Mechanical and microscopical characterisation of bilayer hydrogels strengthened by TiO₂ nanoparticles as a cartilage replacement candidate

Bilayer hydrogels are suggested as a candidate for cartilage replacement due to their similarity to the native cartilage structure. While their water retention in the lubricious layer is an advantage over other counterparts, their mechanical properties have remained under-explored. In this study, indentation, compression and stress relaxation tests were conducted to evaluate mechanical responses of the bilayer hydrogels. Scanning electron microscope was also utilised to visualise the formation of the porous layer with varying TiO₂ nanoparticle (NP) loadings. Our results show that an optimum TiO₂ NPs concentration, improved elastic modulus and hardness of the bilayer hydrogels threefold. 0.2wt% TiO₂ loading in the polyacrylamide/alginate/ acrylic acid hydrogel exhibited the best material stiffness and stress-strain responses. Viscoelastic responses improved for 0.2wt%, 0.4wt% and 0.6wt% NPs loadings compared to the non-reinforced hydrogel. A unique string-like pore on the surface of 0.2wt% NPs sample was observed, and swelling ratio of this sample was at a moderate level (300 %) compared to other concentrations. The NP loaded bilayer hydrogel demonstrated mechanical features similar to those of articular cartilage, hence could be considered as a cartilage replacement candidate.