A key challenge in the world of wearables is comfort. If your new smartwatch is too heavy or bulky, you won’t wear it. If those $300 earbuds just won’t stay in or give you an earache after only an hour of use, who would use them? If your armbands or wristband is so sweaty halfway through a workout that it won’t stay put, what’s the point? Printable wearable devices are looking to close the gap traditionally found between comfort and functionality in the medical technology space.
On the spectrum of personal health technology, ranging from wearables to implants, a new entrant will revolutionize how we monitor our health: biosensors printed directly onto the skin. But what are biosensors, and what are they used for today?
Biosensors are a classification of wearable technology developed in the 1960s as a simple, real-time, selective, and inexpensive food quality and safety monitoring device. The field has since expanded to include medical devices, enabling the monitoring of indicators including temperature, pulse, breathing rate, electrical heart signals, blood glucose, and blood oxygen saturation.
Today, wearable biosensors, referred to as “wearables,” have become ubiquitous with modern healthy living through branded products like Apple Watch, Masimo MightySat, and FreeStyle Libre. A fundamental limitation of these products is that they are not designed to allow for continuous, 24-hour wearability, thus limiting their ability to continuously monitor vital signs. Printable biosensors solve the 24-hour wearability problem.
This breakthrough has been developed by research teams led by Ling Zhang of the Harbin Institute of Technology in China and Penn State University’s Larry (Huanyu) Cheng. Zhang is the first author in the study published in the ACS Applied Materials & Interfaces journal.
Those familiar with biosensors will know Cheng’s group as providing leading research in stretchable and flexible printed circuit boards for wearables. However, a key barrier to broadening use cases for this technology has been its application to the skin.
The process to bond the metallic components of the flexible biosensor, called sintering, typically requires extremely high temperatures of well over 500 degrees Fahrenheit, too high for contact with human skin. This challenge led Zhang and Cheng to seek out a new approach.
The researchers introduced a “sintering aid layer” into the process, a paste containing polyvinyl alcohol, the same ingredients used in over-the-counter peelable facemasks. When applied directly to the skin before the sensor is printed, the sintering aid layer acts as a shield between the biosensors and the skin’s circuitry.
An added benefit of the sintering aid layer is that it provides a smooth surface upon which the thin layer of metal patterns can be printed. It’s this thin, flexible layer that maintains the electrochemical capabilities of the sensor.
The researchers also needed to change the chemical bonding process for the metallic components to enable printing at lower temperatures. This included injecting other nanoparticles into the paste, chiefly aluminum and magnesium oxides, metallic salts, and copper. The metallic components containing silver nanoparticles were then printed onto the sintering layer and bonded with the use of a cool hair dryer to remove excess moisture from the ink.
In addition to their ability to continuously measure temperature, humidity, electrical heart signals, and blood oxygen saturation, printable biosensors can do so with a much higher degree of precision and accuracy over traditional wearables. Epidermal measurements are heavily dependent on good, “sticky” skin contact, which is achieved by printing the biosensors directly onto the skin.
However, the “stickiness” of printable biosensor differs from traditional medical adhesives, which can irritate or damage the skin surface upon removal, a particular issue for people with sensitive skin, like babies or the elderly. Printable biosensors can be peeled away from the skin with hot, shower-temperature water but will remain attached and function in lukewarm water.
A further benefit is that the process of peeling away printable biosensors using hot water does not damage the sensor circuitry, making it possible for future reuse.
“We are interested in applying this multifunctional, wearable sensing technology for diagnostic confirmation and timely treatments for cardiopulmonary diseases, including Covid-19, pneumonia, and fibrotic lung diseases,” says Cheng. Additional uses beyond the medical industry may include monitoring within professional athletics and industries where people work in dangerous or remote locations.
According to Cheng, trials of longer-term use, where the biosensors are worn over weeks or months, have not yet been carried out but, the researchers are hopeful that they will continue to prove safe and effective. Indications on when this technology will come to market and the anticipated manufacturing costs are unknown, but look for this game-changing technology on the horizon.
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