These Wearable Sensors Can Bond With Human Skin

Wearable sensors

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Wearable sensors

These Wearable Sensors Can Bond With Human Skin

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It might not quite meet the sci-fi definition of a cyborg – science fiction’s term for a cybernetic-organism, or human-technological hybrid – but it’s possible to see where this latest technological leap might find a futuristic application.

Scientists at Pennsylvania State University in the United States have developed wearable sensors that not only bend and flex but also bond to human skin through a new and painless printing process.

The new process allows the substantially metal sensors to bond with human skin without the need for heat, allowing the human body to become a part of a printed circuit for the first time.

Led by Professor Huanyu ‘Larry’ Cheng, from the Department of Engineering Science and Mechanics at Penn State University, the development team responsible for the breakthrough technology has released their findings in the latest issue of the peer-reviewed science journal ACS Applied Materials and Interfaces.

While wearable sensors have been around for a while in the form of watches and electrodes that provide reasonably reliable information to a processor, used for example in medical monitoring of heart-rate, blood pressure or other vital signs, this new process provides much more precise biometric measurements as well as being comfortable for users.

Apple Watch, wearables
The Apple Watch

The development, says Professor Cheng, is an extension of his team’s previous development of flexible printed circuit boards for use in wearable sensors. The result is a simple yet universally applicable fabrication technique with the use of a novel sintering aid layer to enable direct printing for on-body sensors,” says Ling Zhang, who is both a colleague of Professor Cheng at Penn State and a researcher at the Harbin Institute of Technology in China.

Previously, printing directly on human skin was hindered by the bonding process for the metallic components in the sensor. Called sintering, this process typically requires temperatures of around 572 degrees Fahrenheit, or about 300 degrees Celsius to bond the sensor’s silver nanoparticles together.

“The skin surface cannot withstand such a high temperature, obviously,” Cheng says. “To get around this limitation, we proposed a sintering aid layer — something that would not hurt the skin and could help the material sinter together at a lower temperature.”

By adding a nanoparticle to the mix, the silver particles sinter at a lower temperature of about 212°F (100°C).

“That can be used to print sensors on clothing and paper, which is useful, but it’s still higher than human beings can stand at skin temperature,” Cheng said, noting that temperatures around 104°F or 40°C could still burn skin tissue. 

“We changed the formula of the aid layer, changed the printing material, and found that we could sinter at room temperature.”

The room temperature sintering aid layer consists of polyvinyl alcohol paste — the main ingredient in peelable face masks — and calcium carbonate, which is made from eggshells. 

The layer reduces printing surface roughness and allows for an ultrathin layer of metal patterns that can bend and fold while maintaining electromechanical capabilities. When the sensor is printed, the researchers use an air blower, such as a hairdryer set on cool, to remove the water that is used as a solvent in the ink.

“The outcome is profound,” Cheng said. “We don’t need to rely on heat to sinter.”

The sensors are incredibly sensitive and capable of precisely and continuously capturing temperature, humidity, blood oxygen levels and heart performance signals, says Cheng. 

The researchers also linked the on-body sensors into a network with wireless transmission capabilities to monitor the combination of signals as they progress.

The process is also environmentally friendly, Professor Cheng says. The sensor remains robust in tepid water for a few days, but a hot shower will easily remove it.

“It could be recycled since removal doesn’t damage the device,” Cheng said. “And, importantly, removal doesn’t damage the skin, either. That’s especially important for people with sensitive skin, like the elderly and babies. The device can be useful without being an extra burden on the person using it or to the environment.”

Next, the researchers plan to alter the technology to target specific applications as needed, such as a precise on-body sensor network placed to monitor the symptoms associated with COVID-19.

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