Coupled chemo-electro-mechanical finite element simulation of hydrogels: I. Chemical stimulation

Polyelectrolyte gels are viscoelastic adaptive materials with enormous swelling capabilities under the influence of different kinds of stimulation, e.g. chemical, electrical or thermal. This unique property makes them very attractive for 'pseudomuscular' actuators. In this paper we investigate the mechanism of the chemical stimulation, by changing the salt concentration in the solution bath surrounding the gel. By applying a fully coupled chemo-electro-mechanical model, the change of the concentrations, of the electric potential and of the displacement are investigated when varying the ambient chemical conditions. The change of the mechanical displacement and the gel geometry is realized by the change of the osmotic pressure difference between the gel and the solution. The volume change of the gel leads to a change in the concentration of bound anionic groups while keeping their mole number constant. It is shown that the full coupling of the mechanical and the chemo-electrical field is necessary and that it is a real improvement to the previously developed one-way chemo-electric to mechanical coupling. It is demonstrated that the fully coupled model works as a kind of limiter for the change of the chemo-electric unknowns and thus for the gel deformation. A qualitative comparison with experimental results shows the validity of the fully coupled chemo-electro-mechanical model for chemical stimulation.