Poly(γ‐glutamic acid)/Silica Hybrids with Calcium Incorporated in the Silica Network by Use of a Calcium Alkoxide Precursor

Abstract

Current materials used for bone regeneration are usually bioactive ceramics or glasses. Although they bond to bone, they are brittle. There is a need for new materials that can combine bioactivity with toughness and controlled biodegradation. Sol-gel hybrids have the potential to do this through their nanoscale interpenetrating networks (IPN) of inorganic and organic components. Poly(γ-glutamic acid) (γ-PGA) was introduced into the sol-gel process to produce a hybrid of γ-PGA and bioactive silica. Calcium is an important element for bone regeneration but calcium sources that are used traditionally in the sol-gel process, such as Ca salts, do not allow Ca incorporation into the silicate network during low-temperature processing. The hypothesis for this study was that using calcium methoxyethoxide (CME) as the Ca source would allow Ca incorporation into the silicate component of the hybrid at room temperature. The produced hybrids would have improved mechanical properties and controlled degradation compared with hybrids of calcium chloride (CaCl2), in which the Ca is not incorporated into the silicate network. Class II hybrids, with covalent bonds between the inorganic and organic species, were synthesised by using organosilane. Calcium incorporation in both the organic and inorganic IPNs of the hybrid was improved when CME was used. This was clearly observed by using FTIR and solid-state NMR spectroscopy, which showed ionic cross-linking of γ-PGA by Ca and a lower degree of condensation of the Si species compared with the hybrids made with CaCl2 as the Ca source. The ionic cross-linking of γ-PGA by Ca resulted in excellent compressive strength and reduced elastic modulus as measured by compressive testing and nanoindentation, respectively. All hybrids showed bioactivity as hydroxyapatite (HA) was formed after immersion in simulated body fluid (SBF).

Timing of calcium nitrate addition affects morphology, dispersity and composition of bioactive glass nanoparticles

“…5b) showed a broad band in the range of 2q ¼ $ [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] , which is a typical characteristic peak for amorphous silicate materials 43 and its existence conrms the amorphous nature of all synthesized BGN. Notably, the broad band of BGN-0 exhibited slight shi compared with other BGN.…”

Hybrid sol–gel inorganic/gelatin porous fibres via solution blow spinning

“…Calcium alkoxide precursors appear a more promising route and have been used to obtain monoliths and fibres with homogeneous silica and calcium networks without the need for thermal treatment [46,72,77,81]. The incorporation of calcium into the porous fibres using calcium alkoxides in the formulations and processing is the focus of current work.…”

Functionalizing natural polymers with alkoxysilane coupling agents: reacting 3-glycidoxypropyl trimethoxysilane with poly(γ-glutamic acid) and gelatin

“…In vivo, chitosan-silica hybrid scaffolds showed good cytocompatibility when implanted subcutaneously and improved tissue regeneration of crushed spinal nerve in a rat model 7 . Highlighting the importance of understanding hybrid systems, a gelatin-silica hybrid material showed dissolution profiles that could be tailored by altering the amount of GPTMS coupling agent 10, 11 yet poly(γ-glutamic acid) (γ-PGA)-silica hybrids did not 40,41 , despite both polymers comprising amino acids with nucleophilic carboxylic groups.…”

“…gelatin), which do not have a single repeating unit but up to 20 different amino acids arranged in different sequences that vary between species, individuals and tissue types, means that chemical characterization by nuclear magnetic resonance (NMR) is challenging. Researchers become reliant on using Fourier transform infrared spectroscopy (FTIR) as a characterization technique 10,46,47 . Unfortunately, the data is not quantitative and vibration band assignment can be controversial.…”