Native B. mori silk is composed of silk fibroin protein coated by sericin proteins. Sericins are adhesive proteins that account for 25-30% of the total silkworm cocoon by weight. The silk fibroin consists of a light chain (Mw approximately 26 kDa) and a heavy chain (Mw approximately 390 kDa) linked by a disulfide bond. Silk fibroin is a block copolymer rich in hydrophobic β-sheet forming blocks linked by small hydrophilic linker segments or spacers. The crystalline regions are primarily composed of glycine-X repeats, where X is alanine, serine, threonine, or valine. Within these domains lie subdomains rich in glycine, alanine, serine, and tyrosine. The result is a hydrophobic protein that self-assembles to form strong and resilient materials. The dominance of the β-sheet-forming regimes within the fibroin structure impart the protein-based materials with high mechanical strength and toughness. The toughness of silk fibers is greater than the best synthetic materials, including Kevlar. In terms of strength, silkworm silk is superior to commonly used polymeric degradable biomaterials such as collagen and poly(L-lactic acid)(PLA). The ultimate tensile strength (UTS) of B. mori silk fibers is 740 MPa. In contrast, collagen has a UTS of 0.9-7.4 MPa and PLA 28-50 MPa. Therefore, silk fibroin is an excellent candidate polymer for biomedical applications.