In areas such as the development of implants, IoT nodes, or electronic bandages, it is often difficult to ensure a stable power supply. Because of this, for several years scientists throughout the world have been looking into a mysterious, ancient principle.
Friction exists everywhere and at all times and is often accompanied by frictional or static electricity. This phenomenon was already known to the Pre-Socratic natural philosopher Thales of Miletus in ancient times. If you rub amber with cotton or a “Greek” cat skin, it absorbs electrons. The negative charge then remains on the surface, where it generates a static electric field which, for example, attracts dust and fluff. Consequently, in upper-class Greek households, the yellow fossil resin was used as a clothes brush. This was long before it gave its name to the electron and, ultimately, electricity (Ancient Greek elektron = amber).
However, although the terms frictional electricity or “triboelectric effect” (Greek tribein = rub) would suggest otherwise, static charging of materials actually requires the separation of surfaces that were previously in “intensive” contact (max. gap 10 nm) with each other. One of the participating materials must be a bad electric conductor (dielectric material). The level of charge is also dependent on the temperature, humidity, and speed of separation. For example, fast separation results in higher charges as there is less return current. The generated potential differences (voltages) can quickly reach the kilovolt range.
Triboelectricity for mini electronics
Naturally, electricity that is available everywhere awakens interest among developers in many areas of modern electronics. However, the mysterious frictional electricity was only made accessible through advances in nanotechnology during the last few years.
As the name electrostatics says, this form of electricity is an electric charge at rest. To set it in motion, scientists at the University of Science and Technology in Beijing use triboelectric nanogenerators (TENG). These “energy harvesters” are simple and inexpensive to produce, efficient, extremely malleable and they are also environmentally friendly.
In principle, the shape-adaptive TENG (saTENG) consists of a salt solution as an electrode and a skin of elastic polymer with a nanostructured surface. Contact with a grounded object and the deformation of the generator cause the electrode to alternately receive and release electrons.
Attached to the sole of a shoe, the generator provides a “thrust” of electrons with every step. Flooring materials or mattresses could also be used to provide electricity. In an experiment, a roughly one square meter “saTENG mat” was able to illuminate 170 LEDs with repeated “intensive” contact with a foot-sized acrylic sheet.
The 3D-printed nanogenerator from the Clemson Nanomaterial Institute (CNI) in South Carolina not only provides triboelectricity, the electrostatic field can also be converted into a remote control.
Goldfinger as an electricity provider
The nanogenerators from two institutes in Beijing, China and Buffalo, New York fit on an index finger. When the finger is bent, a silicone-based polymer and two thin layers of gold rub together. The structure provides up to 124 volts and ten microamperes at a power density of 0.22 milliwatts per square centimeter. Enough to light up 48 LEDs.
Scientists at the University at Buffalo carried out some more detailed research into the triboelectricity phenomenon in a recently published study. According to them, the “hair-raising” phenomenon is due to tiny structural changes that occur on the surfaces of materials when they come into contact with each other. Computer models and physical experiments provided corresponding results and can now help technology companies to develop more sustainable energy supplies for small electronic devices with this technology.