Diels-Alder Click Chemistry as a Dynamic-Covalent Crosslinking Method in Spheroid-Encapsulating Hydrogels for Cartilage Engineering

In cartilage tissue engineering, there is a growing interest in dynamic hydrogels that promote spheroid fusion and cartilaginous matrix deposition, while maintaining sufficient stability for long-term construct maturation. In this study, Diels-Alder click chemistry is employed as a dynamic-covalent crosslinking method to create hydrogels composed of hyaluronic acid, gelatin, and PEG. By adjusting the pH during crosslinking, the tuneability of hydrogel stiffness and stability at pH values around the physiological pH of native cartilage is demonstrated. This pH modulation does not compromise hydrogel functionality, as encapsulated equine articular cartilage progenitor cell spheroids remain viable and functional for a culture period of 28 days. The hydrogel environment supports the deposition of cartilaginous extracellular matrix components, including collagens and sulphated glycosaminoglycans. Enhanced chondrogenesis and deposition of collagen type II are observed at higher spheroid concentrations, corresponding to inter-spheroid distances of 100–150 µm following hydrogel swelling, compared to lower concentrations at a distance of >500 µm. To further improve construct robustness, the hydrogel constructs can be reinforced on day 1 with a melt electrowritten scaffold, increasing the compressive modulus 100-fold by day 28 compared to non-reinforced constructs, highlighting the potential of this system for engineering cartilage implants.