Graphene-sponge touch sensor mimics human skin
The very low cost and compliant touch sensor is made out of a polyurethane sponge dip-coated in a suspension of graphene nano-flakes, then sandwiched between thin film electrodes deposited on a protective PEN film.
As the conductive graphene sponge is squeezed, the piezo-resistive sensor yields a progressive change in conductance (the graphene flakes getting closer). The 15x15x15mm sensor was able to reliably distinguish vertical pressures as low as a few Pa (from placing a 0.11g mass on top) to 20kPa (similar to human pressure perception from 100 to 100 000 Pa). The flexible sensor was also tested in frequency, exhibiting over 20 dB of signal to noise ratio up to a frequency of 50 Hz.
Although the signals gradually decreased with increasing frequency, due to the time lag resulting from elastic deformation and restoration of the sponge, SNR did not drop below 10 dB at up to 500Hz, again corresponding roughly to human sensitivity to vibrations (up to about 400Hz) for texture information.
In their experiments to characterize the sensor’s ability to detect surface roughness, the researchers applied a PET ridge structure (a 80×200μm tip acting as a probe) to the sensor, emulating at a finer scale the ridges of human skin.
When slipped across a set of two ridges separated by 200μm, the sensor was able to distinguish each ridge pattern without mixing the two pressure peaks. Considering the width of the probing tip at 120μm, the researchers estimated the spatial resolution of locally applied pressure to be 80μm.
Breaking free of the inherent limitations of discrete accelerometers and pressure sensors, the graphene-sponge sensor could find very useful applications in robotics and prosthetics, uniquely solving multi-sensor integration in a single element architecture, conclude the researchers.
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