Tuesday, 24 March 2015

‘Smart’ clothes generate electricity

New fabric harvests energy from its wearer.

You’ll get a charge out of the clothes of the future. Scientists in South Korea have developed a flexible, foldable and wearable fabric that generates electricity as it bends and flexes. A person wearing a shirt tailored from the material only has to move around to power a small screen or other electronic devices.

The advance represents an important step toward making wearable power sources a reality, says Yunlong Zi. He’s a physicist at the Georgia Institute of Technology, in Atlanta, who did not work on the new fabric. In his own lab, he studies ways to harvest energy. “Cell phones need batteries, but batteries have limited life,” he notes. With clothing that can generate electricity, he notes, that’s no longer an issue: “You can make power by yourself.”

Sang-Woo Kim led the development of this new material. He works at Sungkyunkwan University in Suwon, South Korea. A shirt made from the new fabric can be worn — even patched — like any other item of clothing. “It feels like an ordinary jacket,” he told Science News for Students.

Fully equipped, it's just a tad on the heavy side, he acknowledges. That added weight comes from the electronic gizmos the researchers wired into the shirt. For tests, these included small screens, lights and even a keyless remote. Press the shirt’s cuff, for instance, and the remote unlocks a car's doors.

How it works

The power-generating material is known as a wearable triboelectric (TRI-bo-ee-LEK-trik) nanogenerator, or WTNG. Here’s what that means: Triboelectricity refers to electricity generated by friction. Friction is the resistance encountered when one material moves over or through another material. People feel friction (in the form of heat) when they rub their hands together. In fact, the prefix tribo comes from the Greek word for rubbing. Meanwhile, nano is a prefix meaning a billionth. The material includes tiny zinc-oxide rods only billionths of a meter long. Those spiky nanoparticles help convert motion into electricity.

Some kinds of triboelectricity are familiar. When a person rubs her head with a balloon, her hair stands on end. That's because the balloon “steals” negatively charged particles called electrons from her hair. The balloon ends up with a negative charge. The hair with a positive charge. Opposite charges attract. So the positive hair stands up to reach the negative balloon. This is static electricity, and it is triboelectric.

The new fabric combines different materials. The top and bottom layers are a cloth coated with silver. The middle layer contains the zinc oxide coated with a polymer. (A polymer is a substance whose molecules are made of long chains of repeating groups of atoms.)

When the fabric bends or moves, the coated rods move back and forth against the silver. The movement produces a reaction similar to that in the hair-and-balloon example. Here, it’s the polymer layer that picks up electrons from the silver layer.

The researchers connected the two outer silver layers with a wire. The wire lets a small electric current run through it. As they compressed and released the fabric, the scientists measured that current. Multiple layers of WTNG produced more electricity than single layers, they showed.

The smart shirt generated enough electricity to power a small screen. It glowed. The shirt also lighted up an array of small light-emitting diodes. And it powered the keyless remote. In the near future, Kim's team wants to develop textile-based batteries. These should be able to store energy. For now, the shirt only works when someone is moving. The researchers also are working on a washable version.

The first flexible electronics appeared only a few years ago, and only in the lab. Researchers hope to use the technology to build wearable electronics, including medical sensors that stick to the skin. These currently require an external power source. By taking mechanical energy that would go to waste and converting it to electricity, WTNGs might one day give people the power to recharge our own electronic devices as we move throughout the day.

Power Words

battery A device that can convert chemical energy into electrical energy.

electric charge The physical property responsible for electric force; it can be negative or positive.

electric current A flow of charge, called electricity, usually from the movement of negatively charged particles, called electrons.

electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.

electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

friction The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage the surface of the materials rubbing against one another.

generator A device used to convert mechanical energy into electrical energy.

light emitting diodes (LEDs) Electronic components that, as their name suggests, emit light when electricity flows through them. LEDs are very energy-efficient and often can be very bright. They have lately been replacing conventional lights in auto taillights and in some bulbs used for home lighting.

nano A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.

physicist A scientist who studies the nature and properties of matter and energy.

polymer Substances whose molecules are made of long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example)

sensor A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

static electricity The buildup of excess electric charge on some surface instead of flowing through a material. This charge buildup tends to develop when two things that are not good conductors of electricity rub together. This allows electrons from one of the objects to be picked up and collected by the other.

triboelectric A term for an electric charge that develops when two things rub against each other, causing friction.

zinc oxide A chemical — with the formula ZnO — made when one atom of zinc bonds to an atom of oxygen. Its reflective properties make zinc oxide useful in a range of products, from paints to sunscreens. Its germ-killing properties make it useful in some medicines or cosmetics. In foods, it can even be used as a source of the mineral zinc.