An example of the new wearable sensor developed at Penn State University

Flexible electronics have opened up some interesting possibilities when it comes to wearable sensors that can be applied to the skin, taking the form of tattoo-like films and sleeves that monitor various aspects of human health. Scientists at Penn State University have now developed one they say can be safely printed directly onto the skin, where it can track things like body temperature and blood oxygen levels, before being washed off once the job is done.

The new printable sensors build on earlier work by the same researchers, in which they developed flexible circuit boards for use in wearable sensors. But a key part of this process involved bonding some of the metallic components together at the kinds of temperatures not well tolerated by the human body, at around 572 °F (300 °C).

This scorching hot sintering process is what had prevented the team from printing their flexible circuit boards directly onto human skin, but it may have now found a way around this problem. The key is what the scientists call a sintering aid layer, which acts as a kind of buffer and enables the materials to bond together at far safer temperatures.

The winning formula for this layer consists of a polyvinyl alcohol paste combined with calcium carbonate, materials found in peelable face masks and egg shells, respectively. This layer serves to smooth out the surface of the skin and allow a very thin layer of metal patterns to be printed directly on top at room temperature, which is then set with an air-blowing device.

This flexible circuit maintains its electrochemical properties and can be tuned to continuously record data on temperature, humidity, blood oxygen and heart signals, according to the team. Once the job is done, the sensor can simply be washed away using hot water.

“It could be recycled, since removal doesn’t damage the device,” says Huanyu Cheng, who led the research. “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 to the person using it or to the environment.”

As it continues to improve its device, the team hopes to tailor the technology to monitor symptoms of COVID-19.