Silk as a Biomaterial

Abstract

Silks are fibrous proteins with remarkable mechanical properties produced in fiber form by silkworms and spiders. Silk fibers in the form of sutures have been used for centuries. Recently regenerated silk solutions have been used to form a variety of biomaterials, such as gels, sponges and films, for medical applications. Silks can be chemically modified through amino acid side chains to alter surface properties or to immobilize cellular growth factors. Molecular engineering of silk sequences has been used to modify silks with specific features, such as cell recognition or mineralization. The degradability of silk biomaterials can be related to the mode of processing and the corresponding content of beta sheet crystallinity. Several primary cells and cell lines have been successfully grown on different silk biomaterials to demonstrate a range of biological outcomes. Silk biomaterials are biocompatible when studied in vitro and in vivo. Silk scaffolds have been successfully used in wound healing and in tissue engineering of bone, cartilage, tendon and ligament tissues.

Figures

Fig. 1

Fig. 1A: Silk fibroin is purified from sericins via boiling in an alkaline solution. The de-gummed or purified silk fibers can be processed into silk cords by twisting [4]; non-woven silk mats by partial solubilization [32]; or dissolved in lithium bromide, dialyzed and formed into aqueous silk fibroin solution [67] for preparation of other material morphologies (See figure 1B). Fig. 1B: Processing of silk morphologies from aqueous silk fibroin solution into non-woven silk fibers [11]; aqueous and solvent based porous sponges [67, 69]; hydrogels [108]; and films [46].

Fig. 1

Fig. 1A: Silk fibroin is purified from sericins via boiling in an alkaline solution. The de-gummed or purified silk fibers can be processed into silk cords by twisting [4]; non-woven silk mats by partial solubilization [32]; or dissolved in lithium bromide, dialyzed and formed into aqueous silk fibroin solution [67] for preparation of other material morphologies (See figure 1B). Fig. 1B: Processing of silk morphologies from aqueous silk fibroin solution into non-woven silk fibers [11]; aqueous and solvent based porous sponges [67, 69]; hydrogels [108]; and films [46].

Fig. 2

Porous gradient silk sponges prepared by stacking a water soluble porogen from smallest to largest (A). Solvent based silk solution is added and allowed to diffuse through the salt crystals (B & C). The porogen is dissolved in water leaving the sponge with pore gradient (D) [109].

Fig. 3

Covalent coupling versus adsorption of proteins on silk surfaces. A: Modifiable amino acid side chains; presence of amine, carboxyl and hydroxyl groups. B: Carboxyl side groups activated by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) for 10–15 minutes. A protein like BMP-2 can be introduced via amine groups reacting with the activated silk to form amide bonds. Cyanuric activated polyethylene glycol (PEG) reacts with amine and hydroxyl groups on silk fibroin surface. C: Coupling of PEG on silk fibroin films generates a more hydrophilic surface and reduced attachment of human mesenchymal stem cells (hMSCs) [81]. D: Coupling BMP-2 to silk fibroin films via carbodiimide coupling results in increased calcium deposition (increased calcein labeling) by differentiated hMSCs [9].