Nanoporous Substrate‐Infiltrated Hydrogels: a Bioinspired Regenerable Surface for High Load Bearing and Tunable Friction

Abstract

Nature has successfully combined soft matter and hydration lubrication to achieve ultra-low friction even at relatively high contact pressure (e.g. articular cartilage). Inspired by this, scientists have used hydrogels to mimic natural aqueous lubricating systems. However, hydrogels usually cannot bear high load because of solvation in water environments and are, therefore, not adopted in real applications. In this work, we developed a novel composite surface of ordered hydrogel nanofiber arrays confined in anodic aluminum oxide (AAO) nanoporous template based on a soft/hard combination strategy. The synergy between the soft hydrogel fibers, which provide excellent aqueous lubrication, and the hard phase AAO, which gives high load bearing capacity, is shown to be capable of attaining very low coefficient of friction (< 0.01) under heavy load (with mean contact pressures in the 2 MPa range). Interestingly, the composite synthetic material was very stable and could not be peeled off during sliding and exhibited the desirable regenerative (self-healing) properties, which can assure long term resistance to wear. Moreover, the crosslinked polymethylacrylic acid (PMAA) hydrogels were shown to be able to promptly switch between high friction (> 0.3) and superlubrication (∼10 −3 ) when their state was changed from contracted to swollen by means of acidbasic actuation. The mechanisms governing ultra-low and tunable friction are theoretically explained via an in-depth study of the chemo-mechanical interactions responsible for the behavior of these substrate-infiltrated hydrogels. These findings open a promising route for the design of ultra-slippery and smart surface/interface materials.

Structural hydrogels

“…Recently, Zhou and coworkers reported on the fabrication a novel soft/hard composite materials by integrating ordered hydrogel nanofibers into anodic aluminum oxide (AAO) nanoporous template [29]. Meanwhile, the dense gel fibers arrays presented high hydration degree and osmotic pressure, while the hard AAO template can effectively eliminate the elastic dissipation in sliding shear on the composite interface.…”

“…Recently, new exploration focused on the construction of 3D free-standing hydrogels nanofibers array has been reported. This reported-system highly relies on the confinement effect from inorganic template and presents potential interesting for biological M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT application [29].…”

“…Recently, Ma et al [29] reported a simple method to fabricate ordered hydrogel nanofibers array. In this work, the acrylic acid (AA) monomer solution (containing initiator and crosslinker) was infused into porous anodic aluminum oxide template and then followed by a thermal-induced free radical polymerization to get a hydrogel filled composite.…”

“…Typical time dependencies of the friction coefficient Vs time in response to weak acid (pH=3, high friction) and weak basic (pH=10, super-lubrication) media (top right). Reproduced with permission from the literature [29].…”

Preparation of CaO-SiO2-CuO bioactive glasses-embedded anodic alumina with improved biological activities

“…Porous anodic alumina (PAA) is typically a selforganized material with ordered nano-porous structures and with promising applications to biotechnology [1,2]. Recently, PAA has been shown to be a superior platform for cell-interface studies due to its excellent chemical stability, controllable dimensions as well as biocompatibility and orthopedic biomimetic properties [3][4][5][6][7][8]. Our research also demonstrates that the nano-pore structure and the pore size are important physical cues for osteoblasts and macrophages, which affect their spreading and cell shape, subsequently regulate their osteogenic functionalities [9,10].…”