A battery that is both flexible and washable is key to designing commercially successful wearable biosensors for medical monitoring. University of British Columbia researchers recently created a prototype battery that does exactly that. It works when twisted or stretched to twice its normal length and even after being tossed in the laundry.
“We don’t know of any other batteries that are machine washable and stretchable,” says electrical and computer engineering professor John Madden, director of UBC’s Advanced Materials and Process Engineering Lab who supervised the work. “It’s what makes this unique.”
In addition to watches and patches for measuring vital signs, the battery might also be integrated into clothing that can emit light, or change colour or temperature.
In normal batteries, the internal layers are hard materials encased in a rigid exterior. Ngoc Tan Nguyen, a postdoctoral fellow at UBC’s faculty of applied science, and his team made the key compounds—in this case, zinc and manganese dioxide—stretchable. They did this by grinding them into small pieces and then embedding them in a rubbery biocompatible poly(styrene – isobutylene – styrene) (SIBS) polymer.
The battery is made up of several ultra-thin layers of this polymer wrapped inside a casing of the same polymer. This construction creates an airtight, waterproof seal that ensures the integrity of the battery through repeated use.
The choice of zinc and manganese dioxide was deliberate. “For devices worn next to the skin, it’s a safer chemistry than lithium-ion batteries, which can produce toxic compounds when they break,” Nguyen says in a news release.
Team member Bahar Iranpour, a PhD student in electrical and computer engineering, suggested throwing the battery in the wash to test its seal. So far, the battery has withstood 39 wash cycles and the researchers expect to further improve its durability.
“We put our prototypes through an actual laundry cycle in both home and commercial-grade washing machines. They came out intact and functional,” says Iranpour.
The battery is rechargeable, although its electrodes may not be easily accessible when incorporated into a wearable device so the team is working on wireless charging. It is also working on boosting the battery’s power.
“Right now, the power is barely enough to drive a low-power Bluetooth device,” says Madden. “We need to reliably power Bluetooth to make it broadly applicable.”
University of California San Diego nanoengineering professor Sheng Xu, who was not involved in the project, says it’s challenging to find a packaging material that meets all the important criteria. “SIBS seems to be perfect for this purpose,” he says.
That said, the technology will need to be refined to further reduce the water vapor transmission rate of SIBS so people can wear the devices or clothing for extended periods. “Ions and biomolecules in the sweat and interstitial fluids may accelerate the water transmission rate in SIBS,” say Xu.
Still, at least one medical device manufacturer has expressed interest in the battery and the team is considering approaching the fashion industry.
“While it will need years of trials before we can try it directly on skin or as an implantable medical device, it is possible to test the product in garments. Clothing doesn’t need to be just a cover anymore,” says Iranpour. “It can be used as a means of communication, health monitoring, and cooling and heating. Beside the technological challenges, smart garments still need to offer comfort and the desirable look to find a market.”