The warp and weft of wearable electronics

 3Q: Zijian Zheng

One of today’s challenges for materials scientists is wearable electronics — smart materials that monitor ailments, harvest energy, track performance or communicate. These remain expensive and hard to produce in bulk, and are often unattractive. Polymer scientist Zijian Zheng takes inspiration from his designer and business colleagues at Hong Kong Polytechnic University’s Institute of Textiles and Clothing. His solution: lightweight electronic yarns that can be made into textiles by adapting existing production processes.

 How do you create wearable electronics?

People need to feel like they’re not wearing electronics, so the materials must be lightweight and flexible. They must also be high-performance, as devices have to charge rapidly, last for a long time and be sweat-proof. Applying all these criteria, we create electronic textiles in which the fabrics themselves form the sensors and devices – from light-emitting diodes, photovoltaics, organic transistors and supercapacitors to batteries. We can make a supercapacitor using conductive yarn, made by coating cotton with nickel, and penetrating it with a form of graphene oxide. If you put a pair of these strands together in parallel, and fill the space between with an electrolyte gel, you can make it work as a supercapacitor storing energy as positively and negatively charged ions collect at the different wires. You could use that to power other devices, such as sensors, or store energy generated from photovoltaics. We’re working on making lithium batteries using the same principles.

When integrating different materials together in an electronic textile, the interfaces create the biggest problems. You can get mismatches between mechanical and thermal expansion properties, and in a flexible system the weakest points are where the device twists or bends. In my group we focus on using polymers to address these issues. For example, we make new polymers that add texture to the surface of textiles, allowing them to be coated in copper at low temperatures for durability. To ensure scalability, our goal is to make textiles that can be integrated with the technology the clothing industry has used for the past 200 years. Our composite yarns can be used in sewing machines, and complicated patterns can be created from them using machine embroidery. From there, you start to add active materials to make devices in ways that are compatible with textile processing. For example, we’re now making photovoltaic cells printable via textile colour-printing technology and encapsulating them with textile-finishing technology. And we are set to make a radio-frequency identification tagging device within a garment, powered by a supercapacitor. We’ve designed it to hide the supercapacitor as an embroidered pattern, like camouflage. We also have a student working with local textile company EPRO Development, trying to put the metallic, conducting textile into real production. Devices will come a bit later as they are ten times more complex to make. Cost is a challenge too: the textile industry cares about every penny. In introducing functional elements into garments such as a breathable section, you might only be allowed to increase production costs by around 10 cents.What are your biggest challenges?

One hank of copper-coated cotton yarns used for making wearable devices and circuits.

KA-CHI YAN

How do the different disciplinary strands in your institute work together?

My institute covers the whole chain of production for textiles and clothing – with materials science and chemistry groups sitting alongside business and design. So we have three streams of students and teaching is totally different for each. The major challenge when I lecture is how to deliver my engineering or scientific-based content to a bunch of artists. We tend to give them an overview to help them understand first, with lots of examples, before we come down into the fundamentals. It’s very different from the physical science students, where we take them through a logical sequence from beginning to end. The artists ask so many questions. Generally they want to know if they can do something with a material, and don’t care about why it functions. They seldom ask “Why does this electron go through there?”