From footfalls to button pushes, scientists seek to generate current to power small devices.

From our footsteps to our button presses, humans are constantly expending energy, and researchers are tapping into these movements to power the world around us.

Harvesting energy from one’s surroundings or activities rather than from a battery or a wall outlet has some key advantages: The electricity sources are free and the devices are more mobile. This is particularly useful for medical electronics like insulin pumps and pacemakers. Energy harvesters could also prolong battery life in smartphones and laptops.

The idea isn’t new. Crystal radios, for example, have been around since the start of the 20th century and do not need a dedicated power source, since they scavenge electricity from radio waves. But current energy harvesting strategies are not very efficient, and the energy in our environment is diffused widely, meaning bright lights, large temperature gradients or long, brisk walks are needed to produce an appreciable amount of power, which may still not be very much.

Now researchers are findings new ways to increase harvest efficiencies and lower costs. Developers are also making electronics that use much less power, so that one day your phone may run on the rustling in your pocket and a few finger taps when you make a call.

At Lawrence Berkeley National Laboratory, scientists demonstrated a device last month that uses viruses to translate pressure into electricity. The device relies on piezoelectricity, a phenomenon where an electrical charge is produced in a material when it is mechanically deformed or stressed. In this case, the team used an engineered M13 virus that typically infects bacteria to make the material.

The fun starts at 50 nanoamps
The device can produce 6 nanoamps of current and 400 millivolts of potential, roughly one-quarter the output of a triple-A battery and enough to briefly activate a small monochrome liquid-crystal display with a square-centimeter sized generator when pressed. The researchers published their report in Nature Nanotechnology.

Ramamoorthy Ramesh, a researcher in the materials science division at LBNL and one of the report’s co-authors, explained that viruses can reproduce themselves and form nanometer-scale structures on their own, making them an attractive low-cost alternative to conventional piezoelectric devices, which can use expensive or toxic chemicals. The virus material can also be sprayed onto a surface, potentially turning any wall or floor into an energy harvester from footsteps and vibrations.

Right now, the virus generator is too weak to provide any practical power, though the researchers are making improvements and say they are not far off from a useful product.

“If we get 50 to 70 [nanoamps], then it’s rock ‘n’ roll time. Then it’s a lot of fun,” Ramesh said.

Researchers in the United Kingdom also recently developed a piezoelectric generator, a knee brace that scrounges electrons from walking. As the wearer’s knee bends, four metal vanes in the device are “plucked,” which then vibrate like a guitar string and produce electricity.

Currently, the device produces about 2 milliwatts of power, but researchers expect to reach 30 milliwatts with a few tweaks.

Michele Pozzi, the project’s lead researcher and a research fellow in energy harvesting at the School of Applied Sciences at Cranfield University, said in a release that he expects the device to cost £10 per unit when production is ramped up. The findings were published earlier this month in Smart Materials and Structures.

But can it power a television?
Still, even with the energy harvesters on the market, the big hurdle is getting enough usable electricity to power a sensor, lamp or screen. This means producing more electricity than the device needs so it has enough to store and remain consistently operational.

“In some cases, you may be only harvesting a microwatt or so,” said David Freeman, chief technologist in the power supply solutions group at Texas Instruments. “If your device requires a microwatt, you aren’t doing anything for anybody.”

For Texas Instruments, the solution is to make the devices that use less electricity as well as to make harvesters more efficient.

“It’s only within the last three, four or five years that the [harvesters] reported enough electricity to be useful and the energy of the devices were low enough to work,” Freeman said.

One potential use for harvesters is building sensors that can monitor air quality. Operators can wirelessly collect that information to efficiently heat or cool their spaces. “The target for most of these applications is ‘peel-and-stick,'” explained Freeman, noting that the best location for a sensor may be far away from any electrical wiring. Such locations may also be hard to get to, so changing a battery frequently would be too inconvenient, making them ideal uses for harvesters.

To this end, the company is making integrated circuits and microprocessors that need much less energy.

“Every generation uses less power than the generation before,” Freeman said. “As we continue to push the power down needed by these devices, then it makes the harvesting piece more practical.”

According to Freeman, the dominant harvesting technology is small photovoltaic panels, since vibrational and radio harvesters don’t yet capture enough power to run both the sensor and the transmitter. The company recently made a prototype wireless keyboard that runs on indoor lighting with a battery life that matches the operating life of the device, roughly three to five years. In addition, Texas Instruments is developing sensors to monitor industrial equipment and roadways, where the regular movement frequencies may be more suited to vibrational harvesters.

For more information regarding this post, please visit ScientificAmerican.com.

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