Flexible Gel Sensor for Foldable Touch Screen

By Benedette Cuffari, M.Sc.Mar 28 2017

Scientists at the University of British Columbia’s (UBC’s) Department of Electrical and Computer Engineering have recently developed a new flexible sensor which has a potential for the development of foldable tough screens¹.

The flexibility of the sensor is attributed to its design, in which a highly conductive material is sandwiched between two silicon layers¹. The sensor is capable of detecting different types of touch, including swiping and tapping, even when it is stretched, folded or bent.

Advances in electronic devices to become more compact and multifunctional have allowed for their application in various gadgets and wearable devices such as touch screen smartphones, tablets, fitness trackers and smart watches.

While some sensing devices are capable of detecting both touch and stretch, most devices are unable to distinguish between touch, stretch or bend. Transparent tactile sensors based on metal mesh, carbon nanotubes (CNTs) and silver nanowires have demonstrated a successful operation in bent configurations. John David Wyndham Madden’s team at UBC took a step forward, and designed a transparent sensor array utilizing a technology that senses finger proximity, including touch, even in conditions of bending and stretching.

Madden’s team created a cross-grid array of ionically conductive hydrogen electrodes with the electrodes capacitively coupled through a silicon elastomer matrix, which allows for the electric field to be produced above the surface of the sensor². The assembly of the array consists of a disc-shaped electrode and its interconnections separated from a loop electrode by a dielectric layer. This layer allows for a better vertical projection of the field as compared to simple crossing of lines. A simple three step process of mold-bond-polymerize (MBP) was used to produce this flexible sensor².

The MBP process involves the curing of polydimethylsiloxane (PDMS) in a mold to form 700 mm deep groves onto a polyacrylamide gel matrix². The layer is then plasma-bonded to an unpatterned 400 mm thick uniform PDMS dielectric sheet to form an assembly with miniature channels². Perpendicular channels are prepared in a similar manner, forming crossover points (taxels or pixels) 5 mm apart. A monomer mixture with a sodium chloride (NaCl) salt is then injected into the channels to form ionically conductive electrodes².

The stretchable and ionically conductive hydrogel electrodes in the sensor allows the projection of the electric field above the sensor, which allows it to couple and sense a finger touch or movement². As a finger approaches the area of the sensor, the electric field is directed more towards the finger, thereby reducing the charge shared between the two electrodes. This results in a decrease in the capacitance between the electrodes, allowing the sensor to sense the proximity of the finger; a phenomenon known as mutual capacitive sensing. The direction and magnitude of the change make it distinguishable from effects due to changes in the electrode geometry resulting from stretch².

The sensors prepared by the MBP process are found to have a 90 % transmittance, which can be further increased to 94 % by adding antireflective coatings². Based on these capacitance changes, these sensors are capable of detecting single or multi-touch². A thirteen percent decrease in capacitance is observed as the finger reaches the sensor, therefore this sensor can also respond to contact gestures, such as a swipe, without requiring a force to receive the signal². Furthermore, the results from the capacitance change with bending showed that the sensor is able to detect the touch, even when it is deformed or stretched².

The transparency of the materials used in the MBP process allows for their future application in touch screen devices to be a viable option. Similarly, the sensors’ ability to resist deformation to sense the proximity of a finger also opens-up new possibilities for these sensors to be used in making flexible touch screen devices that are resistant to deformation. The readily available, low-cost and transparent nature of polyacrylamide and silicon, combined with the amenability of the simple three step manufacturing technique for large-area fabrication makes the manufacturing and scaling up of these sensors an affordable option.

References:

1. "New Flexible Sensor Holds Potential for Foldable Touch Screens." ScienceDaily. ScienceDaily, 15 Mar. 2017. Web. https://www.sciencedaily.com/releases/2017/03/170315140704.htm.
2. Sarwar, Mirza Saquib, Yuta Dobashi, Claire Preston, Justin K. M. Wyss, Shahriar Mirabbasi, and John David Wyndham Madden. "Bend, Stretch, and Touch: Locating a Finger on an Actively Deformed Transparent Sensor Array." Science Advances 3.3 (2017). Web.
3. Image Credit: Shutterstock.com/Chesky

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