Printing Wearable Circuits Directly Onto Skin
Penn State engineers developed a novel technique to produce precise biometric sensors.
The field of wearable sensors presents numerous promising research directions. In biomedical technologies, developing a soft body sensor network in wearable devices integrated to on-body sensors introduces a new potential for physiological signal monitoring. The realization of this premise has previously relied on such sophisticated fabrication approaches as lithography or direct printing on a carrier substrate that is then attached to the body. However, an international research team led by Huanyu “Larry” Cheng of Penn State University has proposed a fabrication technique that allows such sensors to be sintered at room temperature directly onto the skin.
Although Cheng and his colleagues previously created a flexible printed circuit board for use in wearables, printing directly onto the skin has been hindered by the temperatures, as high as 572 degrees Fahrenheit, required to sinter the circuit’s metallic components. The silver particles in the circuit design must be sintered at a temperature too high for the skin to withstand, but the team proposed an aid layer — something non-damaging to skin that could help the materials bond at a lower temperature.
One proposal, adding a nanoparticle to the mix, enabled the silver particles to sinter at about 212 degrees Fahrenheit. While this can be useful for printing on clothing and paper, it is still too high a temperature to allow printing directly onto the body. Altering the formula of the aid layer to consist of a polyvinyl alcohol paste and calcium carbonate, however, gets the materials to sinter at room temperature.
The two components of the aid layer are the main ingredient in peelable face masks and the material that comprises eggshells, respectively. With the aid of this new layer, it is possible to create an ultra-thin layer of metal patterns that can be deformed — can bend and fold — while still maintaining electromechanical properties. As the sensor is printed, the engineers use an air blower to remove the water that is used as a solvent in the ink. There is no need for this new fabrication process to rely on heat at all.
These wearable sensors are capable of precisely and continuously capturing a number of physiological signals, including temperature, humidity, blood oxygen levels, and heart performance signals. The sensor remains robust through washes with tepid water for a few days but is easily removed with a hot shower. This provides a robust sensor with a simple fabrication and application process that is also environmentally friendly. Removal doesn’t damage the device, nor does it damage the skin, making for easy recycling. The researchers intend to next alter the technology to target specific application, including monitoring the particular symptoms associated with COVID-19.
