Development and Application of Naturally Renewable Scaffold Materials for Bone Tissue Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 302

Although there have been numerous studies concluding that collagen is a suitable mate-
rial for tissue engineered bone scaffolds, collagen alone typically does not provide the
mechanical strength needed for an effective bone replacement. For this reason, col-
lagen has been modified from its original form by combining it with other materials.
Collagen-hydroxyapatite composites have been investigated in order to utilize the bio-
compatible, biodegradable, osteoinductive material properties of collagen, while also
providing a more rigid and mechanically stable structure (111-116). Rodrigues et al .
formed a hybrid scaffold made from collagen and hydroxyapatite to create a human
osteoblast-seeded scaffold for bone engineering applications (115). They observed that
osteoblasts exhibited a high degree of proliferation and were securely attached to the sur-
face. In addition, cells migrated through the composite and began covering the surface
of the material 11 days post seeding (115). In order to test the enhanced mechanical
properties of a porous collagen/hydroxyapatite composite, Yunoki et al . showed that
during compression tests at 30% strain, the shape of the specimens were well recovered
(116). They also reported that the composite was able to withstand higher compres-
sive stress, attributed to the reinforcement of hydroxyapatite nanocrystals in the collagen
matrix, than other porous materials with biopolymers.
The emergence of silk as a scaffold material for bone has been extensively developed
by Kaplan and his colleagues (23, 24, 63, 117-124). Studies by Kaplan and others were
initiated due to silk's unique mechanical properties, formability, biocompatibility, and
ability to undergo proteolytic degradation. Initial work was aimed at the extraction of
sericin proteins to limit immunogenic responses and the behavior of human bone marrow
stromal cells on silk fibroin mats (124). Meinel et al . investigated the use of fibroin
films conjugated with RGD peptide sequences for the promotion of integrin adhesion
and subsequent osteogenesis (122). For that work, neat silk scaffolds and collagen gels
were used as controls versus RGD sequenced silk, and all were seeded with human
MSCs. Bone differentiation was comparable on all materials as determined by alkaline
phosphatase levels, scaffold calcification, and expression bone-specific mRNA transcripts
of bone sialoprotein, osteopontin, and BMP-2 (120, 121). Both silk scaffolds expressed
significant increases in calcium content and alkaline phosphatase activity compared to
collagen. The authors attributed the fast biodegradation of collagen to the inhibition
of these markers. Jin et al . electrospun composite silk fibroin mats by blending the
poly(ethylene oxide) (124). They were able to create fibers with diameters 50

that come close to mimicking the natural architecture of the extracellular matrix, and
those matrices maintained cell viability up to 14 days. Work by Li et al . followed up
on that research by producing electrospun nanocomposites that encapsulated BMP-2 and
hydroxyapatite inside the electrospun matrices (123). That nanocomposite displayed the
highest calcium deposition and upregulation of BMP-2. Accordingly, it was a pivotal
moment that began to exemplify the tremendous impact that scaffolds providing the
appropriate morphology, chemical composition, and physical properties have on bone
generation in vitro (123).
To illustrate silk's potential in vivo , Meinel and Kirker-Head investigated silk in both
nonloadbearing (calvarial) and loadbearing (femoral) defects in rodent models (118-120).
Meinel et al . seeded silk fibroin scaffolds with hMSCs and osteogenically differentiated
them in vitro to yield tissue engineered bone prior to implantation in a 4 mm cranial
defect. Microcomputerized tomography, x-ray, and histological analysis were performed

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