Thermoresponsive Hydrogels for the Construction of Smart Windows, Sensors, and Actuators

Conspectus Thermoresponsive hydrogels possess an inherent capacity for autonomous adjustment of their properties in response to temperature variations, eliminating the requirement for external power sources and rendering them suitable for diverse environmental applications. Our discourse commences by establishing a foundational comprehension of the two principal categories governing thermal transitions in thermoresponsive hydrogels, namely, the Lower Critical Solution Temperature (LCST) and the Upper Critical Solution Temperature (UCST). These thermal transitions, LCST and UCST, are pivotal determinants of the physical characteristics and reactivity of hydrogels, as they regulate the response and deformations of temperature-sensitive hydrogels across varying environmental conditions. Moreover, the integration of these hydrogels within the photonic crystal (PC) structures has emerged as a notable approach to modulating dielectric constants or lattice configurations, leading to color change. Due to these remarkable properties, thermoresponsive hydrogels have garnered significant research attention for various smart material applications, including energy-saving technologies, environmental and biometric sensing, and control systems. Despite these distinctive features driving extensive research in smart materials areas, challenges persist due to the inherent water-rich composition and compromised mechanical integrity of hydrogels. These limitations impede their deployment in extreme temperature conditions and make them susceptible to mechanical stress. To address these challenges, innovative strategies, including entanglement-induced reinforcement, incorporation of antifreeze agents, and the application of polyvalent metal ions, have been devised to bolster mechanical robustness and enhance the desired performance metrics of hydrogels. This Account provides readers with comprehensive insights into recent advancements in the field of thermoresponsive hydrogels, with a primary focus on classifying hydrogel categories and elucidating innovative fabrication techniques, particularly with reference to research conducted by our research groups. We systematically expound upon the underlying principles that govern reactions contingent upon thermal transition categories, underscored by illustrative examples of representative hydrogels and the synthetic methodologies employed. Following this, we conduct a comprehensive review of recent innovative property enhancement strategies aimed at broadening the applicability and utility in practical contexts of thermoresponsive hydrogel, addressing existing challenges such as drying, freezing, mechanical properties, and durability. Subsequently, an extensive analysis of applications stemming from the realm of thermoresponsive hydrogels is undertaken with a focus on the latest research trends and accomplishments pertaining to the innovative utilization of these materials in domains such as smart windows, actuators, and sensors. Finally, we reflect on the prevailing challenges and prospects concerning the commercialization and expansion of applications for thermoresponsive hydrogels, thereby concluding this Account.