Nowadays, widely used active cooling systems such as air conditioners consume a lot of electrical energy (~ 15% of global electricity)1,2. Passive radiative cooling (RC), a zero energy-consuming technology for cooling by radiative heat transfer into surrounding environments with a lower temperature and even into the cold out space (nearly 3 K) through the atmospheric window (8–13 µm), has been proposed as a promising cooling technology3–5. RC technology has shown great potential for personal thermal management because the radiative heat transfer is actually the primary heat dissipation pathway for human bodies, which accounts for 40%~60% of the total heat transfer from a human body (Supplementary Fig. 1)6,7.
Various RC materials have been developed for human body cooling in different indoor or outdoor scenarios8–16. According to their cooling mechanism, RC materials can be categorized into two types, transmission-type RC materials mainly used in indoor scenarios (Fig. 1a), and emission-type RC materials mainly used in outdoor scenarios (Fig. 1b). The transmission-type RC textile is transparent to human body radiation in the mid-infrared (MIR) waveband (Fig. 1d)5,6,8. Thus, human bodies (~ 34 ℃) can be cooled by directly dissipating heat through this textile into surrounding environments with low temperature, which has been proven to be the optimal choice for indoor radiative human body cooling8,17. In comparison, an emission-type RC textile emits the human heat into the cold outer space via the atmospheric window (8–13 µm) (Fig. 1e)17–22. Direct heat exchange with outer space and high solar reflection endow emission-type RC materials with the ability to achieve sub-ambient cooling effect under strong direct sunlight, making them an ideal choice for outdoor human body cooling12,15,16,23−26.
It will be ideal for RC materials to enable effective human body cooling for both outdoor and indoor scenarios. However, for the transmission-type RC materials, they exhibit poor cooling performance when exposed to a hot outdoor scenario due to the massive solar thermal load (low solar reflectance) caused by thin thickness limitation5,9,27,28. Although several efforts have been made to improve the solar reflectance of such materials10,17, balancing human body compatibility (such as harmlessness and comfortability) and high cooling performance remains a challenge. Also, for the emission-type RC materials to enable high cooling performance, there need to be a clear atmospheric window to send the radiative heat to the outer space. Therefore, when they are used in indoor scenarios, as the channel for radiative heat transfer into the outer space does not exist, the cooling performance is largely compromised8. In addition, even for the outdoor scenario, this atmospheric window is partially or completely blocked during cloudy days, leading to substantially decreased cooling performance29–31. Therefore, to design a multi-scenario RC material that possess high cooling performances in various scenarios, including cloudy outdoor, sunny outdoor, and indoor, it is critical but remain challenging to precisely tune the optical properties of a material in multiple wavelength bands (including the atmospheric window, non-window of MIR, and solar wavebands) to combine the advantages of both emission-type and transmission-type RC materials.
Selective emission-transmission model
Here, we propose a selective emission-transmission (SET) hybrid-type RC model to achieve multi-scenario human body cooling effect (Fig. 1c). The MIR spectral characteristic is shown in Fig. 1f. The SET hybrid-type RC (SET-type for short) textile exhibits emission-type characteristic in the atmospheric window (8–13 µm) and transmission-type characteristic outside the window, rendering it with an optimal emissive cooling effect by fully exploiting atmospheric window20,28 while retaining transmissive cooling capacity for most human body radiation (~ 61%, inset of Fig. 1f). In addition, its semi-transparent characteristic benefiting from the carful molecular-scale design breaks the thickness limitation of transmission-type textiles and is expected to achieve high solar reflectance similar to emission-type textiles through nanostructured regulation. Thus, in outdoor scenarios, the SET-type textile can achieve human body cooling through both MIR emission and MIR transmission, while in indoor scenarios, its cooling performance is only slightly lower than that of transmission-type textiles but significantly better than that of emission-type textiles. Therefore, the SET-type textile is suitable for both outdoor and indoor scenarios, which is further demonstrated by the following theoretical calculations (Fig. 1g) and the mechanism is shown in Fig. 1c. Cooling performances of different types of textiles (emission-type, transmission-type, and SET-type) in outdoor and indoor scenarios were evaluated and compared numerically by solving steady state heat transfer models (Supplementary Figs. 2,3 and Supplementary Tables 1,2). The results showed that in outdoor scenarios under strong sunlight (800 W/m2), the SET-type textile exhibited a significantly lower skin surface temperature than transmission-type (25.7°C lower) and emission-type (4.2°C lower) textiles (Figs. 1g,h). For indoor scenarios, the skin surface temperature with the SET-type textile was slightly higher (0.6°C) than that with transmission-type textiles but significantly lower (1.7°C) than that with emission-type textiles (Fig. 1i). Therefore, the SET-type textile exhibited good performance in both outdoor and indoor scenarios.
Based on the above results, we developed a polyoxymethylene (POM) nano-textile with the SET characteristics (denoted as SET-type POM textile) for the desired multi-scenario human body cooling. The synthesized SET-type POM textile exhibited 75.7% selective emittance at the 8–13 µm waveband (a high selectivity of 1.67, which is the ratio of average emittance between the 8–13 µm and 4–25 µm ranges), 48.5% transmittance at the 4–25 µm waveband, and 94.6% solar reflectance at the 0.3–2.5 µm waveband, making it an ideal SET-type RC material. As a result, the SET-type POM textile exhibited a significantly enhanced radiative human body cooling performance that was better than those of typical transmission-type, emission-type, and commercial cotton textiles in sunny outdoor (7.8, 2.6, and 8.8°C cooler, respectively), cloudy outdoor (2.9, 0.7, and 3.6°C cooler, respectively), and indoor scenarios (-0.2, 1.2, and 0.5°C cooler, respectively). Furthermore, the SET-type POM textile also exhibited good breathability, high tensile strength, and good anti-humidity capability. The field test demonstrated that a SET-type POM textile-based health hazard protective clothing showed significantly better cooling performance than a commercial counterpart in sunny outdoor (5.4 ℃ cooler), cloudy outdoor (1.3 ℃ cooler), and indoor scenarios (~ 1.0 ℃ cooler).
Material design and characterization
Emission and transmission by organic materials in the MIR region depend on their molecular bonds/functional groups, which have vibrational absorption/emission in different waveband ranges32,33. For the ideal high performance SET-type textiles, the vibrational absorption/emission frequencies of their molecular bonds/functional groups should be restricted to the atmospheric window waveband (8–13 µm). Common functional groups of polymers and their wavelength ranges in the MIR region (4–25 µm) are listed in Fig. 2a. Based on the careful screening and analysis of these functional groups, POM, a widely used polymer with a backbone chain consisting only of C-O-C bonds, was expected to have SET characteristics. This was experimentally confirmed by attenuated total reflection (ATR) mode Fourier transform infrared spectroscopy (FTIR, Fig. 2b)34,35. The characteristic peaks of the FTIR-ATR curve show that the wavelengths of the vibrational absorption/emission of POM are mainly distributed in the atmospheric window (8–13 µm) (Supplementary Table 3), indicating that POM is a promising SET-type textile.
In addition to the selective MIR emission and transmission, a strong solar reflectance is also necessary for RC materials for daytime outdoor scenarios. However, commercial POM products exhibit a low solar reflectance (~ 49.0%, Supplementary Fig. 4). According to the Mie scattering theory8,28, a fiber-based textile would exhibit high solar scattering efficiency when its fiber diameter distribution is close to the waveband of the solar spectrum (Fig. 2a). Therefore, POM nanofibers with a diameter distribution close to the solar waveband are expected to exhibit strong solar reflectance for daytime outdoor scenarios.
Based on the above analysis, a hierarchical SET-type POM nanofiber textile with a thickness of ~ 260 µm was synthesized through electrospinning (Fig. 2c, Supplementary Figs. 5,6). The synthesized POM nanofibers were randomly stacked on each other and exhibited a rough surface (Fig. 2d and Supplementary Fig. 7). The diameter size distribution of these POM nanofibers is close to the main solar waveband of 0.3-1.0 µm (Fig. 2e). In addition to the suitable nanofiber diameters, the disordered arrangement and rough surface of the POM nanofibers also contribute a lot to the high solar reflectance of the POM textile (Supplementary Fig. 8)28. Characterization showed that the synthesized POM textile exhibits a high solar reflectance of 94.6% in the waveband of 0.3–2.5 µm (Fig. 2f), and in the main solar waveband of 0.3–1.5 µm, the average reflectance reached 95.2%. In comparison, in the MIR region of 4–25 µm, the POM textile exhibited an ultra-low reflectance of 6.3% (inset of Fig. 2f), meaning that it has a high MIR emittance/transmission and thus is an ideal non-reflective textile and a prerequisite component for SET-type textiles. More importantly, the SET-type POM textile selectively exhibits a high emittance of 75.9% in the atmospheric window of 8–13 µm (the selectivity reached 1.67) and an average transmittance of 70.0% outside the atmospheric window. Moreover, the average transmittance of the POM textile reaches 48.5% in the entire MIR region of 4–25 µm, indicating that it can transmit nearly half of a human body radiation. These results demonstrate that the synthesized SET-type POM textile exhibits the desired characteristics of the above SET hybrid-type RC model, and can be expected to provide multi-scenario human body cooling.
Thermal measurements
The multi-scenario radiative human body cooling performance of the synthesized SET-type POM textile was investigated with specially designed measurement devices (Figs. 3a,b) in three typical scenarios during hot summer in Nanjing, China (118°57′10″ E, 32°07′14″ N), which were sunny outdoor, cloudy outdoor, and indoor scenarios. Bare skin (uncovered skin simulator) and the skin with the three typical textiles (commercial cotton, emission-type polyvinylidene difluoride (PVDF), and transmission-type nanoporous polyethylene (PE)) were employed for comparison (Supplementary Figs. 9–11). As shown in Figs. 3c, the PVDF and PE textiles were chosen for their high emittance (90.0%) and transmittance (96.9%), respectively, in the entire MIR region, while the former also exhibits a high solar reflectance (95.0%) close to that of the SET-type POM textile. These samples were placed in similar measurement devices with each device consisting of a textile sample, a skin simulator, surrounding insulating foam, and a K type thermocouple for monitoring the real-time temperature of the skin simulator (as shown in Figs. 3a,b). Constant input power (140 W/m2) was applied to the heaters to simulate the metabolic heat production rate of human skin. The skin simulators and thermocouples were carefully calibrated to ensure that the difference between measured temperatures was only caused by the difference between the different samples.
Figures 3d-g clearly show that in the outdoor scenarios (for both sunny and cloudy days, Supplementary Fig. 12), the SET-type POM textile covered skin simulator had the lowest surface temperature of the five samples. In the sunny scenario under strong direct sunlight (> 800 W/m2, with peak solar irradiance of ~ 935 W/m2 and a high Tamb of 34.1 ℃) from 10 a.m. to 1 p.m., the temperature of the SET-type POM textile covered skin simulator was lower than that of the bare skin and the skin covered by cotton, transmission-type PE, and emission-type PVDF by 15.7, 8.8, 7.8, and 2.6 ℃, respectively (Figs. 3d,e and Supplementary Fig. 13). In a hot cloudy outdoor scenario (Tamb = 37.4 ℃), the surface temperature of the SET-type POM textile covered skin simulator was also still much lower than the others by 5.5, 3.6, 2.9, and 0.7°C, respectively (Figs. 3f,g and Supplementary Fig. 14). The results indicate that the SET-type POM textile exhibits the best outdoor cooling performance for a human body. This was due to its higher average solar reflectance than that of bare skin, cotton (~ 68%, Supplementary Fig. 15), and transmission-type PE (~ 48.4%, Supplementary Fig. 11d), as well as its extra transmission effect when compared with emission-type PVDF.
The SET-type POM textile also exhibited good cooling performance in the indoor scenarios, which means allowing a higher air temperature setpoint of active cooling devices while maintaining human thermal comfort, which can save energy by ~ 7% per 1°C increase8,36,37. As shown in Figs. 3h,i, in a room at 30.4 ℃, the temperature of the SET-type POM textile covered skin simulator was lower than that covered with emission-type PVDF and commercial cotton by 1.2 and 0.5 ℃, respectively, while it was only 0.2 ℃ higher than that covered with transmission-type PE (Supplementary Fig. 16). That is, the human body cooling performance of the SET-type POM textile in an indoor scenario is close to that of the transmission-type PE and better than that of both the emission-type PVDF and commercial cotton textiles. A comprehensive consideration of the results in both indoor and outdoor thermal measurements, it is clear that the SET-type POM textile has the most effective cooling effect for the human body.
Wearability testing
In addition to the superior RC performance, the synthesized SET-type POM textile also exhibits many metrics required for wearability, namely, good breathability, mechanical strength, waterproofness, and anti-humidity capability. First, we measured the breathability of the SET-type POM textile, which refers to the ability of a textile to carry human body heat away across the textile by airflow under a pressure difference and is important for human comfort. As shown in Supplementary Fig. 17, when sandwiching the SET-type POM textile between water and air, continuous bubble penetration (Supplementary Movies 1 and 2) without any textile breakage clearly indicates its good breathability. As shown in Fig. 4a, quantitative air permeability tests further showed that the SET-type POM textile exhibits a similarly good breathability as a commercial cotton, which is due to its fluffy fibrous structure and specially designed pores (punched by a commonly used microneedling technique8, Supplementary Fig. 18). In addition, the water vapor transmission rate was also measured, which represents the ability of a textile to transfer water vapor produced from perspiration evaporation. As shown in Fig. 4b and Supplementary Fig. 19, in the performance attributable to the existence of countless nano/micropore channels for water vapor permeation (Supplementary Figs. 6,20), the SET-type POM textile exhibits a high water vapor transmission rate (0.011 g/cm2 hour), which is similar with commercial cotton and the above two types of RC textiles (0.010–0.012 g/cm2 hour).
In addition, the SET-type POM textile also showed a high tensile strength of 13.3 MPa, which is comparable with that of the commercial cotton (14.7 MPa) (Figs. 4c). The high mechanical strength is attributed to the high crystallinity of the POM nanofibers (Supplementary Fig. 21) that have a high inherent strength (70 MPa)38. The elongation of the SET-type POM textile is the highest (~ 300%) among these different textiles (Fig. 4c and Supplementary Fig. 22), indicating its high flexibility for comfortable skin touch. The anti-humidity capability, which is important for keeping a textile dry and clean in humid environments, of these samples was also compared. The water contact angle of the SET-type POM textile reached 138° and still remained at 122°after half an hour, which were much higher than that of the other three textiles (Fig. 4d and Supplementary Fig. 23), indicating that the SET-type POM textile exhibits the highest waterproofness and anti-humidity capability.
We also tested the real performance of the SET-type POM textile on a protective clothing by sewing a POM cloth onto one side of the chest of the commercial protective clothing and the other side was retained for comparison (Figs. 4e,f, and Supplementary Fig. 24). The thermal properties of a researcher wearing the modified protective clothing were recorded in three typical scenarios (sunny outdoor, cloudy outdoor, and indoor scenarios). The clothing surface temperature was captured and visually presented using an infrared camera (Testo, 8–14 µm). As shown in Fig. 4g, in the outdoor scenarios (both sunny and cloudy), the clothing temperature on the SET-type POM textile section was lower than that on the counterpart section in the continuous test. In the sunny outdoor scenario under strong direct sunlight (> 800 W/m2), the surface temperature difference between the two sides of the clothing reached nearly 3.0°C (Supplementary Movie 3). In the cloudy outdoor scenario with a solar irradiation of ~ 200 W/m2, the temperature difference on the two-side cloth surface also reached nearly 1.5 ℃ (Fig. 4g and Supplementary Movie 4). Besides, the corresponding skin temperatures under the clothing were monitored in real-time by bead-probe thermocouples (Figs. 4h-k). In a similar sunny outdoor environment (Figs. 4h,i), the skin temperature difference tested by thermocouples reached 5.4°C, and the maximum difference even exceeded 6.0°C (Fig. 4i). Clearly, the SET-type POM textile shows huge potential in improving human thermal comfort under strong sunlight. The skin temperature difference was 1.3 ℃ in a similar cloudy outdoor environment (Figs. 4h,j). These results together demonstrate a significantly enhanced RC performance of the SET-type POM textile over that of commercial protective clothing in various outdoor scenarios.
Moreover, in the indoor environment, although the surface temperature of the clothing on both sides were similar with each other (Fig. 4g and Supplementary Movie 5), the skin temperature on the POM side was nearly 1.0°C lower than that on the commercial side (Figs. 4h,k). Thus, the SET-type POM textile also exhibited better indoor RC performance. Therefore, the SET-type POM textile exhibited superior cooling performance over that of commercial protective clothing in both outdoor and indoor scenarios and therefore has high commercialization potential.