Biomimicking lubrication superior to fish skin using responsive hydrogels

The slippery mucus produced by fish skin is important to protect fish against predator attack, allowing fish to swim faster and remain elusive because of the ultra-low coefficient of friction (COF) of fish skin. To mimic this slick skin, responsive hydrogels that respond to external stimuli, including pH and temperature, were prepared. These hydrogels were found to perform better than fish skin: not only was an ultra-low COF achieved but multiple tunable COFs from ultra-low to ultra-high were discovered using sequential regulation of pH and temperature. The tunable COF was achieved through conformational changes in the molecular chains in the responsive hydrogel that were induced by the external stimuli. Swelling of both pH- and thermal-responsive polymer chains of the hydrogel resulted in an ultra-low COF; the pH-responsive component, shrink as a result of dehydration caused by a pH change, led to a moderate COF, whereas the two components simultaneous shrink brought out a very high COF. The three levels of COF under different states can be reversibly switched multiple times by sequential regulation of pH and temperature. This reversible tunability in friction performance is likely to have a significant impact on the design of hydrogel-based actuation devices. Fish excrete hydrophilic biopolymers with ultralow ‘coefficients of friction’ onto their skin to improve their mobility in water. Artificial hydrogels, which trap copious quantities of water molecules inside three-dimensional polymer networks, also have low frictional coefficients and have been targeted for use as biomimetic artificial joint materials and actuators. Their natural counterparts — hydrophilic biopolymers — are usually unable to react to external stimuli and therefore unable to alter their friction. Feng Zhou and colleagues from the Chinese Academy of Sciences in China have demonstrated that a hydrogel can adjust its frictional behavior on demand. The authors improved on their previous discovery — a thermally-responsive acrylamide hydrogel containing graphene oxide flakes — by adding pH-sensitive methacrylate components into the polymer network. Modifying environmental conditions enabled the hydrogel to undergo stepwise switching from ultralow to high friction states — a reversible transformation ideal for producing micromechanical surfaces with ‘autobraking’ capabilities. Stimulus-sensitive composite hydrogels with both pH and temperature responsiveness were prepared to simulate the mucus on fish skin and were found to have superior performance to that of the mimicked substance. The coefficients of friction (COFs) of the gels were found to be easily tunable, from a low to moderate, and then a high level. Importantly, the COF can be reversibly switched for many times by the sequential regulation of external pH and temperature.