Written by Catherine Bolgar
Imagine charging your phone with electricity made from your own body. That day isn’t so far away. New technology is being developed to convert our bodies’ sweat, heat and motion into electricity. Here’s how.
“We do energy-harvesting from the body, with sweat as a biofuel,” says Joseph Wang, chair of nanoengineering and director of the Center for Wearable Sensors at the University of California at San Diego.
Researchers used the adhesive, stretchable, flexible tattoo to power a digital watch. “When the person started to sweat, we could see that the watch was turning,” Dr. Wang reports.
The biofuel patch uses an enzyme as a biocatalyst. “The tattoo has two printed electrodes with the enzyme. We later did the same thing on textile that’s in contact with the skin,” Dr. Wang says.
The patch generates 100 microwatts per square centimeter—too weak to charge a phone, but enough for a low-power biomedical device such as a glucose sensor or, eventually, a pacemaker. The device could be woven into headbands or underwear to capture sweat. It might even have military applications by obviating the need to carry batteries.
The beauty is, they’re inexpensive,” Dr. Wang says. “They’re printable devices with low-cost fabrication.”
Lactate isn’t the only potential bodily biofuel. Scientists are studying how glucose can power batteries as well as bodies. Researchers at Université Joseph Fourier in Grenoble, France, are working on implantable biofuel cells that would power artificial organs. And scientists at Virginia Polytechnic Institute and State University in Blacksburg, Virginia, are developing sugar-powered batteries that can store more energy than in lithium-ion batteries.
Heat: “We waste 60% of our energy through heat,” says Gang Chen, head of the mechanical engineering department and professor of power engineering at Massachusetts Institute of Technology. “There is interest in recovering this heat and turning it into electricity.”
One way batteries can convert heat into electricity is by taking advantage of thermodynamic cycles. As temperatures rise, battery voltage decreases. If a battery is charged at a high temperature and then cooled, the voltage increases. “The idea requires you to heat and cool the battery and cycle back and forth,” Dr. Chen explains.
Another method is thermoelectric. It’s possible to generate electricity when one side of a semiconductor is hot and the other cold. Such differentials are everywhere. For example, body temperature is hotter than ambient temperature, Dr. Chen points out, but devices need to be designed to maintain that difference.
The thermoelectric approach is closer to application. A thermoelectric battery can already be used to power a watch, while researchers at Yonsei University and the Korea Institute of Science and Technology, both in Seoul, have developed a wearable thermoelectric generator.
Motion: Piezoelectric systems harvest energy from pressure or vibrations caused by walking, driving cars on roads, or machinery operating in factories.
One recent advance is an energy-generating cloth developed at South Korea’s Sungkyunkwan University. Their piezoelectric nanogenerator is flexible and can be folded, rolled and stretched to capture energy from movement. Indeed, researchers at the École de Technologie Supérieure in Montreal, Canada have created a chin strap that captures electricity from chewing.
Then there’s locomotion. “When we walk, we apply light forces to footwear—around 1,000 newtons, or about 1.3 times the weight of a person,” says Tom Krupenkin, president of InStep NanoPower LLC and professor of mechanical engineering at the University of Wisconsin-Madison. “Up to 20 watts of energy is dissipated as heat into footwear without being used for anything. If we can capture and utilize a small part of that, it can be useful.”
InStep NanoPower uses a process called reverse electrowetting in which fluids harvest energy and convert it into electricity in an insole. Electronics included in the insole, powered by walking, would allow sensors to track basic data such as steps taken. But more important applications might be possible. By integrating GPS, accelerometers and temperature sensors the insoles could help locate, say, a jogger who has suffered a heart attack; someone buried in the rubble of a collapsed building; or a firefighter lost in a smoke-filled building. The insoles could even extend a phone’s battery life by using Bluetooth communications as a relay to cellular networks.
Catherine Bolgar is a former managing editor of The Wall Street Journal Europe. For more from Catherine Bolgar, contributors from the Economist Intelligence Unit along with industry experts, join the Future Realities discussion.
Photos courtesy of iStock