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Towards power-packed wearables

Aug '18
Courtesy: Johan Bodell/Chalmers University of Technology
Courtesy: Johan Bodell/Chalmers University of Technology
Researchers at a Swedish university have developed a fabric that converts kinetic energy into electric power. Subir Ghosh reports.

That one form of energy can be transformed into another is a fundamental law of nature, nay physics. So, when someone says that kinetic energy can be converted into electricity, that would raise no eyebrows. But when someone claims to have developed a fabric which does precisely that, many in the textiles industry would be snapped into rapt attention.

Such a fabric has indeed been developed by researchers at the Chalmers University of Technology, Gothenburg, Sweden. Researchers Anja Lund and Christian Müller recently announced a woven fabric that generates electricity when stretched or exposed to pressure. It can generate enough power to light an LED, send wireless signals or drive small electric units such as a pocket calculator or a digital watch.

According to the university, “The technology is based on the piezoelectric effect, which results in the generation of electricity from deformation of a piezoelectric material, such as when it is stretched. In the study the researchers created a textile by weaving a piezoelectric yarn together with an electrically conducting yarn, which is required to transport the generated electric current.”

Lund had remarked at the time, “The textile is flexible and soft and becomes even more efficient when moist or wet. To demonstrate the results from our research we use a piece of the textile in the shoulder strap of a bag. The heavier the weight packed in the bag and the more of the bag that consists of our fabric, the more electric power we obtain. When our bag is loaded with 3 kilos of books, we produce a continuous output of 4 microwatts. That’s enough to intermittently light an LED. By making an entire bag from our textile, we could get enough energy to transmit wireless signals.”

But this had been in the works for a while. Lund told this writer, “This research started about 10 years ago as my PhD project, and the topic was selected from curiosity. I have a background in both electronic and textile engineering, and I was keen to combine the two. After several years of research, we were able to manufacture piezoelectric textile fibres, which generate an electric voltage when stretched or compressed. The piezoelectric polymer we use had not previously been used in fibre form, so we had to find the right processing parameters in order to induce the piezoelectric effect, as well as find a way to integrate an electrically conducting layer in the fibres to collect the developed electric charges.

“The fibres are thin and flexible, and they can be woven or knitted into ‘conventional’ textiles. The first applications we looked into were towards sensing, e.g. by integrating the fibres in a sock we can register footsteps as well as map the pressure distribution over the sole. Or, by integrating the fibres into a sweater, they can register heartbeat by sensing the minute chest movements. In a way, this sensitivity turned out to be a problem, as the fibres will also register all other movements which causes a lot of noise. To some extent, this can be remedied by signal processing.”

Lund and Müller had worked in cooperation with the Swedish School of Textiles in Borås and research institute Swerea IVF. The results, in a paper titled ‘Energy harvesting textiles for a rainy day: woven piezoelectrics based on melt-spun PVDF microfibres with a conducting core’, were published in the Nature journal Flexible Electronics. The project was funded by the Swedish Foundation for Strategic Research (SSF), the Knut and Alice Wallenberg Foundation, and the European Research Council (ERC).

Previous research had mainly focused on sensors and their ability to generate electric signals through pressure sensitivity. The Chalmers researchers have now gone a step ahead in using the energy to continuously drive electronic components.

When Anja Lund joined Prof Christian Müller’s research group at the university, they were taken in by the idea of using the fibres for energy harvesting, i.e. to convert biomechanical movement to electricity. “For such an application, the ‘noise’ becomes useful and wanted as it adds energy. We developed a woven textile with integrated electrically conducting yarns, that are required to conduct the generated electric charge. As a practical demonstration, the woven textile was used as a shoulder strap for a shoulder case. By wearing this case while walking up and down stairs I was able to generate several microwatts of power, continuously,” she explained.

She cut down on the technicalities to simplify the invention for the lay person:  “A ‘piezoelectric’ material generates an electric voltage when deformed, i.e. through stretching or compression. The piezoelectric effect is present in many different types of materials, and it stems from an asymmetry in the atomic distribution. This results in a polarisation, i.e. the material will have a ‘positive’ and a ‘negative’ side. When such a material is deformed, electric charges will be redistributed within the material, and this can be measured as an electric voltage.”

The question on the minds of everyone (at least, in the textiles and apparel industry) would be whether this can be taken out of a lab and translated into reality for people. Lund said, “The fibre spinning and weaving can easily be upscaled. One practical obstacle is that the fibres must be subjected to a high voltage to become piezoelectric, a process which is at present not used in the textiles industry. A practical difficulty with all electronic textiles, is the hard-to-soft interface which is required to connect the textiles to conventional electronics. This connection inevitably becomes vulnerable to wear and tear.”

So, it might be a while before one sees apparel made from this fabric. As the university had pointed out during the announcement, “It is now mainly up to industrial product developers to find out how to make use of the technology.” In other words, convert the fabric into a “wearable”.

On other feasibility aspects, Lund underlined, “The textiles are fine to be used close to the body, the voltage developed is low and absolutely harmless. Based on the materials used, we think that the cost could be comparable to that of GoreTex-fabrics.” (WE)

Fibre2Fashion News Desk – India

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November 2018

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