Tetra-Sensitive Graft Copolymer Gels as Active Material of Chemomechanical Valves

Stimuli-responsive hydrogels combine sensor and actuator properties by converting an environmental stimulus into mechanical work. Those materials are highly interesting for applications as a chemomechanical valve in microsystem technologies. However, studies about key characteristics of hydrogels for this application are comparatively rare, and further research is needed to emphasize their real potential. The first part of this study depicts the synthesis of grafted hydrogels based on a poly(N-isopropylacrylamide) backbone and pH-sensitive poly(acrylic acid) graft chains. The chosen approach of grafted hydrogels provides the preparation of multiresponsive hydrogels, which retain temperature sensitivity besides being pH-responsive. A pronounced salt and solvent response is additionally achieved. Key characteristics for an application as a chemomechanical valve of the graft hydrogels are revealed: (1) independently addressable response to all stimuli, (2) significant volume change, (3) sharp transition, (4) reversible swelling-shrinking behavior, and (5) accelerated response time. To prove the concept of multiresponsive hydrogels for flow control, a net-poly(N-acrylamide)-g-poly(acrylic acid) hydrogel containing 0.6 mol % poly(acrylic acid)-vinyl is employed as active material for chemomechanical valves. Remarkably, the chemomechanical valve can be opened and closed in a fluidic platform with four different stimuli.