Page 40 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

180 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

SCHEME 3 Ring opening polymerization.

glass fiber, mica, and silica can be used to reinforce poly- 2. Polymer Modification
mers and improve their mechanical properties. There are
problems associated with the use of many of these addi- Polymers are required for use under aqueous conditions
tives because of their low molecular weight (in relation to where the binding of proteins (for recognition by the body)
the polymer phase) as they may be leached from the device and cells is required in order to produce a favourable
causing deleterious effects on the device itself and on the biological response. Polystyrene, polyethylene and ter-
human body. Not only can there be problems due to leach- phthalate, polyfluoroethylene and perfluorinated ethylene
ing of additives but also there may be problems due to the propylene copolymer are poor supports and require post-
very components used in the synthesis of a material. For synthesis modification of their surfaces to improve bio-
example, contaminants found in silicone polymers due to compatibility. Gas plasma (glow discharge) methods have
the synthesis route include; siloxane monomers, platinum become popular as a way of fabricating a range of sur-
catalysts, and peroxide derivatives. This problem is also face chemistries. Surfaces are produced with function-
significant in the production of bioceramics and compos- alities rich in oxygen groups including O, OH, surface
ites for use in biomedical applications. sulfonate and carboxyl which are, in principle, more com-
The mechanical properties of polymers are a conse- patible with biological tissues than the carbon–carbon and
quence of the distribution and packing of the polymer carbon–hydrogen bonds present in the parent polymers.
chains (degree of crystallinity) and the transition temper- It is also possible to use grafting (long used in indus-
ature at which they change from a viscoelastic material tries based on polymers) to modify the surface chem-
to a rigid glass. They cover a wide range of strengths (as istry of polymers. A grafted surface can be produced
measured by values for the Young’s modulus) and can primaril by graft polymerization (often a free radical pro-
therefore be used for a wide range of applications. cess) of monomers or the covalent coupling of existing

FIGURE 3 Schematic of polymer structures possible when two different monomers are used in the synthesis.