Nanogenerators harvest body’s energy to power devices

Ask not what your gadgets can do for you; ask what you can do for your gadgets.

In the race to create bionic humans, researchers are nanometers away from turning people into device chargers. Instruments called nanogenerators can harvest energy from swinging limbs, jiggling skin and ballooning lungs. And that energy can power wearable and implantable gizmos, such as pacemakers, muscle sensors, tumor detectors and even a bone-growing laser.

“This whole area is an incredibly exciting area,” says materials scientist Michael McAlpine of the University of Minnesota in Minneapolis. Merging electronic gadgets directly with the human body is where science is headed, he says, citing examples like a 3-D printed bionic ear made with human cells and a coil antenna. But such body-friendly machinery needs power, and no one wants dangling cords or batteries that require a surgeon to replace.

Nanogenerators could be worn or implanted to endlessly and cordlessly power medical gadgets for everything from diagnosis to long-term treatments, several recent studies suggest. For instance, a nanogenerator on a swinging elbow could power an implantable laser that spurs bone growth, scientists report in the Aug. 25 ACS Nano. Another group, publishing in the July Nature Materials, report designing a nanogenerator that uses tiny tremors in skin to help diagnose tumors.

Such generators come in two varieties: piezoelectric and triboelectric.

Piezoelectric nanogenerators rely on certain materials that create charge when their underlying crystal structures are bent or squeezed (SN: 7/30/11, p. 18). First reported in 2006 by Zhong Lin Wang and Jinhui Song at Georgia Tech in Atlanta, these generators can drive electrical current through electrodes that surround the crystalline material. A handful of materials, including topaz and cane sugar, have this electrifying effect. But lead zirconate titanate, called PZT, is favored for its energy-generating potential, says materials scientist John Rogers of the University of Illinois at Urbana-Champaign.

Rogers and colleagues used nanoribbons of PZT to make a tumor-detecting skin patch. The square-inch sticky patch, about the thickness of a temporary tattoo, includes tiny devices that send vibrations through the skin. When the skin jiggles back, the patch’s PZT nanogenerators wrinkle, generating an electrical signal. By analyzing the pattern and size of that signal, researchers can determine if the skin is springy or stiff. Firm skin is a typical sign of skin tumors that doctors check for by touch, Rogers says. The electrified skin patch lets doctors more precisely map tumors and other skin malignancies.

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After developing their first piezoelectric nanogenerator in 2006, researchers in Wang’s lab continued looking for other ways to collect energy from body motion. In 2012, the group reported a second method: triboelectric nanogenerators. This approach relies on two materials that generate charge when pressed or rubbed together, like creating static electricity by sliding a balloon across hair.

In these nanogenerators, the two rubbing materials are stacked together with a small gap in between that can close when the generator is squeezed. Surrounding layers of metal electrodes harvest current with every press. Since its introduction, researchers have tweaked the design and material pairings, boosting the generator’s initially tiny power haul to 0.05 watts per square centimeter of material. A typical generator with an area of several square centimeters could easily power many types of devices, Wang says, including some pacemakers, which can run on thousandths of a watt.

Last year in Advanced Materials, Wang and colleagues reported implanting one of these triboelectric generators in a rat’s chest, harvesting energy with every little breath. About five rodent breaths could fuel a beat of a human pacemaker, the researchers calculated. Based on that finding, the researchers estimate that in human chests, which can tote a larger generator capable of harvesting more energy, each breath could power a mechanical beat. Wang says that he is now working with colleagues in China to see if implanted nanogenerators can run pacemakers in pigs.                                                                                                

In the meantime, Wang and colleagues have shown that a triboelectric nanogenerator can power an implantable, infrared laser. The laser treatment, still in development itself, spurs bone cells to grow and may help restore broken or degenerating bones, Wang says. By strapping the nanogenerator to a person’s swinging elbow, the researchers calculated that it could power at least one laser zap per minute.

“This technology is promising; it has huge potential,” says materials scientist Sang-Woo Kim of the Sungkyunkwan University in Suwon, South Korea. On August 17, Kim and colleagues reported in ACS Nano that a triboelectric nanogenerator skin patch could monitor skin and muscle movements to diagnose injuries and help track rehabilitation.  But, he cautions, such nanogenerator applications are very young and will need far more testing before they reach clinics.

Materials scientist Xudong Wang of University of Wisconsin–Madison agrees. There’s growing interest in using these nanogenerators in bionics, he says. But with the need to conduct lengthy clinical trials before these devices reach patients, it’s likely that these generators will first show up in places such as shoes that could charge cellphones or smart keyboards that could boost online security by identifying a user’s electronic signature.