How to power wearables without sacrificing energy density

There is a new technology gripping the markets of the future — wearable technology.

Wearables are portable systems that contain sensors to collect measurement data from our bodies. Powering these sensors without wires calls for pliable batteries that can adapt to the specific material and deliver the power the system requires. Microbatteries developed by the Fraunhofer Institute for Reliability and Microintegration IZM provide the technical foundation for this new technology trend.

In medicine, wearables are used to collect data without disturbing patients as they go about their daily business — to record long-term ECGs, for instance. Since the sensors are light, flexible and concealed in clothing, this is a convenient way to monitor a patient’s heartbeat. The technology also has more everyday applications — for example, fitness bands that measure joggers’ pulses while out running.

How to power these smart accessories poses a significant technical challenge. There are the technical considerations — durability and energy density — but also material requirements such as weight, flexibility and size, and these must be successfully combined. This is where Fraunhofer IZM comes in: experts at the institute have developed a prototype for a smart wristband that, quite literally, collects data firsthand.

The silicone band’s technical piece de resistance is its three green batteries. Boasting a capacity of 300 mAh, these batteries are what supply the wristband with power. They can store energy of 1.1 Wh and lose less than 3% of their charging capacity per year.

With these parameters the new prototype has a much higher capacity than smart bands available so far, enabling it to supply even demanding portable electronics with energy. The available capacity is actually sufficient to empower a conventional smart watch at no runtime loss. With these sorts of stats, the prototype beats established products such as smart watches, in which the battery is only built into the watch casing and not in the strap.

Millimetre-sized lithium-ion batteries with interdigital electrodes. Image© Fraunhofer IZM, Volker Mai

Success through segmentation

According to Robert Hahn, a researcher in Fraunhofer IZM’s department for RF & Smart Sensor Systems, segmentation is the recipe for success. “If you make a battery extremely pliable, it will have very poor energy density — so it’s much better to adopt a segmented approach,” he said.

Instead of making the batteries extremely pliable at the cost of energy density and reliability, the institute turned its focus to designing very small and powerful batteries and optimised mounting technology. The batteries are pliable in between segments. In other words, the smart band is flexible while retaining a lot more power than other smart wristbands available on the market.

Mechanically flexible microbattery strip made from segmented battery cells. Image© Fraunhofer IZM

Customer-tailored solutions

In its development of batteries for wearables, Fraunhofer IZM combines new approaches and years of experience with a customer-tailored development process. As explained by Hahn, “We work with companies to develop the right battery for them.”

The team consults closely with customers to draw up the energy requirements. They carefully adapt parameters such as shape, size, voltage, capacity and power and combine them to form a power supply concept. The team also carries out customer-specific tests.

Smart plaster to measure sweat

In 2018, the institute began work on a new wearable technology: the smart plaster. Together with Swiss sensor manufacturer Xsensio, this EU-sponsored project aims to develop a plaster that can directly measure and analyse the patient’s sweat. This can then be used to draw conclusions about the patient’s general state of health. In any case, having a convenient, real-time analysis tool is a useful way to track and monitor healing processes.

Fraunhofer IZM is responsible for developing the design concept and energy supply system for the sweat measurement sensors. The plan is to integrate sensors that are extremely flat, light and flexible. This will require the development of various new concepts. One idea, for instance, would be an encapsulation system made out of aluminium composite foil. The researchers also need to ensure they select materials that are inexpensive and easy to dispose of — after all, a plaster is a disposable product.

Microbattery with metal foil laminated housing. Image© Fraunhofer IZM

Conclusion

For over 25 years, Fraunhofer IZM has been one the world’s leading institutes for applied research and the development and system integration of robust and reliable electronics. Cooperating with industry and academia, the institute is overcoming the challenges associated with wearable technology. And with the sector expected to reach a market value of €72 billion by 2020, we can expect Fraunhofer IZM’s early successes to produce big returns.

Top image caption: Fabrication of microbatteries with side-by-side electrodes on silicon wafer. Image ©Fraunhofer IZM