Biomedical Engineering Reference
In-Depth Information
are dependent on the membrane materials and osmotic agents used in the system,
respectively.
CoatingMaterialsandTechniques
Both synthetic and natural polymers have been used in coating to control the release rate.
Different methods can be applied to coat a particular polymer on different substrates (films,
tablets, granules, etc.). Some of the commonly used materials are summarized, followed by
techniques to coat those materials.
Coating Materials
Collagen
Collagen is a major constituent of connective tissue and a major structure protein of any
organ. It readily self-assembles into higher-order structures (i.e., fibers) in physiological
solutions [79]. Cross-linking of collagen can be achieved by various reactants, and the
degradation of collagen network can be controlled by cross-linking density [80]. Collagen
can be obtained from various sources in different forms.
Gelatin
Gelatin is a natural polymer derived from collagen, and its electric properties can be eas-
ily tailored according to preparation conditions [81]. The most commonly used gelatin
in pharmaceutical industry is capsules, but it is widely used as coating material as
well. The popularity of using gelatin as a coating material for drug delivery systems is
largely because of its biodegradability (100% reabsorbable in vivo) and biocompatibility
(no antigenicity) [82]. Degradation of gelatin happens in a few different ways: (1) dissolu-
tion and (2) enzymatic degradation. Dissolution of gelatin occurs when hydration of the
gelatin coating is high if the molecules are not cross-linked. Alternatively, gelatin can
be cross-linked when carboxyl group and free amines of adjacent molecules are con-
densed. The cross-linking can be induced by water-soluble carbodiimide (such as
N
,
N
-(3-
dimethylaminopropyl)-
N
8-ethyl carbodiimide and
N
-hydroxysuccinimide) and aldehyde
(i.e., glutaraldehyde and formaldehyde) to form hydrogel [83]. Degradation of cross-
linked gelatin occurs when peptide bonds are cleaved. By altering cross-linking density,
degradation time can be tailored from days to months [84].
Alginate
Alginate is a family of un-branched polysaccharides isolated from brown algae found in
coastal water. Alginate is composed of two monomers, namely
β
-d-mannuronic acid (M)
and
α
-l-guluronic acid (G), linked by one to four glycoside bounds. It is found that the
G-monomer and M-monomer join together in a blockwise fashion.
Cross-linking of alginate can be achieved in the presence of di/trivalent ions. It has
been found that Ca
2+
and Sr
2+
mainly form GG linkage, whereas Zn
2+
can form GG, MM,
and GM linkages [85]. The flexibility of alginate in aqueous solutions is in the order:
