When dynamic covalent chemistry meets bioactive glasses – Novel multi-level crosslinked, injectable, self-healing chitosan-based hydrogels

The hydrogels based on dynamic covalent bonding have gained increasing attention in recent years in biomedical applications. This is primarily due to their unique properties, such as self-healing and injectability. In this work, silicate and borate bioactive glasses (BGs) were used as modifiers in the chitosan-based hydrogels crosslinked with dextran dialdehyde via imine bonds. We have shown for the first time that both silicate and borate BGs can play a dual role in the obtained hydrogels – they serve as functional component, modifying hydrogel properties, and providing additional multi-level crosslinking mechanisms. Two main mechanisms of the crosslinking effect of BGs, resulting from the release of their degradation products can be identified. Firstly, boron and silicon provide dynamic covalent ester bonds. Secondly, calcium and sodium-mediated deprotonation of the NH3+ groups of chitosan enables the formation of additional imine bonds. The obtained hydrogels were evaluated in terms of microstructure, mechanical and rheological properties, self-healing, injectability, in vitro mineralization and stability, as well as cytocompatibility with human umbilical vein endothelial cells (HUVECs). The results have shown that BGs significantly improve the stiffness, compressive strength, and viscoelastic characteristics of hydrogels, as well as greatly reduce crosslinking time. Hydrogels modified with BGs showed good injectability and self-healing properties. Hydroxyapatite forming ability of BGs-containing materials in simulated body fluid was confirmed using micro-computed tomography. Furthermore, no cytotoxic effect of hydrogels was observed in direct contact with HUVECs. The developed chitosan-based hydrogels may represent a promising solution for minimally invasive surgery in tissue engineering applications.