The Technion-Israel Institute of Technology, Haifa, Israel

Flexible sensors have been developed for use in consumer electronics, robotics, health care, and spaceflight. One problem with these current flexible sensors is that they can be easily scratched and damaged, potentially destroying their functionality.

Fig.1 - The self-healing device in the form of a bendable and stretchable chemiresistor, where every part is self-healing and has tailored sensitivity toward one or a combination of pressure, strain, gas analytes, and temperature. (Credit: Hossam Haick)

Researchers in the Department of Chemical Engineering at the Technion–Israel Institute of Technology, Haifa, Israel, who were inspired by the healing properties in human skin, have developed materials that, they say, can be integrated into flexible devices to “heal” incidental scratches or damaging cuts that might compromise device functionality. The advancement, using a new kind of synthetic polymer, has self-healing properties that mimic human skin, which means that e-skin “wounds” can quickly heal themselves in less than a day.

“The vulnerability of flexible sensors used in real-world applications calls for the development of self-healing properties similar to how human skins heals,” said co-developer of the sensor, Chemical Engineering Professor Hossam Haick. “Accordingly, we have developed a complete, self-healing device in the form of a bendable and stretchable chemiresistor where every part, no matter where the device is cut or scratched, is self-healing.” (See Figure 1)

How It Works

The new sensor is comprised of a self-healing substrate, high-conductivity electrodes, and molecularly modified gold nanoparticles. “The gold particles on top of the substrate and between the self-healing electrodes are able to heal cracks that could completely disconnect electrical connectivity,” said Prof. Haick.

Once healed, the polymer substrate of the self-healing sensor demonstrates sensitivity to volatile organic compounds (VOCs), with detection capability down to tens of parts per billion. It also demonstrates superior healability at the extreme temperatures of -20°C to 40°C. This property, said the researchers, can extend applications of the self-healing sensor to areas of the world with extreme climates. From sub-freezing cold to equatorial heat, the self-healing sensor is environment-stable.

The healing polymer works quickest, said the researchers, when the temperature is between 0°C and 10°C, when moisture condenses and is then absorbed by the substrate. Condensation makes the substrate swell, allowing the polymer chains to begin to flow freely and, in effect, begin healing. Once healed, the nonbiological chemiresistor still has high sensitivity to touch, pressure, and strain, which the researchers tested in demanding stretching and bending tests.

Another unique feature is that the electrode resistance increases after healing and can survive 20 times or more cutting/healing cycles than prior to healing. Essentially, healing makes the self-healing sensor even stronger, they explained.

The researchers are currently experimenting with carbon-based self-healing composites and self-healing transistors. They believe that one day, the self-healing sensor could serve as a platform for biosensors that monitor human health using electronic skin. Future possible applications could include the creation of electronic skin and prosthetic limbs that would allow wearers to “feel” changes in their environments.