It’s good to be important but, it’s important to be good - - - Anonymous
MECHANICS OF BIOMATERIALS: NEUROLOGICAL IMPLANTED STIMULATORS
FOR CEREBELLAR, NEUROMUSCULAR, SPINAL CORD, PERIPHERAL NERVE
FOR PAIN RELIEF, TEN, INTRACEREBRAL / SUBCORTICAL1
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 .
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 .
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
firstname.lastname@example.org or visit at:
Figure 2. Implant in cerebellar .
The authors are in alphabetical order.
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J1
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 .
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 .
Vision Devices may become a well develop solution for the
blind persons .
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 
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  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 .
percutaneous pedestal. The platinum electrode arrays are
encased in biocompatible plastic (Figure 5).
b. Platinum-group metals
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
platinum [27; 30].
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J2
Figure 5. Electrode layout .
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 .
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 .
arrays are of silicon materials that allow the creation
smaller and more elaborated or complex electrodes as
shown in figures 7 and 8 .
With the communication technology, the wireless medical
devises will be more useful . 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 .
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
Figure 8. The final goal is to develop electrodes capable of
stimulate close to a neurological level .
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 .
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
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 .
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
• 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 .
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 .
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 . 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 .
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
: 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
• 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 ]
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 .
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 .
a. “Stand Up and Walk” Project 
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 .
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
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 .
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J6
Figure 13. Usual places where TENS devises are located .
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 .
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 .
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J7
2. History 
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. 
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) .
3. TENS method 
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)  the nerve blocking the nerve impulse .
Theory: When an electrical current is applied to a painful
area, transmission of the perception of pain (via small b. Opiate-mediated control theory 
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 
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 .
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 .
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 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 .
Figure 18. Dura substitute membrane drawing implantation
1. Dura mater and Dura substitute 
Dura mater is the tough, fibrous membrane that covers and
3. Compatibility 
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
Spinal Cord Titanium, platinum, silver and
Neuromuscular Platinum/gold on aluminum, silver
Stimulators and lithium
Platinum, gel, stainless steel,
neoprint, Dri-Tex and carbon film Figure 19. Pure Titanium .
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 .
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 .
medical specialist designers to select materials and
forms closely tailored to the needs of the application
. 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
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 .
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 .
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  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 .
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 .
the alluvial deposits of the Ural mountains, of Columbia,
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J11
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 (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 .
Figure 22. Platinum cubes .
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 .
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
• 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 .
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 . 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  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 .
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
• 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 . 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 
6. Biocompatibility of implants
There are many factors which influence in implant
biocompatibility, such as implant size, shape, material
composition, roughness and charge .
Figure 25. A miniature battery (left), powers a bion (right) The selection of the materials used in the construction of
. 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 .
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 . 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 .
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 . 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 .
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 .
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
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,
should not degrade at all. A relative degree of implant Brinell
degradation is considered acceptable. The corrosion of Hardness,
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 . Tensile
140 MPa 20300 psi
It is estimated from retrieval studies that stainless steel
alloys constitute approximately 60% of the implants used in Elongation at
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 
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 .
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 .
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 .
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
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 . Elongation at
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’ .
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 .
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 .
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
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
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.
Annealed e. Lithium
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. http://www.mayfieldclinic.com/PE-STIM.htm
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 . Properties of lithium are shown in table 4. 5. http://www.ninds.nih.gov/disorders/sci/detail_sci.htm#
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. http://www.medics-network.com/
etilamine. It is insoluble in hydrocarbons . [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. http://images.search.yahoo.com/search/images?p=titan
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. http://www.medtronic.com/neuro/paintherapies/pain_t
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. http://www.ebimedical.com/pdf/spine/mri_safety.pdf#
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. http://www.kerr-mcgee.com/businesses/chemicals/bus
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. http://www.matweb.com/search/SpecificMaterial.asp?
[Lithium mechanical properties]
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Blind by connecting a television, camera to the visual
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&ei=UTF-8&fl=0&imgsz=all&fr=FP-tab-web-t&b=21 implants that aid neurological disorders]
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[Information of Platinum Today] Lithium]
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50. http://www.mtdaily.com/mt1/news/n020396.html 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.
Magnetic resonance imaging (MRI): An image produced
51. http://www.collagenmatrix.com/products-neurological.htm by use of magnetic fields and radio waves to visualize body
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52. http://translate.google.com/translate?hl=en&sl=de&u= Modulus of elasticity: Is the ratio of applied stress to the
http://www.neuro-competence.de/v-34-9/gore- 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
[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
APPENDIX I: NUMERICAL EXERCISES
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 allow =
n=3 and σ yield = 137 MPa
σ allow = = 45.67 MPa
σ allow =
P = σ allow A
P = (45.67 x10 6 Pa )(4.91x10 −8 m 2 )
P = 45.67 x10 6 2 (4.91x10 −8 m 2 )
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)
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
φ = 2.0 rad
φ = 0.03491 rad
γ allow = 0.0008 rad
γ max = rθ
γ max = r
γ max =
d max =
2(3 mm)(0.0008 rad )
d max =
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
∆d 2 500 x10 −5
ε '= = = 8.333x10 −4
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
PROBLEM 4 CONTINUED
c) POISSON’S RATIO
− d12 =
− 5 .0 2 )
= 6.7853 mm = 6.7853 x10 −6 m 2
( σ < σ y ; Hook’s Law is valid)
σ 45.889 MPa
ε= = = 6.038 x10 − 4
E 76000 MPa
ε ' 8.333x10 −4
ε 6.038 x10 −4
May 2005 Applications of Engineering Mechanics in Medicine, GED -- University of Puerto Rico, Mayaguez J25