The universe’s temperature is near 3 K, while Earth’s surface averages around 300 K. The cold cosmos serves as a natural heat sink for dissipating terrestrial heat. Figure 7a illustrates the energy flow of a radiator on Earth. When exposed to the atmosphere, a radiator absorbs both terrestrial thermal radiation and daytime solar radiation. Net radiative cooling power (Pnet) is given by Pnet = Prad – Patm – Psolar – Ploss, where Prad is thermal radiation from the radiator, Psolar is absorbed solar irradiation, Patm is absorbed atmospheric radiation, and Ploss is intrinsic cooling loss. IR radiation experiences minimal absorption at wavelengths between 8 and 13 µm—the IR atmospheric transparency window (Figure 7b). Radiation in this range passes directly through the atmosphere to space. By aligning an object’s peak emission with this window, IR radiative cooling can be achieved. Since it requires no external energy input, this technology is environmentally friendly and applicable in energy-saving green buildings, personal thermal management, and beyond.
This phenomenon is observed in nature: dew and frost form on plant surfaces even when ambient temperature hasn’t reached dew point or freezing point. Many animals maintain body temperature in extreme heat via passive IR radiative cooling. For example, Saharan silver ants use their silvery hairs to reflect sunlight and emit thermal radiation efficiently, keeping their bodies cool in desert conditions (Figure 7c–i). Their dorsal and lateral body surfaces are covered with dense, uniform hairs that appear silvery (Figure 7c,d). SEM images show these hairs are locally aligned and tapered (Figure 7e), with triangular cross-sections featuring flat bottoms and corrugated tops (Figure 7f,g). At incident angles above 30°, total internal reflection occurs at the flat bottom facets, enhancing reflectivity dramatically. This guides solar radiation toward the ant’s body, where angles approach 90°, reducing reflectivity. Thermodynamic experiments confirmed specimens with natural hairs maintained lower body temperatures than those without. These hairs effectively protect ants from overheating through radiative cooling.
Inspired by nature, developing bioinspired adaptive materials for IR radiative cooling is crucial for advanced thermal management. Systematic research began in the 1960s using conventional materials like SiO-coated aluminum and polyvinylchloride-coated aluminum, which exhibited strong inherent thermal radiation in the IR window. Recent advances in nanoscience and nanofabrication have revived interest in this field. Kirchhoff’s law states that thermal radiation properties can be manipulated by engineering advanced functional materials with hierarchical nanoarchitectures. As a result, numerous traditional materials with well-designed nanostructures have been developed for IR radiative cooling.
This section focuses on bioinspired materials for daytime radiative cooling and their applications in energy-efficient green buildings and smart personal thermal management systems. Daytime IR radiative cooling is challenging because materials must simultaneously possess high solar reflectivity and high IR emissivity in the atmospheric transparency window. Rephaeli et al. proposed a system using a sunlight reflector and two-layer nanophotonic structure to selectively emit thermal radiation in the IR window. Raman et al. developed an integrated photonic thermal device with multilayered HfO₂ and SiO₂ on a silver-coated substrate, designed to reflect ~97% of solar radiation while emitting strongly in the IR window. Other nanophotonic structures have also shown superior performance. Hossain et al. introduced multilayered conical metamaterial pillar arrays composed of alternating germanium and aluminum layers, exhibiting low emissivity outside the IR window and high emissivity within it. Zou et al. demonstrated a dielectric resonator metasurface with silver and phosphorous-doped n-type silicon, showing high emission in the IR window and compatibility with silicon photonic platforms.
Wearable technologies with daytime radiative cooling attributes are highly desired due to flexibility, adaptability, permeability, biocompatibility, moldability, processability, and scalability. Approaches include embedding nanoparticles in matrices, creating functional single- or double-layer coatings, and fabricating polymer-based hybrid metamaterials. Atiganyanun et al. developed random microsphere coatings that reduced substrate temperature by 4.7 °C under direct sunlight—superior to commercial white paint. Huang et al. fabricated a single-layered nanoporous MgHPO₄·1.2H₂O coating with a high emissivity of 0.94 in the IR window and high reflectance of 0.92 in the solar spectrum. Zhai et al. reported large-scale polymeric films loaded with random resonant polar dielectric microspheres, transparent to solar radiation but with IR emissivity >0.93 in the IR window. Coating with silver substrates enabled a radiative cooling power of 93 W/m² under direct sunlight.
Despite progress, most IR radiative cooling materials are static. Developing adaptive systems that respond to environmental changes is essential. Ono et al. proposed a self-adaptive system using phase-changing VO₂ materials and spectrally selective filters, automatically switching cooling based on temperature without external energy. Kort-Kamp et al. reported adaptive cooling near room temperature using VO₂, enabling daytime cooling in hot weather and heating in cold weather. Lee et al. demonstrated colored passive IR radiative coolers using subtractive color films, while Sheng et al. created colored coolers based on optical Tamm nanostructures, combining excellent radiative cooling with subtractive primary colors. These innovations are promising for wearable and portable electronic devices.
In daily life, about 40% of total energy consumption occurs in buildings, especially for indoor thermal management. Passive IR radiative cooling via large cool roofs is an effective way to reduce energy use. Nahar et al. studied eight passive cooling modes, including roof ponds, evaporative cooling, and insulation. White-colored roofs showed superior energy savings. “Cool roofs” are now among the most popular radiative cooling technologies for future green buildings. They combine high solar reflectivity and high IR emissivity, effectively lowering roof temperature and reducing heat transfer into buildings.
Li et al. developed multifunctional passive radiative cooling wood using a scalable bulk process (Figure 10a,b).82-08-6 site The cooling wood displayed a reflective white color from disordered photonic nanostructures and low-loss cellulose fibers (Figure 10c,d).35189-28-7 manufacturer Real-time measurements showed emitted IR energy exceeded received solar energy. This cooling wood was 10.1 times stronger and 8.7 times tougher than natural wood, holding great promise for sustainable green building applications.
Mandal et al. demonstrated hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)] coatings with exceptional daytime radiative cooling performance (Figure 10e). Using a phase-separation technique, they prepared precursor solutions of P(VdF-HFP) and water in acetone. Rapid acetone evaporation induced phase separation, yielding microporous coatings after water evaporation. These coatings exhibited high solar reflectivity and high IR emissivity. Micro- and nanopores scattered solar radiation and enhanced IR thermal radiation (Figure 10f). The material could be easily painted or dip-coated onto diverse substrates and made into freestanding sheets (Figure 10g).
Active radiative cooling systems such as water-based, air-based, and hybrid systems have also been developed. Air-based systems use fans to circulate air; water-based systems use water as a heat transfer medium. Hybrid systems combine daytime IR radiative cooling with other energy-harvesting technologies, such as evaporative cooling, heat pumps, and solar energy harvesting.
Personal thermal management based on IR radiative cooling is vital for reducing energy consumption and improving comfort. Under indoor conditions, IR-transparent textiles allow full transmission of body thermal radiation for passive cooling. However, outdoor use is challenging due to additional solar and body heat. Hsu et al. proposed a nanoporous polyethylene (nanoPE)-based textile with high IR transparency for smart human body cooling (Figure 11a). NanoPE films scatter visible light strongly while being transparent to human body radiation (max 9.5 µm). They can be processed into breathable, flexible, and mechanically robust textiles suitable for wear.
Cai et al. reported the first outdoor IR radiative cooling textile based on spectrally selective nanocomposite films, showing high solar reflection and superior thermal radiation transmission (Figure 11b). Embedding ZnO nanoparticles into nanoPE (ZnO-PE) enabled cooling down simulated skin by up to 10 °C compared to cotton under direct sunlight. During sweat evaporation, it reduced overheating by over 8 °C.
Peng et al. introduced nanoscale porosities into PE microfibers using a large-scale fabrication technique (Figure 11c). The resulting microfibers combined mechanical strength with optical functionality—scattering visible light and transmitting IR radiation—enabling simultaneous visible opacity and IR transparency.PMID:30725879
An ideal wearable thermal management system should passively cool or heat according to needs. Hsu et al. demonstrated a personal thermal management glove based on silver nanowire-embedded cloth (Figure 11c). Compared to a common glove, the silver nanowires reflected most IR thermal radiation from the hand, achieving lower temperatures. The durability and breathability of the original cloth were preserved due to the porous nanowire structure.
Yang et al. introduced smart thermal management into face masks, with fiber/nanoPE exhibiting low pressure drop, high PM capture efficiency, and superior radiative cooling (Figure 11d). These masks can protect against pollutants while keeping the wearer cool in summer and warm in winter.
Cai et al. developed a nanoporous metallized PE textile with tunable IR emissivity, enabling excellent personal radiative heating and good wearability (Figure 11e). Low IR emissivity on the outer surface and interconnected nanoporous metallic film ensured high IR reflectivity. A superior nanophotonic structure was developed by coating an IR-reflective metallic layer on an IR-transparent nanoPE layer.
Zhang et al. designed an IR gating textile that dynamically regulates thermal radiation for smart personal thermal management (Figure 12a). Each yarn contains conductive metafibers responsive to skin temperature and humidity. Under hot/wet conditions, the yarn collapses, shifting emissivity to match the IR transparency window and promoting cooling. Under cold/dry conditions, the opposite occurs, reducing heat loss.
Developing colored textiles with effective infrared management is challenging because organic dyes absorb mid-IR radiation, increasing IR emissivity and reducing cooling/heating effectiveness. Cai et al. overcame this by introducing inorganic pigment nanoparticles into PE textiles (Figure 12b). Optimized size and concentration minimized IR absorption while providing visible color. The resulting knitted fabrics showed excellent IR radiative cooling and superior color stability.
Other bioinspired technologies have been developed for personal thermal management. For example, ultrathin colored textiles with simultaneous solar and passive radiative heating abilities were developed for indoor use. Colored radiative heating textiles consisted of a polydopamine-coated nanoporous textile laminated with a lossy dielectric layer and a reflective metal layer. These textiles exhibited low MWIR emissivity (<0.1) for passive heating and broadband solar absorption for solar heating. Dai et al. demonstrated a bioinspired Janus polyester/nitrocellulose textile with conical micropores for moisture and thermal management, maintaining optimal body temperature and enabling directional sweat transport. Inspired by polar bear hair, Cui et al. developed a thermally insulating textile with excellent breathability and thermal insulation. Coated with electroheating materials like carbon nanotubes, it functions as a wearable heater. In summary, recent developments in bioinspired IR adaptive materials for daytime radiative cooling show great promise for energy-efficient green buildings and personal thermal management. Phase change materials offer dynamic control, polymers provide flexibility and processability, and multifunctional cooling wood enables scalable green building applications. Advanced nanoporous polyethylene-based textiles pave the way for mass production of IR radiative cooling clothing. Despite progress, current materials primarily reduce temperature rather than enable intelligent thermal management. Future systems should adapt to ambient conditions, maximizing solar reflectance and IR emission in hot environments while minimizing heat loss in cold ones—achieving real on-demand thermal regulation. These advancements open new possibilities for widespread adoption in buildings, clothing, and wearable devices, contributing significantly to energy conservation and climate sustainability.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
