You have the power.
That's the idea behind a "wearable microgrid" from the University of California San Diego that harvest and stores energy from your body to keep your electronics going.
The wearable has three components: sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy-storing supercapacitors. All parts are flexible, washable, and can be screen printed onto clothing — in a way that optimizes the amount of energy collected.
- The Biofuel Cells, located in the chest portion of the shirt, harvest energy from sweat. Once the user begins to sweat, the biofuel cells start providing power and continue to do so after the user stops moving. The biofuel cells are equipped with enzymes that facilitate redox reactions of lactate and oxygen in human sweat to generate electricity. The work with sweat-harvesting wearables began in 2013, under the direction of Joseph Wang, director of the Center for Wearable Sensors at UC San Diego. Wang's team later updated the technology to be stretchable and powerful enough to run small electronics.
- Triboelectric Generators, positioned at the outside of the shirt near both the forearm and waist, collect power from the arm's swinging motion during running or walking. The triboelectric generators have a negatively charged material on the forearms and a positively charged material on the torso. As the arms swing against the torso while walking or running, the oppositely charged materials rub against each and generate electricity. The triboelectric generators provide power as soon as the user starts moving.
- Supercapacitors, outside the shirt and on the chest, temporarily store energy from both devices and then discharge it to power small electronics.
The biofuel cells and triboelectric generators are a complementary match. Harvesting energy from both movement and sweat enables the wearable microgrid to power devices quickly and continuously. Because the biofuel cells provide continuous low voltage, however, and the triboelectric generators provide pulses of high voltage, the supercapacitors must temporarily store the energy from both power sources and discharge it as needed.
The technology, reported in a paper published Mar. 9 in Nature Communications, draws inspiration from community microgrids.
“We’re applying the concept of the microgrid to create wearable systems that are powered sustainably, reliably and independently,” said co-first author Lu Yin, a nanoengineering Ph.D. student at the UC San Diego Jacobs School of Engineering . “Just like a city microgrid integrates a variety of local, renewable power sources like wind and solar, a wearable microgrid integrates devices that locally harvest energy from different parts of the body, like sweat and movement, while containing energy storage.”
The wearable microgrid was tested on a subject during 30-minute sessions that consisted of 10 minutes of either exercising on a cycling machine or running, followed by 20 minutes of resting. The moves were enough to power either an LCD wristwatch or a small electrochromic display — a device that changes color in response to an applied voltage — throughout each 30-minute session.
All of the parts are connected with flexible silver interconnections, also printed on the shirt, and insulated by waterproof coating. The performance of each part is not affected by repeated bending, folding, and crumpling, or washing in water—as long as no detergent is used.
In a short Q&A with Tech Briefs below, Yin explains how he envisions these types of wearables being used.
Tech Briefs: The video from UCSD (shown above) says the possibilities for this wearable are “endless.” Can you bring us into one or two specific examples of where you see this kind of wearable microgrid being ideally suited for? What kinds of applications do you think will happen first?
Lu Yin: The concept of microgrids on wearables is a start for future research on self-powered, autonomous wearable systems that do not require external energy input. In this work, we have shown the possibility of harvesting energy to power a microcontroller for measuring sodium concentrations while controlling a low-power electrochromic display. There are much more possibilities in different combinations of energy harvesting, storage, and applications. The mechanical and biochemical energy from the body, as well as the external energy such as heat or sunlight, have the potential to supply much higher energy, which can enable more wearable sensing devices that consume higher power. Such power can be used to power multiplexed sensors for ECG, blood pressure, oximeters, or the measurement of more chemicals such as glucose, lactate, cortisol, uric acid, so the system can continuously monitor people's health.
Another possibility is the integration with low-energy flexible display systems and interface devices (e.g., touch screens, touchpads), so we can perform tactile and visual interaction using textile as the platform. Overall, the concept of microgrid-style energy management can be applied to other miniaturized, self-powered electronics, to remove the need for charging for small devices and simplify our daily workflow.
Tech Briefs: How much power can the body provide with this wearable? Through what action can you generate the most power?
Lu Yin: This work demonstrated the harvesting of using biofuel cells and triboelectric generators; both are generating power on the order of tens to hundreds of microwatts. The power we harvest using current technologies can still be much improved, but it can already power a wide range of electronics, such as low-energy displays, Bluetooth, different microcontrollers, or system-on-chip, which has unlimited applications.
In this work, the system is used for sensing the sodium level in sweat; thus, we designed it to provide the most power from running. Running generates sweat for both sensing and powering biofuel cells, with the most body motion for powering the triboelectric generators. This agrees with the design consideration of "complementary module selection" we discussed in the paper , which advocates the mindful selection of harvesters that are most suitable for the applications. If we have different applications, different harvesters and storage devices should be considered that are best suited for the use case.
Tech Briefs: What were the reactions from test subjects? Are these wearables comfortable? Where did test subjects have the most concerns or questions, do you think?
Lu Yin: For integrating the triboelectric generator on the waist of the shirt, the user has to swing their arm against the body, which may feel strange to some people. The design of the triboelectric generator requires a long-sleeve shirt, which might not be comfortable for people who prefer short sleeves for workouts.
We bought the shirts from online retailer stores, and the devices printed on the shirt are all soft and flexible, so it does not feel any different than wearing a normal shirt that you would have. The integration of the microcontroller to the shirt and its connection to the display can also be further optimized. For sure, since this work is from a small team in a lab where our capability in designing a prototype is limited, and it has more room for improvement as a consumer product.
Tech Briefs: What needs to happen for these wearables to be adopted in a mainstream way?
Lu Yin: For this to be adapted for a mainstream way as a consumer product, the durability of all components and the interconnection between modules needs to be improved. We have demonstrated the harvesters, the supercapacitors, and the displays to be soft, flexible, washable, and mechanically durable, but the standard for a commercial product is more rigorous; the connection between components, as well as the encapsulation of the integrated electronics, needs to be engineered to prevent mechanical or electrical failure.
Tech Briefs: What will you be working on next, in relation to this project?
Lu Yin: The next step will be to implement the microgrid concept in other wearable or miniaturized systems that use other harvesters, storage devices for other types of applications. Different types of sensors for different market needs can be implemented, such as temperature sensors, ECG sensors, or lactate sensors, which are very closely related to sports and athletic purposes. Aside from working on the integration, we are also developing harvesters and storage devices with higher performance. We have recently reported on a record-breaking high energy and power density flexible thin-film-type battery for flexible and wearable electronics (see here ), and it is under further development towards a commercial product. At the same time, we are also working on various harvesters that are more convenient and require less activity from the human body.
What uses do you see for a wearable micro-grid? Share your questions and comments below.