Mechanomutable and reversibly swellable polyelectrolyte multilayer thin films controlled by electrochemically induced pH gradients
We present a new strategy to electrochemically control the swelling state and mechanical properties of a polyelectrolyte multilayer thin film. While a number of pH-responsive polymer films and hydrogels have been developed, biological systems typically will not tolerate substantial deviations in pH. Therefore, to apply such pH-responsive systems for biomedical or other sensitive applications, we developed an electrochemical approach to alter local pH, while maintaining a constant, mild, bulk pH. The polymer film investigated in this work comprises polyallylamine hydrochloride (PAH) and sulfonated polystyrene (SPS) assembled at high pH (>9.0), which is known to exhibit a large pH-induced swelling transition; however, relatively extreme bulk pH values (pH < 4 to swell, and pH > 10.5 to deswell) are required to manipulate the film. Here, we apply negative electric potentials to gold electrodes coated with the film; the potential induces the reduction of dissolved oxygen, which generates hydroxide ions at the electrode surface and raises the local pH. The in situ swelling state and mechanical properties of the film have been probed with a number of techniques. Overall, we have attained reversible 300% volume changes in the polymer thin films, and have reversibly altered the mechanical properties over an order of magnitude (shear modulus between 1.9 MPa and 230 kPa, loss modulus between 620 kPa and 92 kPa, and effective indentation modulus between 19.2 MPa and 3.16 MPa). We maintain that electrochemical control over local pH is a promising strategy to manipulate pH-responsive polymer systems for biomedical and other applications.