Biomedical Engineering Reference
In-Depth Information
An intriguing characteristic of natural polymers is
their ability to be degraded by naturally occurring en-
zymes, a virtual guarantee that the implant will be
eventually metabolized by physiological mechanisms.
This property may, at first glance, appear as a disadvan-
tage since it detracts from the durability of the implant.
However, it has been used to advantage in biomaterials
applications in which it is desired to deliver a specific
function for a temporary period of time, following which
the implant is expected to degrade completely and to be
disposed of by largely normal metabolic processes. Since,
furthermore, it is possible to control the degradation rate
of the implanted polymer by chemical cross-linking or
other chemical modifications, the designer is offered the
opportunity to control the lifetime of the implant.
A potential problem to be dealt with when proteins
are used as biomaterials is their frequently significant
immunogenicity, which, of course, derives precisely from
their similarity to naturally occurring substances. The
immunological reaction of the host to the implant is di-
rected against selected sites (antigenic determinants) in
the protein molecule. This reaction can be mediated by
molecules in solution in body fluids (immunoglobulins).
A single such molecule (antibody) binds to single or
multiple determinants on an antigen. The immunological
reaction can also be mediated by molecules that are held
3.2.8 Natural materials
Ioannis V. Yannas
Natural polymers offer the advantage of being very
similar, often identical, to macromolecular substances
which the biological environment is prepared to rec-
ognize and to deal with metabolically (
Table 3.2.8-1
).
The problems of toxicity and stimulation of a chronic
inflammatory reaction, as well as lack of recognition by
cells, which are frequently provoked by many synthetic
polymers, may thereby be suppressed. Furthermore,
the similarity to naturally occurring substances in-
troduces the interesting capability of designing bio-
materials that function biologically at the molecular,
rather than the macroscopic, level. On the other hand,
natural polymers are frequently quite immunogenic.
Furthermore, because they are structurally much more
complex than most synthetic polymers, their techno-
logical manipulation is quite a bit more elaborate. On
balance, however, these opposing factors have con-
spired to lead to a substantial number of biomaterials
applications in which naturally occurring polymers, or
their chemically modified versions, have provided un-
precedented solutions.
Table 3.2.8-1 General properties of certain natural polymers
Polymer
Incidence
Physiological function
A. Proteins
Silk
Synthesized by arthropods
Protective cocoon
Keratin
Hair
Thermal insulation
Collagen
Connective tissues (tendon, skin, etc.)
Mechanical support
Gelatin
Partly amorphous collagen
(Industrial product)
Fibrinogen
Blood
Blood clotting
Elastin
Neck ligament
Mechanical support
Actin
Muscle
Contraction, motility
Myosin
Muscle
Contraction, motility
B. Polysaccharides
Cellulose (cotton)
Plants
Mechanical support
Amylose
Plants
Energy reservoir
Dextran
Synthesized by bacteria
Matrix for growth of organism
Chitin
Insects, crustaceans
Provides shape and form
Glycosaminoglycans
Connective tissues
Contributes to mechanical support
C. Polynucleotides
Deoxyribonucleic acids (DNA)
Cell nucleus
Direct protein biosynthesis
Ribonucleic acids (RNA)
Cell nucleus
Direct protein biosynthesis
