Conductivity gain for stretchable electronics

2 min read

Networks of spherical nanoparticles embedded in elastic materials could make the best bendable conductors, engineering researchers at the University of Michigan claim.

Flexible electronics have the potential to be used in a range of applications including displays that can be stretched or for medical implants that move with the body.

‘Essentially the new nanoparticle materials behave as elastic metals,’ said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering. ‘It’s just the start of a new family of materials that can be made from a large variety of nanoparticles for a wide range of applications.’

Finding good conductors that still work when pulled to twice their length is a goal of researchers who have so far tried wires in zigzag or spring-like patterns, liquid metals, nanowire networks and more. The team found that spherical gold nanoparticles embedded in polyurethane could out compete the best of these in stretchability and concentration of electrons.

‘We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways,’ said Yoonseob Kim, first author of the study to be published in Nature and a graduate student in the Kotov lab in chemical engineering.

To find out what happened as the material stretched, the team took electron microscope images of the materials at various tensions. The nanoparticles started out dispersed, but under strain, they could filter through the gaps in the polyurethane, connecting in chains as they would in a solution.

‘As we stretch, they rearrange themselves to maintain the conductivity, and this is the reason why we got the amazing combination of stretchability and electrical conductivity,’ Kotov said in a statement.

The team made two versions of their material by building it in alternating layers or filtering a liquid containing polyurethane and nanoparticle clumps to leave behind a mixed layer.

Overall, the layer-by-layer material design is claimed to be more conductive while the filtered method makes for extremely supple materials. Without stretching, the layer-by-layer material with five gold layers has a conductance of 11,000 Siemens per centimetre (S/cm), on par with mercury, while five layers of the filtered material came in at 1,800S/cm.

The blood-vessel-like web of nanoparticles emerged in both materials upon stretching and disappeared when the materials relaxed. Even when close to its breaking point, at a little more than twice its original length, the layer-by-layer material still conducted at 2,400S/cm. Pulled to 5.8 times its original length, the filtered material had an electrical conductance of 35S/cm.

Kotov and Kim chiefly see their stretchable conductors as electrodes. Brain implants are of particular interest to Kotov.

‘They can alleviate a lot of diseases - for instance, severe depression, Alzheimer’s disease and Parkinson’s disease,’ he said. ‘They can also serve as a part of artificial limbs and other prosthetic devices controlled by the brain.’

Rigid electrodes create scar tissue that prevents the electrode from working over time, but electrodes that move like brain tissue could avoid damaging cells, Kotov said.

‘The stretchability is essential during implantation process and long-term operation of the implant when strain on the material can be particularly large.’

Whether in the brain, heart or other organs—or used for measurements on the skin—these electrodes could be as pliable as the surrounding tissue. They could also be used in displays that can roll up or in the joints of lifelike ‘soft’ robots.

The study published in Nature is titled Stretchable Nanoparticle Conductors with Self-Organized Conductive Pathways.