Nanohydrogel Brushes for Switchable Underwater Adhesion

In
nature, living systems commonly adopt the switchable friction/adhesion
mechanism during locomotion. For example, geckos can move on ceilings,
relying on the reversible attachment and detachment of their feet
on substrate surfaces. Inspired by this scientists have used different
materials to mimic natural dynamic friction/adhesion systems. However,
synthetic systems usually cannot work in water environments and are
also limited to single-contact interfaces, while nature has provided
living systems with complex features to perform energy dissipation
and adhere on multiple contact interfaces. Here, for the first time,
we report the design, synthesis, and testing of a novel double-sided
synthetic construct that relies on nanohydrogel brushes to provide
simultaneous friction switching on each side of the membrane that
separates the nanohydrogel fibers. This highly tunable response is
linked to the swelling and shrinkage of the brushes in basic/acid
media. Such a system shows three different friction states, which
depend on the combination of pH control of the two membrane sides.
Importantly, each side of the membrane can independently provide continuous
but stable friction switching from high to ultralow friction coefficients
in a wet environment under high load conditions. An in-depth theoretical
study is performed to explore the mechanisms governing the hydration
state responsible for the observed switching. This novel design opens
a promising route for the development of new solutions for intelligent
devices, which can adapt to multistimulus-responsive complex environments.