aState Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
bHP-NTU Digital Manufacturing Corporate Lab, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
cSingapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
dChina Academy of Aerospace Science and Innovation, Beijing 100094, China
eDepartment of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, China
fCity University of Hong Kong Matter Science Research Institute (Futian), Shenzhen 518045, China
Electromagnetic devices have been widely used in the fields of information communication, medical treatment, electrical engineering, and national defense, and their properties are strongly dependent on the constituent electromagnetic materials. Conversely, electromagnetic metamaterials (EMMs), which are artificially engineered with distinctive electromagnetic properties, can overcome the limitations of natural materials owing to their structural advantages. Three-dimensional (3D) printing is the most effective technique for fabricating EMM devices with different geometric parameters and associated properties. However, conventional 3D-printed EMM devices may lack manufacturing flexibility and environmental adaptability to different physical stimuli, such as electric and magnetic fields. Four-dimensional (4D) printing is an ideal technique for schemes to integrate structural design with intelligent materials environmentally adaptive to external fields, for example, the printed components can change shape under electric stimulation. Given the rapid advancements in the EMM field, this paper first reviews typical EMM devices, their design theories, and underlying principles. Subsequently, it presents various EMM structural topologies and manufacturing technologies, emphasizing the feasibility of combining 3D and 4D printing. In addition, we highlight the important applications of EMMs and their future trends and the challenges associated with functional EMMs and additive manufacturing.
Ruxuan Fang, Xinru Zhang, Bo Song, Zhi Zhang, Lei Zhang, Jun Song, Yonggang Yao, Ming Gao, Kun Zhou, Pengfei Wang, Jian Lu, Yusheng Shi.
3D and 4D Printing of Electromagnetic Metamaterials.
Engineering, 2025, 51(8): 182-205 DOI:10.1016/j.eng.2024.10.017
With the development of the electronic information age, electromagnetic devices have been widely used in several fields, including communication, medical treatment, electrical engineering, and national defense construction. Consequently, the demand for designing novel electromagnetic materials has significantly increased. Electromagnetic metamaterials (EMMs), as new artificial composite structures or composite materials with extraordinary physical properties, overcome the limitations of conventional natural materials and feature special electromagnetic transmission performance. They can regulate the basic physical characteristics such as the frequency, amplitude and phase of electromagnetic waves [1], [2], [3]. In 1968, the theoretical physicist Veselago proposed the concept of metamaterials [4]. Using Maxwell’s equation, Veselago theoretically analyzed the electromagnetic properties of materials with negative permittivity and permeability (double-negative materials), and introduced the novel concept of left-handed materials. Since then, metamaterial design technology has made significant strides. EMMs with a photonic/electromagnetic band gap structure [5], [6], heterotropic media based on transform optics [7], [8], electromagnetic metasurfaces [9], and digital and programmable metamaterials [10], [11] have been developed. Studies have shown that the electromagnetic characteristics of metamaterials can improve the performance of antennas and microwave devices, while also enabling the development of innovative wave-absorbing and wave-transmitting materials [12].
EMMs possess extraordinary electromagnetic properties due to the artificial design of their intricate topological structures. However, the fabrication of these structures remains a bottleneck that limits their practical applications. Presently, the primary methods for EMM fabrication include two-dimensional (2D) technologies, such as printed circuit board (PCB) etching, planar lithography, and mask printing, and three-dimensional (3D) technologies, such as the impregnation method, mechanical assembly, and additive manufacturing (AM) [13]. However, PCB technology, a form of plate engraving, typically requires numerous dielectric components, leading to heavy structures and spatial constraints for the frame [13], [14], [15]. Planar lithography and mask printing involve the creation of patterns on a substrate through mask corrosion or deposition. While planar lithography is associated with high manufacturing costs and is inefficient for producing complex 3D structures, mask printing suffers from accuracy degradation over repeated deposition cycles [16]. As 2D manufacturing processes, these methods are not ideal for fabricating complex conformal structures, thus failing to meet the high-precision and multifunctional integration requirements of EMM production. In the 3D metamaterial preparation process, the impregnation method must account for the bonding strength with the carrier, and surface shape fixation is often problematic. Mechanical assembly, on the other hand, is prone to mismatching, which compromises forming accuracy. Traditional subtractive or equivalent manufacturing methods are similarly inadequate for producing components with complex shapes.
AM [17], [18], commonly known as 3D printing [19], [20], enables the fabrication of complex, high-performance components. AM provides a convenient and efficient approach for the preparation and experimental verification of EMMs. AM methods use media of lower weight compared with other methods and ensure the stability of the forming structure through selective control in the EMM printing process. Furthermore, AM is time-efficient and facilitates multi-material manufacturing as well as gradient structure formation, making it highly suitable for the production of EMM structures. The transition from process-oriented to performance-oriented design significantly enhances the design space, allowing for greater innovation in EMM development.
Modern practical applications are often characterized by complex and extreme environmental conditions. For EMMs to maintain their functional properties, they must exhibit exceptional environmental adaptability [21]. However, fabricating dynamic and environmentally responsive EMM structures with tunable shapes, properties, and functionalities using traditional materials remains a challenge. Consequently, intelligent materials have garnered attention as a new class of environmentally sensitive functional materials [22], [23], [24]. Researchers have been exploring various methods of preparing these materials [25]. As human society advances into the era of intelligence, four-dimensional (4D) printing technology has emerged at an opportune moment, evolving in tandem with 3D printing and intelligent materials. Among these, intelligent materials serve as the core of 4D printing technology. Inspired by bionics, 4D-printed intelligent materials possess sensing, actuation, and control capabilities analogous to those found in biological systems [26]. Therefore, 4D printing represents an integrated intelligent system, with its design, fabrication, and functional applications belonging to the most cutting-edge areas of materials science. 4D printing enables the creation of “dynamic” EMMs that can respond to external stimuli, which represents a key direction for future development.
In this review, we introduce typical EMMs and summarize the types and characteristics of AM methods for EMMs. Then, we explore EMM designs with 2D, 3D, and bionic structures; moreover, we discuss the significance and feasibility of the 4D printing of intelligently responsive EMMs. Finally, we discuss the application and future development of EMMs, as shown in Fig. 1[8], [27], [28].
2. Principles of EMMs
The dielectric constant (ε) and permeability (μ) of materials play an important role in their suitability for EMM design. ε and μ usually consist of real and imaginary parts:
where ε' and μ' are the real parts, representing the energy storage capacity, and ε'' and μ'' are the imaginary parts, representing the energy consumption capacity. j is the basic unit of imaginary numbers. EMMs exhibit varying electromagnetic properties depending on the application. For instance, electromagnetic absorbing metamaterials with strong resonance structures, interference loss characteristics, or high-loss properties are designed to absorb waves and convert incident electromagnetic energy into other forms, such as heat [29]. The absorption performance of EMMs is typically quantified by the absorptivity A(ω). When an electromagnetic wave is incident on a material, it undergoes three primary processes: reflection, absorption, and transmission. The relationship between the A(ω), reflectivity R(ω), and transmissivity T(ω) of a dielectric material is expressed as
To enhance the absorption rate of electromagnetic waves, it is essential to minimize the reflection and transmission of waves by the material. Current electromagnetic absorbing metamaterials generally satisfy the conditions of impedance matching and wave attenuation. The impedance matching relation is as follows:
where , , and Z1 denote the dielectric constant, permeability, and wave impedance of the material, respectively; , , and Z0 denote permittivity, permeability, and wave impedance in free space, respectively; and denote equivalent permittivity and equivalent permeability, respectively. Through impedance matching optimisation, the reflectivity of electromagnetic waves can be minimized, enabling the design of high-performance EMMs [30]. Therefore, in the artificial design of EMMs, the values of ε and μ should be closely matched. Proper impedance matching ensures that all incident waves are absorbed by the material without reflection. The formula for calculating the electromagnetic wave loss factor of a material, tanδ, is as follows:
According to Eq. (5), the attenuation characteristic is related to the imaginary part of the complex equivalent εr and μr. The larger the value of the imaginary part, the better the attenuation performance. Through the design of the imaginary part, the electromagnetic energy conversion efficiency can be increased to the maximum value, to improve the wave absorption performance of EMMs.
Unlike EMMs primarily designed to absorb electromagnetic waves, electromagnetic shielding metamaterials are developed to reduce the transmittance of electromagnetic waves. Electromagnetic shielding operates through three main mechanisms: reflection loss, absorption loss, and internal multiple reflection loss [31]. Reflection loss occurs due to the impedance mismatch between the shielding material and the air, causing a portion of the incident electromagnetic wave to be reflected from the material’s surface, preventing it from reaching the internal field [32]. Since air is an insulator under normal conditions, highly conductive materials, such as gold, silver, copper, and other metals are commonly used for electromagnetic shielding based on reflection loss. In contrast, electromagnetic shielding metamaterials introduce the absorption mechanism, where interactions between the material’s internal substances and the electromagnetic field convert the energy of electromagnetic waves into heat and other forms, thereby preventing secondary electromagnetic pollution [33]. The effectiveness of shielding metamaterials, such as those containing ferrite or carbon nanotubes (CNTs), is mainly attributed to dipole polarization relaxation and the thermal effect of the induced current [34], [35]. Internal multiple reflection losses generally occur in porous materials. However, when a solid material exhibits high absorption loss or when the skin depth of electromagnetic waves is significantly smaller than the material’s thickness, internal multiple reflection loss becomes negligible [36].
The selection of electromagnetic absorbing/shielding metamaterials is pivotal to the properties of EMMs. For instance, to design a complete photonic band gap structure within the optical frequency range using photonic crystal materials, it is essential for the material to exhibit a sufficiently large dielectric constant contrast with air. Thus, choosing appropriate materials is a fundamental aspect of EMM design and can significantly enhance EMM performance. Currently, EMM applications cover nearly the entire electromagnetic spectrum, and the range of materials used is extensive. Common materials include carbon matrix materials [37], [38], metal materials and metal–organic frameworks (MOFs) [39], [40], ceramics materials [41], [42], [43], polymer materials [44], and composite materials (Fig. 2) [45].
3. AM of EMMs
3.1. AM technology
3.1.1. 3D printing
There are various AM methods, each with distinct process characteristics. When selecting an AM process for EMMs, it is important to consider factors such as the type of material to be processed (e.g., powder, filament, liquid, or a mixture) and the process characteristics (e.g., accuracy, build size, and production cost). The chosen process must align with the functional requirements of the application. Below, several typical AM technologies used for EMMs are discussed.
Fused deposition modeling (FDM) [46], also known as fused filament modeling, is a prevalent extrusion-based AM technology. It involves extruding thermoplastic or low-melting-point metal filament from a heated nozzle according to a predefined scanning pattern and deposition rate, to construct a 3D solid structure layer by layer from the bottom up. FDM is characterized by its simplicity and relatively low cost, and it is compatible with a broad range of materials. However, it is primarily suitable for producing small- and medium-sized parts and is known for its lower precision and surface quality. Additionally, the resulting parts often exhibit insufficient strength in the thickness direction. Consequently, FDM is typically employed to create EMM structural samples for experimental verification of new physical properties or to meet performance requirements while minimizing manufacturing costs [47], [48].
Stereolithography (SLA) [49] is a 3D printing process that utilizes a computer-controlled ultraviolet (UV) laser to cure thin layers of photosensitive resin point-by-point, a technique known as photopolymerization. This layer-by-layer curing process is repeated until the 3D solid part is fully formed. SLA is characterized by its high stability, dimensional accuracy of up to 0.1 mm, and excellent surface quality. However, the resulting parts are prone to deformation, and the raw material is limited to photosensitive liquid resin [50]. Currently, SLA technology exhibits high process maturity, with workpiece accuracy reaching the microwave wavelength range. As a result, SLA has promising prospects for the fabrication of EMM structural devices.
Selective laser melting (SLM) [51] and selective laser sintering (SLS) [52] are laser-powder bed processes that utilize high-intensity lasers to melt powder particles. Both methods rely on layer-by-layer scanning principles, but they differ in laser energy density. SLS involves melting a low-melting-point binder followed by powder solidification through cooling, while SLM uses a high-power laser to fully melt the powder, leading to complex physical, chemical, and metallurgical reactions [51]. The powder-based forming process is characterized by high strength and is compatible with a wide range of materials. Additionally, the high-energy laser in SLM can melt most high-melting-point materials, making it suitable for preparing metamaterials with exceptional performance. Consequently, these methods are well-suited for the fabrication of EMMs.
In addition to the major AM technologies previously mentioned, other technologies for fabricating EMMs include direct ink writing (DIW) [53], with a comparable principle to that of FDM, digital light processing (DLP) [54], the Inkjet printing (IJP) [55], projection micro-SLA (PμSL) [56], and two-photon polymerization (TPP) (Fig. 3) [57]. The latter two processes are commonly used for micro- and nano-scale 3D printing. PμSL employs UV-light modulators to create dynamic mask patterns across the entire exposed surface through resin curing, with UV-light typically processed and projected using a digital micromirror device (DMD). Currently, laboratory-scale PμSL can achieve accuracy in the range of hundreds of nanometers, while commercial-scale PμSL provides printing accuracy on the order of several microns. TPP utilizes a nonlinear two-photon absorption effect to solidify photosensitive materials (e.g., resins and gels) within a focal area using an ultrafast pulse laser, achieving printing accuracy of less than 100 nanometers.
The range of applications for EMMs spans the entire electromagnetic spectrum (Fig. 3). In theory, EMMs can be designed for various wavelength ranges, with the scale and morphology of their structural units being determined by the wavelength of the operating band. For microwave EMMs, unit cell sizes typically range from micrometers to centimeters. In contrast, terahertz (THz) metamaterials, which operate in the THz frequency band, generally have micron-scale cell sizes. EMMs designed for infrared and visible wavelengths can have structural units on the nanometer scale. Consequently, the AM of cross-scale EMM is crucial. Many applications require the integration of large EMMs, however, the forming process is constrained by the size limitations of AM equipment. Similarly, enhancing the small-scale machining accuracy of EMMs remains a challenge. Most AM technologies and equipment are currently insufficient for accurately manufacturing the microstructural units of EMMs operating in the visible and infrared bands, and mass production using these technologies is particularly challenging. The advent of micro- to nano-scale AM technologies has significantly improved machining accuracy and is expected to progressively address these challenges.
3.1.2. 4D printing
4D printing, proposed by Tibbits’ team in 2013, is an innovative extension of 3D printing [58]. Compared with 3D-printed components, 4D-printed components possess the additional capability to alter their shape, performance, or functionality. These changes are usually produced by external stimuli such as sound, light, electricity, heat, or mechanical force. This advanced technology enables the fabrication of various intelligent materials, including self-assembly, multifunctional adaptive, and self-healing materials [59]. Typically, the materials used in 4D printing are shape-memory materials, which can be programmed to assume temporary shapes and revert to their original forms when exposed to environmental stimuli [60].
Shape memory materials encompass shape memory alloys, polymers, and ceramics, offering significant design flexibility and enhanced functionality. These materials can be selectively activated by various stimuli, including temperature, magnetic fields, electric fields, and light, depending on specific application requirements. Additionally, composite designs can be integrated into these materials to expand their functional capabilities. The deformation program is inherently embedded within the shape memory material, allowing the printed structure to achieve its intended functionality without the need for external systems. The shape-memory effect, particularly its geometric transformation properties, has been extensively researched; and it can be harnessed for performance optimization and the function transformation in electromagnetic fields.
3.2. AM of EMM structures
The analysis and optimization of EMM design are crucial. In early research, the equivalent media theory was used to customize the media parameters of metamaterials [61], deducing the macroscopic structural properties from the polarization or magnetization characteristics of artificial units. This approach required the size of the equivalent media to be much smaller than the spatial wavelength. The equivalent electromagnetic parameters of EMMs can be obtained using retrieval algorithms [62], [63]. However, several studies [64], [65] have noted the inability of the retrieval method to provide a unique sign for the intrinsic impedance and refractive index. To address this, researchers have analyzed the EMM working mechanisms using the lumped parameter circuit concept and developed the equivalent circuit model [66], [67], [68]. With the increasing complexity of EMM structures, various theoretical approaches have emerged, including multiple reflection interference models [69], the Mie resonance theory [70], Fabry–Pérot resonance [71], [72], and the dispersive model [73]. These analytical methods inform the structural design of EMMs, and the functional range of these structures can be broadened through numerical algorithm optimization and simulation [74]. Consequently, multi-dimensional EMM structures from 2D to 3D can be realized (Fig. 4) [75].
3.2.1. 2D structure
Electromagnetic metasurfaces are 2D functional planar structures designed based on the generalized Snell’s law. These metasurfaces are characterized by low-profile broadband and tuneability [9]. Compared with 3D metamaterials, the metasurface unit features deep-subwavelength thickness, and its electromagnetic wave manipulation ability does not depend on phase accumulation in space, but on the phase gradient between the structure surface to flexibly regulate the amplitude, phase, and polarization of the electromagnetic wave front, with a strong field control ability [76]. Therefore, the use of a metasurface structure is beneficial for the design of ultralight, ultrathin, and conformal electromagnetic devices.
The basis of metasurface design mainly includes the traditional periodic electromagnetic theories (e.g., the Floquet theory and phased array theory) [77], [78], Huygens’ equivalence principle [79], [80] and the new generalized reflection/refraction theorem (generalized Snell’s law) [81]. In 2011, Yu et al. [81] discovered that phase discontinuities at the interfaces between different materials can impart abnormal refraction properties, which they experimentally validated, leading to the development of generalized Snell’s law. Utilizing this theorem, they designed a metasurface composed of V-shaped gold nano-antennas with different structural parameters, as shown in Figs. 5(a) and (b), achieving wavefront phase control from 0 to 2π through the phase shifts induced by each V-shaped phase controller. Building on this, scientists have developed gradient-index (GRIN) metasurfaces and metamaterial Huygens’ surfaces employing specially engineered thin-layer structures to manipulate spatial electromagnetic waves with flexibility [82], [83]. The GRIN metasurface uses phase gradients along the surface tangent direction to guide electromagnetic waves transmitted in free space along the object's surface, achieving nearly 100% surface wave conversion efficiency [82]. Meanwhile, Grbic’s team introduced metamaterial Huygens' surfaces featuring a specialized 2D thin electromagnetic layer, enabling the creation of reflectionless EMMs for applications such as beamforming, deflection, and focusing [83]. Compared to the V-shaped metasurface antennas, Huygens’ surfaces are characterized by high transmittance and transmission mode simplicity. However, the complex periodic structures in electromagnetic metasurfaces pose manufacturing challenges, as traditional methods cannot provide cost-effective fabrication. AM offers a solution by reducing the production time and cost of these structures, making it an effective technology for their fabrication.
For large electromagnetic metasurfaces, such as conformal invisibility cloaks, achieving both superior electromagnetic performance and effective large-area curved surface manufacturing is highly challenging [84]. In Figs. 5(c) and (d), Yin et al. [85] proposed a novel thermal programming AM technology tailored for curved conformal metasurfaces. Their study explored a conformal metasurface that combined rigid and flexible properties by leveraging the glass–rubber transition characteristics of thermoplastic polymers. This conformality not only simplified the manufacturing process but also ensured optimal electromagnetic performance. In their experiment, they utilized polylactic acid (PLA) and Sn–Bi composite materials through FDM to create an arched metasurface carpet cloak composed of easily bendable unit cells. This innovative design provided a practical approach for enhancing the cloaking performance of complex, curved metasurfaces.
Small mushroom-like structural metamaterials typically consist of a metallic top patch connected to a ground surface, where the material beneath the surface must be removed during manufacturing. Traditional manufacturing methods, such as PCB etching, require the addition of ground through-holes, which can be both costly and unreliable. Stuardo et al. [86] directly fabricated a Sievenpiper mushroom-like metasurface using a low-cost 3D printer without any post-processing in Fig. 5(e). The performance of the printed metasurface, specifically in terms of loss and stopband generation, was experimentally verified and compared with numerical simulations. The utilization of 3D printing for fabricating metasurfaces with geometric deformations, such as arches and spherical shells, improves spatial manipulation capabilities, enables 3D control of electromagnetic waves, and advances the development of high-performance electromagnetic metasurfaces [87].
Early studies on electromagnetic absorption metamaterial structures predominantly featured single-layer or multilayer designs that leveraged the resonance principle to enhance electromagnetic wave absorption. However, these metamaterials are limited in their ability to control electromagnetic waves in three-dimensional space due to structural constraints. A common configuration is the “sandwich” metal–dielectric–metal EMM, consisting of three layers: a dielectric layer with an etched metal pattern to achieve electrical resonance, and a bottom metal plate for reflecting electromagnetic waves and inducing secondary absorption. The metal substrate and surface metal structure form a magnetic resonator. While this three-layer structure can achieve nearly perfect wave absorption at the resonant frequency, it is constrained by a narrow absorption bandwidth due to resonance theory. Additionally, such plane structures are sensitive to the incident angle of the electromagnetic waves and their polarization [29], leading to reduced stability. Moreover, the absorption bandwidth of most 2D metamaterial absorbers is fixed post-fabrication, making it challenging to dynamically adjust the absorption frequency in changing environments. To address these limitations, current research on 2D EMMs focuses on multi-frequency and broadband regulation, polarization independence, angle insensitivity, and tunability. AM technology offers the potential to overcome these issues by enabling the cross-scale fabrication of complex micro/macro structures, integrated multi-material metamaterials, and multifunctional coupling metamaterials, thereby maximizing the advantages of 2D structures.
3.2.2. 3D structure
Three-dimensional EMMs present more processing challenges compared to 2D structures. However, AM technology significantly enhances the design flexibility of EMM structures. AM enables the fabrication of 3D metamaterials, which can more effectively control electromagnetic waves, particularly in low-frequency absorption and structural support applications. Using Al2O3/CNT/SiC nanowires (SiCnw)/SiOC composite ceramics, Mei et al. [88] fabricated an EMM with a cross-twisted structure via SLA combined with polymer infiltration and pyrolysis (PIP). The absorption performance was significantly enhanced when the torsion angle was increased to 90°. At a sample thickness of 2.8 mm, the absorption bandwidth reached 3.6 GHz, covering the entire X-band. The torsional structure improved the impedance-matching characteristics of the metamaterial, and the torsion angle increased the propagation path, to enhance electromagnetic wave absorption. Lim et al. [89] prepared ladder-like twisted-cross structure metamaterial absorbents via FDM. They used PLA as the dielectric material for 3D printing and covered the 3D-printed dielectric structure with a silver paste to form conductive patterns. The step structure facilitated the miniaturization of the absorbing metamaterial. In Ref. [90], to improve the operational frequency range and mechanical properties of EMMs, honeycomb-structured 3D EMMs composed of a resistance sheet were fabricated via FDM, resulting in a lightweight structure. Additional studies have fabricated AM metamaterials with mutilayer structures [91], [92], [93], [94], honeycomb structures [95], pyramidal structures [96], and frequency-selective surfaces (FSS) (Table 1) [54], [88], [89], [90], [91], [93], [94], [95], [96], [97], [98], [99].
AM technology can significantly enhance the performance of traditional multilayer EMMs through gradient design and composite design of materials and structures (Fig. 6). Gradient materials can endow EMMs with good impedance matching and enhance their electromagnetic wave regulation ability. Through FDM, Yang et al. [98] combined EMMs with multistage gradient absorbing materials. The geometric parameters of the cell and the distribution of carbonyl iron powder-enhanced polyether ether ketone (CI-PEEK) were adjusted to design a multilayer absorber with characteristic GRIN impedance. The fabricated 3D-printed multilayer metamaterial could effectively absorb electromagnetic waves across a bandwidth of 8.2–18 GHz and at various incidence angles. The composite design of the structure also broadened the operating frequency band of the EMM. In Ref. [100], a combination of non-planar and planar double-layer composite structures was used to fabricate a vertical metamaterial absorber with a magnetic resonance structure, which exhibited superior low-frequency absorption compared to traditional plannar-structure materials. EMMs based on multi-materials are also an active research area. Xie et al. [101] fabricated a 3D metamaterial structure sample of biodegradable polyester and polymer filaments containing conductive copper particles via dual-print head FDM and explored the fabrication technology of 3D metamaterial structures for the integrated printing of metals and non-metals. Therefore, point-by-point or domain-by-domain control in AM represents a crucial strategy for enhancing the structural performance of EMMs.
As the demand for high-frequency electromagnetic wave segments increases, the fabrication of 3D EMMs at the micro/nano scale has emerged as a crucial research area. Owing to the specific response of incident waves to the subwavelength structure of THz metamaterials, the typical size of an artificial unit is approximately tens of microns, and the unit needs to have a special macrostructure. Consequently, developing efficient and cost-effective methods for fabricating THz metamaterials is essential. To meet the forming accuracy requirements of THz metamaterials, researchers [102] have fabricated 3D GRIN metamaterials capable of operating in the THz band, achieving a resolution near the 0.4–0.6 THz diffraction limit, as depicted in Fig. 7(a). The results verified the feasibility of forming THz metamaterials via PμSL. Li et al. [103] proposed a THz metamaterial manufacturing process based on micro/nano PμSL printing combined with magnetron sputtering deposition and coating, as shown in Fig. 7(b). Taking a 3D THz metamaterial based on a vertical U-ring resonator as a prototype, they processed the model using a high-precision micro/nano 3D printing device and then deposited a metal film via magnetron sputtering to achieve structural functionality. The height of the fabricated vertical U-ring metamaterial was 75 μm, which is suitable for the detection of analytes with a certain thickness. Upon loading lactose and galactose powders onto the sample surface, the central frequency of the absorption peak of the metamaterial was effectively shifted, demonstrating the sample's potential applications in refractive index sensing and other fields.
TPP is a common technology through which researchers manufacture metamaterials with operating frequencies at or above the infrared band, such as photonic crystals. In the infrared spectral region, Kenanakis et al. [104] studied a 3D metallic metamaterial structure operating in the infrared spectral region with linearly polarized asymmetric transmission, as shown in Fig. 7(c). The structure was fabricated via laser direct writing and selective electroless silver plating, featuring two vertical split-cube resonators (SCRs). This design endowed the material with asymmetric transmission properties and polarization isolation capabilities. In Fig. 7(d), Sakellari et al. [105] successfully prepared a 3D chiral plasma metamaterial via laser direct writing technology combined with electroless silver plating. This metamaterial unit consisted of a loop structure, known as twisted-omega particles, which integrated a small electric dipole antenna and a split-ring resonator (SRR). These studies underscore the significant potential and advantages of micro/nano scale AM processes in advanced manufacturing applications.
Three-dimensional structures offer significant potential as tunable, multifunctional integrated metamaterials due to their shape-controllability, which facilitates the development of advanced EMMs. However, most studies on 3D EMM design have not addressed the mechanical bearing capacity or tested the corresponding mechanical properties. Further research is required to enhance the electromagnetic absorption properties while simultaneously preserving the structural mechanical integrity of these metamaterials.
3.2.3. Bionic structure
Over millions of years of natural evolution, biological structures have optimized themselves to thrive in extreme environments. Animals, plants, and microorganisms possess a variety of structures with excellent properties, including low resistance, hydrophobicity, impact resistance, and high strength [106], [107], [108], [109]. These structures can serve as valuable sources of inspiration for the design and manufacture of novel materials. Consequently, researchers have increasingly explored biomimetic materials and conducted simulations to understand the structure–activity relationships of these materials, aiming to enhance the efficiency of engineering structures [110]. Particularly for metamaterials driven by structural performance/functionality, biomimetic structures can obtain unique multifunctional combinations [111], [112].
Early studies on biomimetic metamaterials mainly focused on mechanical functions such as motion and structural support. With the upgrading of electromagnetic products, studies on EMMs have gradually begun to combine bionics with the design and preparation of high-performance structures [113]. Models inspired by biological entities have been developed, and these models have undergone qualitative and quantitative analysis to inform EMM preparation. Various materials and processes have been explored to create novel forms and structures. A biomimetic composite material combining radar and infrared stealth technologies was designed and prepared based on the wing model of Pachliopta Aristolochiae in Ref. [114]. By observing and analyzing the complex structure of a diatom cell membrane in an experiment, researchers found that the membrane exhibited excellent mechanical properties and specific surface area [115]. Consequently, some researchers prepared a metamaterial absorber based on a graded nanoparticle array [116]. However, a bionic design is simple in principle but is rather difficult to manufacture. The fabrication of micro/macro cross-scale complex biomimetic structures is particularly demanding. The design freedom afforded by AM is more conducive to the biomimetic structure design of metamaterials, increasing the versatility of metamaterial structures. Sadeqi et al. [117] proposed a hybrid method combining SLA, metal coating, and wet etching to manufacture 2D and 3D metamaterials with complex geometry and new functions. Through this method, a novel metamaterial embedded with geometrical optics (MEGO) was prepared and utilized as a mushroom-like metamaterial in Figs. 8(a)–(d). The structure featured a small metal resonator atop the mushroom stem, enabling microwave absorption at specific frequencies. Inspired by the compound eye of a moth, the researchers created an omnidirectional hemispherical absorber with a moth-eye structure (Figs. 8(e)–(h)). The absorber exhibited frequency selectivity and angle-insensitive omnidirectional absorption and detection functions. Finally, the fusion of frequency-selective metamaterials with optical parabolic reflectors resulted in a unique MEGO device that combined the respective properties of the metamaterials.
In addition to mimicking the surface structure of organisms, the bionic design of their internal organization is also a significant area of research. Inspired by biological micro-blood vessels, Devi et al. [118] printed glass fiber/epoxy resin composites via FDM and formed a vascular biomimetic metamaterial with a highly complex network (Figs. 8(i)–(m)). This metamaterial is dynamically reconfigurable and can be adjusted to filter specific electromagnetic waves within the radio frequency spectrum through the injection/extraction of room-temperature liquid metals, such as gallium and indium, into and from its vasculature. These findings are pertinent to a variety of fields, including electromagnetics, and set a new benchmark for future design optimization and practical applications of multifunctional and adaptive microvascular composite metamaterials.
3.2.4. Intelligently responsive EMMs
Generally, environmental factors significantly influence the electromagnetic properties of materials, making it essential to enhance their adaptability to various conditions. However, traditional metamaterials that are not reconfigurable face limitations in electromagnetic wave regulation, as they typically operate within a fixed environment and offer a single function. Consequently, the research focus has shifted toward tunable and reconfigurable EMMs. Researchers have explored the use of mechanical and physical devices to achieve tuneability [119], [120], aiming to adjust the unit structure and the electromagnetic characteristics of the metamaterial through mechanical deformation (Fig. 9(a-i)). Moreover, microelectromechanical systems (MEMS) have become widely used in tunable EMMs [121], [122], [123], [124]. A MEMS controlled by air pressure is shown in Fig. 9(a-ii). External stimuli that alter the unit cell structure also affect the metamaterial’s equivalent circuit, effectively transforming it into an active integrated device. While these metamaterials often employ flexible substrate to enable reconfigurability, they face challenges regarding installation and integrability with mechanical devices. An environmentally sensitive active metamaterial was proposed in Ref. [125], as shown in Fig. 9(b-i). Additionally, the electromagnetic performance of metamaterials embedded with active components, such as varactor diodes [126] and positive-intrinsic-negative (PIN) diodes [127], [128] can be dynamically regulated by external excitation signals (usually voltage or current). This dynamic regulation holds significant practical value.
However, the introduction of mechanical adjustment or active electronic devices often increases the complexity of the preparation process for EMMs. Consequently, several studies have explored the use of phase-transition and deformable materials to achieve the dynamic regulation of EMM performance. For instance, the conductivity of a graphene metasurface can be modulated through chemical doping or voltage application [129], [130], [131], as shown in Fig. 9(b-ii). Materials such as liquid crystal, vanadium dioxide (VO2) can also influence the phase response of a metamaterial through electrical control of the refractive index [132], [133]. GaAs/Si-based metamaterials, which exhibit illumination-induced photoconductivity, can function as multi-channel dynamic filters [134], [135], as shown in Fig. 9(c-i). In Fig. 9(c-ii), VO2 is also an excellent photoresponsive material owing to its special properties under light [136]. Light can cause changes in surface temperature, so the light response is usually accompanied by a thermal response [137]. For thermal response alone, as shown in Fig. 9(d), temperature-sensitive materials like VO2 and YBa2Cu3O7−δ can be employed as substrates to adjust the dielectric properties of EMMs [138], [139], [140]. Temperature-induced shape changes in materials can result in significant alterations in THz electromagnetic transmittance, with potential changes of up to 50% [140]. Another innovative approach involves adjusting shape or properties of magnetic materials using a direct magnetic field, which enables ultrafast-response deformation (Fig. 9(e)) [141], [142]. Additionally, EMMs can be partially filled with liquid media instead of solid media, allowing for customization of electromagnetic properties through modifications in liquid concentration or replacement [143], [144]. For example, Fig. 9(f-i) demonstrates the regulation of EMMs using liquid mercury. Low-melting-point metal can also be used as replacements [143], [145], altering their shape upon remelting to produce unique reflective responses. In particular, the liquid crystal fluidity can affect the microscopic arrangement of the material, endowing it with various functions [146], as shown in Fig. 9(f-ii). These advancements in metamaterial properties hold promise for applications in electronic communications, semiconductors, and aerospace.
Overall, EMMs can be intelligently controlled through a variety of methods, including mechanical, electrical, optical, thermal, magnetic, and fluidic approaches. The effectiveness of these control methods is often related to changes in unit arrangement, unit shape, and material properties. These regulatory methods and their outcomes are interrelated; for instance, light-induced changes in temperature can simultaneously affect unit arrangement, shape, and material properties. Despite these advances, some regulation methods still lack intelligence and do not possess environmental awareness or self-regulation capabilities. Current research is therefore focused on advancing intelligent regulation toward smart response systems and integrating these intelligent metamaterials into rapid prototyping processes.
With the development of AM technology, the concept of 4D printing has emerged, significantly enhancing the design and application of reconfigurable and functionally adjustable metamaterials. Once traditional EMMs are formed, the related functions and properties are fixed, and the material can only adapt to the application needs in specific situations. Inspired by origami principles, 3D-printed EMMs can be modified through external stretching to achieve varying functions [147]. However, 3D printing does not support the integral formation of intelligent materials with environment perception or self-feedback capabilities. Research into intelligent EMMs for 4D printing is still in its early stages, with current studies focusing primarily on thermal responses [28]. To advance this field, researchers should explore how to achieve transition from static structural integrity to multifunctional intelligent responses. Material–structure–function integration and intelligence have become the main streams of material development. Researchers should rationally select intelligent materials, design structures, and leverage the excellent AM characteristics to manufacture metamaterials with electromagnetic properties sensitive to external stimuli, and finally obtain dynamic wide-bandwidth EMMs suitable for various application scenarios [21].
4. Typical applications
EMMs exhibit unique electromagnetic properties that span the entire electromagnetic spectrum, making them relevant to advanced technologies across diverse fields such as material science, wireless communication, electronic information, and biomedicine. While innovations in EMM design technology have been significant, applications have also been extensively explored. These included metamaterial antennas, lenses, stealth, imaging, energy transmission, and novel nonlinear functional devices [148], [149], [150], [151]. The following section presents several typical application scenarios of EMMs and compares AM-based fabrication methods with other techniques to assess the feasibility and advantages of AM technology.
4.1. Antennas
The primary function of antennas is to facilitate the conversion between free-space electromagnetic waves and local electromagnetic fields while regulating key parameters such as frequency, amplitude, and phase. Metamaterial antenna technology has found widespread applications in engineering, with numerous studies demonstrating its potential benefits in systems equipped with antennas, such as missiles, radars, and spacecraft. The integration of EMMs into these systems has been shown to significantly reduce antenna energy consumption, improve gain, expand bandwidth, and enhance focus and directionality [152], [153]. Current research in this field is primarily focused on two key areas: the development of low-profile, miniaturized antennas, and the advancement of lens antennas using metamaterials.
Due to their subwavelength operation and relatively small scale, EMMs can be integrated into antenna systems to achieve device miniaturization while enhancing performance, such as producing high-gain, low-profile antennas. Common components used for miniaturizing antennas based on EMM designs include high-impedance surfaces, composite right-handed transmission lines, and metamaterial radiation elements. A high-impedance surface, known for its high impedance and in-phase reflection within a specific frequency band, reduces the destructive interference typically caused by the insufficient backplane distance in the traditional directional antenna [154]. Antennas designed in earlier studies [155], [156] incorporated artificial magnetic conductor (AMC) structures or partially reflective surface cladding to enhance gain and reduce profile (Fig. 10(a)). To avoid the bandwidth narrowing associated with AMC resonance, a gradually changing structure can be employed to modify the AMC [157]. Additionally, the zero-order and negative-order resonance characteristics of the composite right-handed transmission lines offer a novel approach to antenna design by eliminating the wavelength limitations of traditional patch antennas [158]. For instance, a microstrip antenna fed by a wideband asymmetric coplanar waveguide, utilizing both left- and right-handed transmission lines, can achieve bandwidth expansion through a combination of the zero-order resonance point and the +1-order resonance point [159]. Besides using metamaterials to design miniaturized antennas, metamaterials can also serve as the antenna's radiation element to further achieve miniaturization (Fig. 10(b)) [160], [161], [162], [163].
The concept of generalized metamaterials has also been proposed, where metamaterials placed at an optimal distance from the antenna can be excited by the antenna’s radiation field, thereby introducing a new resonant frequency significantly lower than that of the antenna itself. This technique facilitates antenna miniaturization while maintaining good impedance-matching bandwidth [164]. The increasing demand for multifunctional antennas has also made the development of small integrated EMM antennas a critical challenge. AM technology, with its unique advantages, promotes the use of metasurfaces in complex, conformal, multifunctional carriers. Integrated antennas represent an effective approach to achieving miniaturization. Li et al. [165] proposed a method combining 3D printing and flexible printing plate technologies for multifunctional integration on a multi-sector metasurface (Fig. 10(c)). This metasurface, based on a three-layer metal transmission structure, achieved efficient cross-polarization conversion and 360°-transmission phase control across a wide frequency range of 8–14 GHz. The researchers designed various phase profiles—parabolic, vortex with topological charges of 1 and 2, and V-shaped linear phase—on four-quarter sectors of the cylindrical metasurface. They then assembled these sectors into a complete cylinder and excited it using an omnidirectional monopole antenna, successfully integrating four distinct functions: highly directive radiation, vortex beams, and dual-beam steering. This innovative design presents a new direction for conformal multifunctional devices and offers valuable insights for the development of small, functionally integrated antennas.
The equivalent refractive index of EMMs can be adjusted by modifying their structural size and element arrangement, enabling the formation of highly directional lenses that are not constrained by the diffraction limit. These lenses can precisely control electromagnetic waves and are widely applied in systems such as high-performance lens antennas, miniaturized phased-array antennas, and super-resolution imaging systems. For instance, GRIN lenses, composed of layers with increasing refractive indices from the exterior toward the center, can emit and receive electromagnetic waves. Using a GRIN metamaterial lens, Chen et al. [166] converted quasi-cylindrical or spherical waves into plane waves through the effect of spatial guided waves, to achieve broadband and high-gain antenna characteristics (Figs. 10(d) and (e)). However, these lenses were not monolithic and required the assembly of multiple PCBs, their widespread use has been limited by material and fabrication process constraints. Zhang [167] used FDM to fabricate a low-cost, lightweight Fresnel zone plate lens, consisting of three medium rings with an empty central ring (Fig. 10(f)). This lens achieved a gain of 7.3–12.8 dB across the 8–12 GHz frequency range. AM technologies also allow for the production of complex, integrated lenses at relatively low costs. Liang et al. [168] used the IJP process to efficiently and accurately fabricate an X-band Luneburg lens antenna. Owing to the integrated forming process of the lens, there was no need for separate production for different layers and complicated assembly, resulting in a time-saving and adaptable process (Fig. 10(g)).
Efficient, convenient and assembly-free production are key advantages of AM EMMs, which play a crucial role in advancing antenna applications. Current research focuses on achieving wide bandwidth, high gain, and miniaturization in antenna designs. This is particularly important for 5G communication, which demands high transmission rates, intelligent beam shaping, and beam energy aggregation—requiring EMMs to exhibit multiple characteristics simultaneously. As a result, AM processes must take into account both material properties and structural design to develop materials with excellent conductivity and integrate them into appropriate structures for optimal performance.
4.2. Invisibility cloak
During the transmission of electromagnetic waves, an induced current is generated on the surface of typical metal devices, causing electromagnetic wave scattering and altering the original spatial distribution of the electromagnetic field. A stealth cloak is designed to suppress or eliminate this disturbance, thereby preserving the electromagnetic field distribution outside the cloak's coverage area. In the microwave field, invisibility cloaks can manipulate radar waves to render objects undetectable, enabling radar stealth. In the optical field, these cloaks can help objects evade detection by visible or infrared light sources, potentially driving a new wave of advancements in weapons, equipment, and combat strategies.
By placing a metamaterial thin layer with a tailored distribution of conductivity and dielectric constant around a metal object, the refractive index of the material gradually changes, allowing the electromagnetic waves to be guided around the object. This deflection enables the waves to propagate in their original direction, thereby achieving electromagnetic stealth. Using an SRR structure, Schurig et al. [169] designed a 2D cylindrical cloak that operated in the microwave frequency band (Figs. 11(a)–(c)) and experimentally verified the electromagnetic stealth characteristics of the metamaterial with an operating frequency of 8.5 GHz. To enhance the stealth bandwidth, Liu et al. [170] designed a low-loss 2D broadband electromagnetic invisibility cloak for ground targets (Figs. 11(d) and (e)), with an operating frequency of 13.0–16.0 GHz. Although these 2D metamaterials possess electromagnetic stealth properties, regulating the electromagnetic wave transmission characteristics in a 3D space is difficult, and the materials suffer from bandwidth and polarization sensitivity. Therefore, 3D metamaterials are increasingly being explored for advanced stealth applications.
EMMs can also be used to manufacture tabletop black holes that absorb light. Based on the theoretical scheme of optical omnidirectional absorbers, Cheng et al. [171] experimentally fabricated optical black holes, which are omnidirectional electromagnetic absorbers with an electromagnetic absorption rates of up to 99% at microwave frequencies in Figs. 11(f)–(h). The device consisted of composite non-resonant and resonant metamaterial structures, which can guide and absorb electromagnetic waves in all directions, without any reflection, through electromagnetic field control. The omnidirectional absorption ability of the device endowed it with electromagnetic blackbody characteristics, and its absorbing property simulated electromagnetic black holes to an extent. This type of device can be used as a heat emission source and electromagnetic wave collector. Ergin et al. [172] designed and implemented a 3D cloaking structure based on transformation optics in the optical band of 1.4 to 2.7 microns and fabricated the structure via laser-based direct-writing technology. However, the cloak was only a simple extension of the 2D case and could exhibit stealth properties only under incoming waves within 60° and meeting certain polarization conditions. Ma and Cui [173] designed a 3D stealth carpet that could exhibit wide-band electromagnetic stealth in the microwave segment for electromagnetic waves incident at different polarization directions and arbitrary directions, and the carpet could be easily extended to the optical band (Figs. 11(i)–(l)). This device leveraged the aperture size of the new artificial electromagnetic material to control the change in refractive index, so that the electromagnetic wave could form a mirror reflection, and a stealth effect could be achieved on the ground target. Additionally, the 3D non-Euclidean metasurface carpet cloak exhibited stable electromagnetic performance due to the perfect symmetry in its cell layout [174], allowing for omnidirectional 3D control of electromagnetic waves in free space.
Research on EMMs has significantly enhanced the applicability of invisibility cloaks. With the advancement of AM technology, the potential for creating high-performance devices that manipulate electromagnetic scattering to produce ideal invisibility cloaks has increased [84], [85], [175]. Tian et al. [175] designed a carpet cloak with a GRIN metamaterial structure and 3D electromagnetic black hole devices with diamond-structured photonic crystals and wood-stacked photonic crystals, and printed them via SLA using photosensitive resin as the matrix material. The researchers experimentally verified the feasibility of using AM technology to fabricate artificial electromagnetic media devices, which lays a foundation for the further promotion and application of the devices.
In conclusion, AM has significantly accelerated advancements in invisibility cloaking and transformation optics. EMMs play a crucial role in facilitating the realization of transformation optics and are essential for developing effective stealth cloaks. While electromagnetic cloaks have been successfully created using naturally anisotropic materials at optical frequencies, these cloaks often feature relatively simple structures and are primarily employed for stealth applications. With ongoing progress in AM technology, it is now possible to achieve more sophisticated functionalities through complex structures, enabling targets to be perceived as entirely different objects.
4.3. Imaging
Metamaterials, with their ability to achieve specific and tunable scattering responses, have emerged as a promising research area in modern optics for imaging systems. Metamaterial-based image sensors can enhance sensitivity, dynamic range, and noise reduction in images. Additionally, micromirrors incorporating metamaterials enable the development of smaller image sensors, leading to improved image resolution and transmission efficiency. As a result, EMMs hold significant potential for applications in imaging optical paths, imaging algorithms, and biomedical imaging. The unique requirements and challenges of different spectral ranges have driven innovations in frequency-specific metamaterials, aiming to enhance existing imaging modalities or develop new approaches through advanced forming processes [176]. Consequently, functionalities such as holographic and lens imaging can be achieved by tailoring the geometric configuration of EMMs across a range of frequencies from radio to optical.
Research into holographic imaging using metamaterials began with Smith’s group, which explored computational imaging through metamaterial apertures [27]. Their experiment involved optimizing the design of an electrically adjustable metamaterial antenna operating in the K-band (18–26 GHz) and developing a mathematical model to predict the system's imaging performance. By varying the pattern of a leaky wave antenna with external bias, they achieved 2D spatial imaging. The team further improved their model (Figs. 12(a)–(d)) by using two single-pole, six-throw metamaterial antennas for signal transmission and a low-gain horn as the receiver, applying a microwave holographic imaging algorithm to image a 3D target [177]. This computational imaging technique eliminates the need for lens focusing, phase shifters, or mechanical scanning devices, offering advantages such as lower cost and a broader frequency range compared to traditional imaging technologies. In 2016, Li et al. [178] advanced metamaterial imaging with the introduction of the first transmission-type 2-bit programmable coding metasurface designed for single-sensor, single-frequency microwave imaging. Their system used a conventional horn antenna to transmit continuous waves, which were dynamically regulated by altering the digital coding state of the metasurface. This approach enabled the generation of various spatial radiation patterns to mimic specific 2D targets, simplifying the imaging system. The device, tested in the 9.2 GHz frequency range, demonstrated promising results (Figs. 12(e)–(h)) [178].
Further research has explored EMM-based real-time holographic imaging, utilizing electronically controlled load diodes to produce multiple holographic images in real time [179]. This technology leverages the beam regulation characteristics of a metamaterial surface encoded by a single digit to provide switchable phase modulation at microwave frequencies, generating high-resolution, low-noise images. Additionally, integrating holographic imaging with machine learning enhances the system’s ability to capture detailed behaviors of test objects by training with sample data, thus facilitating real-time imaging based on programmable metamaterials [180].
As highlighted in the antenna application section, metamaterial-based lenses are not only pivotal for high-gain antennas but also play a significant role in practical imaging systems. Typically, achieving subwavelength-resolution imaging requires large numerical aperture lenses, which are both costly and bulky [181]. Metasurfaces offer a solution by allowing the miniaturization of traditional optical elements into planar lenses. Their ultrathin profile enables the development of conformal optical elements, and their compatibility with conventional micro manufacturing techniques facilitates the integrated fabrication of complex optical systems composed of multiple metamaterials. Metasurface lenses are employed in both reflective and transmissive modes. Reflective metasurface lenses are often constructed from metal–dielectric–metal multilayer structures, with the base metal serving as a total reflector. For instance, Pors et al. [182] fabricated a reflective planar lens with a 50 nm-thick SiO2 layer acting as a dielectric layer. The lens could focus a linearly polarized incident beam within the focal length of its polarization plane (Figs. 13(a) and (b)). In addition, compared with the early transmissive plasma metasurface [183], this type of reflective metasurface based on a multilayer structure has a higher efficiency [82], [184], [185]. However, due to technical constraints and surface scattering, the actual efficiency of reflective lenses can be limited. To address these challenges, research has increasingly focused on high-efficiency transmission metasurfaces. These studies cover a wide range of wavelengths, from near-infrared to long-wave infrared [186], [187], from circular-polarized light based to polarization-independent [188], [189], and from fixed to adjustable imaging range (Figs. 13(c) and (d)) [190].
Achieving complete electromagnetic wave control through dielectric layers demands precise and intricate structures, which often constrain available nano-fabrication methods. Through laser direct-writing 3D printing and etching technology, Liu et al. [191] formed a transmission-type polarization-independent long-wavelength infrared (LWIR) microlens array based on an all-silicon metasurface (Figs. 13(e) and (f)). Metasurface-based microlens arrays can be used in compact thermal imaging systems, considering the characteristics of single-step lithography and standard integrated circuit-compatible manufacturing processes. Processing and cost are major considerations in the nano-field. Therefore, some studies have used 3D printing technology to prepare THz band diffraction gratings and new lenses, which can achieve high-resolution imaging in the corresponding frequency range [102], [192]. Considering the far-field microwave imaging quality, Yurduseven et al. [193] introduced a computational imaging system utilizing additively manufactured frequency-diverse metasurface antennas. This system employs PLA and conductive polymer materials to realize microwave imaging through a simple frequency sweep, eliminating the need for mechanical scanning and active circuit components. This new approach offers a relatively low-cost and time-efficient alternative to conventional manufacturing technologies.
In conclusion, both holographic and lens imaging demand high levels of miniaturization and precision from EMMs. As a result, the accuracy of AM technology plays a crucial role in the production of these materials. Additionally, the cost of the manufacturing process is a significant consideration for manufacturers. Current micro/nano AM technologies are often employed in high-end application scenarios, which poses challenges for large-scale and rapid production of components. Therefore, balancing precision, cost, and scalability remains a key challenge in the advancement of EMMs for imaging applications.
4.4. Wireless power transfer (WPT)
WPT has been a prominent area of research [194]. WPT technology, which relies on radio waves to transmit energy between devices, holds significant potential in applications such as remote control, driverless cars, and mobile devices. EMMs offer the capability to precisely regulate electromagnetic waves and fields, and their integration with WPT systems can enhance energy efficiency and storage capacity. In 2013, researchers achieved near-field energy transfer at a distance of 2 m by utilizing the principle of strongly coupled resonance [195]. Since then, WPT technology has been developed using near-field electromagnetic induction or resonant coupling, as well as far-field radiation systems, and has progressively found applications in various fields of life and engineering [196]. The advent of EMMs has further advanced WPT technology performance. In Ref. [197], scholars designed metamaterial lenses to improve WPT efficiency based on magnetic coupling resonance in Fig. 14(a). However, the operating frequency was only 27 MHz, and the transmission distance was restricted to 50 cm, which constrained the practical use of these lenses. Fan et al. [198] and Ranaweera et al. [199] adopted metamaterial periodic units with a double-layer structure and a triple-helix structure and inserted multiple metamaterial plates between transmitting and receiving coils to improve the WPT efficiency and transmission distance in Figs. 14(b) and (c).
In addition to functioning as intermediate transmission amplifiers, certain specialized metamaterial periodic units, such as SRR structures, can serve as resonators in WPT systems. Resonators based on a broadband circular SRR structure can capture incident electromagnetic waves and channel the collected energy to improve transmission efficiency (Fig. 14(d)) [200]. To further achieve multi-focusing and efficient WPT, Zhang et al. [201] proposed a dual-polarization near-field-focusing (NFF) metasurface with independent regulation characteristics for WPT systems. This system demonstrated a power reception improvement of 15 dB over non-NFF systems under equivalent conditions. (Figs. 14(e)–(h)). Dynamic tunable metamaterials offer greater flexibility in regulating electromagnetic waves, thus meeting the demand for efficient WPT. Smith et al. [202] proposed a power transmission scheme operating in radio frequency range, utilizing a dynamic metasurface aperture platform. This approach significantly extended the transmission range and suppressed electromagnetic radiation in non-essential areas. The metamaterial is a dynamically adjustable reflector that can deflect beams and focus incoming waves to achieve a comparable effect to a wireless communication repeater. The metasurface aperture enabled dynamic focusing at a lower cost than traditional methods such as phased arrays. To avoid the shortcomings of the traditional PCB-printed metasurface, such as the need for specialized substrates and limited process flexibility, one study prepared a metasurface combining two materials via FDM and compared the energy harvesting capabilities of the electromagnetic metasurface fabricated using a PCB and the 3D-printed metasurface in the 2.4 GHz band [203]. The 3D-printed metasurface exhibited lower conductivity and energy conversion efficiency, approximately one-third of that of the PCB-printed metasurface. Consequently, there is a need for techniques to enhance the energy conversion efficiency of 3D-printed metasurfaces to fully utilize their benefits, including rapid performance, multi-material compatibility, and cost-effectiveness.
WPT technology offers significant advantages, such as eliminating the need for traditional wiring, and provides strong flexibility and ease of operation. However, as highlighted in the previous study, the application of additively manufactured metamaterials in WPT faces several challenges, including energy conversion efficiency and process stability. EMMs struggle to fully exploit their benefits at low frequency (< 2 GHz), whereas everyday applications, such as wireless phone charging, operate in the kilohertz (kHz) range. Therefore, further research is necessary to enhance the low-frequency performance of EMMs.
5. Conclusions and outlook
Although research on metamaterials began relatively recently, their development has progressed rapidly, establishing them as a promising avenue for exploring novel properties [204]. In the field of electromagnetism, metamaterials exhibit superior variability owing to their tailored material properties and tunable properties under external stimuli. This variability enables the regulation of electromagnetic wave intensity, phase, and frequency in unconventional ways. As artificial structures have evolved from macroscopic to microscopic scales, the range of EMM applications has expanded to encompass most electromagnetic wavebands. The structural and functional diversity of EMMs endows them with a wide range of attractive applications, such as the 3D printing of technology-supported antennas, invisibility cloaks, imaging devices, and WPT systems [205]. While the theory has been refined, advances in the forming process have also optimized the properties of EMMs. The fabrication technology of high-resolution, cross-scale, and multi-material structures has gained significant traction in the development process of EMMs, and 3D/4D printing technology, which allows for the structural and functional design of intelligent materials, will become the core technology for EMM fabrication.
AM techniques have inherent defects, such as surface roughness, internal porosity, and dimensional deviation, and the impact of these defects on the performance of EMMs remains largely unexplored. Generally, higher printing speeds can lead to poorer forming quality and increased structural porosity. However, internal porosity can sometimes enhance the electromagnetic properties of EMMs, leading to potentially superior performance. Future research should investigate the relationship between various AM processes, macro/micro defects, and electromagnetic properties. Additionally, integrating post-processing techniques to improve the forming quality of EMMs could optimize their electromagnetic properties. Notably, advanced 4D printing combined with artificial intelligence design and optimization presents a promising approach for creating functionally integrated and intelligently responsive EMMs.
5.1. Function integration
EMMs represent a novel class of artificial materials within the electromagnetic field. Enhancing their electromagnetic performance is a crucial area of development. EMMs must be capable of regulating specific electromagnetic waves based on application scenarios, necessitating multi-frequency and broadband regulation as well as frequency-selective abilities. Their permittivity and permeability can be finely tuned within a narrow bandwidth to achieve impedance matching, thereby endowing the EMMs with frequency selectivity. As applications demanding a wide range of operating frequencies become more prevalent, EMMs are increasingly focusing on broadband field. To meet these performance requirements, metamaterial periodic units have been designed to respond to electromagnetic waves and regulate multi-frequency points. These designs include planar multi-frequency response arrangement, integration of lumped circuit elements, multilayer stacking, and integration of resistance films [206], [207], [208], [209], [210], [211], [212], [213], [214], [215]. Research has concentrated on both broadband and multi-frequency characteristics, considering also the polarization and angle-sensitive performance. This includes addressing frequency shifts due to changes in the effective working size of the unit under oblique incidence and impedance mismatches caused by angle variations. Consequently, two primary research directions have emerged: maintaining polarization sensitivity for applications such as filtering, detection, and parameter estimation [3], [72], [216]; and reducing anisotropy by achieving high structural symmetry. Additionally, polarization-insensitive EMMs are being explored for electromagnetic absorption applications [217], [218].
In addition, EMMs are composed of numerous artificial structural elements, and optimizing the engineering of these element arrays—including their mechanical and thermal properties—can also limit their applications. However, many EMMs used in practical scenarios fall short in areas beyond electromagnetic performance. Challenges such as engineering feasibility and ensuring the long service life of large-scale EMMs remain significant. For instance, achieving perfect stealth with an electromagnetic cloak requires addressing lightweight and anti-jamming capabilities. Furthermore, EMMs employed in aircraft must exhibit low wind resistance and easy conformality with the carrier. Currently, accurate simulation and calculation involving numerous metamaterial microstructures are conducted prior to EMM formation, with continuous optimization of operation models [74], [219]. Through simulation experiments and numerical analyses, the electromagnetic and mechanical properties of EMM structures are verified for structural optimization. However, the feasibility of structural machining must be considered, and existing designs often require modification and improvement based on practical constraints, which can limit design flexibility. AM processes align with the “what you see is what you get” principle, facilitating cross-scale manufacturing, ensuring electromagnetic performance, meeting service requirements, and enabling multifunctional integration (Fig. 15).
5.2. Intelligent response
As previously discussed, the development of EMMs has consistently involved innovation and the application of intelligent materials. Currently, integrating intelligent EMMs presents challenges. Combining them with 4D printing is crucial for effectively enhance their intelligence (Fig. 15). This integration requires the development of specialized materials for 4D printing to enable EMMs to respond dynamically to diverse changes in the external environment changes and exhibit enhanced electromagnetic regulation performance. Many 4D printing materials, such as shape-memory polymers, have dielectric constants and permeability characteristics that do not meet the requirements for effective electromagnetic absorption. Therefore, it is necessary to incorporate additional absorbing materials into shape-memory polymers to create composite materials without compromising the response speed and variation of the original materials. Carbon materials, known for their excellent electrical and thermal conductivity, are often used as efficient absorbing phases and can significantly improve electromagnetic energy attenuation and conversion. For example, several studies [220] have utilized the high thermal conductivity of carbon black to enhance the photothermal conversion efficiency of photoresponsive shape-memory polymers, achieving rapid shape recovery triggered by external lighting. Thus, 4D printing of EMMs using materials with multiple physical properties, such as carbonaceous materials, can enable precise and reliable intelligent control. Additionally, expanding the response modes of intelligent materials is essential to enable rapid, quantitative, and controllable deformation of EMMs in complex electromagnetic environments, even from a distance.
Currently, there is a noticeable lack of collaborative innovation in the field of forming methods. Existing 4D printing technologies remain largely based on the traditional 3D printing process and equipment, although significant differences exist in the materials being printed. Like 3D printing, 4D printing operates on two primary processing scales. The first is the macro-scale, which involves large-size printing technologies, such as those used for intelligent EMMs on the surfaces of stealth aircraft. This application requires 4D printing technologies with high manufacturing speeds to shorten the production cycle of EMMs. The second is micro/nano scale 4D printing, which integrates micro/nano scale 3D printing technology with intelligent response materials to create ultra-small dynamic devices that respond to external stimuli. At smaller scales, the sensitivity, response degree, and forms of intelligent micro and nano components differ significantly when exposed to excitation sources. As the application environments for EMM become increasingly complex, single-material solutions are no longer sufficient to meet performance requirements. Consequently, there is a pressing need for high-resolution, high-speed, and multi-material 4D printing processes that can support the rapid production of intelligent EMMs with multi-scale complex geometries.
Finally, the 4D printing of intelligent EMMs usually involves both the printing and transition states, each of which must be accurately controlled simultaneously [221]. To achieve this, it is essential to establish theoretical models and design methods that can accurately predict and optimize deformation, while also employing effective evaluation methods and verification systems. Current methods for dynamic performance testing commonly include in situ measurements and process monitoring. Dynamic online detection involves sampling 4D-printed smart components at various stages of the deformation process and integrating comprehensive measurement data such as geometric accuracy, temperature distribution, and displacement-strain fields. These measurements are compared with simulation results obtained during the design stage to validate and assess the simulation's accuracy. By combining simulation and deformation evaluation, shape control can be achieved based on accuracy and surface integrity, property control can be aligned with the electromagnetic performance of the part, and function control can be tailored to the specific application characteristics of EMMs.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was sponsored by the National Natural Science Foundation of China (52275331 and 52205358), the National Key Research and Development Program of China (2023YFB4604800), the Key Research and Development Program of Hubei Province (2022BAA011), and the Hong Kong Scholars Program (XJ2022014).
VakilA, EnghetaN.Transformation optics using graphene.Science2011; 332(6035):1291-1294.
[3]
GradyNK, HeyesJE, ChowdhuryDR, ZengY, ReitenMT, AzadAK, et al.Terahertz metamaterials for linear polarization conversion and anomalous refraction.Science2013; 340(6138):1304-1307.
[4]
ViktorVG.The electrodynamics of substances with simultaneously negative values of ε and μ.Sov Phys Usp1968; 10(4):509.
[5]
YablonovitchE.Inhibited spontaneous emission in solid-state physics and electronics.Phys Rev Lett1987; 58(20):2059-2062.
[6]
JohnS.Strong localization of photons in certain disordered dielectric superlattices.Phys Rev Lett1987; 58(23):2486-2489.
HollowayCL, KuesterEF, GordonJA, OJ’Hara, BoothJ, SmithDR.An overview of the theory and applications of metasurfaces: the two-dimensional equivalents of metamaterials.IEEE Antennas Propag Mag2012; 54(2):10-35.
[10]
Della GiovampaolaC, EnghetaN.Digital metamaterials.Nat Mater2014; 13(12):1115-1121.
[11]
CuiTJ, QiMQ, WanX, ZhaoJ, ChengQ.Coding metamaterials, digital metamaterials and programmable metamaterials.Light Sci Appl2014; 3(10):e218.
[12]
LiuR, ZhaoZ, JiC, ZhouT.Metamaterials beyond negative refractive index: applications in telecommunication and sensing.Sci China Technol Sci2016; 59(7):1007-1011.
[13]
ShelbyRA, SmithDR, SchultzS.Experimental verification of a negative index of refraction.Science2001; 292(5514):77-79.
[14]
ChiehJCS, DickB, LouiS, RockwayJD.Development of a Ku-band corrugated conical horn using 3-D print technology.IEEE Antennas Wirel Propag Lett2014; 13:201-204.
[15]
HuangGL, ZhouSG, SimCYD, ChioTH, YuanT.Lightweight perforated waveguide structure realized by 3-D printing for RF applications.IEEE Trans Antenn Propag2017; 65(8):3897-3904.
TariqA, ArifZU, KhalidMY, HossainM, RasoolPI, UmerR, et al.Recent advances in the additive manufacturing of stimuli-responsive soft polymers.Adv Eng Mater2023; 25(21):2301074.
[18]
ArifZU, KhalidMY, NorooziR, SadeghianmaryanA, JalalvandM, HossainM.Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications.Int J Biol Macromol2022; 218:930-968.
[19]
DebRoyT, WeiHL, ZubackJS, MukherjeeT, ElmerJW, MilewskiJO, et al.Additive manufacturing of metallic components—process, structure and properties.Prog Mater Sci2018; 92:112-224.
[20]
AboulkhairNT, SimonelliM, ParryL, AshcroftI, TuckC, HagueR.3D printing of aluminium alloys: additive manufacturing of aluminium alloys using selective laser melting.Prog Mater Sci2019; 106:100578.
KhalidMY, ArifZU, TariqA, HossainM, Ahmed KhanK, UmerR.3D printing of magneto-active smart materials for advanced actuators and soft robotics applications.Eur Polym J2024; 205:112718.
[24]
ArifZU, KhalidMY, TariqA, HossainM, UmerR.3D printing of stimuli-responsive hydrogel materials: literature review and emerging applications.Giant2024; 17:100209.
[25]
LendleinA, KelchS.Shape-memory polymers.Angew Chem Int Ed2002; 41(12):2034-2057.
[26]
ZhangX, ChenL, LimKH, GonuguntlaS, LimKW, PranantyoD, et al.The pathway to intelligence: using stimuli-responsive materials as building blocks for constructing smart and functional systems.Adv Mater2019; 31(11):1804540.
[27]
HuntJ, DriscollT, MrozackA, LipworthG, ReynoldsM, BradyD, et al.Metamaterial apertures for computational imaging.Science2013; 339(6117):310-313.
[28]
JeongH, ParkE, LimS.Four-dimensional printed shape memory metasurface to memorize absorption and reflection functions.ACS Appl Mater Interfaces2021; 13(49):59487-59496.
[29]
HaoX, YinX, ZhangL, ChengL.Dielectric, electromagnetic interference shielding and absorption properties of Si3N4-PyC composite ceramics.J Mater Sci Technol2013; 29(3):249-254.
[30]
ImK, KangJH, ParkQH.Universal impedance matching and the perfect transmission of white light.Nat Photonics2018; 12(3):143-149.
[31]
IqbalA, SambyalP, KooCM.2D MXenes for electromagnetic shielding: a review.Adv Funct Mater2020; 30(47):2000883.
[32]
ChungDDL.Materials for electromagnetic interference shielding.J Mater Eng Perform2000; 9(3):350-354.
[33]
YusoffAN, AbdullahMH, AhmadSH, JusohSF, MansorAA, HamidSAA.Electromagnetic and absorption properties of some microwave absorbers.J Appl Phys2002; 92(2):876-882.
[34]
Al-SalehMH, SundararajU.Electromagnetic interference shielding mechanisms of CNT/polymer composites.Carbon2009; 47(7):1738-1746.
[35]
WenB, CaoM, LuM, CaoW, ShiH, LiuJ, et al.Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures.Adv Mater2014; 26(21):3484-3489.
[36]
GonzálezM, PozueloJ, BaselgaJ.Electromagnetic shielding materials in GHz range.Chem Rec2018; 18(7–8):1000-1009.
LiL, WangJ, WangJ, DuH, HuangH, ZhangJ, et al.All-dielectric metamaterial frequency selective surfaces based on high-permittivity ceramic resonators.Appl Phys Lett2015; 106(21):212904.
[42]
YangZ, LuoF, ZhouW, ZhuD, HuangZ.Design of a broadband electromagnetic absorbers based on TiO2/Al2O3 ceramic coatings with metamaterial surfaces.J Alloys Compounds2016; 687:384-388.
WangY, FanZW, ZhangH, GuoJ, YanDX, WangS, et al.3D-printing of segregated carbon nanotube/polylactic acid composite with enhanced electromagnetic interference shielding and mechanical performance.Mater Des2021; 197:109222.
[46]
WickramasingheS, DoT, TranP.FDM-Based 3D printing of polymer and associated composite: a review on mechanical properties, defects and treatments.Polymers2020; 12(7):1529.
KimM, JeongH, LimD, GhoshS, LimS.Low-cost and miniaturized metamaterial absorber using 3D printed swastika symbol.Microw Opt Technol Lett2020; 62(4):1709-1715.
[49]
QuanH, ZhangT, XuH, LuoS, NieJ, ZhuX.Photo-curing 3D printing technique and its challenges.Bioact Mater2020; 5(1):110-115.
[50]
ZhangX, JiangXN, SunC.Micro-stereolithography of polymeric and ceramic microstructures.Sens Actuators A1999; 77(2):149-156.
[51]
HerzogD, SeydaV, WyciskE, EmmelmannC.Additive manufacturing of metals.Acta Mater2016; 117:371-392.
[52]
OlakanmiEO, CochraneRF, DalgarnoKW.A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: processing, microstructure, and properties.Prog Mater Sci2015; 74:401-477.
[53]
MaTB, MaH, RuanKP, ShiXT, QiuH, GaoSY, et al.Thermally conductive poly(lactic acid) composites with superior electromagnetic shielding performances via 3D printing technology.Chin J Polym Sci2022; 40(3):248-255.
RatniB, deLustracA, PiauGP, BurokurSN.3D printed gradient index dielectric metasurface for beam steering applications.In: Proceedings of the 2017 11th European Conference On Antennas And Propagation (Eucap); 2017 Mar 19–24; Paris, France. Piscataway: IEEE; 2017. p. 3402–4.
NelsonEC, Garcia-SantamariaF, BraunPV.Lattice-registered two-photon polymerized features within colloidal photonic crystals and their optical properties.Adv Funct Mater2008; 18(13):1983-1989.
[58]
TibbitsS.The emergence of “4D printing”.Report. Monterey: TED; 2013.
[59]
MomeniF, HassaniM, LiuX, NiJ.A review of 4D printing.Mater Des2017; 122:42-79.
[60]
LiX, ShangJ, WangZ.Intelligent materials: a review of applications in 4D printing.Assem Autom2017; 37(2):170-185.
[61]
SmithDR, SchultzS, MarkoPš, SoukoulisCM.Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients.Phys Rev B2002; 65(19):195104.
[62]
ChenXD, GrzegorczykTM, WuBI, PachecoJ, KongJA.Robust method to retrieve the constitutive effective parameters of metamaterials.Phys Rev E2004; 70(1):016608.
ZiolkowskiRW.Design, fabrication, and testing of double negative metamaterials.IEEE Trans Antennas Propag2003; 51(7):1516-1529.
[65]
OS’Brien, PendryJB.Magnetic activity at infrared frequencies in structured metallic photonic crystals.J Phys2002; 14(25):6383.
[66]
CostaF, MonorchioA, ManaraG.Analysis and design of ultra thin electromagnetic absorbers comprising resistively loaded high impedance surfaces.IEEE Trans Antennas Propag2010; 58(5):1551-1558.
[67]
CostaF, MonorchioA, ManaraG.Efficient analysis of frequency-selective surfaces by a simple equivalent-circuit model.IEEE Antennas Propag Mag2012; 54(4):35-48.
[68]
MarkoPš, SoukoulisCM.Transmission properties and effective electromagnetic parameters of double negative metamaterials.Opt Express2003; 11(7):649-661.
[69]
ChenHT.Interference theory of metamaterial perfect absorbers.Opt Express2011; 20(7):7165-7172.
LubkowskiG, SchuhmannR, WeilandT.Extraction of effective metamaterial parameters by parameter fitting of dispersive models.Microw Opt Technol Lett2007; 49(2):285-288.
[74]
YangY, JiC, DengK.Rapid design of metamaterials via multitarget Bayesian optimization.Ann Appl Stat2021; 15(2):768-796.
[75]
SoukoulisCM, WegenerM.Past achievements and future challenges in the development of three-dimensional photonic metamaterials.Nat Photonics2011; 5(9):523-530.
MartiniE, SardiGM, MaciS.Homogenization processes and retrieval of equivalent constitutive parameters for multisurface-metamaterials.IEEE Trans Antennas Propag2014; 62(4):2081-2092.
[78]
LiuY, JinX, ZhouX, LuoY, SongK, HuangL, et al.A phased array antenna with a broadly steerable beam based on a low-loss metasurface lens.J Phys D2016; 49(40):405304.
[79]
YuN, CapassoF.Flat optics with designer metasurfaces.Nat Mater2014; 13(2):139-150.
[80]
HsiaoHH, ChuCH, TsaiDP.Fundamentals and applications of metasurfaces.Small Methods2017; 1(4):1600064.
[81]
YuN, GenevetP, KatsMA, AietaF, TetienneJP, CapassoF, et al.Light propagation with phase discontinuities: generalized laws of reflection and refraction.Science2011; 334(6054):333-337.
[82]
SunS, HeQ, XiaoS, XuQ, LiX, ZhouL.Gradient-index meta-surfaces as a bridge linking propagating waves and surface waves.Nat Mater2012; 11(5):426-431.
MeiH, YangW, ZhaoX, YaoL, YaoY, ChenC, et al.In-situ growth of SiC nanowires@carbon nanotubes on 3D printed metamaterial structures to enhance electromagnetic wave absorption.Mater Des2021; 197:109271.
JiangW, YanL, MaH, FanY, WangJ, FengM, et al.Electromagnetic wave absorption and compressive behavior of a three-dimensional metamaterial absorber based on 3D printed honeycomb.Sci Rep2018; 8(1):4817.
ZhouY, ShenZ, HuangX, WuJ, LiY, HuangS, et al.Ultra-wideband water-based metamaterial absorber with temperature insensitivity.Phys Lett A2019; 383(23):2739-2743.
[93]
ZhouD, HuangX, DuZ.Analysis and design of multilayered broadband radar absorbing metamaterial using the 3-D printing technology-based method.IEEE Antennas Wirel Propag Lett2017; 16:133-136.
[94]
XiongYJ, WangY, WangQ, WangCQ, HuangXZ, ZhangF, et al.Structural broadband absorbing metamaterial based on three-dimensional printing technology.Acta Phys Sin2018; 67(8):084202.
[95]
GhoshS, LimS.Perforated lightweight broadband metamaterial absorber based on 3-D printed honeycomb.IEEE Antennas Wirel Propag Lett2018; 17(12):2379-2383.
[96]
ChenX, WuZ, ZhangZ, ZouY.Ultra-broadband and wide-angle absorption based on 3D-printed pyramid.Opt Laser Technol2020; 124:105972.
[97]
KjelgårdKG, WislandDT, LandeTS.3D printed wideband microwave absorbers using composite graphite/PLA filament.In: Proceedings of the 2018 48th European Microwave Conference (Eumc); 2018 Sep 23–27; Madrid, Spain. Piscataway: IEEE; 2018. p. 859–62.
[98]
YangD, YinY, ZhangZ, LiD, CaoY.Wide-angle microwave absorption properties of multilayer metamaterial fabricated by 3D printing.Mater Lett2020; 281:128571.
[99]
AnQ, LiD, LiaoW, LiuT, JoralmonD, LiX, et al.A novel ultra-wideband electromagnetic-wave-absorbing metastructure inspired by bionic gyroid structures.Adv Mater2023; 35(26):e2300659.
[100]
LiW, WuT, WangW, GuanJ, ZhaiP.Integrating non-planar metamaterials with magnetic absorbing materials to yield ultra-broadband microwave hybrid absorbers.Appl Phys Lett2014; 104(2):022903.
[101]
XieY, YeS, ReyesC, SithikongP, PopaBI, WileyBJ, et al.Microwave metamaterials made by fused deposition 3D printing of a highly conductive copper-based filament.Appl Phys Lett2017; 110(18):181903.
[102]
ZhouF, CaoW, DongB, ReissmanT, ZhangW, SunC.Additive manufacturing of a 3D terahertz gradient-refractive index lens.Adv Opt Mater2016; 4(7):1034-1040.
[103]
LiS, ZhangL, ChenX.3D-printed terahertz metamaterial absorber based on vertical split-ring resonator.J Appl Phys2021; 130(3):034504.
[104]
KenanakisG, XomalisA, SelimisA, VamvakakiM, FarsariM, KafesakiM, et al.Three-dimensional infrared metamaterial with asymmetric transmission.ACS Photonics2015; 2(2):287-294.
[105]
SakellariI, YinX, NesterovML, TerzakiK, XomalisA, FarsariM.3D chiral plasmonic metamaterials fabricated by direct laser writing: the twisted omega particle.Adv Opt Mater2017; 5(16):1700200.
[106]
HanX, ZhangD, LiX, LiY.Bio-replicated forming of the biomimetic drag-reducing surfaces in large area based on shark skin.Chin Sci Bull2008; 53(10):1587-1592.
[107]
ParkerAR, LawrenceCR.Water capture by a desert beetle.Nature2001; 414(6859):33-34.
[108]
ThielenM, SchmittCNZ, EckertS, SpeckT, SeidelR.Structure-function relationship of the foam-like pomelo peel (citrus maxima)—an inspiration for the development of biomimetic damping materials with high energy dissipation.Bioinspir Biomim2013; 8(2):025001.
[109]
JonesCJ, AizawaS.The bacterial flagellum and flagellar motor: structure, assembly and function.Adv Microb Physiol1991; 32:109-172.
[110]
ZhaiL, BergMC, FÇCebeci, KimY, MilwidJM, RubnerMF, et al.Patterned superhydrophobic surfaces: toward a synthetic mimic of the namib desert beetle.Nano Lett2006; 6(6):1213-1217.
[111]
ZhangZ, ZhangL, SongB, YaoY, ShiY.Bamboo-inspired, simulation-guided design and 3D printing of light-weight and high-strength mechanical metamaterials.Appl Mater Today2022; 26:101268.
[112]
LiX, YangJ, LvK, PapadopoulosP, SunJ, WangD, et al.Salvinia-like slippery surface with stable and mobile water/air contact line.Natl Sci Rev2021; 8(5):nwaa153.
[113]
WuH, JiS, ZhaoJ, JiangC, DaiH.Design and analysis of a five-band polarization-insensitive metamaterial absorber.Int J Antennas Propag2020; 2020:12.
[114]
HuangL, DuanY, LiuJ, ZengY, MaG, PangH, et al.Bionic composite metamaterials for harvesting of microwave and integration of multifunctionality.Compos Sci Technol2021; 204:108640.
PadillaWJ, TaylorAJ, HighstreteC, LeeM, AverittRD.Dynamical electric and magnetic metamaterial response at terahertz frequencies.Phys Rev Lett2006; 96(10):107401.
[135]
DegironA, MockJJ, SmithDR.Modulating and tuning the response of metamaterials at the unit cell level.Opt Express2007; 15(3):1115-1127.
[136]
ZhangY, QiaoS, SunL, ShiQW, HuangW, LiL, et al.Photoinduced active terahertz metamaterials with nanostructured vanadium dioxide film deposited by sol–gel method.Opt Express2014; 22(9):11070-11078.
[137]
CaoT, WeiC, SimpsonRE, ZhangL, CryanMJ.Broadband polarization-independent perfect absorber using a phase-change metamaterial at visible frequencies.Sci Rep2014; 4:3955.
[138]
ChenHT, YangH, SinghR, OJF’Hara, AzadAK, TrugmanSA, et al.Tuning the resonance in high-temperature superconducting terahertz metamaterials.Phys Rev Lett2010; 105(24):247402.
[139]
SeoM, KyoungJ, ParkH, KooS, KimHS, BernienH, et al.Active terahertz nanoantennas based on VO2 phase transition.Nano Lett2010; 10(6):2064-2068.
LeeSH, LeeYK, LeeSH, KwakJ, SongHS, SeoM.Detection and discrimination of SARS-CoV-2 spike protein-derived peptides using THz metamaterials.Biosens Bioelectron2022; 202:113981.
[150]
ZhaoX, DuanG, WuK, AndersonSW, ZhangX.Intelligent metamaterials based on nonlinearity for magnetic resonance imaging.Adv Mater2019; 31(49):1905461.
[151]
HeX, YangY, DengL, LiS, FengB.3D printed sub-terahertz all-dielectric lens for arbitrary manipulation of quasi-nondiffractive orbital angular momentum waves.ACS Appl Mater Interfaces2021; 13(17):20770-20778.
[152]
JiangZH, GregoryMD, WernerDH.A broadband monopole antenna enabled by an ultrathin anisotropic metamaterial coating.IEEE Antennas Wirel Propag Lett2011; 10:1543-1546.
[153]
AlibakhshikenariM, BabaeianF, VirdeeBS, ASïssa, AzpilicuetaL, SeeCH, et al.A comprehensive survey on “various decoupling mechanisms with focus on metamaterial and metasurface principles applicable to SAR and MIMO antenna systems”.IEEE Access2020; 8:192965-193004.
[154]
YangF, Rahmat-SamiiY.Reflection phase characterizations of the EBG ground plane for low profile wire antenna applications.IEEE Trans Antenn Propag2003; 51(10):2691-2703.
[155]
FeresidisAP, GoussetisG, WangSH, VardaxoglouJC.Artificial magnetic conductor surfaces and their application to low-profile high-gain planar antennas.IEEE Trans Antenn Propag2005; 53(1):209-215.
[156]
ZhaiH, XiL, ZangY, LiL.A low-profile dual-polarized high-isolation MIMO antenna arrays for wideband base-station applications.IEEE Trans Antennas Propag2018; 66(1):191-202.
DongY, ToyaoH, ItohT.Compact circularly-polarized patch antenna loaded with metamaterial structures.IEEE Trans Antennas Propag2011; 59(11):4329-4333.
[159]
LeeH, WooDJ, NamS.Compact and bandwidth-enhanced asymmetric coplanar waveguide (ACPW) antenna using CRLH-TL and modified ground plane.IEEE Antennas Wirel Propag Lett2016; 15:810-813.
[160]
LaiA, ItohT, CalozC.Composite right/left-handed transmission line metamaterials.IEEE Microw Mag2004; 5(3):34-50.
[161]
IslamMT, IslamMM, SamsuzzamanM, FaruqueMRI, MisranN.A negative index metamaterial-inspired UWB antenna with an integration of complementary SRR and CLS unit cells for microwave imaging sensor applications.Sensors2015; 15(5):11601-11627.
[162]
GuptaA, ChaudharyRK.A compact dual band short ended metamaterial antenna with extended bandwidth.Int J RF Microw Comput-Aided Eng2016; 26(5):435-441.
[163]
Al-BawriSS, Hwang GohH, IslamMS, WongHY, JamlosMF, NarbudowiczA, et al.Compact ultra-wideband monopole antenna loaded with metamaterial.Sensors2020; 20(3):796.
[164]
OuedraogoRO, RothwellEJ, DiazAR, FuchiK, TemmeA.Miniaturization of patch antennas using a metamaterial-inspired technique.IEEE Trans Antennas Propag2012; 60(5):2175-2182.
LiZ, PalaciosE, ButunS, AydinK.Visible-frequency metasurfaces for broadband anomalous reflection and high-efficiency spectrum splitting.Nano Lett2015; 15(3):1615-1621.
[186]
MuellerJPB, RubinNA, DevlinRC, GroeverB, CapassoF.Metasurface polarization optics: independent phase control of arbitrary orthogonal states of polarization.Phys Rev Lett2017; 118(11):113901.
[187]
FanQ, LiuM, YangC, YuL, YanF, XuT.A high numerical aperture, polarization-insensitive metalens for long-wavelength infrared imaging.Appl Phys Lett2018; 113(20):201104.
[188]
KhorasaninejadM, ChenWT, DevlinRC, OhJ, ZhuAY, CapassoF.Metalenses at visible wavelengths: diffraction-limited focusing and subwavelength resolution imaging.Science2016; 352(6290):1190-1194.
[189]
ArbabiA, HorieY, BallAJ, BagheriM, FaraonA.Subwavelength-thick lenses with high numerical apertures and large efficiency based on high-contrast transmitarrays.Nat Commun2015; 6(1):7069.
YurdusevenO, FlowersP, YeS, MarksDL, GollubJN, FromentezeT, et al.Computational microwave imaging using 3D printed conductive polymer frequency-diverse metasurface antennas.IET Microw Antennas Propag2017; 11(14):1962-1969.
[194]
SongM, BelovPA, KapitanovaP.Wireless power transfer inspired by the modern trends in electromagnetics.Appl Phys Rev2017; 4(2):021102.
[195]
StrassnerB, ChangK.Microwave power transmission: historical milestones and system components.Proc IEEE2013; 101(6):1379-1396.
[196]
ShinoharaN.Power without wires.IEEE Microw Mag2011; 12(7):S64-S73.
[197]
WangB, TeoKH, NishinoT, YerazunisW, BarnwellJ, ZhangJ.Experiments on wireless power transfer with metamaterials.Appl Phys Lett2011; 98(25):254101.
[198]
FanY, LiL, YuS, ZhuC, LiangC.Experimental study of efficient wireless power transfer system integrating with highly sub-wavelength metamaterials.Prog Electromagn Res2013; 141:769-784.
[199]
RanaweeraALAK, DuongTP, LeeJW.Experimental investigation of compact metamaterial for high efficiency mid-range wireless power transfer applications.J Appl Phys2014; 116(4):043914.
[200]
AlShareefMR, RamahiOM.Electrically small particles combining even- and odd-mode currents for microwave energy harvesting.Appl Phys Lett2014; 104(25):253906.
[201]
ZhangP, LiL, ZhangX, LiuH, ShiY.Design, measurement and analysis of near-field focusing reflective metasurface for dual-polarization and multi-focus wireless power transfer.IEEE Access2019; 7:110387-110399.
[202]
SmithDR, GowdaVR, YurdusevenO, LaroucheS, LipworthG, UrzhumovY, et al.An analysis of beamed wireless power transfer in the Fresnel zone using a dynamic, metasurface aperture.J Appl Phys2017; 121(1):014901.
[203]
ViskadourakisZ, TamiolakisE, TsilipakosO, TasolamprouAC, EconomouEN, KenanakisG.3D-printed metasurface units for potential energy harvesting applications at the 2.4 GHz frequency band.Crystals2021; 11(9):1089.
[204]
ZhangL, LiuH, SongB, GuJ, LiL, ShiW, et al.Wood-inspired metamaterial catalyst for robust and high-throughput water purification.Nat Commun2024; 15(1):2046.
[205]
DengY, LuoB, DongM, HuangZ, ChenX, HanW.Low-complexity fingerprint construction for RIS codebook design in millimeter wave systems.In: Proceedings of the GLOBECOM 2023—2023 IEEE Global Communications Conference; 2023 Dec 4–8; Kuala Lumpur, Malaysia. Piscataway: IEEE; 2024. p. 1567–72.
[206]
HuangL, ChowdhuryDR, RamaniS, ReitenMT, LuoSN, TaylorAJ, et al.Experimental demonstration of terahertz metamaterial absorbers with a broad and flat high absorption band.Opt Lett2012; 37(2):154-156.
[207]
ZhangC, ChengQ, YangJ, ZhaoJ, CuiTJ.Broadband metamaterial for optical transparency and microwave absorption.Appl Phys Lett2017; 110(14):143511.
SuZ, YinJ, ZhaoX.Terahertz dual-band metamaterial absorber based on graphene/MgF2 multilayer structures.Opt Express2015; 23(2):1679-1690.
[210]
Thi Quynh HoaN, Huu LamP, DuyTP.Wide-angle and polarization-independent broadband microwave metamaterial absorber.Microw Opt Technol Lett2017; 59(5):1157-1161.
[211]
ChengYZ, WangY, NieY, GongRZ, XiongX, WangX.Design, fabrication and measurement of a broadband polarization-insensitive metamaterial absorber based on lumped elements.J Appl Phys2012; 111(4):044902.
[212]
YuanW, ChengY.Low-frequency and broadband metamaterial absorber based on lumped elements: design, characterization and experiment.Appl Phys2014; 117(4):1915-1921.
GhoshS, BhattacharyyaS, KaiprathY, SrivastavaKV.Bandwidth-enhanced polarization-insensitive microwave metamaterial absorber and its equivalent circuit model.J Appl Phys2014; 115(10):104503.
[218]
WuX, ZhengY, LuoY, ZhangJ, YiZ, WuX, et al.A four-band and polarization-independent BDS-based tunable absorber with high refractive index sensitivity.Phys Chem Chem Phys2021; 23(47):26864-26873.
[219]
KhatibO, RenS, MalofJ, PadillaWJ.Deep learning the electromagnetic properties of metamaterials—a comprehensive review.Adv Funct Mater2021; 31(31):2101748.
[220]
YangH, LeowWR, WangT, WangJ, YuJ, HeK, et al.3D printed photoresponsive devices based on shape memory composites.Adv Mater2017; 29(33):1701627.
[221]
KuangX, RoachDJ, WuJ, HamelCM, DingZ, WangT, et al.Advances in 4D printing: materials and applications.Adv Funct Mater2019; 29(2):1805290.
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