The body electric,Batteries are the weak link for wearable and implantable devices. But what if you could harvest energy from the heat, sweat or vibrations of the wearer?
In “I Sing the Body Electric,” poet Walt Whitman waxed lyrically about the “action and power” of “beautiful, curious, breathing, laughing flesh.” More than 150 years later, MIT materials scientist and engineer Canan Dagdeviren and her colleagues are giving new meaning to Whitman’s poem with a device that can generate electricity from the way it distorts in response to the beating of the heart.
Electronics are now so powerful that a smartphone has more computing power than all of NASA did when it put the first men on the moon in 1969. The extraordinary advances technology has made over time have raised hopes that devices worn on, or even implanted within, the body can also become ever more capable.
One energy-harvesting strategy involves converting energy from vibrations, pressure and other mechanical stresses to electrical energy. This approach, producing what is known as piezoelectricity, is often used in loudspeakers and microphones.
A commonly used piezoelectric material is lead zirconate titanate, whose lead content raises concerns that it might prove too toxic for use with humans. “But for lead to decompose from the structures, they would have to be heated to temperatures higher than 700°C,” Dagdeviren says. “You’ll never reach such temperatures in the body.”
To take advantage of piezoelectricity, Dagdeviren and her colleagues have developed flat devices that can be stuck onto organs and muscles such as the heart, lungs and diaphragm. These devices are “mechanically invisible” in that their mechanical properties are similar to whatever they are laminated onto, so they don’t hinder those tissues when they move.
So far, such devices have been tested in cows, sheep and pigs, all animals with hearts roughly the same size as those of people. “When these devices mechanically distort, they create positive and negative charges, voltage and current — and you can collect this energy to recharge batteries,” Dagdeviren explains. “You can use them to run biomedical devices like cardiac pacemakers instead of changing them every six or seven years when their batteries are depleted.”
Scientists are also developing wearable piezoelectric energy harvesters that can be worn on joints such as the knee or elbow, or in shoes, trousers or underwear. That way, a person can generate electricity for electronics whenever they walk or bend their arms.
When designing piezoelectric gadgets, one might — counterintuitively — not want the materials that are best at generating electricity. Instead of choosing a material that converts 5 percent of all mechanical energy it receives into electricity, for example, you might want a material with only 2 percent or less efficiency. If it converts more, “it might do so by placing more of a load on the body, and you don’t want it to make you tired,” Dagdeviren says.
A different energy-harvesting approach uses thermoelectric materials to convert body heat to electricity. “Your heart beats more than 40 million times a year,” Dagdeviren notes. All that energy is dissipated as heat in the body — it’s a rich potential source to capture for other uses.
Thermometric generators do face some key challenges. They rely on temperature differences, but people usually keep a fairly constant temperature throughout their bodies, so any temperature differences found within are generally not dramatic enough to generate large amounts of electricity. But this is not a problem if the devices are exposed to relatively cool air in addition to the body’s continuous warmth.
Scientists are exploring thermoelectric devices for wearable purposes, such as powering wristwatches. In principle, the heat from a human body can generate enough electricity to power wireless health monitors, cochlear implants and deep-brain stimulators to treat disorders such as Parkinson’s disease.
Static and dynamic
Scientists have also sought to use the same effect behind everyday static electricity to power devices. When two different materials repeatedly collide with, or rub against, one another, the surface of one material can steal electrons from the surface of the other, accumulating a charge, a phenomenon known as triboelectricity. A key virtue of triboelectricity is that nearly all materials, both natural and synthetic, are capable of creating it, giving researchers a wide range of possibilities from which they can design gadgets.
“The more I work with triboelectricity, the more exciting it gets, and the more applications it might have,” says nanotechnologist Zhong Lin Wang at Georgia Tech, the review’s co-author. “I can see myself devoting the next 20 years to it.”