[♪ INTRO] Your body is at the center of a technological revolution.

The human body is one of the most complex machines on the planet. And as part of its everyday functions, it produces loads of data about us.

From the movement of our muscles, to our temperature and even the chemistry of our sweat. That data can help keep tabs on our health, improve our sports performance, and even measure our psychological state. To capture this information, we’re continually developing new, portable, medical devices that connect to our bodies, capable of measuring its different characteristics.

But to operate, most of these devices have one crucial need: electrical power. One of the ways to supply that power is using a battery, but they’re not always ideal. Batteries often need recharging or swapping out, which can be a pain.

Some batteries are also made of hazardous materials, while others might simply be inaccessible when it’s time to replace them. To get around these issues, researchers are coming up with ways to use the body itself as a way of generating power. Here are five of them.

One of the most apparent ways our bodies create energy is heat. You burn calories from the food you eat, and as you do so your body produces heat. Since the average surface temperature of our skin is between 30-40 degrees Celsius, we’re often warmer than our environment, which means that all that heat escapes into our surroundings.

But we can capture some of it and convert it into electrical power. In small electronic medical devices, electrical power is generated from the flow of electrons between two points in a circuit with different electrical potentials. In electrostatics, similar charges repel and like charges attract, because the electric fields they emit act on each other.

The kinetic energy a particle requires to move through that field, or the energy it gains from being moved by it, is what we call electrical potential energy. Some arrangements of charged materials and their electric fields create what we call an electric potential, which is a situation where a force drives charged particles in a current, and generates power. To use heat to drive an electrical current, we can use what are called thermoelectric generators or TEGs.

These are particular combinations of materials in which the electrical potential changes depending on how hot or cold it is. So if one side of the TEG is hot and the other is cold, it creates the difference in electrical potential we need. The temperature difference between our skin and our surroundings can create exactly those conditions.

Researchers have created skin-powered TEGs, capable of producing power in the nano- to micro-Watt range. That’s a lot less than everyday electronics, but enough for a small sensor. One study from 2011 used a TEG to power a sensor that measured oxygen content in blood through the skin of a person’s finger.

The TEG continually charged a small battery, which let the sensor operate in small bursts once it built up enough energy. Another way of using body heat is what’s called pyroelectricity. That’s when the molecular structure of the material changes in response to temperature.

The electrical charges of its component protons and electrons shift as a result, creating an electrical potential which depends on the temperature. Once again, a difference in hot and cold creates a difference in electrical potential, which can generate power. Admittedly, pyroelectric generators aren’t usually as powerful as TEGs.

But, the current a pyroelectric material produces is very sensitive to the temperature difference that causes it. So we can flip the script by measuring that current to estimate the temperature difference between our body and its surroundings! Basically, pyroelectric generators are like body-powered thermometers.

Interestingly, this kind of generator might have its widest uses in front of our mouths rather than on our skin. When we breathe, the air in front of us changes temperature enough for a small pyroelectric generator to produce a current. And since our breathing is tightly linked to the function of our body and sports capacity, a mask with a pyroelectric generator may one day monitor our respiratory health and athleticism.

Another form of energy we produce is the kinetic energy from the motion of our limbs. The right kind of device can convert the energy from that motion into electrical power. And that’s what triboelectric nanogenerators, or TENGs, do.

Triboelectricity, aka static electricity, is when electric charges build up on two different materials after coming into contact in a dry environment, like rubbing a balloon against your hair so it sticks to the walls, or how polystyrene packing peanuts can stick to your arm hairs. It happens because certain materials can transfer electrons to one another when they come into contact. When they separate, the electrons have a tendency to stay on the material they moved to.

So one ends up with an excess of negatively charged electrons, while the other ends up with a positive charge from a lack of electrons. That creates a difference in electrical potential between the two materials… which means we can attach them in a circuit, and use them to produce power! Triboelectricity can be generated by bringing two materials into contact head on and separating them again, or by rubbing them against one another.

TENGs involve exactly this kind of setup, with two materials that either bump into each other or slide when they get jostled around. And the motions of our body, like our limbs when we walk, can provide the movement to generate electrical power. One study attached a contact-separation style of TENG to a person’s arm as they walked.

It produced enough power for a heart rate monitor attached to the same person. In fact, TENGs can produce enough power for some conventional electronics, like LED lights. And just like in the case of heat, we can reverse the process too and use the current generated by a TENG to measure the amount of movement that someone is generating.

That’s because the more vigorous the motion, the more power the TENG will generate. Studies have used TENGs in clinical settings by attaching to someone’s abdomen to monitor their breathing. Asides from big, visible movements like walking or breathing, there are more subtle movements our bodies make, like our pulse or even the motion of our eyes.

Those are capable of powering sensors, too. This time, the answer comes from a different kind of tech, called a piezoelectric nanogenerator or PENG. We swear, we’re not making these up!

PENGs also convert the kinetic energy from motion into power, but in a different way from our previous example. The name comes from the piezoelectric effect. Piezoelectric materials have a regular, repeated pattern of atoms in a crystal structure.

When pressure is applied to the face of the material, or if it’s put under tension, the crystal structure becomes distorted. That means the electric charges within the structure move out of alignment, so that positive and negative charges are slightly skewed towards different sides. And just as we’ve seen for other kinds of materials, that difference creates a potential difference, which means we can connect it up to a circuit to generate power.

PENGs can be used similarly to TENGs and electromagnetic generators to harvest the energy from breathing and walking. But since PENGs don’t require separate moving parts, they can be made thinner and more flexible for certain applications. While they might not generate much power that way, we can also reverse things and use piezoelectricity to measure tiny motions in the body in a self-powered way.

In studies, scientists have used piezoelectric sensors to measure processes like coughing and even a pulse through skin. One study published online in 2020 showed that piezoelectric sensors could detect the motion of an eyeball when attached on the skin nearby, which might pave the way for futuristic eye-sensitive user interfaces. So far, we’ve talked about using the processes of the human body to create power.

Sometimes, however, we might be able to use a conventional battery, but use some clever tricks to make them safer and more stable. In fact, our bodies can help control exactly how the battery turns on in a safe way, using sweat. Sweat is what’s called an electrolyte.

That’s because as well as water, it contains charged particles like sodium ions, which allows it to conduct electrical currents. And since it’s conducting, sweat can be used as a kind of electrical switch to turn a device on by completing a circuit. This setup is called a sweat-activated cell or SAC.

Two parts of a chemical battery are separated with an array of chambers that can soak up small amounts of fluid. Basically a kind of sponge for sweat. It can then be attached to your skin so when you sweat, the fluid gets sucked up into the chamber and conducts a current between two ends of the battery, completing the circuit and turning the device on.

In a 2020 study, researchers used an SAC to power an ECG attached to four participants who were using a stationary bike, measuring the electrical activity and rhythm of their hearts as they exercised. One drawback to this approach is it relies on the user being active enough to sweat, though that might be exactly what you want to preserve the battery life of some devices. All the same, researchers are hoping to develop new SACs in future that rely on just trace amounts of sweat to operate.

Finally, while the generators we’ve mentioned so far have involved being in contact with, or near, your skin, there’s a source of electrolytes inside the body that can be used to help power internal body sensors: stomach acid. Like sweat, stomach acid is conducting and can help complete a circuit, generating a flow of electrical current between two electrical potentials from a battery. In this case, the device itself would be something you swallow, like a pill with some circuitry inside.

There are already devices like these capable of measuring the pressure inside our stomachs, their acidity, and our respiration. They can even fit tiny cameras in to shoot videos of our insides! But most of them currently rely on batteries, which sometimes use materials that could be hazardous if they leaked into the body or were inside us for too long.

To develop a safer alternative, researchers wanted to see if they could use stomach acid as the conducting fluid for a sensor battery. In a 2017 paper, they developed a special pill with a tiny electrode sticking out to use stomach acid for powering a small, wireless thermometer. The battery itself was made of copper and zinc, which are safe for entering the bodies of mammals.

They then fed the pills to pigs, where they stayed inside the animals for about a week, measuring their internal temperature before being passed out the other end. Sure enough, the pigs were fine. That makes sense, since the small amounts of zinc were comparable to the amount we absorb from the food we eat.

But further tests are needed to make sure that the amounts would be safe for humans. Even though the pills only generated less than a microwatt of power, that could be enough to periodically charge up a sensor inside the body if it sticks around inside us for long enough. Eventually, advances like this might overcome all the hurdles so that the idea of an ingestible electronic pill becomes… a little easier to swallow.

Wearable electronics may seem trendy, but the ways we’ve developed to harness our bodies’ energy are seriously impressive. And what that can tell us about our health is even cooler. Thanks for watching this SciShow List Show.

Unfortunately, I don’t have time to list off all of the amazing patrons who helped to make it happen. If you’d like to join the amazing community of people who help make SciShow every day, you can get started at patreon.com/scishow. [♪ OUTRO]