Tuning the properties of silk fibroin biomaterial via chemical cross-linking

Abstract

Protein-based biomaterials with innovative and controlled performance are being sought due to their unique characteristics for use in biomedical fields such as neural implants, drug delivery systems, cellbased therapies and soft tissue engineering. Here, we present a versatile approach for the synthesis of photo-crosslinkable fibroin silk biomaterial with highly tunable mechanical, chemical and biodegradation properties. Unlike the crystalline rich silk fibroin reported previously, the covalently crosslinked fibroin protein photoresist (FPP) via controlled light-induced radical grafting, allows generating a new amorphous biomaterial with tunable properties. It appears that the use of photoreactive acrylate groups to cross-link FPP induces plasticity that can be tuned by changing the photoinitiator concentration of the film. Tensile strength measurements revealed that elasticity was higher for FPP UV-cross-linked materials with higher concentration of photoinitiator. FTIR and relative humidity measurements showed that hydrophilicity was higher for UV-cross-linked FPP. These materials display stiffness between 0.01-1.5 GPa and tensile strains up to 60%, covering a significant portion of the properties of native soft biomaterials. Besides, in vitro proteolytic degradation of the photocrosslinked FPP films demonstrate a tunable degradation rate over a period ranging from hours to weeks. Those biomaterials have been successfully micropatterned by photolithography techniques across several orders of magnitude (μm to cm) and a systematic study of direct patterning of the fibroin protein to form high fidelity and high-resolution structures has been reported. It was also shown that the fabricated protein features are well suited to cell adhesion. The development of protein-based material with controlled and tunable elasticity that can be easily photopatterned into centimeter, micrometer and nanometer components will allow a wide range of applications in biomedical fields requesting a natural functional tissue.

The Mechanical Properties, Secondary Structure, and Osteogenic Activity of Photopolymerized Fibroin

“…Thus, the substrate's elasticity modulus (EM) can regulate cell morphology, motility, and the direction of differentiation. The EM of normal bone extracellular matrix (ECM) is 20-50 kPa [3], whereas for the mineralized bone, the values are as large as [10][11][12][13][14][15][16][17][18][19][20] GPa [4]. The respective values for typical polymer scaffolds are in the kPa range [5]; an increase in EM can be a factor of the osteogenic potency of the material [6][7][8].…”

“…The content, localization, and orientation of crystalline domains may depend on the method of material fabrication; therefore, the mechanical properties would be different [15]. Fb modifications at the side groups are promising for synthesis of polymers by an additional functional network [16,17] with optimized characteristics.…”

“…Chemically modified Fb substrates have been demonstrated to be promising materials for bone regeneration [17]. In this study, a urethane methacrylate Fb derivative was prepared; photopolymerization of this macromonomer yielded covalently cross-linked films.…”

“…The degree of cross-linking negatively correlated with EM values and beta-sheet crystallinity. The urethane methacrylic Fb derivative [17] has been synthesized using a highly reactive isocyanoethyl methacrylate that provided an exhaustive modification of abundant serine residues (~12% of total amino acid content) in the hydrophobic repeats (GAGAGS) that form the crystalline domains of beta sheets. We have proposed another synthetic route for Fb methacrylation, yielding the methacrylamide derivative FbMA [18].…”

“…Thus, FbMA, being a convenient parental compound for the photochemical fabrication (including SLA and DLP additive manufacturing) of covalently cross-linked Fb gels, can provide the scaffolds with a higher EM. This property is important for osteogenic potency and makes FbMA preferable over the materials described in [17].…”