Personalized healthcare requires body area sensor network (bodyNET) that is able to perform continuous, accurate and hands-free monitoring of human physiological signals. To maintain signal integrity and improve wear comfort, soft sensors with good skin conformability made of light-weight and stretchable materials are desired. A key challenge to realize soft bodyNET is to seamlessly integrate soft sensors with rigid silicon readout circuits. Past research that directly connects soft sensors with rigid components often results in poor mechanical robustness. Such failures are mostly due to the strain concentration effect at the soft/rigid interfaces, which is also the major cause of broken cellphone cables in our daily lives. 

In our work just published on Nature Electronics, we tried to change the way how soft sensors and rigid electronics could be linked. We proposed to use wireless communication instead of direct wire connections so that soft/rigid interfaces can be avoided. Finally, we ended up with the passive radiofrequency identification (RFID) technology to make the on-skin part free of rigid batteries and chips. Our RFID technology includes a stretchable on-skin sensor sticker (tag) and a flexible readout circuits (reader) on clothes.

However, this seemingly straightforward design has some intrinsic issues in our bodyNET application. In particular, traditional rigid RFID design uses tags that are resonating at the exact operation frequency with the reader (that is, 13.56 MHz) for effective signal transmission. However, when our soft on-skin tags are under deformation (for example, being bent or stretched), their resonant frequency will significantly shift (due to antenna geometry change) and their quality factor will quickly drop (due to antenna parasitic resistance increase), leading to failed wireless communication.

Given the inevitable deformation of our soft antenna, we had to develop new working mechanisms of wireless communication. After conducing systematic simulations of RFID load modulation, we surprisingly found an unconventional working regime (Regime II) besides the traditional resonant working regime (Regime I). Regime II is located at a higher tag resonant frequency (>30 MHz in Regime II vs 13.56 MHz in Regime I) with larger coupling factors. Within Regime II, the system becomes insensitive to the strain-induced antenna resonance frequency shift and quality factor drop, which is ideal to our bodyNET application.

Guided by the new system design, we fabricated stretchable tags that resonate at Regime II using a scalable printing technology. Our soft bodyNET can maintain full functionality even when subject to 50% strain, an unprecedented performance compared to previous demonstrations. Finally, we built a bodyNET platform consisting of 5 independent pairs of tags and readers. The platform can continuously analyze critical human signals (pulse, respiration and body movement) while sleeping and exercising. Our ultimate goal is to track a wider variety of health indicators (temperature, blood pressure, electro- and chemo-physiological signals) from the whole body with higher accuracy and minimal invasiveness, towards personalized precision healthcare.

By Simiao N. and Naoji M.

Dr. Simiao Niu is a postdoctoral research fellow in Chemical Engineering, Stanford University, under the supervision of Dr. Zhenan Bao. He received his Ph.D. degree in School of Materials Science and Engineering at the Georgia Institute of Technology in May 2016, under the supervision of Dr. Zhong Lin Wang. He earned his master of science degree in Electrical and Computer Engineering at the Georgia Institute of Technology in 2015 and his bachelor of engineering degree in the Institute of Microelectronics at Tsinghua University in 2011 with the highest honors and outstanding undergraduate thesis award. His postdoctoral research interests include stretchable electronics and systems for personalized healthcare. In his doctoral research, his research focuses on theoretical and experimental studies on: mechanical energy harvesting by triboelectric nanogenerators and high-performance piezotronic and piezo-phototronic sensors based on piezoelectric nanowires.

Dr. Naoji Matsuhisa received his BS degree (2012) from the Department of Electrical Engineering in the University of Tokyo and his MS degree (2014) and PhD degree (2017) from the Department of Electrical Engineering and Information Systems in the University of Tokyo. He worked as a research scholar at Prof. Xiaodong Chen's group in Nayang Technological University, and is now working as a postdoctoral scholar at Prof. Zhenan Bao’s group in Stanford University. His research focuses on the development of stretchable conductors and stretchable electronic devices.