Page 349 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

326 BIOMATERIALS

TABLE 13.3 Water Absorption and Its Albumin. Degradation: biodegradable. Albumin is a
Effect on Modulus (E) and Yield Strength of globular, or soluble, protein making up 50 percent of the
Collagen Hollow Fibers 21 protein content of plasma in humans. It has a molecular
weight of 66,200 and contains 17 disulfide bridges. 22
Water absorption, Numerous carboxylate and amino (lysyl) groups are
Humidity, % g/100 g collagen
available for cross-linking reactions providing for a very
wet 240 broad range of mechanical behavior. Heating is also an
90 50 effective cross-linking method, as seen in ovalbumin (egg
80 25 white cooking). This affords another gelling mechanism
60 17 and is finding increasing use in laser welding of tissue,
30 10 where bond strengths of 0.1 MPa have been achieved. 23
As with collagen, the most common cross-linking
Humidity, % Yield stress, psi
agent used is glutaraldehyde, and toxic by-products are of
Wet 3000 concern. Careful cleaning and neutralization with glycine
90 5200 wash have provided biocompatible albumin and collagen
66 13,000 structures in a wide variety of strengths up to tough, very
36 19,000 slowly degradable solids. It should be noted that albumin
8 24,000
and collagen solidification is generally different than
that of fibrin, which gels by a normal biological mecha-
Humidity E, ksi *
nism. The glutaraldehyde methods yield a variety of non-
Wet 44 biologic solids with highly variable mechanical properties.
90 450 This has led to an extensive literature, and very wide
75 750 range of properties for collagen and albumin structures
8 970
which are used for tissue substitutes and drug delivery
* 1 ksi = 1000 psi vehicles.

Oxidized Cellulose. Degradation: bioerosion. Oxidized cellulose is one of the fastest degrading
polymers at physiologic pH. It is classified as bioerodable since it degrades without the help of
enzymes. It is relatively stable at neutral pH, but above pH 7 it degrades. Oxidized cellulose disap-
pears completely in 21 days when placed in phosphate buffered saline (PBS). Similarly, it dissolves
80 percent after 2 weeks in vivo. Cellulose is oxidized using nitrogen tetroxide (N O ). Commercially
4
2
available oxidized cellulose contains between 0.6 and 0.93 carboxylic acid groups per glucose unit,
which corresponds to between 16 and 24 weight percent carboxylic acid. 24
CH 2 OH CH 2 OH
O O
H H
H H
OH H O OH H O Cellulose
H H
H OH H OH n

N 2 O 4

O
CH 2 OH C OH
O O
H H
H H
OH H O OH H O Oxidized cellulose
H H
H OH H OH n