Tough double-network hydrogels with excellent frictional properties based on the in-suit inhibition strategy for biomedical applications

Osteoarthritis (OA) is one of the most common joint disorders all over the world and hydrogels could be an effective tool to solve this problem. In this work, we explore the mechanism of tribology properties of doublenetwork hydrogel (DN hydrogel) under effect of free radical polymerization inhibitors. Fe3+ is added to the DN hydrogel as highly efficient and easily adjustable inhibitor, and the friction coefficient can be reduced to 0.0038 by adjusting the concentration. Even after 6 h of continuous tribological testing, the DN hydrogel remains a surprising sustained superlubricity. Furthermore, the strength of the hydrogel can reach 2.36 MPa, which is 20 times higher than that of the single-network hydrogel (SN hydrogel). The introduction of the inhibitor reduces covalent crosslinking that enables the hydrogel to form a sparser mesh structure conducive to the reduction of the friction coefficient, especially the additional physical cross-linking delivered from the electrostatic interaction between Fe3+ and carboxyl groups increases strength. The optimum concentration of Fe3+ is explored to achieve the synergetic improvement of lubricative and mechanical properties. This work opens innovative technology routes for developing superlubricious and tough hydrogels, which is a brighter future for artificial cartilage applications.