Introducing the knitted circuit board

Lightweight, stretchy, machine-washable – and electrically conductive: it’s a fabric of the future. Researchers at the Hong Kong Polytechnic University have found a new way to incorporate circuit boards into wearable fabrics, using knitting. Led by Xiaoming Tao, the team knitted their electronic fabric with a yarn made of thin strands of copper wire insulated by lycra, they reported in the journal, Proceedings of the Royal Society A, this month.

Although the fabric is essentially made of metal, “it’s very light,” says Tao. “It’s similar to a swimming costume.”

Professional sports, such as swimming, are one area that might benefit most from electronic fabrics. Sensors incorporated into the fabric can be used during training to measure forces and impacts, to alert coaches if an athlete’s technique is putting them at risk of injury. Reactive electronic fibres could even stiffen up to give the body extra support where needed.

Meanwhile, in a medical setting, the fabric could be used to seamlessly embed health-monitoring devices inside clothing.

But making wearable electronics is a knotty problem. Metal is the best material for carrying electricity, but metals don’t stretch. This has sparked research into making conductive, stretchy fibres from other materials, such as elastics mixed with nanoparticles or graphene. What Tao realised is that metal wires can be used after all – if they are knitted.

Tao and her team have trialled their fabric for several applications, including feeding power to a sensor which can detect impact in a bullet-proof vest.

If you pull on a single thread from your jumper it’s not that stretchy. But tug on the garment and you can stretch it out to double or triple its original size. As your granny could tell you, the way the yarn is looped when it is knitted gives the fabric some built-in slack. Starting with non-stretchy copper wire, Tao’s team could knit a fabric which stretched up to 300% without affecting the copper’s electrical conductivity.

The fabric was also robust. Tao’s knitted circuit board could be stretched slightly – by 20% – more than one million times without deteriorating. As she explains, the new approach is a huge improvement on previous efforts in the field, which could only survive about 10,000 stretching cycles. “That means one day’s wear,” she says. By comparison, one million cycles is equivalent to “three months of wear without failure”. Besides the stretching tests, the team also put their fabric through the washing machine 30 times without degrading conductivity. “The results look very promising,” says Shayan Seyedin, a researcher in stretchable conductive textiles at the University of Wollongong. By using very thin copper fibres, about the thickness of a human hair, they’ve made it flexible enough to knit. “They’ve got a really nice approach.”

Although Seyedin is impressed with the fabric’s “outstanding mechanical performance and elasticity”, he points out Tao only tested for small strains. “For wearable electronics applications, 20% strain is low,” he says. “It’s generally considered that textiles can stretch up to 55% during regular daily activities.”

So far Tao and her team have trialled their fabric for several applications, including feeding power to a sensor which can detect impact in a bullet-proof vest. Another of their stretch sensors is designed to aid sports training. “The sensors we use consume very little energy, so one button battery can last about three months,” says Tao.

They’ve also created prototype smart clothing with embedded health monitoring devices, to keep an eye on a patient’s heartbeat, respiration and other vitals.

Some devices need higher levels of energy, she says. “A wearable energy source will be the next thing on our agenda.”