1. Introduction

With rapid industrialization, urbanization, and technological advancement, global energy consumption has surged dramatically, exceeding 600 exajoules annually, with roughly half ultimately converted into thermal energy^[1]. This immense thermal burden intensifies sustainability pressures, including climate change, resource scarcity, and environmental degradation, and exposes a critical yet still unsolved challenge: how to precisely manage ubiquitous heat flows in an integrated, energy-efficient manner. In practice, thermal transport arises from multiple mechanisms, such as conduction, convection, and radiation, that often coexist and interact within the same system. Effective thermal management therefore requires not only improving the performance of individual heat-transfer modes, but also rationally orchestrating their synergistic operation to optimize heat utilization, enhance heat dissipation efficiency, and recycle waste heat, thereby improving energy efficiency, reducing greenhouse-gas emissions, and strengthening environmental resilience. However, conventional thermal management systems are fundamentally constrained by the static, bulk thermal properties of natural materials (e.g., the fixed conductivity of copper or the isotropic insulation of foams), offering little freedom to regulate interactions among different heat-transfer modes or adapt to changing environmental conditions, thereby limiting their effectiveness for complex and dynamic thermal demands across sectors.

To overcome these limitations, the past decades have witnessed a paradigm shift in thermal science with the emergence of thermal metamaterials, which are architected composite materials that manipulate heat transport through artificial structural design rather than chemistry alone^[2-24]. By precisely engineering microstructures, thermal metamaterials enable unprecedented control of thermal fields and heat-flow pathways beyond the capabilities of natural materials, allowing application-specific thermal functionalities that transcend classical thermodynamic constraints. These architected materials leverage advanced methodologies such as transformation thermotics^[25], scattering cancellation^[26], non-Hermitian thermal physics^[27,28], and thermal radiation engineering^[29]. By overcoming conventional limitations, thermal metamaterials enable tunable and synergistic regulation over thermal conduction, convection, and radiation, achieving a transition from empirical, passive adaptation toward model-driven, active designs by utilizing analytical de-homogenization^[30] and topology optimization algorithms^[31]. Furthermore, the integration of machine learning-assisted inverse design^[32] has propelled the field toward data-driven digital twins, enabling predictive and optimized metamaterial architectures.

This perspective systematically explores the emerging paradigm of multimodal thermal control, achieved through the architectural design of synergistic heat transfer, which refers to the coupled interaction among two or more heat-transfer modes, including conduction, convection, and radiation to generate integrated and enhanced thermal responses. Thermal metamaterials are architected materials whose structural length scales are deliberately designed to be smaller than the relevant characteristic thermal lengths (e.g., thermal diffusion length, thermal radiation wavelength, or thermal convection length), enabling effective-medium descriptions and controllable thermal responses^[4]. Within this framework, multimodal thermal metamaterials are defined as a distinct class of architected material systems in which two or more heat-transfer modes are intrinsically and intentionally coupled within a unified microstructural architecture, enabling programmable and predictive regulation of multimodal heat transport. Rather than representing a general collection of composite thermal materials, these systems derive their functionalities predominantly from geometry-, topology-, and architecture-enabled design, giving rise to collective behaviors unattainable from constituent materials alone. We highlight fundamental design principles and recent developments showing how intentional material structuring across multiple length scales enables coupled and programmable regulation beyond intrinsic properties of substances, thereby unlocking function-by-design control over heat transport. To demonstrate the breadth and versatility of this approach, we focus on material-centered regulation across six representative heat-transfer scenarios: conduction, convection, radiation, conduction-convection, conduction-radiation, and conduction-convection-radiation, and examine their transformative applications in cooling, heating, thermal insulation, energy conversion, energy storage, and waste heat recovery (Figure 1). Through analysis of scientific advances and materials innovations underpinning their implementations, we underscore the disruptive potential of multimodal thermal metamaterials in addressing sustainability imperatives, reveal their central role in enabling next-generation energy solutions, and outline future directions to accelerate progress toward global thermal sustainability.

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Figure 1. A conceptual framework for multimodal thermal metamaterials enabling sustainable energy and environmental applications through synergistic heat-transfer architectures. The outermost sectors illustrate application scenarios that impose distinct demands on heat management. The outer ring denotes three heat-transfer modes and their couplings, while the middle ring summarizes six representative thermal functionalities. The inner ring links these functionalities to renewable and nonrenewable energy resources, and the center highlights multimodal thermal metamaterials as the unifying platform underpinning global thermal sustainability.

2. Architectural Design Principles for Synergistic Heat Control

Diffusion metamaterials are architected materials designed to control diffusion-governed transport processes through spatially engineered effective properties. As a critical branch of diffusion metamaterials, thermal metamaterials enable advanced material platforms for precisely tailoring and directing thermal fields. These foundational advancements are illustrated in Figure 2^[33,34], which summarizes the core theoretical principles underpinning the materials design and functional implementation of multimodal thermal metamaterials.

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Figure 2. Fundamental toolkits for the architectural design of thermal metamaterials toward the manipulation of conductive, convective, and radiative heat flows. (a) Transformation theory for heat flow control by tailoring spatial thermal conductivity; (b) Effective medium theory for multilayer composite structures to achieve unconventional parameters; (c) Scattering cancellation theory for core-shell structures; (d) Diffusive pseudo-conformal mapping, which refers to a coordinate transformation approach inspired by conformal mapping; (e) Design of thermal metadevices via topology optimization and machine learning; (f) Design of the one-dimensional Su–Schrieffer–Heeger model and a topological thermal lattice. Republished with permission from^[33]; (g) Schematic of a two-ring model for an anti-parity-time system. Republished with permission from^[34]; (h) Dynamic control of radiative cooling within the atmospheric window.

Transformation theory, originally developed from transformation optics^[35,36], has since been extended to various physical domains including acoustics^[37,38], thermotics^[2,39], mechanics^[40,41], particle dynamics^[42], and plasma physics^[43]. From a materials science perspective, this design principle serves as a powerful tool for manipulating physical processes, as long as their governing equations remain form-invariant under coordinate transformations. The core concept of transformation theory is to emulate physical phenomena from curvilinear to Cartesian coordinates by appropriately tailoring material parameters. For instance, in a conceptual coordinate space (x, y), energy or mass flow can be twisted by geometrically distorting space. Instead of physically deforming space, transformation theory achieves the same effect by modulating material properties to reproduce equivalent flow behavior. This correspondence between physical space (x, y) and virtual space (u, v) is mathematically established through the Jacobian transformation matrix J = ∂(u, v)/∂(x, y). Taking heat conduction as an example, under a two-dimensional transformation from (x, y) to (u, v), the thermal conductivity tensor transforms according to κ' = JκJ^T/detJ. This approach enables advanced functionalities such as thermal cloaking^[2,39], concentration^[44], and rotation^[45] to be implemented in physical space by tailoring material parameters according to the coordinate transformation in Figure 2a. Despite its versatility, transformation theory often requires complex, anisotropic, or even singular material parameters, which may be challenging to implement in practice.

To address this challenge, numerous extended theories have been developed. Effective medium theory, in combination with multilayer composites, provides a practical and scalable strategy for approximating the required anisotropic material properties. For example, a stratified structure composed of n isotropic layers with thermal conductivities κ_n and thicknesses l_n yields effective thermal conductivities κ_v (perpendicular) and κ_p (parallel), forming the diagonal components of the effective thermal conductivity tensor κ = diag(κ_v, κ_p) (Figure 2b). By rotating this layered structure at an arbitrary angle θ and applying transformation theory, a multilayer system with any desired anisotropic thermal conductivity can be achieved^[46], enabling implementation of transformation-based meta-devices. When natural materials cannot meet the required conductivity values, composite materials can be designed using the Maxwell-Garnett or Bruggeman formulas^[47]. Through careful selection of constituent phases and microstructural tuning, these composites can be tailored to exhibit the desired anisotropic thermal response.

Beyond transformation theory, scattering cancellation theory^[48,49] provides a functionality-driven design route. It determines material parameters directly by solving governing equations under specific boundary conditions. For instance, a thermal cloak can be constructed by placing an insulating ring around a protected region and adding a compensating outer shell to eliminate thermal distortion (Figure 2c). The thermal conductivity of the compensating shell is obtained by solving the bilayer Laplace equation under prescribed temperature field boundary conditions. This design principle is realized through the fabrication of single-layer or multilayer core-shell structures, enabling precise manipulation of thermal fields.

In complex or asymmetric systems lacking analytical solutions, pseudo-conformal mapping^[50], together with recent Schwarz-Christoffel-based multiphysics extensions^[51], offers a geometry-driven material design approach for precise thermal and energy-flow control. A two-step mapping can unify isotropic media, ensure interface continuity, and achieve the desired field profiles (Figure 2d). Notably, certain bilayer cloaks derived from scattering cancellation theory can be interpreted within specific geometric mapping frameworks, revealing their underlying design equivalence. Building upon these connections, advanced optimization techniques have been employed to further expand the design space and enhance material versatility. Optimization methods, such as particle swarm optimization^[52,53], topology optimization^[54,55], and machine learning^[32,56], further improve design flexibility by tailoring effective properties from available bulk materials (Figure 2e). These approaches also facilitate the integration of transformation-based and scattering-cancellation-based design principles, thereby advancing the development of thermally functional materials with engineered structures and scalable manufacturability.

The mathematical analogy between the diffusion equation and the Schrödinger equation has drawn increasing attention to topological effects in thermal systems^[57,58], enabling the application of topological models to control and analyze heat transport in engineered material architectures. For example, the one-dimensional Su–Schrieffer–Heeger (SSH) model has been adapted to reveal topological phenomena in the temperature field^[59]. This can be expressed as i∂_tT = H_1DSSH T(t), where ∂_t denotes the time derivative and H_1DSSH represents the thermal analogue of the SSH Hamiltonian. The temperature field T at all discretized sites is given by T = (T_1A, T_1B, T_2A,…, T_NB)^T, where A and B denote the two sublattices, N is the number of unit cells, and the distance between two sites is taken as the unit length (Figure 2f). This model was originally developed to capture the topological features of edge states^[33], and has since been extended to describe higher-order topological insulators in thermal diffusion systems^[60-62].

Incorporating convection extends thermal metamaterials beyond pure conduction, transitioning the system from anti-Hermitian regimes (characterized by purely imaginary spectra) to general non-Hermitian regimes with complex spectra^[63]. Inspired by wave physics, where gain and loss give rise to parity-time symmetry^[64], an analogous anti-parity-time (APT) symmetry emerges in thermal systems due to the interplay between Hermitian (convection) and anti-Hermitian (conduction) components, enabling regimes of asymmetric and nonreciprocal heat transport. A two-ring model^[34] has demonstrated APT symmetry in diffusive heat systems (Figure 2g), where the governing equations exhibit Schrödinger-like behavior. Building on this foundation, higher-order and radiative APT systems have been proposed to further expand the application of APT physics^[65]. Moreover, this approach enables tunable control over thermal conductivity parameters, thereby overcoming the intrinsic limitations of heat conduction in natural materials^[66,67].

Finally, thermal radiation is fundamental to the functionality of metamaterials, especially in the context of radiative cooling. This process enables terrestrial objects to release heat through the atmospheric window into outer space^[68]. Dynamic passive radiative cooling can be achieved by tailoring the spectral emissivity and solar reflectivity of materials^[69,70] (Figure 2h). With advances in atmospheric spectral modulation and integration with optimization frameworks, advanced multispectral camouflage and adaptive emissivity strategies have emerged^[71]. These systems rely on advanced materials, such as nanophotonic structures and infrared-selective coatings, engineered to exhibit wavelength-dependent emissivity and angular selectivity. These developments establish thermal metamaterials as key enablers of sustainable radiative thermal management.

3. Foundational Building Blocks: Single-Mode Regulation

The advancement of thermal conductive, convective, and radiative metamaterials has enabled mode-specific strategies for sustainable thermal management, each designed to optimize a dominant heat transfer pathway. These single-mode approaches encompass enhanced thermoelectric conversion, controlled convective flows for ecological adaptation, and passive radiative cooling for environmental resilience, among other applications, thereby expanding the scope of thermal control across energy, urban, ecological, and other systems.

3.1 Architectural design of conductive pathways for energy harvesting

Metamaterials have substantially advanced the regulation of thermal conduction, significantly enhancing the efficiency of thermoelectric conversion technologies and presenting novel strategies for sustainable energy management. In the context of carbon neutrality, thermoelectric conversion, directly converting thermal energy into electricity^[72-74], is increasingly prominent due to advantages such as silent operation, absence of moving parts, and ease of integration. However, progress in the thermoelectric figure of merit (ZT)^[75] has traditionally been limited by the intrinsic coupling among the Seebeck coefficient, electrical conductivity, and thermal conductivity, making significant breakthroughs challenging to achieve using single materials alone. Recent innovations in thermal conductive metamaterials^[76,77] offer promising solutions by manipulating temperature gradients and heat flux pathways through geometric restructuring, thereby achieving system-level thermoelectric efficiency improvements without altering intrinsic material properties. For instance, a bioinspired extended-plane metastructure, derived from the dual-channel heat dissipation mechanism in stegosaurus dorsal plates, utilizes engineered thermal conduction paths characteristic of thermal conductive metamaterials^[78]. This extended-plane metastructure significantly enhances local temperature gradients without disturbing the ambient thermal field, thereby markedly increasing the thermoelectric driving force. Experimental studies have demonstrated efficiency improvements of up to 59% relative to conventional thermoelectric modules lacking concentration functionality, representing an energy-free, structure-enabled enhancement that yields higher power output under identical thermal conditions and significantly broadens application scenarios of thermoelectric devices in microelectronics cooling and wearable medical devices (Figure 3a). Additionally, multi-stage thermoelectric coolers optimized through geometric matching and heterogeneous material integration effectively mitigate efficiency losses at high-temperature gradients, achieving temperature differences exceeding 160 K^[79]. These developments highlight the thermal conductive metamaterials’ transformative potential in energy conversion and cooling applications, redefining efficiency enhancement through structurally innovative designs.

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Figure 3. Architectural strategies for engineering conductive pathways toward thermoelectric conversion, waste heat management, and super insulation. (a) Enhancement of thermoelectric conversion efficiency via a bioinspired, energy-free extended-plane temperature gradient regulator. Republished with permission from^[78]; (b) Asymmetric heat-transfer system with graded linear conductive material; (c) Bio-derived anisotropic nanowood with hierarchically aligned cellulose nanofibrils as a high-performance super insulator for sustainable building envelopes. Republished with permission from^[86].

Beyond improving thermoelectric conversion efficiency, thermal conductive metamaterials’ remarkable ability to regulate thermal conductivity has begun revolutionizing practices in industrial waste heat management. Globally, large portions of primary energy dissipate as waste heat in industrial processes, underscoring the urgency of transforming these passive losses into manageable resources to facilitate decarbonization and enhance energy efficiency^[80]. Unlike conventional bulky and passive heat exchangers, newly developed structurally functional metamaterials exhibit superior integration capability, responsiveness, and local adaptability, making them highly suitable for complex and distributed waste heat scenarios. For example, phase-change composites embedded with vertically-aligned reticulated graphite nanoplatelets exhibit significantly enhanced thermal conductivity, enabling efficient solar-driven thermal energy storage at temperatures above 186 °C^[81]. Moreover, form-stable phase-change composites utilizing porous graphene aerogels exhibit excellent leakage resistance and thermal cycling stability, enabling rapid and localized heat storage and release under dynamic heat flux conditions, with promising potential for integration into emerging hybrid thermal-electrical energy storage systems^[82]. Recent advances also demonstrate passive asymmetric heat conduction enabled by graded linear thermal conductive metamaterials (Figure 3b), achieving macroscopic thermal rectification without relying on material nonlinearity or external energy inputs, offering new possibilities for directional waste heat harvesting and reutilization^[83]. Further improvements in the controllability of thermal rectification have been achieved using fractal-structured thermal diodes incorporating embedded liquid-metal channels with tunable thermal thresholds, yielding rectification ratios of up to 0.47^[84]. These structurally adaptive and rapidly responsive devices show significant promise in cold-chain logistics, intelligent thermal regulation systems, and refined industrial waste heat utilization. Additionally, flexible phase-change composites based on chemically crosslinked thermoplastic elastomeric networks effectively confine liquid paraffin, enabling modular, deformable, and wearable thermal management systems with high thermal stability and energy efficiency^[85]. These innovative composites not only enhance thermal comfort in wearable systems but also exhibit excellent adaptability to complex, curved surfaces, suggesting potential applicability in industrial thermal interfaces. Collectively, these developments underscore the transformative role of thermal conductive metamaterials in waste heat recovery and thermal energy storage, marking a paradigm shift from traditional material-based methods towards structurally-functional integrated platforms applicable to diverse industrial scenarios, ranging from high-temperature redundancy to medium and low-temperature complexities.

Simultaneously, thermal conductive metamaterials are reshaping thermal insulation technologies for the building sector. As a major global energy consumer, the building sector urgently requires next-generation thermal insulation materials characterized by high performance, lightweight design, and environmental sustainability. Traditional isotropic insulation materials, such as fiberglass and polystyrene foam, approach the thermal conductivity limit of air, yet often fail to satisfy mechanical strength, thickness adaptability, and sustainability requirements for future green buildings. Bioinspired thermal conductive metamaterials offer new structural paradigms for enhancing building energy efficiency. For example, anisotropic nanowood, fabricated from chemically delignified natural wood through freeze-drying, features highly aligned cellulose nanofibrils, enabling efficient directional thermal management^[86]. With thermal conductivity as low as 0.03 W m^-1 K^-1 perpendicular and 0.06 W m^-1 K^-1 parallel to the fiber alignment, this material effectively channels heat along the structural plane while minimizing transverse transfer, thereby significantly reducing thermal loads in building envelope applications (Figure 3c). These anisotropic thermal insulators integrate microscale heat conduction control with macroscale structural compatibility, marking a significant advancement in insulation materials. Nevertheless, compared with incumbent insulators such as fiberglass and expanded polystyrene that benefit from mature continuous manufacturing and established supply chains, nanowood still faces bottlenecks in scalable production, cost competitiveness, and integration into existing insulation-panel workflows. In particular, the current top-down route involving chemical delignification and freeze-drying may impose constraints on feedstock standardization, processing energy intensity, throughput, and solvent management, underscoring that addressing these challenges is essential to enable architected insulation systems that combine enhanced thermal performance with mechanical robustness and environmental compatibility at the building scale. Collectively, this shift underscores the broader potential of thermal conductive metamaterials not only to redefine heat transport pathways across industrial and energy systems, but also to advance material-function integration critical for future sustainable architecture.

3.2 Architectural design of convective flows for ecological systems

Thermal convection is the transfer of energy through the bulk movement of fluids such as air or water, typically categorized into natural or forced convection. Convection-based systems^[87,88] enable energy-efficient control in architectural, ecological, and computational domains. This section highlights three representative applications in sustainable thermal management where convective effects play a dominant role, including thermal ventilation architectures^[89,90], urban heat island mitigation^[91,92], and thermal regulation in data centers^[93]. In each of these areas, the structural configuration and material-dependent functional response of thermal metamaterials are essential for guiding airflow, modulating convective heat flux, and responding to dynamic environmental conditions. By combining precise material selection with geometric optimization, convection-enabled metamaterial systems present new opportunities for passive thermal regulation, enhanced energy efficiency, and environmentally adaptive infrastructure design.

Biologically inspired ventilation architectures exemplify the integration of natural convection principles with biomimetic design. One exemplary source of inspiration lies in termite nests (Figure 4a), whose intricate internal structures, featuring vertical shafts and porous chambers, enable passive airflow and thermal stability under extreme climatic fluctuations^[89]. These self-organizing structures adapt to environmental feedback, offering a decentralized, zero-energy model for responsive ventilation^[90]. This biological principle has been successfully translated into architectural practice, as exemplified by the Eastgate Centre in Harare, which adopts termite-inspired ventilation strategies to significantly reduce mechanical cooling demands, thereby lowering both energy consumption and operational costs. Such porous structures also inspire the development of thermal convective metamaterials. For instance, topology-driven thermal structures can modulate transient thermal responses based on functional requirements^[94], while graded metadevices inspired by black-hole thermodynamics can guide advective heat flow without external energy input^[95]. Moreover, topology optimization techniques^[54,55] facilitate the creation of bionic porous structures capable of advanced thermal convection regulation. Collectively, these bioinspired strategies underscore the potential of integrating natural convection mechanisms with advanced materials engineering, paving the way for the development of high-performance thermal convective metamaterials tailored for efficient cooling applications.

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Figure 4. Architectural strategies for programmable convective flows toward building ventilation, urban cooling, and data-center thermal management. (a) Internal structure of a termite nest, showing ventilation channels that inspire cooling in modern architecture. Republished with permission from^[89]; (b) Reconfigurable and nonreciprocal thermal convective metamaterial designed for building envelope applications. Republished with permission from^[8]; (c) Hall-like active thermal lattice composed of rotatable unit cells, demonstrating anisotropic thermal chirality for programmable channels. Republished with permission from^[99].

At the urban scale, natural convection plays a vital role in mitigating the urban heat island effect. A comprehensive classification of mitigation strategies highlights the importance of urban airflow in promoting convective heat dissipation, identifying features such as green spaces and eco-certified buildings as effective means of redirecting heat-laden air away from dense urban cores^[91]. Crucially, the strategic morphological design of urban green infrastructure, including its size, shape, spatial configuration, location, and connectivity, can significantly reduce ambient air temperatures and lower the energy demand of adjacent buildings^[92]. Convective metamaterials introduce new opportunities to enhance such interventions. Leveraging natural convection, a reconfigurable and nonreciprocal thermal convective metamaterial (Figure 4b) has been developed to steer heat away from critical zones by exploiting temperature-induced asymmetry, thereby enabling site-specific thermal redistribution across urban envelopes^[8]. This device enables seasonal thermal management by utilizing tunable material geometry and orientation, achieving passive cooling in summer and effective thermal insulation in winter. Consequently, integrating thermal convective metamaterials into urban buildings and infrastructure holds great promise for enhancing thermal resilience, improving microclimatic conditions, and supporting sustainable urban development.

In digital infrastructures such as data centers, where thermal loads are both concentrated and dynamic, convection-based thermal management has become essential. Server operation generates substantial heat, increasing the cooling demand and overall energy consumption of data centers^[93]. Forced convection is widely employed to dissipate heat from densely packed servers. Recently, topological transport phenomena have opened new avenues for data centers. The existence of topological edge states (localized high-temperature regions) in the one-dimensional Su–Schrieffer–Heeger (SSH) model has been experimentally demonstrated^[33]. By constructing topological structures around critical components, heat can be confined to non-sensitive zones, thereby mitigating thermal hotspots in core circuits. Furthermore, higher-order topological insulators^[96-98], which support multidimensional topological boundary states, have been proposed for efficient heat dissipation in integrated circuits and multi-core processors. These structures facilitate spatially confined and directional thermal transport, providing distinct functional advantages compared to conventional isotropic materials. Going beyond traditional paradigms, directional heat transport has been realized via a Hall-like mechanism operating at room temperature and without magnetic bias (Figure 4c), laying the groundwork for programmable thermal routing in high-density electronics^[99]. Additionally, asymmetric heat propagation enabled by anti-parity-time (APT) symmetry offers preferential thermal routing to protect sensitive regions^[34]. Complementing these theoretical advances, dynamic control over thermal conductivity has been achieved by varying the rotational speed of engineered structures^[66,67]. Collectively, these innovations leverage structural antisymmetry and reconfigurable design to transform convection from a passive phenomenon into an actively engineered process. This approach enables location-specific thermal responses, targeted insulation, and effective waste heat recovery, offering a paradigm shift in thermal management for next-generation data centers.

In summary, convection serves as a robust and tunable mechanism for sustainable heat transfer, supporting a wide range of applications in architectural systems, urban environments, and computational infrastructures. By employing rationally designed porous structures and engineered topological pathways, the integration of thermal metamaterials into convective systems enables advanced thermal functionalities such as passive cooling, directional insulation, and localized waste heat recovery.

3.3 Architectural design of radiative surfaces for environmental conditioning

Thermal radiative metamaterials have emerged as transformative technologies for sustainable thermal management, particularly through the development of advanced radiative cooling strategies. By exploiting Earth’s atmospheric transparency window (8-13 µm), these materials enable passive heat dissipation into outer space without energy input. Early innovations primarily centered on photonic structures that simultaneously reflect solar irradiation and selectively emit mid-infrared radiation (Figure 5a), achieving temperature reductions of up to 5 °C under peak solar loading^[100]. These pioneering designs not only demonstrated the feasibility of passive daytime radiative cooling but also revealed the far-reaching potential of thermal radiative metamaterials in enhancing urban energy efficiency and alleviating heat island effects, laying a critical foundation for sustainable urban development.

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Figure 5. Architectural strategies for tailoring radiative cooling properties toward passive wavelength-selective emission, scalable fabrication, and thermally adaptive functionality. (a) A photonic multilayer structure enables passive cooling below ambient temperature under sunlight through solar reflection and selective thermal emission. Republished with permission from^[100]; (b) A polymer-based hybrid material with randomly dispersed microspheres achieves daytime radiative cooling and supports scalable roll-to-roll fabrication. Republished with permission from^[101]; (c) A flexible radiative coating utilizing a temperature-driven phase transition adaptively modulates thermal emission for seasonal heat management. Republished with permission from^[103].

Building on these foundational breakthroughs, subsequent research has focused on improving functionality, efficiency, and manufacturability through hybrid metamaterial systems. One notable advancement involves glass-polymer composites embedded with dielectric microspheres, which exhibit near-unity infrared emissivity (> 0.93) and high solar reflectance (~96%) (Figure 5b). Enabled by scalable roll-to-roll manufacturing of large-area thin films, these materials achieved cooling powers exceeding 90 W/m² under direct sunlight, substantially surpassing conventional passive coatings and thereby representing a significant step toward practical deployment^[101]. Further performance gains were realized through multilayer photonic designs, such as SiO₂/Si₃N₄ stacks, that leverage both primary and secondary atmospheric windows, enabling temperature reductions surpassing 12 °C and cooling powers exceeding 140 W/m^2[102]. These successive innovations reflect a vital transition from laboratory-scale demonstrations to scalable, high-performance solutions, thus accelerating the integration of thermal radiative metamaterials into real-world cooling applications. However, translating laboratory-scale films to building-scale envelope deployment requires stringent large-area uniformity and quality control, together with validated outdoor durability, long-term reliability, and cost-effective encapsulation, to ensure robust and reliable urban implementation.

Despite advances in radiative cooling, conventional systems remain inherently static, limiting their effectiveness under fluctuating environmental conditions. Recent developments have thus introduced temperature-adaptive materials capable of passively modulating infrared emissivity in response to ambient temperature. For example, photonic-enhanced coatings based on metal-insulator transitions exhibit sharp emissivity switching across a preset threshold (Figure 5c), enabling autonomous all-season regulation without external energy input^[103]. Building on this concept, printable and scalable designs integrating in-plane optical antenna resonance and albedo optimization have been realized, offering low-cost fabrication, customizable solar absorption, and robust energy performance across diverse climates^[104]. These advances represent a pivotal leap from static coatings to dynamic materials that intelligently respond to environmental changes.

Expanding from materials to systems-level applications, adaptive thermal radiative metamaterials have enabled dual-mode functionalities tailored for year-round thermal comfort. In particular, bilayer films that integrate reflective porous polymers for radiative cooling and graphene-enhanced layers for solar heating demonstrate active switching under seasonal variations^[105,106]. These structures physically alternate between cooling and heating modes, offering significant temperature modulation and global carbon mitigation potential. Beyond structural switching, complementary approaches such as electrochromic, thermochromic, mechanical, and wetting-based actuations further enrich the design toolkit for tuning surface emissivity and solar absorptance in real time^[107,108]. These adaptive strategies have also extended to wearable technologies and bio-inspired materials. For instance, cellulose-based optical wood leverages solvent-driven control of solar transmittance, enabling effective radiative cooling in summer and passive solar heating in winter through sustainable means^[109]. In the domain of personal thermal management, dual-mode radiative textiles dynamically adjust emissivity to ensure thermal comfort without reliance on energy-intensive heating, ventilation, and air-conditioning systems, significantly advancing energy efficiency at the individual scale^[110,111]. Additionally, accurately evaluating radiative cooling performance requires a comprehensive understanding of various atmospheric and environmental factors. Radiative cooling is inherently influenced by a range of dynamic conditions, including geoclimatic variability, seasonal and diurnal fluctuations, local weather patterns, and the surrounding built environment. For example, air pollution and particulate matter produced by wildfire events, can significantly alter atmospheric transmittance and reduce cooling effectiveness^[112]. Such considerations are crucial for optimizing real-world deployment and performance reliability across diverse climatic regions. Collectively, these developments represent a paradigm shift toward radiative cooling systems that are not only adaptive and scalable but also integrative and responsive to both environmental complexity and user demands.

Beyond urban environments, radiative thermal management also offers significant potential for addressing desertification, an escalating ecological challenge driven by climate change and unsustainable land practices. Integrated strategies combining thermal radiation management with renewable energy infrastructure and ecological restoration can effectively mitigate desert expansion and enhance regional sustainability. Research has demonstrated that extensive deployment of photovoltaic and wind farms in deserts can induce significant local climatic changes, enhancing regional precipitation and promoting vegetation recovery^[113]. These climatic transformations, facilitated by reduced surface albedo and increased atmospheric moisture convergence, could substantially enhance precipitation, profoundly impacting arid region rehabilitation efforts. Additionally, studies analyzing photovoltaic and concentrated solar power potential in desert areas highlight the necessity of incorporating local water availability and ecological carrying capacities to ensure sustainable deployment of solar infrastructure^[114]. Global analyses of desertification trends reveal that negative land use practices, compounded by anthropogenic climate change, often outweigh benefits from vegetation greening driven by increased atmospheric CO₂^[115]. Thermal management plays a crucial role in counteracting moisture stress and radiative imbalances that accelerate desertification. Integrative methodologies, such as the Land Multi-degradation Index, underscore the importance of combining radiative cooling, water retention strategies, and soil restoration practices, particularly in regions experiencing severe degradation^[116]. The Intergovernmental Panel on Climate Change (IPCC) Special Report on Climate Change and Land underscores the significance of sustainable land management practices in mitigating desertification impacts and enhancing resilience for populations in vulnerable regions^[117]. Integrating emerging strategies such as radiative thermal management may further support these goals, particularly under climate-induced heat and moisture stress. Accordingly, radiative thermal management is increasingly recognized as a strategic element of integrated desert governance, offering the potential to mitigate extreme heat and support long-term ecological sustainability.

In conclusion, the evolution of thermal radiative metamaterials from static radiative structures to intelligent, multifunctional systems represents a fundamental shift in sustainable thermal management. Spanning applications from urban energy efficiency and personalized thermal comfort to large-scale ecological restoration, these materials illustrate the convergence of scientific innovation and environmental necessity. Their continued advancement not only drives new levels of material functionality but also enables integrated, system-scale solutions that align with global priorities such as carbon neutrality, climate resilience, and long-term ecological sustainability.

4. Synergistic Integration in Multimodal Thermal Architectures

The pursuit of sustainable thermal management has catalyzed significant progress in multimodal thermal metamaterials, enabling solutions to diverse challenges ranging from extraterrestrial exploration and microelectronic cooling to marine heat regulation. In these systems, the cross-mode integration enables synergistic heat-management strategies that bridge wide spatial scales and underpin sustainable energy applications.

4.1 Synergistic conductive-radiative architectures for extreme environments

In extraterrestrial environments, spacecraft face extreme thermal fluctuations, solar irradiation, and vacuum conditions. Multimodal thermal metamaterials provide essential functionalities through dynamic radiative cooling, tunable emissivity, and high-performance insulation. Recent advances in dynamic radiative cooling technologies have significantly enhanced spacecraft thermal regulation. Notably, smart surfaces based on electrically tunable near-field radiative transfer using metal-insulator-semiconductor architectures enable precise emissivity control, which is particularly advantageous for spacecraft with minimal thermal inertia^[118]. Additionally, mechanically tunable photonic structures leveraging stretchable composites offer robust passive modulation, simplifying fabrication and enhancing reliability for space habitats^[119]. Ultralight graphene-boron nitride aerogels further exemplify these innovations, delivering exceptional thermal insulation with minimal thermal conductivity and outstanding mechanical resilience under severe thermal shocks^[120]. This unique combination of properties enables compact deployment and stable thermal performance, positioning these materials as promising candidates for next-generation spacecraft insulation (Figure 6a). The integration of flexible phase-change materials into wearable and structural spacecraft components has also been explored^[68], providing stable and efficient thermal buffering capabilities for extended missions. Nevertheless, translating these material-level advantages into practical spacecraft systems requires a holistic co-engineering framework. For example, the deployment of aerogels must maintain mechanical integrity and interfacial compatibility with spacecraft structural materials (e.g., carbon-fiber composites) under launch-induced vibrations and repeated thermal cycling. Likewise, phase-change materials used for thermal buffering should be co-optimized with mission-level power budgets and mass constraints. Such multifunctional thermal management strategies collectively facilitate deep-space exploration by effectively regulating extreme thermal loads and mechanical stresses, thereby supporting mission longevity and ensuring spacecraft integrity.

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Figure 6. Architectural strategies for synergistic integration of multimodal heat transfer across multiple length scales toward extreme space environments, high-power electronics, and marine energy harvesting. (a) a-BNGA enables ultralight, flexible thermal shielding for lunar bases by suppressing both radiation and conduction, achieving superior insulation under extreme conditions. Republished with permission from^[120]; (b) i) Embedded microchannels positioned beneath high-power electronic hotspots enable targeted convective cooling at the microscale. Republished with permission from^[121]; ii) A topology-optimized microfluidic cooling channel inspired by leaf venation for enhanced convective cooling efficiency. Republished with permission from^[125]; (c) i) An integrated underwater photovoltaic system mounted on an unmanned underwater vehicle for efficient energy conversion. Republished with permission from^[132]; ii) A thermo-hydrodynamic metamaterial capable of functional switching between thermal cloaking and concentration via topological transitions for enhanced waste heat recovery. Republished with permission from^[9].

4.2 Synergistic conductive-convective architectures for microscale cooling

At the microscale, thermal management in electronic devices faces escalating challenges, as traditional cooling methods fail to meet the growing demands for heat dissipation. Microfluidic cooling systems, integrated directly into semiconductor substrates, offer a highly efficient solution due to their superior capacity for convective heat transfer. A particularly impactful strategy involves the co-design of electronics and microfluidics, where embedded cooling channels are positioned beneath high-power hotspots (Figure 6bi)^[121], enabling localized heat extraction and demonstrating up to a 50-fold improvement in cooling performance compared to conventional approaches. To further improve thermal regulation, topology optimization has become a powerful design strategy for developing high-performance microchannel networks within synergistic thermal metamaterials^[122]. Inspired by the fractal venation patterns of natural leaves (Figure 6bii), topology-optimized microfluidic networks maximize convective surface area and promote uniform fluid distribution^[123-125]. The resulting structures exhibit high thermal efficiency and compact geometries, making them well-suited for integration into space-constrained electronic systems.

Beyond structural innovations, thermo-hydrodynamic metamaterials based on porous media models offer active thermal control by tailoring local permeability and thermal conductivity. These materials enable advanced functionalities such as thermal cloaking, concentration, and camouflage^[126-128], effectively redirecting heat away from sensitive regions while facilitating dissipation. Their performance is governed by precisely engineered pore architectures that enable anisotropic and nonlinear thermal transport. Complementing these strategies, nanofluid-based cooling systems under magnetohydrodynamic conditions have demonstrated notable performance gains. In porous metal heat sinks for CPU cooling, saturated nanofluids subjected to strong magnetic fields and high Darcy numbers significantly enhance convective heat transfer efficiency^[129].

Compared with conventional cooling devices based on intrinsic natural materials and fixed channel geometries, metamaterial-based architectures offer greater structural design freedom and programmable heat transport. Integrating metamaterial-inspired microfluidic architectures into microscale platforms demands compatibility with established semiconductor fabrication workflows and constitutes a critical engineering step toward practical implementation. Topology-optimized microchannel networks must be realized through processes aligned with standard semiconductor manufacturing, such as deep reactive-ion etching and wafer bonding, while ensuring leak-tight sealing and reliable electrical isolation. Moreover, their thermal performance is intrinsically coupled to the properties of thermal interface materials that bridge the chip and the heat sink, as excessive interfacial thermal resistance can offset the advantages of advanced microfluidic designs. Finally, achieving manufacturable channel aspect ratios and maintaining yield stability under high-volume processing remain key scalability challenges for realistic chip-level deployment. Together, these developments underscore a synergistic design paradigm that unifies fluid dynamics, material engineering, and structural optimization. Rather than treating heat transport in isolation, next-generation chip cooling systems adopt a holistic approach that co-engineers materials, microfluidic geometries, and dynamic thermal responses to address the increasingly demanding requirements of high-density microelectronic applications.

4.3 Synergistic multimodal architectures for marine energy-water systems

In complex marine environments, thermal management systems must operate under coupled constraints of temperature gradients, fluid dynamics, and solar thermal radiation. Single-mode regulation strategies are insufficient under such conditions; instead, synergistic approaches are essential, integrating thermodynamic cycles, material adaptability, and multifunctional interfaces. Ocean thermal energy conversion (OTEC) and advanced desalination systems exemplify how synergistic strategies involving multimodal thermal metamaterials enable sustainable marine heat management and efficient waste heat recovery.

OTEC systems harness the natural temperature gradient between warm surface seawater and cold deep seawater to generate usable energy, essentially extracting stored solar heat^[130]. These systems operate within closed-loop heat exchangers and their surrounding environment. Recent advances propose hybrid configurations that integrate OTEC with solar and wave energy harvesting technologies^[131,132]. For instance, photovoltaics coupled with triboelectric nanogenerators collect ambient energy from ocean surface dynamics and shadowing effects^[131], while flexible solar cells enhance submerged power conversion by adapting to changing incident angles and mechanical disturbances (Figure 6ci)^[132]. These integrated energy platforms benefit from materials specifically engineered for mechanical flexibility, optical tunability, and long-term durability in marine environments. Beyond system-level integration, synergistic thermal metamaterials introduce novel pathways for dynamic regulation. Adaptive convective cloaks featuring tunable heat gain and loss properties^[133] have been fabricated to enable real-time thermal adjustment in response to dynamic oceanic conditions. To further optimize these structures, advanced machine learning techniques are increasingly employed for inverse design^[32,56], accelerating the discovery of high-performance metamaterial architectures tailored to marine thermal environments.

In parallel, thermal waste generated by OTEC offers a valuable heat source for seawater desalination. Thermally integrated systems combining OTEC with evaporative desalination form closed-loop cycles that co-produce freshwater and power^[134], maximizing the utility of low-grade thermal energy. In this context, pressure-driven gas-layered membranes have been developed to enable selective vapor transport via gas–liquid phase transitions, improving purification efficiency^[135]. At the intersection of thermal regulation and fluid control, thermo-hydrodynamic metamaterials responsive to variations in pressure and temperature offer an alternative approach for enhancing desalination systems^[9,136]. By modulating internal flow through pressure differences, these systems can undergo topological transitions within their virtual space, enabling functional switching between thermal cloaking and concentration modes (Figure 6cii)^[9]. These pressure- and temperature-responsive metamaterials offer a materials-centered strategy for managing oceanic thermal gradients. By employing tailored internal architectures, they enable dynamic regulation of flow direction and thermal distribution, thereby enhancing the utilization of recycled waste heat in seawater desalination and improving overall system-level energy efficiency. Collectively, marine-oriented thermal metamaterials enable dynamic and adaptive heat regulation across energy harvesting and water purification platforms. In OTEC systems, they facilitate convective heat exchange through structurally engineered flow channels and temperature-responsive material architectures. In desalination, these metamaterials offer reconfigurable pathways for coordinated heat and mass transfer, improving both operational efficiency and selectivity. By integrating energy conversion with waste heat recovery, such materials present significant potential for the sustainable utilization of oceanic thermal resources. However, the successful deployment of such metamaterials in real-world marine systems hinges on their seamless integration with existing engineering infrastructure and their ability to withstand harsh oceanic environments over long-term operation. For instance, adaptive convection cloaks and pressure-responsive metamaterials proposed for OTEC and desalination must be integrated with conventional heat-exchanger materials, such as titanium or aluminum alloys, while avoiding galvanic corrosion and excessive thermal contact resistance. Addressing these challenges is not merely an engineering necessity; it represents a critical bridge that transforms laboratory-scale innovations into operationally viable components of marine energy–water systems.

In summary, synergistic thermal metamaterials exemplify transformative regulation across spatial and functional scales, enabling seamless adaptation from extraterrestrial thermal protection to chip-level cooling and marine thermal management. By integrating diverse strategies such as structural design and responsiveness to dynamic environments, synergistic thermal metamaterials significantly extend the capabilities of conventional systems to address emerging demands in energy efficiency. Their broad applicability in cooling, thermal insulation, energy conversion, and waste heat recovery highlights their foundational role in advancing high-performance and adaptable thermal technologies. As multifunctional materials that bridge microscale engineering with macroscale energy systems, multimodal thermal metamaterials are positioned to drive innovation in sustainable heat management and support the global transition toward energy-resilient and environmentally responsible thermal technologies.

5. Outlook: Scaling the Architectural Design of Thermal Systems

Multimodal thermal metamaterials represent a transformative innovation for heat management, offering unprecedented opportunities for advancing sustainable thermal technologies. However, to unlock their full industrial and environmental potential, several critical scientific and engineering challenges must be overcome. Chief among these is the issue of material reliability and scalable fabrication bottlenecks. Multimodal thermal metamaterials typically rely on intricately engineered microstructures that can be vulnerable to environmental stressors such as temperature fluctuations, humidity, mechanical fatigue, and prolonged operational conditions. Achieving long-term structural stability and functional durability under real-world conditions remains a formidable challenge, necessitating systematic materials research and advanced composite engineering strategies^[30,137]. Additionally, translating laboratory-scale prototypes to industrial-scale manufacturing requires cost-effective, reproducible, and scalable standardized fabrication techniques. Addressing these manufacturing barriers will require significant progress in additive manufacturing techniques, modular integration schemes, and embedded quality control systems, thereby accelerating large-scale commercialization and practical deployment. In particular, scalable fabrication should evolve from simple “structure replication” toward “architecture programming”, where processing conditions and functional targets are co-optimized through closed-loop manufacturing to achieve reproducible multimodal performance.

Cross-domain coupling and integration within heterogeneous material systems further intensify the challenges of achieving effective thermal management. Precise thermal regulation typically demands synergistic coordination among conductive, convective, and radiative heat transfer modes^[138-140], especially when embedded within architecturally complex and multiscale material frameworks. Among these mechanisms, thermal radiation plays a pivotal role in large-scale heat modulation. The concept of Dyson Spheres, an imagined megastructure that surrounds a star to harvest its radiative output, symbolizes the ultimate thermal management challenge, demanding optimal spatial material configurations to capture, distribute, and regulate stellar-scale energy flows^[141]. Developing accurate cross-scale coupling models that bridge nanoscale microstructural designs with macroscopic thermodynamic behaviors is essential for precise performance prediction and functional optimization^[142-144]. To address this multiscale complexity, interdisciplinary collaboration across thermal science, materials engineering, computational modeling, and system-level integration is imperative. Furthermore, the convergence of thermal materials with enabling technologies, such as energy harvesting modules, embedded sensing layers, and intelligent thermal control systems, facilitates the development of adaptive thermal platforms capable of real-time tuning and holistic energy optimization^{[123,145-152]}. These advancements are particularly promising for next-generation metamaterials, phase-change composites, and radiative coatings, where structural precision and functional responsiveness are paramount. Looking ahead, a distinguishing direction beyond conventional thermal materials is the development of programmable, self-adaptive, and AI-assisted multimodal thermal architectures that integrate sensing, actuation, and learning within a unified material system. In this paradigm, metamaterials are no longer static media; instead, they function as closed-loop thermal platforms that can sense environmental inputs, trigger material-level responses, and iteratively optimize control policies via embedded computing and data-driven feedback. Such material–algorithm co-design can continuously recalibrate against uncertainty, aging, and evolving boundary conditions, thereby improving robustness and expanding the operational envelope beyond passive materials. Conceptually, this direction motivates digital twins and data-driven constitutive models that couple multimodal heat-transfer physics with measurable state variables, enabling real-time prediction, fault diagnosis, and adaptive reconfiguration in practical infrastructures.

Beyond technical and scientific innovation, it is also important to acknowledge that certain advanced thermal functionalities, such as thermal cloaking, thermal concentration, and thermal illusion, may raise dual-use and safety considerations. Addressing these concerns requires establishing clear ethical guidelines, robust oversight mechanisms, and broader governance frameworks to mitigate potential misuse and promote responsible research and application practices. Emphasizing peaceful and sustainability-oriented applications will help ensure that progress in multimodal thermal metamaterials primarily serves energy efficiency, climate resilience, and environmental sustainability, thereby aligning future advances with beneficial and responsible global energy solutions.

Tackling these critical challenges through targeted materials research and interdisciplinary cooperation will be pivotal in advancing the practical viability, scalability, and long-term sustainability of multimodal thermal metamaterials. Strategic integration of design innovation, fabrication techniques, and functional testing, combined with contributions from materials chemistry, thermal physics, nanomanufacturing, and systems engineering, can unlock new frontiers in high-performance thermal management. Ultimately, overcoming these multifaceted obstacles holds transformative potential, facilitating the widespread implementation of advanced thermal management strategies and accelerating the global transition toward sustainable and energy-efficient technologies and infrastructures. In this vision, the “architecture” of thermal systems extends beyond geometry to become programmable and adaptive, enabling thermal materials to operate as intelligent, resilient, and scalable infrastructure elements rather than passive components.

Acknowledgements

Jiping Huang gratefully acknowledges the helpful discussions with Professors Fabio Marchesoni and Jian-Hua Jiang. ChatGPT was used solely for language polishing. The authors take full responsibility for the final manuscript.

Authors contribution

Feng H: Writing-original draft, writing-review & editing.

Huang J: Conceptualization, writing-review & editing, funding acquisition, supervision.

Chen Y, An Z, Tan P, Qi Y, Li X: Writing-review & editing, funding acquisition, supervision.

Conflicts of interest

Jiping Huang is an Editorial Board Member of Thermo-X. The other authors declare no conflicts of interest.

Ethical approval

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Funding Information

This work was supported by the National Natural Science Foundation of China (Grants No. 12320101004, No. 12274086, No. 11991060, No. 12027805, No. 12425503, No. 12174071, No. 12474042, No. 12174072, No. 2021hwyq05, No. 12174068, and No. 11934002), the Innovation Program of Shanghai Municipal Education Commission (Grant No. 2023ZKZD06), the Innovation Program for Quantum Science and Technology (Grant No. 2024ZD0300103), the Shanghai Science and Technology Committee (Grant No. 23DZ2260100), the National Key Research and Development Program of China (Grants No. 2022YFA1404204, No. 2022YFA1403402, and No. 2021YFA1400900), the Space Application System of China Manned Space Program (Grant No. KJZ-YY-NLT0501), the Science and Technology Commission of Shanghai Municipality (Grants No. 24LZ1400100, No. 23JC1400600), and the China Postdoctoral Science Foundation (Grant No. 2024M760482).

Copyright

© The Author(s) 2026.