Biologic Biomaterials: Silk - Biomaterials - page 168

fibroin scaffolds and hMSCs offers options for the repair of critical-sized bone defects, where the contri-
bution of host cells is not sufficient for a proper healing. Aqueous-derived silk fibroin scaffolds showed
improved bone-tissue engineering outcomes when compared to HFIP-derived silk fibroin scaffolds in
vitro [85]. This has implications for silk protein processing modes related to biomaterial matrix interac-
tions with stem cells for tissue engineering. New approaches to combine micron-range silk powders
with HFIP regenerated silk to fabricate reinforced high-strength reinforced protein-protein composite
scaffolds for in vitro and in vivo applications [120].
7.5.2 Silk-Based Cartilage Tissue Engineering
Adult articular cartilage has limited self-repair capacity due to low cell density, low cell proliferation,
slow matrix turnover, and a lack of a vascular supply. Damage in articular cartilage tissue due to devel-
opmental abnormalities, trauma, or age-related degeneration such as osteoarthritis often result in
extensive chronic pain, gradual loss of mobility, and disability. Current treatment methods are often not
sufficient to achieve timely recovery of normal cartilage functions [121]. Most synthetic polymers used
in cartilage tissue engineering, especially poly(lactide) (PLA), poly(glycolide) (PGA), or copolymers
poly(lactide- co -glycolide) (PLGA), can induce inflammation in vivo [122,123]. For biomaterials consid-
ered for this tissue, collagen suffers from rapid degradation [86] and high swelling [82], while alginate
also has limitations including fast degradation, insufficient mechanical properties, inhibitory effects
on spontaneous repair, and unfavorable immunological responses [124,125]. The useful combination of
high strength, porosity, processability, good biocompatibility, and ability to support cell adhesion, pro-
liferation, and differentiation as described above suggests 3D porous silk fibroin scaffolds as candidates
for stem-cell- and chondrocyte-based cartilage tissue engineering [86,126,127]. 3D HFIP-derived silk
fibroin scaffolds (pore size ~200 mm) and hMSCs were used for in vitro cartilage tissue engineering and
outcomes were compared with unmodified and cross-linked collagen scaffolds [86].
Similar to the studies conducted for bone-tissue engineering [83,118], the structurally stable, slow
degrading scaffolds (cross-linked collagen scaffolds, silk, and RGD-modified silk scaffolds) were essen-
tial to maintain sufficient cell density and to promote the formation of cartilage-like ECMs, based on
DNA content and glycosaminoglycan deposition. hMSCs in the porous silk fibroin scaffolds deposited
higher amounts of cartilage-specific ECM components (GAGs and Col-II) and expressed higher levels
of Col-II mRNA than hMSCs in the collagen-based scaffolds. 3D porous aqueous-derived silk scaffolds
(pore size ~550 mm) were also used for in vitro cartilage tissue engineering using MSCs and chon-
drocytes [126,127]. MSCs successfully adhered, proliferated, and differentiated along the chondrogenic
lineage in the aqueous-derived silk fibroin scaffolds, based on confocal microscopy, real-time PCR, his-
tology, and immunohistochemistry.
In 3D cultivation with highly porous, aqueous-derived silk fibroin scaffolds, within 3 weeks the
majority of MSCs were embedded in lacunae-like spaces and acquired a spherical morphology, which
has been found to be essential for the synthesis of ECM components related to cartilage tissue [128].
In the presence of inducers like dexamethasone and TGF-b3, the proliferation of MSCs peaked and
switched to a more active differentiating stage. Further, within 3 weeks, the MSCs expressed high lev-
els of cartilage-related ECM transcripts [Col-II, aggrecan (AGC), Col-X, and Col-II/Col-I ratio] and
deposited an ECM rich in Col-II protein and sulfated proteoglycans based on immunohistochemistry.
No calcium deposition occurred, confirming the absence of osteogenesis. These results supported the
presence of chondrogenesis under the cultivation conditions within silk scaffolds. A rather homoge-
neous cell and ECM distribution was achieved due to the unique features of these aqueous-derived
scaffolds, including a rough, hydrophilic surface and high pore interconnectivity [126,127]. The distri-
bution of Col-II protein in the 3D constructs also showed a zonal pattern with more protein deposited
in the outer regions, an architecture similar to native articular cartilage tissue. In a follow-up study,
combined adult human chondrocytes (hCHs) with aqueous-derived porous silk fibroin scaffolds (pore
size ~550 mm) were used for in vitro cartilage tissue engineering [126]. After cell seeding, the hCHs