Everything from industrial equipment to the human body loses some of the energy it uses to things like heat and vibrations. The ability to harvest some of this energy is usually pretty limited, as small heat differences and weak movements are difficult to concentrate into significant amounts of useful energy. But even an inefficient conversion can be sufficient to provide power for small energy-efficient devices, such as medical implants and short-range transmitters, so researchers are working on developing materials that can convert environmental noise into small amounts of useful energy. In a recent example of this work, researchers have demonstrated that they can print a bio-compatible device that can harvest the stress created when it's flexed to produce over 10 nanoAmps of current.
The device relies on the piezoelectric effect. A number of crystals, when stressed, create small amounts of current. That stress doesn't have to be extreme—the vibrations, flexing, and twisting that normally occur in many situations is sufficient to create a small charge. The individual events may not be enough to do much, but combined with a good rechargeable battery or capacitor, they can be sufficient to provide enough power for devices that only operate intermittently. The lab behind the new work, for example, created a piezoelectric device that filled capacitors with enough power to run an LED.
There have been a number of hurdles to clear in order to make these devices, however. If they're overly large, the crystals can break under the strain of typical flexing, ruining the hardware. So both the piezoelectric material, along with the wiring necessary to harvest the potential that develops in it, have to be made at very small scales (on the order of tens of micrometers). For common usage, we'd also have to avoid using any materials that have problems with toxicity, especially if we're considering these for use in medical devices.
The new work involves barium titanate (BaTiO3), which is apparently biocompatible and comes from a family of materials that have excellent piezoelectric performance. But, perhaps more significantly, it describes how to print a large number of piezoelectric crystals onto a flexible substrate.
The first step in the process is a layer-by-layer deposition of the barium titanate and two conductive metals on top of a hard silicon substrate. That material is then etched to both cut it free from the underlying silicon (which doesn't flex well) and to create an array micrometer-sized piezoelectric device. This is where the printing comes in. A flexible plastic stamp can be used to pick up these pieces and deposit them in an organized array within some epoxy, which is then cured to lock them in place. Wiring is dropped on top to link everything up, and then a new layer of epoxy seals the whole thing up. A flexible plastic sheet gives the whole thing some robustness.
With everything wired up, the researchers put it to the test by having someone pick it up and flex the plastic (don't worry—he or she was wearing gloves). Each flex triggered a short pulse of current (about 10 nanoAmps); releasing the strain produced a similar burst with an opposite polarity. Each of these was about a third of a volt. The authors calculate the power density of their device as about seven milliWatts per cubic centimeter.
Again, that's not going to power the next-generation electric car. But it could be sufficient to charge a small device that only needs to operate sporadically. The researchers envision harvesting power from things like the flexing of hiking boots or the changes in the chest that accompany breathing.
The authors wrap up by pointing out that there are other materials similar to barium titanate that have even better piezoelectric properties, so this new device may be on the low end of what's possible. They're not the only lab working on this, so these flexible devices may find their way into some real-world applications before too long.