Biomedical Engineering Reference
In-Depth Information
PRG4,whilethoseinthemiddlelayerdidnot,indicatingbiomimetic
stratification.
36.3 Protein-Based Matrices
The other key component of the articular cartilage extracellular
matrix (in addition to the proteoglycans) is the proteins, which
make up the collagen network. This network is made primarily of
collagen type II but also contains minor components of collagen
type XI, IX, XI, and others. This cross-linked protein network resists
the swelling pressure of the proteoglycans and provides the ten-
sile properties.
56
The variations in collagen organization are impor-
tant to the pronounced cartilage anisotropy and depth dependent
properties. Protein-based hydrogel matrices can mimic parts of the
natural cartilage matrix, including natural sites for cell attach-
ment, enzymatic degradation, and incorporation into existing
networks.
36.3.1
Collagen or Gelatin
As collagen is the most abundant protein in cartilage, it is not sur-
prising that collagen gels have been investigated for cartilage tissue
engineering purposes. Collagen gels can be formed by pH neutral-
ization. Cells can be suspended in the collagen solution and incor-
porated into the hydrogel. However, collagen gels typically have the
issue of cell-induced shrinkage due to the contractile forces of the
cells and the weak mechanical properties of the gel. One method
to overcome the problem of shrinkage is through incorporation of
short collagen fragments in the gel.
57
Another interesting approach
to improving the mechanical properties is through nonenzymatic
glycation, which commonly occurs in the native cartilage matrix, in
the solution before gel formation.
58
This treatment resulted in bet-
ter glycosaminoglycan retention and modest increase in compres-
sivemodulus.
The type of collagen used can have a major influence on
chondrogenesis, with collagen II hydrogels leading to better
mesenchymal stem cell (MSC) chondrogenesis than collagen I
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