Wearable Sensors: Engineers Develop New, Heat-Free Sintering Process

Printing sensors on skin used to be a difficult endeavor, considering the temperature development during the process. Now, an international team of engineers has come up with a new way to print wearable sensors directly on skin without heat.

Wearable sensors are not a new thing, a team of researchers developed wearable UV sensors with active ink back in 2018. By now, the technology is evolving from watches and electrodes to bendable devices that provide far more precise biometric measurements and users can wear them more comfortably. In the middle of 2020, we reported on a research team printing tattoo electrodes on human skin, an innovation which could revolutionize brain signal measurement. Now, an international team of researchers combined this technology with heat-free technology for flexible sensors, creating a novel technique to produce precise, high-performing biometric sensors and printing them directly on human skin.

Sintering Process Optimization

The team, led by Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Professor at the Department of Engineering Science and Mechanics at Penn State, previously developed flexible printed circuit boards for wearable sensors. Printing their technology directly on skin has been hindered by the bonding process for the metallic components in the sensor though. The solid state sintering process – a technique of forming metal parts from powders by pressing and sintering them – typically requires temperatures of around 300 degrees Celsius, or 572 degrees Fahrenheit, to bond the sensor’s silver nanoparticles together. Obviously, the human skin surface cannot withstand such a high temperature. To work around this issue, the team worked with a sintering aid layer that wouldn’t hurt the skin and could be able to help the material sinter together at a much lower temperature. With this nanoparticle added to the process, the required silver particles can now sinter at a lower temperature of about 100 C, or 212 F.

The Temperature Issue

With the improvement, the technology can be used to print sensors on clothing and paper, but of course, it’s still higher than our human skin can endure. Even a temperature as low as 40 C, or about 104 F, could still burn skin tissue. Therefore, the team needed to find another access point to change the parameters: they changed the formula of the aid layer, tried a different printing material and even discovered they were able to sinter at room temperature.

“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,”

says Cheng. With this layer, printing surface roughness can be reduced. It also allows for an ultrathin layer of metal patterns which can bend and fold while maintaining electromechanical capabilities. To remove the water that is used as a solvent in the ink when the sensor is printed, the researchers work with an air blower, like a common hair dryer on a cool setting. Cheng is very satisfied with the results, calling the outcome “profound”. With their new technique, there is no longer a need to rely on heat for the sintering process.

A Novel Technique

Cheng and his colleagues published their results in ACS Applied Materials & Interfaces. First author Ling Zhang, a researcher in the Harbin Institute of Technology in China and in Cheng’s laboratory, said:

“In this article, we report a simple yet universally applicable fabrication technique with the use of a novel sintering aid layer to enable direct printing for on-body sensors.” 

Not unlike the printed tattoo electrodes, Francesco Greco, head of the Laboratory of Applied Materials for Printed and Soft electronics (LAMPSe) at the Institute of Solid State Physics at Graz University of Technology, developed for EMG, ECG and EEG, the sensors are capable of monitoring human body functions. According to Cheng, they can precisely and continuously capture temperature, humidity, blood oxygen levels and heart performance signals. The researchers also linked the on-body sensors into a network with wireless transmission capabilities to monitor the combination of signals as they progress, opening them up to even more areas of application in the future.

Easy To (Re)Use

Like many other new technologies these days, the developers of this innovative process also had sustainability in mind: the process is also environmentally friendly. It could be recycled, since the removal of the sensor doesn’t damage the device. The sensors are designed to remain robust in tepid water for a few days, but will easily come off with a hot shower. This way, removal doesn’t only keep the device intact but also doesn’t damage the skin, which is especially important for people with sensitive skin, like the elderly and babies. Cheng explained:

“The device can be useful without being an extra burden to the person using it or to the environment.”

The researchers plan to alter the technology to target specific applications as needed in the future, such as a precise on-body sensor network placed to monitor the particular symptoms associated with COVID-19. Yet another way a print technology is working on aiding in the fight against the global pandemic.

Could you imagine wearing a sensor, printed on your skin, to collect data on your health?
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