The response of current hydrogel devices mainly depends on the diffusion of stimuli.However, diffusion is a slow transport mechanism compared to advection, which therefore limits the response speed of hydrogel devices. To overcome this limitation, we introduce a capillary network and elastic instability mechanism. Particularly, an open surface capillary delivers and distributes solvent, thus triggering the swelling and bending of curved polymeric beams. To demonstrate this concept, we fabricate these polymeric microstructures using projection micro-stereolithography (PμSL). Combined with instability criteria analysis based on static beam theory, this device is designed to exhibit two-way snap-through behavior. Our analysis provides the minimum dimensionless stiffness β for the beam device to snap during solvent actuation. Here, β is a well-defined dimensionless parameter in our analysis that indicates whether the device can provide sufficient axial force to trigger the snap-through of the beam. The actuation displacement can be as high as 45% of the length of the beam. We observe a maximum midpoint speed of 3.1 cm s−1 for a beam 2 mm long—20 times higher than that for a beam without an elastic instability mechanism. This device can be used in artificial muscle and as the key component for fluidic-to-mechanical signal transduction in active micro-fluidic circuits.
Figure： (A) The CAD design of the polymer gel bistable device.
(B) A half section view of the curved beam. The arrows indicate the direction of the solvent flow.
(C), (D) Optical images of the microfabricated sample. Scale bars are 500 μm.
(E) The mechanism of solvent-introduced bending of hydrogel beams.