How to bounce back from stretched out stretchable sensors

September 24, 2020

Elastic can stretch too far and that could be problematic in wearable sensors. A team of researchers at Yokohama National University has proposed a fix to prevent too much stretching while improving the sensing ability of electronics. This could lead to advanced prosthetics or disaster recovery robotics.

They published their results on July 29 in the Scientific Reports.

"Stretchable physical sensors are crucial for the development of advanced electrical systems, particularly wearable devices and soft robotics," said Hiroki Ota, paper author and associate professor in the Faculty of Engineering at Yokohama National University. "However, current stretchable pressure sensors composed of elastic materials can be highly deformed during the strain of the devices."

The bend of an elbow or a knee can push the sensor past its structural integrity, producing a large error on the pressure movement measurement. This stops the sensor from being able to measure pressure and strain at the same time, but as independent variables.

To help combat this, the researchers proposed a monolithic array of pressure and strain sensors that can simultaneously and independently detect the force and bend deformation of motion. They used two different materials--one soft and one hard--to protect the sensor's ability to stretch and still accurately measure movement. They placed a hard silicone, called PDMS, along electrodes over the array. At the center of each PDMS placement, they positioned soft porous silicone, which senses pressure.

"The PDMS around the pressure-sensing elements prevents the development of large deformations of the elements during the developed device tension," Ota said.

The soft porous silicone pressure sensor is contained within the hard shell of the PDMS, so it can measure the force of pressure without being overextended past reliable margins of error. The containment also allows the sensors to identify and measure both pressure and strain as independent contributors to movement.

"In addition, resistances of column and row electrodes in the matrix of the mapped array are much lower than the ones of the pressure sensors," Ota said. "This substrate and control of electrode resistances can prevent stretch deformation of the device from affecting the sensing of pressure."

The electrodes in the stretchable array can measure strain at a much lower rate than is required to detect pressure.

"We could recognize pressure and strain sensing of our device independently," Ota said.

Ota said the team plans to apply the new stretchable sensor approach to a physical keyboard that can be mounted on the surface of a body, which could bend with the strain of the body and still detect fingertip pressure, as well as a physical sensor on a soft robot. They also hope to use the sensor to better understand the motion and touch of the human hand.

"In the future, by molding this sensor into a glove shape, it can be applied to the device which electronically analyzes the finger movement and tactile sense of the hand," Ota said.
Other contributors include Ryosuke Matsuda, Satoru Mizuguchi, Fumika Nakamura, Takuma Endo, Yutaka Isoda and Go Inamori, all of whom are affiliated with the Department of Mechanical Engineering at Yokohama National University.

This research was supported by the Japan Science and Technology Agency, PRESTO Grant Number JPMJPR18J2, MIC/SCOPE (Number: 181603007).

Yokohama National University (YNU or Yokokoku) is a Japanese national university founded in 1949. YNU provides students with a practical education utilizing the wide expertise of its faculty and facilitates engagement with the global community. YNU's strength in the academic research of practical application sciences leads to high-impact publications and contributes to international scientific research and the global society. For more information, please see:

Yokohama National University

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