carbon nanotubes, which can be reversed by translating direction when different temperatures are applied. This phenomenon makes it possible to use carbon nanotubes as a heat engine working between two heat sources. Macroscopic applications Nanotube sheet electrodes as actuators Researchers of AlliedSignal initially demonstrated the possibility of electrically powered actuators fabricated by carbon nanotube sheets. They taped carbon nanotube sheets on two sides of a double sided scotch tape and applied potential on the nanotube sheets in a NaCl electrolyte solution. Nanotube sheets are used as electrolyte-filled electrodes of a supercapacitor. Nanotube sheets are electrically charged by the double layer formation at the nanotube-electrolyte interface without any need of ion intercalation. Therefore, electrically driven actuators of nanotube sheets are superior to the conjugated polymer actuators which involve solid-state dopant diffusion and structural changes limiting rate, cycle life, and energy conversion efficiencies. On the other hand, ferroelectric and electrostrictive materials are also very useful for direct energy conversion, but they require high operation voltages and ambient temperature of a limited range. Nanotube sheet actuators were shown to operate at low voltages (≈1 volts or less) and provide higher work densities per cycle than other alternative technologies. Later Baughman et al. showed that actuator response can be observed up to switching rates of 1 kHz and cycling the nanotube actuator at constant rate of 1 Hz for 140,000 cycles decreases the stroke by ≈33%. 0.75 MPa of stress were measured on the nanotube sheet actuators, which is greater than the maximum stress (0.3 MPa) that can be loaded on a human muscle. The maximum actuator strain for electrically driven actuators of carbon nanotube sheets can be improved up to 0.7% in a 1 M electrolyte once the sheets are annealed in an inert atmosphere at very high temperatures () in contrast to once-reported 0.1% or less for low electrochemical potentials (≈1 V or less). The maximum strain for the carbon nanotube sheet actuators at low voltages is greater than that of the high-modulus ferroelectric ceramic actuators (≈0.1%), but it is lower than that of the low-voltage (≈0.4 V) conducting polymer actuators (≈3% film direction, 20% thickness direction). Strokes were reported as high as 215% for strain-biased low-modulus electrostrictive rubbers under biases greater than 1 kV (corresponding to an electric field 239 MV/m for the geometry mentioned in the reference paper). Spinks et al. realized pneumatic actuation from the carbon nanotube sheets in