Electrospinning Machine | Bioinspired Colored Films with Humidity-Induced Dynamic Reflectivity and Emissivity for Self-Adaptive Efficient Radiative Cooling

Existing colored radiative cooling materials struggle to adapt to weather changes due to static optical properties, with cooling capacity significantly decreasing especially when humidity rises. Current research mostly focuses on temperature response, leaving humidity-adaptive materials a blank. Addressing this, Professor Tang Shaochun's team at Nanjing University, inspired by chameleons' skin adaptation to the environment, proposed a humidity-driven dual-band regulation strategy. Through material composition design and structural optimization, they developed an environmentally humidity-adaptive colored radiative cooling film. This film achieves dynamic environmental humidity-responsive dual-band spectral regulation in solar and mid-infrared bands, adaptively enhancing cooling efficiency within a 25%-90% humidity range. These findings were published online in the renowned international journal ACS Nano under the title "Bioinspired Colored Films with Humidity-Induced Dynamic Reflectivity and Emissivity for Self-Adaptive Efficient Radiative Cooling" (ACS Nano, 2025, DOI: 10.1021/acsnano.5c09133). Nanjing University is the sole corresponding institution. He Jiajun, a direct PhD student at the School of Modern Engineering and Applied Science, Nanjing University, is the first author of the paper, and Professor Tang Shaochun is the corresponding author.

The research background indicates that traditional cooling technologies, represented by air compression, consume large amounts of electrical energy and are not suitable for outdoor scenarios. To significantly reduce the energy consumption of traditional cooling methods, it is particularly urgent to develop new cooling technologies that are highly efficient, energy-saving, simple, rapid, and environmentally friendly. Radiative cooling is a zero-energy-consumption passive cooling technology capable of providing cooling both at night and even during the day without requiring external energy input. Its working principle involves reflecting sunlight in the 0.3-2.5 μm band to reduce heat absorption, while simultaneously allowing the material to spontaneously transfer heat in the form of infrared radiation through the 8-13 μm "atmospheric transparency window" towards outer space, which is near absolute zero, thereby achieving temperatures below the ambient environment.

Reported radiative cooling materials often exhibit a bright white or silver appearance. The monotonous silver-white color can easily cause glare effects and create light pollution. The emergence of colored radiative coolers has effectively alleviated the aforementioned problems. However, traditional colored radiative coolers possess high absorption rates in the solar band, and their radiative cooling performance is limited by their own optical properties. When facing high humidity environments, the actual cooling performance of colored radiative coolers significantly decreases or even fails. Some studies have attempted to improve the weather adaptability of radiative coolers by introducing phase change materials, Janus structures, etc., but complex material designs, durability issues, and other problems restrict the large-scale application of such materials.

To address the above challenges, the team drew inspiration from the environmental adaptive characteristics of chameleon skin. They proposed a dual-spectral regulation strategy that responds to dynamic environmental humidity in both the solar and mid-infrared bands, and developed a colored radiative cooling film with environmental humidity-adaptive characteristics. Under low humidity conditions, the film exhibits low absorption characteristics in the solar band, enabling excellent colored radiative cooling performance. As environmental humidity increases, the film demonstrates dynamically enhanced solar reflectivity (increasing from 89% to 93%) and ultra-high infrared emissivity (~99%), resulting in a 180% improvement in cooling power. Particularly, the film can adaptively and reversibly regulate itself in complex humidity environments. The novel humidity-driven dual-band regulation strategy validated in this study provides new insights for the development of humidity-adaptive radiative coolers.

Figure 1: (a) Transmittance of the atmospheric transparency window under different environmental humidities. (b) Working diagram of traditional colored radiative coolers under different environmental humidities. (c) Working principle of the bionic colored radiative cooling film. (d) Actual photo of the film in a low humidity environment. (e) Actual photo of the film in a high humidity environment. (f) CIE chromaticity coordinates of the film before and after moisture absorption.

As shown in Figure 1a, as the ambient relative humidity increases, the transmittance of the atmospheric transparency window gradually decreases. Simulation results show that when the ambient relative humidity increases from 10% to 70%, the average transmittance of the atmospheric transparency window decreases by 50%. High humidity greatly weakens the cooling power of traditional colored radiative coolers (Figure 1b). As shown in Figure 1c, inspired by the environmental adaptation of chameleon skin, the team designed and developed a colored cooling film with humidity-adaptive characteristics. Its solar reflectivity and infrared emissivity can change with dynamic changes in ambient humidity, ensuring high-efficiency and stable cooling performance in complex and changing humidity environments. Figures 1d and 1e are actual photos of the film under different humidity environments. As humidity increases, the film's appearance changes from colored to white, and its solar reflectivity significantly improves. Figure 1f reveals the change in its chromaticity coordinates under different humidity environments, reflecting the regulatory effect of environmental humidity on solar reflectivity.

Figure 2: (a) SEM image of the film. (b) EDS spectrum of the film. (c) Statistical distribution of fiber diameters inside the film. (d) FDTD scattering efficiency simulation calculation. (e) FTIR transmittance curves. (f) XRD pattern. (g) XPS full spectrum of the film. (h) XPS spectrum of the Cl element in the film. (i) Thermogravimetric analysis (TGA) curve.

The bionic film is prepared using an electrospinning process, comprising upper and lower layers. The top layer material is P(VdF-HFP) doped with cobalt-based functional complexes. Figures 2a-c characterize the film's top layer; cobalt and chlorine elements are uniformly distributed in the P(VdF-HFP) fibers, indicating uniform doping of the cobalt-based complex into the matrix fibers. The film's bottom layer is pure P(VdF-HFP) fiber, with a fiber diameter distribution (0.1-0.7 μm) wider than the top layer fibers (diameter distribution 0.1-0.5 μm). According to scattering efficiency simulations (Figure 2d), this gradient diameter distribution design enables efficient Mie scattering over a broader solar band, thereby improving the film's reflectivity in the solar band. Figure 2e shows the Fourier transform infrared (FTIR) spectroscopy transmittance curves of the film before and after moisture absorption. The characteristic hydroxyl absorption peak (at 3390 cm⁻¹) of the film changed significantly before and after moisture absorption. Meanwhile, FTIR spectroscopy and XRD patterns (Figure 2f) also prove the doping of the cobalt-based complex. Figures 2g-h are XPS spectra. The shift in the binding energy of the Cl element in the film (from 199.8 eV to 197.9 eV) proves the successful coordination between cobalt and ethanolamine. The thermogravimetric (TG) curve (Figure 2i) shows that the film's components are stable within 100°C, ensuring component stability during practical application without decomposition.

Figure 3: (a) Absorption curve of the cobalt-based complex solution in the visible light band. (b) Trend of solar reflectivity change during the moisture absorption process of the film. (c) Schematic diagram of the molecular structure of P(VdF-HFP) matrix and cobalt-based complex. (d) Solar reflectivity and infrared emissivity curves of the film and comparison samples. (e) Comparison of average solar reflectivity and infrared emissivity performance of the film.

To investigate the influence of environmental humidity on the solar reflectivity of the bionic film, the team analyzed the spectral characteristics of the cobalt-based complex. As shown in Figure 3a, when the cobalt-based complex is dispersed in ethanol, the solution color is blue; as water is continuously added, the solution color begins to fade, eventually becoming colorless. UV-Vis-NIR spectroscopy curves show that the ethanol solution of the cobalt-based complex has a distinct absorption peak at 600-700 nm, which disappears after adding water. This is because water can form a new coordination structure with the cobalt-based complex. Figure 3b shows the change in the solar reflectivity of the bionic film during the moisture absorption process: when the film is exposed to a high humidity environment, its solar reflectivity gradually increases, especially in the 600-700 nm band. At relatively low humidity (RH=25%), the average solar reflectivity of the film is ~89%; in a high humidity environment (RH=60%), it reaches ~93%, and this process is completely spontaneous and reversible. As shown in Figure 3c, due to the high emissivity of P(VdF-HFP) and the abundance of chemical bonds such as N-H and C-O in the cobalt-based complex, the average emissivity of the film at low relative humidity (RH=25%) is as high as ~98%, and it can achieve ultra-high infrared emissivity (~99%) after moisture absorption. Compared with traditional fabrics, the film exhibits excellent optical performance (Figure 3d). Figure 3e shows a comparison of the average solar reflectivity and infrared emissivity of the bionic film, proving that it has superior optical performance compared to traditional colored radiative coolers.

Figure 4: (a) Schematic diagram and actual photo of the outdoor cooling experiment setup. (b-c) Temperature, humidity, solar radiation data from the outdoor cooling experiment on a sunny day. (d-e) Temperature, humidity, solar radiation data from the outdoor cooling experiment on a cloudy day. (f) Temperature, humidity data from the nighttime outdoor cooling experiment. (g) Average cooling values of the film under different environmental humidities.

As shown in Figure 4a, the team studied the service performance of the bionic film under different weather conditions and environmental humidities. Figures 4b-c show the test results on a sunny day: when the ambient relative humidity was 34% and the solar radiation intensity was 1222 W/m², the film still achieved a cooling effect of ~4.7°C below ambient temperature. Compared to commercial white and colored cotton fabrics, the film effectively reduced temperature by ~6.4°C and ~7.8°C, respectively. Figures 4d-e show the test results on a cloudy day: when the ambient relative humidity was 63% and the solar radiation intensity was 1245 W/m², the film achieved a cooling effect of ~5.8°C below ambient temperature and exhibited dynamically enhanced cooling performance in high humidity environments. Figures 4f-g show the nighttime outdoor test results: compared to traditional commercial cotton fabric, the film achieved a higher cooling amplitude. Notably, due to its ultra-high infrared emission characteristics at high humidity, the film can achieve efficient radiative cooling performance within the high humidity range (RH=60-90%). Even when the ambient humidity was 89%, it still achieved cooling of ~4.5°C below ambient temperature.

Figure 5: (a) Actual photo and thermal image of the film used as a human body cooling fabric. (b) Actual photo and thermal image of the film used as sunshade fabric. (c-d) Temperature, humidity, solar radiation data from the outdoor sunshade cooling experiment. (e) Annual energy savings and energy saving proportion for Nanjing. (f) Energy savings in different months for Nanjing. (g) Annual energy savings for various cities.

As shown in Figure 5a, the bionic film is used as a fabric for human body thermal management. It exhibits a dual-band spectral response to local humidity changes induced by sweat, and compared to commercial colored cotton fabric, the film surface temperature is lower. Particularly, when local sweating occurs, the bionic film provides an additional ~5.7°C cooling effect. This characteristic holds potential for developing a new generation of humidity-responsive intelligent thermal management fabrics. Furthermore, the film is colored, effectively avoiding glare effects and reducing light pollution, and can be used for sunshade applications. As shown in Figures 5b-d, compared to traditional commercial sunshade nets, this film achieved a cooling effect of up to ~8.3°C. Calculations using Energy Plus software show that using this bionic film for building sunshade cooling can bring significant energy-saving benefits. Taking Nanjing City as an example (Figures 5e-f), applying this film could save ~0.176 GJ of electricity annually for the city, achieving ~11.2% electricity savings. When extended to other regions (Figure 5g), this bionic film could save annual energy consumption by ~22.8% in Lanzhou City and ~11.7% in Guangzhou City.

Summary and Outlook
Inspired by the environmental adaptation characteristics of chameleons, the team successfully developed a humidity-sensitive colored film. It achieves dynamic dual-band spectral regulation in the solar and mid-infrared bands through a humidity-sensitive cobalt-based complex, adaptively enhancing cooling efficiency within a 25%-90% humidity range. In high humidity environments, its solar reflectivity increases from 89% to 93%, its mid-infrared emissivity reaches ~99%, cooling power improves by 180%, and the entire response process is completely spontaneous and reversibly regulated without external energy. This humidity-driven dual-band regulation strategy ensures that the colored radiative cooling film can adapt to complex humidity environments, exhibiting excellent cooling performance under both high and low humidity conditions. The humidity-driven dual-band regulation strategy proposed in this study can be extended to other radiative cooling materials, providing new ideas for developing a new generation of humidity-adaptive radiative coolers.

Literal Translation:Jiajun He, Yu Chen, Rui Guo, Shaochun Tang*. Bioinspired Colored Films with Humidity-Induced Dynamic Reflectivity and Emissivity for Self-Adaptive Efficient Radiative Cooling. ACS Nano, 2025, DOI: 10.1021/acsnano.5c09133.

Paper link: https://pubs.acs.org/doi/10.1021/acsnano.5c09133