Flexible sensors, like the ones used in skin patches, offer a more precise and comfortable health monitoring option than larger wearable devices with rigid electrodes and wires. Expanding on these advantages, researchers at the Penn State Department of Engineering Science and Mechanics have developed a process for safely printing sensors directly onto the skin.

Printed sensors aren’t new, but until now they have required high temperatures to successfully bond metallic components together during printing, using a process called “sintering.” On its own, sintering requires a temperature of around 572F. Printing sensors onto clothing, paper, and other surfaces requires a “sintering aid layer” that enhances heat conduction between metallic nanoparticles; this reduces the sintering temperature to about 212F (the temperature of boiling water). This is still too hot to be applied directly to a subject’s skin.

The research team used a unique sintering layer to bring the sintering temperature low enough to print onto human skin. This layer contained polyvinyl alcohol, a mild adhesive found in cosmetic face masques, and calcium carbonate, a molecule in eggshells. The new layer facilitated sintering at room temperature and therefore suitable for on-skin printing.

The eco-friendly sensor — which resembles a metallic marker drawing — remains intact in room-temperature water, facilitating handwashing and chores that require water. It washes off under hot water at temperatures typical of a hot shower. Neither the printing process nor removal of the sensor causes any damage to the skin.

On-skin printed sensor monitoring can precisely record body temperature, blood oxygen levels, heart rate, and humidity. When connected to a wireless network, these combined biometrics offer a robust, real-time monitoring capability. 

The research team published its results in the journal ACS Applied Materials & Interfaces. The researchers plan to develop their technology further. The next step is to calibrate the sensor components to detect specific symptoms, including those associated with COVID-19. The novel sintering layer and successful low-temperature sintering could eventually see broad application across a range of industries.