Bioactive γCaPGA Sol-Gel Hybrids for Bone Regeneration

Abstract

Bioactive glasses have the ability to bond to bone in vivo, but they are brittle and cannot be used in load bearing applications. In this thesis, a process was developed to toughen bioactive glasses by forming a hybrid material for bone tissue engineering using the sol-gel process. As a first step, in preparation for polymer incorporation into the sol-gel process, the pH of the sol-gel synthesis had to be raised to milder pH conditions to prevent acid chain scission hydrolysis of the polymer. Solgel glasses were synthesised under the modified conditions and no adverse effects were found due to raising the pH of synthesis from pH < 1 to 5.5. These mild pH conditions were then used to synthesise hybrids of silica and calcium salt poly(γ-glutamic acid) (γCaPGA). γCaPGA was used as the toughening agent and as a low temperature calcium source with 3-glycidoxypropyl trimethoxysilane (GPTMS) providing covalent coupling between the inorganic and organic components. Hybrids of 40 wt% γCaPGA of all molecular weights tested (120 to 30 kDa) had large strain to failure (> 26 %) which showed that γCaPGA hybrids successfully softened the brittle behaviour of sol-gel glasses. However, the polymer dissolved preferentially due to its hydrophilic nature. All γCaPGA hybrids were found to form hydroxycarbonate apatite (HCA) within one week in SBF, even though they contained a low calcium concentration of 5 wt% when compared with 17.5 wt% Ca in Bioglass®. Formation of HCA is the first step in bonding to bone in vivo which is a fundamental requirement of materials for bone tissue engineering. Calcium was not only important for bioactivity, but also for ionic crosslinking, which improved compressive strength and reduced strain to failure when compared with identical hybrids made without ionic crosslinking. Although hybrids synthesised with γCaPGA dissolved too quickly for bone applications, calcium chelating polymers have been shown to offer great promise for bone tissue engineering.