Wearable electronics are getting smaller, more comfortable and increasingly capable of interfacing with the human body. To achieve a truly seamless integration, electronics could someday be printed directly on people's skin. As a step toward this goal, researchers reporting in ACS Applied Materials & Interfaces have safely placed wearable circuits directly onto the surface of human skin to monitor health indicators, such as temperature, blood oxygen, heart rate and blood pressure.
The latest generation of wearable electronics for health monitoring combines soft on-body sensors with flexible printed circuit boards (FPCBs) for signal readout and wireless transmission to health care workers. However, before the sensor is attached to the body, it must be printed or lithographed onto a carrier material, which can involve sophisticated fabrication approaches. To simplify the process and improve the performance of the devices, Peng He, Weiwei Zhao, Huanyu Cheng and colleagues wanted to develop a room-temperature method to sinter metal nanoparticles onto paper or fabric for FPCBs and directly onto human skin for on-body sensors. Sintering -- the process of fusing metal or other particles together -- usually requires heat, which wouldn't be suitable for attaching circuits directly to skin.
The researchers designed an electronic health monitoring system that consisted of sensor circuits printed directly on the back of a human hand, as well as a paper-based FPCB attached to the inside of a shirt sleeve. To make the FPCB part of the system, the researchers coated a piece of paper with a novel sintering aid and used an inkjet printer with silver nanoparticle ink to print circuits onto the coating. As solvent evaporated from the ink, the silver nanoparticles sintered at room temperature to form circuits. A commercially available chip was added to wirelessly transmit the data, and the resulting FPCB was attached to a volunteer's sleeve. The team used the same process to sinter circuits on the volunteer's hand, except printing was done with a polymer stamp. As a proof of concept, the researchers made a full electronic health monitoring system that sensed temperature, humidity, blood oxygen, heart rate, blood pressure and electrophysiological signals and analyzed its performance. The signals obtained by these sensors were comparable to or better than those measured by conventional commercial devices.