Bioactive silica–poly(γ-glutamic acid) hybrids for bone regeneration: effect of covalent coupling on dissolution and mechanical properties and fabrication of porous scaffolds

It is well known that bone has excellent mechanical properties through its hierarchical structure and that design of new materials should take inspiration from its structure, especially those that will be used as synthetic bone grafts. However it has not yet been possible to mimic the complex structure. Here we present new organic–inorganic hybrid scaffolds produced by a sol–gel foaming process with the aim of producing materials that mimic the trabecular structure of bone and take inspiration from the nanocomposite structure. Poly(γ-glutamic acid) (γ-PGA) was chosen as the organic component because it is a polypeptide that can be functionalised and can degrade by enzyme action. The γ-PGA chains and bioactive silica were covalently bonded to each other (class II hybrid) through an organosilane coupling agent, glycidoxypropyl trimethoxysilane (GPTMS). This was key to obtaining a hybrid with good mechanical properties and homogeneous integration of inorganic and organic chains at the nanoscale. The effect of the degree of covalent coupling on mechanical properties and dissolution rate of the hybrids was investigated. Hybrids with a covalent coupling between 2 and 10 glutamic acid units per GPTMS molecule showed optimal mechanical properties and polymer release into TRIS-buffer solution. Polymer release was much slower from these hybrids than from those with lower amounts of covalent coupling. Monoliths of these compositions were not toxic to the SaOs-2 osteosarcoma cell line and supported cell attachment and growth. Foaming of the optimised hybrid composition was performed to produce porous 3D scaffolds. The modal pore and interconnect diameters were quantified to be 297 and 111 μm respectively, using micro-computed tomography (μCT) and image analysis, showing the pore network to mimic that of cancellous bone.