Researchers Bring Sintering Heat to Room Temperature to Print Sensors on Human Skin

Wearable sensors are no longer confined to smartwatches and other relatively bulky electronics. Today, they transcend several applications with new research efforts constantly bringing novel developments to the table. 

Now, a joint team of researchers led by Professor Huanyu “Larry” Cheng from Penn State University has described a new fabrication technique that enables direct printing of on-body sensors. The study is published in the journal ACS Applied Materials & Interfaces.

 

A Simple, Universally Applicable Fabrication Technique

Although a soft body area sensor network would present promising new avenues for wearable devices, such as on-body sensors for advanced physiological and flexible printed circuit boards (FPBCs) for wireless transmission, its success rests on several complicated fabrication methods. 

 

With the help of a novel aid layer to help the metallic components bond, a joint research team has succeeded in printing sensors directly on human skin. Image used Ling Zhang, Penn State University

 

According to the joint team’s research, this could be about to change thanks to what they’re calling a “simple” and “universally applicable” fabrication technique that involves the use of a novel sintering aid layer to achieve direct printing for on-body sensors.

 

Wearable Sensors at Room Temperature

Previous works by the same team have culminated in the development of FPCBs for use in wearable sensors. However, the bonding process for the metallic components in the sensor has prevented them from being printed directly onto the skin. Known as sintering, the process typically requires temperatures of roughly 300 degrees Celsius so that the sensors’ silver nanoparticles can be bonded together.  

 

In previous studies, the same research team explored computational power for smart bandages, artificial organs, and "health tattoos." Image used courtesy of Penn State University

 

“The skin surface cannot withstand such a high temperature, obviously,” says Cheng.

To get around this challenge, the researchers came up with a sintering aid layer that would not hurt the skin but could still help the material sinter together at a lower temperature. By adding a nanoparticle to the mix, the silver particles sinter at around 100 degrees Celsius instead. And although this was great for printing sensors on fabric and paper, it was still too high for the skin. 

But by changing the formula of the aid layer and the printing material, Cheng and his team found that they could sinter at room temperature. The room temperature aid layer consists of a polyvinyl alcohol paste and calcium carbonate. It reduces printing surface roughness and allows for a thin layer of metal that can bend and fold while maintaining electromechanical capabilities.

 

Wireless Transmission Capabilities

According to the research paper, the sensors are capable of ultra-precise and continuous monitoring of temperature, humidity, blood oxygen levels, and heart performance, giving them a plethora of potential applications in on-body health monitoring applications.

The research also points to potential applications in “on-body networks,” which would see several of these sensors with wireless transmission capabilities used to monitor the combination of signals as they progress. 

The researchers plan to refine this technology so that it’s able to target specific applications as needed, such as monitoring COVID-19 symptoms.