Conclusions
The wide range of molecular structures, remarkable mechanical properties, morphology control, versatile process ability and surface modification options make silk fibroin an attractive polymeric biomaterial for design, engineering and processing into scaffolds for applications in controlled drug delivery, guided tissue repair and functional tissue engineering. 3D porous or fiber silk fibroin scaffolds with surface morphology, useful mechanical features, biocompatibility, and ability to support cell adhesion, proliferation, and differentiation have expanded silk-based biomaterials as promising scaffolds for engineering a range of skeletal tissues like bone, ligament, and cartilage as well as connective tissues like skin. The generally slow rates of degradation of silk fibroin in vivo, coupled with the versatile control of structure, morphology and surface chemistry, offer a range of utility for this family of protein polymers in many needs in biomaterials and tissue engineering. In addition, since these structures can be sterilized by autoclaving or ethylene oxide treatment, suitable options are available to prepare the Fig. 4. Three-dimensional distribution of human bone marrow derived mesenchymal stem cells in porous aqueous derived silk fibroin scaffolds at week 3. Cells were stained using 2 mM calcein AM (green for live cells) and 4 mM EthD-1 (red for mostly silk fibroin and also dead cells) (Molecular
Probes, Eugene, OR) for 30-45 min at room temperature and evaluated using a Bio-Rad MRC 1024 confocal microscope with Laser sharp 2000 software. Y. Wang et al. / Biomaterials 27 (2006) 6064–6082 6075 materials for in vivo studies. To date most of the impact with silk-based biomaterials has been with only one source of silk, the fibroin from B. mori silkworm. As new sources of silk proteins become available, such as from spiders and via genetic engineering and modification of native silk sequence chemistries, the range of material properties can be generated and utilized for biomaterials can be expected to further expand options and lead to additional medical impact. For example, genetically engineered nanocomposites of spider silk with mineralizing domains have recently been described and offer new mechanical properties as well as interfacial properties, along with osteoconductivity or
osteoinductivity depending on design [170]. Future directions to improve the incorporation and delivery of cell signaling factors via the aqueous processing modes available during the formation of silk biomaterial matrices, or to induce vascular networks in silks in vivo, will further enhance impact for this family of protein biomaterials. Finally, hybrid or composite systems with other biopolymers offer novel options to match complex mechanical and biological functions with tissue-specific needs.