Biomedical Engineering Reference
In-Depth Information
attached to, proliferated, and redifferentiated in the scaffolds based on cell morphology, expression of
cartilage-related gene transcripts, and the presence of a cartilage-like ECM rich in GAGs and Col- II.
Compared to MSCs, the hCHs attached more slowly on the aqueous silk fibroin 3D scaffolds. Cell den-
sity was critical for the differentiation of culture-expanded hCHs in the 3D aqueous-derived silk fibroin
scaffolds. Significant levels of cartilage-related transcripts (AGC, Col-II, Sox 9, Col-II/Col-I ratio) were
upregulated, and uniform deposition of cartilage-specific ECM components (Col-II and GAGs) were
observed. The hCH-based constructs were significantly different than those from MSCs with respect to
cell morphology and zonal structure. Collectively, these studies demonstrated the potential of porous
3D silk fibroin scaffolds in autologous cell-based cartilage tissue engineering.
7.6 Silk Degradation
The degradation of biomaterials is important related to full regeneration or restoration of full tissue
structure and function
in vivo
. Thus, control over the rate of degradation is an important feature in
designing functional tissue scaffold biomaterials [129]. Commerical silk fibroin fibers (which are rou-
tinely coated with waxes and dyes) retain more than 50% of their mechanical properties after 2 months
of implantation
in vivo
; thus, they are defined as a nondegradable biomaterial by the
United States
Pharmacopeia
[130]. Other polymers such as PLA, PGA, and PLGA hydrolyze at rates based on the
polyester composition, purity, and processing conditions. The degradation of these polymers is usually
controlled by varying the ratio of monomers or by altering molecular weight and crystallization [131].
Further, degradation products of polymers like PLA generate low-molecular-weight acids, which can
decrease local pH and result in inflammation [132].
Natural polymers like collagen and silks degrade via the action of proteases. Typically, the rate of col-
lagen degradation is altered by cross-linking in order to reduce the rate of enzymatic degradation. Cross-
linking of collagen impact cell interactions and may also reduce immunogenicity [133]. The degradation
byproducts of collagen are peptides and amino acids. In the case of silks, the rate of fibroin degradation
depends on the structure, primarily β-sheet content. Degradation of silk fibroin films and fibers has been
explored using several types of proteases, including --chymotrypsin and collagenases [130,134]. A cor-
relation between
in vitro
and
in vivo
rates of degradation of silk fibroin fibers has also been established
[130]. Similarly, the degradation of silk fibers and silk films when exposed to different enzymes has been
assessed [134]. Films degraded faster than the fibers based on weight loss. The weight loss was accompa-
nied by a change in average molecular weight of silk from 120 kDa for control silk films to 53 kDa for silk
films degraded with --chymotrypsin over 17 days. Tensile strength properties of silk fibers decreased
without a significant change in molecular weight of the silk protein [134]. Silk fibroin porous sponges
from regenerated
B. mori
fibers degraded differently with different processing conditions [82]. Aqueous
processed 3D sponges, with similar pore sizes, degraded more slowly upon exposure to proteases with an
increased percent silk present during the processing. Solvent-processed sponges degraded more slowly
than the aqueous-processed systems with similar pore sizes: 65% mass remained after 21 days when com-
pared to aqueous-based sponges, which degraded completely in 4 days upon exposure to proteases. The
difference in degradation was likely due to differences in content or distribution of crystallinity [82]. It is
generally understood that silk fibroin degradation can be regulated by changing crystallinity [135], pore
size, porosity, and molecular weight distribution (MWD). A change in MWD can be achieved by treat-
ing the silk fibroin under alkaline conditions and with heat. A decrease in MWD may disrupt ordered
structures and reduce cross-links, potentially resulting in faster degradation.
7.7 Silk Immunological Responses
Sutures made from virgin silk (nondegummed silk with sericin) compared with sutures from degummed
silk (without sericin) showed differences in hypersensitivity [5]. In early studies, the inflammatory
response of degummed silk fibroin
in vitro
was comparable with polystyrene and poly(2-hydroxyethyl
