IEEE Spectrum – scientists at Seoul National University’s Multiscale Biomimetic Systems Laboratory showed off a pressure-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 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.”
ABSTRACT – Flexible skin-attachable strain-gauge sensors are an essential component in the development of artificial systems that can mimic the complex characteristics of the human skin. In general, such sensors contain a number of circuits or complex layered matrix arrays. Here, we present a simple architecture for a flexible and highly sensitive strain sensor that enables the detection of pressure, shear and torsion. The device is based on two interlocked arrays of high-aspect-ratio Pt-coated polymeric nanofibres that are supported on thin polydimethylsiloxane layers. When different sensing stimuli are applied, the degree of interconnection and the electrical resistance of the sensor changes in a reversible, directional manner with specific, discernible strain-gauge factors. The sensor response is highly repeatable and reproducible up to 10,000 cycles with excellent on/off switching behaviour. We show that the sensor can be used to monitor signals ranging from human heartbeats to the impact of a bouncing water droplet on a superhydrophobic surface.
Multiplex, flexible strain-gauge sensor based on the reversible interlocking of Pt-coated polymer nanofibres. a, Schematic of the assembly and operation of a flexible sensor layer sandwiched between thin PDMS supports (~500 μm thickness each). b, Photograph showing the flexibility of the assembled sensor