Robot with human skin

Researchers create chipless, wireless ‘e-skin’

Image credit: Foto 9019151 © Leifstiller |

MIT scientists have fabricated a chip-free, wireless electronic 'skin' that could transmit health-monitoring information to a smartphone.

The device is able to send signals related to pulse, sweat, and ultraviolet exposure, without using bulky chips or batteries.

The team’s sensor design is a form of electronic skin, or 'e-skin' – a flexible, semiconducting film that conforms to the skin like electronic Scotch tape. The heart of the sensor is an ultrathin film of gallium nitride, a material that can both produce an electrical signal in response to mechanical strain and mechanically vibrate in response to an electrical impulse.

The research team's design, detailed in the journal Science, harnessed gallium nitride’s two-way piezoelectric properties for both sensing health indicators and sending the information to an external device through wireless communication.

Currently, most wireless sensors communicate via embedded Bluetooth chips powered by small batteries. However, as health-tracking wearables increase in popularity, scientists have been searching for a way to design next-generation sensors, which are taking on smaller, thinner, more flexible forms.

The MIT engineers' new design does not require onboard chips or batteries.

“Chips require a lot of power, but our device could make a system very light without having any chips that are power-hungry,” said the study’s author, Jeehwan Kim. “You could put it on your body like a bandage, and paired with a wireless reader on your cellphone, you could wirelessly monitor your pulse, sweat, and other biological signals.”

The researchers hypothesised that a gallium nitride-based sensor, adhered to the skin, would have its own inherent, “resonant” vibration or frequency that the piezoelectric material would convert into an electrical signal. Any change to the skin’s conditions, such as from an accelerated heart rate, would affect the sensor’s mechanical vibrations, and the electrical signal that it automatically transmits. 

“If there is any change in the pulse, or chemicals in sweat, or even ultraviolet exposure to skin, all of this activity can change the pattern of surface acoustic waves on the gallium nitride film,” Kim noted. “And the sensitivity of our film is so high that it can detect these changes.”

To test their idea, the researchers produced a thin film of pure, high-quality gallium nitride and paired it with a layer of gold to boost the electrical signal. The gallium nitride and gold, which they consider to be a sample of electronic skin, measured just 250 nanometers thick – about 100 times thinner than the width of a human hair.

The scientists placed the new e-skin on volunteers’ wrists and necks, and used a simple antenna to wirelessly register the device’s frequency without physically contacting the sensor itself. The team also paired the device with a thin ion-sensing membrane that allowed it to sense variation in sodium levels as a volunteer held onto a heat pad and began to sweat. 

The results of the study showed that the device was sensitive enough to vibrate in response to the volunteers' heartbeat, as well as the salt in their sweat, and that the material’s vibrations generated an electrical signal that could be read by the antenna. 

The findings could be a first step toward chip-free wireless sensors, and the researchers said they envision that the current device could be paired with other selective membranes to monitor other vital biomarkers, such as glucose, or cortisol related to stress levels. 

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