Bilayer PAM-PVA cartilage-inspired hydrogels with layer-selective CNT and nanodiamond interfacial reinforcement for...

Double-network hydrogel systems are developed to enhance the structural and mechanical stability of single-layer hydrogels, which are used as biological or load-bearing materials. In this work, bilayer hydrogel systems comprising polyacrylamide (PAM) and poly(vinyl alcohol) (PVA) were fabricated, followed by layer-selective reinforcement with multi-walled carbon nanotubes (CNTs) and nanodiamonds (NDs) to obtain PAM-CNT/PVA-ND and PAM-ND/PVA-CNT constructs. The experimental and theoretical analyses revealed a strong correlation between bilayer architecture, nanoscale reinforcement, and the resulting structure-property relationships. Swelling studies showed that water uptake followed a combination of Fickian and non-Fickian diffusion mechanisms, with diffusion exponents (n) ranging from 0.307 to 0.350. The swelling kinetics were well described by a pseudo-second-order model, with the swelling rate constants increasing from 0.0108 for PAM/PVA to 0.0323 for PAM-ND/PVA-CNTs, indicating faster swelling in reinforced bilayer systems. The equilibrium swelling ratio decreased from 14.15 for PAM/PVA to 6.21 for PAM-ND/PVA-CNTs, reflecting reduced water uptake due to nanoscale reinforcement. The nanocomposite bilayers exhibited significantly improved stability compared to the unreinforced PAM/PVA hydrogels. After degradation testing, the PAM-ND/PVA-CNT hydrogel retained approximately 90% of its original mass, demonstrating strong interfacial cohesion and resistance to hydrolytic degradation. Mechanical evaluation revealed that the PAM-CNT/PVA-ND bilayer achieved the highest compressive strength of 0.42 MPa, highlighting efficient stress transfer across the reinforced interface. The PAM-CNT/PVA-ND bilayer hydrogel showed the highest compressive strength and higher interfacial adhesion strength, which implies effective stress transfer across the interfaces. The synergistic effect of the reinforcement with CNTs in the load-bearing layer and nanodiamonds' contribution to the interfacial cohesion is due to this enhanced performance. Cytocompatibility studies confirmed more than 99% viability of MG-63 cells, indicating excellent biological compatibility. Consequently, enhanced stress transfer in the CNT-reinforced PAM layer, together with hydrogen-bond-mediated interfacial cohesion from nanodiamonds, governs the structure-property response of the bilayer system. These findings establish layer-selective nanoscale reinforcement in bilayer PAM-PVA hydrogels as an effective strategy to control swelling behavior, mechanical performance, biological activity, and stability for tissue engineering and regenerative medicine applications.