We report the design of hydrogels
that can act as “smart” valves or membranes. Each hydrogel
is engineered with a pore (about 1 cm long and <1 mm thick) that
remains closed under ambient conditions but opens under specific conditions.
Our design is inspired by the stomatal valves in plant leaves, which
regulate the movement of water and gases in and out of the leaves.
The design features two different gels, active and passive, which
are attached concentrically to form a disc-shaped hybrid film. The
pore is created in the central active gel, and the conditions for
opening the pore can be tuned based on the chemistry of this gel.
For example, if the active gel is made from <i>N</i>-isopropylacrylamide
(NIPA), the actuation of the pore depends on the temperature of water
relative to 32 °C, which is the lower-critical solution temperature
(LCST) of NIPA. The concentric design of our hybrid provides directionality
to the volumetric transition of the active gel, i.e., it ensures that
the pore opens as the active gel shrinks. In turn, contact with hot
water (<i>T</i> > 32 °C) opens the pore and allows
the water to pass through the gel. Conversely, the pore remains closed
when the water is cold (<i>T</i> < 32 °C). The gel
thereby acts as a “smart” valve that is able to regulate
the flow of solvent depending on its properties. We have extended
the concept to other stimuli that can cause gel-swelling transitions
including solvent composition, pH, and light. Additionally, when two
different gel-based valves are arranged in series, the assembly acts
as a logical “AND” gate, i.e., water flows through the
valve-combination only if it simultaneously satisfies two distinct
conditions (such as its pH being below a critical value and its temperature
being above a critical value)
