Figure 5.

Hydrogel valve designs. (a– c) A two-dimensional hydrogel valve design. (a) A hydrogel plug is polymerized in the gap between two adjacent parallel channels. The hydrogel composition is the same as that described in Fig. 4e, with a pH volume transition at pH = 5. The sensing channel (600-μm wide) was constructed wider than the regulated channel (300-μm wide) to keep the sensing channel open when the regulated channel is completely shut off. The microchannel is made of EPON (Nano XP SU-8 50; Microchem, Newton, MA). Transparent adhesive tape (regular packaging tape) was used to seal the 200-μm-deep EPON microchannels. (b) A pH 5.7 buffer is pumped through the sensing channel while water is pumped through the regulated channel. The hydrogel valve expands to seal off the flow in the regulated channel. The arrows denote the direction of the fluid flow. (c) The valve reopens the regulated channel when a pH 3.8 solution was pumped through the sensing channel. (d– h) A 3D PDMS/hydrogel hybrid valve design. (d) A schematic of the PDMS/hydrogel hybrid valve. (e and f) The top view and the side view of the hydrogel structure when it expands and deforms a membrane, blocking the flow in an adjacent channel. The fluid in the blocked channel has been dyed for visualization purposes. (g and h) The hydrogel valve contracts, and the membrane returns to a position that allows flow in the adjacent channel. [Bars = 400 μm (a– c) and 250 μm (e– h).] Note in the side views that the edge of the hydrogel actuator has been outlined for clarity.