Physical crosslinked hydrogel-derived smart windows: anti-freezing and fast thermal responsive performance

Cellulose-based hydrogel simultaneously possessing solar and evaporative cooling performances for energy-saving window and personal thermal management

Thermochromic smart windows have been widely developed for building energy saving. However, most smart windows suffer from limited energy-saving performance, fixed phase transition temperature, and are not suitable for the temperature regulation needs of different application scenarios. Herein, a unique self-adaptive thermochromic hydrogel (HBPEC-PNA) with self-moisture-absorbing performance is reported that assembles solar energy cooling and evaporative heat dissipation. Importantly, the hydrogel shows excellent energy-saving performance by using the synergistic interaction of solar and evaporative cooling. The HBPEC-PNA hydrogel has an adjustable critical transition temperature (21.8-33.9 °C), a high solar modulation (ΔTsol = 78.90 %), and a high light transmittance (Tlum = 90.15 %). Interestingly, the hydrogel harvests water molecules from the surrounding air, enabling it to be directly used in circumstances without losing its ability to dynamically regulate solar energy transmission due to water loss. Furthermore, the indoor simulation experiments confirmed that the dual-cooling system can reduce 14.2 and 2.4 °C, respectively, compared to ordinary glass and conventional smart window. Dramatically, HBPEC-PNA hydrogel can be utilized to personal thermal management due to its superior cooling property. This work provides an attractive strategy that will contribute to the development of thermochromic materials with excellent temperature regulation ability.

Fast-Developing Dynamic Radiative Thermal Management: Full-Scale Fundamentals, Switching Methods, Applications, and Challenges

Rapid population growth in recent decades has intensified both the global energy crisis and the challenges posed by climate change, including global warming. Currently, the increased frequency of extreme weather events and large fluctuations in ambient temperature disrupt thermal comfort and negatively impact health, driving a growing dependence on cooling and heating energy sources. Consequently, efficient thermal management has become a central focus of energy research. Traditional thermal management systems consume substantial energy, further contributing to greenhouse gas emissions. In contrast, emergent radiant thermal management technologies that rely on renewable energy have been proposed as sustainable alternatives. However, achieving year-round thermal management without additional energy input remains a formidable challenge. Recently, dynamic radiative thermal management technologies have emerged as the most promising solution, offering the potential for energy-efficient adaptation across seasonal variations. This review systematically presents recent advancements in dynamic radiative thermal management, covering fundamental principles, switching mechanisms, primary materials, and application areas. Additionally, the key challenges hindering the broader adoption of dynamic radiative thermal management technologies are discussed. By highlighting their transformative potential, this review provides insights into the design and industrial scalability of these innovations, with the ultimate aim of promoting renewable energy integration in thermal management applications.

Multi-functional smart bulk hydrogel panels with strong near-infrared shielding and active local control

Thermochromic hydrogel is a versatile smart material that can be used in various applications. In this paper, we present a new concept of smart windows to passively regulate light transmittance and reduce energy consumption while functioning as an information display. By incorporating passive solar regulation and active local control, this window is devised through the multilayer assembly of tailored poly(N-isopropylacrylamide) (PNIPAM) hydrogels and surface-modified photonic crystal films. The modified surface tension of solvent tunes the scattering center size of the hydrogel, and the addition of the photothermal films (PT films) imparts a high near-infrared (NIR) shielding and light-to-heat conversion, which is needed for low-latitude smart window application. Together with high writing speed, clarity, and repeatability for local writing. This new smart hydrogel engineering can have broad applications, allowing more functionalities in designing building façades.

High-Performance Triple-Network Hydrogels Derived from Chrome Leather Scraps: Ultrahigh Compressive Strength, Adhesion, and Self-Recovery

The development of engineered hydrogels with high strength, self-recovery, and adhesion is essential for applications requiring resistance to large deformations and cyclic loading. Herein, a triple-network (TN) hydrogel with ultrahigh compressive strength, strong adhesion, and good self-recovery was constructed by using tannic acid-modified chrome leather scrap hydrolysate as the first network, polyacrylamide as the second network, and poly-2-propenamide-2-methylpropanesulfonic acid as the third network. The ultrahigh (70 MPa compressive strength and 95% compression deformation) TN hydrogels were effectively created, which is attributed to the synergy of the three networks. The TN hydrogels display adhesion (adhesion strength > 20 kPa) ascribed to the introduction of phenolic hydroxyl groups in tannic acid. Intriguingly, the TN hydrogels exhibit excellent self-recovery performance (93.6% dissipated energy recovery at 70 °C) and shape memory performance (restored to the original shape in 20 s). These properties are essential for the development of high-performance hydrogels and promote the resource utilization of leather waste.

Tunable thermoresponsive and stretchable hydrogel sensor based on hydroxypropyl cellulose for human motion/health detection, visual signal transmission and information encryption

Thermoresponsive hydrogels can be used as smart flexible sensors. However, the design and facile preparation of multifunctional thermoresponsive hydrogel sensors still face great challenges. Herein, a tunable thermoresponsive, thermochromic and stretchable poly(2-hydroxypropyl acrylate-co-acrylamide) (P(HPA-co-AM))/hydroxypropyl cellulose (HPC)/lithium chloride (LiCl) hydrogel with the networks constructed from non-covalent interaction was fabricated by photopolymerization. PHPA exhibits excellent thermoresponsiveness. HPC endows the hydrogel with outstanding mechanical performance and enhanced temperature-sensitivity. LiCl not only provides good conductivity, but also regulates the lower critical solution temperature (LCST) of the hydrogel. The hydrogel shows tensile strength up to 300 kPa and maximum strain up to 790 %. The LCST value of the hydrogel can be adjusted from 38 to 75 °C. Therefore, the thermoresponsive conductive hydrogel can realize the information encryption, and be used as sensor through strain and temperature changes in the external environment to realize the motion and health detection, and visual signal transmission. This work is expected to provide ideas for the next generation of smart multifunctional electronic skin and information encryption device.

Tunable thermoresponsive hydrogels for temperature regulation and warning in fruit and vegetables preservation

Fresh fruit and vegetables usually suffer from quality deterioration when exposed to inappropriate temperatures. Common energy-input temperature regulation is widely applied but there remain challenges of increasing energy consumption. Passive temperature management regulates the heat transfer without energy consumption, showing a sustainable strategy for food preservation. Here, thermoresponsive hydrogels were constructed by incorporating NaCl and sodium dodecyl sulfate (SDS) micelles into a poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAM-co-AM)) network. Due to the excellent mechanical properties and reversible thermochromism at 14 °C and 37 °C, Gel-8 wt%-NaCl could inhibit temperature rise and avoid sunburn damage to peppers under direct sunlight by blocking the input of solar energy and accelerating moisture evaporation. Additionally, hydrogels could act as a feasible sensor by providing real-time visual warnings for inappropriate temperatures during banana storage. Based on the self-adaptive thermoresponsive behaviour, the prepared hydrogels showed effective performance of temperature regulation and quality preservation of fruit and vegetables.

Thermochromic hydrogel-based energy efficient smart windows: fabrication, mechanisms, and advancements

Thermochromic smart windows are regarded as highly cost-effective and easily implementable strategies with zero energy input among the smart window technologies. They possess the capability to spontaneously adjust between transparent and opaque states according to the ambient temperatures, which is essential for energy-efficient buildings. Recently, thermochromic smart windows based on hydrogels with various chromic mechanisms have emerged to meet the increasing demand for energy-saving smart windows. This review provides an overview of recent advancements in hydrogel-based thermochromic smart windows, focusing on fabrication strategies, chromic mechanisms, and improvements in responsiveness, stability and energy-saving performance. Key developments include dual-responsiveness, tunable critical transition temperatures, freezing resistance, and integrations with radiative cooling/power generation technologies. Finally, we also offer a perspective on the future development of thermochromic smart windows utilizing hydrogels. We hope that this review will enhance the understanding of the chromic mechanism of thermochromic hydrogels, and bring new insights and inspirations on the further design and development of thermochromic hydrogels and derived smart windows.

Zwitterionic cellulose nanofibers-based hydrogels with high toughness, ionic conductivity, and healable capability in cryogenic environments

Extreme environmental conditions often lead to irreversible structural failure and functional degradation in hydrogels, limiting their service life and applicability. Achieving high toughness, self-healing, and ionic conductivity in cryogenic environments is vital to broaden their applications. Herein, we present a novel approach to simultaneously enhance the toughness, self-healing, and ionic conductivity of hydrogels, via inducing non-freezable water within the zwitterionic cellulose-based hydrogel skeleton. This approach enables resulting hydrogel to achieve an exceptional toughness of 10.8 MJ m-3, rapid self-healing capability (98.9 % in 30 min), and high ionic conductivity (2.9 S m-1), even when subjected to -40 °C, superior to the state-of-the-art hydrogels. Mechanism analyses reveal that a significant amount of non-freezable water with robust electrostatic interactions is formed within zwitterionic cellulose nanofibers-modified polyurethane molecular networks, imparting superior freezing tolerance and versatility to the hydrogel. Importantly, this strategy harnesses the non-freezable water molecular state of the zwitterionic cellulose nanofibers network, eliminating the need for additional antifreeze and organic solvents. Furthermore, the dynamic Zn coordination within these supramolecular molecule chains enhances interfacial interactions, thereby promoting rapid subzero self-healing and exceptional mechanical strength. Demonstrating its potential, this hydrogel can be used in smart laminated materials, such as aircraft windshields.

Machine Learning Assisted Self-Powered Identity Recognition Based on Thermogalvanic Hydrogel for Intelligent Security

Identity recognition as the first barrier of intelligent security plays a vital role, which is facing new challenges that are unable to meet the need of intelligent era due to low accuracy, complex configuration and dependence on power supply. Here, a finger temperature-driven intelligent identity recognition strategy is presented based on a thermogalvanic hydrogel (TGH) by actively discerning biometric characteristics of fingers. The TGH is a dual network PVA/Agar hydrogel in an H2O/glycerol binary solvent with [Fe(CN)6]3-/4- as a redox couple. Using a concave-arranged TGH array, the characteristics of users can be distinguished adequately even under an open environment by extracting self-existent intrinsic temperature features from five typical sites of fingers. Combined with machine learning, the TGH array can recognize different users with a high average accuracy of 97.6%. This self-powered identity recognition strategy is further applied to a smart lock, attaining a more reliable security protection from biometric characteristics than bare passwords. This work provides a promising solution for achieving better identity recognition, which has great advantages in intelligent security and human-machine interaction toward future Internet of everything.

Micro/nanofabrication of heat management materials for energy-efficient building facades

Advanced building facades, which include windows, walls, and roofs, hold great promise for reducing building energy consumption. In recent decades, the management of heat transfer via electromagnetic radiation between buildings and outdoor environments has emerged as a critical research field aimed at regulating solar irradiation and thermal emission properties. Rapid advancements have led to the widespread utilization of advanced micro/nanofabrication techniques. This review provides the first comprehensive summary of fabrication methods for heat management materials with potential applications in energy-efficient building facades, with a particular emphasis on recent developments in fabrication processing and material property design. These methods include coating, vapor deposition, nanolithography, printing, etching, and electrospinning. Furthermore, we present our perspectives regarding their advantages and disadvantages and our opinions on the opportunities and challenges in this field. This review is expected to expedite future research by providing information on the selection, design, improvement, and development of relevant fabrication techniques for advanced materials with energy-efficient heat management capabilities.

LCST ion gels fabricating "all-in-one" smart windows: thermotropic, electrochromic and power-generating

Smart windows always respond to single stimuli, which cannot satisfy various needs in practical applications. Smart windows that integrate thermotropic, electrochromic and power-generating functions in one device is highly challenging yet important in satisfying on-demand light modulation and energy efficiency in practical applications. Herein, a thermoresponsive lower critical solution temperature (LCST) ion gel was fabricated via a facile in situ polymerization of butyl acrylate in a conventional ionic liquid to explore "all in one" smart windows. The ion gel-assembled smart windows are thermotropic and electrochromic with a reliable adjustment of light transparency as well as power-generating, enabled by the ionic Soret effect of ionic liquids. Additionally, the ion gels demonstrated self-defensive robust mechanical properties, thermal insulating and antifogging properties. With such an interdisciplinary and comprehensive study of the ion gels, the LCST ion gels could fulfil the requirements of genius windows with high energy-saving potential and exceptional climate adaptability, such as shut-down of light transmission in summer, daily solar energy collection, and colour changes on demand. It conceptually updates smart windows from an energy saving to an energy supplier in buildings. It is the first time to explore the "all in one" smart windows based on integrated multifunctional ionic liquids, which could greatly bridge the gap between the materials and buildings to accelerate practical applications of smart windows.

Engineering a polyvinyl butyral hydrogel as a thermochromic interlayer for energy-saving windows

Achieving mastery over light using thermochromic materials is crucial for energy-saving glazing. However, challenges such as high production costs, limited durability, and recyclability issues have hindered their widespread application in buildings. Herein, we develop a glass interlayer made of a polyvinyl butyral-based hydrogel swollen with LiCl solution. In addition to a fast, isochoric, and reversible transparency-to-opacity transition occurring as ambient temperatures exceed thermally comfortable levels, this hydrogel uniquely encompasses multiple features such as frost resistance, recyclability, scalability, and toughness. The combination of these features is achieved through a delicate balance of polyvinyl butyral's amphiphilicity and the suppression of network-forming phase separation. This design endows a nanostructured polyvinyl butyral-LiCl composite gel with swollen molecular segments linked by dispersed cross-linking sites in the form of hydrophobic nano-nodules. Upon laminating this hydrogel (a thickness of 0.3 mm), the resultant glazing product demonstrates approximately 90% luminous transmittance even at sub-zero temperatures, along with a significant modulation of solar and infrared radiation at 80.8% and 68.5%, respectively. Through simulations, we determined that windows equipped with the hydrogel could reduce energy consumption by 36% compared to conventional glass windows in warm seasons. The widespread adoption of polyvinyl butyral in construction underscores the promise of this hydrogel as a thermochromic interlayer for glazing.

Thermochromic Hydrogels with Adjustable Transition Behavior for Smart Windows

With the fast economic development and accelerating urbanization, more and more skyscrapers made entirely of concrete and glass are being constructed. To keep a comfortable indoor environment, massive energy for air conditioning or heating appliances is consumed. A huge amount of heat (>30%) is gained or released through glass windows. Using smart windows with the capability to modulate light is an effective way to reduce building energy consumption. Thermochromic hydrogel is one of the potential smart window materials due to its excellent thermal response, high radiation-blocking efficiency, cost-effectiveness, biocompatibility, and good uniformity. In this work, polyhydroxypropyl acrylate (PHPA) hydrogels with controllable lower critical solution temperature (LCST) were prepared by photopolymerization. The transition temperature and transition rate under "static transition" conditions were investigated. Unlike "static" conditions in which the transition temperature was not affected by the initial and final temperature and heating/cooling ramp, the transition temperature varied with the rate of temperature change under dynamic conditions. The "dynamic" transition temperature of the PHPA hydrogel gradually increased with the increase of the heating rate. It was the result of the movement of the molecular chains lagging behind the temperature change when the temperature change was too fast. The results of the solar irradiation experiment by filling PHPA hydrogels into double glazing windows showed that the indoor temperature was about 15 °C lower than that of ordinary glass windows, indicating that it can significantly reduce the energy consumption of air conditioning. In addition, a wide range of adjustable transition temperatures and fast optical response make PHPA hydrogels potentially applicable to smart windows.

Thermoelectric Hydrogel Electronic Skin for Passive Multimodal Physiological Perception

Electronic skins (e-skins) are being extensively researched for their ability to recognize physiological data and deliver feedback via electrical signals. However, their wide range of applications is frequently restricted by the indispensableness of external power supplies and single sensory function. Here, we report a passive multimodal e-skin for real-time human health assessment based on a thermoelectric hydrogel. The hydrogel network consists of poly(vinyl alcohol)/low acyl gellan gum with [Fe(CN)6]4-/3- as the redox couple. The introduction of glycerol and Li+ furnishes the gel-based e-skin with antidrying and antifreezing properties, a thermopower of 2.04 mV K-1, fast self-healing in less than 10 min, and high conductivity of 2.56 S m-1. As a prospective application, the e-skin can actively perceive multimodal physiological signals without the need for decoupling, including body temperature, pulse rate, and sweat content, in real time by synergistically coupling sensing and transduction. This work offers a scientific basis and designs an approach to develop passive multimodal e-skins and promotes the application of wearable electronics in advanced intelligent medicine.

Double Cross-Linked Hydrogel Dressings Based on Triblock Copolymers Bearing Antifreezing, Antidrying, and Inherent Antibacterial Properties

Bacterial infections typically invade the living tissue of wounds, thereby aggravating the inflammatory response, delaying wound healing, or causing further complications. In this paper, the antibacterial hydrogel (PNVBA) with antifreezing and antidrying properties was prepared by a two-step method using N-isopropylacrylamide (NIPAM), 1-butyl-3-vinylimidazolium bromide (VBIMBr), and 3-acrylamidophenylboronic acid (AAPBA). PNVBA hydrogels exhibited a high adsorption capacity of 280 mg·g-1 for bovine serum albumin (BSA) and can adhere to the surface of different materials through ion-dipole or hydrogen-bonding interactions. Meanwhile, the PNVBA hydrogels exhibited high viscoelasticity and good adhesion after freezing at -20 °C or heating at 70 °C for 24 h with a sterilizing rate of up to 98% against multidrug-resistant (MDR) Escherichia coli and methicillin-resistant Staphylococcus aureus (MRSA). Moreover, a survival rate of up to 90% after incubation with L929 cells over 24 h was observed. Therefore, this inherent antibacterial hydrogel can be used as an excellent alternative material for wound dressings.

Bidirectional optical response hydrogel with adjustable human comfort temperature for smart windows

Smart windows are effective in reducing the energy consumption of air conditioning and lighting systems, while contributing to maintaining the comfort zone of temperature in the indoor environment. Currently used smart windows mainly rely on traditional single-phase thermochromic material in which only one abrupt optical change occurs during temperature changes, and their inherent characteristics may not be suited for a practical balance of energy saving and privacy protection. Here, we developed a novel bidirectional optically responsive smart window (BSW) with unique bidirectional optical response features by introducing sodium dodecyl sulfate (SDS)/potassium tartrate (PTH) micelles into PNIPAM hydrogel to form a composite hydrogel, which was encapsulated in two glass panels. The upper critical solution temperature (UCST) and lowest critical solution temperature (LCST) of the material can be individually adjusted and are capable of matching the human comfort zone of temperature. In addition, the smart window exhibits remarkable transparency (92.5%), visible light transmission ratio (Tlum = 91.31%), and excellent solar modulation (ΔTsol,UCST = 76.34%, ΔTsol,LCST = 76.75%). Moreover, it possesses selectivity in transmitting light in the infrared band of solar radiation and can complete the "transparent-opaque" transition in a very narrow temperature range (<1 °C). When at comfortable temperatures, the highly transparent smart windows facilitate interior light and appreciation of the view. At low temperatures, SDS/PTH micelles aggregate to form large micelles, blocking the transmission of light and protecting customer privacy. At high temperatures, PNIPAM can undergo a "sol-gel" transition, thus blocking incident solar radiation. Taken together, these proposed materials with bidirectional optical response characteristics would be harnessed as a promising platform for building energy conservation, anti-counterfeiting, information encryption, and temperature monitoring.

Thermochromic Smart Windows with Ultra-High Solar Modulation and Ultra-Fast Responsive Speed Based on Solid–Liquid Switchable Hydrogels

Thermo-responsive hydrogels can dynamically modulate incident light, providing a broad prospect for development of smart windows, which are of pivotal importance for energy conservation in buildings. However, these hydrogels normally exhibit slow response speed and tend to contract over extended phase transition, compromising structural integrity of smart windows. In this study, a solid–liquid switchable thermochromic hydrogel, denoted as SL-PNIPAm, was synthesized by cross-linking PNIPAm with AMEO through dynamic imine bonds. Due to its distinctive solid–liquid transformation characteristics, SL-PNIPAm demonstrates rapid response time (within 5 s) and retains structural integrity without undergoing shrinkage during heating/cooling and freezing/thawing cycles. SL-PNIPAm can also be encapsulated within 2 glass panels to prepare smart windows, which showed extraordinary luminous transmittance ( T lum = 96.8%) and solar modulation ability (Δ T solar = 89.7%) and effectively reduced the indoor temperature (22 °C) in a simulated indoor experiment. Energy consumption simulation investigations are performed in diverse cities. The results reveal that SLW is capable of achieving a remarkable 54% reduction of HVAC energy consumption, leading to substantial decrease in CO 2 emissions by up to 40 kg m −2 annually. This work develops a new hydrogel system with outstanding durability for smart windows and will promote the development and renovation of thermochromic smart windows.

Freezing-resistant poly(N-isopropylacrylamide)-based hydrogel for thermochromic smart window with solar and thermal radiation regulation

Adaptive regulation of solar and thermal radiation by windows in diverse (hot and cold) climates is essential to reduce building energy consumption. However, conventional hydrogel-based thermochromic smart windows lack thermal radiation regulation, and have difficulty to combine high solar regulation with excellent freezing resistance. It is challenging to integrate the above performance into one hydrogel-based thermochromic window. Here, we firstly prepared poly(N-isopropylacrylamide-co-N, N-dimethylacrylamide)/ethylene glycol (PNDE) hydrogels with tunable and excellent freezing resistance (below -100 °C) by adding the anti-freezing agent ethylene glycol, and assembled PNDE hydrogels, polyvinylidene fluoride and polymethyl methacrylate-silver nanowires panels into a freezing-resistant smart window with solar and thermal radiation regulation (STR). PNDE hydrogels had an excellent thermochromic performance with luminous transmittance (Tlum) of 89.3 %, solar regulation performance (ΔTsol) of 80.7 % and tunable phase change temperature (τc, 22-44 °C). The assembled STR window showed high Tlum of 68.2 %, high ΔTsol of 62.6 %, suitable τc of ∼30 °C and freezing resistance to low temperature of -27 °C. Moreover, the different thermal emissivity (0.94 and 0.68) of the two sides of the STR window gave it the ability of radiative cooling in hot climates and warm-keeping in cold climates. Compared to the conventional thermochromic windows, the STR window promotes heat dissipation in hot conditions while reduces heat loss in cold conditions and is applicable to diverse climates, which is a promising energy-saving device for reducing building energy consumption.

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.