Embroidering power-generating yarns onto fabric has allowed researchers to embed a self-powered, numerical touchpad and movement sensors into clothing. This novel embroidery technique offers a low-cost, scalable method for making wearable devices.
“Our technique uses embroidery, which is pretty simple – you can stitch our yarns directly on the fabric,” said Rong Yin, assistant professor of textile engineering, chemistry and science at North Carolina (NC) State University. “During fabric production, you don’t need to consider anything about the wearable devices. You can integrate the power-generating yarns after the clothing item has been made.”
In a paper on this work in Nano Energy, the researchers report testing multiple designs for their power-generating yarns. To make yarns durable enough to withstand the tension and bending of the embroidery stitching process, the researchers ultimately used five commercially available copper wires covered in a thin polyurethane coating. Then, they stitched them onto cotton fabric with a polymer material called PTFE.
“This is a low-cost method for making wearable electronics using commercially available products,” Yin said. “The electrical properties of our prototypes were comparable to other designs that relied on the same power-generation mechanism.”
The researchers took advantage of a method for generating electricity called the ‘triboelectric effect’, which involves harnessing electrons exchanged by two different materials, as happens with static electricity. They found that the PTFE fabric had the best performance in terms of voltage and current when in contact with the polyurethane-coated copper wires, compared with other types of fabric that they tested, including cotton and silk. They also tried coating the embroidery samples in plasma to increase the effect.
“In our design, you have two layers – one is your conductive, polyurethane-coated copper wires, and the other is PTFE, and they have a gap between them,” Yin explained. “When the two non-conductive materials come into contact with each other, one material will lose some electrons, and some will get some electrons. When you link them together, there will be a current.”
The researchers tested their yarns as motion sensors by embroidering them with the PTFE fabric on denim. They placed the resulting embroidery patches on the palm, under the arm, at the elbow and at the knee to track electrical signals generated as a person moves. They also attached fabric with their embroidery on the insole of a shoe to test its use as a pedometer, finding that the electrical signals varied depending on whether the person was walking, running or jumping.
Lastly, they tested their yarns in a textile-based numeric keypad on the arm, which they made by embroidering numbers on a piece of cotton fabric, and attaching them to a piece of PTFE fabric. Different electrical signals were generated for each number that a person pushed on the keypad.
You can embroider our yarns onto clothes, and when you move, it generates an electrical signal, and those signals can be used as a sensor,” Yin said. “When we put the embroidery in a shoe, if you are running, it generates a higher voltage than if you were just walking. When we stitched numbers onto fabric, and press them, it generates a different voltage for each number. It could be used as an interface.”
Since textile products will inevitably need to be washed, the researchers also tested the durability of their embroidery design in a series of washing and rubbing tests. After hand washing and rinsing the embroidery with detergent, and then drying it in an oven, they found either no difference or a slight increase in voltage. For the prototype coated in plasma, they found weakened but still superior performance compared with the original sample. An abrasion test revealed there was no significant change in the electrical output performance of their designs after 10,000 rubbing cycles.
In future work, the researchers plan to integrate their sensors with other devices to add more functions. “The next step is to integrate these sensors into a wearable system,” Yin said.
Source: Materials Today