Thiol–Yne Crosslinked Nanocomposite Hyaluronic Acid Hydrogels as In Situ-Forming Bone Grafts for the Treatment of Irregular Bone Defects

Biopolymer-based nanocomposite injectable hydrogels represent an optimal solution for bone filling and regeneration, providing an effective tool for treating bone fractures or defects, particularly in the maxillofacial and calvarial regions. To overcome the typical limitations of biopolymer-based hydrogels, such as excessive swelling and rapid degradation, this study presents a series of hydrogels synthesized through photoinduced thiol–yne click chemistry between alkyne-functionalized hyaluronic acid (HA) and ethoxylated trimethylolpropane tris(3-mercaptopropionate) (ETTMP). The viscoelastic properties of the gelling solution enable the incorporation of nanohydroxyapatite (nHAp) at concentrations ranging from 10 to 30 wt % relative to the hyaluronan derivative. Gelation occurs within seconds upon blue light irradiation (405 nm) by using a commercial dental lamp. The resulting hydrogels retain approximately 80% of their initial weight after 28 days of incubation in phosphate buffer, while maintaining a stable swelling degree throughout the same period. Following the photoinduced gelation, hydrogels exhibit optimal viscoelastic behavior as well as good adhesiveness to various materials and tissues. Micro-CT analyses confirm that the hydrogels maintain their structural integrity and optimal filling properties over 28 days of incubation in ex vivo bone specimens. Furthermore, cytocompatibility studies conducted on preosteoblastic cells demonstrated 100% cell viability after 48 h of incubation, highlighting the excellent biocompatibility of the developed hydrogels. Preliminary mineralization studies using mesenchymal stem cells and Alizarin Red showed signs, revealing that the presence of hydroxyapatite confers osteogenic properties to the sample