To design novel architectures with unique properties that surpass those of natural matter, scientists have developed diverse structures/materials by incorporating artificial structures of periodic/aperiodic nano-, micro-, and macro-scale, so called metamaterials. These metamaterials have special properties such as a negative refractive index [1] and negative Poisson’s ratio [2], which are important for acoustic, thermal, electromagnetic, mechanical, and other fields. They offer a high degree of design freedom in terms of the material composition and structural topology. However, these unique properties require the structure to have an exact geometric structure or cell. Small changes in structural dimensions or alignment orientation can dramatically affect performance [3]. The evolution of metamaterials has rendered conventional production methods inadequate for high-precision needs.

Additive manufacturing is a technology that discretizes a three-dimensional model into a two-dimensional plane, shapes the two-dimensional plane from points to lines, and finally manufactures layer-by-layer into a three-dimensional part. It uses computer-aided design technology to create three-dimensional objects and then sends the digital objects to a printer [4]. The complexity of structures designed with computer-aided design technology depends on creativity, which offers great potential for structural development. In addition to the advantages of manufacturing forms, a variety of raw materials can be used for additive manufacturing, such as powders, liquids, and wires. Thanks to its unique manufacturing method, additive manufacturing can theoretically create structures of any complexity. This approach allows for more precise and integrated manufacturing than conventional subtractive manufacturing and fulfills the requirements for metamaterials. The development of metamaterials is closely linked to the advances in additive manufacturing technology. The relationship between development trends of additive manufacturing components and additive manufacturing metamaterials is illustrated in Fig. 1.

Depending on the characteristics of the additive manufacturing components, the development of additive manufacturing can be roughly divided into three stages: the early stages of mold structures (structural components), the middle stages of complex integrated structures (material–structure-integrated components), and the current functional/intelligent structure (material–structure–function-integrated components).

Conventional casting technology is essential for the production of important metal parts in the aviation, aerospace, and automotive industries. The production of molds is critical to the casting process as it directly affects the efficiency and accuracy of the casting. With advances in design and improved integration of parts, molds are becoming increasingly precise and complex. Owing to the high degree of mold customization, conventional methods are costly, and the production of complex mold manufacturing is challenging. Currently, two common additive manufacturing techniques, selective laser sintering and binder injection three-dimensional printing, are used to overcome this challenge [5]. The precision of structures made by additive manufacturing has not yet attained the submillimeter level generally necessary for metamaterials.

Additive manufacturing offers several advantages for the fabrication of integrated and complex structures. The development of laser powder bed fusion technology has facilitated the integrated manufacturing of basic aerospace structures. This can effectively avoid errors and reduce costs associated with structural assembly, shortening the distance between design and manufacturing.

The manufacturing accuracy of functional structures has a significant effect on their performance. There are more than 20 types of additive manufacturing processes, including printable materials such as polymers, metals, and ceramics. Printing can cover scales from nanometers to meters [4]. For example, nanoscale manufacturing can be done by photopolymerization [6], sub-micron manufacturing through one-photon micro-stereolithography [7], and meter-scale manufacturing through wire-arc direct energy deposition [8]. The various additive manufacturing methods mentioned above have greatly promoted the development of functional metamaterials. In addition to functional structures, intelligent structures developed by Massachusetts Institute of Technology researchers in 2014 [9] are a focal point of research. An intelligent structure that needs to make specific changes under different stimuli can also be effectively controlled by additive manufacturing.

In summary, various additive manufacturing technologies offer numerous opportunities for the production of personalized, integrated, and complex structures. Advances in additive manufacturing technology have significantly expanded the design possibilities and potential applications of metamaterials.

In 1968, Veselago [10] proposed the concept of left-handed materials. These materials possess unique properties, including negative refraction and electromagnetic invisibility, which enable them to functionalization. The concept of phononic crystals emerged in the 1990s [11], and Liu et al. [12] subsequently developed three-dimensional phononic crystals for the first time. Fan et al. [13] proposed a thermal metamaterial and it was experimentally confirmed by Narayana and Sato [14]. With the advances in manufacturing technologies and related theories, researchers are increasingly investigating the potential of metamaterials [15]. Metamaterials have potential applications in various industries, such as medicine, automotive, construction, and aerospace. Depending on their function, they can be categorized into four types: electromagnetic metamaterials, acoustic metamaterials, thermal metamaterials, and mechanical metamaterials.

Electromagnetic metamaterials have gone through three important stages: the development of concepts, the experimental realization, the establishment of the theory of negatively refracting materials and the application of metamaterials [4]. As an artificial material, electromagnetic metamaterials can be characterized by two physical parameters: electrical permittivity ε and the magnetic permeability μ. These physical parameters can be modified by the selection of the base material, structural design, and optimization of the topological structures [16]. By adjusting the values and polarities of these two physical parameters, four electromagnetic materials with different properties were obtained [17]. Due to their unique properties, metamaterials are suitable for specific purposes such as electromagnetic cloaks, electromagnetic wave metamaterials, and terahertz electromagnetic metamaterials.

Despite the unique properties of electromagnetic metamaterials, their applications are limited. For example, electromagnetic stealth is usually achieved by constructing split-ring resonators to obtain gradient permeability, which are usually fabricated using a printed circuit board. However, the magnetic resonators of critical components in cloaks often have a larger volume than that of the cloaked object [18]. By introducing a specific metamaterial or metasurface, the volume of electromagnetic cloaks is drastically reduced [19]. Although the metasurface effectively reduces the volume, it was susceptible to the angles. The key to achieving the practical use of electromagnetic cloaks is to design for a small volume and invisibility at wide angles. Printed circuit board manufacturing methods that take advantage of two-dimensional metamaterials encounter significant challenges due to the complexity of these structures. However, additive manufacturing techniques, including fused deposition modeling and stereolithography, have successfully facilitated the fabrication of intricate electromagnetic metamaterials [20].

Acoustic metamaterials have been under development for over 20 years since they were first proposed in 2000 [12]. Similar to electromagnetic metamaterials, the properties of acoustic metamaterials can also be described by two physical parameters: effective density ρ and bulk modulus Κ. By designing materials and structures, different equivalents ρ and Κ are obtained, resulting in acoustic metamaterials with varying properties. Acoustic control has various applications in noise and vibration control, acoustic lensing, acoustic imaging, and acoustic cloaking [4]. Thus, there is a great need for acoustic materials.

Noise control is closely related to human life and has been extensively researched. Long-term exposure to noise, especially low-frequency noise, can significantly affect the human body [21]. Noise control can usually be achieved by acoustic isolation and absorption. The realization of sound isolation is usually based on the control of the sound source. The key is that the volume of the acoustic isolation material must be as small as possible to meet the isolation requirements of the corresponding sound source frequency. Conventional materials such as porous materials can be used for sound absorption [22]. Porous materials are effective for high-frequency acoustics. However, to achieve the same effectiveness for low-frequency sounds an inappropriate volume of materials is required, which is a major challenge for applications. The proposal for acoustic absorber metamaterials, such as microperforated plates, split tube resonators, and decorated membrane resonators, is expected to solve this problem. Acoustic absorber metamaterials are capable of achieving perfect acoustic absorption at specific frequencies. However, this also presents a new challenge: Noise normally occurs in a complex acoustic environment. The frequency range in which acoustic absorber metamaterials can achieve perfect absorption is narrow compared to that of a complex acoustic environment.

In summary, broadband acoustic absorption is an important research area in the field of acoustic metamaterials. The combination of units with varying absorption frequencies is an effective approach to achieve broadband sound absorption. Although additive manufacturing is promising for cross-scale production [6], balancing the structural size with the accuracy of the cells presents another challenge.

Compared to other metamaterials, thermal metamaterials have recently attracted much attention owing to their complex physical processes [23]. In 2008, Fan et al. [13] discovered a shaped-graded material with thermal rectification that can be used as a thermal cloak. It was the first case in which thermal metamaterials were introduced and the theory of steady-state transformation thermotics was presented. Subsequently, Guenneau et al. [24] developed unsteady transformation thermotics. Since then, many researchers have focused on thermal metamaterials as they are crucial applications in the fields of thermal cloaking/camouflage, thermal protection, thermal management, and thermal information [23].

Thermal conductivity is a critical physical property of materials. In the case of diffusive transport, thermal conductivity significantly influences the transmission efficiency. The key factors in the design of thermal metamaterials are structural design and material selection. Transformation thermals can also be designed in a manner similar to that of transformation acoustic metamaterials. As the choice of material significantly influences heat transfer, the selection of a suitable material is crucial for efficient heat transfer. The significant variation in thermal conductivity between different materials facilitate the development of desirable thermal metamaterials through material and structural optimization. However, the substantial differences between different materials pose a challenge for the manufacture of metamaterials from different materials. Selective laser sintering, stereolithography, fused deposition modeling, and other methods can enable the manufacture of multi-materials, which is a practical manufacturing method for thermal metamaterials [25], [26]. However, the manufacture of thermal metamaterials from different materials and with complex structures remains a challenge. The balance between the manufacturability and functionality of thermal metamaterials is an important issue that deserves attention.

Mechanical metamaterials have been developed to achieve unconventional mechanical properties not found in nature. These properties include vanishing shear modulus, zero or negative Poisson's ratio, negative stiffness, and negative compressibility [23]. This can be achieved by adjusting equivalent parameters such as Young's modulus, shear modulus, bulk modulus, and Poisson's ratio, which can be obtained by structural design and topological optimization [27]. Mechanical metamaterials can fulfill various functions, such as energy absorption, energy storage, and vibration reduction [16].

Auxetic metamaterials, one of the earliest and most widely used mechanical metamaterials, typically exhibit a zero or negative Poisson's ratio [28]. A material with this particular property can resist local indentation resistance well when subjected to indentation resistance. It can also resist flexural deformation well as it exhibits an “arch” shape during deformation, which helps it resist bending [16]. Mechanical metamaterials can improve the physical properties necessary for engineering applications, such as hardness, shear resistance, and energy absorption [29], [30], [31]. Another type of mechanical metamaterial is the pentamode metamaterial, which is known for its high bulk modulus and low shear modulus. It could be rationally applied in the underwater acoustic cloak and seismic isolation [32], [33]. The structures of mechanical metamaterials mainly depend on their unique properties. They have great potential for various applications because of their unique properties. Another key to their practical application lies in combining them with the specific performance requirements of applications such as acoustic or biological functions [34], [35]. Therefore, the further development of mechanical metamaterials depends on how they are designed and manufactured to adapt to the unique uses and inherent properties.

Advances in additive manufacturing technology have reduced the disparity between theoretical and practical manufacture of metamaterials. The functionality of the metamaterials can be verified experimentally. Advances in manufacturing technology and theoretical knowledge have opened up limitless possibilities for the realization of metamaterials. The widespread use of metamaterials will drive the development of personalized and customized structures. In the future, the following opportunities may arise for additive manufacturing metamaterials:

(1) The development of multifunctional coupling represents a prospective trend in metamaterials. Load-bearing capacity is a necessary property of most metamaterials, such as thermal and acoustic metamaterials. For mechanical metamaterials, the future is not only about their unique mechanical properties but also about incorporating special designs that couple other functions [36], [37], [38], [39].

(2) Significant differences between the different materials have been incorporated into the metamaterial design. The inherent attributes of a material have a significant effect the ultimate performance of the metamaterial. Selecting appropriate materials for the specified locations (such as manufacturing in the form of multiple materials, and gradient materials) for different metamaterial components would simplify their complexity, achieve special performance, and improve their performance.

(3) Advances in additive manufacturing technology provide opportunities for the cross-scale manufacturing of metamaterials. It is characterized not only by the production of intricate and precise components but also boasts capabilities in integrated manufacturing. Additive manufacturing will potentially facilitate the integration of metamaterial structures from the micrometer to the meter scale, thus promoting the application of metamaterials.

(4) Four-dimensional (4D) printing opens up creative possibilities for the design of metamaterials. 4D printing is the targeted further evolution of a three-dimensional (3D) printed structure, in terms of shape, property, and functionality. The 4D printed metamaterials continue to evolve, exhibit intelligent behavior, and enable special functions such as self-assembly and stimuli-response reconfigurations [40]. Some intelligent structures can be printed directly using the 4D printing technology. This unique capability expands the design space for metamaterials and enables them to adapt their structure in response to various external stimuli and perform the corresponding functions as the external conditions change.

(5) The development of artificial intelligence can accelerate high-throughput design of metamaterials. The development of metamaterials using artificial intelligence can significantly reduce the time required to obtain optimal structures for complex and complicated design spaces. At the same time, by combining the ability of artificial intelligence to design new structures with data from the literature, a database for the metamaterial structural parameter and performance can be created to achieve rapid acquisition of metamaterials.

Additive manufacturing technology offers a viable solution for the fabrication of metamaterials. Nonetheless, this technology also brings unique challenges to the application of metamaterials. These challenges include unresolved issues inherent to additive manufacturing, but also those arising from the unique characteristics of the technology. In addition, the planned advances in metamaterials mentioned above are not immediately attainable, and the road ahead is full of challenges. Below are some summaries of these issues:

(1) There are significant challenges in developing metamaterials that can achieve multifunctional coupling. The different functions of metamaterials may have conflicting structural design requirements. Balancing these diverse functions places higher demands on the design of metamaterials.

(2) The trade-off between structural dimensions and the accuracy is another problem. In additive manufacturing technology, high-precision structures often imply small-scale components. For example, the fabrication of hierarchical metamaterials from nanometers to centimeters has been achieved by photopolymerization [6]. However, this size is still far from what is required for certain applications. Scaling up to larger sizes usually leads to lower precision. The production of metamaterials that combine high precision with large dimensions remains a constant challenge for additive manufacturing technologies.

(3) The manufacture of multi-material, cross-scale integrated metamaterials is a formidable challenge. A key advantage of additive manufacturing is its integrated manufacturing capability. However, it is difficult that manufacturing cross-scale integrated metamaterials because of the limitations of additive manufacturing equipment and additive manufacturing characteristics, such as rapid cooling and heating capabilities. Furthermore, the significant variations in material properties, particularly between metallic and non-metallic materials, pose a major challenge in combination with additive manufacturing.

(4) Cost remains a major challenge in the development of additively manufactured metamaterials and hinders their industrialization. This is largely due to the high cost of equipment and materials used, as well as the time and expertise required to operate the software and technology associated with additive manufacturing machines [41]. In addition, the production of parts using additive manufacturing usually takes longer than using conventional methods [42]. The cost problem therefore mainly arises from the inherent problems of additive manufacturing technology.

In summary, additive manufacturing metamaterials remain an area of research that offers both challenges and potential. It is important to determine the intrinsic functions of metamaterials and their coupling with other functions. Equally important is the further development of additive manufacturing technologies involving multiple materials and cross scales, which are necessary for the future development of metamaterials.

This work was financially supported by the National Key Research and Development Program of China (2023YFB4604800), the National Natural Science Foundation of China (52275331), and acknowledges financial support from the Hong Kong Scholars Program (XJ2022014).