Recent advances in bio-inspired radiative cooling film designs for sustainable building applications

Abstract

Amid escalating global warming, radiative cooling technology has emerged as a transformative approach to sustainable building cooling and energy efficiency applications. Conventional inorganic non-metallic and polymer-based radiative cooling films, however, face intrinsic limitations, including suboptimal cooling performance, poor environmental durability, and inferior optical selectivity. Recently, several promising candidates, such as carbon compounds, metamaterials, and hydrogels, have been regarded as favorable radiative cooling materials, and these studies have made great contributions to the development of the green building domain. However, the search for high-performance materials that can achieve a superior cooling effect still requires exploration. Nature offers inspiration: diverse organisms employ sophisticated micro–nano architectures for thermoregulation, motivating bio-inspired design paradigms in radiative cooling film development. However, systematic reviews of the scientific advancements and characteristics of bio-inspired radiative cooling films are still rare. To fill this knowledge gap, this review systematically examines the operational mechanisms of radiative cooling techniques, identifies critical limitations of conventional film materials, and evaluates recent advancements in bio-inspired design strategies for radiative cooling films. Finally, the perspectives of bio-inspired radiative cooling films are discussed in detail.

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1. Introduction

The energy crisis and greenhouse effect caused by the consumption of fossil fuels are the two primary triggers that result in an increase in demand for cooling technology (Fig. 1a–c).^1–3 However, several traditional cooling systems, including steam compression refrigeration and absorption refrigeration, consume large amounts of electric energy and emit greenhouse gases (e.g., CO₂ and CH₄), thereby causing secondary damage to the environment (Fig. 1d and e). It is reported that conventional air conditioners and electricity account for one-fifth of the global electricity usage in buildings (Fig. 1f).⁴ In this scenario, it is imperative to explore new refrigeration technologies. Among various candidates, radiative cooling technology has garnered significant attention due to its passive operation, exceptional efficiency, and precise thermal regulation in built environments.⁵ Generally, radiative cooling technology utilizes the atmospheric window (8–13 μm) to emit heat infrared radiation into the cold universe, thereby cooling objects on the Earth without a medium.⁶ Currently, the radiative cooling technology is mainly achieved through the optimization of materials and the design of photonic structures.^7,8 In principle, the working mechanism of radiative cooling films is based on the physical rules governing thermal radiation. For example, a building's surface collects solar radiation during the day and emits heat into space via radiation at night. Among the various components of radiative cooling devices, film materials play a pivotal role by effectively reflecting and emitting heat radiation, which can substantially reduce the temperature of buildings. However, the conventional radiation coatings (e.g., imitation paint insulation, aerogel thermal insulation, and interior wall latex paint coatings) applied to building surfaces normally exhibit numerous drawbacks (Fig. 1g–i). Firstly, traditional coating materials contain volatile organic compounds (VOCs) that degrade air quality and contribute to environmental pollution during application and curing procedures. Secondly, their inferior adhesion properties can lead to the detachment of coating materials from the substrate surface. Additionally, their periodic maintenance and recoating requirements in harsh climates can increase long-term costs. Thirdly, conventional coatings may degrade through discoloration, blistering, or peeling under high temperatures.⁹ Therefore, it is necessary to search for desired radiative cooling films with a superior cooling effect.¹⁰

Fig. 1 Traditional cooling methods and building coatings. (a) Rate of change in land surface temperature during 1981–2020. Reprinted with permission from ref. 2 Copyright 2022, Elsevier. (b) Global energy consumption and (c) Global CO₂ emissions. Reprinted with permission from ref. 3 Copyright 2023, Oxford University Press. (d) Steam compression and (e) absorption refrigeration model. (f) Share of global electricity demand growth during 2018–2050. Reprinted with permission from ref. 4 Copyright 2018, Word Energy Outlook. (g) Stone imitation paint insulation, (h) aerogel thermal insulation, and (i) interior wall latex paint coating materials.

Recently, carbon compounds, metamaterials, and hydrogels that possess exceptional optical properties have been utilized as radiative cooling films and have gained favorable advancements.^11–13 However, the advancement of radiative cooling technology is hindered by inherent limitations in key material systems: (1) carbon materials demonstrate undesirable brittleness and insufficient mechanical strength; (2) metamaterials typically exhibit environmental sensitivity in their microstructures, particularly to temperature and humidity fluctuations; and (3) hydrogel materials suffer from both low mechanical resilience and susceptibility to structural degradation.^14,15 These collective material deficiencies critically constrain the development of practical and durable radiative cooling technologies. Hence, it is urgent to develop novel materials with superior radiative cooling performance.

Nature serves as the paramount blueprint for interfacial material design, where biological systems exhibit outstanding thermal regulation behavior via their unique architectures.¹⁶ For instance, silkworm cocoons (Fig. 2a) leverage hierarchically structured silk fibers to achieve sub-ambient radiative cooling via broadband light scattering (97.3% solar reflectance) and high infrared emissivity (∼93%), effectively shielding pupae from predators and thermal fluctuations.¹⁷ Similarly, the tropical white beetle Goliathus goliatus (Fig. 2b) employs a dual-functional scale microstructure combining thin-film interference and Mie resonance to achieve >90% visible light reflection while enhancing mid-infrared emissivity (95%) via anti-reflective hollow cylinders, enabling a 7.8 °C body temperature reduction below ambient conditions.¹⁸ Moreover, human skin demonstrates adaptive thermoregulation through folded epidermal microstructures that modulate sweat evaporation and radiate ∼60% of metabolic heat, achieving 91.5% emissivity within the atmospheric transparency window (8–13 μm, Fig. 2c).¹⁹ These biological strategies – spanning passive spectral selectivity, structural emissivity enhancement, and active thermal adaptation – provide foundational insights for developing bio-inspired radiative cooling coatings. However, current research studies lack systematic analyses of biomimetic radiative cooling mechanisms, particularly in reconciling multiscale structural replication with dynamic environmental responsiveness, thereby hindering the optimization of durable coatings for real-world applications.²⁰

Fig. 2 Principle of radiative cooling and organisms in nature. (a) Temperature regulation mechanism of the silkworm and its cocoon. Reprinted with permission from ref. 17 Copyright 2023, Springer Nature. (b) Thermoregulation mechanism and infrared spectral characteristics of Goliathus goliatus. Reprinted with permission from ref. 18 Copyright 2019, Royal Society of Chemistry. (c) Human skin surface temperature regulation mechanism and spectral emissivity at 8–13 μm. Reprinted with permission from ref. 19 Copyright 2021, Elsevier.

This review systematically summarizes biological radiative cooling mechanisms in nature, grounded in the fundamental principle of radiative cooling. By analyzing micro/nano-structural adaptations and behavioral strategies in diverse species, we elucidate how organisms achieve thermoregulation via radiative cooling, offering bio-inspired methods for developing advanced radiative cooling materials.²¹ Building upon this foundation, we analyze three conventional thin-film radiative cooling materials (carbon-based, metamaterial, and hydrogel systems), highlighting their respective merits in optical performance, alongside inherent limitations in environmental stability, scalability, and mechanical durability. Finally, this review also proposes future research directions, application potential, as well as material preparation techniques for bio-inspired radiative cooling.

2. Basic principles of radiative cooling

2.1 Basic principles of thermal radiation

Heat transfer occurs via three modes: thermal radiation, convection, and conduction. Thermal radiation, distinct from the others, involves heat emission as electromagnetic waves, independent of a medium and operable in a vacuum. As depicted in Fig. 3, all objects above 0 K emit such radiation, with the wavelength and intensity dependent on temperature and emissivity, and higher temperatures typically enhancing radiation capacity.²²

Fig. 3 Basic principles and spectral characteristics of thermal radiation. (a) Heat radiation from the blackbody into free space and (b) Thermal radiation between two blackbodies. Copyright 2020, Acta Physica Sinica. (c) Blackbody heat radiation intensity at various temperatures. Reprinted with permission from ref. 22 Copyright 2018, Optica. (d) Core energy transfer process in the Sun/universe-Earth system and (e) Ideal photothermal selective material. Copyright 2024, Wiley. (f) Schematic of heat radiation from the Earth's surface. Copyright 2023, Royal Society of Chemistry. (g) Distribution of solar illuminance measured above the atmosphere as a function of wavelength or photon energy. Copyright 2021, Chinese Physical Society.

Thermodynamically, an object is defined as a blackbody if it absorbs all external rays. In 1900, M. Planck defined the functional relationship between blackbody radiation E_bλ,max, wavelength λ, and thermodynamic temperature T, as depicted in the following equation:²³

Here, C₁ is Planck's first constant (3.743 × 10⁸ W μm⁻⁴ m⁻²), and C₂ is Planck's second constant (1.439 × 10⁴ μm K). The blackbody spectral distribution of Planck's law is displayed in Fig. 3. Each curve represents the change in the spectral radiant force of the blackbody with wavelength at the same temperature. It is evident from Fig. 3c that as the temperature increases, the blackbody radiation force E_b and blackbody spectral radiation force E_bλ increase rapidly, and the peak wavelength λ_max shifts to the shortwave direction.

In 1891, Wien et al.²² derived from Planck's law via thermodynamics the relationship between the blackbody radiation's peak wavelength λ_max and thermodynamic temperature T, expressed as:

λ_maxT = 2897.6 μm K (2)

The formula shows that as the temperature T increases, the peak wavelength λ_max, corresponding to the maximum spectral radiant flux, shifts to shorter wavelengths. For radiative heat transfer calculations, determining the blackbody radiant flux E_b is critical. The Stefan–Boltzmann law states that this flux is proportional to the fourth power of thermodynamic temperature, expressed as:

In the formula, C_b is called the blackbody radiation coefficient (5.67 W (m² K⁴)⁻¹). According to the Stefan–Boltzmann law, the blackbody radiation force will increase by 16 times when the thermodynamic temperature of the blackbody is doubled. It can be seen that radiation heat transfer will become the main way of heat exchange as the temperature increases.

These thermodynamic laws reveal the intrinsic linkage of thermal radiation to temperature, requiring sufficient temperature differences and blackbody properties for practical efficacy. The Sun, Earth's primary energy source, sustains its climate and biodiversity. With the Earth's surface averaging 300 K (due to atmospheric retention) versus space's 3 K, the universe acts as an ideal blackbody cold sink, absorbing all Earth-emitted radiation to enable passive, medium/energy-free cooling, providing a novel refrigeration research avenue.²⁴

2.2 Radiative cooling mechanism

The Earth's surface can maintain a constant temperature due to the presence of a layer of atmosphere composed of a variety of substances on the Earth's surface, in which substances such as water vapor and carbon dioxide, have a high absorption rate in multiple bands (Fig. 4a and b).²⁵ These substances will weaken or even block the spread of surface heat radiation to outer space,^26–28 and the atmosphere itself will also radiate heat to objects on the surface.^29,30 However, when the electromagnetic wave segment of radiation heat transfer is between 8–13 μm, the atmosphere has slight weakening and absorption effect on the electromagnetic wave, which allows the electromagnetic wave to radiate energy directly through the atmosphere to outer space. Consequently, the “atmospheric transmission window” is another name for the 8–13 μm band.^31,32 The atmospheric transmission window (ATW) coincidentally contains the peak value of 300 K blackbody radiation, which is approximately 9.7 m, according to formula (1). The atmospheric window's spectral transmission map also closely resembles the 300 K blackbody radiation curve, as shown in Fig. 4c.

Fig. 4 Basic principles of radiative cooling. (a) Schematic of outdoor heat transfer between the surface and the environment. Reprinted with permission from ref. 25 Copyright 2024, Wiley. (b) Modeled atmospheric transmittance. Reprinted with permission from ref. 30 Copyright 2016, Wiley. (c) Blackbody radiation profile at 300 K (solid black line) and atmospheric transmission spectra in the infrared region (blue background). Reprinted with permission from ref. 1 Copyright 2019, AIP. (d) Variation of radiant cooling power with temperature difference and (e) Standard AM1.5 solar spectrum. Reprinted with permission from ref. 30 Copyright 2016, Wiley. (f) Spectra of normalized solar irradiation (AM 1.5G), blackbody thermal irradiation (300 K), and atmospheric transmittance. Reprinted with permission from ref. 25 Copyright 2024, Wiley.

The majority of heat radiation from terrestrial objects that can traverse the atmosphere occurs within this band, exemplifying that the Earth maintains a relatively steady surface temperature year-round by radiating heat into the universe through an atmospheric window.^33,34 The whole radiant heat transfer process has no additional power and other energy input; thus, a passive refrigeration technology that does not require energy input to achieve a cooling effect – radiative cooling – came into being. Radiative cooling technology utilizes the atmospheric window to dissipate heat radiation into outer space without energy consumption, theoretically offering over 100 W m⁻² of cooling power, making it a highly promising cooling method. Initial investigations into radiative cooling technology concentrated on nighttime, as daytime solar radiation is excessively intense. If the radiative cooling film material fails to adequately mitigate thermal gain from solar radiation, the sun can readily negate its cooling efficacy. The solar energy received at various longitudes and latitudes on the Earth's surface differs, leading to the planet's diverse climates. In recent years, scholars have proposed the standard atmospheric environment of AM1.5 to facilitate research, under which the average solar radiation intensity exceeds 1000 W m⁻² (Fig. 4d–f).^35–37 The majority of solar energy is concentrated within the 0–2.5 μm range, with a peak at approximately 0.5 μm.^38,39

The core of radiative cooling technology is achieved through material optimization and photon structure design.^40,41 In construction, radiation coating is mostly employed to attain a cooling effect. Recently, advancements in material preparation technology and extensive studies on radiative cooling have led to the realization of daylight radiative cooling technology.^42,43 Conventional building coatings and films evidently fail to create a comfortable and habitable living environment. Various materials for radiative cooling, including carbon materials, metamaterials, and hydrogel materials, can enhance the cooling effect of buildings, thereby mitigating the energy crisis and environmental pollution issues.

3. Current radiative cooling film materials and structural design

3.1 Carbon materials

Carbon materials refer to a class of non-metallic substances synthesized through specific processing techniques using organic precursors (e.g., coal, petroleum derivatives, or biomass) with carbon constituting their principal component. Recent research on micro and nano-scale materials has led to the creation of several synthetic carbon structures, including fullerenes, carbon nanotubes, graphene, and graphdiyne, thereby fostering significant advancements in nanomaterials and nanotechnology.^44,45 With the unceasing advancement in processing, synthesis, and preparation technologies, the advent of novel carbon materials has furnished fresh impetus for the development of scientific research. New carbon materials denote a category of carbon-based substances synthesized through innovative technologies and methodologies, possessing distinct physical and chemical properties or microstructures, which markedly outperform conventional carbon materials in performance, and exhibit unique application potential and extensive developmental prospects across various domains. Novel carbon materials have emerged as a prominent category that includes various carbon-based substances, featuring structural modifications and performance improvements from traditional materials, such as activated carbon, synthetic graphite, and diamond.^46,47 This category also encompasses carbon quantum dots, carbon fibers, graphene aerogel, and other innovative structures, alongside high-performance applications in display, sensing, energy storage, and other domains, as illustrated in Fig. 5.^48,49 Novel carbon materials, including carbon quantum dots and carbon fibers, typically exhibit high emissivity, particularly within the 8–13 μm “atmospheric projection window”. Additionally, these carbon materials possess excellent thermal conductivity, facilitating rapid heat transfer from the object's surface to the coating, thereby effectively radiating heat into space and reducing the object's temperature.

Fig. 5 Current emerging carbon materials and their applications. (a) Infrared spectra of CNT at different scales and the modification mechanism of CNT by surface optimization. Reprinted with permission from ref. 44 Copyright 2023, Elsevier. (b) Application of CDs in doped carbon synthesis, photocatalytic water decomposition and digital anti-counterfeiting. Reprinted with permission from ref. 45 Copyright 2021, Elsevier. (c) Core-shell structure of nano-diamond and its application properties. Reprinted with permission from ref. 46 Copyright 2024, Springer Nature. (d) Schematic of composite phase change materials prepared by carbon nanospheres. Reprinted with permission from ref. 47 Copyright 2021, Elsevier. (e) Fabrication of polybithiophene electrochemically polymerized thin-film modified densely packed C60-ethylenediamine adduct microparticle film. Reprinted with permission from ref. 48 Copyright 2023, Royal Society of Chemistry. (f) Structural model of functionalized graphene materials. Reprinted with permission from ref. 49 Copyright 2022, Elsevier.

Recent advancements in radiative cooling coatings have demonstrated innovative methods for thermal management in buildings. For instance, to address the environmental pollution issues existing in traditional photoluminescent materials, Lu et al. designed a photoluminescent carbon quantum dot-based nanocoating with adaptive solar reflectance modulation (Fig. 6a).⁵⁰ This system utilizes ultraviolet-to-visible photoluminescent conversion and intensity-dependent reflectance enhancement for self-regulating cooling (Fig. 6b). Field tests demonstrated 10–20% superior cooling performance compared to conventional coatings, pioneering sustainable building envelope solutions (Fig. 6c). Similarly, to meet the demands for regulating radiative heat across diverse seasons throughout the year, Zhang et al. designed a dual-function asymmetric coating (Fig. 6d) with an ultra-white radiative cooling layer (98% solar reflectivity, 97% mid-IR emissivity) and an ultra-black solar-heating layer (96% solar absorption, 98.5% visible absorption).⁵¹ The top porous structure achieves 7.6 °C sub-ambient daytime cooling (11.2 °C max) with 98.8 W m⁻² net cooling power (Fig. 6f), while the bottom carbon nanotubes and carbon black (CNTs/CB) layer enable 12.8 °C above-ambient heating and 745 W m⁻² heating power. Compared to commercial paints, this season-adaptive system reduces annual building cooling/heating energy consumption by 7.39% and 1%, respectively (Fig. 6e). Meanwhile, Wang et al. proposed a bifocal phase change composite film (PCCF) integrating radiative cooling (RC) and phase change materials (PCMs) for all-sky cold energy harvesting, storage, and utilization (Fig. 6g).⁵² This bi-functional system exhibits high solar reflectance (94%), strong atmospheric transparency window emissivity (96%), and substantial phase change enthalpy (≈138 kJ kg⁻¹), enabling nocturnal cold energy redirection to mitigate daytime deficits in buildings (Fig. 6h). Through hyperspectral selectivity and optimized cold storage, the phase change composite film (PCCF) achieves continuous sub-ambient cooling with 180 W m⁻² cooling power and −11.95 °C temperature reduction at daytime regulation (Fig. 7i).

Fig. 6 Carbon coating and its cooling effect. (a) Microstructure of the SARC coating, (b) SARC coating concept diagram and (c) Temperature measured for ambient air, SARC coating and normal RC coating. Reprinted with permission from ref. 50 Copyright 2024, Elsevier. (d) SEM micrograph of the PMMA coating, (e) Diagram of an ultra-white asymmetric coating in cooling mode and (f) Measurement of air temperature and cooling effect of asymmetric coatings. Reprinted with permission from ref. 51 Copyright 2024, Elsevier. (g) Digital photos of a top view and vertical section of PCCF, the enlarged images showing the SEM micrograph of PMMA/BaSO₄ coating on PCC@EG, (h) Schematic of bifunctional PCCF for realizing PMMA/BaSO₄-based cold energy harvesting and PCC@EG-based cold energy storage and (i) Temperature evolutions of a PCCF house and an RC house during the daytime and nighttime in summer. Reprinted with permission from ref. 52 Copyright 2024, Wiley.

Fig. 7 Classification of metamaterials based on their functionalities. (a) Mechanical metamaterials based on lattices. Reprinted with permission from ref. 56 Copyright 2022, Elsevier. (b) Acoustic metamaterials. Reprinted with permission from ref. 57 Copyright 2020, Elsevier. (c) Thermal metamaterials. Reprinted with permission from ref. 58 Copyright 2022, Wiley. (d) Imitation lattice metamaterials. Reprinted with permission from ref. 59 Copyright 2023, Multidisciplinary Digital Publishing Institute.

Despite these aforementioned achievements, carbon materials are susceptible to rapid oxidation and corrosion at temperatures exceeding 400 °C, and their scratch resistance is inadequate, significantly diminishing their utility.^53–55 Moreover, the inherent deeper pigmentation of many carbon materials, particularly black, facilitates sunlight absorption when applied as a cooling coating on building exteriors, leading to elevated temperatures that diminish the cooling efficacy. Notably, although advanced carbon materials, such as carbon quantum dots (CQDs) and carbon fibers, exhibit exceptional infrared radiative properties, their synthesis typically involves complex procedures requiring specialized instrumentation and technical expertise, resulting in limited practical applications.

3.2 Metamaterials

Metamaterials comprise artificially designed nanostructures that demonstrate unique optical properties not found in nature, with their extraordinary physical characteristics originating from precisely engineered subwavelength architectures rather than the inherent properties of constituent materials.⁵⁶ The microstructure of metamaterials can establish an electromagnetic resonance at the target frequency to augment heat radiation in the ATW. Metamaterials are engineered to display features within specified wavelength ranges that natural materials cannot achieve, including negative refractive index, perfect absorption, and hyperlens effects.^57,58 To realize these extraordinary occurrences, numerous designs of single-component or hybrid material architectures have been proposed, consisting of either homogeneous or graded distributions of functional units. Based on their functions, the metamaterials can be categorized into mechanical metamaterials, acoustic metamaterials, thermal metamaterials, and imitation lattice metamaterials, as shown in Fig. 7.⁵⁹

Metamaterial radiative cooling coatings are typically composed of multilayer structures.⁶⁰ This encompasses layers of materials exhibiting strong infrared emissivity, including oxide compounds like silicon dioxide (SiO₂) and hafnium oxide (HfO₂), which possess favorable radiation characteristics in the mid-infrared spectrum.^61,62 The infrared emissivity of the coating inside the air transmission window can be augmented by designing nanostructures, including nanoparticles and nanopore configurations.⁶³ Typically, metamaterial coatings incorporate a layer of substances characterized by minimal absorption of solar light, including some transparent polymers or thin films with unique optical properties. These materials can reflect or transmit the majority of solar energy, thereby inhibiting the absorption of solar heat.

In 2015, Hossain et al. demonstrated a conical structure of alternating aluminum and germanium metal dielectrics, as exhibited in Fig. 8a and b, with nearly 100% emissivity over the wavelength range of 8–13 μm.⁶⁴ Their approach demonstrates that effective thermal radiation can be produced using specially engineered metal–dielectric microstructures, eliminating the need for traditional bulk materials. The material they present can achieve a temperature reduction of 12.2 °C below ambient conditions during the day, delivering a cooling power of 116.6 W m⁻². Meanwhile, Zhu et al. proposed a periodic nanostructured metamaterial consisting of orthogonal silicon nanowires, quartz rods at the top and Al layers at the bottom (Fig. 8c and d).⁶⁵ Due to its excellent transparency and substantial thermal radiation emission, it will facilitate efficient radiative cooling while maintaining the color of the object. Zou et al. demonstrated a metamaterial made of a metal-loaded doped silicon resonator, as shown in Fig. 8e and f.⁶⁶ The configuration indicated that with a 100 nm silver layer as the uppermost layer, the material exhibited a solar absorption rate of merely 3% and a peak net cooling power of 68.9 W m⁻². At thermal equilibrium, the minimum temperature may be decreased by 7.4 °C. The application of solar-reflective coatings modifies surface optical properties, enabling effective daytime radiative cooling through spectral selectivity. For example, Eden et al.⁶⁷ prepared a metamaterial with an emission layer composed of α quartz and SiC at the top, and a reflector layer composed of TiO₂, MgF₂ and Ag at the bottom, with peak emissivity within 8–13 and 20–30 μm.⁶⁷ The net cooling power in excess of 100 W m⁻² can be achieved at ambient temperatures (Fig. 8g and h).

Fig. 8 Metamaterial-based radiative cooler. (a) Structural design diagram of multilayer CMM column array heat transmitter and (b) Calculated emissivity spectra for different bottom diameters of the CMM pillars. Reprinted with permission from ref. 64 Copyright 2015, Wiley. (c) Schematic of the modified periodic nanostructured metamaterials, with quartz bar array on top of the original structure of silicon nanowires and (d) Black curve is the emissivity/absorptivity spectrum of the modified structure. Reprinted with permission from ref. 65 Copyright 2013, AIP. (e) Metal-loaded dielectric resonator meta-surface and its false-color scanning electron microscope image of the fabricated sample and (f) Simulated and measured absorptivity (emissivity) of the radiative cooling meta-surface. Reprinted with permission from ref. 66 Copyright 2017, Wiley. (g) Ultrabroadband photonic structures and optimized daytime radiative cooler design. Reprinted with permission from ref. 67 Copyright 2023, American Chemical Society. (h) Scaled AM1.5 solar spectrum (yellow) and atmospheric transmittance t(λ) (blue). Reprinted with permission from ref. 67 Copyright 2023, American Chemical Society.

Notwithstanding their exceptional properties and diverse application potentials, metamaterials present several inherent limitations and technical challenges. The preparation technology of metamaterials is intricate and necessitates high-precision methods and equipment, resulting in high cost. Micro and nano-scale processing necessitates specialized methods and equipment, such as electron beam etching or mask printing, which are expensive to implement and maintain. The properties of metamaterials are strongly dependent on their microstructural features and material constituents, requiring continuous investigation and optimization to enhance their structural stability and functional reliability. The operational reliability of metamaterials in extreme environments remains experimentally unvalidated, while their limited long-term durability substantially constrains practical deployment.

3.3 Hydrogel materials

Hydrogel is a material characterized by distinctive features, utilizing water as a dispersion medium and establishing a three-dimensional network structure via the cross-linking of polymer chains.^68,69 Water-soluble polymer networks are chemically modified through the incorporation of both hydrophobic moieties and hydrophilic functional groups. The hydrophilic residues interact with water molecules, facilitating their connection within the network, while the hydrophobic residues expand upon contact with water, resulting in the formation of cross-linked polymers.^70,71 This flexible material maintains structural integrity post-hydration while exhibiting significant water uptake capacity and shape retention properties. Due to its superior performance, its use has extended into diverse sectors like agriculture, forestry, animal husbandry, horticulture, desert reclamation, healthcare, biomedicine, construction, petrochemicals, consumer goods, food production, electronics, and environmental conservation, and is still expanding to a wider range of applications (Fig. 9).^72–74

Fig. 9 Structural properties and applications of hydrogel materials. Reprinted with permission from ref. 70 Copyright 2024, Elsevier.

Hydrogel coating is a surface application technique utilizing sol-gelation technology, merging hydrogel characteristics with coating benefits to demonstrate distinctive qualities across many applications.⁷⁵ The fundamental principle of hydrogel coatings is the creation of a hydrogel film on the substrate's surface through the sol preparation, gel formation, and coating application. The film is permeable, pliable, and moist, resembling biological tissues.^76,77 The performance attributes of the hydrogel covering are mostly evident in the following areas: Firstly, exceptional biocompatibility: the hydrogel coating materials predominantly consist of hydrophilic polymers, which exhibit favorable biocompatibility and can mitigate rejection reactions with biological tissues.^78,79 Secondly, excellent lubricity: the hydrogel coating's surface is moist, facilitating a super-lubricated interface that diminishes friction and wear.^80,81 Thirdly, adjustable mechanical properties: the mechanical characteristics of the hydrogel can be modified to align with the tissue by altering the cross-linking density and composition.⁸² Lastly, excellent adherence: the hydrogel coating exhibits strong adhesion to the substrate, ensuring that the coating remains securely attached during use.^83,84

Gan et al. engineered a spectrally selective self-imbibing polyacrylate (PAAS) film using diaper-derived hygroscopic agents, enabling scalable hybrid passive cooling via synergistic evaporative-radiative mechanisms (Fig. 10a).⁸⁵ The dry PAAS film achieves 96% solar reflectance and 99% atmospheric window emissivity (Fig. 10b), driven by its polymer chain vibrations. Under 800 W m⁻² solar irradiance, the film reduces the surface temperature by 5 °C under overcast conditions (Fig. 10c). This integrated passive cooling strategy could mitigate 118.4 billion kilograms of annual global carbon emissions by replacing conventional vapor-compression air conditioning systems. To achieve bidirectional regulation of heating and cooling, Guo et al. designed a spectrally adaptive coating combining a high-emissivity polymer, thermochromic hydrogel (HPC), and solar-absorbing black layers for bidirectional passive thermal regulation (Fig. 10d).⁸⁶ The PET/HPC/black multilayer structure enables solar spectrum modulation via hydrogel-driven visible-light transmittance/reflectivity switching (Fig. 10e). At 1-sun intensity, the system transitions from 675.0 W m⁻² heating at 20 °C to 86.2 W m⁻² cooling at 60 °C (Fig. 10f). Its visible-light absorption mechanism maintains cooling capacity nocturnally, achieving 20.3% annual energy savings versus conventional coatings, with enhanced summer performance through radiative cooling synergy.

Fig. 10 Hydrogel materials for radiative cooling. (a) SEM image of sodium polyacrylate (PAAS) photonic film, (b) Cooling mechanism of PAAS films and (c) Temperature variation map for outdoor testing. Reproduced with permission ref. 85 Copyright 2023, Nature Communications. (d) Schematic of the switchable coating, (e) Measured spectra of the switchable coating in heating and cooling modes and (f) Temperature changes of different coatings in outdoor experiments. Reproduced with permission ref. 86 Copyright 2024, Elsevier. (g) SEM images of HPHG, (h) Refrigeration diagram and (i) Outdoor test cooling effect. Reproduced with permission ref. 87 Copyright 2024, Elsevier. (j) SEM images of the intelligent flexible porous double-layer membrane, (k) Schematic of the refrigeration process and (l) Cooling power of the bilayer and PDMS at different solar irradiation levels. Reproduced with permission ref. 88 Copyright 2024, Wiley.

Wang et al. engineered a lightweight hierarchical porous hydrogel-graphene (HPHG) composite through rational design, demonstrating superior thermal regulation under extreme conditions via architecture multiscale porosity (Fig. 10g and h).⁸⁷ Indoor testing revealed a maximum sub-ambient temperature reduction of 22.4 °C (average ΔT = 15.9 °C) in enclosed spaces, significantly outperforming conventional radiative materials. Under open-air conditions at 45 °C ambient temperature, HPHG maintained an average 8.9 °C cooling (peak 14.7 °C), while benchmark materials lost the cooling capacity (Fig. 10i). The structural hierarchy enables enhanced thermal dissipation mechanisms inaccessible to traditional radiative coolers. Qiu et al. engineered a flexible bilayer porous film for autonomous day–night cooling mode switching, addressing the static limitations of radiative cooling.⁸⁸ The hierarchically porous PDMS upper layer (Fig. 10j) achieves 93% solar reflectance and 95.2% infrared emissivity, while the hygroscopic hydrogel substrate (Fig. 10k) enables atmospheric water harvesting and evaporative cooling. Field tests demonstrate peak daytime cooling power of 424.4 W m⁻² with ΔT = 10.4 °C sub-ambient under solar maxima (Fig. 10l). Nocturnally, the bilayer maintains 1.09 °C–3.96 °C thermal elevation versus single-layer radiative films, preventing overcooling through synergistic vapor sorption-thermal buffering mechanisms.

Hydrogel-based materials are significant in various domains owing to their distinctive structure and physical and chemical characteristics. Nevertheless, their application is constrained by several issues, including inherent softness and low mechanical strength, which restrict their employment in harsh environments.⁸⁹ Secondly, hydrogel materials are significantly influenced by external conditions, with parameters such as temperature, pH, and ionic strength potentially compromising their function and leading to instability.⁹⁰ Moreover, the hydrogel is prone to water loss in arid environments due to evaporation, leading to diminished performance; thus, it must be maintained under suitable humidity levels.⁹¹ A further constraint is the concern of biodegradability; while many hydrogels are biodegradable, the rate and method of disintegration within the body or the environment may differ.⁹² Ultimately, the cost of synthesizing and processing particular functional hydrogels restricts their prevalence in certain commercial applications.⁹³ Finally, the microstructure and characteristics of the coating influence the efficacy of radiative cooling, and given the rapid advancement of materials science, there is an urgent need to investigate novel radiative cooling materials.

While carbon materials, metamaterials, and hydrogels have advanced radiative cooling technology with their unique optical properties, such as high solar reflectivity, strong mid-infrared emissivity, and adaptive thermal regulation, their inherent limitations significantly hinder practical deployment in sustainable building applications. Concretely, carbon materials suffer from brittleness, poor mechanical strength, susceptibility to oxidation at high temperatures, and deep pigmentation that increases solar absorption, while their synthesis often requires complex procedures and specialized equipment. Metamaterials, despite their exceptional spectral selectivity, face challenges including intricate fabrication processes, high production costs, environmental sensitivity to temperature and humidity fluctuations, and unproven long-term durability in extreme conditions. Hydrogels, though promising for evaporative–radiative synergies, are plagued by low mechanical resilience, structural degradation under varying pH or ionic strength, water loss in arid environments, and high synthesis costs.

These collective drawbacks-insufficient cooling power, poor environmental adaptability, mechanical fragility, and scalability issues underscore the urgent need for novel radiative cooling materials. However, nature offers a wealth of inspiration: organisms ranging from silkworm cocoons and Saharan silver ants to white beetles have evolved sophisticated micro–nano structures that enable efficient thermal regulation through optimized solar reflection and infrared emission. Emulating these biological strategies, bio-inspired design emerges as a transformative approach to overcome the limitations of traditional materials, promising enhanced cooling performance, durability, and adaptability for sustainable building applications.

4. Bio-inspired film designs in radiative cooling

Although the aforementioned three novel radiative cooling materials have led to significant advancements in radiative cooling technology compared to traditional materials, the inherent defects of these materials pose challenges to achieving further improvements in radiative cooling efficiency. For instance, carbon materials exhibit poor stability, limited environmental adaptability, and relatively low mechanical strength. Metamaterials suffer from complex fabrication processes and high manufacturing costs, while hydrogel materials tend to experience water loss, chemical instability, susceptibility to corrosion and degradation. Therefore, the exploration of novel radiative cooling film materials is imperative. Numerous organisms in nature possess remarkable thermal management properties, providing new insights into the design of radiative cooling films. This section presents the radiative cooling phenomena observed in nature and, in conjunction with biomimetic design principles, elaborates on the structural design of biomimetic radiative cooling films and their application in temperature regulation and energy savings in buildings.

4.1 Phenomena of radiative cooling in nature

Numerous instances of radiation cooling occur in nature, exemplified by the temperature disparity between day and night in desert environments. The predominant constituent of deserts, silicon dioxide (SiO₂), exhibits high emissivity within the atmospheric window. Consequently, in the absence of solar radiation at night, the desert naturally emits heat into outer space, resulting in a temperature variation that can attain several degree Celsius. The dew on leaves in the early morning results from the radiative heat exchange of plants with space at night, which lowers the leaf surface temperature and condenses atmospheric moisture.^94,95 Moreover, certain creatures in nature, such as silver ants inhabiting deserts and beetles residing in tropical rainforests, can endure extreme temperatures due to their unique biological structures that minimize solar radiation absorption and facilitate heat dissipation into the atmosphere.^96–98 Consequently, by studying the coating characteristics of creatures in nature, contemporary architecture can employ bio-inspired principles to create more effective radiation coatings that enhance temperature regulation and decrease energy consumption in buildings.

4.2 Basic principles of bio-inspired film material designs

The fundamental ideas of bio-inspired design primarily involve the emulation and utilization of natural biological structures to enhance the efficiency and flexibility of technological goods.⁹⁹ Bio-inspired materials are artificial materials designed and manufactured with reference to the pattern of living systems and the rules of organ materials, as exhibited in Fig. 11. To enhance the performance of radiative cooling materials in high-temperature environments, inspired by the porous multilayer porous biological structure of Dictyophora (Fig. 11a and b), Wu et al. proposed a novel hollow@porous radiation-cooled membrane, which combines hollow particles with a porous polymer (Fig. 11c).¹⁰⁰ The hollow@porous flexible films have high solar reflectivity (93.7%), strong infrared emissivity (89.1%) and ultra-low thermal conductivity (17.56 mW mK⁻¹). The experimental results also show that the daytime cooling performance of the prepared cooler is significantly reduced to 17.4 °C when the peak solar intensity is 980 W m⁻². Meanwhile, with the aim of achieving the direct application of plant transpiration in hydroelectric power generation, motivated by the transpiration of lotus leaves (Fig. 11d), Zhou et al. developed a living lotus leaf-based transpiration generator (LTG), which can use the water cycle process of leaf transpiration to directly capture environmental latent heat to achieve continuous electricity generation, revealing the water-volt effect of plant transpiration for the first time (Fig. 11e).¹⁰¹ Given the widespread existence of plant transpiration and its huge energy reserves, this study will provide new perspectives for the development of green energy in the future. To address the challenge that superhydrophobic surfaces struggle to maintain superhydrophobicity under deep-water and fluid-impact conditions, roused by the fluff-like microstructure of the leaves of aquatic plants (Fig. 11f and g), Deng et al. developed a design strategy for a bio-inspired adaptive superhydrophobic surface (Fig. 11h).¹⁰² The water pressure resistance was improved by about 183%, and the adhesion force was reduced by about 80% after compression. This adaptive superhydrophobic material provides a new design idea for further improving the operational performance of amphibious aircraft and other cross-medium vehicles. Furthermore, owing to the increasing demand for drag-reducing surfaces in numerous engineering fields, influenced by the dermal structure of osteichthyan fish, Xue et al. developed a dual-functional hydrogel coating (JHC) through a two-step ultraviolet irradiation process under ambient conditions (Fig. 11i–k).¹⁰³ The engineered JHC demonstrates remarkable shear adhesion strength (103.3 ± 17.5 kPa) at its basal interface while exhibiting superior mechanical robustness, anti-swelling resistance, fouling inhibition, and drag reduction capabilities on its upper surface. This bio-inspired design paradigm presents promising potential for multidisciplinary applications, spanning pipeline transportation systems, biomedical engineering, and marine vessel technologies.

Fig. 11 Bio-inspired materials and systems inspired by plants and animals. (a and b) Microstructure of the neck of Dictyophora and (c) Radiative cooling and thermal insulation film inspired by Dictyophora. Reprinted with permission from ref. 100 Copyright 2023, American Chemical Society. (d) Transpiration of lotus leaves and (e) Transpiration generator based on live lotus leaves. Reprinted with permission from ref. 101 Copyright 2024, Nature Water. (f and g) Duckweed leaves and their microstructure and (h) Bio-inspired adaptive superhydrophobic surface. Reproduced with permission ref. 102 Copyright 2024, Wiley. (i and j) Microstructure of fish scales and (k) imitation fish skin Janus hydrogel coating. Reproduced with permission ref. 103 Copyright 2023, Wiley.

Nature has imparted significant experience for the advancement of human society, with various animal forms and the micro and nano structures of plants offering innovative concepts for the design and fabrication of radiative cooling materials. The effectiveness and efficacy of radiative cooling coatings in buildings can be enhanced by bio-inspired designs, addressing the current issues associated with them.

4.3 Radiative cooling materials for bio-inspired films

The distinctive micro and nano structures of many animal coats and plants in nature can effectively utilize radiative cooling to adapt to climatic demands and address the obstacles posed by harsh climates. The implementation of bio-inspired designs in the formulation of radiative cooling coatings can significantly boost the performance of coating materials and improve the efficacy of radiative cooling. Recently, inspired by the biological mechanism of Cyphochilus, which evolved tilted triangular nanosheets on the surface and internal porous structures to synergistically achieve high-efficiency light regulation for adapting to tropical high-temperature environments, Tang et al. designed a bioinspired polymer film with surface pyramid arrays and 3D hierarchical pores (Fig. 12a and b).¹⁰⁴ Its surface-ordered pyramid arrays enhance the reflection of sunlight at high incident angles through total internal reflection, while the internal 3D hierarchical pores (nanopores with diameters ranging from 0.1 to 1 μm and micropores with diameters ranging from 1 to 18 μm) achieve scattering, covering the entire solar spectrum via Mie scattering. The synergistic effect of these two structures enables the film to attain a solar reflectance of 98% and a mid-infrared emissivity of 96%. Outdoor tests show that it can achieve a temperature reduction of approximately 8.8 °C during the daytime, with a net cooling power of 96.6 W m⁻². Additionally, it exhibits excellent mechanical strength, flexibility, and hydrophobicity, providing a feasible solution for high-efficiency passive radiative cooling (Fig. 12c). Meanwhile, to enhance the environmental adaptability of radiative cooling films, motivated by the synergistic biological mechanism of Saharan silver ants, which evolved hair structures with triangular cross-sections (enhancing solar reflection via total internal reflection) and surface wrinkles (strengthening broadband scattering through high-order Mie resonance) to adapt to extreme desert heat, Zhang et al. created a Saharan silver ant-inspired radiative cooling film with a dual-scale photonically engineered structure (Fig. 12d). In this structural design, the embedded glass bubbles achieve full solar spectrum scattering through high-order Mie resonance by virtue of the refractive index difference and broad size distribution (with an average of approximately 10.4 μm), resulting in a solar reflectance of 0.93. The binary photonic structure on the surface, composed of 8 μm-wide triangular prism arrays and 500 nm silica nanospheres, combined with the vibrations of chemical bonds in the PDMS matrix, enhances the long-wave infrared emissivity to 0.94. The synergistic effect of the two enables the film to achieve a daytime temperature reduction of 6.9 °C under a solar irradiance of 746 W m⁻² (Fig. 12e and f).¹⁰⁵ Moreover, with the aim of meeting the aesthetic requirements, high infrared emissivity, and fire safety requirements of radiative cooling materials in practical applications, Fei et al. engineered chameleon-inspired thermoresponsive polyurethane coatings (Fig. 12g) through a strategic integration of phase-change microcapsules and 2D nanosheets (Fig. 12h).¹⁰⁶ Silicon–boron modified thermochromic microcapsules regulate colorant binding with temperature changes, increasing ultraviolet-visible light reflectance by 27.8–32.9%. Boron nitride and montmorillonite nanosheets enhance dipole moment changes of relevant bonds through strong interfacial interactions, and combined with free C–O bonds of molten fatty alcohols, raise mid-infrared emissivity to 98.1%. Montmorillonite forms a ceramic protective layer at high temperatures, synergizing with boron nitride to enhance mechanical properties and flame retardancy. The three synergistically enable the coating to achieve a maximum daytime temperature reduction of 8.3 °C and a nighttime sub-ambient temperature reduction of 7.6 °C, balancing aesthetics and practicality (Fig. 12i).

Fig. 12 (a) Radiative cooling material for bio-inspired white beetles, (b) SEM image of white scales and (c) Outdoor daytime cooling effect. Reprinted with permission from ref. 104 Copyright 2024, American Chemical Society. (d) Schematic of the bioinspired model of the Saharan silver ant, (e) SEM image of the material and (f) outdoor daytime cooling effect. Reprinted with permission from ref. 105 Copyright 2024, American Chemical Society. (g) Schematic of radiative cooling material design for bio-inspired chameleon, (h) SEM image of the material and (i) temperature curves at nighttime for PUA/SiB-TCM/BN/MMT (blue), control sample, and tenvir, 2. Reprinted with permission from ref. 106 Copyright 2024, Wiley.

In addition, drawing on the synergistic biological mechanism of scarab beetles, which have evolved a cuticle layer with nanostructures and an underlying porous layer (regulating body temperature through broadband reflection) in their elytra to survive in high-light environments, Li et al. developed scarab elytra-inspired hierarchically structured radiative cooling (RC) films (Fig. 13a), featuring embedded photonic crystals and polymethyl methacrylate (PMMA) micro-pits (Fig. 13b) to address interfacial adhesion issues in conventional multilayer RC materials.¹⁰⁷ The ordered surface micropits and internal disordered porous structures synergistically extend the light scattering path, and combined with the selective reflection of photonic crystals, a high solar reflectance of 93.4% is achieved; the C–O–C stretching vibration of PMMA and the C–H bending vibration of polystyrene nanoparticles enhance infrared emission in the 8–13 μm band (92.3%); the synergistic effect of the two enables the material to achieve a cooling effect of 10.2 °C at night and 7.2 °C at noon, while the micropit structure improves color stability and mechanical robustness (Fig. 13c).

Fig. 13 (a) High brightness color daytime radiative cooling film inspired by chafer, (b) SEM image of the SC–RC thin film structure and (c) Outdoor measured cooling effect. Reprinted with permission from ref. 107 Copyright 2024, Cell Press. (d) Radiative cooling material inspired by the micro-scale tree-like structure of morpho butterfly scales, (e) Micro-scale tree structure and (f) Emission spectrum. Reprinted with permission from ref. 108 Copyright 2018, Semantic Scholar. (g) Radiative cooling coating inspired by the natural plant fiber structure. Reprinted with permission from ref. 109 Copyright 2023, Springer Nature. (h) Micrograph of top and cross-section views of microporous polypropylene sheet and (i) Reflection and emission characteristics of microporous polypropylene sheets. Reprinted with permission from ref. 109 Copyright 2023, Springer Nature.

The micro-tree-like structures in Morpho butterfly scales achieve selective emission in specific wavelength bands through photon interference between ridges and lamellae, and this biological mechanism has inspired the design of artificial micro-tree structures. Based on this, Krishna et al. designed a periodic structural material in 2018, drawing on the microscale tree-like structures of Morpho butterfly scales (Fig. 13d and e).¹⁰⁸ Metallic microtrees achieve a high emissivity in the 8–13 μm wavelength range (enabling a cooling power of 136 W m⁻²) through designs, such as a 10 μm ridge period and an 8 μm lamella period, leveraging the interference of photons between ridges and lamellae. Meanwhile, they maintain a low emissivity in the solar spectrum (0.2–2.5 μm) to enhance reflection. Ceramic microtrees (e.g., alumina), with structures like a 1 μm ridge period and a 3 μm lamella period, enhance solar spectrum absorption and reduce infrared emission, achieving a heating power of 12 W m⁻². This morphology-driven selective emission characteristic allows metallic surfaces to cool by 10 K and ceramic surfaces to heat up by 8 K, breaking through the performance limitations of traditional materials (Fig. 13f).

Furthermore, motivated by the structure of natural plant fibers (Fig. 13g), Yang et al. constructed random three-dimensional pores in a polymer matrix by phase separation.¹⁰⁹ Utilizing styrene-butadiene-styrene (SBS) and styrene-ethylene-propylene-styrene (SEPS), the disparity in SEPS compatibility with polypropylene (PP) facilitated the manufacture of two polypropylene sheets exhibiting distinct internal porosity (Fig. 13h). Both materials can efficiently scatter sunlight (97%) and exhibit excellent emissivity in the mid-infrared spectrum (67∼81%) (Fig. 13i), demonstrating effective radiative cooling properties and possible applicability in building energy saving.

Current bio-inspired radiative cooling technology has advanced rapidly and diversified. Research into the synergies of biological materials, structures, and behaviors has yielded novel materials and systems, boosting the cooling power. With unique structural advantages, these films are crucial for cooling in buildings, energy conservation, agriculture, and electronics.

From the aforementioned examples, it can be concluded that bio-inspired radiative cooling materials exhibit superior advantages in the domains of building cooling and energy conservation. Nevertheless, in recent years, as the overlap between radiative cooling and smart window technologies has increasingly intensified, electrochromic radiative cooling devices-by virtue of actively regulating the optical properties via electric fields and enabling switching between “cooling” and “heating” modes in response to ambient temperatures and practical demands-have evolved into another sophisticated approach for the utilization of radiative cooling technology. The differences between them in core mechanisms, performance characteristics, and application scenarios are of great significance for technology selection.

In terms of core mechanisms and regulation modes, PRC films achieve passive cooling by virtue of bio-inspired micro–nano structures. By simulating the spectral selectivity of natural organisms (such as Goliathus goliatus and Saharan silver ants), they optimize solar reflectivity (0.3–2.5 μm) and mid-infrared emissivity (8–13 μm), thereby realizing continuous heat dissipation without external energy input. Electrochromic devices dynamically regulate spectral characteristics through reversible changes in the optical properties of materials driven by electrical signals (e.g., redox reactions). For example, WO₃-based devices can switch between “cooling mode” (high infrared emissivity + high solar reflectivity) and “transparent mode” (high visible light transmittance), balancing cooling and lighting requirements,^110,111 with response times typically ranging from seconds to minutes.¹¹²

When it comes to the comparison of performance and applications, the advantages of PRC films lie in zero energy consumption, low cost, and large-scale potential. For example, plant fiber-inspired polypropylene films can achieve 97% solar reflectivity with simple preparation processes, making them suitable for static scenarios, such as large-scale building roofs and outdoor sunshades. The core value of electrochromic devices lies in their dynamic regulation capability, which can adapt to diurnal or seasonal changes. For instance, the device in ref. 111 reduces building energy consumption by over 20% through switching, making it suitable for scenarios with large temperature differences in temperate zones or requiring intelligent integration (such as smart windows). Nevertheless, they require low-voltage driving (1–5 V), involve complex preparation processes, have high costs, and their long-term cycle stability (usually <10⁴ cycles) needs improvement.¹¹²

PRC films and electrochromic devices are not substitutes for each other. In the future, they may achieve complementary advantages through composite designs.^113,114 For example, integrating bio-inspired structures with electrochromic layers could form an efficient system characterized by “passive cooling as the mainstay and active regulation as the supplement,” balancing low cost and intelligent control requirements.¹¹⁵

To more intuitively illustrate the advantages of bio-inspired radiative cooling coatings over other radiative cooling materials and electrochromic radiative cooling technologies, the comparative results are summarized in Table 1.

Table 1 Comparison between bio-inspired radiative cooling films and other cooling technologies

Material type	Specific examples	Reflectivity	Mid-infrared emissivity	Maximum cooling power (W m^−2)	Maximum temperature reduction extent (°C)
Traditional materials	Stone-like coating, interior wall latex paint^9	60–80%	60–80%	<30	2–4
	Aerogel insulation material^10	70–85%	70–85%	30–50	3–5
Advanced materials	Carbon materials^50–52	94%	96%	180	11.95
	Metamaterials^64–67	>90%	>90%	116.6	12.2
	Hydrogel materials^85–87	93%	>90%	>80	5
Bio-inspired materials	Bio-inspired white beetle coating^104	98%	96%	96.6	8.8
	Bio-inspired Saharan silver ant coating^105	93%	94%	78.1	6.9
	Bio-inspired chameleon coating^106	Adjustable	93%	—	8.3
	Bio-inspired scarab elytra coating^107	97%	>90%	—	10.2
	Bio-inspired butterfly coating^108	>90%	>90%	136	10
	Bio-inspired plant fiber coating^109	96%	>90%	—	11
Advanced switchable device	Electrochromic radiative cooling devices^110–112	Adjustable (heating at 20 °C: 675 W m^−2, cooling at 60 °C: 86.2 W m^−2)	High emissivity	86.2 (cooling mode)	—

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Compared with traditional materials, bio-inspired materials have achieved significant optimizations in solar reflectivity (with an improvement of 15–30%) and mid-infrared emissivity (with an improvement of 10–20%), leading to a 1–3 fold enhancement in cooling efficiency. When compared to advanced materials, bio-inspired materials not only maintain high cooling performance but also overcome the inherent drawbacks of carbon materials (such as oxidation), metamaterials (such as high production costs), and hydrogel materials (such as water loss). They exhibit particular advantages in environmental adaptability (e.g., pollution resistance and tolerance to temperature and humidity fluctuations) and functional integration (e.g., structural coloration and fire resistance). For instance, the 98% solar reflectivity of the film inspired by white beetles outperforms most metamaterials, and its mechanical flexibility makes it more suitable for applications on curved building surfaces; the superhydrophobic property of the film inspired by Saharan silver ants is absent in carbon materials and hydrogels. Through these comparisons, it is evident that the core innovation of bio-inspired design lies in the “synergetic optimization of structure and function”; it not only draws inspiration from the efficient light-regulation structures of natural organisms (such as the dual-layer photonic structure of white beetles) but also integrates engineering requirements (such as pollution resistance and mechanical properties), thus filling the gap in the “high performance-practicality” balance among existing advanced materials.

4.4 Applications of bio-inspired films in building cooling fields

The utilization of bio-inspired films in building cooling signifies a revolutionary method for meeting the increasing need for energy-efficient and sustainable cooling solutions. These films emulate natural systems by incorporating mechanisms like passive cooling, evaporative cooling, and radiative cooling, providing novel methods to diminish heat absorption and enhance thermal comfort in structures. For instance, Yu et al. developed a bio-inspired micropyramidal structural model influenced by the distinctive hair morphology of thermoresistant species.¹¹⁶ Complex designs were replicated using silicon templates, and radiation-cooled films featuring specialized micropyramid structures were produced by combining high dielectric constant materials with polymers and applying PVDF coatings (Fig. 14a and b). The resultant coating exhibits a solar reflectivity of 97.3% and an infrared emittance above 98% within the atmospheric window. According to empirical daylight radiative cooling trials, the PRCM exhibited enhanced radiative cooling efficacy, attaining an average temperature reduction of 7.3 °C for the coated unit. Meanwhile, Li et al. achieved comprehensive delignification and densification of wood to produce a specialized structural material (Fig. 14c and e).¹¹⁷ The nanofibers exhibited an average reflectance of 96% for solar light and an average infrared emissivity exceeding 90%, with negligible emissivity variation on spectral response angle. In the continuous outdoor thermal measurement conducted over 24 hours, the cooling power of the wood during the night and from 11 AM to 2 PM is 63 W m⁻²and 16 W m⁻², respectively. The average cooling power over the 24-hours period is 53 W m⁻², with the average night temperature being over 9 °C lower than the ambient temperature and more than 4 °C lower at noon. It demonstrates significant potential in cooling and energy conservation. Moreover, to achieve the efficient production of radiative cooling materials, Huang et al. presented an innovative approach for the design of structured rod-like particles (RLP) featuring textured surfaces packed with random nanopores for the production of highly efficient radiation-cooled coatings (Fig. 14f and h).¹¹⁸ The coating possesses a straightforward structure and production procedure akin to paint, and can be applied to the outside walls of structures. The coating exhibits exceptional solar reflectivity (97%) and long-wave thermal emissivity (95%), facilitating excellent subambient temperature reductions of 5.7 °C at night and 4.3 °C during the day under sun intensity approaching 1000 W m⁻². These distinctive bio-inspired films possess extensive application potential in building cooling, offering significant advantages in energy conservation, environmental preservation, and comfort enhancement, hence anticipated to foster the sustainable advancement of the construction sector.

Fig. 14 Application of bio-inspired films in building cooling. (a) Schematic of bio-inspired micro-pyramid structure thin film and (b) Cooling effect of building model. Reprinted with permission from ref. 116 Copyright 2025, Wiley. (c) Cooling wood. Reprinted with permission from ref. ¹¹⁷ Copyright 2019, AAAS. (d) Architectural model and (e) Cooling effect of the building with wood coating. Reprinted with permission from ref. 117 Copyright 2019, AAAS. (f) Photos of coating materials, (g) Cooling comparison of building models and (h) Building cooling effect test diagram. Reprinted with permission from ref. 118 Copyright 2022, Elsevier.

In summary, these bio-inspired films provide a sustainable and energy-efficient avenue to building cooling via utilizing natural principles to minimize heat absorption and enhance thermal comfort. Cool roofs, window coatings, and evaporative and radiative cooling systems have proven their capacity to substantially decrease energy consumption and carbon emissions in buildings. A typical example is that bio-inspired radiative cooling films possess unparalleled advantages over commercial alternatives (e.g., white paints and conventional radiative films) in terms of cooling power and reduction of air conditioning loads, with the specific comparison results shown in Table 2.

Table 2 Advantages of bio-inspired radiative cooling films over commercial alternatives

Performance indicators	Bio-inspired films^104–109	White paint^9,10	Traditional radiation film^44,45,56,57,70
Solar reflectivity	93–98%	80–85%	90–92%
Mid-infrared emission rate (8–13 μm)	89–96%	70–80%	85–90%
Cooling power (W m^−2)	78.1–96.6	30–50	60–80
Typical temperature drop (°C)	6.9–8.8	2–4	4–6
Air conditioning load reduction	20–30%	—	15–20%

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It can be observed from the comparison that the cooling power of bio-inspired films is 56–120% higher than that of white coatings and 18–36% higher than that of conventional radiative films. They can reduce air conditioning loads by approximately 20–30%, which is significantly higher than traditional radiative cooling materials.¹¹⁹ Their core advantage stems from the synergistic optimization of solar reflection and infrared emission by the bio-inspired structures. Although bio-inspired radiative cooling materials currently exhibit significant advantages in terms of cooling performance and energy-saving potential, they are confronted with certain issues, such as insufficient adaptability under different climatic conditions and limitations in compatibility with special scenarios.^120,121

Despite the existing hurdles, continual progress in material design and technology is facilitating wider usage, positioning bio-inspired films as an essential element of sustainable construction techniques. In light of the ongoing challenges posed by climate change and increasing energy needs, bio-inspired films offer a viable solution for creating a more sustainable and resilient built environment.

4.5 Scalability of fabrication methods

In terms of the potential for scalable manufacturing, some bio-inspired materials have adopted simple processes that are close to industrialization.¹²² For example, the structured rod-like particle (RLP) coating developed by Huang et al.,¹¹⁸ featuring a surface design with random nanopore textures, has a preparation process similar to that of traditional coatings. It can be directly applied to building exteriors without high-precision equipment, and the raw materials are low-cost, providing feasibility for large-scale construction. Its performance (97% solar reflectivity and 95% long-wave emissivity) also verifies its practical value. Similarly, the PAAS film prepared by Gan et al.⁸⁵ using diaper-derived hygroscopic agents achieves high spectral selectivity through simple regulation of polymer chain vibrations. With wide sources of raw materials and a process compatible with existing film production lines, it shows potential for low-cost scaling. In addition, inspired by plant fibers, Yang et al.¹⁰⁹ constructed random three-dimensional porous structures in a polymer matrix through phase separation. Using common polymers such as styrene-butadiene-styrene (SBS), the process is simple and allows mass production by adjusting the proportion of components. Its 97% sunlight scattering rate demonstrates a balance between performance and scalability.

However, scalability still faces significant challenges. On the one hand, the preparation of high-precision micro–nano structures is costly.^123,124 For instance, periodic materials mimicking the tree-like structure of Morpho butterfly scales rely on electron beam etching or mask printing, with a single set of equipment costing over one million dollars. Moreover, the processing efficiency is low, making it difficult to meet the large-area needs of buildings. On the other hand, material stability restricts long-term large-scale applications: hydrogel-based bio-inspired materials (such as the thermochromic coating developed by Guo et al.⁸⁶) are prone to dehydration and degradation in arid environments, requiring additional encapsulation processes, which increases production complexity. Additionally, the synthesis of advanced carbon materials like carbon quantum dots (CQDs) requires high-temperature and high-pressure reactors, with a batch-to-batch performance variation rate of up to 15%, affecting industrial quality control. Furthermore, most existing prototypes are centimeter-level samples, lacking data on large-area preparation above the square-meter level. For example, the dual-photonic structure film developed by Tang et al., which mimics the scales of white beetles, achieves a cooling effect of 8.8 °C in the laboratory. However, when scaled up to 10 square meters or more, the uniformity of the surface microstructure decreases, leading to a cooling power loss of over 30%.

5. Conclusions and prospects

Radiative cooling is a contemporary passive cooling technique that transfers energy to outer space as thermal radiation via the “atmospheric window” by spectrum management, lowering the temperature of things. Compared to traditional refrigeration technology, it is characterized by its lack of power or energy consumption and is seen as a promising new energy technology. This paper initially examines the fundamental principles of radiative cooling and delineates the advantages and limitations of conventional building thin-film materials. It is alongside novel radiative cooling thin-film materials exemplified by carbon materials, metamaterials, and hydrogels. Then, drawing inspiration from the distinctive temperature regulation systems of natural creatures, it examines the advancements in bio-inspired radiative cooling films and their applications in building cooling and energy conversion. Finally, it anticipates the future advancement of bio-inspired radiative film materials.

The bio-inspired radiative cooling film material significantly lowers building surface temperature and diminishes air conditioning energy consumption by emulating the natural heat dissipation mechanisms of organisms, thereby providing significant energy-saving and environmental protection advantages. However, the current radiative cooling coating materials have several limitations and challenges, as outlined below:

(1) The primary issue is that the radiative cooling power is insufficient, resulting in a poor cooling rate and suboptimal cooling impact. In the future, we should draw inspiration from the temperature-regulating mechanisms of many animals and plants in nature, integrating the principles of biomimetic design to create superior building coating materials.

(2) The seasonal adaptation of radiative cooling materials must be improved to broaden their use contexts and range.

(3) The advent of diverse advanced material manufacturing techniques has facilitated the integration of sophisticated micro and nano processing technologies in bio-inspired radiative cooling materials, enabling efficient and cost-effective large-scale manufacturing of these materials.

Radiative cooling films offer a novel approach to building cooling. The advancement of bio-inspired radiative cooling coatings will enhance comfort and hold significant strategic importance in mitigating global warming and addressing the energy issue. Future research must enhance the material and fabrication process, augment the performance and stability of the bio-inspired film, and decrease the production cost. Simultaneously, it is essential to enhance the integration of bio-inspired films and intelligent building technology to further its application in intelligent construction. It is anticipated that the ongoing advancement of technology will render bio-inspired films increasingly significant in the construction sector, aiding in the achievement of sustainable building objectives.

In summary, bio-inspired radiative cooling films offer a highly promising solution for building cooling and energy conservation by mimicking the thermal regulation mechanisms of natural organisms. However, they still face challenges in cooling power, seasonal adaptability, and large-scale manufacturing. Looking ahead, with in-depth exploration of the thermal regulation principles of natural biological structures, combined with advanced material preparation technologies and intelligent building integration concepts, bio-inspired radiative cooling films are expected to make breakthroughs in improving cooling performance, enhancing environmental adaptability, and reducing production costs. The maturity of this technology will not only significantly reduce building energy consumption and carbon emissions but also provide crucial support for addressing global warming and the energy crisis, driving the field of sustainable buildings toward a more efficient and environmentally friendly new stage.

Author contributions

Conceptualization: Huaping Mei, Qianzhi Gou; methodology and investigation: Guanfeng Xue, Qianzhi Gou, Zhaoyu Chen, Kaixin Wang, Sida Zhang, Kangning Xiong, Juanxiu Xiao; writing–reviewing & editing: Huaping Mei, Guanfeng Xue, Qianzhi Gou; supervision & funding acquisition: Qianzhi Gou, Bingye Song, Meng Li; project administration: Meng Li.

Conflicts of interest

The authors declare no interest in conflicts.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported by research grants from the Natural Science Foundation of China (52173235 and 52106110), Key Project of Chongqing Technology Innovation and Application Development Special Project (CSTB2025TIAD-KPX0020), the Hainan Province Science and Technology Special Fund (ZDYF2024SHFZ038), and the Research Start-up Fund of Xi'an University of Architecture and Technology (No. 196032407). The segment of Earth/atmosphere picture that appears in the graphical abstract is reprinted with permission from ref. 29; Copyright 2020, the American Association for the Advancement of Science.

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