Biomedical nanotechnology Physics World  March 2021

Nanomesh pressure sensor preserves skin’s sense of touch

Light touch This polyurethane and gold sensor can resist shear forces and rubbing. (Courtesy: Someya et al.)

Takao Someya and colleagues at the University of Tokyo have developed the first artificial-skin patch that does not affect the touch sensitivity of the real skin beneath it. The new ultrathin sensor could be used in applications as diverse as prosthetics and human-machine interfaces.

“A wearable sensor for your fingers has to be extremely thin,” explains Tokyo’s Sunghoon Lee. “But this obviously makes it very fragile and susceptible to damage from rubbing or repeated physical actions.” For this reason most e-skins developed to date been relatively thick and bulky.

In contrast, the sensor developed by the Tokyo team is thin and porous and consists of two layers (Science 370 966). The first layer is an insulating mesh-like network comprising polyurethane fibres around 200–400 nm thick. The second layer is a network of lines that makes up the functional electronic part of the device – a parallel-plate capacitor. This is made of gold on a supporting scaffold of polyvinyl alcohol (PVA), a water-soluble polymer often found in contact lenses. Once this layer has been fabricated, the PVA is washed away to leave only the gold support. The finished pressure sensor is around 13 μm thick.

When a finger covered with this sensor grasps an object, the dielectric nanomesh layer deforms, producing a change in the capacitance. The device’s sensitivity as determined by the slope of its capacitance change-pressure curve, and the team found values that were comparable to the grip forces measured for a bare finger.

“We performed a rigorous set of tests on our sensors with the help of 18 test subjects,” Lee says. “They confirmed that the sensors were imperceptible and affected neither the ability to grip objects through friction, nor the perceived sensitivity compared to performing the same task without a sensor attached.”

The nanomesh sensors continued to work even after being compressed repeatedly and the capacitance changed by just 0.15% after they had been squeezed 1000 times. The conductivity of the gold electrode also remained relatively stable.

The sensor is resistant to friction damage and could be rubbed 300 times with a 50 g object without breaking. The sensor’s electrical characteristics changed only slightly during these tests, and the device remained sensitive to applied pressures.

The researchers now plan to increase the number of sensing points in their device and determine how pressure is spatially distributed across it.

Isabelle Dumé