Citric acid-enhanced chitosan/hyaluronic acid interpenetrating hydrogel network as a bioinspired adhesive for osteochondral scaffold fabrication

Osteochondral scaffolds often employ multilayered structures to mimic the cartilage–bone interface, but weak interfacial adhesion and insufficient mechanical robustness remain key limitations. To address this challenge, we developed a mussel-inspired interpenetrating hydrogel network (IHN) composed of catechol-functionalised chitosan (Cat–CS) and ultra-high molecular weight methacrylated hyaluronic acid (MeHA) as a bioadhesive phase for osteochondral scaffold fabrication. Leveraging the hydrogen-bonding capability of selected natural polymers, we hypothesised that the acidity of the hydrogel-forming solution could regulate Cat–CS/MeHA network formation, mechanical stability, swelling resistance, adhesion and scaffold bonding. Systematic solvent screening identified citric acid (CA) as the most suitable biocompatible medium, enabling full dissolution of Cat–CS and MeHA and promoting polyelectrolyte complexation (PEC). Increasing CA concentration from 1 to 2 M yielded IHNs with markedly improved macroscopic properties, with Young's modulus increasing from 1.26 ± 0.69 to 7.32 ± 1.84 kPa and swelling ratio decreasing from 359% to 259%. Adhesion strength increased from 0.53 kPa (1 M) to 1.51 kPa (2 M), enabling stable bonding of the hydrogel to a 3D-printed polyhydroxybutyrate (PHB) scaffold after rehydration. Additional cyclic compression, NaOH stability and attenuated total reflection Fourier-transform infrared spectroscopy (ATR–FTIR) analyses were consistent with the formation of a photo-crosslinked MeHA-based IHN reinforced by Cat–CS/MeHA PEC and citrate-mediated secondary interactions. Cytocompatibility assays showed that, after neutralisation, the hydrogels supported fibroblast viability and proliferation over 7 days. Collectively, these findings establish CA-mediated Cat–CS/MeHA IHNs as bioinspired adhesive interfacial layers for integrated osteochondral scaffold fabrication.