SENSORS AND ACTUATORS BASED ON CARBON NANOTUBES AND THEIR COMPOSITES: A REVIEW Chunyu Li, Erik T. Thostenson and Tsu-Wei Chou Department of Mechanical Engineering, University of Delaware, Newark, Delaware, USA KEYWORDS: carbon nanotubes, sensors and actuators, electrical conductivity, percolation. The emergence of carbon nanotubes (CNTs) has stimulated considerable interest in investigating their physical and mechanical properties toward the development of potential technological applications. It has been confirmed theoretically and experimentally that nanotubes possess remarkably high stiffness and strength. Carbon nanotubes also have exceptionally high electrical and thermal conductivities. The unique mechanical and physical properties of nanotubes combined with their high aspect ratio and low density have brought about extensive research in creating composite material systems to exploit these properties. Considerable interest has focused on utilizing nanotubes as passive reinforcement to tailor mechanical, electrical and thermal properties [1, 2]. Increasingly there is a demand for designing materials that have both tailored structural and functional properties. With advances in nanotechnology enabling us to structure new materials at the nanoscale, the opportunity exists for developing novel material systems and devices that are capable of self-sensing and active response. Carbon nanotubes and their composites are uniquely suited as potential intelligent materials. This distinctive form of carbon shows extraordinary mechanical and physical properties. For the development of these material systems with multi-functional constituents for sensing and actuation a fundamental knowledge of their structure/property relations is necessary. In this article we review some of the recent advances in nanotube and nanotube-based composite sensors and actuators, with a particular emphasis on their electro-mechanical behavior. Unlike conventional piezoelectric materials, where dipoles are oriented by applying high electric fields at elevated temperatures, electromechanical coupling of carbon nanotubes results as a consequence of charge injection. The influence of local nanotube mechanical deformation on their electrical properties was investigated and large, reversible changes in conductivity were observed [3]. It was also reported that bending deformation altered the band structure of the nanotube, depending on nanotube chirality [4]. This intrinsic coupling of electrical properties and mechanical deformation in carbon nanotubes makes them ideal candidates for future multifunctional material systems that combine adaptive and sensory capabilities. While the development of nanoscale devices and nanoelectromechanical systems (NEMS) utilizing the unique properties of carbon nanotubes is an evolving area of nanotechnology, there is considerable interest in making macroscopic engineering materials that can exploit these novel material properties. The coupling of physical properties with mechanical deformation particularly has widespread application in the development of multifunctional materials for sensing and actuation. Of particular importance in the development of sensors and actuators based on carbon nanotube composites is their electrical conductivity. Polymeric or ceramic matrix of composite is usually considered as non-conductive material because of its extremely low electrical conductivity (in the order of 10 -10 ~10 -15 S/m). Dispersing conductive materials into the non-conductive matrix can form conductive composites. The electrical conductivity of a composite is strongly dependent on the volume