Biomedical Engineering Reference
In-Depth Information
microspheres loaded with HRP demonstrated controlled and sustained release of active enzyme over
10-15 days [113]. Growth factor delivery via silk microspheres in alginate gels was more efficient in deliv-
ering BMP-2 than IGFs, probably due to the sustained release of the growth factor [114]. Additionally,
growth factors were reported to successfully form linear concentration gradients in scaffolds to control
osteogenic and chondrogenic differentiation of hMSCs. A new mode to generate micro- and nanopar-
ticles from silk was reported based on blending with PVA [115]. This method simplifies the overall pro-
cess compared with lipid templating and provides high yield and good control over feature sizes, from
300 nm to 20 μm, depending on the ratio of PVA/silk used [115]. Silk fibroin microparticles containing
BMP-2, BMP-9, and BMP-14 were prepared by dropwise addition of ethanol and exhibited mean diam-
eters of 2.7 ± 0.3 μm, encapsulation efficiencies of 67.9-97.7% depending on the type and amount of
BMP loaded, and slow release of BMP over 14 days [116].
7.5 Applications of Silkworm Silk for Bone and Cartilage
Tissue Engineering
7.5.1 Silk-Based Bone Tissue Engineering
Bone consists of a highly mineralized ECM, leading to tissue rigidity and strength. The complexity
of bone tissues and their morphological, structural, and functional diversity impart difficulties to
the repair of critical-sized bone defects. Cortical (compact) bone provides mechanical and protective
functions, whereas cancellous (spongy) bone mainly provides metabolic functions [117]. In addition,
bone is essential to calcium homeostasis. Despite immune compatibility, bone repair using autologous
tissue is often not the best treatment option as it is associated with disadvantages such as limited donor
tissue supply, repeated surgery, second site morbidity with additional pain, and long rehabilitation times
[78,84]. Silk fibroin scaffolds for the repair of bones have been explored [45,83-85,111,118]. As previously
reported, silk fibroin hydrogels [111] and membranes/nets [45] without preseeded cells have been used
for guided bone regeneration. In recent years, the technique has evolved to use 3D porous silk fibroin
scaffolds and MSCs for the repair of critical-sized bone defects [83-85,118,119].
HFIP- and aqueous-derived 3D porous silk fibroin scaffolds have been used for hMSC-based bone
tissue engineering
in vitro
and
in vivo
[83-85,118]. Prior to cell seeding, the hMSCs were characterized
for the expression of surface markers and the capacity to differentiate into cells of multiple lineages, with
hMSCs staining positive for CD105, CD44, and CD71 and negative for CD34 and CD31 [83-118]. When
cultured in BMP-2-containing osteogenic medium under static conditions for 4 weeks, the hMSCs
seeded in HFIP-derived porous 3D silk fibroin scaffolds (pore size ~200 mm) showed enhanced osteo-
genic differentiation over the controls (collagen scaffolds) based on real-time RT-PCR for bone-related
gene markers, by immunohistochemistry and microcomputerized tomography for calcium deposition.
With RGD modification of the scaffolds, enhanced differentiation of hMSCs was observed, and more
organized ECM structures were formed [118]. When cultured under dynamic conditions, the stabil-
ity of the HFIP-derived silk fibroin scaffolds were found beneficial in terms of maintaining high cell
density and promoting the differentiation of hMSCs [83-118]. In spinner flask cultures (5 weeks) at
60 rpm, hMSCs generated trabecular-like bone networks with an ECM similar to that of bone [83]. This
engineered bone-like tissue was implanted into critical-sized calvarial bone defects in nude mice and
compared with hMSCs freshly seeded on the scaffolds, scaffolds alone and unfilled defects as controls.
After 5 weeks of implantation, the engineered bone implants and freshly seeded scaffolds were inte-
grated with the surrounding tissue and stained positive for bone sialoprotein, osteopontin, and osteo-
calcin, which were absent in the controls (scaffolds alone and unfilled defects). Compared to hMSC
freshly seeded implants, the tissue-engineered bone implants showed more substantial bone forma-
tion. Within 5 weeks, these tissue-engineered implants started to transform from trabecular-like bone
networks to structures similar to the physiological healing process of intra-membraneous bone [83].
Collectively, these observations suggested that a tissue engineering approach combining 3D porous silk
