Development of an injectable hydrogel loaded with hydroxyapatite nanoparticles for enhanced in situ bone regeneration

Bone fractures pose a significant burden on worldwide healthcare systems, with over 900,000 fractures reported every year in the UK (1). Various tissue engineering techniques are being researched which mimic native extracellular tissue and allow the proliferation and differentiation of cells, ultimately leading to bone repair. The use of in situ forming hydrogels presents a minimally invasive technique for sustained local drug delivery and is an attractive alternative to invasive surgery, which can be costly and result in complications. We have synthesised a thermo-responsive N-isopropylacrylamide (NIPAAm) hydrogel grafted onto a Chitosan (Cs) backbone. Chitosan is biocompatible, biodegradable, and has a structure similar to that of native extracellular matrix (2). This hydrogel undergoes in situ sol-gel transition at body temperature and degrades over an 8-week period to allow sustained cargo delivery. Furthermore, this hydrogel has been loaded with RALA nanoparticles containing hydroxyapatite (HA). HA is commonly used in fracture repair but has limited bioavailability. Our patented RALA peptide forms nanoscale particles encapsulating HA to allow its cellular entry and increase its bioavailability. The aim of this study was to synthesise Cs-g-PNIPAAm hydrogel loaded with RALA-HA nanoparticles (NP) for bone regeneration in vitro and in vivo.