Mechanical and structural responses of silk fibroin scaffolds to MechanoCulture T6® bioreactor loading and enzymatic degradation.

Abstract

Silk fibroin is a semicrystalline biopolymer derived from Bombyx mori cocoons that can be fabricated into various biomaterials including aligned porous scaffolds. Silk fibroin is an ideal biomaterial polymer due to its tunable pore sizes and mechanical properties, which match those of aligned soft tissues, as well as its biocompatibility, non-toxic effects, and tunable degradation. This in vitro platform was developed from ice-templated anisotropic silk fibroin scaffolds toward modeling aligned soft tissues using mechanical loading via a bioreactor. Different scaffold fabrication post-lyophilization parameters gave varied self-assembly of the amino acid building blocks of silk fibroin, as explored through crystalline structures and in vitro degradation. X-ray scattering revealed a longer-range order of the crystalline domains when scaffolds were subjected to slower post-lyophilization processing. This structural difference also manifested in different rates of enzymatic degradation, where protease XIV was able to cleave the amorphous regions between smaller crystalline domains more rapidly. MechanoCulture T6 bioreactor stimulation occurred for 5, 10, or 25 days at 1 Hz, 10% strain for 30 min with 11.5 h of rest periods to mimic skeletal muscle stimulation for hypertrophy. Hydrated uniaxial rheology was used to assess Young's modulus (E), ultimate tensile stress (UTS), and strain at break. The mechanical properties and internal porosity were found to be independent of storage, loading, and time. Scanning electron microscopy (SEM) and nano-computed tomography (nano-CT) showed minimal differences in scaffold structural properties after bioreactor loading. Ice-templated silk fibroin scaffolds were shown to be suitable for new approach methods requiring mechanical stimulation.