If our bodies burn 2,000-2,500 calories per day anyway, that could power a smartphone.

Someday, battery may not be such an issue for smartwatches (and other wearables).

Technology has always been intimately linked to the human body. From sharpened flint to smartphones, we've been carrying our inventions for millennia—but the relationship is about to get even closer. The next generation of electronic devices might not just be near our bodies, they could be powered by them.

Staying alive guzzles energy. In order to keep us ticking, our bodies need to burn between 2,000 and 2,500 calories per day, which is conveniently enough to power a modestly used smart phone. So if just a fraction of that energy could be siphoned, our bodies could in theory be used to run any number of electronic devices, from medical implants to electronic contact lenses—all without a battery in sight. Recently, researchers have taken important strides toward unlocking this electric potential.

Untapped potential

To start, the energy in our bodies exists in various forms. Most of them need some manipulation before they can be used to power an electronic device. But not all do.

For instance, the ears of mammals contain a tiny electric voltage called the endocochlear potential (EP). Found inside the cochlea, a spiral-shaped cavity in the inner ear, the EP aids hearing by converting pressure waves into electrical impulses. It’s vanishingly weak—about a tenth of a volt—but still strong enough, in theory, to power hearing aids and other aural implants.

Harvesting the EP had long been considered unthinkable due to the extreme sensitivity of the inner ear. But using a combination of surgical prowess and technological innovation, researchers in Massachusetts managed to do just that in 2012.

The team developed an “energy harvester chip,” about the size of a fingernail, which was designed to extract electrical energy directly from the EP. They tested the chip in a guinea pig, implanting it into the animal’s inner ear where it generated enough electricity to power a radio transmitter. The minute electric power produced by the chip—about a nanowatt (a billionth of a watt)—is still about a million times too low to power an electronic implant. But it’s a nanowatt more than had been generated before, making this an important proof-of-concept. If the power output of future prototypes can be boosted, the natural voltage of the inner ear could someday be used to power hearing aids; it could even allow the development of implants to treat diseases which originate there, such as Ménière's disease.

Outside the cochlea, however, free-flowing electricity is (perhaps fortunately) rare in our bodies. Most biological energy is locked up in other forms. And one way to release it is by recycling.

Feet and Heat

We’re built to move. Other than powering basic functions in our cells, the bulk of our energy expenditure goes toward muscle motion; heartbeats, breathing, and getting places. (I’m sure you’ll agree, these are vital things.) To anyone who's used a bicycle generator or wind-up torch, the idea of converting this kinetic energy into electricity won't be new, but things have gotten a tad more complex.

In the past few years, researchers have started to exploit a unique property of some materials, known as piezoelectricity, to generate electricity from human movement. Piezoelectric materials spontaneously generate electric charge when exposed to stress (the Greek word piezo means to squeeze or press). These materials are already used in countless industrial applications, and even the humble cigarette lighter (that “click” you hear in the electronic kind is the sound of a piezoelectric crystal being struck). But their next use could be in energy-generating fabrics.

One of the most advanced of these was developed in 2013 by a Chinese-US research team that invented an elastomer-based piezoelectric fabric able to generate electricity using only the kinetic energy of human locomotion. When a piece of this fabric was worn as a shoe insole by a volunteer, walking generated enough electricity to illuminate 30 LEDs. What’s more, when the same fabric was applied onto a shirt that was then artificially moved, it charged a lithium-ion battery in a matter of hours.

The potential of piezoelectric materials goes even deeper. They’re also being used to harvest energy from internal organs. Last year, US-based researchers successfully generated electricity from the beating hearts, lungs, and diaphragms of (sedated) cows and sheep, all by attaching an ultra-thin piezoelectric material to the organs. Impressively, the implanted fabric generated about a microwatt of power (one millionth of a watt)—roughly the amount needed to run a cardiac pacemaker.

If all this walking seems like too much effort, and you don’t like their idea of people wrapping fabric around your heart, never fear—you’re also full of hot air. Smart fabrics are being developed which incorporate “thermoelectric” materials to generate electric charge from a heat difference. This year, researchers from Australia and China synthesized the first-ever fabric capable of turning thermal energy into electricity. It hasn't been integrated into a garment yet, but during a trial in a heated room, the material generated an electric current when heated to body-like temperatures. It only produced about a nanowatt—a fraction of what piezoelectric fabrics are capable of—but much like the EP harvester chip, it’s a world first. Thermoelectric fabrics are definitely a space to watch.

Recycling energy from our bodies could provide a previously untapped source of power for electronic devices. But there's an even more abundant energy source under our skin that’s showing just as much, if not more, potential. It's the chemical fuels that our bodies burn...

Blood

In order to function properly, our cells require a continuous supply of chemical energy. Accordingly, our insides are brimming with it. If recent research is anything to go by, this internal fuel supply could soon be powering more than just your metabolism.
PROTEIN POWER

Like all fuel cells, enzymatic fuel cells (EFCs) work by extracting electrons from a fuel source via a chemical reaction known as oxidation. But unlike most fuel cells, which use metals like nickel and platinum to catalyze this fuel oxidation,EFCs use (you guessed it) enzymes—in this case, proteins, that normally play a vital role in cellular metabolism.
At the anode, an enzyme normally involved in sugar metabolism oxidizes glucose or some other molecule, freeing a hydrogen ion in the process. At the cathode, an enzyme called hydrogenase combines these hydrogen ions with oxygen to produce water. Electrons are released by the reaction at the anode and can be used for current before being transferred to water at the cathode.

Using enzymes as their catalysts gives EFCs great advantages over standard fuel cells. Not only does it allow them to extract power from biological fuel sources like glucose, but it also makes them more sustainable. The metals used in most fuel cells, such as platinum, are finite resources, which must be mined at great cost; enzymes, on the other hand, can be easily produced in the lab—or even borrowed for free from within a living being.
Probably the single biggest step toward harnessing the power of our bodies has been the development, in the last few decades, of enzymatic biofuel cells (EFCs)—small, battery-like devices which can generate electricity by breaking down the energy-rich chemicals in bodily fluids (see sidebar). The technology to create EFCs has existed for more than a decade, but in the past five years, researchers have begun to test them on—and in—living creatures.

When it comes to energy-rich bodily fluids, blood is hard to beat. Plasma, the liquid component of blood, is constantly suffused with dissolved glucose, our cells’ primary source of energy. Most EFCs that have been developed to date target this molecule. The first EFC that could draw power directly from an organism's bloodstream was created in 2010. Its French developers implanted the inch-long device into the abdomen of a live rat, where it operated successfully for 11 days—apparently without much discomfort on the part of the host. During this time, it continually generated around two microwatts of power, which is more than enough to power a pacemaker in theory.

By 2012, a far more powerful glucose EFC had been developed. Another French team (including researchers from the 2010 effort) constructed an improved, carbon nanotube-based EFC. When this was implanted into a rat’s abdomen, it generated around 40 microwatts of power, which the team actually used to operate both an LED and a digital thermometer.

Blood-glucose powered EFCs have yet to be tested in humans. But based on their success in animals (as well as rats, EFCs have also been shown to work in rabbits, lobsters, and cockroaches), these self-sustaining fuel cells could someday replace conventional batteries in medical implants, removing the need for risky replacement surgery that is currently needed.

For all its potential pros, using blood to generate electricity still comes with a serious con: you need to cut somebody open. And to access the quantities of blood needed for an EFC to work, a pinprick won’t do. While a one-time surgical implant might be acceptable for some patients, the risks and inconvenience attached to such procedures makes finding a less-invasive way to get at the body's chemical energy highly desirable. Thankfully, though, we're oozing with the stuff.