Reversible change in volume of thin hydrogel layer deposited on electrode surface using Cu(II)↔Cu(I) process

Thin hydrogel layers, based on a negatively charged cross-linked polymeric network, were deposited on conducting surfaces by means of an electrochemically induced free-radical polymerization method. The presence of ionized carboxylic groups in the polymer network allowed for the effective accumulation of Cu(II) cations in the hydrogel layer; these Cu(II) cations acted as an additional electroactive cross-linker and caused a substantial shrinking of the layer. It was found that the Cu(II) complexed by the polymer network could be electrochemically reduced to Cu(I) and that led to substantial swelling of the layer. Subsequent electrooxidation of Cu(I) led to the reformation of the crosslinking complexes, and again the shrinking of the layer was observed. The electrochemically active hydrogel layer deposited on the electrode surface was able to act as a microactuator due to its fast action, high stability, and reversibility of the shrinking/swelling transformation. The changes in hydrogel layer thickness were monitored by an electrochemical quartz crystal microbalance and confirmed by electron microscopy.