Transparent capacitive touch sensor flexes, stretches and detects 3D shapes

July 27, 2017 // By Julien Happich
Leveraging the versatile properties of graphene, a team of Korean researchers has designed a large transparent, flexible and stretchable capacitive touch sensor that will not only operate in multi-touch, spread, and scroll modes as commonly used on smartphones, but can also detect 3D shapes as they approach its surface.

Only a few tens of micrometres thick and easily scalable, the polymer-based film sensor can operate equally on a flat surface or wrapped around someone's forearm or on the palm of a hand, making it suitable for numerous wearable applications, as well as for robotics and 3D gesture sensing systems.


Caption

The multi-layered sensor described in their paper "Graphene-Based Three-Dimensional Capacitive Touch Sensor for Wearable Electronics" published in the ACS Nano journal consists of an array of top and bottom transparent graphene electrodes (chemically doped) separated by a dielectric polymer sheet layer (25μm thin) and protected on the top and the bottom by ultrathin sheets of PET only 5μm thick. A graphene monolayer at the bottom of the stack eliminates electrical noise caused by the surface charge on human skin and prevents sweat from penetrating into the active device layers.

As a proof of concept, the researchers fabricated a 40×60mm2 sensor embedding and 8×8 array of graphene electrodes 3mm wide and 4mm apart, providing 64 channels. To compensate for parasitic capacitance from the lead wires and the surrounding environment, they connected it to an interface board with an offset-calibration, touch-data was then transferred to a display. Their device was operated using conventional driver circuits as used for ITO-based capacitive touch screens.


The 3D sensor on the palm with a finger near (left).
On the right, the resulting capacitance changes 
15 and 5mm away.

What they found under a rigorous battery of tests, in accordance with solid mechanical modelling, is that the resistance of the graphene electrodes and the capacitance of the dielectric layer remained virtually unperturbed even at a bending radius of 1.5mm (only a few percent changes). Then the sensors recovered their original electrical properties after returning to an unbent state.

Measuring changes in mutual capacitance between the top and bottom electrodes, the sensor exhibited a good sensing capability in contact as well as in noncontact modes (22dB SNR at 7cm), enabling accurate finger proximity measurements and even the 3D mapping of conductive objects.