There has been a large shift in the last two decades towards IoT devices. This has resulted in a massive increase in devices requiring mobile power supplies.
The conventional approach to solving the mobile power demand was to manufacture batteries, which have a few problems associated with them. Batteries have limited sustainability and over time will need to be replaced. If a device only needs a few batteries to retain power, it may not be difficult to replace them; however, the more batteries the device requires, the more difficult the task becomes to replace them. Batteries can also potentially cause environmental issues due to the chemicals batteries use to create power.
Thus far, the limitations that batteries present have resulted in a development shift towards devices that consume small amounts of power. However, they've also created a research demand for devices that use alternatives to batteries, such as self-powered systems that harvest energy from the environment around them. This emerging interest has resulted in a shift towards micro and nano power sources.
One such energy-harvesting power source, invented in 2012, has since gotten an upgrade that may help solve the issue of powering wearable devices.
Harnessing the Triboelectric Effect
In 2012, a new type of electrostatic generator was invented called a triboelectric nanogenerator— or TENG for short. Since then, the initial researchers Zhong Lin Wang, along with his colleagues from the Georgia Institute of Technology have been continually researching the technology, seeking greater efficiency, adaptability, as well as means of production.
A TENG unit. Image courtesy of the Georgia Institute of Technology.
A TENG is based on the principles of the triboelectric effect, which is a method of generating electricity from contact friction, also known as contact electrification.
A set of materials can be “rubbed” together creating frictional contact that results in the transfer of electrons from the atoms in each material. The electron affinity of different materials, for example fur and glass, will cause the two materials to exchange electrons. Since the glass has a higher affinity, it will become positively charged; and the fur will gain a negative charge since it will obtain an excess of electrons after contact.
In 2012, the initial research team arranged materials in a way that would generate electricity by pressing them together. The materials were designed with alternating ridges that entangled them together, ultimately making use of the mechanical energy of friction to generate electricity. The energy conversion ratio has ranged anywhere from 50-85% in TENG technology. However, the technology has been limited to use in rigid and rigid-flex circuitry.
The research was recently furthered when it was augmented to be used in flexible as well as arbitrarily surfaced electronics.
Gif courtesy of Electro Static Technology.
The Self-Adapting TENG (saTENG)
The Georgia Tech researchers went one step further and created a new type of TENG which they coined the Self-Adapting TENG (saTENG.) The new approach greatly increases the stretchability, deformation ability, and scalability of energy harvesters.
Instead of using the TENG's solid-based nanogenerator material, the research team adapted the material to use a conductive liquid electrode and an elastic polymer cover. The liquid electrode combined with the elastic cover increases the strain capabilities of the newly adapted saTENG to over 300% without degradation of electrical capabilities.
The device is also capable of using water as an electrode, significantly enlarges areas of development and potential prospects for the device.
The unit primarily consists of a rubber layer containing a liquid electrode.
Representation of a basic saTENG unit. Image courtesy of the Journal of Science Advances.
Wearable Energy Harvesting
The saTENG is also capable of running in various different modes of operation varying from freestanding mode, single electrode mode, and attached electrode mode (contact and sliding). When the triboelectric parts are displaced vertically in contact and horizontally in sliding mode, they create a difference in electric potential, forcing electrons to flow through the device load.
Various simulations and tests were run with extraordinary results. The research team tested the range of applications of the device including attaching a saTENG unit to a human wrist. The saTENG was able to function with moving parts quite impressively, as it was capable of driving more than 80 LEDs by harvesting the mechanical energy of a tapping motion.
The saTENG as an energy-harvesting wearable. Image courtesy of the Journal of Science Advances.
In each demonstration, the researchers used ordinary tap water as an electrode and a human body as a ground due to our natural size and conductivity. The devices are also capable of taking the energy generated from mechanical motion and storing it in batteries and capacitors.
Image courtesy of the Journal of Science Advances.
The saTENG has not only been used as a wearable energy device but also as an energy harvester operating on flowing water. This research impressively broadens design convenience and significantly increases manufacturing opportunities ranging from biomedical sensors to energy harvesters.
The original research paper can be found in the Journal of Science Advances.