Presenting their novel approach in the Advanced Functional Materials Journal under the title “Energy-Autonomous, Flexible, and Transparent Tactile Skin”, the researchers disclose a seemingly simple and highly scalable manufacturing process for their novel capacitive pressure sensor, that they are confident could be scaled up to provide comprehensive haptic feedback to robotics and prosthetics.
The actual capacitive touch sensor consists of single-layer graphene coplanar interdigitated capacitive (IDC) electrodes on a 125µm-thick PVC substrate, connected to Ti/Au (10nm/100nm) contacts deposited on the edges of the electrodes. The single layer graphene is first transferred to the PVC substrate through a hot lamination process (with a copper foil on which graphene is originally grown), before etching out the copper foil. Then metal contacts are deposited on the edges of the graphene layer (using electron beam evaporation and a shadow mask) and finally the graphene is patterned into interdigitated electrodes using a computer-controlled plotter blade.
The sensor is completed by a 25μm-thick layer of polymer (PDMS) spin-coated on top of the graphene channel. This ultimate layer not only serves as a deformable dielectric layer, it also encapsulates the device. The researchers tried various electrode patterns before settling for squared shape meanders which they report exhibited the maximum capacitance response, along with a wide range of pressures.
Characterizing the novel capacitive sensor, they found that it had a stable response for a wide range of pressures (contrarily to conventional coplanar or layered structures which can only sense the presence or absence of touch but not the pressure). They also found that the pressure sensitivity could be mainly attributed to the change of the PDMS dielectric constant under compression (due to the polymer’s porous structure).
Interestingly, over the 0 to 60kPa pressure range, the tested sensors presented a slightly varying sensitivity which remained in the same order of magnitude: 9.3×10−3 kPa−1 from 0 to 20kPa, then 4.3×10−3 kPa−1 between 20 and 60 kPa and a sensitivity of 7.7×10−3 kPa−1 at pressures over 60kPa.
To prove the usability of their sensors in real world e-skin applications, the team integrated them at the intermediate and proximal phalanges of a state-of-the-art bionic hand and converted the capacitive variation of the graphene sensors to a voltage through a simple readout interface circuitry designed and implemented in a flexible polyimide PCB. They could demonstrate real-time pressure mapping as the bionic hand grabbed a soft ball and the read-outs were even used to enable a real-time haptic feedback loop, controlling the hand’s grasp.
For a self-powered solution, the researchers then stacked the transparent sensor on top of a commercially available amorphous silicon solar cell. They demonstrated that while the 39.6×22.9mm2 cell they used in this work could produce 160μW cm−2 of power, the sensor consumed only 31 and 55nW before and during touch, respectively. The ultralow power consumed by the sensing layer, 20nW cm−2, is orders of magnitude less than what the photovoltaic energy harvesting provided.
This leads the researchers to conclude that large transparent touch sensors stacked with flexible and stretchable photovoltaic cells could not only provide a self-powered prosthetics skin but could also contribute to improved system efficiency, by storing the excess power or using it to drive the actuators in a robotic hand. Such sensors may also find their way into functional clothing (including helmets, gloves where gathering pressure data might be useful).