Outdoor personal heating and cooling by a Janus textile

Enhancing the personal thermal comfort in outdoor environment is of substantial significance to ameliorate the health conditions of pedestrian and outdoor laborer. However, the uncontrollable sunlight, substantial radiative loss, and intense temperature change in the outdoor environment present majestic challenges to outdoor personal thermal management. To date, a wearable device with optional passive heating and cooling abilities to abet people combat extreme temperatures in outdoor spaces, is lacking. Here, we report an eco-friendly passive textile which converts the challenges into opportunities and harvests energy from the sun and the outer space for optional localized heating and cooling. Compared to conventional heating/cooling textiles like black/white cotton, its heating/cooling mode enables a skin simulator temperature increase/decrease of 11.3 °C/14.5 °C respectively under sunlight exposure. Meanwhile, the temperature gradient created between the textile and skin simulator allows a continuous electricity generation with thermoelectric modules. Owing to the exceptional outdoor thermoregulation ability, this Janus textile is promising to help maintain a comfortable microclimate for individuals in outdoor environment and provide a platform for pervasive power generation. Outdoor space, which is an inevitable part of our daily lives, accommodate numerous human activities and industrial operations. However, extreme weather conditions and acute temperature changes in the outdoor spaces are serious threats to public health, which adversely impact the well-being and sustainability of society. In the context of climate change, the diurnal temperature variation in the outdoor space can exceed 25 °C and the figure is still elevating. Accompanied by the mortality burden due to the heat/cold stresses and the temperature variations has been increased and is not likely to decrease in the near future. Since it is uneconomical and impractical to implement air conditioners or ventilations in the outdoor environment, a desired solution for maintaining personal thermal comfort is managing the temperature of the immediate space around human body, which is popularly known as outdoor personal thermal management. To realize eco-friendly passive outdoor personal thermal management, there are three remarkably great challenges to be tackled (Fig. 1a). Firstly, the outdoor infrared atmospheric window allows a large fraction of body thermal emission to be transmitted to the cold outer space (~ -270 °C), resulting in much higher radiative loss for personal heating compared to the indoor scenario (~ 150 W·m vs. ~ 35 W·m for 0.6 mm cotton). Secondly, the outdoor sunlight irradiation (~ 1000 W·m for 1sun, AM1.5) leads to extra heat load, making it difficult to realize personal cooling. Thirdly, the outdoor temperature changes are drastic and uncontrollable, while the indoor room temperature can be controlled with air conditioners. Though there have been attempts on personal management, most of the studies are focused on indoor applications. The few passive outdoor textiles are demonstrated with single fixed thermal functionality of either solar heating or radiative cooling, showing moderate localized temperature regulation ranges. An integration of the two seemingly opposite thermal functions, which can adequately protect people from excessive hot/cold outdoor environment and intense temperature changes, has not been achieved yet. Moreover, as such a textile is expected to be able to create a temperature gradient between itself and the human skin, it can also pave a sustainable and practical pathway for continuous thermoelectric power generation. In this paper, we demonstrate a wearable solution towards passive all-day outdoor personal thermal management and electricity generation with a Janus textile. This textile presents several distinct advantages by converting the challenges into opportunities. (i) It functions with extra high sunlight absorptivity, low mid-infrared (MIR) emissivity for localized heating and high solar reflectivity, high MIR emissivity for cooling. (ii) Outdoor daytime measurement shows that compared to black/white cotton, this textile enables a skin simulator temperature increase/decrease of 11 °C/4.5 °C during heating/cooling modes, respectively. It is also well suitable for nighttime and indoor personal thermal management. (iii) Based on its thermoregulation abilities, the integration with the thermoelectric modules showed that a maximum power generation capacity of 5.7 mW·m at daytime and 17.1 mW·m at nighttime can be achieved. (iv) The textile is breathable and washable due to porous structure and the hydrophobic surface, it affords scalable manufacturing regarding low material costs and facile fabrication methods. Consequently, this work lays a foundation for feasible implementation of harvesting substantial outdoor energy by wearable device and explores its broad applications in thermal management and pervasive electricity generation.

the context of climate change, the diurnal temperature variation in the outdoor space can exceed 25 ℃ and the figure is still elevating 1-3 . Accompanied by the mortality burden due to the heat/cold stresses and the temperature variations has been increased and is not likely to decrease in the near future 4 . Since it is uneconomical and impractical to implement air conditioners or ventilations in the outdoor environment, a desired solution for maintaining personal thermal comfort is managing the temperature of the immediate space around human body, which is popularly known as outdoor personal thermal management.
To realize eco-friendly passive outdoor personal thermal management, there are three remarkably great challenges to be tackled (Fig. 1a). Firstly, the outdoor infrared atmospheric window allows a large fraction of body thermal emission to be transmitted to the cold outer space (~ -270 ℃), resulting in much higher radiative loss for personal heating compared to the indoor scenario (~ 150 W·m -2 vs. ~ 35 W·m -2 for 0.6 mm cotton). Secondly, the outdoor sunlight irradiation (~ 1000 W·m -2 for 1sun, AM1. 5) leads to extra heat load, making it difficult to realize personal cooling. Thirdly, the outdoor temperature changes are drastic and uncontrollable, while the indoor room temperature can be controlled with air conditioners 5 . Though there have been attempts on personal management [6][7][8][9][10][11][12][13][14][15][16][17] , most of the studies are focused on indoor applications.
The few passive outdoor textiles are demonstrated with single fixed thermal functionality of either solar heating 13 or radiative cooling 18 , showing moderate localized temperature regulation ranges. An integration of the two seemingly opposite thermal functions, which can adequately protect people from excessive hot/cold outdoor environment and intense temperature changes, has not been achieved yet.
Moreover, as such a textile is expected to be able to create a temperature gradient between itself and the human skin, it can also pave a sustainable and practical pathway for continuous thermoelectric power generation 19-23 . In this paper, we demonstrate a wearable solution towards passive all-day outdoor personal thermal management and electricity generation with a Janus textile. This textile presents several distinct advantages by converting the challenges into opportunities. (i) It functions with extra high sunlight absorptivity, low mid-infrared (MIR) emissivity for localized heating and high solar reflectivity, high MIR emissivity for cooling. (ii) Outdoor daytime measurement shows that compared to black/white cotton, this textile enables a skin simulator temperature increase/decrease of 11 ℃/4.5 ℃ during heating/cooling modes, respectively. It is also well suitable for nighttime and indoor personal thermal management. (iii) Based on its thermoregulation abilities, the integration with the thermoelectric modules showed that a maximum power generation capacity of 5.7 mW·m -2 at daytime and 17.1 mW·m -2 at nighttime can be achieved. (iv) The textile is breathable and washable due to porous structure and the hydrophobic surface, it affords scalable manufacturing regarding low material costs and facile fabrication methods. Consequently, this work lays a foundation for feasible implementation of harvesting substantial outdoor energy by wearable device and explores its broad applications in thermal management and pervasive electricity generation.

Results
Working mechanism of the Janus textile.
To numerically anticipate the required optical properties of the Janus textile, consider the total heat dissipation rate q of the human body as 13 is the thermal radiation power emitted by the outer surface of the textile which is proportional to its emissivity ε and atm q is the absorbed power due to the radiation of ambient air/objects. In indoor environment (Fig. 1a, environmental temperature Te=25 °C), the human skin emits thermal radiation at around Ts=34 °C and meanwhile, absorbs radiation from the environment at Te=25 °C. In comparison, a great amount of emission power from the human body directly passes through the atmospheric window at the wavelength range of 8 μm to 14 μm in outdoor environment (where atm q is around zero) [24][25][26][27][28][29][30][31][32][33][34][35] , leading to a much higher radiative heat dissipation (Highlighted in Fig. 1b, cyan area). From the perspective of outdoor thermal management, this extra radiative heat loss emphasizes the importance of suppressing thermal emissivity ε for outdoor personal heating. Meanwhile, a suitable opportunity arises for outdoor cooling.
2) Outdoor sunlight: Whereas the radiative heat loss poses great challenge on personal heating in outdoor space, the outdoor solar irradiation shed light on ecofriendly localized heating. Accordingly, the solar irradiation intensity sun q according to AM1.5 spectrum is 1000 W·m -2 , indicating even a very slight amount of sunlight would be able to compensate the heat dissipations of human body (q~231.6 W·m -2 for ε=1 and q~97.1 W·m -2 for ε=0, Fig. 1c). For an ideal heating textile of ε=0, a sun q of 200 W·m -2 would be able to heat up the immediate space around human body when its absorptivity α is larger than 0.3 (Fig. 1c). For the aim of outdoor thermal management, high sunlight absorptivity α substantially improves the heating ability of textiles, while high sunlight reflectivity r (r=1-α when textile is opaque to sunlight) is crucial for personal cooling.
Ultimately, when we choose 32° to 36 °C to be the comfortable temperature range of the skin simulator (heat dissipation rate q = 200 W·m -2 ), an ideal Janus textile ( Apparently, it outperforms same thickness cotton in terms of both outdoor and indoor thermal management ability. As can be derived from the analysis results (Fig. 1d), the design of such an all-environment (outdoor and indoor) thermal management wearable requires critical multiband control of sunlight absorptivity α and thermal emissivity ε.
Accordingly, the localized heating requires a selective solar absorbing surface (high α and low ε, Fig.1e) and the localized cooling requires a radiative cooling surface (low α and high ε, Fig.1e). Hence, an ideal Janus textile should be engineered with asymmetric sunlight to thermal optical properties at two surfaces to synergistically realize these two functions within itself. Moreover, to avoid the crosstalk of these two functions, the textile should be opaque for all wavelengths of interest.

Design and optical properties
Guided by the theoretical analysis results, we adopted a layered design based on porous fibrous polymers for the dual-functional outdoor personal thermal management (Fig. 1e). In this design, nanoporous polyethene (nPE) is chosen to be the substrate for selective solar absorbing plasmonic structures formed by copper (Cu) and zinc (Zn) nanoparticles. To enhance the mechanical stability of the solar absorbing nanostructures and avoid particle agglomeration when immersed in water, we performed graft polymerization of fluorinated acrylate monomer, 1H,1H,2H,2H-nonafluorohexyl-1acrylate (F4) onto the nanostructure 36  The optical properties of the Janus textile were then measured using Fourier transform infrared (FTIR) and ultraviolet-visible-near-infrared spectroscopy with an integrating sphere. The measured spectra show that the solar absorbing heating side, which consists of Cu/Zn nanoparticles sized several hundred nanometers, manifests high absorptivity (α > 0.8) over solar spectrum and low emissivity (ε ~ 0.16) between 6 and 20 μm where the body thermal radiation is centralized (Fig. 2a, Fig. 2e and Fig.   S3). When flipped, the cooling side shows high sunlight reflectivity (r ~ 0.91) and high emissivity (ε ~ 0.87, Fig. 2b), which satisfies the criterion for outdoor radiative cooling.
The outdoor thermal management ability is then validated by wearing it on the human body and captured by optical and thermal cameras. In optical imaging, the Janus textile features dark grey color in heating mode and white color in cooling mode (Fig. 2c), which corresponds to the superior absorptivity and diffuse reflectivity for visible light, respectively. In thermal imaging, the textile shows cold color in heating mode and warm color in cooling mode (Fig. 2d), rendering low radiative heat loss and high radiative heat loss, respectively. To demonstrate the thermal regulation ability of Janus textile with two thermal functions, we first built a setup for measuring the temperature of the surface which mimics the skin (termed as TAS) covered by different textiles (Fig. 3a, schematic presented in Fig. S4). The setup was placed on the rooftop, facing the clear sky. To avoid the influence of the outdoor wind flows, a thin broadband transparent polyethene film was used to cover the thermal chamber. During daytime measurement when the whole setup was exposed by the sunlight (the solar irradiance was also recorded) and no power supply is applied to the skin simulator 13 . Compared to conventional heating/cooling textile like black/white cotton of 1mm/0.5mm thickness (see Fig. S5 for their optical properties), the heating/cooling mode enables a surface temperature increase/decrease of 11.3 ℃/14.5 ℃ under mid-day sunlight exposure (12:00 am), indicating its ability in protecting people from both cold and hot environments.
Throughout the measurement duration of 10:00 am-15:00 pm, the thermal regulation ability of the Janus textile (both heating and cooling modes) outperforms the conventional cottons with regard to the corresponding thermal functions.
The same setup with daytime measurement was adopted for nighttime tests and DC power was applied for simulating metabolic heat generation. In the nighttime test, we gradually increased the input power to the skin simulators to demonstrate the difference in thermoregulation abilities of two modes. The skin simulator temperature reaches 35.7 ℃ (Fig. 3c) at 400 W·m -2 input power density with heating mode, while the cooling mode textile requires 500 W·m -2 input power to reach the similar temperature (33.6 ℃). This result shows that this Janus textile has the ability of enhancing (or suppressing) more than 25% of total outdoor personal heat loss rate by switching the working mode at nighttime, which is in good agreement with the calculation results (see Supporting Information for calculation details).

Thermoelectricity generation.
Subsequently, the dual-functional outdoor textile thereby provides a reliable platform for further energy-related integrations. As an example, we attached the Janus textile onto a commercial thermoelectric generator (TEG) to demonstrate the all-day electricity generation ability (Fig. S6). To mimic metabolic heat generation, we put a silicone heater and an aluminum block with multiple fins beneath the TEG (As illustrated in Fig. 3d). During daytime test under sunlight, the temperature difference between the skin simulator (TAS) and the hot side (TH, TEG covered with Janus textile in heating mode) reaches a value of around 3 ℃ when the TAS is around 36 ℃ (Fig. 3e, see Fig. S7 for real-time solar irradiance power density). Correspondingly, the output voltage is around 115 mV and the calculated maximum output power density is around 5.7 mW·m -2 . We further tested the thermoelectricity generation ability of the whole device at night. By applying appropriate voltage to the silicone heater, the temperature difference between the skin simulator (TAS=34.5 ℃) and the cold side (TC, TEG covered with Janus textile in cooling mode) is around 4 ℃ (Fig. 3f). The corresponding output voltage is around 200 mV and maximum output power density is 17.1 mW·m -2 .
Noted that there are ways to improve the power generation capacity based on the Janus textile: (1) A direct use of the Janus textile in heating mode as hot side and the Janus textile in cooling mode as cool side. We have also demonstrated that by linking these textiles with a home-made flexible thermal electricity generator (10 pairs of AZO/PEDOT:PSS as n-type and PEDOT:PSS as p-type), a temperature difference of 45 ℃ and an output voltage of 26 mV is achievable (Fig. S8); (2) A combination with stacked TEGs or TEG with lower thermal conductivity to further increase the temperature of the hot side (Fig. S9), thereby improve the overall performance of the whole device.

Wearability tests.
We further characterized the wearability of the Janus textile and these tests are mainly focused on washability and breathability of the resulting textile. The washability of the Janus textile is tested through clear water stirring of different duration time. It can be inferred that with the help of the grafted fluorinated groups, the resulting textile retains super-hydrophobicity which makes the laundry induced deterioration negligible.
The sunlight absorptivity of the heating side witnesses almost no change and its emissivity show only around 0.01 increment after 60 minutes washing (Fig. 4a). The solar reflectivity and emissivity of the cooling side decreases by 0.02 and 0.09 after 60 minutes washing, respectively (Fig. 4b). Apart from the laundry durability, the treatment by F4 also contributes to various advantages in terms of multifunctional smart textile. At heating mode, such a waterproof heating side also prevent water droplets adhesion in outdoor environment and thereby reduce the heat loss due to evaporation ( Fig. 4c). At cooling mode, the hydrophobicity difference of two surfaces has great potential to be combined with engineered micropores to accelerate sweat transportation process and further enhance the cooling ability 11 . The breathability test shows that with the well-distributed nanopores (Fig. 2e and Fig. S10), the water vapor transmission rate (termed as WVTR in Fig. 4d) of the Janus textile is comparable to traditional textile such as cotton, being much better than the Mylar blanket (aluminum coated polyethylene terephthalate film), which is a commercial radiative textile to prevent radiative heat loss.

Discussion
In summary, we develop a Janus textile with remarkable localized heating/cooling performance in outdoor environment. First, from the perspective of wearables devices, this work offers an unprecedented solution for harvesting energy from the sun, cold space for passive thermal management. Second, the Janus textile is able to implement two modes both day and night, thereby exceeds or matches the state of art for all-day and all-environment thermal management. Third, the perceptible temperature differences between the skin and the textile surface created when the textile works in different modes makes the textile a reliable building block for continuous thermoelectric generation. Last, the facile fabrication method of the flexible textile and its low material costs makes it suitable for scalable fabrication, facilitating potential applications such multi-functional camouflage, sensing, anti-freezing, anti-fogging, and powering on-body electronics. Ultimately, the described flexible material affords new possibilities for various modern technologies where spatial temperature regulation and continuous green energy solution is needed. , the number of thermocouples n in the thermoelectric generator is 1148, the measured Seeback coefficient S is 43.5 μV/K, the measured resistance per thermocouple is 0.042 Ω and area size A is 11 cm×11 cm.

Fabrication of the Cu/Zn
Water vapor transmission rate measurement. The test was performed using ASTM E96 with modification. Petri dishes filled with 10 ml distilled water were sealed by the textile samples using rubber bands. These sealed dishes were then put into an environmental chamber whose temperature was kept at 30°C and relative humidity at 40%. The dishes were weighed periodically with an electronic balance (OHAUS, AR1502CN). The water vapor transmission rates were calculated from the mass loss, which was equal to the mass of evaporated water.
Water contact angle measurement. The static water contact angles were measured by a DropMeter A-200 contact angle system (MAIST Vision Inspection & Measurement Co. Ltd., China) in the ambient environment to evaluate the wettability of the textile.

Data Availability
The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files.