e-skins beats human touch with CNT-based structured capacitors

November 26, 2018 // By Julien Happich
In a paper titled "A hierarchically patterned, bioinspired e-skin able to detect the direction of applied pressure for robotics" published in Science Robotics, researchers from Stanford University describe how they have designed a layered three-dimensional structure that precisely mimics the interlocked dermis-epidermis interface in human skin.

Getting their inspiration from the biological structure of actual human skin, the researchers embedded a grid of molded square carbon nanotube (CNT) pyramids into a polyurethane (PU) matrix to form the top electrodes of an array of capacitors. The bottom electrodes were also lined up with CNTs, forming a two-dimensional array of molded hills which mimicked the spinosum layer in human skin. These two electrode layers separated by a thin-film dielectric layer allowed the formation of conformable multi-pixel capacitors (a 5x5 top grid centred over each bottom hill) whose electrical response is highly dependent on the skin's deformations under external pressure.

The spinosum (left) is a layer found between the dermis and the epidermis, forming interlocked microstructures (hills) responsible for tactile signal amplification. Because of the 3D structure of the spinosum, the hills concentrate forces onto the mechanoreceptors differently depending on the direction of applied force. On the soft biomimetic e-skin (right), CNT electrodes (black) embedded in elastomer (blue) are separated by a thin-film dielectric layer (gray). Bottom and top electrodes are aligned so as to form an array of capacitors, with each hill corresponding to 25 capacitors.

The e-skin's configuration is such that for each hill about 1mm in diameter and 200μm high corresponds 25 tiny capacitors each only 90μm2 in size, with one capacitor precisely located at the top of the hill, four on the slopes, four on the "corners" and 16 surrounding the hill. This high density of mechanoreceptor-like sensors ensured the e-skin would respond differently depending on the pressure's direction or even based on drag deformation, being able to measure and discriminate in real time between normal and shear forces.

Cross-sectional views of a five-by-five capacitors e-skin (centred around one hill) to which various forces are applied. The forces can be characterized by measuring the pressure response map (relative changes in capacitance across the 25 capacitors).

In various experiments including one with robot fingers grabbing a delicate raspberry, the researchers were able to establish a capacitance map around each hill, which allowed them to differentiate several types of applied forces with a response time in the order of the microsecond, adapting its grip force to the detection of shear forces (to adjust against slip).

"It is possible, by looking back at a recorded signal, to evaluate the nature of an unknown stimulus based on the combination of amplitude, shape, and frequency of the signal by referring to a previously known library of stimuli response curves", the authors report.

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