Engineers at the University of California San Diego have developed a thin, flexible, stretchy sweat sensor that can show the level of glucose, lactate, sodium, or pH of your sweat — at the press of a finger. It is the first standalone wearable device that allows the sensor to operate independently — sans any wired or wireless connection to external devices — to directly visualize the measurement’s results.
The small, disk-shaped patch includes two integrated batteries, a microcontroller, sensors, the circuit, and a stretchable display. This covers all functionalities essential for operating a wearable sensor, from powering up to showing the user the results.
“We are trying to address the issue of practicality in wearable technologies,” said Lu Yin, a Nanoengineering Postdoctoral Researcher at the UC San Diego Jacobs School of Engineering. “We have seen so many inventions of wearable sensors with great novelty but with limited practicality.”
Besides the microcontroller, all the components and interconnections are fabricated using customized elastomeric inks, which can be printed with low-cost and high-throughput screen printing onto stretchable polymer sheets.
The fabrication of the device involves the formulation of nine different types of stretchable inks, which were used to print the batteries, circuits, display panel, and sensors. The device is printed by layer onto stretchable polymer sheets and then assembled with hydrogels and microcontroller chips. Each ink was optimized to ensure its compatibility with other layers while balancing its electrical, chemical, and mechanical performance.
The researchers stress-tested each component of the system, making sure that the display, sensors, and batteries can be stretched for up to 20 percent over 1,500 cycles with little to no performance effects. The batteries were packed with enough power to last over a week of non-stop usage.
The researchers demonstrated four different types of sensors that work with the patch: a sodium sensor, a pH sensor, a lactate sensor, and a glucose sensor. Each sensor type measured different metrics in the sweat during exercise. The research team also included a fluidic channel made of adhesives, to adhere the patch to the skin and guide the sweat to flow across the sensor. A small switch mechanism was included on the patch, and the concentration of the chemical being measured will instantly show on the display as soon as the user presses the switch.
To visualize the data from the sensors, the team designed a special type of nonlight-emitting display called electrochromic display. The electrochromic display technology uses materials that change color upon applying an electrical pulse.
The researchers designed a display panel comprising 10 individual pixels, which is programmed to display the concentration of the chemicals by turning on different numbers of the pixel. After optimizing the display’s operation condition, each pixel can be turned on and off reversibly over 10,000 cycles. The pixels only take 500 ms to change color, during which time they consume on average 80 microwatts of power.
Currently, the patch is not rechargeable and works with just one sensor at a time. The team aims to develop a more advanced version of the integrated sensor, which would allow the battery to be rechargeable and even harvest energy from the body, to extend its lifetime.
“This is a big step forward in developing wearable electronics that are practical and user-friendly, but this is just a start,” Yin said.