Dielectric elastomers are in a special class of electro-active polymers known for generating expansion in plane and contraction in thickness under voltage application. This paper advances the understanding of a planar contractile dielectric elastomer actuator (cDEA) that is distinct from conventional multi-layer cDEAs but generates comparable contractile strains. Its structure has a rod-constrained rhombus-shaped electrode region, which undergoes simultaneous in-plane contraction and extension during actuation depending on the configuration of the rod-constraining. It is demonstrated that when the planar cDEA is driven by high voltages, off-plane deformation (i.e., wrinkling) in the direction of contraction causes the rod-constrained electrode region to lose tension and extend in the lateral direction, resulting in a significant increase in contraction strain. It also demonstrates that the contraction strain can be increased further by having biased bi-axial pre-strains. By incorporating both effects, the new cDEA generates a maximum contraction strain of 13%, twice that reported previously for planar cDEAs. A modified planar cDEA, having an additional rigid frame to maintain the pre-strain in the lateral direction to contraction was also developed to demonstrate contractile force actuation. Finally, a stability study shows that the planar cDEA has a primary failure mode of electrical breakdown close to the corners of the rod-constrained electrode region. Having inactive regions around the active cell is essential for generating contraction and eliminating buckling of the planar cDEA in the lateral direction.
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