Flexible electronics boost with stretchiest conductor ever made

US researchers have made the stretchiest electrical conductor yet using gold nanoparticles embedded in an elastic polymer. The new material can stretch to over five times its size while still conducting well enough to power small devices.

Finding materials that can conduct when stretched is a huge challenge within flexible electronics. Existing approaches, which involve coiling wires, liquid inks or embedding conductive particles in stretchy materials, have achieved limited success. The biggest hurdle, says lead author Nicholas Kotov from the University of Michigan, US, is combining two properties that counteract each other. ‘If we increase the stretchability of a material we automatically decrease the conductivity because we are increasing the gap between the conducting elements,’ he says. ‘On the other hand, adding more conductive elements increases stiffness and reduces stretchability.’

Kotov and his group have overcome this trade-off using spherical gold nanoparticles dispersed through sheets of polyurethane. When this approach has been tried with other conductors, such as carbon nanotubes, conductivity drops as the material is stretched. But instead of spreading further apart when the material is stretched, the gold nanoparticles form a branching network of conductive chains so conductivity stays high.

‘We knew from our experience with nanoparticles in solutions that a lot of particles have the ability to self-organise – it’s intrinsic to nanoscale matter,’ says Kotov. ‘Our approach worked really well – because of this self-organisation we were able to get high conductivity and amazing stretchability.’ He also says nanoparticles other than gold could be used in the same way.

Without stretching, the material has a high conductance of 11,000S/cm. When stretched to over twice their original length, conductance is 2400S/cm. It even conducted when stretched to 5.8 times its length at 35S/cm, which is still enough to power some small devices. This is a significant improvement on existing stretchy carbon nanotube containing conductors, few of which can even stretch more than twice their length, let alone maintain conductivity.

There are several potential applications for such an elastic conductor. Kotov says it could feature in flexible gadgets or soft robots, and his group are currently trying to use it to create soft, stretchable medical implants and sensors, in particular for use in the brain.

John Rogers, a nanofabrication expert at the University of Illinois, Urbana-Champaign, US, agrees this is a promising development. ‘These types of conducting materials could provide new options in engineering design,’ he says. ‘The next steps will be to determine routes for integrating these materials into functional systems and assessing their performance compared to alternatives.’