Smart hydrogels are promising materials for soft actuators in biomedical applications thanks to their varied responses to external stimuli. Light is a particularly attractive trigger for contactless stimulation of hydrogels that can induce reversible morphological changes without damaging the fragile gels. To meet the varied needs of applications in microfluidics, soft robotics, biomedicine, and other fields, there is significant demand for novel valve designs that are highly tunable, miniaturizable, and respond quickly to stimuli while maintaining their function over many activation cycles. Additionally, it is crucial to develop a more quantitative understanding of the mechanics of valve operation in response to different stimuli, especially when active hydrogels are combined with other materials in multicomponent devices. Here, stimulus-responsive valves are fabricated using active hydrogels deformed upon temperature changes and exposure to near-infrared radiation. Gold nanorods (AuNRs) acting as photothermal transducers are embedded inside cross-linked poly N-isopropylacrylamide (PNIPAM), allowing local morphological changes in response to light with high spatiotemporal control. These changes are described precisely as a function of the valve's confinement, aspect ratio, and the parameters of the stimulus using quantitative image analysis, providing novel mechanistic insights. Changing the aspect ratio of the valves and the degree of confinement of the hydrogel causes valves to either open or close during heating and can be used to control the magnitude of their response to different stimuli. These varied morphological changes are due to local, inhomogeneous deformations of the gel. The use of light as a trigger enables local confinement of the valve, reversible opening and closing, and fast response times on the order of seconds. The valves are shown to withstand hydrostatic pressures of up to 18 kPa, providing high potential for biomedical applications where precise pressure control and quick switching between open and closed states is critical.
Mittelholzer et al. (Mon,) studied this question.