Photo-switchable control of pH-responsive actuators via pH jump reaction

We propose a new approach to fabricate reversible self-bending actuators utilizing a photo-triggered pH jump reaction. A photo-initiated proton-releasing agent of o-nitrobenzaldehyde (NBA) was successfully integrated into bilayer hydrogels composed of a polyacid layer, poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) and a polybase layer, poly(N-isopropylacrylamide-co-N,N'-dimethylaminopropylacylamide) (P(NIPAAm-co-DMAPAAm)), where the adhesion of both layers was achieved via electrophoresis of semi-interpenetrating polyelectrolyte chains. The NBA-integrated bilayer gels demonstrated quick proton release upon UV irradiation, allowing the pH within the gel to decrease below the volume phase transition pH in 30 seconds. By controlling the NBA concentration and the gel thickness, the degrees and the kinetics of bending were easily controlled. Reversible bending was also studied with respect to the NBA concentration in response to 'on-off' UV irradiation. Additionally, self-bending of the non-UV irradiated region of the gel was also achieved because the generated protons gradually diffused toward the non-irradiated region. The proposed system can be potentially applied in the fields of mechanical actuators, controlled encapsulation and drug release, robotics and microfluidic technologies because control over autonomous motion by both physical and chemical signals is essential as a programmable system for real biomedical and nano-technological applications.