Recent advancements in additive manufacturing (AM) techniques have significantly expanded the potential applications of magnetic materials and devices. This review summarizes various AM methods, including ink‐based and ink‐free processes, and their use in fabricating complex magnetic structures with specific properties tailored for different fields. Key applications discussed include energy‐harvesting devices enhanced with magnetic nanoparticles, water decontamination through magnetically guided microswimmers, and magnetic soft composites in robotics and medical devices. In addition, the integration of AM in producing wearable and flexible magnetic sensors is highlighted, demonstrating its transformative impact on human‐machine interactions. Furthermore, rare‐earth‐free magnets and electric motor designs enabled by AM techniques are also discussed. Despite material compatibility and scalability challenges, AM provides opportunities for creating multifunctional, sustainable devices with reduced waste. Future research should focus on optimizing these techniques for complex applications and large‐scale production, particularly in eco‐friendly and industrial settings.
]]>and its compatibility with a wide variety of materials, such as composites, metals, polymers, and ceramics. [1] AM typically involves building objects by adding material layer by layer from 3D models. [2,3] Unlike traditional manufacturing methods, such as subtractive manufacturing (SM) and formative manufacturing, where the material is removed through machining, drilling, grinding, or molding, AM offers significantly greater design flexibility. The capability to produce complex components in a singlemachine operation has driven many scientific communities and businesses to explore AM as a viable technique for end-use product manufacturing. Recently, research and industry adoption has advanced AM from rapid prototyping to rapid tooling, and it is now poised to become a key player in direct manufacturing. [4,5] Other reasons that have fueled the development of AM technologies are the ability to print multi-material, multi-function structures quickly, the potential to reduce printing flaws, and the enhancement of mechanical properties. [2] AM encompasses a variety of methods, each offering unique advantages for fabricating complex structures and functional components. As one of the most commercially mature AM processes, fused deposition modeling (FDM) leverages thermoplastic filaments that are extruded layer by layer to create solid objects, making it popular for prototyping and producing durable parts. Direct ink writing (DIW), on the other hand, uses a viscoelastic ink extruded through a nozzle, allowing for the creation of intricate designs with tailored material properties. [6] Stereolithography (SLA) and digital light processing (DLP) rely on a light source to selectively cure liquid resin into solid forms, offering high precision and smooth surface finishes ideal for detailed and small-scale objects. [7] Binder jet printing (BJP) involves depositing a liquid binder onto a powder bed to create objects layer by layer [8] while powder bed fusion (PBF), including selective laser sintering (SLS) and selective laser melting (SLM), uses a laser or electron beam to fuse powdered materials, enabling the production of strong and durable parts for industries like aerospace and medical devices. Each of these AM methods is chosen based on the specific material requirements, desired precision, and application of the final product. [2,9] These AM techniques have opened new possibilities for the application of functional materials such as magnetic composites in diverse fields, including robotics, biomedical engineering, and aerospace. [1,10,11] With the use of AM technology, sophisticated designs with unprecedented levels of functionality and complexity can be created without the need for conventional dies, molds, or machining processes, thereby saving substantial amounts of time and resources. Furthermore, the ability to print with multiple materials and achieve fine shape control allows for the creation of structures that were previously impossible to manufacture. [12] However, while AM has been extensively applied to develop structural materials, far less research has been devoted to developing AM techniques specifically for magnetic materials, leaving significant opportunities for innovation and advancement in this area.
AM of magnetic materials provides unique advantages by allowing for complex geometries that optimize magnetic properties. [13,14] Unlike traditional methods, AM can produce intricate designs like lattices and specialized coil structures that improve magnetic field paths and heat dissipation. [15] This flexibility is beneficial for applications like motors, transformers, and inductors, where custom magnetic profiles enhance efficiency. Another key benefit of AM is the ability to customize magnetic properties within a single part, enabling tailored performance in applications requiring specialized magnetic profiles. AM also allows for efficient use of materials like rare-earth elements, by minimizing waste through a layer-by-layer approach. This efficiency not only reduces costs but also supports sustainable manufacturing, especially in industries like electric vehicles and renewable energy. [16,17] Additionally, AM speeds up the prototyping process, reducing development time and costs by eliminating complex molds and dies. This faster iteration is especially valuable in fields like consumer electronics and medical devices, where innovation cycles are rapid. Lastly, AM's ability to integrate magnetic materials directly into assemblies simplifies production and enables more compact, lightweight designs. [18,19] This freedom is essential for industries like aerospace and automotive, where reducing weight and space while maintaining performance is crucial. [15] The magnetic properties of materials are quantified through analysis of their hysteresis loops, which define the relationship between material magnetization (M) and the externally applied magnetic field (H), commonly referred to as the M-H loop. Magnetic materials are systematically classified according to critical parameters-including remanence magnetization (M r ), coercive field (H c ), and Curie temperature (T c )-which determine whether they are categorized as soft or hard magnetic materials based on their ability to retain magnetization and resist demagnetization. Soft magnetic materials, such as ferrites, silicon steel, and amorphous alloys, have low coercive fields, making them ideal for transformers, inductors, and electromagnetic shielding. In contrast, hard magnetic materials, like alnico, samarium-cobalt (SmCo), and neodymium-iron-boron (NdFeB), retain their magnetization when the external field is removed, making them suitable for permanent magnets in motors, sensors, and electronics. [20] Figure 1 shows the statistics of the number of publications each year under the topic of AM of magnetic materials from 2009 to 2023. It is clearly shown that AM of magnetic materials for a variety of applications has attracted increasing research atten- tion across various disciplines. The recent advancements in AM of different magnetic materials, i.e., ferromagnets, ferrimagnets, paramagnets/super-paramagnets, and diamagnets, have opened new possibilities for innovative applications and improved performance across various fields. These magnetic materials have the potential to be used in a wide range of applications since they provide possibilities for controlled motion, thermogenesis, deformation sensing, mechanical strengthening, and efficiency improvements. Magnetic materials are of great and increasing importance in a very broad range of products, such as electronic devices, spinning electrical machines, electric cars, wind turbines, magnetic cooling, electromagnetic shielding microphones, mobile phones, laptops, etc. Research on AM of magnetic materials is driven by the possibility of producing novel magnetic components with enhanced performance and reduced processing costs. [18] In this review, we aim to provide peers with a comprehensive summary of different AM methods, along with recent progress and emerging trends in printed magnetic materials for applications in the fields of energy, environment, healthcare, and industry. Specifically, Section 2 gave an overview of different AM methods. It highlights the key techniques and their roles in advancing the fabrication of magnetic materials. In Section 3, we provided an overview of AM-processed magnetic materials used for energy harvesting, specifically focusing on thermoelectric (TE) generators, piezoelectric devices, and mechanoelectrical systems. In Section 4, we discussed the use of AM-processed magnetic materials for environmental remediation, covering novel systems such as magnetically guided, self-propelled devices for pollutant adsorption and degradation, as well as other magnetically assisted pollutant removal tools. In Section 5, we summarized several representative applications of AM-processed magnetic materials in healthcare, focusing on areas such as magnetic soft composites for disease treatment, targeted drug delivery, hyperthermia, and the development of other novel medical devices. In Section 6, we covered the use of AM-processed magnetic materials in various industrial applications, including magnetic sensors, Table 1. Printing process comparison for magnetic materials. The primary advantages of these printing methods are design flexibility, material efficiency, and the ability to create complex, application-specific geometries. DIW: direct ink writing; FDM: fused deposition modeling; AJP: aerosol jet printing; IJP: inkjet printing; BJP: binder jet printing; SLA: stereolithography; TPP: two-photon polymerization; TPC: two-photon crosslinking; SLS: selective laser sintering; PBF: powder bed fusion; H c : coercivity; M s : saturation magnetization; M r : remanent magnetization; (BH) max : maximum energy product; μ r : relative permeability; PVA: polyvinyl alcohol; PEG: polyethylene glycol; PDMS: polydimethylsiloxane; PPS: polyphenylene sulfide; DEG: diethylene glycol; SR349: bisphenol A ethoxylate diacrylate; TPO-L: 2,4,6-trimethylbenzoylphenyl phosphinate; nBA: n-butyl acrylate; AAHAQ: 2-amino-3-hydroxyanthraquinone; SZ2080: zirconium-silicon photopolymer; PLA: polylactic acid; TPU: thermoplastic polyurethane; PA12: polyamide 12; MWCNTs: multi-walled carbon nanotubes; rGO: reduced graphene oxide.
Method Structural materials Magnetic material Magnetic properties Applications Refs. DIW PVA + PEG BaFe 12 O 19 Density = 5.38 g cm -3 M s = 64.94 emu g -1 H c = 159 kA m -1 Data storage and high frequency application [25] DIW PDMS, MWCNTs, rGO Nd 2 Fe 14 B H c = 0.6747 T Targeted drug delivery using magnetic soft robot [26] FDM PPS SrFe 12 O 19 M r = 39.7 emu g -1 M s = 41.8 emu g -1 H c = 299.85 kA m -1 Anisotropic magnet for automotive industry [27] AJP Polyimide Matrix Ni 0.5 Zn 0.5 Fe 2 O 4 -Microwave circuits [28] AJP Urethane-acrylate based ink Fe 3 O 4 -Small-scale soft robots with programmable functions [29] IJP -F e 3 O 4 / Ag -Spin Seebeck Effect (SSE) thermopile devices [30] BJP DEG Nd 2 Fe 14 B infiltrated with Nd 3 Cu 0.25 Co 0.75 H c = 1345 kA m -1 M r = 0.31T High-temperature applications of NdFeB [31] BJP -Fe-Si-B alloy H c = 47.8 A m -1 μ r = 4447 Improved soft magnetic properties [32] SLA SR349 + TPO-L Ni nanopowder M s = 15.82 emu g -1 M r = 6.7 emu g -1 H c = 7.8 kA m -1 Magnetic sensors or magnetically actuated switches [33] DLP BA + Ebecryl 8232 Fe 3 O 4 -Magneto-responsive structure with programmable complex functions [34] TPP SZ2080 Fe 3 O 4 M s = 43.81 emu g -1 Micro-optical chopper [35] TPC nBA + AAHAQ Fe 3 O 4 -Magnetic micro-actuators [36] SLS PLA + TPU Fe 3 O 4 -Magneto-responsive shape memory composite [37] PBF PA12 Nd 2 Fe 14 B with filler Sm 2 Fe 17 N 3 M r = 0.338T H c = 675.613 kA m -1
Anisotropic magnets [38] the automotive industry, the development of rare earth-free magnets, and potential uses of AM within the automotive industry. This review concludes with a discussion of the challenges and opportunities in AM-processed magnetic materials for real-world applications.
AM techniques have enabled the fabrication of magnetic materials with controlled structures and encoded properties. One unique advantage of AM is its design freedom that allows for developing novel magnetic devices such as programmable magnetically actuated structures, [21] microswimmers with a catalytic reaction mechanism, [22,23] and advanced nanorobots [24] which are almost inaccessible from conventional manufacturing. AM often involves a computer-guided, layer-by-layer process that integrates functional materials into desired patterns, shapes, and compositions, significantly accelerating the speed of device prototyping while reducing the fabrication cost. AM of magnetic materials, based on the deposition mechanism, can be primarily categorized into two types: ink-based and ink-free AM. For ink-free AM, we will focus our discussion on light-/laser-based AM. This sec-tion will review the printing strategies employed for magnetic materials and compare their advantages and disadvantages in the context of these two distinct printing processes. Representative examples of AM-processed magnetic materials are presented in Table 1, along with typical advantages and disadvantages of various AM methods provided in Table 2.
Direct ink writing (DIW) is an extrusion-based AM process [39] that deposits viscoelastic inks layer by layer to form 3D structures (Figure 2A). Initially developed by Cesarani et al. at Sandia National Laboratories and significantly advanced by Lewis et al., [57,58] their pioneering work has expanded DIW's capabilities, making it a versatile tool for materials processing and engineering, particularly for ceramics and functional materials. The DIW inks typically require specific rheological characteristics, such as shear-thinning behavior and sufficient yield strength, to mitigate printed structures from collapsing during the drying process. DIW offers high versatility in material selection, enabling the fabrication of multi-material, [59] custom structures essential for applications in flexible electronics, [60,61] tissue engineering, [62] and microfluidics. [63] Furthermore, DIW's adaptability extends to handling inks with various amounts of magnetic particle loading, thus enabling the fabrication of complex structures with tailored magnetic properties. For example, Figure 2B shows images of DIW-printed barium (Ba) and strontium (Sr) hexaferrite magnets with encoded structures which are difficult to achieve by traditional methods. [64] To date, DIW has been used in diverse applications, such as high-gradient magnetic separators, [65] shapeshifting magnetic robotics, [21] and magnetic epicardial patches [66] for rapid vascular reconstruction. Fused deposition modeling (FDM) [67,68] utilizes thermoplastic filaments such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and ethylene ethyl acrylate (EEA) [69] that can be mixed with magnetic particles to create composite filaments. The resulting composite filament is then extruded through the heated nozzle of the FDM printer to build polymer-bonded mag-netic objects. FDM has been used to print permanent magnets where, by precisely controlling the magnetic filler ratio and specific magnetic orientations, fine-tuned magnetic properties are achieved, [70] facilitating near-net shapes and intricate geometries [71,72] for magnetic application. [27,73] However, the addition of non-magnetic polymers in the process compromises the magnetic properties of the printed magnets. To achieve desired magnetic properties, high magnetic filler ratios of above 65 vol% are often suggested. [74] Aerosol jetting, also known as aerosol jet printing (AJP), is a non-contact AM method that can deposit aerosolized materials onto both flat and non-planar surfaces with high precision as small as 20 μm. [75] In this process, the material is atomized into small aerosol droplets and then transported in a gas stream to the substrate. These aerosol droplets are aerodynamically focused by sheath flow gas, allowing the deposition of fine features with a high degree of control. AJP can handle a relatively wide range of [64] copyright 2019 Elsevier B.V. (C) reprinted with permission from ref., [30] copyright 2023 Wiley-VCH GmbH. (D) reprinted with permission from ref., [32] copyright 2022 Elsevier B.V. , [33] licensed under CC BY-NC-ND 4.0. (C) reprinted with permission from ref., [35] copyright 2023 Optica Publishing Group. (D) reprinted with permission from ref., [96] copyright 2023 Elsevier B.V. ink viscosities, up to 2500 mPa s, enabling it to work with various types of materials. [76][77][78] In the context of magnetic material, magnetic particle loading is limited by the need to balance particle concentration for an effective curing process in the polymer matrix, without compromising print quality. [28,29] Additionally, AJP's capability for high-resolution deposition, along with its compatibility with a broad range of ink viscosities, makes it particularly effective for fabricating thin-film devices such as flexible electronics, sensors, and antennas. [28,79] While AJP deals with ink aerosols, inkjet printing (IJP) uses ink droplets to create structures or patterns on substrates. IJP is typically used for printing on flat surfaces with high deposition accuracy. IJP can employ a variety of actuators, including piezoelectric, thermal, solenoid, pneumatic, and acoustic actuation mechanisms. Thermal IJP and piezoelectric IJP are the most common ways to distribute functional materials due to the controllable droplet size and the high-frequency response. It is worth mentioning that most IJP-based inks require low viscosity, usually less than 40 mPa s. [78] IJP has been employed to produce magnetic materials (particularly thin films), [80] which can provide mechanical flexibility to the printed parts (see Figure 2C). [30,81,82] Binder jet printing (BJP) utilizes liquid binder droplets to selectively bond powder particles layer by layer, enabling the printing of various material powders, such as metals and ceramics. Unlike the IJP which deposits material directly, BJP deposits a binder solution that connects material powders and is often followed by thermal sintering. Despite the post-processing shrinkage due to the binder removal, BJP can print complex 3D structures with powder supports for magnets and magnetic shape memory alloys. [32,83,84] For example, in Figure 2D, an optimized soft magnetic Fe-Si-B ring, fabricated with this technology, is presented with improved magnetic properties despite the challenge of brittleness at high Si contents.
As a template-based or mask-based AM process, screen printing has also been used to fabricate magnetic materials for various applications such as magnetics-enhanced TE [85] and electrochemical [86] devices. This process involves transferring functional inks through a permeable mesh screen by controlled squeegee strokes to print 2D patterns on substrates. [87] The success of screen printing depends on the development of inks with desirable rheological properties (e.g., high viscosity), which help mitigate ink spreading, prevent clogging in mesh, and ensure strong adhesion to substrates. [88]
In addition to ink-based printing methods, light-or laserbased AM processes have also been utilized for fabricating magnetic materials and devices. As shown in Figure 3A, vatphotopolymerization (VPP) processes, such as stereolithography (SLA), digital light processing (DLP), and two-photon polymerization (TPP), use a specific light source to selectively polymerize liquid photosensitive monomers or pre-polymers layer-bylayer to form a solid object. These VPP processes enable highresolution fabrication and quality surface finish.
For example, magnetic SR349-Ni composites have been fabricated using SLA techniques, enabling magnetically active and electrically conductive features in the final objects (Figure 3B). [33] While SLA uses a single laser to cure the photopolymer resin, tracing each layer point by point (e.g., layer pixel), DLP employs a digital micro-mirror device (DMD) to project an image of the entire layer and cures it all at once. [89] This leads to a faster printing speed of DLP compared to SLA, [90] especially for large objects, as the curing time for a layer is less demanding when it comes to the device size or complexity. [91] However, the amount of magnetic material added to the resin needs to be carefully balanced as excesses can lead to inadequate curing due to particleinduced absorption and light scattering. [92,93] The particle sizes typically range from sub-microns to a few micrometers, depending on the application. [94,95] While SLA and DLP offer intricate structure designs up to 100 μm, TPP [97] enables precision at the nanoscale, [98,99] utilizing the near-infrared femtosecond laser pulses to achieve simultaneous absorption of two photons by the photo-initiator, unlike single-photon polymerization. TPP, like other VPP techniques, involves embedding magnetic particles within the printed structures, [92,100] sometimes using a magnetic field to trap and orient magnetic particles to manipulate their magnetization directions. [92] In Figure 3C, magnetically-driven microoptical choppers are fabricated utilizing TPP and are presented to showcase their rapid response and potential for integration into diverse micro-optical systems. [35] Recent advancements in TPP technologies have given rise to a new technique called two-photon crosslinking (TPC) [101] which can layer different polymers atop one another, producing multifunctional materials, unlike traditional TPP techniques. [36] TPC utilizes a prepolymer containing photochemically reactive groups which undergoes crosslinking and surface bonding upon two-photon excitation in the glassy state, using C, H insertion crosslinking reactions. [101] Due to the common presence of C and H elements in polymers, TPC offers great design flexibility by forming crosslinked networks with various matrix components. [102] In the context of magnetic materials, TPC has the potential to lead the way for direct light-based writing to create functional microstructures. [103] Powder bed fusion (PBF) [104][105][106] is an AM process where a power source, such as an electron beam or laser, is used to selectively fuse powder materials to build up objects layer by layer. PBF includes techniques like selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), and multi-jet fusion (MJF), each varying in the mechanism of powder fusion and the type of energy source used. [107] This process includes engineering alloys and high-performance oxides, making it ideal for aerospace, medical, and automotive applications. PBF's capability for in-process support generation enables the creation of complex designs with overhanging features overcoming difficulties in removing supporting material. [105,106] Additionally, PBF can manufacture components that are near-net shapes, reducing the need for extensive post-processing. [108] PBF systems typically use powder particles sized between 20 and 50 μm, which allows for smoother surfaces and higher resolution. Microstructural features like grain size, texture, and porosity affect the magnetic properties of the printed structure. Thus, optimal heating or post-processing is required to reduce stresses, promote grain growth, and lower the hysteresis losses. [18] In Figure 3D, a Ni-Mn-Sn-Co magnetic shape memory alloy with complex geometries (relative density ≈96%) was produced via the PBF technique for a solid-state refrigeration system. Its precise control over composition and grain structure is crucial for optimizing the alloy's magnetic properties. [109] Direct energy deposition (DED) [110] fuses materials (e.g., metals/metal alloys) via the focused thermal energy from a laser or plasma arc during deposition. [15] Similar to other powderbased techniques, DED is also able to create near-net-shape components with enhanced magnetic functions. [111][112][113][114] In DED, larger powder particles (50-150 μm) are typically employed to enhance flowability and elemental powder blends are used to facilitate custom alloy development and compositional grading. However, variations in melting points and rapid solidification rates can result in compositional inhomogeneity within the final structure. [18] Careful control of printing parameters, as shown by Sun et al., can mitigate defects such as porosity which leads to cracks and balling, and further optimize the mechanical and magnetic properties of silicon-iron alloys. [114] The unique advantage of DED is due to its coaxial system which offers a flexible design, reliable feeding mechanism, and real-time fabrication. It is commonly used for repairing structures or integrating material into final products, [115] and DED has also been proven effective for creating functionally graded magnetic materials. [116]
Flexible film TE devices are widely applied in wearable electronic devices due to their high stability, silent operation, ecofriendliness, lightweight, compactness, and portability. [75,[117][118][119][120] Studies have shown that the embedding of magnetic nanoparticles such as Fe, Co, Ni, etc. in TE materials to form thermoelectromagnetic materials offers an innovative approach to regulate TE performance. Zhao et al. found that adding magnetic nanoparticles (Fe, Co, and Ni) into CoSb 3 enhances its TE performance by over 30% due to the synergistic effects of electron scattering and magnetic domain thermal perturbations. [121] The transition between ferromagnetic and superparamagnetic states in these nanoparticles along with phonon scattering in nanostructures, helps to regulate electron and phonon transport, improving the material's TE properties. Zhao et al. showed that the interaction between ferromagnetic Co nanoparticles and Te in Bi 0.5 Sb 1.5 Te 3 (BST) increases electrical conductivity and maintains a high Seebeck coefficient in flexible TE films. [122] Recent works have reported the fabrication of Fe/BST/epoxy flexible thermo-electromagnetic films containing dot magnetic arrays fabricated using screen printing (Figure 4A). [85] These printed thin films achieved high charge carrier mobility because of their high coercivity, leading to significantly better electrical conductivity while still maintaining a high Seebeck coefficient, as shown in Figure 4B. The authors of the study observed that hexagonal dot magnetic arrays promoted the electrical transport properties of printed thermo-electromagnetic films. At room temperature, these films achieved a power factor of 1.51 mW m -1 K -2 , with an increase of 33.6% and 36.1% in comparison to the standard TE film and thermo-electromagnetic film with a continuous magnetic layer, respectively. These results highlight the critical role of AM-enabled controlled deposition of magnetic materials in TE devices.
The piezoelectric effect is a material property that generates an electrical voltage in response to applied mechanical stress and , [85] copyright 2023 American Chemical Society.
vice versa. This phenomenon is due to the material's crystal structure lacking symmetry at the center but balanced in an equilibrium state, which under mechanical stress causes an asymmetric charge distribution, leading to an electrical potential or voltage across the material. In the area of energy harvesting, systems combining magnetic and piezoelectric properties have been demonstrated to harness magnetic energy and increase mechanical-to-electrical conversion. [123][124][125][126][127] The applied magnetic field causes the magnetoactive layer to respond magnetically and interact mechanically with the piezoelectric layer. This mechanical stimulation under a magnetic field generates electricity in the piezoelectric layer, converting mechanical stress into electrical charges. Under varying magnetic field strength, the generated electrical signal also changes, and this enables the measuring of the change in a magnetic environment.
Brito-Pereira et al. reported an AM-processed, self-powered sensing device combining a piezoelectric polymer with CoFe 2 O 4 nanoparticles/wax composite which showed the ability to harness magnetic energy along with a maximum output power density of 9.7 mW cm -3 , detecting magnetic fields with a sensitivity of 30 V T -1 (Figure 5A). [128] A pre-magnetized layer was used against the direction of the applied magnetic field to cause ad-ditional strain. The device was optimized for the output voltage by varying its magnetic layer properties with a wt.% gradient of CoFe 2 O 4 (Figure 5B). It also achieved an output power of ≈160 μW and increased to around 350 μW when it was closer to magnets causing additional mechanical interaction (Figure 5C). Overall, this magnetic field sensing/energy harvesting performance provides considerable potential for use in self-wearable electronics with Bluetooth connectivity and applications related to the Internet of Things (IoT). [128]
In the context of magnetic materials, mechanoelectrical devices generate electricity when the magnetic structure is deformed by applied stress and electromagnetic induction occurs due to changes in the surrounding magnetic field, whereas piezoelectric materials directly generate electricity from mechanical stress applied to the material due to the magnetic field. In particular, soft mechanoelectrical devices are gaining recent interest in engineering and materials science because of their ability to deform under force, altering the magnetic field , [128] copyright 2021 Elsevier Ltd.
distribution and thus having the potential for mechanoelectrical energy conversion. [129] For example, Li et al. reported a magnetic soft material (MSM) composed of Nd 2 Fe 14 B particles and thermoplastic urethane (TPU) with variable Poisson ratios. [130] Using selective laser sintering (SLS), the unique structural design of MSMs demonstrated a mechanoelectrical conversion, particularly those with negative Poisson's ratios showing the highest electrical output (7.42 mV at the strain of 40% and compression velocity of 40 mm s -1 ). The authors indicated that SLS has the potential for fabricating highly efficient mechanoelectrical conversion devices and self-powered sensors (Figure 6).
Zhang et al. developed a flexible, superhydrophobic, magnetic device (3D-printed magnetic device, 3DMD) with adaptable mechanoelectrical conversion approaches for raindrop interactions. [131] These devices consist of a lotus-leaf-like magnetic top (Nd 2 Fe 14 B particles-TPU composite) and a stem-like elastic bottom fabricated using FDM (Figure 7A) and poly-jet printing, respectively. As presented in Figure 7B,C, when a water droplet falls on the device, it causes the magnetic top to move, changing the magnetic flux through a coil inside the elastic stem and thus, generating electricity. With the design flexibility enabled by AM, it is anticipated to see additional exciting research progress on the frontier of novel energy harvesting, energy management, and energy conversion applications.
Chemical and biological water contamination worldwide, especially in developing nations, is a serious health risk, with a large amount of the population negatively impacted by uncontrolled industrial waste spills and inadequate sewer infrastructure. Every year, human activity produces over 1500 cubic kilometers of wastewater, which is six times the amount of all the rivers combined on Earth. Many illnesses can be brought on by contaminated water, ranging from toxic chemical-derived conditions like cancer and chronic poisoning to pathogen-induced ailments like cholera and dysentery. To ensure complete water safety, successful water remediation solutions must address the removal of toxic chemicals as well as the elimination of dangerous microbes such as bacteria, viruses, and protozoa. [132] The potential of AM approaches interconnected with magnetic technologies for the development of smart devices able to mitigate or analyze contaminants in environmental matrices has been demonstrated in the last decade. [22,[133][134][135][136][137][138] It should be noted that there exist multiple studies dealing with the design and fabrication of magnetically enabled, AM-processed devices for environmental applications. Nevertheless, in some cases, the AM-processed systems are not intrinsically magnetic but are integrated with additional structures or materials to provide them with a magnetic response. For example, some of these structures are equipped with magnetic properties by the addition of ferromagnetic materials after the manufacturing process (i.e., Fe 3 O 4 nanoparticles, magnets, etc.), preventing more complex functionalization of the devices. [135] However, the capability to fabricate magnetic responsive devices via AM methods directly, by using magnetic inks, is highly desired for designing and customizing multifunctional, intelligent, small-scale devices. In light of the promising prospects of this technique, some groups have reported the design, fabrication, and utilization of multifunctional, AM-processed systems using magnetic inks and have shown their potential for environmental remediation. [22,[133][134][135][136][137][138] , [130] 2023 Elsevier B.V. [131] licensed under CC BY-NC-ND 4.0. Figure 8. A) Stereomicroscopy image of a magnetically guided catalytic fish (scale bar: 2 mm). B) Schematics of the sequentially printed individual layers of the multifunctional fish architecture. C) Dependence of the fish speed upon the peroxide fuel concentration using different Pt loadings (15%, green fish; 10%, yellow fish; 5%, orange fish). D) Tracking line of the magnetically guided motion of the fish in a predetermined "C" shape over 3 secs based on the schematic illustration shown in E) using external magnetic manipulation of the fish. Scale bar: 10 mm. F) Absorbance spectra of p-nitrophenol, a product from the hydrolysis of methyl paraoxon, showing the comparison between a catalytically propelled fish and a static fish after a 20 min treatment (the standard represents the spectra obtained after complete degradation of the toxin). Figure reprinted with permission from ref., [133] copyright The Royal Society of Chemistry 2015.
Recently, novel studies have addressed the manufacture of systems resembling the morphology of fishes, able to swim in the water matrix while at the same time decontaminating it. For example, Kumar et al. AM-processed self-propelled, catalytic fish swimmers using a versatile screen printing approach and different modified inks (Figure 8). [133] More specifically, artificial bioinspired fish-shaped motors with a size in the millimeter scale (3 mm -10 mm) capable of swimming autonomously while at the same time performing additional functions (modified propulsion efficiency, control of the fish directionality, removal of contaminants via adsorption, etc.) were designed (Figure 8A). The authors demonstrated that screen printing could be employed to incorporate different characteristics and functionalities at specific areas of the swimmers by the sequential printing of specific layers based on different functional materials, as presented in Figure 8B. A variety of multifunctional swimmer architectures were manufactured using novel inks and their efficiency was tested. To provide the swimmers with magnetic controllability, the authors used inks based on Ni, and, for the removal of aqueous contaminants, activated carbon was used. Additionally, different Pt loadings on the tail of the swimmer provided propulsion by converting chemical energy into movement via a catalytic re-action (i.e., decomposition of hydrogen peroxide to generate oxygen bubbles that generate momentum when detached from the fishtail, as presented in Figure 8E). Figure 8D shows the tracking line of the magnetically guided motion of the swimmer. The swimmers were able to reach a speed of 175 mm s -1 that corresponds to ≈20 body lengths per second (Figure 8C).
For environmental remediation, these devices showed great potential being capable of removing methylene blue from water at a concentration of 12 μM as well as degrading the highly toxic methyl-paraoxon at a concentration of 30 μM. In comparison to static fishes, the active swimmers provided a better performance at adsorbing methylene blue due to their high speeds, which improved the solution mixing and hence, the adsorption rate. Moreover, active fishes offered coverage of large, polluted areas improving the purification efficiency in comparison to their static counterparts (57% and 11%, respectively, after 60 min). In a different experiment, the self-propelled catalytic fish was evaluated for the rapid peroxide-based oxidative detoxification of the nerve agent simulant methyl paraoxon, resulting in a 72% degradation of the toxin compared to 17% in the case of its static counterpart over a 20 min period (Figure 8F).
Analogously, the same group also addressed the AM of magnetically guided microswimmers for toxin neutralization. [22] In their work, the authors employed microscale continuous optical printing approaches to manufacture hydrogel devices , [138] licensed under CC BY-NC-ND 4.0.
featuring biomimetic structures, locomotive capabilities, and functionalized nanoparticles. The AM-processed micro-fishes exhibited highly efficient propulsion, were chemically powered, and magnetically guidable. The incorporation of nanoparticles in the structures (i.e., polydiacetylene nanomaterial) enabled their utility in toxin-neutralization applications. Specifically, nanoparticles made from the self-assembly of 10,12-pentacosadiynoic acid (PCDA) were employed for melittin sensing and detoxification. Once the toxins bound to the surface of the nanoparticles, they disrupted the structure of the surface polymer, emitting fluorescence. Thus, changes in fluorescence intensity over time could serve as an indicator of detoxification efficiency. Additionally, the authors demonstrated that there was a significant difference between the mobile and stationary micro-fish, highlighting the importance of the microswimmer movement for enhancing the detoxification process.
Finally, Bernasconi et al. reported on the fabrication and evaluation of magnetically controlled cylindrical microdevices with photocatalytic and anti-bacterial properties. [138] In their work, the authors described the AM of plate or spiral morphologies using the SLA technique. SLA was opted for due to its low cost, the excellent physical integrity of the produced microdevices, and their reproducible architecture with small dimensional deviation. Subsequently, the microdevices were coated with four functional layers to provide the following characteristics: magnetic actuation, photocatalytic, and anti-microbial properties, as presented in Figure 9A. The first layer consisted of 400 nm of pure copper and was achieved by electroless deposition. The role of the first layer was to provide electrical conductivity to the surface of the microdevices for subsequent electrolytic deposition of the second layer. The second layer, measuring 5 nm, was comprised of CoNiP, a semi-hard magnetic alloy that can be pre-magnetized, thus, allowing for the programming of its magnetization axis resulting in the rolling actuation of the microdevices. The magnetic layer was deposited using barrel deposition, a technique where many objects are placed inside a container able to conduct electricity. For the third layer, another 400 nm of pure Cu was coated via electrolytic deposition to provide adhesion for the final layer. For the fourth and outermost layer, the authors selected an Ag/TiO 2 composite to leverage the anti-microbial behavior of Ag with the photocatalytic properties of TiO 2 for water cleaning purposes. The layer of TiO 2 particles on an Ag substrate was achieved via electrochemical co-deposition and verified using SEM.
The obtained microrobots were then evaluated on their watercleaning performance based on their photocatalytic activity, biocidal attributes, and magnetic locomotion (Figure 9B). In terms of photocatalytic activity, Rhodamine B photodegradation by UV radiation was found to be enhanced up to 6 times in comparison to a reference solution. Moreover, for their biocidal attributes, the microrobots had a demonstrated anti-bacterial effect, as well as the time of treatment and degree of contamination in the water. Using the percentage of gram-negative bacteria left as a metric, only 11% remained after treatment. It is important to note that total water disinfection (i.e., 0% bacteria left) is typically not possible using such techniques. To evaluate their locomotion in the presence of magnetic fields, a step test was performed where it was determined that the microrobots could overcome a glass step with a maximum height equal to their radius. In summary, the microrobots described in the work by Bernasconi et al. could be applied to water purification inside small reservoirs or canalizations provided for a scaled-up magnetic field. Their most targeted application would be in cases where both photocatalytic and antimicrobial agents are required to perform in a highly localized and precise manner. , [134] copyright 2021 Elsevier B.V.
Besides microswimmers, AM-processed static magnetic devices and systems have been also employed for water treatment with a high decontamination efficiency. In some cases, the magnetic materials present in the devices have been proven beneficial and enhanced the water treatment process via various effects. In one study, Fernández-Velayos et al. fabricated, using the FDM method, zero-valent iron (ZVI) coated with polylactic acid (PLA) for tetracycline (TC) degradation via magnetically assisted persulfate activation, as presented in Figure 10A. [134] For the antibiotic degradation process, the generation of oxidizing species using heating as well as the ZVI was explored and the authors analyzed the effect of two different heating methodologies: thermal bath and contactless heating promoted by magnetic induction heating (MIH) using a solenoid (Figure 10B). It was demonstrated that the non-contact heating of ZVI by MIH (using fields of 12 mT -62 mT) reactivated the catalyst over time by renewing the surface iron content exposed to the pollutant, which made the material reusable (up to 10 cycles) with no efficiency reduction (Figure 10C,D). In contrast, by using a conventional thermal bath, the kinetic constant gradually decreased over the cycles due to superficial iron consumption, resulting in a 5-fold decrease in the kinetic constant of the degradation reaction. The capability of MIH to generate heat directly to the catalyst through electromagnetic energy conversion on magnetic materials was proven a useful technique, which could overcome the limits of heat transfer found in classical "contact" heating reactors. This breakthrough technology can be employed in future studies to mitigate the problems of poor energy transfer and heat dissipation phenomena in other advanced oxidation processes, improving the hydrodynamics of the reactor and making any transformation safer, cleaner, and more reproducible.
A similar material was developed by the same group in another work for the degradation of ofloxacin. [137] Briefly, the development of 3D monoliths from a powder blend of iron oxide and PLA were prepared as water treatment agents, achieving a degradation efficiency of >50%, which was kept constant even after 100 h of operation.
AM-processed magnetic systems have also been applied for metal extraction. In a work reported by Calderilla and co-workers, the authors designed and manufactured rotating-disk sorptive extraction devices using SLA. [136] Specifically, extraction disks and small magnets were immobilized into the device before the polymerization reaction was completed, which after UV postcuring, became permanently attached to the system. These devices were applied to the simultaneous extraction of 14 trace metals before analysis via ICP-OES (inductively coupled plasma optical emission spectroscopy), featuring detection limits between 0.03 and 1.27 μg L -1 and extraction yields >90% for most of the compounds. The developed method was validated using certified wastewater and soil reference samples, and satisfactory spiking recoveries were obtained in the analysis of highly polluted solid waste treatment plant leachates (89-110%).
Magnetic soft composites are materials created by embedding ferromagnetic particles within a soft matrix, typically an elastomer. [139] This integration allows the normally flexible material to exhibit magnetic properties, making it suitable for controlled actuation through magnetic fields. Unlike traditional materials, these composites can be easily deformed yet quickly respond to magnetic stimuli with untethered actuation and control, providing essential functionality in applications ranging from robotics to medical devices. [140,141] The choice of magnetic particles is critical in these composites, particularly the distinction between soft and hard magnetic materials. [141,142] Soft magnetic materials are preferred for applications requiring rapid magnetization and demagnetization without retaining a remanence magnetization, which is essential for dynamic control in actuation systems. In contrast, hard magnetic materials maintain nonzero magnetization after the removal of external magnetic fields. The manufacturing of different scales from micrometers to centimeters using magnetic soft composites incorporates various advanced fabrication technologies. [143][144][145][146] AM, specifically SLA and FDM, allows for precise control over the spatial distribution of magnetic particles as well as the overall geometry and structure of microactuators. Effective design ensures that magnetic soft actuators respond predictably to external magnetic fields, achieving desired locomotion and forces. [147,148] In this section, a selection of state-of-the-art studies on magnetic soft composites that utilize different AM and control strategies are reviewed. These methodologies are applied to design, fabricate, and control devices made from magnetic soft composites with their applications in disease treatment.
Bozuyuk et al. developed magnetically powered microswimmers designed for targeted drug delivery. [143] Figure 11(A: a,b) shows that these microswimmers, featuring a double-helical structure with a diameter of 6 μm and a length of 20 μm, could be actuated using a rotating magnetic field. Additionally, they could actively release the chemotherapeutic drug doxorubicin on demand through the application of an external light stimulus. The magnetic microswimmers were fabricated using two-photon 3D printing, which allowed the integration of superparamagnetic iron oxide nanoparticles (SPIONs) into a biodegradable chitosanbased polymer matrix, forming microswimmers with a doublehelical structure tailored for efficient actuation. The actuation of these microswimmers was achieved through a low-amplitude 10 mT, rotating magnetic field (1 Hz -6 Hz), enabling refined control over their movement within fluidic environments. This magnetic steering was critical for navigating the microswimmers to targeted locations inside the body for the precision delivery of the drug. The light-triggered release mechanism involved a UVresponsive photocleavable linker that bound the chemotherapeutic drug doxorubicin to the microswimmer. Upon exposure to UV light, the linker cleaved, enabling controlled drug release at the targeted site. This method not only ensures precise drug dosing but also leverages the biodegradable and biocompatible properties of chitosan, promising significant advancements in targeted drug delivery.
Giltinan et al. tackled the challenge of the limitations of existing magnetic materials used in microrobots, which are either toxic or exhibit weak magnetic responses. [150] By synthesizing and utilizing iron platinum (FePt) nanoparticles, which are both ferromagnetic and biocompatible, a potent alternative was developed for creating clinically viable microrobots. The method was demonstrated by the AM of micro helical robots via the TPP technique. The manufacturing process began with the synthesis of FePt nanoparticles through a modified one-pot method that allowed for large-scale production. These nanoparticles were then incorporated into a biocompatible polymer, trimethylolpropane ethoxylate triacrylate (PETA), during the 3D printing phase. The method allowed for the precise construction of helical microswimmers with embedded nanoparticles, ensuring uniform distribution and strong magnetic properties. Figure 11C shows that the 30-μm long helical magnetic microswimmers could swim at speeds of 180 μm s -1 under a 200 Hz, 10 mT rotational magnetic field. The microswimmers' speeds were tested under various magnetic field strengths from 5 to 10 mT, showcasing their capability to navigate and maneuver efficiently at speeds significantly higher than those achieved by microswimmers using traditional soft magnetic nanoparticles, SPIONs. The tests also confirmed the biocompatibility of the FePt nanoparticles through in vitro cytotoxicity assays, highlighting their safety and efficacy for potential in vivo applications.
To overcome the challenge of manufacturing functional hydrogel structures with complex geometries and prescribed functionalities, Podstawczyk et al. developed magnetic hydrogel actuators using a novel magnetic 3D printing ink. [149] This bionanocomposite, consisting of alginate, methylcellulose, and iron oxide magnetic nanoparticles, enabled the precise creation of a functionally responsive hydrogel matrix. The formulated ink exhibited thixotropic properties, crucial for maintaining the shape fidelity of the printed structures. This behavior allowed the ink to flow under pressure during extrusion and quickly regain viscosity when the pressure was removed, preventing the structure from collapsing during the building process. The magnetic ink was carefully extruded layer by layer, building up the 3D structure as designed in computer-aided design (CAD) software. Each layer's deposition was controlled for thickness and alignment to ensure structural integrity and functional performance. After printing, the structures underwent a post-processing step involving immersion in a calcium chloride (CaCl 2 ) solution to crosslink the alginate, solidifying the hydrogel. This step was essential for achieving the desired mechanical properties and ensuring the hydrogel's stability in aqueous environments. Cuboid samples were fabricated for actuation tests with varying infill densities and patterns, such as grid and honeycomb, to observe different responses based on the structural configuration, as shown in Figure 11B. By changing the proximity and orientation of a neodymium magnet, the directional control and movement of the hydrogels were shown to demonstrate their potential as actuators in devices that require precise and reversible actions.
Makridis et al. developed a novel approach to enhance magnetic hyperthermia treatment through the precise fabrication of magnetic composite filaments using AM techniques. [151] Figure 11D shows how the filaments were created by integrating magnetite nanoparticles with biocompatible PLA and a ferromagnetic PLA variant, utilizing FDM to ensure high precision and uniformity in filament production. The magnetic properties of these filaments were tailored to optimize their responses to external magnetic fields, which is a critical feature for the targeted heating required in hyperthermia treatments which aim to destroy cancer cells without harming surrounding healthy tissue.
The fabrication protocol began with the compounding of magnetite nanoparticles into PLA, followed by the extrusion of this composite into filaments suitable for 3D printing. These filaments were then used to print scaffolds that could be precisely controlled and activated by an alternating magnetic field to achieve localized heating. The effectiveness of these scaffolds in hyperthermia applications was validated through a series of experiments that included structural and magnetic characterization and specific absorption rate (SAR) tests. Structural integrity was assessed using SEM, which provided detailed images of the filament and scaffold microstructures to ensure uniform distribution of magnetite nanoparticles within the PLA matrix. Magnetic properties were analyzed using vibrating sample magnetometry (VSM), measuring the magnetic responsiveness essential for controlled heating in hyperthermia treatments. The SAR, indicating the scaffold's ability to convert electromagnetic energy into heat, was evaluated under varying magnetic field conditions to determine the heating efficiency, critical for effective cancer treatment. These tests collectively confirmed that the 3D-printed scaffolds could achieve and maintain therapeutic temperatures accurately and reliably.
In a study reported by Kim et al., the authors used the DIW approach for 3D printing ferromagnetic domains in soft materials, allowing complex 3D shapes to undergo rapid transformations via magnetic actuation. [21] The ink is an elastomer com-posite containing ferromagnetic microparticles. During printing, a magnetic field is applied to the dispensing nozzle, aligning the particles along the field to establish specific magnetic polarity patterns within the printed filaments. Which enables precise programming of ferromagnetic domains within complex 3Dprinted soft structures, facilitating transformation modes previously unattainable. Among these are remotely controlled auxetic behaviors in mechanical metamaterials, which exhibit negative Poisson ratios. Notably, the actuation speed and power density of these printed materials surpass those of current 3D-printed active materials.
These printed active structures can undergo a variety of controlled deformations when exposed to an external magnetic field. For example, filaments with alternating magnetic domains deform into predetermined shapes, such as bending into an "m" shape, when exposed to a uniform magnetic field. These shape changes are fast and reversible, allowing the material to return to its original form once the field is removed. This rapid response is a direct outcome of the alignment and programming of magnetic particles within the material during the printing process. The study quantifies magnetic actuation by measuring the magnetic moment density of printed samples, showing how variables like NdFeB concentration, applied field strength, and nozzle diameter influence the material's magnetic response. For example, increasing the particle concentration results in a higher magnetic moment density, enhancing the material's responsiveness under magnetic fields. This quantitative control allows for the creation of complex structures with tailored actuation properties, enabling diverse applications.
Compared to other actuation methods, such as shape-memory polymers or hydrogels that typically require heat or direct contact, magnetically actuated soft materials offer the benefit of remote, contactless actuation. This feature is particularly useful in constrained environments, such as biomedical contexts, where mechanical interaction may be impractical or infeasible. Moreover, the power density and actuation rate of these printed materials exceed those of conventional shape-transforming materials, offering a faster response and greater reliability in dynamic applications.
Their study provides solutions to a range of functional applications, including reconfigurable soft electronics, a mechanical metamaterial that can jump, and a versatile soft robot capable of crawling, rolling, catching fast-moving objects, and transporting pharmaceutical doses. The AM method can be adapted to various composite inks with different elastomer and hydrogel matrices and magnetic particles. By printing ferromagnetic domains in soft materials, the study introduces new design parameterssuch as domain patterns, magnetization strength, and actuation fields-into the creation of shape-programmable soft materials. The remote actuation of these untethered, complex, and fast shape-shifting materials through magnetic fields opens new [143] copyright 2018 American Chemical Society. (B) reprinted with permission from ref., [149] copyright 2020 Elsevier B.V. (C) reprinted from ref., [150] licensed under CC BY 4.0. (D) reprinted from ref., [151] licensed under CC BY 4.0. possibilities for applications in flexible electronics, biomedical devices, and soft robotics.
Among the different magnetic material-based applications, magnetic sensors play a crucial role in detecting magnetic fields which are essential to many real-life applications. [152,153] The information gathered via magnetic sensors, which record variations in magnetic fields, can be used to track objects' locations, orientations, rotations, angles, and the existence of electric currents, among other parameters. Because of this, magnetic sensors are widely used in robotics, space technology, automotive technology, geophysics, as well as other industrial applications. Various technological needs across these applications are driving an increasing demand for durable, multipurpose new magnetic sensors. [152,154,155] Generally, sensors can be produced through both conventional manufacturing techniques and modern AM methods. AM offers greater flexibility in product customization and the ability to create complex designs. Additionally, AM presents safety benefits by minimizing exposure to harsh and potentially unsafe work environments compared to traditional manufacturing methods. [156] As a result, AM-processed magnetic sensors are increasingly being adopted across various industries, including robotics, electronics, biomedical engineering, and energy and environmental applications. This innovative approach not only enhances the performance and efficiency of magnetic sensors but also opens up new possibilities for their use in advanced technological applications. [157]
AM has revolutionized sensor applications in human-computer interactions (HCI) by enabling the creation of highly customizable and efficient sensors. These innovations significantly enhance HCI interfaces by improving precision, functionality, and user experience. Notable applications include wearable sensors that monitor physiological parameters for healthcare and fitness, prosthetics and exoskeletons with embedded sensors that translate muscle movements into actions, and tactile feedback devices that enhance virtual and augmented reality experiences by providing haptic feedback. [10,[158][159][160] Additionally, AM-processed sensors in robotics enable more precise and adaptable interactions with their environment, while assistive devices like Braille readers and speech-generating tools offer customized support for individuals with disabilities. [161] For example, Zhang et al.'s work described the AM of selfpowered magnetic sensor gloves for underwater and smoke environmental HCI using a homemade multi-extrusion-head manipulator. [159] To manufacture these flexible magnetoelectric sensor gloves, multiple ink types are often required, each needing different curing techniques.
The fabrication of the self-powered magnetic sensor gloves involved a multi-step process using three different types of specially developed inks. First, the glove-shaped base was printed using elastic ink, which was immediately cured by UV light as it was extruded. Next, conductive coils with a 12 mm diameter and related circuits were printed on four fingers using liquid metal (LM) ink, protected by a nitrogen gas environment. Following this, the magnetic component, a 12 mm square, was printed on the thumb using magnetic ink. Finally, a second layer of elastic ink was printed over the entire glove to seal the conductive parts, resulting in a fully wearable glove produced through this integrated DIW process. The elastic ink, made from UV-curable 50A resin, was transparent and flexible, with excellent curing, bending, and stretching properties. Vapor silica was added to improve its rheology. The conductive ink was a mixture of PDMS pre-polymer and LM, which had excellent deformability, low toxicity, and low resistance, but required adjustment to reduce surface tension for DIW. This mixture was cured by infrared heating under nitrogen protection. The magnetic ink was composed of NdFeB, a strong magnetic material, mixed with a PDMS pre-polymer. This combination resulted in good rheological properties and could also be cured using infrared radiation, making it suitable for creating the magnetic components of the glove.
Figure 12A,C depict the multi-extrusion-head manipulator that they used to create a variety of materials, such as magnetic, conductive, and elastic components. Six jaws-three syringe needles, an IR laser transmitter, a UV light transmitter, and a nitrogen protection gun-were uniformly spaced, as seen in Figure 12B. The robot's six degrees of freedom allowed each claw to move independently in space under the direction of the programmed software. The intricate 3D printing of a glove and its HCI in an invisible environment is depicted in Figure 12D.
As shown in Figure 13A,B, the optical images depict a printed glove securely attached to the volunteer's hand. By clicking the fingers, the self-powered gesture recognition system was activated. Its self-powered HCI operated based on Faraday's law of electromagnetic induction, which is seen in Figure 13C, and its behavior was simulated by the Ansys Maxwell software (Figure 13D). The magnetic portion was printed on the polymeric region of the thumb, and four conductive coils were printed on the remaining four fingers. The 3D-printed glove could be used for HCI in smoky and underwater conditions without experiencing sealing issues because of the integrated manufacturing (see Figure 13E-H). Moreover, to accomplish sophisticated HCI operations, the nine-key alphabetic typing input method keys could be mapped to the distinctive electrical signals of various movements. Their research specifically sheds light on how to build human-computer interfaces in harsh conditions for printed gloves with integrated manufacturing elements.
Giant magnetoresistive (GMR) devices are one type of spintronic device that has been widely used as magnetic sensors in the industry and biomedical areas. [155,162] GMR sensors are made of alternating metallic multilayers of ferromagnetic and nonmagnetic materials, which exhibit a significant change in electrical resistance while exposed to any magnetic field. This giant electrical resistance change is known as the GMR effect. [163,164] The giant change in the resistance of the GMR devices below Curie [159] copyright 2023 Elsevier B.V.
temperature is explained through the Mott two-channel model. The Mott two-channel model explains that if the spin direction of the ferromagnetic (FM) layers and the electron spinning directions are opposite, it causes the scattering of the conduction electrons while passing through the FM layers, which contributes to the increment of the resistance. Based on the relative alignment of the neighboring FM layers' magnetizations, the GMR ratio is calculated based on the scattering of the conduction electrons. For instance, if the FM layers are aligned parallelly, there is less scattering of the conduction electrons, which causes a small change in the resistance R p , and if they are aligned antiparallelly, there is significant scattering of the conduction electrons causing high resistance R AP ; the GMR ratio is then calculated using the ratio of change of the resistivity, R AP -R P , divided by R p . Several applications can benefit from the GMR effect, for instance, noncontact stress [165,166] and position sensing, [167] biosensing, [168][169][170] and hard disks, [171] etc.
Conventional GMR sensors have been fabricated on the rigid Si wafer, limiting their use for versatile applications. By con-trast, the current deposition method for GMR sensor manufacturing using flexible and bendable substrates has opened doorways for magnetoelectronics. For instance, Ha et al. reported the first flexible GMR multilayered [Py/Cu] 30 micro-flakes printed on ultrathin foils capable of withstanding bending radii of up to 16 micrometers without degrading performance. [172] The adhesion of micro-flakes to the styrene-butadiene-styrene (SBS) matrix allowed for deformation at low bending strain, resulting in superior bending ability. Under severe bent states, energy was dissipated by slippage at the interface between micro-flakes and the SBS matrix, preventing mechanical fracture and crack propagation of micro-flakes. Their reported sensitivity was 3 T -1 in a low magnetic field of 88 mT.
Furthermore, Karnaushenko et al. reported the first printable GMR sensor. They developed a magnetic ink with GMR micro-flakes, which could be used for printing on any substrate, such as paper, polymer, or ceramic. [173] Moreover, they also demonstrated contactless switching through the integration of a printable GMR ([Co/Cu] 50 ) sensor into an electrical circuit, as [159] copyright 2023 Elsevier B.V. [174] licensed under CC BY-NC 4.0. (B) reprinted with permission from ref., [173] copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
illustrated in Figure 14B. The change in resistance of the printable GMR sensor altered the open/close state of the transistors, thereby regulating the current flow in the circuit as the GMR sensor related to the transistor in the amplification cascade. The resistance in the GMR sensor was changing due to the permanent magnet that was attached to the postcard. When the postcard was closed, the LED was off Figure 14(B: a); when it was opened, the LED was turned on, as shown in Figure 14(B: b). Also, this fabricated sensor operated at room temperature and exhibited a GMR change of up to 8%.
Later, Karnaushenko et al. also achieved further improvements in printing high-performance flexible magnetic sensors, reporting an electrical resistance change of up to 37%. [174] Their GMR sensor was prepared by regular brush painting and GMR paste to a polyimide-based flexible printed circuit (FPC) board. In Figure 14(A: a), the array of GMR sensors is realized by the GMR paste printed using a brush onto the FPC board, and Figure 14(A: b) exhibits the effect of bending on the GMR ratio. Moreover, these flexible GMR sensors could bend up to a bending radius of 12 mm without sacrificing sensitivity, and importantly these bending radii were limited by the thickness of the available FPC boards, not the sensor itself. Hence, the use of thinner, flexible foils in these sensors significantly improves the mechanical stability of GMR sensors.
Traditionally, spintronic devices, such as GMR, are fabricated using thin-film technologies that are expensive and dependent on nanofabrication facilities, often unavailable in underdeveloped regions. However, recent studies reviewed in this sub-section demonstrate that spintronic devices can also be fabricated using low-cost, accessible printing methods. In these examples, the magnetic sensors were transferred to various substrates through brush painting, but other methods, like spray coating and screen printing, can also be used. These pioneering works pave the way for future AM of spintronic devices on diverse substrates.
Modern cars are powered by a diverse array of technologies categorized into three main areas: infotainment, safety, and powertrain. Currently, the auto industry is heavily focused on enhancing ergonomics and vehicle electronics to meet consumer demands and prepare for emerging applications. One significant trend in the automotive sector is the increasing demand for flexible printed sensors, driven by their rapid manufacturing capabilities, flexibility, and ability to accommodate intricate geometries. Despite their potential, there has been limited research on optimizing capacitive flexible printed sensors to improve overall performance metrics. Notably, the design and production of suitable flexible sensors for automotive infotainment systems remain underexplored areas. Addressing these gaps can lead to substantial advancements in the performance and integration of flexible sensors in modern vehicles, ultimately enhancing user experience and vehicle functionality. [175] Elaskri et al. explored AM technology in the development of cutting-edge DC electric motors. [176] Yet, no fabricated device with electromechanical capability has been developed from parts that are made solely utilizing AM technology. With an emphasis on an iron core, winding, insulation, and a permanent magnet that were all additive-manufactured by different methods including laminated object manufacturing (LOM) and direct metal laser sintering (DMLS), they reported a DC electric motor where the only parts that were not 3D printed were the wire coils. On the other hand, copper foil, which can be manufactured using the LOM process, can be used in place of the coils. The pancake motor armature winding was shaped by the Circuit Explore Air 2 machine. The radial and axial flux DC electric motor components were all made using the above-mentioned AM methods, and their functionalities suggested that a fully AM-processed motor might be produced with reasonably priced, widely accessible machinery. Even though the motors in these samples were constructed manually, the systems show what may be accomplished with a 3D printer that can print in multiple materials utilizing various technologies. One exciting prospect for the use of such a device in the future is to aid in the development of a self-replicating machine, which is seen to be an essential instrument for space exploration.
Rare-earth elements like Nd, Sm, Dy, Pr, and La are extensively incorporated into magnetic materials, especially permanent magnets, to enhance their magnetic properties. However, the increasing demand for these magnets brings challenges related to the cost and environmental impact of scalable magnet production. [177][178][179] Concerns about the availability of rare-earth elements, difficulties in extraction, and their strategic and unstable supply origins also raise national security issues. Additionally, operational temperatures and magnetic energy density are crucial for permanent magnet-based applications. The rising cost of rare-earth metals exacerbates these issues, leading to a shift in the magnetic materials community toward developing new magnetic materials free from rare-earth elements. [180,181] These rare-earthfree materials, with high magnetocrystalline anisotropy and energy products, show great potential as permanent magnets. [182] Key types of these materials include Alnico magnets, composed of Al, Ni, and Co, known for their high magnetizations and temperature stability. [183] Ferrite magnets, made from iron oxide mixed with barium (Ba) or strontium (Sr), are widely used due to their low cost and corrosion resistance. Mn-based magnets, such as Mn-Al and Mn-Bi, exhibit high magnetocrystalline anisotropy, making them suitable for permanent magnet applications. [184] Fe-Co-based magnets combine Fe and Co to deliver high saturation magnetization and thermal stability, ideal for aerospace and military technologies. [185,186] Emerging Fe-N and Fe-P magnets show potential due to their high magnetization and outstanding mechanical properties. [187][188][189] Additionally, nanocomposite mag-nets, consisting of nanoscale grains of magnetic materials, offer high-energy products through the interaction between hard and soft magnetic phases. These diverse types of rare-earth-free magnets are crucial for advancing technology while mitigating the reliance on scarce rare-earth elements. [183] AM allows for precise control over the microstructure of these materials, optimizing their intrinsic properties, such as saturation magnetization (M s ), magnetic anisotropy (K), and Curie temperature (T c ), as well as extrinsic properties like remanent magnetization (M r ) and coercivity (H c ). This control results in improved magnetic performance, making these materials suitable for various applications, including renewable energy technologies like wind turbines and wave power buoys, as well as other sectors such as transportation and medical devices. [190] In Adrianna Kania et al.'s research, using electron microscopy, spectroscopy, and magnetometry, stable magnetic pastes consisting of nanocomposite materials-materials that are ideally suited for robocasting technology-were created by the DIW method and characterized. The magnetite (Fe 3 O 4 ) nanoparticles (see SEM/EDS analysis in Figure 15A) were added to a polymeric matrix, such as silicone gel or PEG-PVB (polyethylene glycolpolyvinyl Butyral), to create the nanocomposites. [191] By adjusting the volume proportion of magnetic nanoparticles and the kind of polymer matrix (sample index: S20F (20 wt.% of Fe 3 O 4 in a silicone matrix), P20F (20 wt.% of Fe 3 O 4 in a PEG-PVB matrix), and S20F-MGN), each composite sample demonstrated a strong ferromagnetic response in the form of printouts and provided control over magnetic characteristics. As a result, they could be used to create solid objects with intricate shapes. Figure 15B shows the photographs of the manufactured P20F cuboid, S20F cuboid, and S20F-MGN cone.
Small, thin layers are vulnerable to deformations brought on by the magnetic field-associated particle arrangement during the production process when a magnetic field is applied. Quickhardening materials with thicker layers will exhibit significantly less distortion when printed in the applied external field. To prevent geometric distortion and investigate greater control over particle arrangement, matrices with quick hardening after extrusion are still required. The precise geometry and magnetic characteristics of the printed pieces are found to be influenced by rheological and polymer matrix alterations. Specifically, these authors demonstrated that the coercive field of the nanocomposites based on the silicone matrix with higher viscosity was smaller than that of the PEG-PVB polymer matrix, measuring 10.7 mT -12.6 mT as opposed to 15.1 mT -20.9 mT, respectively. Stronger interparticle interactions were indicated by relative differences as great as 40%. The samples based on a silicone matrix that was printed in a magnetic field with the highest concentration of Fe 3 O 4 particles-60%-had the highest remanence value (M r /M s = 0.143), as expected. The samples printed in the magnetic field had the highest remanence value (M r /M s = 0.185) among those using a PEG-PVB matrix. Compared to PEG-PVB composites, silicone-based composites had a squareness ratio that was ≈20-25% lower (Figure 15(C: a-f)).
The produced pieces' shape anisotropy was shown for both types of polymer matrices. When it comes to rheological qualities, which determine a material's overall printability and its capacity to solidify at the appropriate time to guarantee the best print quality possible, the concentration of the nanoparticles is [191] licensed under CC BY-NC-ND 4.0.
crucial. Because of the effects of filament extrusion and composite synthesis on particle distribution and interparticle interactions, the volume fraction of magnetic particles in the polymer matrix has a major impact on the magnetic properties and weak deterioration of the effective response in a 3D printed particlepolymer composite.
The recent advancements in AM techniques, especially in manufacturing magnetic materials and devices, have triggered an increasing list of applications. This review provides peers with a summary of various AM methods, along with examples of these AM-processed magnetic materials and devices for applications in different fields.
In Section 2, we have covered the ink-based and ink-free AM processes. Ink-based AM such as DIW, FDM, AJP, and BJP technologies, deposits functional inks to create complex magnetic materials and devices. Whereas ink-free methods, such as SLA, DLP, PBF, and DED, use a light source to polymerize the photosensitive polymer or thermal energy to fuse materials for highresolution designs. Each method offers unique benefits and limitations in terms of precision, material compatibility, and magnetic performance to fabricate advanced magnetic devices.
In Section 3, we have discussed the integration of magnetic particles in energy-harvesting devices fabricated via AM tech-niques, focusing on TE, piezoelectric, and mechanoelectrical devices. For TE applications, embedding magnetic particles like Fe, Co, and Ni, enhances TE performance by improving electron and phonon transport. For example, screen-printed Fe/BST/epoxy films show significant improvements in power factor due to the controlled deposition of magnetic materials. In piezoelectric devices, combining magnetic and piezoelectric properties enables higher energy conversion, as demonstrated in a 3D-printed sensing device with CoFe 2 O 4 particles that can detect magnetic fields and harness magnetic energy. Mechanoelectrical devices generate electricity when magnetic structures deform under stress and can be fabricated using different AM methods. Examples include soft materials with variable Poisson ratios and bioinspired devices that generate electricity from raindrop interactions.
Magnetically assisted devices fabricated with AM approaches are promising candidates for water decontamination and detoxification. In Section 4, we have reviewed recent research in self-propelled microswimmers that can be magnetically guided within the volume of water to be treated, which reduces the amount of time required for the contaminant adsorption/degradation due to the presence of active mixing, in comparison to static materials that rely only on passive mixing via diffusion for contaminant removal. Moreover, magnetic induction has been proposed to increase the temperature of the solution via contactless heating when magnetic materials are incorporated for the water treatment process within the solution, achieving a Fe/BST/epoxy flexible thermo-electromagnetic films Screen printing TE generators [85] Self-powered sensing device combining a piezoelectric polymer with CoFe 2 O 4 nanoparticles/wax composite Screen printing and molding Magnetic field sensing and energy harvesting [128] Magnetic soft material composed of Nd 2 Fe 14 B particles-TPU composite with variable Poisson ratios SLS Mechanoelectrical conversion devices [130] Lotus-leaf-like magnetic top composed of Nd 2 Fe 14 B particles-TPU composite and a stem-like elastic bottom FDM and poly-jet printing Mechanoelectrical conversion devices for raindrop interactions [131] Self-propelled, catalytic fish swimmers, the magnetic ink is based on Ni and Pt Screen printing Magnetically guided, self-propelled devices for pollutant adsorption and degradation [133] Micro-fishes Continuous optical printing Melittin sensing and detoxification [22] Microdevices coated with four functional layers to provide the following characteristics: magnetic actuation, photocatalytic, and anti-microbial properties SLA Water cleaning [138] Zero-valent iron (ZVI) coated with polylactic acid (PLA) FDM TC degradation via magnetically assisted persulfate activation [134] 3D monoliths from a powder blend of iron oxide and PLA FDM Catalyst for antibiotic degradation [137] Rotating-disk sorptive extraction devices SLA Simultaneous extraction of 14 trace metals [136] Magnetically powered microswimmers featuring a double-helical structure with a diameter of 6 μm and a length of 20 μm TPP Targeted drug delivery, release the chemotherapeutic drug on demand through the application of an external light stimulus [143] 30-μm long helical magnetic microswimmers (FePt nanoparticles embedded in a PETA) TPP Biocompatible microswimmers for in vivo applications. [150] Magnetic hydrogel actuators (alginate, methylcellulose, and iron oxide magnetic nanoparticles embedded in hydrogel matrix) Direct printing Actuators [149] Scaffolds that contain magnetite nanoparticles embedded in biocompatible PLA FDM Magnetic hyperthermia treatment [151] Complex 3D-printed soft structures with ferromagnetic microparticles embedded in soft materials DIW Soft robotics [21] Flexible magnetoelectric sensor gloves, where the magnetic ink is composed of NdFeB embedded in PDMS pre-polymer DIW Self-powered magnetic sensor gloves for underwater and smoke environmental HCI [159] Iron core, winding, insulation, and a permanent magnet LOM and DMLS DC electric motors [176] Magnetic pastes consisting of Fe 3 O 4 in a PEG-PVB matrix DIW Rare-earth-free magnets [191] better kinetic rate of pollutant degradation as well as improved recyclability in comparison to conventional, thermal baths. Additionally, these devices have been manufactured using multiple materials and can provide different functionalities within the system (controlled propulsion and directionality, pollutant adsorption or degradation via photocatalysis, anti-bacterial properties, biocompatibility, fluorescence emission as indicator of decontamination progress, etc.). Magnetic soft composites are materialsconsisting of ferromagnetic particles embedded inside a soft matrix, such as elastomers. These composites, characterized by their pliability and rapid reaction to magnetic stimuli, are well-suited for use in robotics and medical devices. These materials can be fabricated using AM processes, allowing for exact manipulation of their magnetic characteristics and structural design. In Section 5, we have included some examples of these systems including drug-delivery microswimmers, biocompatible microrobots, and magnetic actuators. AM of magnetic devices also enables the development of functional and adaptable devices in medical treatments and fluid manipulation systems.
In Section 6, we have focused on the applications of AMprocessed magnetic materials for various established industries. The ability to classify and manipulate different types of magnetic materials has been key to enhancing performance, particularly in magnetic sensors, which are vital for the precise detection and measurement of magnetic fields in diverse technological fields. The integration of AM has revolutionized the production of magnetic sensors and materials, allowing for the creation of complex, customized structures that meet specific technological needs. This has led to innovative applications, especially in human-machine interactions and flexible electronics, where GMR sensors on flexible substrates offer high sensitivity and durability. Furthermore, AM has been employed to fabricate rareearth-free magnets, offering precise control over the microstructure of these materials to enhance their intrinsic properties. This level of control improves the magnetic performance, making these magnets ideal for a wide range of applications, including renewable energy technologies such as wind turbines and wave energy converters, as well as in industries like transportation and healthcare.
In Table 3, we have summarized all the AM-processed magnetic materials and devices that are reviewed in Sections 3-6.
AM provides several advantages in the fabrication of magnetic materials and devices, facilitating innovation across multiple fields. It enables complex geometries that optimize magnetic designs and allow for strategic placement of costly or rare materials, reducing waste. It allows precise control over microstructure and grain orientation, enabling customization of magnetic properties for specific applications. Additionally, AM supports rapid prototyping, speeds up design iterations, and integrates multiple functions into single structures, streamlining assembly and lowering material usage. Importantly, as many magnetic materials are toxic, AM can improve safety by reducing exposure to hazardous substances.
However, challenges remain. AM systems often have size limitations, restricting the scale of components that can be produced. Post-processing is usually necessary to achieve desired properties, as AM can introduce stresses and inhomogeneities that degrade performance. Layer-by-layer construction may cause inconsistencies in magnetic properties, impacting reliability in critical applications. Not all magnetic materials are AM-compatible, and some exhibit altered properties when printed, requiring further research. High precision is crucial for applications in energy, healthcare, and sensors, but achieving this with AM is complex and costly. Scalability to industrial demand and the energyintensive nature of some AM processes also pose sustainability challenges.
Nonetheless, AM offers unique opportunities, such as the development of highly specialized applications, including personalized medical devices and innovative energy-harvesting systems. The material efficiency enabled by AM helps reduce waste and enables the production of sustainable, rare-earth-free magnets. In environmental applications, AM allows for the fabrication of multifunctional devices that combine magnetic properties with additional functionalities, like photocatalysis or antibacterial action, enhancing their utility. These fabrication techniques can also advance industries such as sensor technology, energy, transportation, and robotics by supporting the development of highperformance magnetic sensors and reducing reliance on rareearth materials, contributing to more cost-effective and sustainable solutions.
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