Date: 22nd October 2020
Wearable sensors have gained huge traction due to their potential for non-invasive health monitoring. The field is now evolving such that bendable devices may provide far more precise biometric measurements and comfort to user than watches and electrodes but the bonding process of the metallic components (sintering) in the sensor has limited direct printing on the skin and has required various sophisticated fabrication approaches. Now researchers report a simple yet universally applicable fabrication technique with the use of a novel sintering aid layer to enable direct printing for on-body sensors.
The team, led by Huanyu Cheng from The Pennsylvania State University, US, had previously developed stretchable and flexible biosensors that could easily integrate with the changing contours of the human body. However, the sintering process which allows the silver particles in the sensor to bond, typically occurs at 300oC, and is therefore not compatible with on-skin printing. Now, the team have created a sintering aid layer — designed not to burn or damage the skin and that could help the material sinter together at a lower temperature.
The team start by incorporating a nanoparticle to the mix, which allowed the silver particles to sinter at a lower temperature however, this was still too hot for the skin. Further alterations to the formula of the aid layer and the printing material meant the team could finally sinter at room temperature.
The sintering aid layer comprised of polyvinyl alcohol (PVA) paste, which is safe to use on the skin and is an ingredient in many face masks, and nanoadditives in the water such as calcium carbonate. The sensors could now be printed on paper or fabric to create flexible printed circuit boards or could be directly printed on the skin. Once printed, a cool hair dryer was used to remove the water solvent in the ink.
The team also found the aid layer decreased the printing surface roughness and allowed for the incorporation of an ultrathin layer of metal patterns that could bend and fold with body movements whilst maintaining electromechanical capabilities.
The team then demonstrated that the on-body sensors were capable of precisely monitoring a variety of health parameters such as temperature, humidity, blood oxygen levels and the electrical activity of the heart. By linking the sensors into a network with wireless transmission, the team were able to observe the parameters in real time.
It was crucial that the sensors were stable on the skin for several days, without irritating the user or peeling off. Indeed, this was the case, the sensors were stable in warm water for example whilst washing hands, but could be easily removed under hot water without damage to itself or the skin, meaning they could be recycled and reused.
Conclusions and future applications
The team here present an environmentally friendly, recyclable sensor than can be printed directly on the body without the need for heat. It provides a simple yet universally applicable fabrication technique which could accelerate and dramatically improve healthcare monitoring.
There is currently much interest and investment into the development of biosensors. At the beginning of this month we saw the development of motion-charged biosensors to monitor sweat biomarker. However, whilst this application offered an early insight into the possibilities of self-powered wireless personalised health monitor, the ability to print sensors directly on to the skin offers a tremendous advancement for this technology.
Looking to the future the team are hoping to develop the technology to target specific applications. Their first focus will be to develop the on-body sensor network to monitor particular symptoms associated with COVID-19.
For more information please see the press release from Penn State University
Zhang, L., H. Ji, H. Huang, N. Yi, X. Shi, S. Xie, Y. Li, Z. Ye, P. Feng, T. Lin, X. Liu, X. Leng, M. Li, J. Zhang, X. Ma, P. He, W. Zhao and H. Cheng (2020). “Wearable Circuits Sintered at Room Temperature Directly on the Skin Surface for Health Monitoring.” ACS Applied Materials & Interfaces 12(40): 45504-45515.