Stretchable, multifunctional electronic skin for tactile sensing

Photo courtesy of The Johns Hopkins University Applied Physics Laboratory (JHU/APL)

Flexible sensor array mimics some of the tactile properties of human skin

Picking up an object is not a simple process. It relies on a range of data inputs – visual clues help you to gauge the object’s size and shape, allowing you to adjust your grip. Your fingers can determine if an object is hard or soft, and as you begin to lift, you get an indication of its weight from the subtle tug of skin on your fingertips. Most humans can do all of this without thinking, but the same is not true for robots. Even the best industrial robotic systems are limited to picking up objects with well-defined dimensions and weight from a specific location. Going beyond that, into tactile intelligence – where a robot can successfully pick-and-place any object on its first attempt – is a considerable challenge.

But a group of researchers from Huazhong University, China, have developed a sensor array that may help robots mimic the tactile sensing capability of human skin. Their system makes use of the triboelectric effect, and sandwiches a micro-structured dielectric PDMS layer between two liquid metal electrode arrays, producing a stretchy, ~500 µm-thick stack. Writing in Nano Energy [DOI: 10.1016/j.nanoen.2019.05.046], the team say that their multilayer device can simultaneously operate in capacitance mode and as a triboelectric nanogenerator (TENG), allowing it to act as a self-powered sensor.

The design provides other benefits. While both electrode arrays consist of circular pads of liquid metal connected by straight-line contacts, the geometry of the arrays differs slightly – the top electrodes have a diameter of 4 mm, while those on the bottom measure 1.5 mm. This produces an electric field that extends beyond the surface of the device, allowing it to act as a proximity sensor for conductive objects. A number of different objects were tested; a human finger could be detected at a distance of 10 cm. The sensitivity of the device was also found to increase sharply when objects were within 5 mm of the surface. The highly-flexible dielectric layer, comprised of an array of hollow PDMS micro-pillars, allow the device to act additionally as a sensor that can differentiate between applied pressure and tensile strain.

The team integrated their sensor arrays into the palm and fingertips of a bionic hand in order to test their performance ‘in-situ’. The instrumented hand successfully detected the presence of an orange from a distance of 10 cm, and its approximate shape could be deduced from capacitance contour maps produced by the sensor arrays. The voltage output of the sensors operated in TENG mode also provided a measure of grasping forces, and these matched closely with measurements taken by commercial thin-film force sensors in the same location.

The authors say that their electronic-skin sensor array “….is of great promise for applications in dexterous manipulation, prosthetics, rehabilitation and service robots.”

This story originally appeared in Materials Today on 2nd August 2019. You can see it here: https://www.materialstoday.com/energy/news/stretchable-electronic-skin-for-tactile-sensing/

Research paper ($): Cheng Zhang, Shaoyu Liu, Xin Huang, Wei Guo, Yangyang Li, Hao Wu. “A stretchable dual-mode sensor array for multifunctional robotic electronic skin” Nano Energy 62 (2019) 164–170. DOI: 10.1016/j.nanoen.2019.05.046