Energy-Efficient Smart Window Based on a Thermochromic Hydrogel with Adjustable Critical Response Temperature and High Solar Modulation Ability

Thermochromic smart windows realize an intelligent response to changes in environmental temperature through reversible physical phase transitions. They complete a real-time adjustment of solar transmittance, create a livable indoor temperature for humans, and reduce the energy consumption of buildings. Nevertheless, conventional materials that are used to prepare thermochromic smart windows face challenges, including fixed transition temperatures, limited solar modulation capabilities, and inadequate mechanical properties. In this study, a novel thermochromic hydrogel was synthesized from 2-hydroxy-3-butoxypropyl hydroxyethyl celluloses (HBPEC) and poly(N-isopropylacrylamide) (PNIPAM) by using a simple one-step low-temperature polymerization method. The HBPEC/PNIPAM hydrogel demonstrates a wide response temperature (24.1-33.2 °C), high light transmittance (Tlum = 87.5%), excellent solar modulation (ΔTsol = 71.2%), and robust mechanical properties. HBPEC is a functional material that can be used to adjust the lower critical solution temperature (LCST) of the smart window over a wide range by changing the degree of substitution (DS) of the butoxy group in its structure. In addition, the use of HBPEC effectively improves the light transmittance and mechanical properties of the hydrogels. After 100 heating and cooling cycles, the hydrogel still has excellent stability. Furthermore, indoor simulation experiments show that HBPEC/PNIPAM hydrogel smart windows have better indoor temperature regulation capabilities than traditional windows, making these smart windows potential candidates for energy-saving building materials.

A review of recent advances in biomedical applications of smart cellulose-based hydrogels

In biomedical engineering, smart materials act as media to communicate physiological signals inspired by environmentally responsive stimuli with outer indicators for timely scrutiny and precise therapy. Various physical and chemical processes are applied in the design of specific smart functions. Hydrogels are polymeric networks consisting of hydrophilic chains and chemical groups and they have contributed their unique features in biomedical application as one of the most used smart materials. Numerous raw materials can form hydrogels, in which cellulose and its derivatives have been extensively exploited in biomedicine due to their high hydrophilicity, availability, renewability, biodegradability, biocompatibility, and multifunctional reactivity. This review collates cellulose-based hydrogels and their extensive applications in the biomedical domain, specifically benefiting from the "SMART" concept in their design, synthesis and device assembly. The first section discusses the physical and chemical crosslinking and electrospinning techniques used in the fabrication of smart cellulose-based hydrogels. The second section describes the performance of these hydrogels, and the final section is a comprehensive discussion of their biomedical applications.

Engineering Self-Adaptive Multi-Response Thermochromic Hydrogel for Energy-Saving Smart Windows and Wearable Temperature-Sensing

Buildings account for ≈40% of the total energy consumption. In addition, it is challenging to control the indoor temperature in extreme weather. Therefore, energy-saving smart windows with light regulation have gained increasing attention. However, most emerging base materials for smart windows have disadvantages, including low transparency at low temperatures, ultra-high phase transition temperature, and scarce applications. Herein, a self-adaptive multi-response thermochromic hydrogel (PHC-Gel) with dual temperature and pH response is engineered through "one-pot" integration tactics. The PHC-Gel exhibits excellent mechanical, adhesion, and electrical conductivity properties. Notably, the low critical solubility temperature (LCST) of PHC-Gel can be regulated over a wide temperature range (20-35 °C). The outdoor practical testing reveals that PHC-Gel has excellent light transmittance at low temperatures and radiation cooling performances at high temperatures, indicating that PHC-Gel can be used for developing energy-saving windows. Actually, PHC-Gel-based thermochromic windows show remarkable visible light transparency (Tlum ≈ 95.2%) and solar modulation (△Tsol ≈ 57.2%). Interestingly, PHC-Gel has superior electrical conductivity, suggesting that PHC-Gel can be utilized to fabricate wearable signal-response and temperature sensors. In summary, PHC-Gel has broad application prospects in energy-saving smart windows, smart wearable sensors, temperature monitors, infant temperature detection, and thermal management.

Phase-Changing Polymer Film for Smart Windows with Highly Adaptive Solar Modulation

Adjusting sunlight and thermal radiation from windows is important in efficient energy-saving applications. A high solar modulation (ΔT_sol) capability as well as a high luminous transmittance (T_lum) are the ultimate aim of smart windows. In this study, hydroxypropyl cellulose (HPC), W-doped VO₂, and poly-N-iso-propylacrylamide (PNIPAm) composite hydrogel films were produced. The sample was prepared between two glasses to constitute a smart window with a sandwich construction structure, which exhibits a high T_lum of 87.16%, a ΔT_sol of 65.71%, and a lower critical solution temperature (LCST) of 29 °C. The practical applications of conventional PNIPAm hydrogels are limited by the volume contraction of phase transition. Here, this challenge is addressed by the simple method of combining with HPC. The PNIPAm-2.5 wt % HPC hydrogels possess thermo-responsive contractility with a volume shrinkage rate of 8.5%. Even after 100 high- and low-temperature cyclic durability tests, the smart windows still exhibit a high solar modulation capability.

Recent advances in cellulose microgels: Preparations and functionalized applications

Microgels are soft, deformable, permeable, and stimuli-responsive microscopic polymeric particles that are now emerging as prospective multifunctional soft materials for delivery systems, interface stabilization, cell cultures and tissue engineering. Cellulose microgels are emerging biopolymeric microgels with unique characteristics such as abound hydroxyl structure, admirable designability, multiscale pore network and excellent biocompatibility. This review summarizes the fabrication strategies for microgel, then highlights the fabrication routes for cellulose microgels, and finally elaborates cellulose microgels' bright application prospects with unique characteristics in the fields of controlled release, interface stabilization, coating, purification, nutrition/drug delivery, and bio-fabrication. The challenges to be addressed for further applications and considerable scope for development in future of cellulose microgels are also discussed.

Self-rolling of vanadium dioxide nanomembranes for enhanced multi-level solar modulation

Thermochromic window develops as a competitive solution for carbon emissions due to comprehensive advantages of its passivity and effective utilization of energy. How to further enhance the solar modulation ([Formula: see text]) of thermochromic windows while ensuring high luminous transmittance ([Formula: see text]) becomes the latest challenge to touch the limit of energy efficiency. Here, we show a smart window combining mechanochromism with thermochromism by self-rolling of vanadium dioxide (VO₂) nanomembranes to enhance multi-level solar modulation. The mechanochromism is introduced by the temperature-controlled regulation of curvature of rolled-up smart window, which benefits from effective strain adjustment in VO₂ nanomembranes upon the phase transition. Under geometry design and optimization, the rolled-up smart window with high [Formula: see text] and [Formula: see text] is achieved for the modulation of indoor temperature self-adapted to seasons and climate. Furthermore, such rolled-up smart window enables high infrared reflectance after triggered phase transition and acts as a smart lens protective cover for strong radiation. This work supports the feasibility of self-rolling technology in smart windows and lens protection, which promises broad interest and practical applications of self-adapting devices and systems for smart building, intelligent sensors and actuators with the perspective of energy efficiency.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Near-Infrared-Activated Thermochromic Perovskite Smart Windows

Perovskite-based thermochromic smart windows that can change color have attracted much interest. However, the high transition temperature (>45 °C in air) hinders their practical application. Herein, a near-infrared (NIR) activated thermochromic perovskite window that enables reversible transition cycles at room temperature is proposed. Under natural sunlight (>700 W m-2 ), it efficiently harvests 78% NIR light to trigger the thermochromism of perovskites, blocking the heat gain from both the visible and NIR light. Meanwhile, it also exhibits a low mid-infrared emissivity of <0.3, suppressing thermal radiation to the indoor environment. A field test demonstrates that this smart window can reduce the indoor temperature by 8 °C compared to a normal glass window at noon. The near-room-temperature color change, multispectral thermal management, outstanding energy-saving ability, and climate adaptability, and solution-based process of this window make it unique and promising for real applications.

Application of New Energy Thermochromic Composite Thermosensitive Materials of Smart Windows in Recent Years

Thermochromic smart windows technology can intelligently regulate indoor solar radiation by changing indoor light transmittance in response to thermal stimulation, thus reducing energy consumption of the building. In recent years, with the development of new energy-saving materials and the combination with practical technology, energy-saving smart windows technology has received more and more attention from scientific research. Based on the summary of thermochromic smart windows by Yi Long research groups, this review described the applications of thermal responsive organic materials in smart windows, including poly(N-isopropylacrylamide) (PNIPAm) hydrogels, hydroxypropyl cellulose (HPC) hydrogels, ionic liquids and liquid crystals. Besides, the mechanism of various organic materials and the properties of functional materials were also introduced. Finally, opportunities and challenges relating to thermochromic smart windows and prospects for future development are discussed.

Reversible and repeatable phase transition at a negative temperature regime for doped and co-doped spin coated mixed valence vanadium oxide thin films

Smooth, uniform mixed valance vanadium oxide (VO) thin films are grown on flexible, transparent Kapton and opaque Al6061 substrates by the spin coating technique at a constant rpm of 3000. Various elements e.g., F, Ti, Mo and W are utilized for doping and co-doping of VO. All the spin coated films are heat treated in a vacuum. Other than the doping elements the existence of only V⁴⁺ and V⁵⁺ species is noticed in the present films. Transmittance as a function of wavelength and the optical band gap are also investigated for doped and co-doped VO thin films grown on a Kapton substrate. The highest transparency (∼75%) is observed for the Ti, Mo and F (i.e., Ti–Mo–FVO) co-doped VO system while the lowest transparency (∼35%) is observed for the F (i.e., FVO) doped VO system. Thus, the highest optical band gap is estimated as 2.73 eV for Ti–Mo–FVO and the lowest optical band gap (i.e., 2.59 eV) is found for the FVO system. The temperature dependent phase transition characteristics of doped and co-doped VO films on both Kapton and Al6061 are studied by the differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition is noticed in the range of −24 to −26.3 °C.

Smooth, uniform mixed valance vanadium oxide (VO) thin films are grown on flexible, transparent Kapton and opaque Al6061 substrates by the spin coating technique at a constant rpm of 3000.