Sew-lar Power

The last thing that most developers of wearable electronic devices want to talk about is power delivery. The fact of the matter is that all of the advancements in computing and sensing technologies that have enabled the development of tiny, yet powerful, wearables that are comfortable to wear all day long have not been met with corresponding advances in battery technology. As such, those otherwise incredibly useful devices are rendered impractical by rigid and bulky batteries or the need for frequent recharges.

This present state of affairs has led to a renewed interest in both generating and storing electricity in fabrics. An efficient system of this sort would allow us to transparently carry batteries everywhere we go by simply getting dressed as we normally do. It would also free us from having to remember to charge our devices — which typically also spells downtime for a wearable. Unfortunately, this dream has yet to materialize. Existing fiber batteries and energy harvesting systems generally perform pretty poorly.

Everything we know about fiber-based power delivery may soon change, however, thanks to the efforts of a team at the Johns Hopkins Applied Physics Laboratory. In a pair of recently published papers, the lab has provided a blueprint for building both batteries and solar cells into fibers that can be woven directly into fabrics. Owing to their innovative methods, the fiber-based devices are highly performant, durable, and easy to fabricate.

To produce the batteries, thin sheets of anode and cathode electrodes are separated by an insulating polymer. These layers are laminated by a heated roll press to create a stacked design that is then cut into strands that are just 700 micrometers wide with a laser cutter. This design is reminiscent of the pouch batteries that are highly prevalent in today’s smartphones and other portable electronics, which gives these fiber-like batteries excellent performance characteristics.

Solar cells are very difficult to build into flexible fibers, as they are normally built into large, rigid sheets. The team’s approach involved making lots of very tiny solar cells on top of miniature, flexible circuit boards. These tiny beads of solar cells were then encased in a polymer to protect them, before being adhered to a strand of fiber. When present in large quantities, these mini solar cells can produce significant amounts of electricity, much like their larger counterparts.

By weaving both types of fibers into a fabric, functional materials can be created that both generate electricity, and store the power that they produce. Experiments proved that these fibers could be bent thousands of times and be exposed to a month of constant sunlight without negatively impacting their performance. This indicates that they might be able to be incorporated into articles of clothing in the future.

In a test of the technology, a small circuit was designed to blink an LED. This circuit was connected to a small strip of the solar battery fabric and placed under a lamp. Within a few seconds, the LED was observed to start blinking. Looking ahead, the researchers believe that additional components, like sensors and LEDs, could also be incorporated into fabrics using similar techniques, further extending the utility of the approach.