Nucleation and propagation of voltage-driven wrinkles in an inflated dielectric elastomer balloon.

Dielectric elastomer (DE) transducers frequently undergo voltage-induced large deformation, which may lead to mechanical instabilities. Here, we investigate wrinkle formation and propagation on the surface of a DE membrane mounted on an air chamber and subjected to a step voltage. Our experiments show that the geometric characteristics of the wrinkle morphology and the nucleation sites depend on the inflation pressure and the applied voltage. As the inflation pressure increases, the critical voltage used to nucleate the wrinkle decreases, while the location where the wrinkle nucleates shifts from the center to the boundary of the membrane. Moreover, by increasing the amplitude of the applied voltage, wrinkle morphology changes from stripe-like wrinkles to labyrinth-like wrinkles. Furthermore, we develop an analytical model to validate the experimental observations and map out the various wrinkle morphologies as a function of the applied pressure and voltage. A three dimensional phase diagram is constructed to help design new soft actuators.

Characterization of human female breast and abdominal skin elasticity using a bulge test.

Characterization of material properties of human skin is required to develop a physics-based biomechanical model that can predict deformation of female breast after cosmetic and reconstructive surgery. In this paper, we have adopted an experimental approach to characterize the biaxial response of human skin using bulge tests. Skin specimens were harvested from breast and abdominal skin of female subjects who underwent mastectomy and/or reconstruction at The University of Texas MD Anderson Cancer Center and who provided informed consent. The specimens were tested within 2 h of harvest, and after freezing for different time periods but not exceeding 6 months. Our experimental results show that storage in a freezer at -20 °C for up to about 40 days does not lead to changes in the mechanical response of the skin beyond statistical variation. Moreover, displacement at the apex of the bulged specimen versus applied pressure varies significantly between different specimens from the same subject and from different subjects. The bulge test results were used in an inverse optimization procedure in order to calibrate two different constitutive material models - the angular integration model proposed by Lanir (1983) and the generalized structure tensor formulation of Gasser et al. (2006). The material parameters were estimated through a cost function that penalized deviations of the displacement and principal curvatures at the apex. Generally, acceptable fits were obtained with both models, although the angular integration model was able to fit the curvatures slightly better than the Gasser et al. model. The range of the model parameters has been extracted for use in physics-based biomechanical models of the breast.