Hydrogels - a macroscopic approach based on microscopic physics

As a smart material, hydrogels react to different stimuli. These could be - for example - of chemical, electrical, mechanical or thermal nature. The hydrogels consist of a more or less crosslinked polyelectrolytic polymer network, depending on the conditions, components, and time spent for its fabrication. The reaction of these stimulated hydrogels is to uptake or deliver ions with hydration shells or solvent, followed by e.g. an elongation or bending deformation of the gels. Gels can reach enormous swelling ratios and are applicable as actuators, energy converters or sensors. In the present approach, the underlying physical phenomena of the chemical, electrical as well as of the mechanical field are incorporated in a homogenized way by using different partial differential equations. Local effects, e.g. the osmotic pressure differences in the mechanical field, are derived over reference and local concentrations. Depending on the type of the stimulus, the hydrogel reaction is more or less sensitive. For example, the mechanical reaction under chemical stimulation is far beyond the reaction under electrical stimulation. The applied coupled multi-field formulation is capable of providing local concentrations, electric potential distributions and displacements. The hydrogel model of a finger gripper is formulated by the use of finite elements. For the simulation the Newton-Raphson method and implicit Euler time integration are applied. Here only small volume changes, corresponding swelling ratios and deformations are considered. The results to different kinds of stimulation will be presented.