Actuating materials capable of producing useful movement and forces are recognized as the "missing link" in the development of a wide range of frontier technologies including haptic devices, [1] microelectromechanical systems (MEMS), [2] and even molecular machines. [3] Immediate uses for these materials include an electronic Braille screen, [4] a rehabilitation glove, [5] tremor suppression, [6] and a variable-camber propeller. [7] Most of these applications could be realized with actuators that have equivalent performance to natural skeletal muscle. Although many actuator materials are available, none have the same mix of speed, movement, and force as skeletal muscle. Indeed, the actuator community was challenged to produce a material capable of beating a human in an arm wrestling match. [8] This challenge remains to be met.One class of materials that has received considerable attention as actuators is low-voltage electrochemical systems utilizing conducting polymers [9][10][11] and carbon nanotubes. [12,13] Low-voltage sources are convenient and safe, and power inputs are potentially low. One deficiency of conducting polymers and nanotubes compared with skeletal muscle is their low actuation strains: less than 15 % for conducting polymers and less than 1 % for nanotubes. It has been argued that the low strains can be mechanically amplified (levers, bellows, hinges, etc.) to produce useful movements, [7] but higher forces are needed to operate these amplifiers. In recent studies of the forces and displacements generated from conducting-polymer actuators, it has become obvious that force generation is limited by the breaking strength of the actuator material. [14][15][16] Baughman [17] has predicted that the maximum stress generated by an actuator can be estimated as 50 % of the breaking stress, so that for highly drawn polyaniline (PAni) fibers, stresses on the order of 190 MPa should be achievable. However, in practice the breaking stresses of conducting-polymer fibers when immersed in electrolyte and operated electromechanically are significantly lower than their dry-state strengths. [15,16,18] The reasons for the loss of strength are not well known, but the limitations on actuator performance are severe. The highest reported stress that can be sustained by conducting polymers during actuator work cycles is in the range 20-34 MPa [16,19] for polypyrrole (PPy) films. However, the maximum stress that can be sustained by PPy during actuation appears to be very sensitive to the dopant ion and polymerization conditions used, [16] with many studies showing maximum stress values of less than 10 MPa. [4,[14][15][16]20] The low stress generation from conducting polymers, limited by the low breaking strengths, mean that the application of mechanical amplifiers is also very limited. To improve the mechanical performance, we have investigated the use of carbon nanotubes as reinforcing fibers in a polyaniline (PAni) matrix. Previous work has shown that the addition of singlewalled nanotubes (SWNTs) and multi-walled nanotu...
