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A large-displacement electrochemical actuator was designed, fabricated, and tested. The large displacement is obtained by using a corrugated membrane made by physical vapor deposition of Parylene sandwiched with an intermediate layer of sputtered platinum. The layered structure is approximately 8-/spl mu/m thick, with 26 grooves approximately 120-/spl mu/m deep, and with a radial period of 350 /spl mu/m. The electrochemical cell consists of platinum electrodes with a 1 M H/sub 2/SO/sub 4/ solution. Hydrogen and oxygen gas is generated to displace the membrane. Although the actuator can operate at a voltage as low as 1.23 V, the experimentally determined efficiency of converting electrical energy to mechanical work is only 0.37%. The governing equations for the conservation of mass, momentum (equilibrium), energy, and the entropy generation rate were formulated assuming that the gas bubbles either nucleate without growth or grow without nucleation. For the nucleation case, simulations were performed for constant pressure isothermal actuation, and the average experimental efficiency was bounded by simulations with gas bubble radii between 1/spl times/10/sup -6/ m and 1/spl times/10/sup -6/ m. The predicted ratio of the power dissipated to the electrical power supplied is 1.37 for isothermal actuation.