Enabling touch sensing in skin-conformable wearable electronics

by | Jun 2, 2020

A transparent and stretchable touch sensor could enable robust touch input mapping under either static or dynamic deformation.

Image credit: D. Gao

We are in an era of evolving and humanizing electronics, from monolithic computing units to intelligent systems which can perceive, think, and interact naturally with human beings. For example, the maturity of capacitive touch sensing technology over the past decade has reshaped how we operate consumer electronics — by simply touching, sliding, and zooming in and out, tasks could be done in an intuitive and efficient manner. As the technology continuously iterates, touch screens nowadays have obtained satisfactory refresh rate, taxel resolution, and touch sensing precision.  

Beyond all these advancements, the challenge remains in realizing compliant and soft touch sensors to incorporate interactive features in skin-conformable wearable devices. Transforming a rigid touch panel into a deformable counterpart requires convoluted materials development and fabrication processes.

To address these challenges, a research team led by Professor Pooi See Lee at Nanyang Technological University has demonstrated a highly transparent and stretchable touch sensing matrix that can perceive touch input even under large deformations. Central to the unique properties of this device is the development of a gel-elastomer complex, wherein an ionically conductive gel is directly patterned onto an elastomeric substrate to form an array of capacitive sensing taxels. Compared to electronic conductors, ionic gels are intrinsically transparent, soft, and able to withstand significant deformation without losing ionic conductivity. Therefore, the stretchable matrix exhibits both skin conformability and over 94% transmittance to visible light.

In order to achieve high-throughput and efficient production of the devices, an inkjet printing protocol was developed to deposit the gel at a feature resolution of 40 µm. The direct ink writing process gives the researchers full freedom to design printing patterns and study how the electrode geometry could influence the sensors’ performance.

In wearable devices, softness is often improved at a cost of signal quality. Irregular deformations, including stretching, bending, or twisting, will always introduce strain artifacts to capacitive signals. The research team prepared devices with variant electrode configuration, and successfully integrated a coplanar “interlocking-diamond” taxel that led to 4 times reduction in noise level, compared to electrodes with regular rectangular shape. Consequently, the sensor delivers a record touch sensitivity of 47% under 40% uniaxial tensile, and a high signal-to-noise ratio against dynamic stretch.  

This soft touch sensor is expected to be a solid basis for fully stretchable electronics with touch sensing capability.

Reference: D. Gao, et al. ‘Inkjet‐Printed Iontronics for Transparent, Elastic, and Strain‐Insensitive Touch Sensing Matrix.’ Advanced Intelligent Systems (2020). DOI: 10.1002/aisy.202000088

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