This study investigated a parameter that determines an optimum condition of the content of the ionic group and the concentration of outer solution for highperformance electro-driven polymer hydrogel membranes. The optimum condition for quick bending was determined by a simple method that identified the initial conditions based on Donnan equilibrium theory. Since the bending behavior depends on the initial conditions of the ionic group content and the concentration of the outer solution, it can be predicted by the ratio of the ionic concentrations at the membrane-solution interface; the inverse of the Donnan ratio (1/K) at the initial condition. The bending rate of the membranes showed a maximum value at around 1/K = 0.15. The relationship between several interrelated control factors and the bending dynamics of the gel membranes was established by using the initial system parameters alone. 1/K is the effective simple parameter to determine the optimum condition of the content of the ionic group and the concentration of the outer solution for high-performance membranes. (C) 2000 John Wiley & Sons, Inc.