CNT Actuators and Sensors
The focus of my PhD thesis has been the study of artificial muscles, specifically those made from carbon nanotubes, conducting polymers and dielectric elastomers. The aim is to create low-cost actuator technologies that exhibit stresses, strains, power densities and efficiencies similar to natural muscle. Many medical applications exist for technologies that are able to replace or assist skeletal or cardiac muscles. The capabilities of micro- and nano-scale mechanical devices are currently restricted by the relative weakness of electrostatic actuators, the small strains of piezoelectric materials, and the high voltage requirements of both. Actuators with muscle-like properties will greatly enhance micro-device performance and enable new nanoscale applications. Muscle-like actuators will substantially reduce the size, weight and cost of such devices. Ultimately, high performance artificial muscles may provide an inexpensive, efficient and environmentally-friendly replacement for electric motors and combustion engines.
The double-layer interaction mechanism allows yarns of carbon nanotubes to act as both mechanical force sensors and actuators.
We reported for the first time how the twisted structure of our carbon nanotube yarns can convert a tensile force into a change in the electrochemical double-layer capacitance of the yarn due to torsional effects. The tensile force compacts the yarn in the radial direction, resulting in more interaction between the electrochemical double-layers on adjacent carbon nanotubes. The increase in interaction means less charge can be stored in the double-layers, resulting in a decrease in the yarn capacitance. Therefore, applying a tensile force to an electrochemically charged yarn results in a change in the cell current or the open-circuit potential. These quantities can be measured electronically, and the yarn can thus be used as a force sensor. Since the tensile strength of the yarns is about 1 GPa, they can be used as force sensors under unprecedented levels of stress. See the sensor cycle in the illustration.
Our work can be considered the first report of an application of the interaction between electrochemical double-layers in a macroscopic scale, owed to the special structure of nanotube yarns.
A scanning electron micrograph of a two-ply twisted yarn of multi-walled carbon nanotubes