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housings
Poly(vinyl chloride) Transparency, good scuff Blood bags, catheters, cannulae, corrugated tubing, renal
resistance care products, transfusion supplies, face masks
Polyurethane Good chemical resistance, Catheters, tubing, I.V. connectors, drug delivery systems
toughness, good
processability
Polyetherimide Autoclavibility, chemical Sterilization trays
resistance
Polysulfone High heat resistance Medical trays
7.10.2 Nontoxic Sterilizable Items
Polymers used in nontoxic sterilizable items, such as tubing, artificial organs, and wound coverings, must
be able to withstand sterilization by ethylene oxide, steam autoclave, or gamma radiation. For medical
products sterilized by gamma irradiation, only plastics that do not degrade or discolor on exposure to
radiation, such as polyester and polycarbonates, can be used.
The biological compatibility of materials used in the nontoxic category is vital and is the subject of
continuing research. Since no material is completely inert, it is the level of interaction between an implant
and the surrounding tissue that determines the acceptability of the material. Several other facts that are
important in determining acceptability include mechanical properties of the polymer, e.g., wear
resistance and fatigue, and bulk chemical properties such as resistance to degradation by hydrolysis,
sensitivity to enzymes, and the way it reacts to the deposition of protein.
PVC is used for nearly all surgical tubing. Recent developments include flexible PVC compounds with
greater resistance to body fluids, and x-ray-opaque PVC compounds that enable PVC catheters and
tubing to be traced after insertion into the body. Esmedica-V, a plasticizer-free flexible PVC compound
produced by Sekisui, Japan, reportedly meets all requirements for medical use, including ability to stand
ethylene oxide sterilization. ICI has produced Welvic VK-2004 for coronary dilation catheters. It is
reported to offer high flexibility, high-pressure resistance, and low elongation.
The development of artificial wound coverings has received much attention. The major requirements
for such coverings are protection from microbial attack from the environment, optimal water
permeability, capability of adhering well to the wound, and ready removability without causing tissue
damage. Other important factors are prevention of excessive formation of granulation tissue and optimal
elasticity to facilitate an intimate cover of the wound.
Sustained-release wound dressings capable of delivering antibiotics or other biomedical agents in small
concentrations have been obtained by microcapsulating a variety of drugs into a UV curable urethane
q 2006 by Taylor & Francis Group, LLC
Trends in Polymer Applications 7-41
elastomer. Thermedics Incorporated has developed special UV-curable polyurethane elastomers for
such applications.
Developments related to medical implants, artificial organs, and prosthesis have improved the quality
of life and increased the life expectancy of many individuals.
Silicone rubber is a highly biocompatible thermosetting elastomer that has found applications in
prosthesis for ophthalmology, neurology, facial reconstruction, replacement of finger, toe and wrist
joints, cardiovascular applications, such as pacemaker coatings and lead wires, and tendon replacements.
It is also used in drug delivery systems and tubes for carrying blood, drugs, and nutrients.
The artificial heart, Jarvik-7, which has been successful in keeping a recipient alive for more than a year,
has valves made from modified polypropylene and the two ventricles made of polyurethane supported on
an aluminum base. For arterial replacements or bypass of clogged blood vessels, polyester fiber such as
Du Pont s Dacron remains the preferred material.
Ultrahigh molecular weight polyethylene (UHMW PE) is most commonly used for articulation
surfaces in joints. It is also used for prosthesis components in total hip replacement. In the latter
application, it may be reinforced with carbon fibers to increase wear properties.
Expanded polytetrafluoroethylene (PTFE) grafts have gained increasing popularity as synthetic or
nontextile grafts for reconstructive procedures, such as, above- or below-the-knee bypasses for
limb salvage.
7.10.3 Biodegradable Polymers
Generally, biodegradable polymers are those that can be broken down by nature either by hydrolytic
processes or enzymatic processes producing nontoxic by-products. ICI offers a biodegradable plastic
under the tradename Bipol. It is a polyester made from hydroxybutyric and hydroxyvaleric acids. Because
of its much higher price than conventional mass polymers, Biopol will not find wide use as long as purely
economic considerations determine the use of plastics. A niche market exists, however, for the medical
grade of the polymer. For example, fibers can be used for surgical sutures. The compound is absorbed in
the body and does not invoke immune reactions. The molecular weight lies between 30,000 and 750,000.
The mixture of the two hydroxyacids is produced by a bacterium of the type alcaligenes eutrophus.
Biodegradable polymers are very interesting for tissue engineering (see Chapter 5) applications
because they can be absorbed gradually by the human body without permanently retaining traces of
residuals in the implantation site, and are compatible with tissue repair. Thus cell transplantation using
scaffolds of biodegradable polymer offers the exciting possibility of creating, in vivo, completely natural
new tissue with the required mechanical or metabolic features to restore the function of tissues such as
cartilage, bone, skin, nerve, kidney, and liver. In this process [54], biodegradable polymer scaffolds act as
temporary substrates to which cells can adhere, proliferate, and retain their differential function (see
Degradable Polymers, Chapter 5.)
7.10.4 Conducting Polymer Nanotubes
Conducting polymers are of considerable interest for a variety of biomedical applications [55]. Their
response to electrochemical oxidation or reduction can produce a change inconductivity, color and
volume. A change in the electronic charge is accompanied by an equivalent change in the ionic charge,
which requires mass transport between the polymer and electrolyte [56]. When counterions enter a
polymer it expands and when they exit it contracts, with extent of expansion or contraction depending
on the number and size of ions exchanged. Electrochemical actuators using conducting polymers based
on this principle have been developed [57]. They can be doped with bioactive drugs, and can be used in
actuators such as microfluidic pumps [58].
Microelectrode neural probes facilitate the functional stimulation or recording of neurons in the central
nervous system and peripheral nervous system. Minimizing the electrode impedance is an important
requirement for obtaining high quality signals (high signal-to-noise ratio). It has been shown [59,60] that
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7-42 Plastics Technology Handbook
conducting polymers such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) can
decrease the impedance of the recording electrode sites on neural prosthetic devices. It has also been
demonstrated [61] that the impedance of the neural microelectrodes can be further decreased significantly
(by about two orders of magnitude) and the charge-transfer capacity significantly increased (about three
orders of magnitude) by creating conducting polymer nanotubes on the microelectrode surface. The
conducting nanotubes that have well-defined internal and external surface texture decrease the electrode
impedance by increasing the effective surface area for ionic-to-electronic charge transfer to occur at the
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