Hydrogel Smart Windows: Working Mechanisms, Recent Developments, and Perspectives

Smart windows (SWs) with energy-saving capabilities can dynamically regulate light transmittance and window optical characteristics, which is crucial for building energy efficiency. As a kind of typical wet-soft material, hydrogels have programmable network structures, excellent mechanical flexibilities, and multifunctionality, thus enabling them to be one of the most promising candidates to construct multifunctional SWs. However, although the rapid advance of SWs technology, various types of SWs, including thermo-, electro-, photochromic SWs, have been extensively reported, the recent research development, current challenges, and future perspectives of hydrogel-based smart windows (HSWs) have been rarely demonstrated. This review systematically summarizes the research progress of HSWs, comprising thermochromic, electrochromic, photochromic, humidity-responsive, mechanochromic, magnetochromic, and salt-responsive HSWs in recent years. First, the fabrication strategies for every HSW are presented, particularly focusing on functional materials, working principles, and property enhancement. Meanwhile, the strengths and weaknesses of every HSW are summarized and compared. Then, some emerging technologies, such as double-stimulus-triggered HSWs and multifunction-integrated devices, are discussed. Finally, the existing issues and future opportunities for HSWs are demonstrated to facilitate future scientific research and practical applications.