Biomedical Engineering Reference
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Fig. 3.2.8-3 Illustration of the variety of porous structures that can be obtained with collagen-GAG copolymers by adjusting the kinetics of
crystallizaton of ice to the appropriate magnitude and direction. Pores form when the ice dendrites are eventually sublimed. SEM.
(Courtesy of MIT.)
components proceeds in the wound bed, these compo-
nents are being synthesized de novo by cells at the same
anatomical site. Eventually, new architectural arrange-
ments of collagen fibers, such as scar tissue, are synthe-
sized. Although it is not a replica of the intact tissue, scar
tissue forms a stable endpoint to the healing process and
acts as a tissue barrier that allows the healed organ to
continue functioning at a nearly physiological level. One
of the frequent challenges in the design of collagen im-
plants is to modify collagen chemically in a way that the
rate of its degradation at the implantation site is either
accelerated or slowed down to a desired level.
An effective method for reducing the rate of degrada-
tion of collagen by naturally occurring enzymes is by
chemical cross-linking. A very simple self-cross-linking
procedure, dehydrative cross-linking, is based on the fact
that removal of water below ca. 1 wt.% insolubilizes col-
lagen as well as gelatin by inducing formation of interchain
peptide bonds. The nature of cross-links formed can be
inferred from results of studies using chemically modified
gelatins. Gelatin that had been modified either by esteri-
fication of the carboxylic groups of aspartyl and glutamyl
residues, or by acetylation of the 3-amino groups of lysyl
residues, remained soluble in aqueous solvents after ex-
posure of the solid protein to high temperature, while
unmodified gelatins lost their solubility. Insolubilization of
collagen and gelatin following severe dehydration has
been, accordingly, interpreted as the result of drastic re-
moval of the aqueous product of a condensation reaction
that led to formation of interchain amide links. The pro-
posed mechanism is consistent with results, obtained by
titration, showing that the number of free carboxylic
groups and free amino groups in collagen are both signifi-
cantly decreased following high-temperature treatment.
Removal of water to the extent necessary to achieve
a density of cross-links in excess of 10 5 mol cross-links/g
dry protein, which corresponds to an average molecular
weight between crosslinks, M c , of about 70 kDa, can be
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