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).