Stretchable packaging developed to protect batteries from gas, moisture

Wearable batteries have garnered attention for their potential as sensors that could monitor various biomarkers and as medical devices.

In order for these wearable devices to be functional, they need to have stretchable and deformable batteries. Because wearable devices are exposed to the atmosphere, it is important to extent battery life while protecting the batteries from atmospheric moisture and gases. Researchers from Yokohama National University in Japan have developed a stretchable packaging film for these batteries with a high gas and moisture barrier functionality. Their results were published in ACS Applied Materials & Interfaces.

Corresponding author Hiroki Ota of the Department of Mechanical Engineering at Yokohama National University said that although soft and stretchable batteries have been studied in the world, they cannot be used in air due to the high gas and moisture permeability of the packaging materials of stretchable batteries. “Currently, the use of solid and large batteries for stretchable devices is a problem in stretchable electronics; that is, while sensors and interfaces are soft, batteries still use hard batteries,” Ota said.

To create this flexible film with a high gas barrier, the researchers coated a thin layer of liquid metal onto a gold-deposited thermoplastic polyurethane film using the layer-by-layer method. This method allowed for the desired deformability, unlike aluminium-laminated films, which were used previously to address the issue of gas and moisture permeability but failed to allow for the needed flexibility. According to researchers, the resulting film demonstrates oxygen gas impermeability under mechanical strain, and low moisture permeability. The stretchable lithium-ion battery that they assembled in the study was also able to operate reliably in air due to the stretchable gas barrier film that they developed.

“It is exciting that in addition to the development of a stretchable battery, which could be used in the next generation of smart devices, including future wearable devices, films with high gas and moisture barrier properties can be achieved by using a novel material called liquid metal,” Ota said.

This research could enable the use of batteries that have high energy density, high working voltage and long-term stability and that are also highly deformable — as opposed to bulky and inflexible — in wearable devices. As a result, the findings bring wearable devices closer to becoming more practical, thereby opening up opportunities in medicine and health. “This research contributes to the social implementation of stretchable devices,” Ota said.

Next, the researchers aim to enhance the moisture protection ability of the film by modifying the materials. Another future direction involves improving the stability of the performance of the batteries, even under deformation, by developing materials better suited for their parts. Making the film cost-effective will also contribute to eventual scalability. “Further cost reductions of the developed film will lead to the implementation of stretchable batteries. In addition, the film could be useful as a barrier film for organic electronics and so on,” Ota said.

Image credit: Yokohama National University