Page 41 - 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

Biomaterials, Synthetic Synthesis, Fabrication, and Applications 181

polymer molecules onto the substrate polymer surface. A as amides, enamines, enol-ketones, esters, ureas, and ure-
terminal group reactive to functional groups present on thaneswhenpresentinpolymersallshowsomebiodegrad-
the substrate polymer surface is required for the poly- ability.
mer chains to be used in the coupling reaction, while Materials being developed include lactide/glycolide
the graft polymerization method needs active species on polymers, polyorthoesters, derivatives of pseudo-
the substrate polymer to initiate radical polymerization. and poly-amino acids such as poly(l-glutamate),
An apparently grafted surface can also be produced by polyphosphazenes, poly(∈-caprolactone) and tyrosine–
physical adsorption of existing polymers. The adsorbed polycarbonates. Advantages of these materials lies in
chains are readily desorbed upon immersion in solvents their ease of processability and their biocompatibility
or washing with surfactants unless the adsorptive force with extracellular matrix components present in the body.
is remarkably strong. Polyethyleneoxide (PEO) has been They may be made as high surface area, porous devices
used to modify artificial surfaces by the above chemical thereby allowing cell movement and adhesion within the
surface modification methods with the establishment of device both important for the assimilation of the material
PEO-surface and PEO-interpenetrating networks. An al- within the body and in the eventual replacement of the
ternative simple and cost-effective approach is to use the biodegradable component with the bodies own tissue.
meltblendtechniquewheresmallamountsofproteincom- Future applications of degradable polymers may in-
patible additives, such as PEO, polyvinyl alcohol (PVA), volve their use in a foam form as scaffolds for the regen-
poly(ethyl oxazoline) (PEOX), and poly(vinyl pyrroli- eration of specific tissues such as liver, bone, cartilage, or
dine) (PNVP) have been used to modify the substrate vascular walls.
polymer. The base polymer could be a single polymer
or mixture of ethylene–vinyl acetate (EVA), polypropy- B. Polymers from Natural Sources
lene (PP), glycol modified poly(ethylene terephthalate)
(PETG), poly(methylmethacrylate) (PMMA), styrene– In the search for biocompatible materials scientists
butadiene copolymer and polyamide-based copolymers. have taken naturally occurring polymers and modifed the
Materials modified by these techniques can show en- structure of the materials for a range of end uses. Materials
hanced protein adhesion important for general biocompat- based on collagen and hyaluronic acid are in common
ibility or show reduced protein adsorption which is useful use for both dental, ophthalmological and maxillofacial
in the production of blood-contacting devices, chromato- reconstructive work where the “natural” polymer phase
graphic supports, coatings to minimize biofouling, sepa- is used to fill out defects in the bone thereby providing
ration membranes, contact lenses, immunoassays, protein a matrix around which bone will develop. Collagen is
drug-contacting materials, etc. used in film, membrane, solution, gel, and sponge forms.
It is also used in conjunction with glucosaminoglycans,
tricalcium phosphate, hydroxyapatite, allogeneic bone,
3. Biodegradable Polymers
cells, and with drugs such as tetracycline.
Although there are many medical applications for poly- Most implant procedures where collagen has been used
mers which require materials to be stable over a long time have required the implant to be a permanent feature in the
period, there are also devices such as sutures, small bone human body. The “collagen” used was usually partially
fixation devices, skin grafts, and drug delivery systems degraded, enzyme-extracted collagen, or had been stabi-
where polymers that break down in a controlled fashion lized by cross-linking it with cytotoxic glutaraldehyde or
are required. Materials that can be tailored to break down chromium salts, or else had been assembled into non-
either unaided or by enzyme-assisted pathways under the natural polymeric structures such as films and sponges.
conditions inherent in the location where the implant is An alternative approach is to maintain as much of the bio-
found are desired. logical and organizational structure of collagen as possible
Two main factors affect the biodegradability of poly- using continuous collagen threads to make collagen fibers
mers. The first is the chemical composition of the ma- which can then be used to knit and weave fabrics with a
terial and the presence of hydrolysable and/or oxidizable structure more akin to that found naturally. Many differ-
chemical groups in the main chain, suitable substituents in ent materials have been fabricated each with different bulk
side chains, stereoconfiguration, balance of hydrophilicity and extensibility properties.
andhydrophobicity,andconformationalflexibilityallcon- Materials derived from hyaluronic acid are convention-
tribute to the biodegradability of synthetic polymers. The ally esterified (either 100% or fractional amounts) and
second aspect is the morphology of the polymer sample the acid esters so produced have physicochemical prop-
with amorphous polymer regions degrading prior to crys- erties which are significantly different from those of the
talline and cross-linked regions. Functional groups such parent molecule. Ethylhyaluronic acid ester (HYAFF-07)