Page 668 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Selected Topics 631

when the polymer is used as a carrier such as in controlled release of drugs, removal of unwanted
materials, or where the materials purpose is short lived such as in their use as sutures and as frames
for natural growth.
While the nature of the material is important, the surface of the material is also often critical.
The human body wants to wrap around or connect to bodies within its domain. In some cases, the
desired situation is little or no buildup on the polymer. Here, surface “slickness” is needed. Siloxanes

and flurinated materials such as polytetraflouroethylene (PTFE) are generally slick materials but

other materials can be made slick through surface treatments that presents to the body few voids and
irregularities at the atomic level. In other cases, bodily buildup is desired and surfaces and materials
that assist this growth are desired. Surface hydrophobicity/hydrophilicity, presence/absence of ionic
groups, chemical and physical (solid or gel) surface are all important considerations as one designs
a material for a specifi c application.
Ability to function long term is an ongoing problem. In general, polyurethanes degrade after
about 1.5 years, nylons lose much of their mechanical strength after about 3 years, and polyacrylo-
nitrile loses about 25% strength after 2 years. On the long side, PTFE loses less than 10% strength
after about a year and siloxanes retain most of their properties after 1.5 years.
Following is a brief look at some of the varied uses, real and projected, for synthetic polymers.
These are intended to be illustrative only. The field of biomedical materials is rapidly growing and

it is extensive.

There has been a lot of effort to construct artificial hearts for human use. These hearts are largely
composed of polymeric materials. Even with the problems associated with organ transplant, the use

of artificial hearts may never be wide spread. This is because of several trends. First, many biological
solutions to biological problems resides with the use of biological materials to solve biological problems.
With the advent of increased effectiveness of cloning and related advances suitable biological replace-
ments may be grown from a person’s cells thus ensuring compatibility. Second, related to the fi rst, regen-
eration of essential parts of vital organs is becoming more practical. Third, because of nanotechnology
and related electrical and optical advances, surgery to locate and repair imperfections is improving.
Forth, our autoimmune system is divided into two main systems—one guards against “small” invad-
ing organisms such as viruses, bacteria, and pollen. The second group acts to reject whole organisms
such as the heart through rejection of foreign body tissue. Purine nucleoside phosphorylase (PNP), is a
human enzyme that serves at least two major functions. First, PNP acts to degrade unleashed molecules,

including foreign nucleic acids. PNP is necessary to our immune system as it fights disease. Some anti-
cancer drugs are synthetic nucleosides and nucleotides such as polyIpolyC that are employed to directly
attack selected cancerous tumors. PNP degrades such nucleic acid-related materials before they reach
the tumor. Neutralization of PNP just before administration of the synthetic nucleic acid-related material
would allow lower dosages to be used. Second, PNP is an integral part of the body’s T-cell immunity
system that rejects foreign tissue. Effort is currently underway to effectively inhibit only the PNP action

allowing the first autoimmune system to work while allowing organ transplantation without rejection.
Recently, through the use of zero gravity conditions, crystals of PNP have been grown of suffi cient size
to allow structural determination. With this structure determined, efforts are underway to detect sites of
activity and drugs that would allow only these sites to be neutralized when needed.
Even so, synthetic polymers have been important in replacing parts of our essential organs. Thus,
silicon balls are used in the construction of mechanical heart valves. Many of these fail after some
time and they are being replaced by a flap valve made from pyrolytic carbon or polyoxymethylene.

Aneurisms can be repaired through reinforcement of the artery wall with a tube of woven PTFE
or polyester (PET). Replacement of sections of the artery can be done using a tube of porous PTFE.
One remaining problem is the difference in elasticity between the woven and porous materials and
the arteries themselves.

Carbon-fiber composites are replacing screws for bone fracture repair and joint replacements.

These fiber composites are equally as strong and are chemically inert. By comparison, the metals
they replace are often alloys that may contain metals that the patient may be allergic to.

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