Biomedical Engineering Reference
In-Depth Information
Figure 6.4
Polymers: (a) UHMWPE hip, (b) polyurethane catheters, and (c) polyglycolic matrix.
poly(ethylene oxide) and poly(lactic-co-glycolic acid) are notable exceptions. Syn-
thetic polymers are often associated with inflammatory reactions, which limit their
use to solid, unmoving, impermeable devices. Most commonly used synthetic poly-
mers for catheters, tubings, contact lenses, and hip joints include polyurethane,
polyethylene, polypropylene, polyimides, polytetrafluoroethylene, and polymethyl-
methacrylates, poly (vinyl chloride), polyethylene, polypropylene, and polystyrene;
and its copolymers with acrylonitrile and butadiene, polyesters, polyamides or
nylons, polyfluorocarbons, polyurethanes, natural and synthetic rubbers, silicone
polymers, polyacetal, polysulfone, and polycarbonates.
A majority of the polymers made of carbon-carbon backbone alone tend to
resist degradation. However, if polymers contain other atoms in the backbones,
they tend to be biodegradable. Using this basic concept, one can synthesize poly-
mers to a desirable degradation rate by the addition of chemical linkages such as
anhydrides, esters, or amide bonds, among others. For example, copolymers of
poly(ethylene oxide) and poly(butylene terephthalate) have been developed. These
materials are subject to both hydrolysis (via ester bonds) and oxidation (via ether
bonds). One could control the degradation rate by adjusting the poly(ethylene ox-
ide) molecular weight and content.
Hydrogels
are also polymers that can swell without dissolving when placed in
water or other biological fluids. At equilibrium, hydrogels typically comprise of
60-90% fluid and only 10-30% polymer. The structural changes are reversible
and repeatable upon additional changes in the external environment. Hydrogels
are attractive because of their high water content, tissue-like mechanical properties,
and ability to be polymerized in vivo under physiological conditions. The ability to
polymerize the hydrogel transdermally would minimize the invasive procedure that
is normally required for plastic and orthoscopic surgery. The injectable hydrogel
would allow implantation through a large needle or arthroscopic instrument. The
key to this method is the ability to inject the gel into the joint, control the shape of
the gel, and be able to polymerize the gel beneath the skin.
6.2.4 Biological Materials
Materials include natural skin, arteries, veins, cord blood vessel, excised diseased,
and defective tissues. Tissue-derived substances or modified extracellular matrix
