the device has been
implanted. This information channel may be used to switch
the output of a device on or off, or to cycle through a series
of pre-set programs. The devices are currently under
investigation to provide an experimental basis for the
clinical exploitation of electrically stimulated skeletal
muscle in cardiac assistance, sphincter reconstruction or
functional electrical stimulation of paralyzed limbs [35].

a. “Stand Up and Walk” Project [26]

The project “Stand Up and Walk” of the European Union
directed by Pierre Rabischong has implanted in paraplegics
electric stimulator to make possible that persons that has
being confined to a wheelchair for more that 10 years,                   Figure 11. Spinal cord stimulation system [4].

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez             J5
Accounts of spinal cord injuries and their treatment date           conditions and modeling scenarios for the lumbar/torso
back to ancient times, even though there was little chance          area has demonstrated that the implantable spinal fusion
of recovery from such a devastating injury. The earliest            stimulator (Figure 12) will not present a hazard to a patient
case is found in an Egyptian papyrus roll manuscript                under going MR imaging with the respect to heating or
written in approximately 1700 B.C. that describes two               induced electric fields during the use of conventional MR
spinal cord injuries involving fracture or dislocation of the       pulse sequences and parameters.
neck vertebrae accompanied by paralysis. Centuries later in
Greece, treatment for spinal cord injuries had changed              The implantable spinal fusion stimulator consists of a direct
little. According to the Greek physician Hippocrates (460-          current generator with a lithium iodine battery and solid
377 B.C.), there were no treatment options for spinal cord          state electronics encased in a titanium shell, partially coated
injuries that resulted in paralysis. Those patients were            with platinum that acts as an anode. The generator weighs
destined to die. But Hippocrates did use rudimentary forms          10 g and has dimensions of 45 x 22 x 6 mm. Two
of traction to treat spinal fractures without paralysis. The        nonmagnetic silver / stainless steel leads insulated with
Hippocratic Ladder was a device that required the patient to        Silastic provide a connection to two titanium electrodes that
be bound, tied to the rungs upside-down and shaken                  serve as the cathodes. A continuous current is provided by
vigorously to reduce spinal curvature. The Hippocratic              the device. The cathodes are composed of insulated wire
Board also allowed the doctor to apply traction to the              leads that terminate as bare wire leads, which are embedded
immobilized patient's back using either his hands and feet          in pieces of bone grafted onto the lateral aspects of fusion
or a wheel and axle arrangement.                                    sites. The generator is implanted beneath the skin and
                                                                    muscle near the vertebral column and provides the full
In about 200 A.D., the Roman physician Galen introduced             rated current for approximately 24-26 weeks.
the concept of the central nervous system (CNS). He
proposed that the spinal cord was an extension of the brain
that carried sensation to the limbs and back. In his
influential anatomy textbook published in 1543, the
Renaissance physician and teacher Vesalius described and
illustrated the spinal cord with all its parts. The illustrations
in his books, based on direct observation and dissection of
the spine, gave physicians a way to understand the basic
structure of the spine and spinal cord and what would
happen when it was injured. The words we use today to
identify segments of the spinal cord (cervical, thoracic,
lumbar, sacral, and coccygeal) come directly from

With the widespread use of antiseptics and sterilization in
surgical procedures in the late nineteenth century, spinal
surgery could finally be done with a much lower risk of
infection. The use of X-rays, beginning in the 1920s, gave
surgeons a way to precisely locate the injury and to
accurately make diagnosis and prediction of outcome. By
the middle of the twentieth century, a standard method of
treating spinal cord injuries was established: Reposition the
spine, fix it in place, and rehabilitate disabilities with
exercise. In the 1990s, the discovery that the steroid drug
methylprednisolone could reduce damage to nerve cells if
given early enough after injury gave doctors an additional
treatment option.

In the 1960’s Medtronic engineers and C Norman Shealy
M.D., developed the first neurostimulation system. The
first system was implanted in 1967. Neurostimulation
technology has undergone significant advances, including
refinements in patient selection criteria, and equipment
design, flexibility, reliability and lifespan.

1. The use of Magnetic Resonance (MR) Imaging

Recent studies using extreme exposures to MR related
electromagnetic fields under highly specific experimental                     Figure 12. Spinal stimulator fusion [10].

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez                 J6
                                  Figure 13. Usual places where TENS devises are located [47].

The implantable spinal fusion stimulator was suspended by           electrical activity cause the body to release its own pain
a 30 cm long piece of thread that was attached to the               easers called beta endorphins [45].
estimated center of the device. The thread was then
attached to a plastic protractor so that the angle of
deflection from the vertical could be measured. The
accuracy of this measuring device is based on the ability to
read the protractor in the MR system. The implantable
spinal fusion stimulator contains small ferromagnetic
electronic components, while the leads, cathodes, and
battery are each made from nonferromagnetic materials.

1. Transcutaneous electrical nerve

Transcutaneous Electrical Nerve Stimulator (TENS) is a
type of electrotherapy equipment that works by sending
small electrical pulses through the skin via ten unit
electrodes to the underlying peripheral nerves. TENS work
in two ways. With "high frequency" continuous, mild,
electrical activity blocks the pain signal traveling to the
brain. Brain cells perceive pain. If the pain signal does not
get through to the brain, the pain is not "felt." The other
way is by stimulating the body's own natural pain-control
mechanism. "Low frequency" or short bursts of mild                         Figure 14. Electrodes placed on the back [46].

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez               J7
2. History [48]

There is evidence that ancient Egyptians used electrogenic
fish to treat ailments in 2500 B.C., although the Roman
Physician Scribonius Largus is credited with the first
documented report of the use of electrogenic fish in
medicine in 46 A.D. (Kane and Taub, 1975). The
development of electrostatic generators in the eighteenth
century increased the use of medical electricity, although
its popularity declined in the nineteenth and early twentieth
century due to variable clinical results and the development
of alternative treatments (Stillings, 1975). Interest in the
use of electricity to relieve pain was reawakened in 1965 by
Melzack and Wall (1965) who provided a physiological
rationale for electro analgesic effects. They proposed that
transmission of noxious information could be inhibited by
activity in large diameter peripheral afferents or by activity
in pain-inhibitory pathways descending from the brain (Fig.
17.2). Wall and Sweet (1967) used high-frequency
percutaneous electrical stimulation to activate large
diameter peripheral afferents artificially and found that this
relieved chronic pain in patients. Pain relief was also                      Figure 15. TENS blocking pain signals. [47]
demonstrated when electrical currents were used to
stimulate the periaqueductal gray (PAG) region of the
midbrain (Reynolds, 1969), which is part of the descending
pain-inhibitory pathway. Shealy, Mortimer and Reswick
(1967) found that electrical stimulation of the dorsal
columns, which form the central transmission pathway of
large diameter peripheral afferents, also produced pain
relief. TENS was used to predict the success of dorsal
column stimulation implants until it was realized that it
could be used as a successful modality on its own (Long,
1973, 1974) [47].

3. TENS method [46]

The TENS method is based on a hypothesis of Melzack &
Wall from 1965. They proposed that activity in coarse,
afferent nerve fibers (A-beta-fibers that convey pressure,
touch, and vibration) inhibits impulse transmission in pain
pathways at spinal cord level. The coarse nerve fibers have
a low threshold for electrical stimulation and are therefore
simple to activate by stimulation using electrodes placed on
the skin. Usually stimulation frequencies of 50–120 Hz are
used for conventional TENS.
                                                                   Figure 16: TENS use electrical impulses to hyperpolarize
a. Gate control theory (Melzack and Wall, 1985) [48]               the nerve blocking the nerve impulse [47].

Theory: When an electrical current is applied to a painful
area, transmission of the perception of pain (via small            b. Opiate-mediated control theory [48]
diameter fibers) to the brain in inhibited by the activity of
the large diameter, fast-conducting highly myelinated,             Theory: Electrical stimulation is thought to relieve pain to
proprioceptive sensory nerve fibers --- closing the gate to        some degree by promoting endorphin release.
the pain perception to the brain.
                                                                         •      The brain can secrete its own analgesic
The CNS can only interpret and transmit one form of                             substance such as endorphins to modulate pain.
sensory stimulus at a time. There a two sets of afferent
(incoming) nerve fibers that enter the spinal cord:                      •      Endorphins are neuropeptides that act on the
                                                                                CNS and peripheral nervous system to reduce
 A-beta fibers – larger diameter (faster) – carry touch                         pain. They have the similar pharmacological
sensation. C and A-delta fibers – smaller diameter (slower)                     effect as morphine.
carry pain sensation

May 2005          Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez             J8
4. Results [48]

Good results are observed in acute and long-term pain
conditions of many kinds, but above all when the pain
originates in joints, the skeleton, muscles, skin, viscera, or
nervous system (i.e., nociceptive and neurogenic pain). In
some cases, TENS on its own may be sufficient, and in
other cases it may provide a valuable complement to other
forms of treatment. TENS has also proved to provide pain
relief and improved healing of wounds in connection with
peripheral circulation disorders and is effective with
treatment of nausea [46].

5. Hazards

Unlike many drugs, TENS devices have no known side
effects and there are many tens unit electrodes to choose
from for varying skin types. Patients may experience skin
irritation with TENS such as reddening beneath or around
the electrodes. This is commonly due to dermatitis at the          Figure 17. DuraMatrix™           collagen    dura   substitute
site of contact with the electrodes resulting from the             membrane [51].
constituents of electrodes, electrode gel or adhesive tape
(Corazza et al., 1999; Fisher, 1978; Meuleman, Busschots
and Dooms Goossens, 1996a, b). The development of
hypoallergenic electrodes has markedly reduced the
incidence of contact dermatitis. Patients should be
encouraged to wash the skin (and electrodes when indicated
by the manufacturer) after TENS and to apply electrodes to
fresh skin on a daily basis [45,47].

                   DURA SUBSTITUTE

Dura substitute is an inert and biocompatible material
(polytetrafluoroethylene [ePTFE]) used to replace dura
matter removed in neurosurgical cases of tumor extraction
or trauma repair. Preclude dura substitute has been cleared
for marketing by the FDA and is intended to be used
instead of dura implants extracted from cadavers thereby
eliminating concerns about the possibility of viral disease
transmission in such cases [50].
                                                                   Figure 18. Dura substitute membrane drawing implantation
1. Dura mater and Dura substitute [49]

Dura mater is the tough, fibrous membrane that covers and
                                                                   3. Compatibility [49]
protects the brain and spinal cord. When the dura mater is
damaged due to surgery or trauma, surgeons look to
repairthe injury with a dura substitute material. Collagen         The safety and effectiveness of DuraMatrix are supported
                                                                   by a clinical study and a large scale animal study. The
Matrix has developed a collagen based membrane that
                                                                   animal study, including 64 rabbits, was a comprehensive
possesses key characteristics of an ideal dura substitute that
                                                                   dural repair study comparing the effectiveness of
can be implanted as an onlay graft or as a suturable graft.
DuraMatrix is biocompatible, resorbable, has a thickness           DuraMatrix with currently marketed collagen-based dural
                                                                   substitutes. DuraMatrix was evaluated against DuraGen®
similar to that of native dura, and conforms to the contours
                                                                   Dural Repair Graft manufactured by Integra Life Sciences
of the brain. The unique conformability of the membrane
                                                                   and DuraGuard® Dural Repair Patch manufactured by
combined with its mechanical strength allows the
membrane matrix to be implanted with or without sutures.           Synovis Surgical Innovations. DuraMatrix was the only
                                                                   dural substitute that demonstrated an ability to facilitate the
The characteristics of the device, including its pore
                                                                   healing with balanced in vivo resorption of the device and
structure, are designed to aid and support the healing of the
                                                                   regeneration of host tissue. The clinical study confirmed
dura mater in accordance with the accepted principle of
guided tissue regeneration. DuraMatrix is easy to handle           the findings of the animal study with no CSF leakage
                                                                   observed in any of the patients and no device related
and can be trimmed, either dry or hydrated, to fit the size of
                                                                   adverse effects reported.
the defect as shown in figures 17 and 18.

May 2005          Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez               J9
The following materials are used in different situations:

  Medical Device                    Materials

 Cerebellar           Platinum and Biocompatible
 Implants             Polymers

 Spinal Cord          Titanium, platinum, silver and
 Stimulators          lithium

 Neuromuscular        Platinum/gold on aluminum, silver
 Stimulators          and lithium

                      Platinum, gel, stainless steel,
                      neoprint, Dri-Tex and carbon film                         Figure 19. Pure Titanium [40].

 Dura substitute      Highly purified type I collagen            b. Titanium performance in medical applications

                                                                 'Fit and forget', is an essential requirement where
From the above list, we shall only present Titanium,             equipment in critical applications, once installed, cannot
Platinum, Silver and Lithium.                                    readily be maintained or replaced. There is no more
                                                                 challenging use in this respect than implants in the human
  DESIGN AND FUNCTIONAL REQUIREMENTS                             body. Here, the effectiveness and reliability of implants,
                                                                 and medical and surgical instruments and devices is an
1. Titanium                                                      essential factor in saving lives and in the long term relief of
                                                                 suffering and pain. Most metals in body fluids and tissue
The high, low weight, outstanding corrosion resistance           are found in stable organic complexes. Corrosion of
possessed by titanium and titanium alloys (Figures 19 and        implanted metal by body fluids, results in the release of
20) have led to a wide and diversified range of successful       unwanted metallic ions, with likely interference in the
applications which demand high levels of reliable                processes of life. Titanium is judged to be inert and
performance in surgery and medicine as well as in                immune to corrosion by all body fluids and tissue, and is
aerospace, automotive, chemical plant, power generation,         thus bio-compatible [7].
oil and gas extraction, sports, and other major industries.
More than 1000 tones (2.2 million pounds) of titanium            The natural selection of titanium for implantation is
devices of every description and function are implanted in       determined by a combination of most favorable
patients worldwide every year. Light, strong and totally         characteristics including immunity to corrosion, bio-
bio-compatible, titanium is one of few materials that            compatibility, strength, low modulus and density and the
naturally match the requirements for implantation in the         capacity for joining with bone and other tissue - Osseo
human body.                                                      integration. The mechanical and physical properties of
                                                                 titanium alloys combine to provide implants which are
                                                                 highly damage tolerant. The human anatomy naturally
Medical grade titanium alloys have a significantly               limits the shape and allowable volume of implants. The
higher strength to weight ratio than competing stainless         lower modulus of titanium alloys compared to steel is a
steels. The range of available titanium alloys enables           positive factor in reducing bone restoration [7].
medical specialist designers to select materials and
forms closely tailored to the needs of the application
[7].                                                             Titanium is regularly used for pacemaker cases and
                                                                 defibrillators, as the carrier structure for replacement heart
                                                                 valves, and for intra-vascular stents. Titanium is suitable
a. Titanium properties [7;8;37]                                  for both temporary and long term external fixations and
                                                                 devices as well as for orthotic calipers and artificial limbs,
It has low density, good strength, can be easy                   both of which use titanium extensively for its light weight,
fabricated, and has excellent corrosion resistance. It is        toughness and corrosion resistance. A wide range of
ductile only when it is free of oxygen. The metal,               surgical instruments are made in titanium. The metal's
which burns in air, is the only element that burns in            lightness is a positive aid to reducing any fatigue of the
Nitrogen. It is considered to be physiologically inert.          surgeon. Instruments are frequently anodized to provide a
Titanium is as stronger as steel, but 45% lighter, it is         non reflecting surface, essential in microsurgical
60% heavier than aluminum, but twice as strong. The              operations, for example in eye surgery. Titanium
price of titanium metal powder is about $100/lb.                 instruments withstand repeat sterilization without
                                                                 compromise to edge or surface quality, corrosion resistance

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J10
                                                                  Figure 21. Platinum electrode for peripheral nerve
                                                                  stimulation [6].

                                                                  and of certain western American states. Sperrylite,
                                                                  occurring with the nickel-bearing deposits of Sudbury,
                                                                  Ontario is the source of a considerable amount of metal

              Figure 20. Titanium dioxide [8].
                                                                  Platinum (Figures 21 and 22) when pure, is malleable and
or strength. Titanium is non magnetic, and there is               ductile. It has a coefficient of expansion almost equal to
therefore no threat of damage to small and sensitive              that of soda-lime-silica glass, and is therefore used to make
implanted electronic devices.                                     sealed electrodes in glass systems. The metal does not
                                                                  oxidize in air at any temperature, but is corroded by
                                                                  halogens, cyanides, sulfur, and caustic alkalis. It is
Titanium and its alloys offer:                                    insoluble in hydrochloric and nitric acid, but dissolves
                                                                  when they are mixed as aqua regia, forming chloroplatinic
     •    Availability in all forms.                              acid.
     •    Comparable cost to other high performance
          materials.                                              Platinum has the ability, in certain chemical forms, to
     •    Ready weld ability and mach inability.                  inhibit the division of living cells. The discovery of this
     •    Weight saving - as strong as steel, but half the        property in 1962 led to the development of platinum-based
          weight.                                                 drugs to treat a wide range of cancers. Cisplatin, the first
     •    Fire and shock resistant.                               platinum anti-cancer drug, began to be used in treatment in
     •    Favorable cryogenic properties.                         1977. Testicular cancer was found to be susceptible to
     •    Bio-compatibility and non-toxicity.                     treatment with cisplatin and there were other successes with
                                                                  drugs to treat a wide range of cancers [14].
Titanium has replaced heavier, less serviceable or less cost
effective materials. Designing with Titanium taking all           Pacemakers, used to treat heart disorders, usually contain at
factors into account has resulted in reliable, economic, and      least two platinum-iridium electrodes, through which
more durable systems and components, which in many                pulses of electricity are transmitted to stabilize the
situations have substantially exceeded performance and            heartbeat. Platinum electrodes are also found in pacemaker-
service life expectations.                                        like devices which are used to help people at risk of fatal
                                                                  disturbances in the heart's rhythm. This risk can be
c. Medical device statement [11]                                  minimized by implanting a device known as an Internal
                                                                  Cardioverter Defibrillator (ICD) which sends a massive
Tronox titanium dioxide is not suitable for class III medical     electric charge to the heart as soon as it detects a problem
devices or human implant devices. Tronox titanium dioxide         [14].
has not been specifically designed for implantable medical
devices. Tronox titanium dioxide has not had supporting           Catheters, flexible tubes which can be introduced into
FDA certification testing conducted for implantable               the arteries, are widely used in modern, minimally-
medical devices. The use of Tronox titanium dioxide for           invasive treatments for heart disease. Many catheters
implantable medical devices is not recommended for use on         contain platinum marker bands and guide wires,
life-sustaining or life-supporting applications in humans.        which are used to help the surgeon guide the
                                                                  device to the treatment site. The radio-opacity of
2. Platinum                                                       platinum, which makes it      visible in x-ray images,
                                                                  enables doctors to monitor the position of the catheter
Platinum occurs in nature, accompanied by small quantities        during treatment. Platinum never tarnishes. Its intense
of iridium, osmium, palladium, ruthenium, and rhodium, all        luster remains intact over the years, and it is
belonging to the same group of metals. These are found in         completely hypoallergenic [14].
the alluvial deposits of the Ural mountains, of Columbia,

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez            J11
                                                                 3. Silver

                                                                 Silver is a soft brilliant white transition metal that can take
                                                                 a high degree of polish. It is very ductile and malleable and
                                                                 it has the highest electrical and thermal conductivity of all
                                                                 metals. The principal use of silver is as a precious metal
                                                                 and its halide salts, especially silver nitrate, are also widely
                                                                 used in photography (which is the largest single end use of
                                                                 silver) [31].

                                                                 Silver (Figure 24) is found in native form, combined with
                                                                 sulfur, arsenic, antimony, or chlorine and in various ores
                                                                 such as argentite (Ag2S) and horn silver (AgCl). The
                                                                 principal sources of silver are copper, copper-nickel, gold,
                                                                 lead and lead-zinc ores obtained from Canada, Mexico,
                                                                 Peru, Australia and the United States. Silver is also
                                                                 produced during the electrolytic refining of copper [31].

             Figure 22. Platinum cubes [53].

Platinum is extremely dense and remarkably heavy. This
property of platinum enhances and ennobles the quality of
the jewelry from which it is created. The ultimate stability
of platinum over the years is unmatched. It does not wear,
and its extreme level of durability offers a profound
guarantee of strength and longevity.

a. Some other usages of platinum are:

    •    In pollution devices.
    •    In the jewelry industry.
    •    In the manufacturing of explosives like TNT.
    •    In the processing of nitric acids.
    •    In the manufacturing of nitrogen based fertilizers.
    •    Used to make aqua regis, a powerful acid used in                    Figure 23. Platinum medical wire [14].
         the chemical and scientific community.
    •    In the processing of eyeglasses.
    •    Used in the petrochemical industry, in processing
         of unleaded gasoline and jet fuel.
    •    Petrochemical usages include the manufacturing
         of plastics and polyester.
    •    In the pharmaceutical industry in the
         development of anti-cancer and many other
         drugs. It's used to manufacture other medical
         devices like endoscopes and catheters.
    •    Used extensively in the manufacturing of
         pacemakers (Figure 23).
    •    It is a key component of jet and rocket engines.
    •    Used in extremely sensitive scientific devices like
         light and oxygen sensors. In the communications
         network like the telephone and internet fiber
         optics system.
    •    In the manufacturing of LCD (liquid crystal
         displays) used in lap top computer and other
         small display electronic devices.
                                                                        Figure 24. Photograph of pure silver [31;33].

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez               J12
While silver itself is not considered to be toxic, most of its    These are lightweight and are not as toxic as lead and
salts are poisonous. Silver compounds can be absorbed in          cadmium batteries. These batteries have applications as
the circulatory system and reduced silver deposited in the        small as watch batteries and as large as military and space
various tissues of the body. A condition, known as argyria,       vehicles [43].
results, with a grayish pigmentation of the skin and mucous
membranes. Silver has germicidal effects and kills many           Lithium is the lightest metal and has a density that is only
lower organisms effectively without harm to higher                half that of water. Like all alkali metals, lithium reacts
animals. It is used as ingredient in dental alloys [34].          easily in water and does not occur freely in nature due to its
                                                                  activity, nevertheless it is still less reactive than the
                                                                  chemically similar sodium. When placed over a flame, this
Some other uses of silver [31]                                    metal gives off a striking crimson color but when it burns
                                                                  strongly, the flame becomes a brilliant white. Lithium is a
     •    Electrical and electronic products, which need          univalent element [44].
          silver's superior conductivity, even when
          tarnished. For example, printed circuits are made
          using silver paints, and computer keyboards use         Because of its large specific heat (the largest of any solid),
          silver electrical contacts.                             lithium is used in heat transfer applications. It is also an
     •    Mirrors which need silver's superior reflectivity       important battery anode material due to its high
          for visible light are made with silver as the           electrochemical potential. Other uses:
          reflecting material. Common mirrors are backed
          with aluminum.                                               •    Lithium chloride and lithium bromide are
     •    Silver has been coined to produce money since                     extremely hygroscopic and frequently used as
          700 BC by the Lydians, in the form of electrum.                   desiccants.
          Later, silver was refined and coined in its pure             •    Lithium stearate is a common all-purpose high-
          form. The words for "silver" and "money" are the                  temperature lubricant.
          same in at least 14 languages.                               •    Lithium is an alloying agent used to synthesize
     •    The metal is chosen for its beauty in the                         organic compounds, and also has nuclear
          manufacture of jewelry and silverware, which are                  applications.
          traditionally made from the silver alloy known as
                                                                       •    Lithium is sometimes used in glasses and
          Sterling silver, which is 92.5% silver.
                                                                            ceramics including the glass for the 200-inch
     •    The malleability, non-toxicity and beauty of                      (5.08 m) telescope at Mt. Palomar.
          silver make it useful in dental alloys for fittings
          and fillings.
                                                                       •    Lithium hydroxide is employed to extract carbon
                                                                            dioxide from the air in spacecraft and
     •    Silver's catalytic properties make it ideal for use
          as a catalyst in oxidation reactions; for example,
          the production of formaldehyde from methanol                 •    Alloys of the metal with aluminum, cadmium,
          and air by means of silver screens or crystallites                copper, and manganese are used to make high
          containing a minimum 99.95 weight-percent                         performance aircraft parts.
          silver.                                                      •    Lithium niobate is used extensively in the
     •    Used to make solder and brazing alloys, electrical                telecoms market, such as mobile phones and
          contacts, and high capacity silver-zinc and silver-               optical modulators.
          cadmium batteries.                                           •    The high nonlinearity of lithium niobate also
     •    Silver sulfide, also known as Silver Whiskers, is                 makes a good choice for nonlinear applications.
          formed when silver electrical contacts are used in
          an atmosphere rich in hydrogen sulfide.                 Lithium is widely distributed but does not occur in nature
     •    Silver fulminate is a powerful explosive.               in its free form. Because of its reactivity, it is always found
     •    Silver chloride can be made transparent and is          bound with one or more other elements or compounds. It
          used as cement for glass.                               forms a minor part of almost all igneous rocks and is also
     •    Silver iodide has been used in attempts to seed         found in many natural brines.
          clouds to produce rain.
     •    In legend, silver is traditionally seen as harmful      Like the other alkali metals, lithium in its pure form is
          to supernatural creatures like werewolves and           highly flammable and slightly explosive when exposed to
          vampires. The use of silver fashioned into bullets      air and especially water. Lithium metal is also corrosive
          for firearms is a popular application.                  and requires special handling to avoid skin contact.
     •    Silver oxide is used as a positive electrode            Lithium should be stored in a non-reactive compound such
          (cathode) in watch batteries.                           as naptha or a hydrocarbon. Lithium compounds play no
                                                                  natural biological role and are considered to be slightly
4. Lithium                                                        toxic. When used as a drug, blood concentrations of Li+
                                                                  must be carefully monitored [44]. A new miniature battery
                                                                  is powering tiny, implantable devices that could help
Lithium has a number of different uses in different sectors
                                                                  millions who suffer from a variety of neurological
of society. The most common use is in lithium batteries.

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J13
                                                                  may cause violent combustion or explosion. The substance
                                                                  may spontaneously ignite on contact with air when finely
                                                                  dispersed. Upon heating, toxic fumes are formed. Reacts
                                                                  violently with strong oxidants, acids and many compounds
                                                                  (hydrocarbons, halogens, halons, concrete, sand and
                                                                  asbestos) causing fire and explosion hazard. Reacts
                                                                  violently with water, forming highly flammable hydrogen
                                                                  gas and corrosive fumes of lithium hydroxide [42]

                                                                  6. Biocompatibility of implants

                                                                  There are many factors which influence in implant
                                                                  biocompatibility, such as implant size, shape, material
                                                                  composition, roughness and charge [36].

Figure 25. A miniature battery (left), powers a bion (right)      The selection of the materials used in the construction of
[38].                                                             implants is basically focused on their ability to maintain
                                                                  mechanical, chemical and structural integrity on various
disorders, such as urinary urge incontinence. The devices,        characteristics which allow this function to substitute any
called "bionic neurons" or bions, are implanted near nerves,      organ or tissue properly and exhibit safe, effective
where they emit electrical micropulses that stimulate             performance within the body. Biocompatibility is by no
nearby muscles and nerves. The strength and frequency of          means a measurable entity [36].
the stimulation can be programmed from outside the body,
and the tiny batteries can be recharged wirelessly with an        For a material to be deemed biocompatible, any adverse
electrical field [44].                                            reactions which may ensue at the blood/material or
                                                                  tissue/material interface must be minimal, while resistance
                                                                  to biodegeneration must be high.
The entire bion system is only about 1/35 the size of a
standard AA battery and can be implanted using minimally
                                                                  Implantable materials should not:
invasive techniques as shown in figure 25. Competing
systems for treating these kinds of muscular impairments
require large batteries and invasive surgery to implant wires          •    Cause thrombus-formations.
and electrical leads in the body.
                                                                       •    Destroy or sensitize the cellular elements of
5. Health effects of Lithium
                                                                       •    Cause adverse immune responses.
Lithium is flammable. Many reactions may cause fire or
explosion. Gives off irritating or toxic fumes (or gases) in a         •    Cause cancer.
fire. Explosion: Risk of fire and explosion on contact with
combustible substances and water. Inhalation: Burning                  •    Produce toxic and allergic responses.
sensation, cough, labored breathing, shortness of breath,
sore throat, symptoms may be delayed. Skin: Redness, skin         To the present date, there are no known materials which
burns, pain, blisters. Eyes: Redness, pain, severe deep           totally satisfy these criteria. So when a foreign material is
burns. Ingestion: Abdominal cramps, abdominal pain,               placed into a biological environment, inevitable reactions
burning sensation, nausea, shock or collapse, vomiting,           occur which are detrimental to both host and material [36].
weakness. Routes of exposure: The substance can be
absorbed into the body by inhalation of its aerosol and by
ingestion. Inhalation risk: Evaporation at 20°C is                             MECHANICAL PROPERTIES
negligible; a harmful concentration of airborne particles
can, however, be reached quickly when dispersed. Effects          There are three main factors which will influence the
of short-term exposure: The substance is corrosive to the         performance of biomaterials in the human body:
eyes, the skin and the respiratory tract. Corrosive on            Biocompatibility, mechanical properties and degradation.
ingestion. Inhalation of the substance may cause lung
oedema [42].                                                      By convention the mechanical properties of a material
                                                                  are described in terms of the deformation or strain
 The symptoms of lung oedema often do not become                  produced by an applied stress. Such behavior can be
manifest until a few hours have passed and they are               plotted on a stress-strain diagram. Normal stress is
aggravated by physical effort. Rest and medical observation       defined as the applied force divided by the cross-
is therefore essential. Immediate administration of an            sectional area over which it is acting, while he
appropriate spray, by a doctor or a person authorized by          resultant strain is the change in length per unit
him/her, should be considered. Chemical dangers: Heating          original length.

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez            J14
        Figure 26. Standard stress/strain curve [20].

Area under the stress-strain curve (Figure 26) reflects the       Figure 27. Stress/strain curve for Titanium as a function of
toughness or the energy absorbed (work required) to               grain size [20].
produce failure. Brittle materials, although often exhibiting
high ultimate stress and high modulus, typically have a           rejection of an implant by certain patients. Materials
smaller area under the curve owing to low breaking strain         failures may result from deficiencies in engineering design,
and therefore are not as tough, for example, as a "strong"        manufacturing processing, or handling in the operating
ductile material having a moderate ultimate stress but large      room.
ultimate strain. For structural implant purposes the latter is
preferred [20].
                                                                              Table 1. Titanium specifications [12;37].
Fatigue is a mode of failure that results from repeated stress
at magnitudes lower than that required to cause failure in a
single application (ultimate stress). Implant materials
                                                                       Property            Metric             English
necessarily must have a high degree of fatigue resistance to
perform over the long term.
                                                                    Density              4.5 g/cc      0.163 lb/in³
The most important non-mechanical requirement of
biomaterial is "inertness." Ideally an implant material             Hardness,
                                                                                         70            70
should not degrade at all. A relative degree of implant             Brinell
degradation is considered acceptable. The corrosion of              Hardness,
                                                                                         60            60
metals in biologic fluids is an electrochemical reaction that       Vickers
results in the release of metal ions into the surrounding           Tensile
aqueous electrolyte. Other requirements of metallic and             Strength,            220 MPa       31900 psi
nonmetallic biomaterials include manufacturing, fabric              Ultimate
ability, and cost [20].                                             Tensile
                                                                                         140 MPa       20300 psi
                                                                    Strength, Yield
 It is estimated from retrieval studies that stainless steel
alloys constitute approximately 60% of the implants used in         Elongation at
                                                                                         54%           54%
the United States. The mechanical properties of surgical-           Break
grade stainless steel are considered to be good relative to         Modulus of
                                                                                         116 GPa       16800 ksi
other implant alloy systems but certainly not outstanding in        Elasticity
the general engineering field.
                                                                    Poisson's Ratio      0.34          0.34
a. Materials complications [20]
                                                                    Electrical           5.54e-005     5.54e-005
                                                                    Resistivity          ohm-cm        ohm-cm
Clinically, implant failure may be defined as a failure of the
implantation procedure to produce satisfactory results.
                                                                    Heat of Fusion       435.4 J/g     187 BTU/lb
Causes of such failure can be grouped into three categories:
surgical, material, or idiosyncratic. Surgical failures relate                           0.528 J/g-
to errors in surgical judgment or application technique             Heat Capacity                      0.126 BTU/lb-°F
including surgically introduced complications such as
infection. Material failures stem from either the chemistry,        Thermal
                                                                                         17 W/m-K      118 BTU-in/hr-ft2-oF
structural metallurgy, or engineering design deficiencies of        Conductivity
the implant. Idiosyncratic failures refer to the selective

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez            J15
b. Titanium                                                       Titanium alloys exhibit modulus of elasticity values which
                                                                  are approximately 5O% of steel. This low modulus means
For medical implants, Titanium is considered one of the           excellent flexibility which has been the basis for its use in
most biocompatible materials available, especially where          dental fixtures and human prosthetic devices (hip joints,
direct contacts to tissue or bone is required. Properties are     implants, etc.). Titanium possesses a coefficient of
shown in table 1.                                                 expansion which is significantly less than ferrous alloys.
                                                                  Titanium is the world's fourth most abundant structural
Titanium has a melting point of 1668°C and a density of           metal.
4.54 gm/cc. Titanium is immune to corrosive attack by salt
water or marine atmospheres. It also exhibits exceptional         c. Platinum
resistance to a broad range of acids, alkalis, natural waters
and industrial chemicals [37].
                                                                  Platinum can be fabricated into very tiny, complex
The combination of high strength and low density results in       components. As it is inert, platinum does not corrode inside
exceptionally favorable strength-to-weight ratios for             the body, while allergic reactions to platinum are extremely
titanium-based alloys. These ratios for titanium-based            rare. Platinum also has good electrical conductivity, which
alloys are superior to almost all other metals and become         makes it an ideal electrode material. Properties are shown
important in such diverse applications as surgical implants       in table 2 [14].
in the medical field. Yield strengths range from 25,000
psi (172 MPa) commercially pure (CP) Grade 1 to above             The platinum group metals are well suited for use at
200,000 psi (1380 MPa) for heat treated beta alloys. Stress       extremely high temperatures under mechanical loads and
versus strain relationship for titanium is shown in figure 28.    simultaneous corrosive attack. They have high melting
                                                                  points, excellent chemical stability and are highly resistant
Titanium has excellent resistance to oxidizing acids, such
as nitric and chromic, over a wide range of temperatures                      Table 2. Platinum specifications [16].
and concentrations. Titanium exhibits low corrosion rates
in nitric acid over a wide range of conditions. At boiling
temperatures and above, titanium's corrosion resistance is
                                                                        Property               Metric            English
very sensitive to nitric acid purity. Generally, the higher the
contamination and the higher the metallic ion content of the
acid, the better titanium will perform. This is in contrast to      Density               21.45 g/cc      0.775 lb/in³
stainless steels which are often adversely affected by acid
contaminants [37].
                                                                    Hardness, Vickers     40              40

Titanium offers superior resistance to erosion and                  Tensile Strength,
cavitations. Titanium is at least twenty times more erosion                               143 MPa         20700 psi
resistant than the copper-nickel alloys. Titanium is most
commonly associated with jet engines and airframes, but             Tensile Strength,
                                                                                          140 MPa         20300 psi
the most recent media attention has been given to fittings          Yield
for prosthetic devices and implants [37].                           Elongation at
                                                                                          35%             35%
                                                                    Modulus of
                                                                                          171 GPa         24800 ksi

                                                                    Poisson's Ratio       0.39            0.39

                                                                    Shear Modulus         62 GPa          8990 ksi

                                                                    Electrical            1.06e-005       1.06e-005
                                                                    Resistivity           ohm-cm          ohm-cm

                                                                    Heat of Fusion        113 J/g         48.6 BTU/lb

                                                                    Heat Capacity         0.134 J/g°C     0.032 BTU/lb°F

                                                                    Thermal               69.1 W/m-K      480 BTU-in/hr-ft²-°F
Figure 28. Tensile stress as a function of strain for cold          Conductivity
worked polycrystalline silver, ‘99.99% pure’ [20].

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez             J16
to oxidation. When using these materials in the design of                         Table 4. Lithium specifications [13].
components it is necessary to have data available on their
elastic properties as a function of temperature. In this
paper, investigations are presented into the temperature
                                                                         Property               Metric               English
dependence of Young's modulus, the modulus of rigidity
and Poisson's ratio for platinum, platinum alloys, rhodium            Density               0.53 g/cc         0.0191 lb/in³
and iridium [39].
                                                                                            2950 oC           5340 oF
It has the third highest density, behind osmium and iridium.
Platinum is unaffected by air and water, but will dissolve in         Hardness,
                                                                                            Max 5             Max 5
hot aqua regia, in hot concentrated phosphoric and                    Vickerss
sulphuric acids, and in molten alkali. It is as resistant as          Tensile Strength,
gold to corrosion.                                                                          15 MPa            Max 2180 psi

d. Silver                                                             Electrical            8.4e-006          8.4e-006
                                                                      Resistivity           ohm-cm            ohm-cm
Silver is used in medical implants as material for the                                      5e-007            5e-007
production of wires because it is a good electrical
conductor of all known metals and it is very ductile and              Heat of Fusion        137 J/g           58.9 BTU/lb
malleable. In its pure state, is not toxic. Silver compounds
can be slowly absorbed by body tissues, with the
consequent bluish or blackish skin pigmentation. Soluble              Heat Capacity         3.305 J/g-°C      0.79 BTU/lb-°F
silver salts, especially AgNO3, are lethal for health. Silver
wires are covered with silicon to avoid direct contact with
                                                                      Thermal               71.2 W/m-K        494 BTU-in/hr-ft²-°F
the body and give extra [33;34].
                                                                      Poisson’s Ratio       0.36              0.36
            Table 3. Silver specifications [31;32].

                                                                    Silver is used in numerous applications such as contacts,
     Property                 Metric            English             fuse elements, lead wires, battery plated, etc. Fine silver
                                                                    has a high thermal and electrical conductivity, good
 Density                 10.491 g/cc     0.379 lb/in³               resistance to oxidation and corrosive attack, excellent
                                                                    ductility and is easily joined by welding or brazing. Pure
 Hardness, Vickers       25              25                         silver is used extensively in the preparation of high-
                                                                    temperature superconductor wires, tapes, films, and other
 Tensile Strength,                                                  configurations in which the silver not only shields the super
                         140 MPa         20300 psi                  conducting material from the surrounding materials, but
                                                                    also provides a degree of flexibility and strain relief, as
 Tensile Strength,
                         137 MPa         19800 psi                  well as stabilization and low-resistance electrical contact.
                                                                    Silver is relatively expensive, but at this stage of
 Elongation at                                                      superconductor development, its unique combination of
                         33%             33%
 Break                                                              properties seems to offer the only reasonable means of
 Modulus of                                                         achieving usable lengths of conductor. Properties are
                         76 GPa          11000 ksi
 Elasticity                                                         shown in table 3. Stress-strain curve is shown in figure 28.
 Poisson's Ratio
                         0.37            0.37
 Annealed                                                           e. Lithium
 Poisson's Ratio
                         0.39            0.39
 Hard Drawn                                                         Lithium has a melting point of 180.54°C, boiling point of
                                                                    1342°C, specific gravity of 0.534 (20°C). It is the lightest
 Shear Modulus           27.8 GPa        4030 ksi                   of the metals, with a density approximately half that of
                                                                    water. It has the highest specific heat of any solid element.
 Electrical              1.55e-006       1.55e-006                  Metallic lithium is silvery in appearance. It reacts with
 Resistivity             ohm-cm          ohm-cm                     water, but not as vigorously as does sodium. Lithium
                                                                    imparts a crimson color to flame, although the metal itself
 Heat of Fusion          105 J/g         45.2 BTU/lb
                                                                    burns a bright white. Lithium is corrosive and requires
                                                                    special handling [41,42].
 Heat Capacity           0.234 J/g-°C    0.0559 BTU/lb-°F
                                                                    Lithium is used in heat transfer applications. It is used as an
 Thermal                 419 W/m-K       2910 BTU-in/hr-ft²-°F      alloying agent, in synthesizing organic compounds, and is
 Conductivity                                                       added to glasses and ceramics. Its high electrochemical

May 2005           Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez               J17
potential makes it useful for battery anodes. Lithium            4.
chloride and lithium bromide are highly hygroscopic, so are           [Information and photo of a spinal cord stimulation
used as drying agents. Lithium is used as a high-                     system]
temperature lubricant. Lithium has medical applications, as
well [41]. Properties of lithium are shown in table 4.           5.
Between the most significant properties of lithium we find            %20of%20Spinal%20Cord%20Injury
its high specific heat (calorific capacity), the huge                 [Information about spinal cord injury, how does the
temperature interval in the liquid state, high termic                 spinal cod work, anatomy]
conductivity, low viscosity and very low density. Metallic
lithium is soluble in short chain aliphatic amines, like         6.
etilamine. It is insoluble in hydrocarbons [42].                      [Photo of Platinum electrode for peripheral nerve
Today’s technology is having an impact on medicine, and               [Information about Titanium Medical Applications]
in the case of neurological implants, advances are being
made that are aiding people with diverse problems. This          8.
specific area of medicine is quite risky for the patient and          ium&sm=Yahoo%21+Search&fr=FP-tabt&toggle=1&
great care must be taken when implants of this kind are               ei=UTF-8
placed in the brain and spinal cord. The materials discussed          [Photo of Titanium dioxide]
in this project have been successful at their purpose, but
new materials are necessary for the implants to become           9.
more effective, more durable, and cheaper to manufacture.             reatment_ladder/neurostimulation/screening/neuro_scr
                       SUMMARY                                        [Information of Implant Techniques]

Every material must meet a number of properties before it        10.
can be considered appropriate for an implant. The materials          search=‘Implantable%20Spinal%20Fusion%20Stimul
all share the most important characteristic, which is                ator:20Assessment’
biocompatibility. If the material does harm to the patient,          [Information and photo of spinal fusion stimulator;
then the whole purpose of the implant is lost. But in                assessment of magnetic resonance safety]
addition to being biocompatible, the material must also
have some properties which make it useful for the function       11.
it is designed to do. The material must be chosen by                 _ch_ stewardship/bus_ ch_ medicaldevice.htm
carefully taking into account each of its properties and             [Information of Titanium dioxide in Medical Device]
comparing their advantages over other kinds of materials.
              ACKNOWLEDGEMENTS:                                      [Titanium mechanical properties]

We thank Dr. Megh R. Goyal for his guidance and help.            13.
                                                                     [Lithium mechanical properties]
                    BIBLIOGRAPHY                                      [Photo of Platinum medical wires and information of
                                                                      biomedical components]

1.         15.
     reatment_ladder/neurostimulation/neuro_neurostimula             78.html
     tion.html                                                       [Information of properties, uses, sources of Platinum]
     [Information of neurostimulation system components
     and implantation]                                           16.
2.                     [Platinum mechanical properties]
     [Photo of X-rays of a person with a spinal stimulator
     and photo of a CAT and MRI scans of a person with           17.
     spinal stimulator in place]                                     T/Welcome.htm
                                                                     [Information of Platinum Project]
     cfm                                                         18.
     [Information of : What is spinal cord stimulation?]             [Photo and information of implants in the spinal cord]

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez            J18
19.    35. http;//
    rticles/WISE%20-%202004.pdf                                       tics06dec00.shtml
     [Photos and information of         Wireless Implantable          [Information of microchip implants, mind control, &
     Microsystems]                                                    cybernetics]
20. “Ti-2003:Proc 10th World Conference on Titanium, July 13-     36. http;//
    18, Hamburg, Germany. N.Saunders, X. Li, A.P. Miodownik
    and J.-P. Schille
                                                                      [Biocompatibility ofimplants]
     [Information and photo of titanium and stress
                                                                      [Titanium properties applications and mechanical
    [Photos and information of Artificial Vision for the
    Blind by connecting a television, camera to the visual
22.         [Photo and information about battery powers tiny,
    &ei=UTF-8&fl=0&imgsz=all&fr=FP-tab-web-t&b=21                     implants that aid neurological disorders]
     [Photo of implant in cerebellar]
    [Information of Pain definitions]                                 sum=8F202B4D205C4CC2A8F7F746EEE2B799&cookie=1
24.                [Information of high temperature              mechanical
                                                                       properties of the Platinum group metal]
     [Information of Applications and properties]
                                                                  40. http://www-
    [Information of Artificial Vision for the Blind Part II]          .jpg
                                                                      [Photo of Pure Titanium]
26.      41.
    taje4.html                                                        [Information of Lithium, All Info About Chemistry]
     [Information of “Stand Up and Walk” Project]
27.                en.htm
    001_04.html                                                       [Information about properties and health effects of
    [Information of Platinum Today]                                   Lithium]
28.              Projects/ElementWebSites/lithium/uses.html
    [Information of sight restoration for individuals with            [Information of Lithium uses]
    profound blindness]
29.              [Information of applications, characteristics and
    tesis.html                                                        mechanical properties of Lithium]
    [Information about Neuroprotesis]
30.              [Information of T.E.N.S.]
    [Materials properties and possible applications of            46.
    precious metals and alloys]                                       [Photos of electrodes placed on the back, and
                                                                      information of TENS]
31.                           47.
    [Information and photo of Silver, characteristics,                books/pdf/135.pdf
    applications, precautions and health effects]                     [Photos and information of usual places where TENS
32.                devises are located, TENS blocking pain signals]
    [Silver mechanical properties]                                48.
                                                                      Information of 4-Theories of how does TENS work,
                                                                      parameters and contraindications]
     [Photo of pure Silver and information about health
     effects of silver]
                                                                      [Information of Dura Sudstitute Membrane]
    [Information about chemical facts of silver]

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J19
50.                  Lumbar: The part of the spine in the middle back, below
    [Information of new releases of new medical terms,            the thoracic vertebrae and above the sacral vertebrae.
    Dura Substitute]
                                                                  Magnetic resonance imaging (MRI): An image produced
51.       by use of magnetic fields and radio waves to visualize body
    [Photo of collagen dura substitute membrane]                  structures.

52.          Modulus of elasticity: Is the ratio of applied stress to the                   resultant strain in the linear (elastic) portion of the stress-
    preclude-dura-                                                strain curve and in tensile testing is designated the
    substitute.html&prev=/search%3Fq%3Ddura%2Bsubs                "Young's modulus, 'E'.
    %3DGGLD,GGLD:2005-05,GGLD:en%26sa%3DN                         Neurological: Involving the nerves or nervous system.
    [Photo of dura substitute membrane drawing                    Source: European Union
                                                                  Neurostimulation: The act of stimulating neurons with
53. http://www-                                                   electrical impulses delivered via electrodes attached to the           brain.
    [Photos of Platinum cubes]                                    Pain: An unpleasant sensory and emotional experience
                                                                  associated with the stimulation of specialized nerve endings
                                                                  that signal actual or potential tissue damage, or is described
                        GLOSSARY                                  in terms of such damage.

Catheters: Flexible tubes which can be introduced into the        Peripheral nerves: Nerves that run from your spinal cord
arteries.                                                         to all other parts of your body. They transmit messages
                                                                  from the spinal cord and the brain to and from other parts
Cervical: The part of the spine in the neck region.               of your body and send sensory signals back to the spinal
                                                                  cord and brain.
Chronic pain: Pain that persists beyond the time of normal
healing and can last from a few months to many years. It          Peripheral nerve stimulation: A surgical treatment for
can result from disease, such as arthritis, or from an injury     pain in which specific nerves are stimulated rather than the
or surgery. It also can occur without a known injury or           general area of the spinal cord.
                                                                  Spinal cord: The bundle of nerves that extends from the
Creep: Materials that undergo to a progressive deformation        base of the brain to the small of the back. It processes
with time, under constant stress.                                 information and conducts impulses between the brain and
Deep brain or intracerebral stimulation: Is considered            the rest of the body.
an extreme treatment and involves surgical stimulation of
the brain, usually the thalamus. It is used for a limited         Spinal cord stimulators: Are devices which are designed
number of conditions, including severe pain, central pain         to provide relief from chronic pain. A compact electronic
syndrome, cancer pain, phantom limb pain, and other               device is surgically implanted in the wall of the lower
neuropathic pains.                                                abdomen, and connected by wire to a strip of electrodes
                                                                  placed next to the back of the spinal cord.

DuraMatrix: Is a nonfriable, conformable and resorbable
                                                                  T.E.N.S.(Transcutaneous Electrical Nerve Stimulator):
membrane matrix engineered from highly purified type I
                                                                  Is a type of electrotherapy equipment that works by sending
                                                                  small electrical pulses through the skin via ten unit
Electrode: Component of the pacing system which is at the
                                                                  electrodes to the underlying peripheral nerves.
distal end of the lead. It is the interface with living cardiac
tissue across which the stimulus is transmitted.
                                                                  Ultimate stress: Is the stress associated with complete
General anesthesia: A controlled state of unawareness,            mechanical failure of the test specimen. The ultimate strain
induced by an injected or inhaled medication, that renders a      is the fractional deformation at failure and is occasionally
patient unconscious and unresponsive to surgical pain.            called the ductility.
Hardness: Material's resistance to indentation either by a
ball (Brinell test) or by a pyramidal diamond (Vickers test).     X-ray: Electromagnetic vibrations of short wavelength that
Implant: A prosthesis placed permanently in tissue.               penetrate most matter and can produce an image on film.

Leads: A small medical wire that has electrodes at one end.       Yield stress: Maximum stress that can be applied without
Electrical current passes from a battery, along the wire, to      producing permanent deformation after removal of the
the electrodes. Two types; percutaneous and surgical leads.       stress,sometimes termed the elastic limit.

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez              J20

                                              A silver wire with a diameter of 0.25 mm and yield stress of 137 MPa
                                              is used for a spinal cord stimulator fusion implant. What is the
                                              maximum tension that the wire can hold with a safety factor of 3.


                   (0.25 mm)          2

            A = 0.0491 mm 2

            A = 4.91x10 −8 m 2

                        σ yield
            σ allow =

            n=3      and          σ yield = 137 MPa

                        137 MPa
            σ allow =           = 45.67 MPa

            σ allow =

            P = σ allow A

            P = (45.67 x10 6 Pa )(4.91x10 −8 m 2 )

                             N 
            P =  45.67 x10 6 2 (4.91x10 −8 m 2 )
                            m 

            P = 2.24 N

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez       J21
                                                    A platinum electrode with a diameter of 1mm is going to be
                                                    implanted on the visual cortex of a blind. Find the expression that
                                                    calculates the stress in the electrode and where it is more likely to



                    N ( y)
            σc =

            N ( y) =

            ∴σ c =

                      i.       At the tip (y = 0)   ⇒σc = 0
                      ii.      At the base (y = L)    ⇒σc =

            The electrode is more likely to break at the base, rather than the tip.

May 2005          Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez             J22
                                                  A titanium rod of length L = 3mm from a spinal cord
                                                  implant is tested in a laboratory being twisted by torques
                                                  (T) until the angle of rotation between the ends of the rod
                                                  is 2.0Ε. If the allowable shear strain for titanium is 0.0008
                                                  rad, what is the maximum permissible diameter of the


            L = 3 mm

            φ = 2.0 o

                       π 
            φ = 2.0         rad
                       180 

            φ = 0.03491 rad

            γ allow = 0.0008 rad

            γ max = rθ

            γ max = r

            γ max =

                       2Lγ max
            d max =

                       2(3 mm)(0.0008 rad )
            d max =
                           0.03491 rad

            d max = 0.13749 mm

May 2005         Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez    J23
                                 A hollow silver cylinder from a spinal cord implant is compressed by a force
                                 P caused by the person’s own weight (see figure). The cylinder has an inner
                                 diameter d1 = 5.0 mm and an outer diameter d2 = 6.0 mm, and a modulus of
                                 elasticity E = 76 Gpa. When the force P increases from zero to 311.375 N the
                                 outer diameter of the cylinder increases by 500 x 10^(-5) mm.

                                          a) Determine the increase in the inner diameter.
                                          b) Determine the increase in the wall thickness.
                                          c) Determine Poisson’s ratio of the silver.


            d1 = 5.0 mm
            d 2 = 6.0 mm
            t = 1 mm
            E = 76 x10 9 Pa
            P = 311.375 N
            ∆d 2 = 500 x10 −5 mm
            LATERAL STRAIN

                 ∆d 2 500 x10 −5
            ε '=     =           = 8.333x10 −4
                  d2    6.0

                   a) INCREASE IN INNER DIAMETER

                              ∆d1 = ε ' d1 = (8.333x10 −4 mm)(5.0)

                                   = 416.67 x10 −5 mm
                   b) INCREASE IN WALL THICKNESS

                              ∆t = ε ' t = (8.333x10 −4 mm)(1.0)

                                 8.333 x10 −4 mm

May 2005        Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez   J24

              c) POISSON’S RATIO

                       Axial stress


                             π                        π
                               (d     2
                                      2       )
                                          − d12 =
                                                        (6.0   2
                                                                   − 5 .0 2   )
                                       1m 
                          = 6.7853 mm           = 6.7853 x10 −6 m 2
                                       1000 mm 
                                               

                       ( σ < σ y ; Hook’s Law is valid)

                       Axial strain

                             σ        45.889 MPa
                       ε=        =               = 6.038 x10 − 4
                             E        76000 MPa

                         ε ' 8.333x10 −4
                       V= =
                         ε 6.038 x10 −4
                          = 1.3801

May 2005   Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez   J25