Thermochromic hydrogels for synergistic mechano-optical properties and global energy saving potential

Abstract

As building energy consumption and carbon emissions continue to rise, minimizing energy loss through windows—major heat exchange pathways—has become an urgent challenge. To reduce building energy consumption by improving windows’ thermal insulation and light modulation, this study develops a thermoresponsive hydrogel named PDH, based on a poly(N-isopropylacrylamide) backbone, for use as a window coating. The hydrogel exhibits considerable visible light transmittance (97.92%) and high solar modulation ability (81.70%), along with favorable mechanical properties, which enable its stable and direct application onto single-pane glass surface, thereby simplifying the conventional glass–hydrogel–glass structure in smart windows. In practical building applications, PDH hydrogel shows significant thermal regulation, with an average daytime indoor temperature reduction of 6.95 °C and energy savings of 384.04 kJ m⁻². Furthermore, a global energy-saving and carbon-reduction model is developed using climate data to support the assessments and applications of PDH hydrogels across various climate regions.

It is challenging to design thermally regulating windows to reduce building energy consumption. Here the authors designed a thermoresponsive hydrogel-based window balancing mechanical, optical, and thermal properties.

Introduction

With the rapid increase in global energy consumption and carbon emissions from the building sector, enhancing building energy efficiency has become a crucial pathway toward achieving carbon neutrality¹. Windows, serving as major channels for energy exchange in buildings, significantly contribute to energy loss and unwanted heat gain, especially amid rising demand for air conditioning². Therefore, the development of high-performance window solutions particularly smart windows with light- and thermal-regulating functions has emerged as a central focus in improving building energy efficiency. Smart window technologies include photochromic^3,4, electrochromic^5,6, and thermochromic^{7, 8, 9–10} systems that enable dynamic regulation of solar radiation and indoor temperature. Among them, photochromic windows rely on expensive fabrication processes and perform poorly under low-light or adverse weather conditions¹¹; electrochromic windows require external power sources and complex circuitry, resulting in high costs and ongoing energy consumption, which limits their widespread application¹². In contrast, thermochromic windows can autonomously switch between transparent and opaque states based on ambient temperature without external energy input¹³. Their advantages such as zero energy consumption, multi-wavelength light modulation, simple fabrication, low cost, and scalability render them ideal candidates for energy-efficient building applications¹⁴.

Common thermochromic materials for smart windows include vanadium dioxide^15,16 (VO₂), liquid crystals^17,18, perovskites^19,20, ion gels^21,22, and hydrogels^23,24. Compared to other materials, hydrogels offer several distinct advantages, including a critical transition temperature (τ_c) closer to ambient conditions²⁵, lower cost, facile fabrication²⁶, intrinsic flexibility²⁷, and high environmental compatibility²⁸. Notably, hydrogels possess prominent optical properties. Studies have shown that at below the transition temperature, the visible light transmittance (T_lum) of hydrogels^{29, 30, 31–32} is approximately 1.5 to 3 times higher than that of VO₂ and liquid crystals, allowing greater solar heat gain and improved natural lighting; meanwhile, their solar modulation capability (ΔT_sol) is about four times that of perovskites and ion gels. However, conventional hydrogel materials often suffer from inadequate adhesion, low tensile strength, and suboptimal elastic modulus, which compromise their interfacial bonding and structural integrity during long-term service³³. As a result, such hydrogels are typically encapsulated in a sandwich structure composed of glass–hydrogel–glass, providing additional mechanical support and interfacial protection to enhance overall stability and durability¹⁴. Although strategies such as increasing crosslinking density³⁴, incorporating reinforcing agents (e.g., silica³⁵ or graphene³⁶), or reducing water content can enhance the mechanical strength of hydrogels, these modifications often lead to a deterioration in visible light transmittance, thereby compromising the solar modulation ability. This trade-off reveals that T_lum, ΔT_sol, and mechanical strength are not independently tunable parameters, but are intrinsically coupled through the hydrogel’s composition and structure. Addressing this coupling is essential because improvements in one property often compromise the others, making it fundamentally difficult to achieve simultaneous optimization. As a result, balancing these properties remains a core challenge in the development of high-performance thermoresponsive hydrogels.

In this work, we propose a thermoresponsive hydrogel (PDH hydrogel) based on a composite crosslinked network structure synthesized via radical polymerization³⁴ of N-isopropylacrylamide (NIPAm), N, N-dimethylacrylamide (DMAA), and 2-hydroxyethyl acrylate (HEA). The PDH hydrogel exhibits valuable comprehensive performance, including high visible light transmittance (T_lum = 97.92%), remarkable solar modulation ability (ΔT_sol = 81.70%), and remarkable mechanical properties (adhesion strength of 54.63 kPa, tensile strength of 34.31 kPa, and an elastic modulus of approximately 129.35 kPa), successfully achieving simultaneous optimization of multiple properties. Benefiting from the enhanced mechanical properties, the PDH hydrogel can be directly coated onto single-layer glass, thus eliminating the need for the conventional glass–hydrogel–glass sandwich configuration commonly employed in thermochromic smart windows, simplifying the structure and improving practicality. Practical applications demonstrate that the PDH smart window outperforms commercial low-emissivity and conventional glass by reducing the average daytime indoor temperature by 6.95 °C and saving up to 384.04 kJ m⁻² per day, indicating its promising potential. Moreover, the PDH hydrogel shows rapid thermal response, fully transitioning to the opaque state within 11.27 seconds at elevated temperatures, reliable cycling durability over 1000 thermal cycles, and long-term stability with a mass retention rate of 93.86 % after 180 days. Additionally, benefiting from the broad transition temperature range of the PDH hydrogel, a theoretical predictive model is established based on global climatic data to assess its energy-saving and carbon-reduction (ESCR) performance. These results provide a foundation for the application of PDH hydrogels in diverse regions, demonstrating their potential in building energy conservation.

Results

Preparation, thermochromic mechanism of PDH hydrogels

In this study, a thermoresponsive hydrogel was prepared using an ionic liquid-assisted synergistic copolymerization strategy. The preparation procedure is as follows: N-isopropylacrylamide (NIPAm) and N, N-dimethylacrylamide (DMAA) were mixed at a mass ratio of 4:1, followed by the addition of ultrapure water and a small amount of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄). The mixture was stirred at room temperature for 20 minutes. Subsequently, the crosslinker N, N’-methylenebisacrylamide (MBA) and the functional monomer 2-hydroxyethyl acrylate (HEA) were added, and stirring continued for another 10 min. Finally, the initiator ammonium persulfate (APS) and the co-initiator sodium bisulfite (NaHSO₃) were introduced. After rapid stirring for 1–2 min, the mixture was poured into molds and crosslinked at room temperature (Fig. 1a).

Fig. 1 Preparation process and working mechanism of PDH hydrogel. [Images not available. See PDF.]

a Preparation process of PDH hydrogel (indicated sequentially in a clockwise direction by blue arrows). This figure was created with Microsoft PowerPoint. b Real-life image of PDH hydrogel in the transparent state, with the text beneath clearly visible. c Schematic illustration of the internal crosslinking mechanism of PDH hydrogel. I Poly(N-isopropylacrylamide) (PNIPAm) chains exhibit weak hydrophilicity; II Incorporation of N, N-dimethylacrylamide (DMAA) and 2-hydroxyethyl acrylate (HEA) introduces amide and hydroxyl groups, which enhance hydrogen bonding interactions with water molecules, thereby improving hydrophilicity. The short-dashed lines represent hydrogen bonds, with the process illustrated in a locally magnified view. This figure was created with Microsoft PowerPoint. d Schematic illustration of the thermochromic mechanism of PDH hydrogel. I When the temperature is below the transition temperature (τ_c), the hydrogel remains in a hydrated swollen state, with polymer chains uniformly distributed in the aqueous phase, forming an almost single-phase system with negligible light-scattering interfaces, resulting in high transparency (see magnified view I). II When the temperature exceeds τ_c, polymer chains dehydrate and aggregate to form hydrophobic microdomains (magnified view II, red lines represent polymer chains intertwining and clustering together). In this process, the internal network transforms from a hydrophilic extended state to a compact aggregated state, leading to pronounced phase separation. The size of these hydrophobic microdomains is comparable to the wavelength of visible light, causing anisotropic scattering and leading to a transition from transparent to opaque or whitish appearance. This figure is generated using Autodesk 3ds Max and further processed with Microsoft PowerPoint. e Scanning electron microscopy (SEM) images: I Internal structure of PNIPAm hydrogel; II PDH hydrogel in the transparent state; III PDH hydrogel in the opaque state. Scale bars, 20 μm.

It is noteworthy that, while maintaining a consistent addition sequence, precise control over the component ratios critically influences the final performance of the PDH hydrogel. For instance, experimental results show that insufficient DMAA content leads to a significant reduction in optical transparency, accompanied by the emergence of numerous microscopic wrinkle-like structures on the surface. Given that the copolymerization of N-isopropylacrylamide (NIPAM) with hydrophilic monomers typically leads to an increase in the lower critical solution temperature (LCST), whereas copolymerization with hydrophobic monomers results in a decrease, and considering that 2-hydroxyethyl acrylate (HEA) is a hydrophilic monomer³⁷, excessive incorporation of HEA compromises the thermochromic performance of the PDH hydrogel, rendering the phase transition less effective and thereby reducing its applicability in real-world scenarios. Similarly, a high concentration of APS elevates the radical concentration and accelerates the polymerization reaction but may also result in uneven crosslinking and irregular gelation, adversely affecting the gel morphology and properties. Detailed synthesis ratios and procedures are provided in the Methods section and Supplementary Table 1. The resulting samples exhibit good structural integrity, smooth surfaces, and high transparency at ambient temperature (Fig. 1b).

The notable performance of the PDH hydrogel originates from its precise molecular design and multimodal crosslinking mechanism. During chemically initiated free radical polymerization, NIPAm, DMAA, and HEA undergo copolymerization in the presence of ammonium persulfate (APS) and sodium bisulfite (NaHSO₃), while MBA serves as the crosslinker to establish a three-dimensional chemical network³⁸. The incorporation of DMAA improves the spatial distribution of polymer chains, enhancing the dispersion and uniformity of PNIPAm segments²⁵, which facilitates the regularization of the network structure and the formation of a porous architecture (Fig. 1e II). This, in turn, strengthens the hydrogel’s thermal responsiveness and pore regulation capabilities. Moreover, the introduction of HEA plays a crucial role in modulating the hydrogel network. Due to its steric volume, HEA introduces inter-network gaps during polymerization and increases overall hydrophilicity³⁷, promoting the formation of a homogeneous, highly porous three-dimensional network (Fig. 1c). This multidimensional network, constructed from diverse monomers, significantly improves the mechanical properties (Supplementary Fig. 4), compressive strength (Supplementary Fig. 5), and structural stability of the hydrogel.

The thermochromic behavior of the PDH hydrogel arises from its temperature-driven reversible phase transition. Below the critical transition temperature (τ_c), the hydrogel remains in a hydrated, swollen state, with polymer segments uniformly distributed in the aqueous phase, resulting in a single-phase system with negligible light scattering interfaces, thus exhibiting high transparency³⁹. When the environmental temperature exceeds τ_c, a hydrophilic-to-hydrophobic phase transition occurs within the hydrogel, characterized by dehydration and aggregation of polymer chains into dense hydrophobic microdomains (as illustrated in Fig. 1d and Supplementary Movie 1). During this process, the internal network transforms from an expanded hydrophilic state to a compact aggregated state, demonstrating pronounced phase separation. The size of these hydrophobic microdomains is typically comparable to the wavelength of visible light; according to Mie scattering theory⁴⁰, incident light interacting with structures of similar scale undergoes anisotropic scattering, leading to the hydrogel’s transition from transparent to opaque or milky white. This thermoresponsive optical switching mechanism underpins the PDH hydrogel’s capability to regulate solar radiation and manage thermal energy. To validate this mechanism, scanning electron microscopy (SEM) was employed to characterize the internal morphology of the hydrogel before and after phase transition. The results reveal a uniform network structure with well-defined pores at low temperature (Fig. 1e II), whereas at elevated temperatures, polymer chains significantly contract to form dense aggregated microstructures (Fig. 1e III). This structural reorganization provides direct physical evidence of Mie scattering and confirms the hydrogel’s thermochromic behavior.

Optical and mechanical properties of PDH hydrogels

In this study, the effects of varying DMAA content on the transmittance (T_{sol, 200-2500 nm}) and thermochromic behavior of PDH hydrogels were systematically investigated. A series of hydrogel samples were prepared using 1.6 g of NIPAM with different amounts of DMAA (0 g, 0.45 g, 0.55 g, and 0.65 g), and their optical transmittance was measured. The results revealed a significant enhancement in transmittance with increasing DMAA content. Specifically, the transmittance (T_sol) of pure PNIPAm hydrogel was approximately 20.43%, which increased to 62.28% upon incorporation of 0.45 g DMAA. Further increases in DMAA content led to transmittance values of 76.52% (0.55 g DMAA) and 79.11% (0.65 g DMAA), respectively (Fig. 2a). However, the improvement in transmittance tended to plateau with excessive DMAA addition, indicating a saturation effect. This phenomenon may be attributed to the nonlinear influence of DMAA on the hydrogel’s internal structure and properties, thereby stabilizing the transmittance enhancement. Moreover, the introduction of excessive DMAA could lead to competitive hydrogen bonding (Fig. 2f), affecting the network configuration and mechanical strength of the hydrogel—a mechanism that will be further discussed in subsequent sections. Thus, considering the balance between improved optical properties and mechanical integrity, 0.65 g of DMAA was determined to be the optimal formulation.

Fig. 2 Optical, thermal, and mechanical properties of PDH hydrogels. [Images not available. See PDF.]

a Transmittance of PDH hydrogels with different N, N-dimethylacrylamide (DMAA) contents (0.00, 0.45, 0.55, 0.65 g) in the ultraviolet (UV), visible (Visible), and near-infrared (NIR, solar spectrum range) regions. The bar chart shows the AM1.5 G solar spectrum distribution in different wavelength ranges. b Photographic images of PDH-0.65 hydrogel at different temperatures. The temperature increases clockwise, with red arrows indicating heating and blue arrows indicating cooling. This thermochromic transition is reversible and has been tested over 1000 heating–cooling cycles. c Temperature-dependent transmittance of PDH hydrogel (15–55 °C) in the UV, visible, and NIR regions. The bar chart illustrates the AM1.5 G solar spectrum distribution across the corresponding wavelength ranges. d Variation of visible light transmittance with temperature for PDH hydrogels with different DMAA contents (0.00, 0.45, 0.55, 0.65 g). The transition temperature of each hydrogel is defined as the temperature at which visible light transmittance decreases to 50%. e Stress–strain curves of hydrogels with different compositions: P denotes PNIPAm hydrogel, PD denotes PNIPAm-co-DMAA hydrogel, and PDH denotes PNIPAm-co-DMAA-co-HEA hydrogel. The elastic modulus of each hydrogel is indicated. f Fourier-transform infrared (FTIR) spectra of hydrogels with different compositions (nomenclature as above). The absorption peaks corresponding to the stretching vibrations of carbonyl (C=O) and amino (N–H) groups are marked with their wavenumbers.

The thermochromic behavior of PDH hydrogels was also evaluated by subjecting the samples to a controlled temperature environment. The hydrogels exhibited a semi-transparent appearance at 30 °C, transitioning to an opaque, milky white state at 50 °C, demonstrating promising potential for practical applications (Fig. 2b and Supplementary Movie 1). Further temperature-dependent transmittance measurements showed that the transmittance decreased with increasing temperature: at 15 °C, T_sol reached approximately 79.11%, but dropped to about 13.07% at 55 °C (Fig. 2c). In the ultraviolet (200–400 nm) and visible (400–760 nm) regions, PDH hydrogels exhibited high transmittance, while effectively blocking radiation in the near-infrared (NIR, 760–2500 nm) region. Given that visible light is essential for indoor lighting and that NIR radiation contributes primarily to solar heat gain⁴¹, the ability to selectively block NIR through smart windows could substantially reduce indoor temperatures and enhance energy efficiency.

To further quantify the phase transition temperature, the relationship between visible light transmittance (T_{lum, 380-750 nm}) and temperature was examined. The data revealed that transmittance decreased with rising temperature, and that this temperature-dependent optical transition became more pronounced with increasing DMAA content. Notably, the thermochromic transition range broadened as the DMAA content increased, indicating an extended temperature span for the transparent-to-opaque phase shift. It is important to note that the observed transition temperature refers to a temperature range rather than a single critical point (τ_c). According to the definition in the reported work⁴², the transition temperature was defined as the temperature corresponding to a 50% decrease in transmittance. For pure PNIPAm hydrogel, the 50% transmittance point occurred at approximately 32 °C; for PDH-0.45, PDH-0.55, and PDH-0.65, the transition temperatures were about 37 °C, 45 °C, and 49 °C, respectively (Fig. 2d). This increasing trend in transition temperature with DMAA content may be due to the changes in the crosslinking density and network structure introduced by DMAA, which also contributed to improved transparency. Furthermore, as a hydrophilic monomer, DMAA may alter the microenvironment of the hydrogel, enabling a delayed and more pronounced thermoresponsive transition at higher temperatures (Supplementary Fig. 6). Therefore, an increased DMAA content leads to an upward shift in the transition temperature and enhanced thermal responsiveness.

To assess the mechanical properties, the tensile strength of PNIPAm (P), PNIPAm-co-DMAA (PD), and PDH hydrogels was evaluated. The elastic modulus of the P hydrogel was approximately 8.09 kPa, that of PD was 7.70 kPa, while the PDH hydrogel exhibited a markedly higher modulus of 12.94 kPa (Fig. 2e). These results are closely related to the composition of the hydrogels, where hydrogen bonding plays a critical role in enhancing mechanical strength. As one of the key intermolecular forces, hydrogen bonds can increase the degree of physical crosslinking, thereby reinforcing the overall structural stability of the hydrogel network⁴³.

Fourier-transform infrared (FTIR) spectroscopy further confirmed the influence of hydrogen bonding on mechanical performance. The amide group (–CONH–) in DMAA contains both –NH and C=O functional groups capable of forming hydrogen bonds with the amide groups in NIPAm. Additionally, the hydroxyl groups (–OH) in HEA provide additional hydrogen bonding sites, further promoting intermolecular interactions (Supplementary Fig. 7). In the FTIR spectra of the three hydrogels, the characteristic absorption of the C=O (carbonyl) stretching typically appears between 1640 cm⁻¹ and 1690 cm⁻¹, and that of the N–H stretching lies between 3300 cm⁻¹ and 3400 cm⁻¹ ⁴⁴. The formation of hydrogen bonds during polymerization leads to decreased vibrational frequencies of these groups, resulting in a shift to lower wavenumbers—a phenomenon known as redshift. The redshift of absorption bands serves as a clear indication of hydrogen bond formation. Specifically, the C=O absorption peak shifted to 1632.93 cm⁻¹, and the N–H peak to 3276.95 cm⁻¹ (Fig. 2f), reflecting the enhanced intermolecular interactions.

In hydrogels containing DMAA, the newly introduced amide groups can form hydrogen bonds with those in PNIPAm, competing with the original hydrogen bonding network. This competition disrupts the pre-existing hydrogen bond architecture and weakens the overall interaction strength. Consequently, while DMAA provides additional bonding sites, the net hydrogen bonding intensity may not increase significantly, leading to only slight redshifts in the absorption peaks and a slight reduction in mechanical strength, as observed in the PD hydrogel’s lower elastic modulus. Upon introduction of HEA, a substantial number of hydroxyl groups are incorporated into the hydrogel, providing abundant hydrogen bonding sites that substantially reinforce the intermolecular crosslinking network. As a result, significant redshifts of the C=O and N–H absorption peaks to 1623.76 cm⁻¹ and 3266.30 cm⁻¹, respectively, were observed (Fig. 2f), confirming the strengthening of hydrogen bonding interactions. This enhancement in hydrogen bonding contributed notably to the improved mechanical strength of the PDH hydrogel.

Performance of PDH smart windows

Based on the preceding investigations into the properties of PDH hydrogels, smart windows fabricated using the PDH-0.65 hydrogel exhibited comparable performance characteristics. When the temperature exceeded the transition temperature, the PDH smart window transitioned from a highly transparent state to a semi-transparent state, eventually becoming fully opaque with a milky-white appearance. In its transparent state, the PDH smart window allowed partial transmission of ultraviolet (UV) light and complete transmission of visible light (VIS), while effectively blocking the majority of near-infrared (NIR) radiation. In contrast, under the opaque state, the transmittance across nearly all spectral regions decreased significantly (Fig. 3a).

Fig. 3 Laboratory evaluation of the optical and thermoresponsive properties of PDH smart windows. [Images not available. See PDF.]

a PDH smart windows allow most visible light to pass through while effectively reflecting near-infrared (NIR) radiation when the temperature is below the transition temperature (τ_c). When the temperature exceeds τ_c, the smart windows block the majority of solar radiation. This figure was created with Microsoft PowerPoint. b Outdoor demonstration of smart windows. The left panel shows the opaque state captured under high ambient temperature, while the right panel shows the transparent state photographed under lower temperature. c Response time of PDH smart windows during heating and cooling cycles. The upper panel represents the heating process, and the lower panel represents the cooling process. d Effect of hydrogel thickness on the luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) of PDH smart windows. e Comparative analysis of the optical performance of thermochromic hydrogel-based smart windows, including poly(N-isopropylacrylamide) (PNIPAm), polyacrylamide (PAM), cellulose, and ionic gel systems, as well as multiple PNIPAm-based composites (e.g., PNIPAm–PAM, PNIPAm–hydrogenated benzothiadiazole (HBPEC), PNIPAm–2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), PNIPAm–N,N-dimethylacrylamide (DMAA), and PNIPAm composites incorporating sodium dodecyl sulfate (SDS), up conversion nanoparticles (UCNPs), hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), polyethylene terephthalate (PET), antimony-doped tin oxide (ATO), and 2-aminoethyl methacrylate (AEMA)). The results highlight PDH smart windows as among the top-performing candidates within this class of materials. f Thermal cycling durability test of PDH smart windows. Red dots indicate transmittance in the high-temperature state, while blue dots represent transmittance in the low-temperature state. The smart windows maintained stable performance after 1000 heating–cooling cycles.

To ensure the long-term stability and functionality of hydrogels in smart window applications, their mechanical properties must meet multiple criteria. First, the hydrogel should exhibit strong adhesion strength; an adhesion strength exceeding 50 kPa is sufficient to ensure firm bonding to glass or coated interfaces, thereby preventing delamination or edge peeling during prolonged use and maintaining stable optical and thermal regulation performance⁴⁵. Second, high tensile strength is a critical requirement to accommodate stress variations induced by external factors such as thermal expansion, contraction, or mechanical disturbances, thereby preventing tearing or fracture and ensuring structural integrity under dynamic conditions⁴⁶. In addition, the hydrogel should possess an appropriate elastic modulus—typically in the range of 35–144 kPa—to accommodate slight deformations of the glass substrate caused by temperature fluctuations, and to mitigate interfacial stress concentrations that could otherwise lead to material failure or detachment⁴⁷. To meet the above mechanical performance requirements, the designed PDH hydrogel exhibited solid comprehensive mechanical properties, including the adhesion strength of 54.63 kPa, the tensile strength of 34.31 kPa, and the Young’s modulus of 129.35 kPa (Fig. 2e and Supplementary Fig. 8). These mechanical parameters satisfy the critical thresholds for smart window applications, ensuring reliable interfacial stability and mechanical adaptability under practical conditions.

Owing to its enhanced mechanical robustness, the PDH hydrogel can be directly applied onto single-pane glass surfaces, eliminating the need for the conventional laminated glass–hydrogel–glass structure typically required in thermochromic smart windows (Supplementary Fig. 9 and Movie 2). This design not only eliminates the need for hydrogel replacement but also significantly simplifies maintenance procedures, thereby facilitating the real-world deployment of thermoresponsive hydrogel coatings in smart window systems and expanding their potential application scenarios.

As shown in Fig. 3b, the smart window exhibited two distinct states: transparent and milky-white opaque. Outdoor testing at 114.347°E, 30.517°N demonstrated this thermochromic transition. Under direct sunlight with ambient temperatures above the transition threshold, the window turned milky-white opaque. As the solar angle changed and the temperature fell below the threshold, it reverted to its transparent state. To assess the response speed, laboratory-based thermal cycling tests were performed. Immersing the PDH smart window in hot water (above its transition temperature, τ_c) triggered a transparent-to-opaque transition in 11.27 seconds, while immersion in cool water (near room temperature) restored the transparent state within 22.09 seconds (Fig. 3c, Supplementary Movie 3). These results demonstrate the rapid and reversible thermal responsiveness of the PDH smart window. Experimental results indicated that the thickness of the hydrogel layer significantly influences the optical performance of the smart window. To investigate this effect, PDH smart windows of various thicknesses were fabricated and compared (Fig. 3d). Excessively thin hydrogel layers exhibited diminished thermochromic contrast upon heating, yielding a lower ΔT_sol (78.28%). Conversely, overly thick layers compromised visible light transmittance (T_lum), reducing it to 85.31% and thus also decreasing ΔT_sol. Therefore, a hydrogel thickness of approximately 1 mm was identified as optimal for balancing transparency and solar modulation performance.

The optical performance of the PDH smart window was quantitatively evaluated through visible light transmittance (T_lum, 380–750 nm), Solar light transmittance (T_sol, 200–2500 nm) and solar modulation ability (ΔT_sol), calculated using the following equations⁴⁸:

Visible light transmittance:

1

$$T_{lum/sol}=\frac{\int {\phi }_{lum/sol\left(\lambda \right)T\left(\lambda \right)d\lambda }}{\int {\phi }_{lum/sol\left(\lambda \right)d\lambda }}$$

Solar modulation ability:

2

$${\mathrm{\Delta }T}_{sol}=T_{transparent,solar}-T_{opaque,solar}$$ Where ${\mathrm{\phi }}_{\mathrm{lum}/\mathrm{sol}\left(\mathrm{\lambda }\right)}$ denotes the solar spectral irradiance in the visible range (380–750 nm) or the solar range (200-2500 nm), $\mathrm{T}\left(\mathrm{\lambda }\right)$ represents the spectral transmittance of the smart window, ${\mathrm{T}}_{\mathrm{transparent},\mathrm{solar}}$ is the integrated transmittance over 200–2500 nm in the transparent state, and ${\mathrm{T}}_{\mathrm{opaque},\mathrm{solar}}$ is the corresponding value in the opaque state. Integrating the measured transmittance data against the AM1.5 G solar spectrum, the visible transmittance and solar modulation ability of the PDH smart window were calculated to be 97.92% and 81.70%, respectively (Supplementary Fig. 10). These values surpass those of comparable PNIPAm-based smart windows, such as PNIPAm–polyacrylamide^{34,49, 50, 51–52} (PAM), PNIPAm–hydrogenated benzothiadiazole^33,53 (HBPEC), PNIPAm–2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)⁵⁴, PNIPAm–dimethylacrylamide (DMAA) as well as PNIPAm-based composites incorporating SDS⁵⁵ (sodium dodecyl sulfate), UCNPs⁵⁶ (upconversion nanoparticles), HPMC (hydroxypropyl methylcellulose), PVA²³ (polyvinyl alcohol), PET⁵⁷ (polyethylene terephthalate), ATO³⁶ (antimony-doped tin oxide), and AEMA³² (2-aminoethyl methacrylate), (Supplementary Table 2), placing the PDH smart window among the top-performing candidates in its class (Fig. 3e).

The PDH smart window also demonstrated solid durability in cyclic use. It maintained its optical switching capability with negligible performance loss after 1000 thermal cycles (Fig. 3f), highlighting its reliable reusability. Furthermore, during a six-month laboratory-based aging test simulating seasonal changes from summer to winter, the PDH smart window retained 95.94% of its original mass, indicating good long-term stability (Supplementary Fig. 11). These findings confirm the robust long-term stability and application potential of PDH smart windows in building thermal management systems.

Application of PDH hydrogels in buildings

To evaluate the practical potential of the PDH smart window in building energy-saving applications, we integrated it into a self-constructed experimental building model and conducted comprehensive outdoor tests to assess its thermal regulation performance and energy-saving capacity. The test structure employed a roof-window design, with thermal insulation foam lining the interior walls to minimize heat conduction through the envelope (Fig. 4a). This setup ensured that heat exchange occurred primarily through the window, allowing a more accurate evaluation of the material’s influence on the indoor thermal environment.

Fig. 4 Real-world performance and energy-saving assessment of PDH smart windows. [Images not available. See PDF.]

a Experimental house model. In the control group, the interior was lined with thermal insulation, and the roof was fitted with a conventional glass window and temperature sensors; the experimental group was arranged identically except that the roof window was replaced with a hydrogel smart window. This figure was created with Microsoft PowerPoint. b Schematic illustration of the PDH hydrogel film (PHF) and the PDH hydrogel packaged film (PHPF). c Transmittance spectra of PHF smart window (SW) and PHPF smart window in the solar wavelength range. d Comparison of the temperature regulation performance of PHF and PHPF smart windows, along with real-time ambient temperature (yellow curve) and solar irradiance (bar chart). e Differences in service life and mass retention among PNIPAm hydrogel film, PHF, and PHPF. Red circles indicate the time point at which the smart window (SW) lost its thermochromic capability. f Photographs of windows made from PHPF, low emissivity (Low-E) film, and conventional glass in real building applications, shown here with the PHPF smart window in its opaque state. g Comparison of the temperature regulation performance of windows made from PHPF, Low-E film, and conventional glass, together with real-time ambient temperature (yellow curve) and solar irradiance (bar chart). h Variation of heat flux density of the PHPF smart window (experimental group) over time and its estimated daily energy savings.

Considering the inevitable water loss of hydrogels during real-world operation, hydrogel-based smart windows directly coated onto glass substrates typically suffer from dehydration-induced loss of thermochromic functionality, resulting in a limited-service life. To address this issue and enhance long-term stability and durability, a widely adopted hydrogel packaging strategy³⁹ was employed in this study. In this structure, the hydrogel film was combined with an organic polymer sheet exhibiting high visible light transmittance, good chemical stability, and significant weather resistance, enabling its direct integration into existing building structures. Specifically, polyvinylidene chloride (PVDC), which meets the above requirements, was selected as the encapsulation material and laminated with the PDH hydrogel film (PHF) to fabricate the PDH hydrogel-packaged film (PHPF) (Fig. 4b). Upon encapsulation with PVDC, the visible light transmittance of the PHPF smart window decreased slightly from 97.92% to 93.85% (Fig. 4c), a marginal reduction that does not compromise its daylighting performance, thereby confirming the feasibility of the packaging strategy.

Subsequently, we exposed both the PVDC-encapsulated PHPF and the unencapsulated PHF smart windows to actual outdoor conditions and conducted comparative evaluations of their thermal regulation performance and operational lifetimes. Throughout the exposure period, key environmental parameters—including ambient temperature, relative humidity, solar irradiance, and wind speed—were continuously monitored using a professional weather station (Supplementary Fig. 12) to ensure accurate correlation between environmental conditions and device performance. The results demonstrated that PHPF outperformed PHF in both aspects. During daytime testing, the PHPF group exhibited an average indoor temperature 3.39 °C lower than that of the PHF group (Fig. 4d). Furthermore, the PHPF smart window lost its thermochromic function after 264 hours of continuous operation due to water evaporation under conditions of an average relative humidity of 53.01% and wind speed of 0.31 m s⁻¹ (Supplementary Table 3)—a significant improvement over the 120-hour and 60-hour service lifetimes observed for the PHF and conventional PNIPAm-based smart windows, respectively. (Fig. 4e). The operational lifetime was evaluated using both the thermochromic switching ability and the retained mass ratio, defined as the ratio of the current mass (m) to the initial mass (m₀), R = m/m₀. These two indicators jointly reflect the environmental stability and functional durability of the smart windows under practical conditions.

To further assess performance in a realistic setting, the PHPF smart window, commercial Low-E glass, and conventional single-pane glass were installed on the test buildings (Fig. 4a) and subjected to a two-day outdoor evaluation under identical geographical, altitudinal, and solar radiation conditions (Fig. 4f). During peak solar radiation, the average ambient temperature was 41.22 °C, and the average solar irradiance was 717.74 W m⁻². The recorded average daytime indoor temperatures were 51.44 °C for the single-pane group, 46.07 °C for the Low-E glass group, and 43.15 °C for the PHPF smart window group. The PHPF group exhibited an average temperature reduction of 6.95 °C compared to ordinary glass (Fig. 4g), significantly outperforming traditional glazing materials. Taking the single-pane glass as a reference, we combined temperature differences with measured irradiance data to estimate the cooling load reduction (details in Methods). Under typical sunny conditions, the PDH smart window achieved an energy savings of approximately 384.04 kJ m⁻² (Fig. 4h). A rough estimation based on this daily energy-saving data indicates that its annual energy savings reach 140.16 MJ m⁻². Regarding the reference standards for energy savings, an annual energy saving of 100 MJ m⁻² is considered a high level, while exceeding 500 MJ m⁻² is regarded as an extremely high level⁵⁸. Thus, the annual energy savings of PDH smart windows have already met the high-level standard, confirming their promising potential and practical applicability in building energy conservation.

Global energy-saving potential of PDH hydrogels

The results in Fig. 2d indicate that the PDH hydrogel exhibits a broad phase transition temperature range, which endows it with the potential for year-round thermal regulation under diverse climatic conditions. This study focuses on its climate adaptability and energy-saving potential as a building window material.

To evaluate the global thermal regulation capability of PDH smart windows, we developed a climate-coupled simulation framework that integrates experimentally obtained optical and thermal parameters with global environmental datasets. Specifically, we employed 2024 surface climate data—including 2 m air temperature and downward solar radiation—and incorporated these data into the simulation model to calculate the monthly average energy-saving and carbon-reduction (M-ESCR) performance of PDH hydrogel across different regions. The climate datasets cover summer, winter, and the full year of 2024, sourced from the global climate dataset (ERA5-Land monthly averaged data from 1950 to present). This evaluation framework enables spatial quantification of the energy-saving and carbon mitigation effects of hydrogel windows under realistic irradiance and thermal conditions, and presents the results as global distribution maps.

Based on the previous reference standards⁵⁸ and considering the differences in monthly climatic conditions, it can be inferred that a monthly energy saving of 10 MJ m⁻² is regarded as a high level, while exceeding 40 MJ m⁻² is considered an extremely high level. Simulation results show that in the equatorial core zone (0°-10° N/S), such as the Malay Archipelago and the Amazon Basin, the average monthly energy savings reach 45.66 MJ m⁻² (Fig. 5a). In this region, the solar altitude angle is close to 90°, leading to intense solar radiation, which becomes the primary driver of building cooling demands⁵⁸. Energy-saving windows can effectively block most of the solar heat. In the low-latitude peripheral zone (10°-30° N/S), including the Sahara Desert and the South China Sea islands, the average monthly energy savings are 30-35 MJ m⁻² (Fig. 5a). In the tropical desert climate (e.g., the Sahara), summer surface temperatures exceed 50 °C, and building cooling demand is significant⁵⁹. Through infrared reflection and reduced heat conduction, energy-saving windows achieve a monthly savings of up to 43.24 MJ m⁻² (Fig. 5e). In winter, when temperatures drop to around 20 °C, energy savings drop to around 10 MJ m⁻² (Fig. 5c). In the mid-latitude transitional zone (30°-60° N/S), represented by northern China and the Midwestern United States, the average monthly energy savings are 10-20 MJ m⁻² (Fig. 5a). In temperate continental climates (e.g., Xinjiang), winter heating dominates, with substantial building heat loads⁶⁰. Energy-saving windows must balance transparency and insulation, resulting in a winter monthly energy savings of only 5.36 MJ m⁻² (Fig. 5c). In summer, enhanced solar radiation increases savings to 23.64 MJ m⁻² (Fig. 5e). In high-latitude regions, such as Siberia and Greenland, the annual average energy savings are less than 10 MJ m⁻² (Fig. 5a). In frigid climates, winters last up to six months, with building heating demands remaining high⁶⁰. To utilize solar heat, the window transmittance must be relatively high, which results in reduced heat-blocking efficiency. The peak summer monthly energy savings are approximately 5 MJ m⁻² (Fig. 5e), while in winter, due to weak solar radiation, energy savings are nearly zero (Fig. 5c). More than 80% of annual savings occur during summer, highlighting a distinct unidirectional distribution under extreme climates.

Fig. 5 Predicted Global Energy-Saving and Carbon-Reduction Potential of the PDH Smart Window. [Images not available. See PDF.]

a Global distribution map of the monthly average energy savings (MJ m^-2) of PDH hydrogel for the year 2024. The closer the color is to blue, the lower the energy savings; the closer it is to orange, the higher the energy savings, as indicated in the legend on the left. b Global distribution map of the monthly average carbon reduction (kg m^-3) of PDH hydrogel for the year 2024. The darker the green, the higher the carbon reduction, as indicated in the legend on the right. c Global distribution map of the monthly average energy savings of PDH hydrogel during the winter of 2024. d Global distribution map of the monthly average carbon reduction of PDH hydrogel during the winter of 2024. e Global distribution map of the monthly average energy savings of PDH hydrogel during the summer of 2024. f Global distribution map of the monthly average carbon reduction of PDH hydrogel during the summer of 2024. And each map is annotated with latitude and longitude along the bottom and left sides. All the maps above were created with MATLAB R2024b.

The energy-saving distribution is not solely governed by latitude, but rather results from the combined effects of climate, altitude, topography, and local environments. For instance, in tropical low-latitude regions, the Amazon Basin features a humid rainforest climate with high temperatures, heavy rainfall, and stable solar radiation, leading to high but steady monthly energy savings⁶¹. In contrast, the Sahara Desert, with its hot, arid, and intensely sunny environment, exhibits both high energy savings and significant seasonal fluctuations⁶¹, with winter values notably 40%-60% lower than those in summer (Fig. 5c, e). Altitude also plays an important role. On the Qinghai–Tibet Plateau, where the average elevation exceeds 4000 m, the thin atmosphere and weak atmospheric back-radiation cause rapid heat loss and low temperatures⁵⁹. Compared with low-lying plains at the same latitude, high-altitude areas have much lower cooling demand in summer, with energy savings reduced by approximately 30–40% (Fig. 5c).

Overall, the spatial distribution of M-ESCR for PDH hydrogels reflects the combined effects of latitude, climate, and altitude, rather than being determined by latitude alone. This indicates that the deployment of PDH smart windows should be tailored to specific environmental conditions—for example, by adjusting optical properties or LCST thresholds to align with local factors, incorporating hybrid energy-saving coatings, or integrating passive cooling technologies—to achieve optimal energy efficiency across various climate zones and seasonal variations. These findings provide a basis for the application of PDH hydrogels in different regions and reveal their potential contribution to building energy-saving strategies.

Discussion

This study presents a high-performance thermoresponsive hydrogel (PDH hydrogel) tailored for smart window applications, featuring a composite crosslinked network structure synthesized from NIPAm, DMAA, and HEA. The PDH hydrogel simultaneously achieves high visible light transmittance (T_lum = 97.92%), exceptional solar modulation ability (ΔT_sol = 81.70%), and robust mechanical strength, enabling direct application onto single-layer glass surfaces and eliminating the need for traditional sandwich encapsulation. Through PVDC film encapsulation, the hydrogel’s environmental durability is significantly enhanced, extending its service life to 264 hours without compromising optical functionality. Outdoor experiments validate its effective thermal regulation and energy-saving performance, achieving an indoor temperature reduction of up to 6.95 °C and daily energy savings of 384.04 kJ m⁻². The material also demonstrates fast thermal responsiveness, high cycling stability, and long-term structural integrity. Furthermore, the PDH hydrogel exhibits a broad phase transition temperature, making it well-suited for year-round operation in diverse climates. A global predictive model of its monthly energy-saving and carbon reduction potential (M-ESCR) confirms its broad applicability, particularly in equatorial and arid regions. Collectively, these findings establish the PDH hydrogel as a promising, scalable material for next-generation smart window systems and offer valuable guidance for its integration into climate-responsive, energy-efficient building technologies.

Methods

Preparation of PDH-0.65 hydrogel

The PDH-0.65 hydrogel was synthesized via free-radical polymerization at room temperature. N-isopropylacrylamide (NIPAM, 1.60 g, 14.1 mmol, 98% purity, irritant, CAS: 2210-25-5, Aladdin, Shanghai, China), N,N-dimethylacrylamide (DMAA, 0.65 g, 6.57 mmol, 99% purity, irritant, CAS: 2680-03-7, Aladdin), ultrapure water (9.00 g, 500 mmol, resistivity ≥18.2 MΩ cm, Millipore Milli-Q system, USA), and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄, 0.10 mL, 0.53 mmol, ≥98% purity, potentially toxic, CAS: 174501-65-6, Tokyo Chemical Industry, Japan) were sequentially added into a 50 mL beaker containing a magnetic stir bar and stirred for 20 min. Subsequently, N, N′-methylenebisacrylamide (MBAA, 0.010 g, 0.065 mmol, 99% purity, suspected carcinogen, CAS: 110-26-9, Aladdin) and 2-hydroxyethyl acrylate (HEA, 0.100 g, 0.86 mmol, 96% purity, irritant, CAS: 818-61-1, Aladdin) were introduced, followed by stirring for 10 min. Then, ammonium persulfate (APS, 0.080 g, 0.35 mmol, ≥98% purity, strong oxidizer, CAS: 7727-54-0, Aladdin) was added and dissolved within 2 min, and sodium bisulfite (NaHSO₃, 0.040 g, 0.38 mmol, ≥98% purity, reducing agent, CAS: 7631-90-5, Aladdin) was added immediately afterward, followed by brief mixing for 30 s. The resulting precursor solution was rapidly poured into a pre-cleaned mold and allowed to polymerize undisturbed for 12 h at ambient temperature until gelation was complete. The hydrogel was removed from the mold, washed thoroughly with ultrapure water to remove unreacted species, and dried to a constant weight. The isolated mass was 10.4 g, corresponding to an overall yield of ≈92% based on total monomers.

Calculation of daily energy savings from outdoor experiments

In the outdoor experiments evaluating the application of PDH smart windows in actual buildings, temperature sensors were installed on the bottom surface of the room. For these sensors, the heat transfer process can be approximated as natural convection over a flat plate. Based on the experimental conditions, a set of parameters was defined, including the air density ρ = 1.225 kg m⁻³, air velocity u = 0.31 m s⁻¹, characteristic length of the bottom plate $\mathrm{L}=0.48\mathrm{m}$ , dynamic viscosity $\mathrm{\mu }=1.81\times {10}^{-5}\mathrm{Pa}\cdot \mathrm{s}$ , specific heat capacity at constant pressure ${\mathrm{c}}_{\mathrm{p}}=1005\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ , and thermal conductivity k = 0.026 W m⁻¹ K⁻¹.

Based on these parameters, the Reynolds number (Re) and Prandtl number (Pr) were calculated as follows:

3

$$\mathrm{Re}=\frac{\mathrm{\rho }\mathrm{uL}}{\mathrm{\mu }}=10072$$

4

$$\mathrm{Pr}=\frac{\mathrm{\mu }{\mathrm{c}}_{\mathrm{p}}}{\mathrm{k}}=0.707$$

Since the Reynolds number indicates laminar flow ( ${\mathrm{R}}_{\mathrm{e}}<5\times {10}^5$ ), the external laminar convection Nusselt number correlation for a flat plate was adopted:

5

$$\mathrm{Nu}=0.023{\mathrm{Re}}^{\frac{4}{5}}\cdot {\mathrm{Pr}}^{\frac{1}{3}}=32.6$$

The convective heat transfer coefficient (h) can then be determined by:

6

$$\mathrm{h}=\frac{\mathrm{Nu}\cdot \mathrm{k}}{\mathrm{L}}=1.77\mathrm{W}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$$

By multiplying the instantaneous temperature difference at each time point by the convective heat transfer coefficient h, the instantaneous heat flux density (q) was obtained. Subsequently, integration of the heat flux density over time provided the daily energy savings E = 384.04 kJ m⁻².

Mathematical model of global energy saving by PDH hydrogel

To estimate regional energy-saving potential, a simplified thermal balance model was constructed, focusing on convective and radiative heat exchange between air and glass. The key assumptions and parameter values are as follows:

Air density, specific heat capacity, and air velocity: $${\mathrm{\rho }}_{\mathrm{a}}=1.18\mathrm{kg}{\mathrm{m}}^{-3},{\mathrm{c}}_{\mathrm{a}}=1100\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1},\mathrm{u}=0.31{\mathrm{ms}}^{-1}$$

Stefan–Boltzmann constant and emissivity: $$\mathrm{\sigma }=5.67\times {10}^{-8}{\mathrm{W\; m}}^{-2}{\mathrm{K}}^{-4},\mathrm{\epsilon }=0.85$$

Radiative heat transfer coefficient:

7

$${\mathrm{h}}_{\mathrm{r}}=\mathrm{\sigma }\cdot \mathrm{\epsilon }\cdot \left({\mathrm{T}}_{\mathrm{g}}^4-{\mathrm{T}}_{\mathrm{amb}}^4\right)$$

Tilt angle of the glass is assumed to be: $$\mathrm{\theta }={45}^{\circ }$$

View factors to sky and ground are simplified as:

8

$${\mathrm{F}}_{\mathrm{sky}}=0.5\left(1+\cos \mathrm{\theta }\right),{\mathrm{F}}_{\mathrm{ground}}=1-{\mathrm{F}}_{\mathrm{sky}}$$

Effective radiation temperatures are assumed as:

9

$${\mathrm{T}}_{\mathrm{sky}}={\mathrm{T}}_{\mathrm{amb}}-20\mathrm{K},{\mathrm{T}}_{\mathrm{ground}}={\mathrm{T}}_{\mathrm{amb}}$$

The air temperature ${\mathrm{T}}_{\mathrm{a}}$ is updated over time according to:

10

$${\mathrm{T}}_{\mathrm{a}}^{\left(\mathrm{t}+1\right)}={\mathrm{T}}_{\mathrm{a}}^{\mathrm{t}}+\frac{\mathrm{\Delta }\mathrm{t}}{{\mathrm{\rho }}_{\mathrm{a}}{\mathrm{c}}_{\mathrm{a}}}\left[\mathrm{u}\left({\mathrm{T}}_{\mathrm{a}}-{\mathrm{T}}_{\mathrm{amb}}\right)+{\mathrm{h}}_{\mathrm{r}}\left({\mathrm{T}}_{\mathrm{a}}-{\mathrm{T}}_{\mathrm{amb}}\right)\right]$$

Acknowledgements

This study was sponsored by the National Natural Science Foundation of China (NSFC52106268, S.L.), the National Natural Science Foundation of Hubei Province, China (No. 2022CFB316, S.L.), and Hubei Province Youth Science and Technology Talent Training Project (2025DJA060, S.L.).

Author contributions

S.L. and B.Y. conceived and designed the experiments; B.Y., E.Z., W.L., and Z.W. performed the experiments and analyzed the data; B.Y. and S.L. provided guidance relating to data analysis and interpretation; B.Y. and S.L. wrote the manuscript; S.L. provided overall guidance of the project; Others reviewed the paper. All authors read and approved the manuscript.

Peer review

Peer review information

Nature Communications thanks Jun Fu, Zhengming Sun and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data are available from the corresponding author upon request. The majority of data generated in this study have been deposited in the Figshare repository under https://doi.org/10.6084/m9.figshare.30147484.v1. These data are openly accessible. For any additional data or specific requests not included in the repository, access can be obtained by contacting the corresponding author.

Code availability

All codes used for the transmittance calculation and global energy savings estimation have been deposited in Code Ocean for public access and reproducibility: Bowen Yang (2025) Global Distribution of Energy-Saving Potential of PDH Smart Windows 2.0. https://doi.org/10.24433/CO.1096641.v2. The transmittance calculation code is available at https://doi.org/10.24433/CO.4271496.v1. These repositories enable other researchers to reproduce, verify, and build upon the findings of this work.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s41467-025-65071-w.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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