Moisture-thermo dual-responsive hydrogel structure for liquid composition change detection and programmed actuation via vertical crosslinking gradient based defocusing photolithography

Abstract

• Dual-responsive hydrogel structure is designed via defocusing photolithography. • Curvature of the structure is changed according to moisture contents. • Liquid composition changes are detected by utilizing the moisture responsive property. • Structural curvature varies in accordance with thermal changes. • Thermally controlled and programmed structural actuations are implemented. Multifarious detection and response mechanisms have been introduced and implemented because responsive materials, particularly hydrogel polymers, adopt certain transformations by external reactions and recover their original appearance. Heterogeneous structures are usually fabricated to obtain responsive hydrogels. In addition, component frame gradation methods have been applied to overcome the limitations of material composition. In this study, we present versatile responsive hydrogel structures that perform as sensor or actuator. The structures are fabricated using a defocusing maskless photolithography system with an objective lens, and consist of a single hydrogel-retained polymeric crosslinking density gradient. The hydrogel structures immersed in anhydrous hygroscopic solutions fail to swell, thereby maintaining their curved shapes. The solutions, improperly stored and left unattended, naturally absorb ambient moisture, and the resulting increase in water content enhances water–polymer interactions proportionally. This enhanced interaction induces swelling of the hydrogel, leading to curvature changes, enabling the structure to function as a sensor for detecting changes in liquid composition. By utilizing the structure, a change in moisture content of approximately 3% is easily verified without mechanical assistance. In contrast, temperature-dependent property changes in ethanol solutions with minimal water content predominantly affect solution–polymer interactions rather than polymer–polymer interactions. Reversible structural responses of the hydrogel are analyzed under repeated thermal cycling, and actuators such as a gripper and walking robot operating via thermal switching are successfully developed.