Molecular mechanics and failure mechanisms in B. mori Silk Fibroin-hydroxyapatite composite interfaces: Effect of crystal thickness and surface characteristics.

Abstract

Bombyx mori Silk Fibroin-hydroxyapatite (B. mori SF-HA) bio-nanocomposite is a prospective biomaterial for tissue engineered graft for bone repair. Here, B. mori SF is primarily a soft and tough organic phase, and HA is a hard and stiff mineral phase. In biomaterial design, an understanding about the nanoscale mechanics of SF-HA interface, such as interfacial interaction and interface debonding mechanisms between the two phases is essential for obtaining required functionality. To investigate such nanoscale behavior, molecular dynamics method is a preferred approach. Present study focuses on understanding of the interface debonding mechanisms at SF-HA interface in B. mori SF-HA bio-nanocomposite at nanometer length scale. For this purpose, nanoscale atomistic models of SF-HA interface are also developed based on the HA crystal size and HA surface type (Ca²⁺ dominated and OH^- dominated) in contact with SF. Mechanical behavior analysis of these SF-HA interface models under pull-out type test were performed using Molecular Dynamics (MD) simulations. Surface pull-off strength values in the range of 0.4-0.8 GPa were obtained for SF-HA interface models, for different HA crystal thicknesses, wherein, the pull-off strength values are found to increase with increase in HA thicknesses. Analyses show that deformation mechanisms in SF-HA interface deformation, is a combination of shear deformation in SF phase followed by disintegration of SF phase from HA block. Furthermore, higher rupture force values were obtained for SF-HA interface with Ca²⁺ dominated HA surface in contact with SF phase, indicating that SF protein has a higher affinity for Ca²⁺ dominated surface of HA phase. Current work contributes in developing an understanding of mechanistic interactions between organic and inorganic phases in B. mori SF-HA composite nanostructure.