Swelling of electrochemically active polyelectrolyte multilayers

This article is focused on the swelling of redox-active polyelectrolyte multilayers. Upon electrochemical stimulation, the influx of counterions and solvent molecules causes an increase of osmotic pressure in the film which results in a volumetric expansion. Motivated by potential biomedical and micromechanical applications, the current experimental and theoretical understanding of the process is described. Electroactive polyelectrolyte multilayers show a rich thermodynamic behavior which is dependent on both the properties of the polyelectrolytes, such as the pKa and the complexation enthalpy, and of the electrolyte solution, such as the pH and the type of counterions. Although, at present no complete theoretical description of the swelling phenomenon is available, a simple qualitative model can be used to explain the influence of various different external parameters on the swelling.