Power/Performance Bits: April 27

Wearables: energy-harvesting shirt; power from radio waves; conductive cellulose thread.


Energy-harvesting shirt
Engineers at the University of California San Diego developed a ‘wearable microgrid’ that harvests and stores energy from the human body to power small electronics.

The microgrid consists of three main parts: sweat-powered biofuel cells, motion-powered triboelectric generators, and energy-storing supercapacitors. All parts are flexible, washable and can be screen printed onto clothing.

“We’re applying the concept of the microgrid to create wearable systems that are powered sustainably, reliably and independently,” said 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.”

Biofuel cells that harvest energy from sweat are located inside the shirt at the chest. Triboelectric generators are positioned outside the shirt on the forearms and sides of the torso near the waist to harvest energy from the swinging movement of the arms against the torso while walking or running. Supercapacitors outside the shirt on the chest temporarily store energy from both devices and then discharge it to power small electronics.

The biofuel cells use enzymes that trigger a swapping of electrons between lactate and oxygen molecules in human sweat to generate electricity. They are stretchable and powerful enough to run small electronics.

The wearable microgrid uses energy from human sweat and movement to power an LCD wristwatch and electrochromic device. (Credit: Lu Yin / UC San Diego)

Using energy from both movement and sweat allows the wearable microgrid to provide power quickly and continuously. As soon as the wearer starts moving, the triboelectric generators provide power. Once the wearer is sweating, the biofuel cells start providing power and continue to do so after the user stops moving.

“When you add these two together, they make up for each other’s shortcomings,” Yin said. “They are complementary and synergistic to enable fast startup and continuous power.” The entire system boots two times faster than having just the biofuel cells alone, and lasts three times longer than the triboelectric generators alone.

The biofuel cells provide continuous low voltage, while the triboelectric generators provide pulses of high voltage. The supercapacitors help in combining and regulating into one stable voltage.

The parts are connected with flexible silver interconnections that are 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 without detergent.

Yin said that the integration of the different components is what gives the garment potential. “We’re not just adding A and B together and calling it a system. We chose parts that all have compatible form factors (everything here is printable, flexible and stretchable); matching performance; and complementary functionality, meaning they are all useful for the same scenario (in this case, rigorous movement).”

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 system was able to power either an LCD wristwatch or a small electrochromic display throughout each 30-minute session.

While the current system is designed for athletic activity, Yin said the team is working on other designs to harvest energy under different circumstances, such as sitting or during gentle movement. “We are not limiting ourselves to this design. We can adapt the system by selecting different types of energy harvesters for different scenarios.”

Power from radio waves
Researchers from Pennsylvania State University, Heriot-Watt University, and Hebei University of Technology are finding ways to power wearable electronics with radio waves.

Instead of replacing other methods of energy harvesting, such as solar and triboelectric, they are trying to augment them. “We don’t want to replace any of these current power sources,” said Huanyu “Larry” Cheng, Professor in the Penn State Department of Engineering Science and Mechanics. “We are trying to provide additional, consistent energy.”

The researchers developed a stretchable wideband dipole antenna system capable of wirelessly transmitting data that is collected from health-monitoring sensors. The system consists of two stretchable metal antennas integrated onto conductive graphene material with a metal coating. The wideband design of the system allows it to retain its frequency functions even when stretched, bent and twisted.

This system is then connected to a stretchable rectifying circuit, creating a rectified antenna, or rectenna, capable of converting energy from electromagnetic waves into electricity. The energy is used to power the sensing modules on the device, which track temperature, hydration and pulse oxygen level, or could be used to charge batteries or supercapacitors. While it produces less energy than other methods, it can work continuously.

“We are utilizing the energy that already surrounds us — radio waves are everywhere, all the time,” Cheng said. “If we don’t use this energy found in the ambient environment, it is simply wasted. We can harvest this energy and rectify it into power.”

“Our next steps will be exploring miniaturized versions of these circuits and working on developing the stretchability of the rectifier,” Cheng said. “This is a platform where we can easily combine and apply this technology with other modules that we have created in the past. It is easily extended or adapted for other applications, and we plan to explore those opportunities.”

Conductive cellulose thread
Researchers at Chalmers University of Technology, Aalto University, KTH Royal Institute of Technology, University of Borås, and Chung-Ang University developed thread made of conductive cellulose for electronic textiles.

“Miniature, wearable, electronic gadgets are ever more common in our daily lives. But currently, they are often dependent on rare, or in some cases toxic, materials. They are also leading to a gradual build-up of great mountains of electronic waste. There is a real need for organic, renewable materials for use in electronic textiles,” said Sozan Darabi, doctoral student at the Department of Chemistry and Chemical Engineering at Chalmers University of Technology and the Wallenberg Wood Science Center. “This cellulose thread could lead to garments with built-in electronic, smart functions, made from non-toxic, renewable and natural materials.”

Sewing the electrically conductive cellulose threads into a fabric using a standard household sewing machine, the researchers produced a thermoelectric textile that produces a small amount of electricity when it is heated on one side, such as by a person’s body heat. At a temperature difference of 37 degrees C (98.6F), the textile can generate around 0.2 microwatts of electricity.

The researchers made the thread conductive through dyeing it with PEDOT: PSS, an electrically conductive polymeric material. The researchers’ measurements show that the dyeing process gives the cellulose thread a record-high conductivity for cellulose thread in relation to volume of 36 S/cm-, which can be increased to 181 S/cm by adding silver nanowires. The thread could handle at least five machine washes without losing its conductivity.

The dark yarn is the cellulose yarn and the lighter one is a commercially available silver-plated yarn, both of which are electrically conductive. The researchers have sewn the two threads separately into the fabric, in a special way that gives the fabric its thermoelectronic properties. (Credit: Anna-Lena Lundqvist/Chalmers University of Technology)

By integrating the cellulose yarn into an electrochemical transistor, the researchers were able to demonstrate its electrochemical function.

“Cellulose is a fantastic material that can be sustainably extracted and recycled, and we will see it used more and more in the future. And when products are made of uniform material, or as few materials as possible, the recycling process becomes much easier and more effective. This is another perspective from which cellulose thread is very promising for the development of e-textiles,” said Christian Müller, a professor at the Department of Chemistry and Chemical Engineering at Chalmers University of Technology.

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