Bilayer PAM-PVA Cartilage-Inspired Hydrogels with Layer-Selective CNT and Nanodiamond Interfacial Reinforcement for Structure-Property Control

Abstract

Double-network hydrogel systems are developed to improve the structural and mechanical stability of single-layer hydrogels 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 (CNT) and nanodiamonds (ND) 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 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 swelling rate constants increasing from 0.0108 for PAM/PVA to 0.0323 for PAM-ND/PVA-CNT, 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-CNT, reflecting reduced water uptake due to nanoscale reinforcement. The nanocomposite bilayers exhibited significantly improved stability compared to 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. 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 layerselective nanoscale reinforcement in bilayer PAM-PVA hydrogels as an effective strategy to control swelling behaviour, mechanical performance, biological activity and stability for tissue engineering and regenerative medicine applications.