The bellows-like motions that a person makes while breathing, for example, can generate 0.83 watts of power; the heat from a person’s body, up to 4.8 watts; and the motions of a person’s arms, up to 60 watts. That’s not nothing when you consider that a pacemaker needs just 50 millionths of a watt, a hearing aid needs a thousandth of a watt, a smartphone requires one watt.*
Now Dagdeviren and others are designing machines that use the human body itself as their source of energy. Increasingly, researchers are testing such wearable or implantable devices in animal models and people.
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 degrees Celsius,” 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.