MIT develops a wearable sensor able to target any biomarker

Using a gold-gallium "band-aid" could prove the next generation of biological monitoring as researchers from the Massachusetts Institute of Technology have devised a new kind of wearable sensor capable of communicating wirelessly without the need for microchips or even batteries.

Wearable sensors are ubiquitous due to wireless technology, which enables the monitoring of glucose concentrations, blood pressure, heart rate, etc., to be transmitted seamlessly and near-instantly from the sensor to computers or even smartphones.

Most wireless sensors today communicate via an embedded Bluetooth microchip powered by small batteries. However, these conventional chips, albeit incredibly small, are considered too bulky for the envisioned next-generation of sensors, which are taking on smaller, thinner, and more flexible forms.

Now, MIT engineers have developed a new kind of wearable sensor by harnessing the near-liquid state of gallium and the conductivity of gold to use the natural vibrations of the body to generate the energy and information for it to function and transmit.

The team's sensor design is a form of electronic skin, or "e-skin"-a flexible, semiconducting film that adheres to the skin like electronic masking tape. The heart of the sensor is an ultrathin, high-quality film of gallium nitride, a material that is known for its piezoelectric properties, meaning it can generate an electric charge in response to applied mechanical stress. This offers the ability to both produce an electrical signal in response to mechanical force and mechanically vibrate in response to an electrical impulse.

"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 corresponding author, Jeehwan Kim, an associate professor of mechanical engineering and of materials science and engineering, as well as principal investigator at MIT's Research Laboratory of Electronics.

In the study, the team produced pure, single-crystalline samples of gallium nitride, which was then paired with a conducting layer of gold – both of which are non-toxic and biocompatible – to boost any incoming or outgoing electrical signal.

Using their prototype, the engineers found that the device was sensitive enough to vibrate in response to a person's heartbeat and that the material's vibrations generated an electrical signal that could be read by a nearby receiver.

In this way, the device was able to wirelessly transmit sensing information without the need for a chip or battery.

"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," Kim added.

Electrical resistance

Kim's group previously developed a technique called remote epitaxy – a cost-effective method to produce semiconducting films from materials that outperform silicon. Using this technique, they were able to quickly grow and peel away ultrathin, high-quality semiconductors from wafers coated with graphene.

Through this technique, they have fabricated and explored various flexible, multifunctional electronic films.

The team looked to use a pure film of gallium nitride as both a sensor and wireless communicator of surface acoustic waves, which are essentially vibrations across the film – like how a spider senses prey on its web.

The patterns of these waves, however, vary and can indicate a person's heart rate or, even more subtly, the presence of certain compounds on the skin, such as salt in sweat.

The researchers hypothesized that a gallium nitride-based sensor, adhered to the skin, would have its own inherent "resonant" vibration or frequency that the resistance would then be able to convert into electrical signals, which any kind of wireless receiver could register.

Any change to the skin's conditions, such as accelerated heart rate, would affect the sensor's mechanical vibrations, and the preceding electrical signal would then automatically transmit to the receiver.

"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," said Yeongin Kim, first author and former MIT postdoctoral, now an assistant professor at the University of Cincinnati. "And the sensitivity of our film is so high that it can detect these changes."

To test their idea – although the gallium nitride has intrinsic properties that make it ideal as a biocompatible device – it needed something that would enable a stronger signal.

A layer of gold was used to boost the electrical signal. Depositing the gold in a repeating dumbbell-like pattern – a lattice configuration that imparted some flexibility to the ordinarily rigid metal – the gallium nitride and gold patch ultimately ended up at around 250 nanometers thick, roughly 100 times thinner than the width of a human hair.

From a test group, the team placed the e-skin on the volunteers' wrists and necks and used a simple antenna held nearby to wirelessly register the device's frequency without physically contacting the sensor itself.

Through this, the device was shown to sense and wirelessly transmit changes in the surface vibrations of the skin that related to their heart rate.

In addition, the researchers paired the device with a thin ion-sensing membrane – a material that selectively attracts a target ion, and in this case, sodium. With this alteration, the device could then determine and transmit changing sodium levels as a volunteer held onto a heating pad and began to sweat.

"We showed sodium sensing, but if you change the sensing membrane, you could detect any target biomarker, such as glucose, or cortisol related to stress levels," said co-author and MIT postdoctoral Jun Min Suh. "It's quite a versatile platform."

The MIT engineers see these results as the first step toward chip-free wireless sensors, and they envision that the current device could be paired with other selective membranes to monitor other vital biomarkers.