There is an increasing need to establish skin-like ultra-thin sensors which can directly be deposited (e.g. sprayed) on a soft active structure representing the limbs (e.g. fingers) of prosthetic devices such as a prosthetic hand. Such sensors are essential to control the actuation behaviour of the soft structure, and to obtain information about kinematics of the structure as much as temperature of an object which the finger is touching. This study employed an inexpensive and scalable technique to place elastomeric sensors directly onto a monolithic structure representing the finger of a soft robotic prosthetic hand. The chemical affinity between the solvent used, and the soft elastomers (thermoplastic urethane, TPU, and styrene-butadiene-styrene, SBS) allowed the blending of the TPU and the SBS at the finger interface, creating a highly compliant piezoresistive sensor, that was directly deposited in the desired place.
The bending movement of the three interphalangeal joints of the finger were recorded simultaneously. The sensing elements (i.e. soft strain sensors) showed an outstanding linearity between the change of resistance and the strain during the finger bending, with a gauge factor of ~1, repeatability and stability (>10,000 cycles), fast response and low creep, with negligible mechanical and electrical hysteresis. These sensors have a significant potential to establish prosthetic devices with built-in sensing elements, paving the way towards prosthetic limbs compatible with natural control.