Biomedical Engineering Reference
In-Depth Information
polysaccharides, and nucleic acids. They are hydrophilic, and most of them are
water-soluble. Proteins are polypeptides, whose amide (peptide) bonds are highly
stable against spontaneous (nonenzymatic) hydrolysis under neutral conditions but
can be degraded by enzymatic hydrolysis. For example, collagen, gelatin, and fibrin
have been used as biodegradable medical material [
20
-
22
]. However, proteins of
nonhuman origin may cause immunogenetic problems. In addition, both human-
origin and animal-origin proteins may cause infection problems, such as bovine
spongiform encephalopathy (BSE) and human immunodeficiency virus (HIV).
Polysaccharides are highly hydrophilic polymers having many hydroxyl groups.
Some polysaccharides have other functional groups, such as carboxylic acid, sulfate,
amino, and acetamide groups. The glucoside (ether) bonds of polysaccharides are
also highly stable against spontaneous (nonenzymatic) hydrolysis under neutral
conditions but can be degraded by enzymatic hydrolysis. Polysaccharides are gener-
ally nontoxic and have no or low immunogeneticity, and are also used as biomedical
materials. Nucleic acids (DNA and RNA) are highly water-soluble anionic polymers
and have not been used very often as biomedical materials. The phosphorotriester
linkage of DNA is relatively stable under physiological conditions, but is very sensitive
to enzymatic (nuclease) degradation. These natural polymers are hydrophilic and
not suitable for solid-state (bulk) materials having firm physical properties.
On the other hand, many synthetic biodegradable polymers have been developed
for biomedical materials. Some of them are semicrystalline or noncrystalline
polymers having strong physical properties, and can be used as biodegradable
plastics. Typical examples of synthetic (artificial) biodegradable polymers are
polyamides (including synthetic polypeptides), polyesters [
23
,
24
], polyanhydrides
[
25
-
28
], polycarbonates [
29
], poly(ortho ester)s [
30
-
32
], polyacetals [
33
,
34
],
polyphophazenes [
35
,
36
], and polyphosphoesters [
37
-
40
]. They have various
degradation rates and physical properties based on their molecular structures.
In the design of biodegradable biomaterials, many important properties must be
considered [
5
]. These materials must (1) not evoke a sustained inflammatory
response; (2) possess a degradation time coinciding with their function; (3) have
appropriate mechanical properties for their intended use; (4) produce nontoxic
degradation products that can be readily resorbed or excreted; and (5) include
appropriate permeability and processability for designed application.
Among these synthetic biodegradable polymers, aliphatic polyester [poly
(hydroxyl acid)s] such as poly(lactic acid) (polylactide, PLA); poly(glycolic acid)
(polyglycolide, PGA); poly(
e
-caplolactone) (PCL); and their copolymers have been
used often as implantable biomaterials (e.g., absorbable sutures, bone fixation
materials, and drug delivery devices) because these aliphatic polyesters can provide
favorable degradation rates, high mechanical properties, low- or nontoxic metabo-
lizable degradation products, and are FDA-approved for clinical use [
8
-
11
].
Recently, biodegradable polymers have been used to fabricate macro- and nano-
meter scale self-assembled systems such as microspheres (MSs), nanospheres (NSs),
polymer micelles, nanogels, and polymersomes (Fig.
1
). These have attracted growing
interest because of their potential utility for drug delivery systems (DDS), tissue
engineering, and other applications. To construct these self-assembled systems
