Contraction response of a polyelectrolyte hydrogel under spatially nonuniform electric fields

One of the major challenges in contemporary materials science is to build synthetic structures that can mimic responsive biological tissues, such as artificial skins and muscles. Polyelectrolyte hydrogels can provide such mechanoelectrical responses under external electric fields. Yet, their response to such stimuli, particularly at the molecular scales, is not fully revealed. Here, we study the mechanical response of a semi-infinite polyelectrolyte hydrogel slab to transient and spatially nonuniform sinusoidal electric fields by using extensive coarse-grained molecular dynamics simulations. Our simulations show that if the electric field is exerted on a small volumetric section of the hydrogel slab, the entire slab contracts reversibly and uniaxially in the direction perpendicular to the field. The hydrogel contracts almost half of its field-free initial length before retracting its original size, and eventually, its size fluctuations decay similarly exponentially. A contraction maximises when the electric field is applied away from the boundaries of the hydrogel slab. Further, by tuning the electric field frequency and amplitude, both contraction times and efficiency can be controlled. Analyses of contraction times and efficiency with implicit solvent simulations for various electrostatic parameters and salt concentrations confirm the robustness of the phenomenon while highlighting the importance of hydrodynamics. Our results demonstrate the effectiveness of spatially nonuniform electric fields' effectiveness in manipulating homogenous hydrogel structures for applications requiring rapid and soft actuation components.