In a series of demonstrations (published in Nature Materials), scientists at Seoul National University’s Multiscale Biomimetic Systems Laboratory showed off apressure-sensing membrane that is sensitive enough to feel the fall of water droplets, a human pulse in the wrist, and even the whisper-light tread of a lady-bug walking across the “electronic skin.”
True to its “biomimetic” creed, the group took its cue from the signal transduction systems found in the ear, intestines, and kidney—nanoscopic hairs that interlock and produce signals by rubbing one another when their base membranes dent, ripple, or twist. They also added a self-assembly feature inspired by the locking mechanism on a beetle’s wing.
The device features two sheets of polyurethane acrylate. The sheets, which can be as big as 9 by 13 centimeters, are molded onto dense arrays of minute polymer hairs, each 100 nanometers in diameter and 1000 nm tall. Each of the hairs is coated with a 20 nm layer of platinum and bonded to a basement membrane (polydimethylsiloxane treated to enhance conductivity).
The two ciliated sheets are then mated, face to face, like two pieces of Velcro. The fibers in the top layer mesh with those on the bottom. But instead of mechanical hook-and-loop binding, the sheets are held together strongly (but reversibly) by Van der Waals attraction. The nanofiber sandwich conducts current between layers, and the resistance changes as the total contact area between the meshing hairs varies. A touch, push, or twist of the basement membrane makes the meshed nanohairs rub and bend, and the changing current shows what’s going on. Indeed, since orthogonal pressure, lateral shear, and torsion produce different response curves, the device can tell the difference between a push, a rub, and a twist.
The system’s gauge factors—the change in resistance due to changes in strain—were about 11.5 for direct pressure, 0.75 for shear, and 8.53 in response to torsion. By comparison, direct-pressure sensors based on graphene-film have a gauge factor of about 6.1, and for conventional metal foil sensors, the factor is about 2.0. (Note that these other sensors pick up strain in one direction only. In order for them to detect pressure, shear, and torsion they must be specially fabricated with separate sensors for each direction of strain.)
In sum, the researchers say, the “nano-interlocking mechanism requires no complex integrated nanomaterial assemblies or layered arrays, thus allowing a simple, cheap, yet robust sensing platform for high-performance, large-area strain-gauge sensors.”
