Wednesday, 29 January 2014

Stretchy Gold Electronics Could Someday Live Inside Your Brain

What looks like a shiny piece of gold foil is actually a new stretchy conductive material that could one day be fashioned into electrode implants for the brain or pacemakers for the heart. Crafted from gold nanoparticles and an elastic polymer, the material retains its conductivity even when stretched to four times its original length.
“It looks like elastic gold,”  a chemical engineer at the University of Michigan.But we can stretch it just like a rubber band. When it stretches, it retains all the properties of a metal, including the ability to transport electrons. 
Normally, stretching a circuit disrupts the interatomic connections that keep electrons flowing from one end to the other. Most existing stretchable electronics overcome this difficulty by using accordion- or spring-like folding wires that can expand and contract. But in the new material, no folds or convolutions are needed.
Its secret? Self-organizing gold nanoparticles that have been embedded into an elastic polymer, polyurethane.
When the shiny material is stretched, the nanoparticles self-organize into conductive chains, scurrying to fill the gaps in the elongating material. It’s the first material that relies on nanospheres to achieve intrinsic stretchable conductivity.
Looking at the substance under electron microscopes revealed that the spheres snapped into chains under pressure, producing structures electrons could flow through. And when you release the stress, they pretty much come back to their original position.
The process is repeatable. And although conductance at maximal stretch is decreased to less than 10 percent of the original, it’s still enough to provide power to some devices, the team reports.“The results suggest some very interesting, unexpected effects of nanoparticle-elastomer composites,” said John Rogers, a materials scientist at the University of Illinois. Rogers and his lab have developed an array of super-cool flexible, silicon-based circuits that use serpentine wires and buckled folds for stretching and contraction. “These types of conducting materials could provide new options in engineering design,” he said.
                               Someday, the gold-and-polyurethane material might live inside your head – in the form of implantable electrodes for treating movement disorders or other conditions. Or maybe, it will find its place on your heart, as part of a device that regulates cardiac activity. Scientists have been searching for ways to make pliant, biocompatible electronics that can bend and stretch and mold to the human body’s many curved surfaces, whether in the form of temporary tattoos or circuits that hug the ridges on the brain’s surface.

                   Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 123). Furthermore, implantable devices or stretchable displays4 need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. 
                                       However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals. This conductivity–stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent
                       Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition, or with specific alignment (the way the fillers are arranged in the matrix). However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment15. Pre-strained substrates, buckled microwires and three-dimensional microfluidic polymer networks have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.


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