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

Abstract

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(Nisopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) and a polybase layer, poly(N-isopropylacrylamide-co-N,N 0 -dimethylaminopropylacylamide) (P(NIPAAm-co-DMAPAAm)), where the adhesion of both layers was achieved via electrophoresis of semiinterpenetrating 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.

Reversible patterning and actuation of hydrogels by electrically assisted ionoprinting

“…Hydrogels are networks of crosslinked hydrophilic polymers capable of absorbing and releasing large amounts of water while maintaining their structural integrity. They are a diverse class of soft, biocompatible materials composed of a tissue-like aqueous matrix with the capacity to change shape in response to external stimuli such as salinity 5 , pH 5,14 , temperature 6,16,17 or light 18 . The potential to locally pattern hydrogels with chemical or physical means can provide control over their physical response, which is useful in numerous applications such as soft actuators [5][6][7][8] , drug administers 14,15 , microvalves 19 and cell encapsulation 17 .…”

Stimulus-active polymer actuators for next-generation microfluidic devices

“…The authors focused mainly on pH-and temperature-sensitive gels, but of course these 'smart' membranes can also be designed to respond to any other stimulus. The concepts presented so far utilize changes in the pH value of the surrounding medium to induce movement of the hydrogel structures; in [114], an alternative approach was presented: The authors used a phototriggered pH jump reaction of an integrated protonreleasing agent to induce local changes in the pH value within the gel structure and thus reversible movement. Although the concept is insufficiently robust-after a few actuation cycles, the bending capability decreased-and it has not yet been applied to microfluidics, it may be an interesting approach for microfluidic technologies.…”

Biomimetic photo-actuation: progress and challenges

“…To achieve this we are employing a metastable state photoacid (long lived yet reversible fall in pH upon light exposure) as the sensing and signal transmission element and a pH-responsive hydrogel as an actuating element. Techawanitchai et al used a similar combination of components to create a light sensitive smart material actuator, although a non-reversible photoacid generator was employed, significantly limiting the material's cycle life 7,8 . Shi et al attempted to overcome this limitation by actuating a pH-responsive hydrogel with a meta-stable state photoacid they developed, but were unable to achieve reversibility from the hydrogel used 9 .…”