(Invited) Soft Conductors from Carbon Nanotubes

Abstract

Since their infancy, carbon nanotubes (CNTs) were viewed as an exciting new material that combined superior transport properties with great mechanical strength and stiffness—a classical “hard material”. These expectations were based on single-molecule measurements; yet, translation to macroscopic materials proved arduous because CNT did not dissolve easily in liquids, while solvent-less assembly yielded largely disordered structures. I will present an overview of the past fifteen years of research on assembling well-ordered macroscopic CNT materials, much of it conducted in close collaboration with Bob Hauge. From the perspective of soft matter, CNTs can be treated as hybrids of rodlike polymers and cylindrical colloids. CNTs dissolve spontaneously in acids, and form liquid crystals with intriguing morphology. Because dissolution is spontaneous, CNTs do not suffer mechanical or chemical damage and can reach high concentrations that enable material processing into fibers, thin films, tapes, and three-dimensional structures. Intriguingly, these macroscopic CNT structures display mechanical and solid-state properties on par with the best metal and carbon fibers—even in the absence of covalent coupling between the CNTs, making them a viable lightweight, corrosion-resistant replacement for metals. Yet, the most surprising property—and perhaps the one that will prove most useful in future applications—was not predicted in the early days (and could not have been predicted based on single-CNT properties): macroscopic assemblies of CNTs display a “softness” that is completely uncharacteristic of highly conductive materials. They can be knotted, sewn with a common sewing machine, woven with a common loom, and still retain their properties—making them a great platform for electronic textiles and wearable devices. Moreover, these materials have intrinsic nanoscale porosity and display extremely low impedance in contact with biological tissue. This makes them a natural interface to the electrical function of the body, where they could be used as restorative patches for electrically damaged heart tissue as well as electrodes for stimulating and sensing the activity of the brain.