The deployment of electroactive ionic polymer hydrogel–metal composites in artiŽ cial muscle and BioMEMS applications has recently been intensively investigated. In order to analyse their electromechanical responses to externally applied electrical Ž elds, it is critical to develop a constitutive model linking the macro-mechanical moduli with the micro-mechanical characteristics, and to determine the geometric size and shape of the micro-structural cluster and investigate the effect of cluster morphology on the effective electro-elastic moduli of the polymer hydrogels. As a typical ionic polymer-based hydrogel, the NaŽ on membrane is studied in this work. Based on the Biot poroelasticity theory, a multi-scale constitutive model which includes both macro and micro characteristicsis developed using an asymptotic homogenisationmethod. The effect of water-volume fraction on the effective elastic moduli of the hydrogelmembrane is examined for different equivalent weights. Numerical investigations show that the simulated effective constitutive moduli agree well with experimental data. The presently developed constitutive model is thus validated. In order to determine the micro-structuralshape of the polymerskeleton subject to  uid pressure, a representative volume element (RVE) is designed by topology optimisation of the periodic microstructures of the NaŽ on hydrogels, through the minimisation of the electro-elastic interaction energy between the polymer-based  uorocarbon matrix and the surrounding  uid. This optimal RVE correctly predicts the geometric shapes of the clusters.