Spray coated tactile sensor holds promise for electronic skin

Technology News | September 26, 2018

By Rich Pell

Because the sensor’s electrical response under strain is decoupled from any pressure input, a robotic hand equipped with the new sensor and separate pressure sensors could differentiate these two mechanical inputs. Such a robotic hand could manipulate delicate objects with precise grasp control, adapting its grip based on shear detection, while applying the lowest possible pressure.

The stretchable pressure insensitive strain (SPIS) sensor described in the ACS Nano journal paper titled “Pressure Insensitive Strain Sensor with Facile Solution-Based Process for Tactile Sensing Applications” is in fact a porous multiwalled carbon nanotube (MWCNT)-polydimethylsiloxane (PDMS) composite. The flexible compound is made by mixing a water-based MWCNT solution into an oil-phase PDMS solution (which creates micrometre-sized droplets of the water-based MWCNT solution).

After two sequential heat treatments, one at 70°C to evaporate the PDMS solvent and cure the polymer, the other at 120°C to evaporate away the water from the MWCNT-laden droplets, remains a porous and conductive elastomeric structure with pores about 23μm in diameter.

A close examination of the structure under strain and under compression shows that while compression merely closes the empty pores and do not significantly alter the percolation network of MWCNTs (and afferent conductivity), tensile strain does create microcracks on the wall of the pores, which alters the compound’s conductivity, yielding a relatively large change in the resistance. Conductivity returns to its initial state when removing the tensile strain, as the cracks close again.

Further measurements confirmed the SPIS sensor shows nearly no response to pressure up to 140kPa (at a 70% compressive strain), but high sensitivity to tensile strain (gauge factor of 55.8 at a 70% tensile strain).

Because the compound is obtained through an easily scalable solution-based process, it could be produced in volume to dip-coat complex three-dimensional objects, or applied via spray coating. Another interesting result reported in the paper is that by connecting multiple electrodes at the periphery of the sensor, it becomes possible to spatially map the local strain through electrical impedance tomography (EIT), without having to rely on an array of pre-patterned electrodes.

That makes the sensor very easy to implement on complex shapes, as an electronic skin for robotics or in wearable electronic applications to detect motion. To prove the idea, the authors measured the conductivity distribution within a 11x11cm² SPIS sensor patch equipped with 24 electrodes at its periphery, while pressing or stretching the material. They were able to process the data and generate the relevant colour maps at 30Hz, above video rates, making this new sensor suitable for highly-reactive real-time robotics.

Connecting 24 electrodes at the periphery of a 11x11cm² patch of SPIS sensor enables the spatial mapping of strain through electrical impedance tomography. The sensor is shown under pressure (d) and strain (e). Conductivity maps below the photos show no change under pressure (g), while the conductivity map turns blue locally (h) under strain (decreasing).

Other experiments involved spray-coating a hand figurine to accurately measure strain across the finger joints and nozzle-printing the SPIS sensor on a cotton bandage connected serially to a 9V battery and a LED. The light would only dim when stretching the bandage but not when pressing it with a weight.