By Will Soutter
Image credit: Joseph Xu, University of Michigan
Flexible electronics is one of the most exciting areas of technology at the moment. Recent advances in materials technology are allowing manufacturers to consider the possiblities of making flexible devices, from solar panels to smartphones.
Nanomaterials are playing a key role in the development of this technology - materials like graphene, nanowire networks and liquid metals are all being used to create touch screens, batteries and electronic circuits that maintain their properties when bent. However, these materials do not react well to being stretched. Even if they remain physically intact, their electrical properties are often compromised when stretched, which limits the possiblities for the devices they will be used in.
Now, however, researchers at the engineering department at the University of Michigan may have found a solution. They have created a material made of spherical gold nanoparticles embedded in flexible polyurethane, which stays electrically conductive when it is stretched.
Yoonseob Kim, a graduate student at the University of Michigan and the lead author on the paper published in Nature, explains how this works:
"We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways.
"As we stretch the material, they rearrange themselves to maintain the conductivity, and this is the reason why we got the amazing combination of stretchability and electrical conductivity."
Two electron microscope images of the gold nanoparticles embedded in polyurethane, at the same magnification. The image on the left is the material in it's relaxed state; the image on the right has been stretched, and the network of gold nanoparticle chains which forms is clearly visible. Image credit: Nicholos Kotov, University of Michigan
The team observed the nanoparticle chain formation within the stretchy material using a state-of-the-art electron microscope. The web of nanoparticles forms spontaneously upon stretching, and is completely reversible, collapsing back to its original state when the material relaxes.
The nanoparticle-polymer composite can be made in two ways - either by building it up layer-by-layer, or by mixing the polyurethane and gold nanoparticles together in a liquid suspension, then filtering them out into a single, mixed layer.
These two methods produce quite different properties, which may allow the material to be tailored for particular applications. The layering method produces good conductivity - around 11,000 S/cm, around that of mercury. The filtered material is still conductive, but only at about 1,800 S/cm.
The layered version also performs better under stretching, with conductivity dropping to just 2,800 S/cm when the material is stretched to twice it's original length, and still conducting at 35 S/cm at nearly 6 times the length - unprecedented performance, and still plenty for many applications. The filtered material, however, is much more flexible, and still has reasonable conductivity.
Yoonseob Kim, and the research group leader Prof. Nicholas Kotov, have commented that they see many potential applications for this technology, such as less harmful electrode brain-implants for treating Parkinson's, epilepsy, and other diseases.
The team at UMich will mow be working on adapting this technique to other, perhaps less expensive nanoparticle materials, to further expand the range of applications and investigate the properties of this new type of stretchable conducting material.
"Essentially the new nanoparticle materials behave as elastic metals - 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."
Professor Nicholas Kotov