January 20, 2016 at 00:19This is Science: wearable electronics and triboelectricity. Part 1
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Just some 10-15 years ago, scientists and engineers only dreamed that wearable electronics (fitness bracelets, fashionable smart watches and various sensors) would be firmly rooted in everyday life. Today, we are already discussing the Internet of Things (IoT) and smart devices that can be used to control our home via the Internet.
In fact, here and now we are witnessing the transition of “wearable” electronics to the “built into the human body” stage, when the same fitness bracelet will be enough to stick on the wrist before leaving the house, or to embed a chip with additional memory directly into the brain .
Just a year ago it was publishedarticle on triboelectricity , in which the basic principles of the so-called tribo-electric generators (TEG) were demonstrated. Following her, due to the abundance of material (articles were published and continue to be published in the most prestigious scientific journals with enviable regularity, just like cars from the Henry Ford factory) a whole review of this technology was already followed . Therefore, due to the abundance of material accumulated over the past year, this article will be divided into two parts devoted to two main issues of wearable electronics: in fact, where to get electricity and which devices can be powered with this energy.
Here, for example, the battery in a modern fitness bracelet can occupy half or even more than its volume. Perhaps, with the elements or, I would say, “ways” of nutrition, we will begin.
Triboelectricity- the potential difference and, accordingly, the separation of charges that occurs when two materials rub against each other. This phenomenon is somewhat reminiscent of the well-known static electricity. A triboelectricity generator can be made by a simple combination of two materials that can be charged negatively or positively (panel a) in the figure below). Simple movements, such as bending, stretching and compression, displace the two selected materials, resulting in a potential difference being generated, and, if you like, the charge is redistributed between the materials in contact, which is essentially an electric current (panel b).
Flexible triboelectric generators are an alternative method of converting the surrounding mechanical energy (vibration, motion) into electrical energy.
a) Examples of the most common triboelectric materials capable of accumulating positive and negative charges. b) A schematic description of the principle of operation of a triboelectric generator (TEG). c) The main sectors of application of triboelectric generators - from smart lenses to charging smart watches and glasses, as well as the required power level of the generator.
Currently, Korean researchers are actively engaged in these developments, who have already created a huge number of the most diverse triboelectric generators, covering power values from fractions of microW / cm 2 (we talked about them in the previous review ) to tens of W / cm 2, which, you see, is already enough to power smart glasses or watches. Well, let's talk about some interesting examples in this area in a little more detail.
The review article , Triboelectric Generators and Sensors for Self-Powered Wearable Electronics, is published in ACSNano ( DOI: 10.1021 / acsnano.5b01478 ).
Textiles as a basis for triboelectric generators
Have you ever wondered how many times a day our clothes undergo bends, squeezes and stretches when we move ?! It seems to us trifling, however, the generated electricity during the deformation of clothes may well be enough to charge a fitness bracelet or sensors on the body.
For example, researchers from the Center for Human Interface Nanotechnology, located in South Korea, together with their colleagues from the Australian Institute for Superconducting and Electronic Materials created a special textile material that allows you to turn the mechanical energy of movements into electrical energy.
To create such a triboelectric nanogenerator (TriboElectric NanoGenerator - TENG), a conductive fabric was used, onto which zinc oxide rods (ZnO) coated with a polymeric dimethylsiloxane insulating sheath (PDMS) were then applied . The resulting textiles can withstand up to 12,000 cycles of load (squeezing), while producing about 170 V and 65 microA, which is equivalent to 1.1 mW of power. In this case, the required force is only 10 kgf (~ 100 H).
(ab) Schematic representation of the fabric created: TENG was placed between layers of silver textured, in other words, conductive textile. (c-d) Photographs of a real prototype at the micro and macro level, respectively.
The principle of operation of the device is shown in the diagram below. When squeezing, two layers of the device are in contact (a), the redistribution of charges (b), causing electric current, if we weaken the external effect (c). Then the system returns to the equilibrium state (d) and with the next pressing the current is already running in the opposite direction (e).
Step-by-step scheme of the triboelectric nanogenerator (TENG) operation.
The resulting textile in its characteristics is not much different from ordinary fabric. It can be embedded in clothes as (yet ?!) a bracelet and used to power various devices built into smart clothes - for example, various identification marks or even a remote control from a car!
The introduction of triboelectric nanogenerators using smart clothes as an example
The original article “Nanopatterned Textile-Based Wearable Triboelectric Nanogenerator” is published in ACSNano ( DOI: 10.1021 / nn507221f ).
Another group of South Korean scientists from the universities of Ulsan (Ulsan National Institute of Science and Technology) and Suwon (Sungkyunkwan University) together with their American colleagues from Atlanta (Georgia Institute of Technology) proposed a way to create a non-sandwich from various structured fabrics, as we saw earlier, and the fibers themselves, which are already woven into any fabric (in the described article, water-repellent fabric was used).
The basis of such fibers is aluminum foil with a nanostructured layer of the same insulating polymer PDMS and an aluminum micro-size wire coated with zinc oxide rods ZnO.
A schematic representation of a new type of thread woven into a fabric to create triboelectric generators - something like a worm that absorbs harvesters from Dune 2000, isn't it ?!
This development has several advantages. For example, it is not necessary to create a new type of fabric and new production, it is enough just to weave the obtained fibers into almost any modern fabric material. On the other hand, even for individual fibers, the researchers managed to obtain a power of about 4 mW (40 V at a current of 120 microA), while the material can stretch to 25% in one direction.
Electrical characteristics of the obtained triboelectric tissue samples
Moreover, the obtained triboelectric tissue can be simultaneously used as a sensor, since the generated current is proportional to the degree of extension. This development, with a high degree of probability, will be of interest to athletes and medical professionals for the accurate detection of the movements of athletes and patients.
Triboelectric fabric motion sensor: current proportional to tensile strength
The original article , Highly Stretchable 2D Fabrics for Wearable Triboelectric Nanogenerator under Harsh Environments, is published in ACSNano ( DOI: 10.1021 / acsnano.5b02010 ).
High power for smart watches
And finally, we got to the most powerful devices based on triboelectric generators. Chinese researchers and their American associates managed to create a TEG that produces up to 10 4 W / m 2 of power at a current of 10 4 A / m 2 . This is an absolute record for such devices today!
One version of such a triboelectric generator is shown in the figure below. This is the so-called rotary TEG, in which, according to scientists, a longer contact time of the “electrodes” is achieved to remove the accumulated charge and, consequently, the current increases proportionally.
a) Diagram of the presented device. b) The dependence of the generated current on the speed of rotation. c) Generated current at 1600 rpm and an external load of 500 Ohms installed after the diode bridge.
Created by TEG is quite compact. The area of two rotating planes is only 25 cm 2 (that is, the linear dimensions of such a generator are 5-7 cm).
Schematic diagram of the operation of the device based on the presented triboelectric generator
It is hoped that further developments in this field will create an even more powerful, but at the same time miniature triboelectric generator for integration with existing portable devices.
The original article , Pulsed Nanogenerator with Huge Instantaneous Output Power Density, is published in ACSNano ( DOI: 10.1021 / nn403151t ).
Instead of output
So, today we got acquainted with the main achievements in the field of creating triboelectric generators covering a wide range of capacities of produced electricity: from mW / m 2 to W / m 2 (and in some cases, kilowatts!). Over the past two years, many articles have been published, the generators themselves have evolved from a huge box to miniature fibers woven into ordinary fabric. And, knowing the Koreans, we can confidently count on the imminent appearance of triboelectric generators in real smartphones or fitness bracelets.
Wait and see!
And in the next part, we turn to examples of real wearable electronics that have already been created in laboratories and are waiting in line at the doors of the engineering bureaus of microelectronics manufacturers.
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PS: On this significant day, we opened the Prestigio page on GT . Some of the community members already know and use Prestigio devices and products, someone else has yet to get to know them, which, of course, we will help, but we plan to write on the most varied topics: from wearable electronics to an honest review of products.
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This is Science: wearable electronics and triboelectricity. Part 1
This is Science: Wearable Electronics and Triboelectricity. Part 2