See-through flexible e-skin is powered by underlying photovoltaics

March 24, 2017 // By Julien Happich
Researchers at the University of Glasgow have devised a graphene-based transparent and flexible capacitive touch-sensor that not only delivers touch-detection but also pressure-sensing. Because it is transparent, the tactile sensitive layer can be stacked on top of a rigid or flexible photovoltaic cell so as to be self-powered in operation.

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 flexible and transparent graphene-based
touch sensors, only 125µm-thick.

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: 9.3x10−3 kPa−1 from 0 to 20kPa, then 4.3x10−3 kPa−1 between 20 and 60 kPa and a sensitivity of 7.7x10−3 kPa−1 at pressures over 60kPa.