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Hydrogels have been developed to respond to a wide variety of stimuli, but their use in macroscopic systems has been hindered
by slow response times (diffusion being the rate-limiting factor governing
the swelling process). However, there are many natural examples of chemically
driven actuation that rely on short diffusion paths to produce a rapid response. It is therefore expected that scaling down hydrogel objects to
the micrometre scale should greatly improve response times. At these scales,
stimuli-responsive hydrogels could enhance the capabilities of microfluidic
systems by allowing self-regulated flow control. Here we report the fabrication
of active hydrogel components inside microchannels via direct photopatterning
of a liquid phase. Our approach greatly simplifies system construction and
assembly as the functional components are fabricated in situ, and the
stimuli-responsive hydrogel components perform both sensing and actuation
functions. We demonstrate significantly improved response times (less than
10 seconds) in hydrogel valves capable of autonomous control of local flow.