Foldable touch sensor

(Top) UBC team leader John Madden (left) and first author Mirza Saquib Sarwar examine a sample of the group’s stretchable, bendable touch sensor. (Bottom) A closeup of the flexible sensor, showing its array of cross-hatched electrodes with “loop-disk” junctions. [Images: University of British Columbia]

A surface that remains sensitive to touch and gesture, even as it’s actively being bent and deformed into arbitrary shapes, would open up a range of application possibilities—from flexible, foldable interactive tablet displays to highly sensitive “electronic skin” for health applications and robotics. Researchers in the electrical and computer engineering department of the University of British Columbia (UBC), Canada, now report a step toward such bendable, stretchable touch sensors: a transparent material, based on ion-containing hydrogel electrodes, that can distinguish the touch of a finger and respond to gestures even as the surface is actively being stretched and deformed (Sci. Advances, doi: 10.1126/sciadv.1602200).

Distinguishing touch from stretch

Many touch sensors work by detecting changes in capacitance or resistance as a user’s finger moves across the surface. Recent years have also seen considerable progress in sensor surfaces that detect bending and strain. But putting the two together—that is, developing a touch sensor that can distinguish the touch and swipe of a finger from mechanical bending or stretching of the sensor itself—has posed a bigger challenge. That’s because the capacitance and resistance used to identify the finger’s touch are also affected by the stretching and bending of the material, and the mechanical deformation gums up the signal from the finger’s pressure.

The UBC team attempted to solve the problem by borrowing from a concept well established in the world of rigid, indium-tin-oxide–based sensors: so-called mutual capacitive sensing. In this scheme, rather than responding to touch at the actual surface of the device, the sensor reads changes in the “projected” electric field several millimeters above that surface.

The approaching finger, acting as an electrode in its own right, leads to a drop in capacitance across the device electrodes. And, because the interaction happens not at the device surface but above it, the resulting capacitance changes have a specific directionality and magnitude—and can thus, in principle, be distinguished from capacitance changes related to mechanical bending or stretching of the sensor surface itself.

Hydrogel electrodes

To put these principles into action in a transparent, foldable surface, the UBC researchers began with a three-layer silicone polymer matrix, with the 700-µm-thick top and bottom layers containing, respectively, horizontal and vertical channels, and the middle layer forming a 400-µm-thick dielectric separator. They then injected an inexpensive ion-containing hydrogel, polyacrylamide—widely used in the lab in DNA gel electrophoresis—into the channels, to turn them into ionically conducting electrodes. The junctions of the cross-hatched electrodes featured a “loop-disk” pattern that allowed the creation of a local, projected electric field stretching several millimeters above each junction.

The team then carefully measured and mapped the response of the surface to the approach and touch of a finger, based on capacitance changes attributable to disturbances in the projected field. They found that these changes could easily be distinguished from the very different pattern of capacitance changes attributable to bending or stretching of the device surface. They were even able to demonstrate smartphone-style “multitouch” gesture sensing in the surface while actively bending and stretching it.

Low-cost process

Beyond the system’s capabilities, another potential advantage, the UBC engineers point out, is its cost—on the order of US$1/m2 for materials, with a scalable low-cost manufacturing method. That could give it applications not only in small devices, but in much more expansive visions.

“It's entirely possible to make a room-sized version of this sensor for just dollars per square meter,” noted the study’s first author, UBC Ph.D. student Mirza Saquib Sarwar, in a press release, “and then put sensors on the wall, on the floor, or over the surface of the body—almost anything that requires a transparent, stretchable touch screen.” Sarwar adds that the low cost suggests that the technology could even find a place in “disposable wearables like health monitors.”