Wearable sensors are ubiquitous thanks to wireless technology that enables a person’s glucose concentrations, blood pressure, heart rate and activity levels to be transmitted seamlessly from sensor to smartphone for further analysis.
Most wireless sensors communicate via embedded Bluetooth chips that are powered by small batteries, but these conventional chips and power sources may be too bulky for smaller, thinner and more flexible next-generation sensors.
The team’s new sensor design, detailed in Science, is a flexible ‘e-skin’ — a semiconducting film that conforms to the skin like electronic Scotch tape.
According to the team, the heart of the sensor is an ultrathin, high-quality film of gallium nitride, a material known for its piezoelectric properties, producing an electrical signal in response to mechanical strain and mechanically vibrating in response to an electrical impulse.
The researchers said they found they could harness gallium nitride’s two-way piezoelectric properties and use the material simultaneously for sensing and wireless communication.
In the study, the team produced pure, single-crystalline samples of gallium nitride which they paired with a conducting layer of gold to boost any incoming or outgoing electrical signal.
They showed that the device was sensitive enough to vibrate in response to a person’s heartbeat, as well as the salt in their sweat, and that the material’s vibrations generated an electrical signal that could be read by a nearby receiver.
“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 co-author, Jeehwan Kim, associate professor of mechanical engineering and of materials science and engineering at MIT.
“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.”
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Kim’s group previously developed a technique called remote epitaxy, which they employed to quickly grow and peel away ultrathin, high-quality semiconductors from wafers coated with graphene. Using the technique, they have fabricated and explored various flexible, multifunctional electronic films.
In their new study, the engineers used the same technique to peel away ultrathin single-crystalline films of gallium nitride. They looked to use a pure film of gallium nitride as both a sensor and a wireless communicator of surface acoustic waves, which are essentially vibrations across the films.
The patterns of these waves can indicate a person’s heart rate or the presence of certain compounds on the skin, such as salt in sweat.
Researchers hypothesised that a gallium nitride-based sensor, adhered to the skin, would have its own ‘resonant’ vibration or frequency that the piezoelectric material would simultaneously convert into an electrical signal, the frequency of which a wireless receiver could register.
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 to the receiver.
When pairing the gallium nitride with gold, the team deposited the gold in the pattern of repeating dumbbells — a lattice-like configuration that imparted some flexibility to the normally rigid metal.
The gallium nitride and gold, which they consider to be a sample of electronic skin, measures just 250nm thick.
Researchers said they placed the e-skin on volunteers’ wrists and necks, and used a simple antenna held nearby to wirelessly register the device’s frequency without physically contacting the sensor itself. The device sensed and wirelessly transmitted changes in the surface acoustic waves of the gallium nitride on volunteers’ skin related to their heart rate.
They also paired the device with a thin ion-sensing membrane, a material that selectively attracts a target ion and in this case, sodium. With this enhancement, the device could sense and wirelessly transmit changing sodium levels as a volunteer held onto a heat pad and began to sweat.
The team sees the results as a first step toward chip-free wireless sensors, and envisions that the device could be paired with other selective membranes to monitor other target biomarkers such as cortisol related to stress levels.
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