It’s not just an episode from your favourite sci-fi anymore. From Madonna’s light-bulb cape, U2’s laser diode jacket to Lady Gaga’s “Living Dress”, wearable technology is the buzz phrase among fashionistas, techies and sports lovers these days. Solar panel couture that charges your cell phone, smart socks that check your running posture, intelligent compression tank top or sports bra that monitors your heart rate and blood pressure are all ingenious ideas. But before these ideas can be realized, there needs to be an electric circuit that connects the sensors, microchips and signal emitters. There have been many different ways to create electric circuits in clothing in the past, but most of them are too fragile for repeated uses, not to mention the vigorous wash cycle in machines. Dr Zijian Zheng, Associate Professor, Institute of Textiles and Clothing and his research team have recently developed the first washable high-performance e-textile that is based on natural materials such as cotton and linen. It can be machine washed up to 30 times without losing its conductivity, is proven 100% safe, and is cheaper than similar options. It sure does open up a whole world of possibilities in the smart sportswear and “fashionology” realms.
The quest for washable e-textile
To conduct electricity in a piece of clothing, the most straight-forward way is to connect the components with wires sewn under a lining. Yet, it’s obvious that such wires cannot be folded many times before breaking down, meaning the clothing has to be stored flat to be functional. With many wires running under a garment, it also adds to its thickness and stiffness, sacrificing wearing comfort. Other methods to build electric circuits on fabrics include screen printing or inkjet printing with conductive ink, metal wire embroidery, weaving metal wires into yarns, coating yarns in conductive polymers, and even carbon nanotubes (CNTs) twisted into the yarns or fabrics dip-coated in CNT ink.
“All of these existing methods have their problems. Most of them need a synthetic substrate to adhere well and can’t apply on natural fabric. Another objective of our research is to achieve machine washability. We want an e-textile that can be treated just like any clothes without painstaking special care. Most of the said methods render the garment unwashable because the vigorous wash cycle may physically break the brittle wires. Exposure to water may also oxidize the contacts. Conductive polymers are salts that are unstable under atmospheric moisture and thus complicated and expensive to process. Similarly, sputtering, plasma deposition and CNTs are too expensive to be used in mass scale as they entail extreme conditions such as high temperature and high pressure. The U.S. government also warned about the potential respiratory hazard of exposure to CNTs1. We need to balance among cost, safety and performance,” said Dr Zheng.
Low-cost, washable, durable
Dr Zheng and his team found that coating copper on yarns with electroless deposition keeps the cost down while ensuring high conductance. The metal-coated yarns can then be woven into fabric using regular weaving machines. But they have another problem with securely adhering the metal onto the fibre. “We came up with a process known as polymer-assisted metal deposition. Polymer brushes are grafted on the yarn first to increase the surface area for adhesion. Then the metal is coated over the polymer brushes,” explained Dr Zheng. The resulting fabric woven with copper-coated cotton yarns is rather inexpensive as the production doesn’t involve any expensive catalyst, specific lab requirements or special equipment. It still functions well and remains comfortable to touch after multiple machine washing cycles, making it an ideal candidate for producing wearable electronics in various areas such as sports, healthcare, entertainment, military equipment and energy harvesting.
The conductive fabric won a silver medal in the 43rd International Exhibition of Inventions of Geneva, Switzerland in April 2015.
[1] DHHS (NIOSH). Current Intelligence Bulletin 65: Occupational exposure to carbon nanotubes and Nanofibers, April 2013. http://www.cdc.gov/niosh/docs/2013-145/ Retrieved on 9 Oct 2015.
