Page 347 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

324 BIOMATERIALS

Chitosan. Degradation: biodegradable.

CH 2 OH CH 2 OH
O O
H H
H H
OH H O OH H O
H H
H NHCOCH 3 H NHCOCH 3 n

Chitin: Poly(1, 4'-β-N-acetyl-2-amino-2-deoxy-D-glucopyranose)

CH 2 OH CH OH
2
O O
H H
H H
OH H O OH H O
H H
H NH 2 H NH 2 n

Chitosan: Poly(1, 4'-β-2-amino-2-deoxy-D-glucopyranose)

Chitin is a polysaccharide that is the major component of the shells of insects and shellfish. Chitosan
is deacetylated chitin. Deacetylation is accomplished using basic solutions at elevated temperatures.
Chitin is not 100 percent acetylated and chitosan is not 100 percent deacetylated. The degree of
acetylation has a large influence on properties, in particular solubility. Chitin is difficult to use as a
biomaterial since it is difficult to process. It cannot be melt processed and is insoluble in most aque-
ous solutions and organic solutions. It is soluble only in strong acid solutions. Chitosan, on the other
hand, is soluble in dilute organic acids; acetic acid is most commonly used. Chitosan has a positive
charge due to the primary amines in its structure. The positive charge is significant because most
tissues are negatively charged. Chitosan has been used for artificial skin, sutures, and a drug delivery
matrix. 18
Chitosan absorbs a significant amount of water when placed in aqueous solutions. Equilibrium
water content of 48 percent was determined by immersing chitosan films in deionized water. Tensile
testing on these wet films resulted in an ultimate tensile stress of approximately 1600 psi with
70 percent elongation at break. 19

Collagen. Degradation: biodegradable. Collagen is the major structural protein in animals and
exists in sheet and fibrillar form. Collagen fibrils consist of a triple helix of three protein chains. Type
I collagen is a fibrillar form of collagen that makes up 25 percent of the protein mass of the human
body. Due to its prevalence and ability to be separated from tissues, type 1 collagen is most often
used in medical devices. Collagen fibrils are strong and biodegradable and collagen is hemostatic,
making it useful in a variety of applications. Table 13.2 shows many of the applications for collagen.
Collagen is usually obtained from bovine corium, the lower layer of bovine hide. Bovine collagen is
nonimmunogenic for most people, but immune response may be triggered in those with allergies to
beef. 20
Both water-soluble and water-insoluble collagen can be extracted from animal tissues. Water-
soluble collagen can be extracted from collagen using salt solutions, organic acids, or a combination
of organic acids and proteases. Proteases break down cross-links and nonhelical ends, yielding
more soluble collagen than acid alone or the salt solutions. Water-soluble collagen finds little use in