Smart Hydrogel-Based Valves Inspired by the Stomata in Plants

Abstract

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 N-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 (T > 32 °C) opens the pore and allows the water to pass through the gel. Conversely, the pore remains closed when the water is cold (T < 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).

Onion-like multilayered polymer capsules synthesized by a bioinspired inside-out technique

“…NIPA is well-known to be a thermoresponsive polymer, i.e., NIPA gels shrink when heated above the polymer’s lower critical solution temperature (LCST) of 32 °C 45 , 50 . DMAA, on the other hand, is not affected by temperature 48 . Figure 5b shows a three-layer capsule where layer 1 and 3 are DMAA and layer 2 is NIPA (the core is alginate/Ca 2+ , as before).…”

“…It is well-known that ionic polymer gels exhibit a different response to pH compared to nonionc polymer gels 43 – 45 . For example, an anionic gel based on a monomer such as sodium acrylate (SA) will be swollen at high pH when all its carboxylate groups are ionized and shrunken at low pH when the same groups lose their charge 46 – 48 . Nonionic gels, on the other hand, will exhibit the same volume at low and high pH.…”

Self-Stabilizing Transpiration in Synthetic Leaves

“…49,50 A recent work demonstrated hydrogel-based valves that dynamically respond to environmental stimuli, analogous to natural stomata. 51 Given that our only design consideration is to increase the humidity within the substomatal chambers, it was not necessary for our synthetic stomata to have a dynamic response. For the substomatal chambers, each chamber exhibited a cross-sectional diameter of 60 μm and arrays of chambers were organized in a square lattice configuration with the center-to-center pitch between chambers being 150 μm (Figure 2d).…”

Coumarin‐4‐ylmethyl‐ and p‐Hydroxyphenacyl‐Based Photoacid Generators with High Solubility in Aqueous Media: Synthesis, Stability and Photolysis

“…Nevertheless, Gargava et al . used diphenyliodonium nitrate as PAG to regulate the pH‐dependent pore size of bioinspired hydrogel valves . Also Feng et al .…”