Engineers develop sensors and receiver that detects physiological skin signals

Stanford engineers have developed a way to detect physiological signals emanating from the skin with sensors that stick like band-aids and beam wireless readings to a receiver clipped onto clothing.

Researchers stuck sensors to the wrist and abdomen of one test subject to monitor the person's pulse and respiration by detecting how their skin stretched and contracted with each heartbeat or breath. Likewise, stickers on the person's elbows and knees tracked arm and leg motions by gauging the minute tightening or relaxation of the skin each time the corresponding muscle flexed.

Zhenan Bao, a chemical engineering professor, thinks this wearable technology called BodyNet, will first be used in medical settings such as monitoring patients with sleep disorders or heart conditions.

Her lab is already trying to develop new stickers to sense sweat and other secretions to track variables such as body temperature and stress. Her ultimate goal is to create an array of wireless sensors that stick to the skin and work in conjunction with smart clothing to more accurately track a wider variety of health indicators than the smart phones or watches consumers use today.

Bao said: "We think one day it will be possible to create a full-body skin-sensor array to collect physiological data without interfering with a person's normal behaviour."

Postdoctoral scholars Simiao Niu and Naoji Matsuhisa led the 14-person team that spent three years designing the sensors. The team’s goal was to develop a technology that would be comfortable to wear and have no batteries or rigid circuits to prevent the stickers from stretching and contracting with the skin.

The eventual design met these parameters with a variation of the RFID - radiofrequency identification - technology used to control keyless entry to locked rooms. When a person holds an ID card up to an RFID receiver, an antenna in the ID card harvests a tiny bit of RFID energy from the receiver and uses this to generate a code that it then beams back to the receiver.

The BodyNet sticker is similar to the ID card: It has an antenna that harvests a bit of the incoming RFID energy from a receiver on the clothing to power its sensors. It then takes readings from the skin and beams them back to the nearby receiver.

But to make the wireless sticker work, the researchers had to create an antenna that could stretch and bend like skin. They did this by screen-printing metallic ink on a rubber sticker. However, whenever the antenna bent or stretched, those movements made its signal too weak and unstable to be useful.

To get around this problem, the Stanford researchers developed a new type of RFID system that could beam strong and accurate signals to the receiver despite constant fluctuations. The battery-powered receiver then uses Bluetooth to periodically upload data from the stickers to a smartphone, computer or other permanent storage system.