Magnetically-assisted digital light processing 4D printing of flexible anisotropic soft-Magnetic composites

ABSTRACT Flexible anisotropic soft-magnetic composite (FASMC) presents superior magnetic properties in one or more specified directions, showing great potential in the application of microwave absorption, soft robots, and other smart sensors/actuators. However, the fabrication of FASMC using additive manufacturing is challenging due to a trade-off between magnetic properties of the composites enhanced by iron particles and printability during printing. Here, we developed a 4D printing scheme using flexible soft-magnetic photosensitive resin consisting of flexible long-chin acrylic resin monomer and soft magnetic iron particles. Multiple complex structures with good spatial resolution of ∼170 μm were fabricated using magnetic field-assisted digital light processing (MF-DLP). Directional magnetic field was applied during printing, enabling the fabrication of FASMC with strong anisotropic magnetic properties. FASMC with high CIP (carbonyl iron powder, CIP) concentration of up to 45 wt.% was fabricated with excellent tensile strength and elongation up to 460%. Strong anisotropic magnetic properties were demonstrated through a series of stimuli-response testing such as large deformation, anti-deflection, controlled motion, variable stiffness metamaterial, and array assembly, under external magnetic field. This study demonstrates the feasibility and potential of MF-DLP technique for fabrication of FASMC, shedding light on the design and fabrication of next-generation sensors and actuators.


Introduction
Photosensitive resin and process development in 3D printing of functional materials are of great interest to the additive manufacturing community to achieve higher printing resolution, strength with reliable functionalities.Projection microstereolithography based 3D printing technique has been used with combined with nanoscale coating and postprocessing to fabricate ultralight and ultrastiff micro-lattices [1].Hollow-tube nickelphosphorus and aluminium oxide three-dimensional structures with ∼5 μm resolution and ∼40 nm thick truss shell have been achieved.Stimuli-responsive structures such as piezoactive metamaterials [2][3][4], selfmorphing structures [5,6] and shape reversible structures [7] have been fabricated and studied.However, traditional 3D printed stimuli-responsive structures usually can only perform one-way actuation, while field-assisted 3D printing could enable two-way actuation [8].Lu et al. did the early work on magnetic field-assisted DLP.Nano/ micro-sized ferromagnetic particles were aligned using an external magnetic field.Smart materials with complicated and heterogeneous properties were fabricated.Li et al. [9] fabricated a bioinspired painless microneedle array using magnetic field-assisted DLP.The alignment of iron oxide particles could provide anisotropy and enables the fabrication of microneedles with sharp and long features as well as good mechanical properties.Ma et al. [10] aligned short steel fibres in resin matrix via magnetic assembly during DLP process.The mechanical properties and friction behaviour show directionally dependent anisotropy.Safaee and Chen [11] used magnetic field-assisted DLP to fabricate functionally graded composites with embedded magnetic particles.
Flexible magnetic composites have attracted great research interest in recent years due to their high stimuli-response efficiency and geometric compliance, which are promising for used as smart materials in soft robots [12][13][14][15], biomedicine [16], bionics [17,18], and reinforcement materials [19,20].Flexible magnetic composites are usually prepared by embedding magnetic particles (neodymium iron boron (NdFeB), barium ferrite, strontium ferrite, Fe 3 O 4 and CIP particles, etc.) into a flexible polymer matrix (silicone rubber, polydimethylsiloxane (PDMS), polyurethane (PU) and hydrogel, etc.) [12,[21][22][23].The magnetisation process is indispensable for hard magnetic material system, endowing structure with permanent magnetisation in a specific orientation before being used [24][25][26][27].The magnetised hard magnetic samples have a fixed magnetisation orientation, so they can perform a series of complex stimuli-response under external magnetic field [28].However, it's difficult for soft magnetic composites such as Fe 3 O 4 or CIP reinforced flexible composites to achieve such high-actuating capability, despite of advantages of high permeability and easy magnetisation.The low coercivity of soft magnetic particles makes them lose magnetisation at the absence of external magnetic field due to random orientation of the internal magnetic dipoles, making it difficult to gain unidirectional magnetic properties within the composites.Therefore, further research on flexible anisotropic softmagnetic composites fabrication is of great need.
Both geometric and magnetic anisotropy could be utilised to gain magnetic anisotropy in soft magnetic composites [26,29,30].Geometric method is to design structures with a high aspect ratio, containing randomly distributed soft magnetic particles.However, spatial design space is usually limited for practical applications.Magnetic anisotropy could be achieved using external magnetic field to arrange the magnetic particles during fabrication, forming magnetic particle chains inside the matrix material [20,[31][32][33].In this way, the anisotropic magnetic performance of the structure is not limited by its geometry.For example, Kuhnt et al. used rotating magnetic field to align ellipsoidal Fe 3 O 4 magnetic nanoparticles in hydrogel during DLP 3D printing for fabrication of magnetic anisotropic hydrogel [16].Magnetic particle content inside this magnetic hydrogel was severely limited to a low level (5 wt.%), also resulting in limited mechanical properties.In summary, developing reliable magnetic resin for magnetic field-assisted DLP process and improving the performance of flexible magnetic anisotropic composites with high solid content remain a challenge.In addition, the actuation of flexible magnetic composites has yet been well studied.
In this work, we developed a novel magnetic photosensitive resin, and fabricated FASMC with excellent anisotropic magnetic properties, flexibility, and stretchability, using the developed magnetic field-assisted DLP 4D printing process.A flexible and reliable magnetic resin was developed, enabling printing parts with high flexibility as well as strong adhesion to the metal printing substrate.A directional magnetic field was applied during the printing to align CIP within the liquid resin.CIPs with a high permeability enable the stimuli-response to the external magnetic field during printing.Then, MF-DLP process printed flexible complex samples were tested against their anisotropic magnetic properties and mechanical properties.Finally, FASMC with highly-oriented magnetic particles and multi-functionalities ware fabricated and demonstrated.
Preparation of magnetic photosensitive resin: Firstly, CIPs (5-45 wt.%), 2 wt.%TPO, 1 wt.%BYK 163, and 2 wt.%Disparlon 6900-20X were added to 4-HBA (50 g) and mixed thoroughly with mechanical stirring for 5 min at a speed of 500 r/min.Second, 4 wt.%fumed silica was added to the precursor solution and stirred further for 20 min until forming a smooth liquid surface.Then, the as-prepared magnetic resin was vacuum degassed and sealed for storage in a dark environment.Figure s3 shows that the magnetic resin exhibits good dispersion and stability, and there was no obvious precipitation within 24 h.
Magnetic field stimuli-response of FASMC: For jumping tests, an N35 cylindrical magnet was placed at a distance of 30 mm above the cubic FASMC sample whose size is 5 mm* 5 mm* 5 mm.Then, the magnet was moved toward the sample at a speed of 120 mm/min until the sample was attracted to the magnet.For the large deformation tests using an external magnetic field, one end of the cubic FASMC samples of 20 mm* 20 mm* 2 mm was fixed and the other end is freely suspended.The magnet moves vertically toward the sample from a distance of 90 mm at a speed of 120 mm/min, until the moving distance reached 45 mm.In this process, a high-speed monochrome CCD camera (BFS-U3-50S5M-C, FLIR, USA) was used to record the sequential deformation of the sample with a sampling frequency of 30 Hz.A tesla metre (TD8620, Tianheng, China) was used to measure the magnitude of the magnetic field.For controlling deflection tests of the solid vase, as shown in Figure 5, a cylindrical magnet (10 mm diameter and 10 mm thickness) was located below the solid vase and fixed on a customised three-axis moving system.The solid vase was supported and isolated by a polyvinyl chloride plate (PVC, 2 mm thickness).The pre-designed moving paths were imported into control software to realise the programmed motion of the solid vase controlled by the magnet.For rotation tests actuated by the external magnetic field, as shown in Figure s9, rotation of the magnetic field was generated by two rectangular N52 magnets (64.5 mm*54.3mm*36.2mm) mounted in parallel, and the FASMC samples were supported in the middle.Two types of magnet motion, rotation and translation, were used as driving modes.All experiments were conducted at least three times to ensure the reliability and accuracy.
4D printing of FASMC: As shown in Figure 1a,b, the MF-DLP printing scheme installs two N52 permanent magnets on both sides of a resin tank such that a relatively uniform magnetic field was formed in the effective printing area, where CIPs could be aligned into chains along the magnetic lines during printing.Exposure time was set to 12 s and the layer thickness was set to 50 μm.The magnetic field is activated until the entire part has been printed.
All samples were post-processed under UV radiation for 30 min at a temperature of 60°C.
Characterisation and mechanical testing: The viscosity of the resin composite was measured by a dynamic shear rheometer (Kinexus Pro+, Malvern Instruments Ltd., UK) with a shear ratio ranging from 0.1 to 100 s −1 at a temperature of 25 °C.The microstructure and surface morphology were characterised by a field emission scanning electron microscopy (FESEM) (SU8010, Hitachi, Japan) at an acceleration voltage of 3 kV, which was equipped with an energy dispersive X-ray spectroscopy (EDS) for analysing the elementary composition of the FASMC.The magnetic properties of the FASMC composites with a dimension of 2*2*2 mm were used for the vibrating sample magnetometer (VSM) (Lakeshore 7404, LakeShore, USA) at room temperature.Magnetic hysteresis loops were recorded in a magnetic field range from −1.5 T to 1.5 T. Anisotropic magnetic properties along and perpendicular to the chain alignment direction of the FASMC were measured.The matter phase patterns were recorded by X-ray diffraction (XRD) (Bruker D8 Advance, Germany) with the diffraction angle from 10 to 90 o with an increment step of 0.15.Tensile tests were performed using an electronic universal testing machine (WDW-02, Shijing Corp, China) with a load cell of 200 N and at a speed of 50 mm/ min.Each test was performed at least five times and the average value was calculated.

Results and discussions
3.1 MF-DLP 4D Printing of FASMC composites MF-DLP printing was conducted on a lab-made DLP 4D printer (the principle was shown in Figure 1a and the equipment was shown in Figure s1, the Supplementary information).In this printer, one permanent magnet was installed on each side of the resin tank with a distance of 240 mm to generate a uniform magnetic field in the central printing area (Figure 1b) with an average magnetic field strength of 11.6 mT.CIPs can be aligned into chains with the activated magnetic field.The distribution of the magnetic field in the printing area (① in Figure 1b) was measured using a Gauss metre with a scanning speed of 50 mm/min.Measured and simulation results are shown in Figure 1c.The maximum magnetic field fluctuation along the centreline is about 1.1 mT, which is evaluated against the printing quality (Figure 1f, h-k).
The magnetic photosensitive resin developed in this work was made by embedding magnetic CIPs (∼7 μm for diameter) in an elastomeric matrix (Figure 1d).The soft elastomeric matrix consists of acrylic monomer 4-HBA, ensuring high stretchability and flexibility due to its long-chain structure with a highly reactive hydrophilic OH group at the end of chains [34,35] (Figure s2, the Supplementary information).Upon exposure to the ultraviolet radiation with a wavelength of 405 nm, a significant quantity of free radicals was generated by the TPO which initiates the process of self-crosslinking polymerisation of 4-HBA monomer molecules.The self-crosslinking polymerisation was described in Figure s2 in the Supplementary information.The high CIP density of 7.8 g/cm 3 could cause severe sedimentation and agglomeration in liquid resin.To solve this issue, hydrophilic fumed silica was added to slow the sedimentation of CIPs while not sacrificing the viscosity (Figure 1e and Figure s3, the Supplementary information).Figure 1e shows that all magnetic resin exhibit shear thinning behaviour at room temperature, which could be contributed to the chainlike structure of fumed silica with a large number of isolated silanols [36,37].The addition of CIPs will disturb the connectivity between hydrogen bonds, resulting in a decreasing viscosity compared with pure resin.
CIPs have strong UV absorbability [38] that would hinder the layer curing process.Figure s4 in the Supplementary information shows the effects of exposure time on the curing depth and printing accuracy of magnetic resin with different CIP concentration.The curing depth increased with the increase of exposure time of UV light due to higher cross-linking density.For high CIP concentration of up to 45 wt.%, a curing depth of 78.49 μm was obtained (Figure s4a, the Supplementary information), indicating excellent curing performance.In addition, magnetic resin with 45 wt.% CIP concentration was used to print several simple structures (Figure s4b).The exposure time was fixed at 12 s.Geometric accuracy achieved were 99.75%, 99.20% and 99.70% for these three simple structures, respectively, exhibiting good printability.
The limited size of magnets could lead to high magnetic field gradient in the printing zone, resulting in pushing the magnetic particles towards the magnetic poles.Thus, a smaller magnetic field gradient could ensure a better alignment.Figure s5 in the Supplementary information presents distribution patterns of CIP particles with 5 and 45 wt.% CIP concentration under different magnetic strength.For both concentrations, good alignment was obtained for a magnetic field of ∼12 mT.Further increase the magnetic field could drive the particles to the magnetic poles.Thus, the distance was fixed at 240 mm (generate magnetic field about 12 mT) for all sample during printing.A list of optimised printing parameters was summarised in Table 1.
Under the activation of magnetic field, CIPs could be aligned in liquid resin and cured during printing, as shown in Figure 1f.The morphology and distribution of CIPs on the top and side surfaces of the sample (Figure 1g) with a CIP content of 45 wt.% were shown in Figure 1h-k.It can be seen that CIPs are distributed uniformly, and no aggregates were formed, showing good dispersion of the CIPs.Moreover, the EDS results show that the main content elements in FASMC are C, O, Si, Fe, and N. The aligned iron particles can be observed.The X-ray diffraction confirmed the presence of CIP phase and SiO 2 phase in the FASMC composites (Figure s6, the Supplementary information).

Printability and mechanical properties of FASMC
Several complex structures such as diamond lattice, minimum surface lattice, pinecone, and dragon were fabricated using a high solid content magnetic resin up to 45 wt.% CIP concentration (Figure 2a).The results show that the proposed MF-DLP technique can print complex structures with fine features, also maintain high flexibility.The feature resolution is at ∼170 μm level, which is suitable for most applications (Figure 2b).FASMC presented an outstanding load-carrying capacity, about 5000 times (500 g to 0.0103 g) compared to its own weight (Figure 2c).Furthermore, large deformation of the 4D printed FASMC was demonstrated in Figure 2d, where a balloon with a diameter of 10 mm was fabricated and inflated using a syringe, achieving a 320% diameter change from 10 mm to 32.42 mm.
The mechanical properties of FASMC printed using MF-DLP with different CIP concentration were studied, and the results are shown in Figure 2e,i.High-content of CIPs could lead to brittleness due to decreased bonding strength, resulting in a decrease in tensile strength and elongation (Figure 2d,e).Figure 2g shows the stress-strain curve of FASMC with a CIPs concentration of 5 wt.% with progressive strain increment (20% to 50% to 80%).Small residual strains, 0.26%, 0.88%, and 1.72% were remained.Furthermore, FASMC with high CIP content (45 wt.%) was subjected to 100 cycles of tensile-unloading at a 50% maximum strain to test its durability.The results in Figure 2h showed that the area enclosed by the stress-strain curve during the first cycle was larger due to the local fracture of molecular chains and the local detachment of CIP from the resin matrix.For 20, 50, and 100 cycles, the mechanical properties of the material reached at a stable state.Furthermore, as shown in Figure 2i, the residual strain after the 1st, 20th, 50th, and 100th cycles were 6.58%, 9.25%, 9.88%, and 10.23%, respectively.It demonstrated that even at high CIP concentration, FASMC exhibits good durability.
The energy dissipation during cyclic testing can be characterised by the degree of overlap of the hysteresis loops [13].Figures s7 a and b in the Supplementary information show complete 100-cycles tensile tests of the FASMC sample with 5 and 45 wt.% CIP concentration, respectively.The results indicate that both configurations almost overlapped, demonstrating stable energy dissipation and durability.

Anisotropic stimuli-responsive behaviours of FASMC composites
Clear chain-like arrangements of CIP particles indicates that FASMC could have high magnetic anisotropy.To further study its anisotropy, as shown in Figure 3a, a cylindrical permanent magnet (NdFeB, N35 grade, 20*10*5 mm) was gradually approached a cubic FASMC sample (5*5*5 mm) with two configurations, one with the magnetic field parallel and the other perpendicular to its CIP chains.The cylindrical permanent magnet is initially hold at 20 mm from the platform.Then, it is moved down at a speed of 120 mm/min until FASMC sample is attracted to its bottom.The distance h of the magnet movement was recorded.For example, for the 45 wt.% CIP content sample, the displacement curve of the magnet is shown in Figure 3c, and distance h is 3.15 mm.A high-speed CCD camera was used to capture the movement as the magnet was approached, the process is shown in Figure 3d,e and Supplementary video s1.It was found that when the CIP chains are parallel to the magnetic field the sample will be directly attracted.While the perpendicular sample will flip 90 o to align the CIP chain to the direction of magnetic field.The movement distance h of the permanent magnet for perpendicular samples is ∼42.68%larger than that of parallel samples (Figure 3f).The movement distance h of FASMC samples decreases with an increase of CIP concentration.Magnetisation is the direct feedback of a magnetic material for the external magnetic field.The magnetisation of FASMC in both configurations was studied, and a vibrating sample magnetometer was used to test the magnetic hysteresis loop at the room temperature.Figure 3g and Figure s8 in the Supplementary information show that all samples exhibited superparamagnetic behaviour.Before reaching the saturation magnetisation state, magnetic susceptibility in the parallel direction (slope in Figure 3g) is larger than that of perpendicular direction since the easy axis of CIPs is aligned with the external magnetic field in the resin tank.It can also be found that the magnetisation along the aligned direction is greater than that in perpendicular direction when magnetic field is lower than 750 mT.The maximum surface magnetic intensity generated by the NbFeB magnet in this study is ∼665.2mT, such that the FASMC sample cannot reach the saturation magnetisation under such a driving magnetic field.The FASMC sample can be regarded as a small magnet.As the magnet approaches the FASMC sample, its magnetic pole direction (CIP chain direction) will align with the direction of external magnetic field, causing the flipping.
Figure 3h shows that saturation magnetisation increases linearly with the increase of CIP concentration in both configurations.At the same CIP concentration, saturation magnetisation is slightly higher in the parallel samples.FASMC with a CIP concentration of 45 wt.% reaches the maximum saturation magnetisation of 68.82 and 68.48 emu/g in the parallel and perpendicular directions, respectively, with a difference of about 0.5%.The difference under other concentration is less than 1%, which indicates that the saturation magnetisation is independent on the chain orientation but increases linearly with the concentration of CIP.
FASMC samples have exhibited great flexibility and stretchability.In this section, we tested the deformability of FASMC of multiple shapes using magnetic stimuliresponse.As shown in Figure 4a, a FASMC cantilever beam with a size of 20 *20 *2 mm was used for magnetic stimuli-response deflection tests.The same two configurations were still adopted, as shown in Figure 4b.FASMC samples will bend when the magnet approaches.Figure 4c and Supplementary video s2 show the significant difference in stimulus-response behaviour for the two configurations.At the same CIP concentration, the bending degree of the perpendicular samples is significantly larger than that of parallel samples.In the parallel configuration, the components present tilting, which is attributed to the tendency of CIP chains to align with the magnetic induction lines.This reorientation behaviour is due to energy minimisation and state stabilisation.The results of finite element simulation are shown in Figure 4d,-e, which are found consistent with the experimental results.The maximum stress occurs at the clamping position between the sample and fixture.Bending distance and bending angle for FASMC samples with both configurations of different CIP content were shown in Figure 4f,g.It showed that the bending angle and distance increased with the increase of CIP content in both configurations.In the perpendicular configuration, the bending distance and angle of the FASMC sample increased from 3.06 mm and 15.83°(concentration of 5 wt.%) to 13.85 mm and 78.72°(concentration of 45 wt.%), respectively.At the same CIP concentration, the bending distance and angle of the parallel configuration were smaller than those of the perpendicular configuration.In the parallel configuration, the CIP chain in the FASMC sample is aligned with the magnetic induction line and needs to overcome the resistance from the fixture.Therefore, the bending distance and angle of the parallel configuration are smaller than those of the perpendicular configuration.In the test, the probe of the Gauss metre is located below the FASMC sample (Figure 4a) to detect the changes of magnetic field when magnet approaches.Figure 4h shows the magnetic field amplitude below the sample with the magnet moving distance.When the moving distance h is 28.13 mm, the magnetic field measured in the perpendicular configuration, parallel configuration, and the absence of FASMC sample was 177.49, 210.28, and 222.37 mT, respectively, indicating that the magnetic shielding ratio of the two configurations was 20.18% and 5.44%, respectively.Compared with the parallel configuration, most CIP chains in the vertical configuration were aligned with the magnetic induction lines, resulting in a larger magnetic shielding effect.
It has been demonstrated that FASMC composites have strong magnetic anisotropy.In this study, a solid vase with 45 wt.% CIP content was fabricated with its CIP chain direction aligned along the axisymmetric axis, as shown in Figure 5a.When applied vertical external magnetic field, the solid vase 'stood up' immediately.If the vase is pressed down, it could recover when removing the force.In addition, as shown in Figure 5b, the solid vase was placed on a polyvinyl chloride (PVC) plate with a thickness of 2 mm and driven by a N52 cylindrical magnet under the plate.The magnet was moved forward, backward, left and right as shown in Figure 5c to control the vase movement and tilting (Figure 5d and Supplementary video s3).The tilting direction is consistent with the moving direction of the magnet, and the tilting angle can be precisely controlled.For example, Figure 5e shows when the moving distance is 7.5 mm, the tilting angle of the vase reaches 68.5 o , demonstrating excellent motion control performance of FASMC.
Micro-lattices with engineered structures could exhibit extraordinary physical properties [39][40][41].3D printing techniques are suitable to fabricate micro-lattices.Here, a magnetic anisotropic micro-lattice was printed (Figure 5f, Gyroid-type three-period minimal surface structure with a size of 20 mm*20 mm*20 mm) using the proposed MF-DLP technique.The compressive properties of the metamaterial for parallel and perpendicular to the CIP chain direction were studied under external vertical magnetic field (Figure 5g,h).When the compression direction is parallel to CIP chains, the compression load is higher than that of the perpendicular direction.The stiffness in the parallel and perpendicular directions are 0.39 and 0.32 N/mm, respectively, with an increment of 21.88%, showing that the MF-DLP technique can be used to print FASMC with adjustable stiffness.

FASMC Array assembly driven by magnetic field
Anisotropic magnetic properties of the MF-DLP fabricated FASMC have been demonstrated.In this work, inspired by the compass, we study the response of a series of FASMC samples with different shapes and CIP chain directions under the stimuli of a magnetic field.As shown in Figure 6a, arrow samples were fabricated with four different chain alignment configurations (90°, 45°, 0°and −45°, respectively) and were placed on a substrate.The samples were floated in a container filled with sodium chloride solution (Figure 6b,c).As shown in Figure 6c and Supplementary video s4, when the magnet approached slowly at a speed of 120 mm/min, FASMC samples will turn towards the external magnetic field direction with respect to their own chain directions, even for low CIP concentration of 5 wt.%, implying strong anisotropic magnetic properties and actuation capabilities.The difference angle between the arrow and magnetic field changed from 70.58 to 0.73°when the magnetic field amplitude increases from 0.09 mT to 1.96 mT for sample I (Figure 6d).
To demonstrate the multi-objects actuation capability, three letters 'H', 'I' and 'T' ('HIT' is the abbreviation of Harbin Institute of Technology) were fabricated to construct a 3*3 array as shown in Figure 6e,f.The letter samples in row 1, 2 and 3 have 90°, 45°, and 0°CIP chain orientation, respectively.In the test, all letters rotated rapidly from a random state to align their chain directions to the magnetic field (∼5.3 mT), exhibiting an accurate and fast response (see Supplementary video s5).Deviation occurred in row 1 (marked in a blue line) due to the spatial limitation of non-uniform magnetic field generated by two magnets.In addition, disturbance tests were conducted on the aligned samples (letter 'D' in Figure 6g) with an external magnetic field of ∼7 mT.After artificial rotation, the sample can re-align to its stable state (Figure 6g and Supplementary video s6), showing high stability and robustness.Furthermore, the FASMC samples of letter 'D' successfully rotated with the rotating magnetic field from 0 to 90°at a speed of 10 rad/s (Figure 6h and Supplementary video s7).
Rotating sequence could occur for magnetic cell arrays as shown in Figure 6i and Supplementary video s8 when magnet approaches the magnetic cell arrays (see the inset in Figure 6i).A more rapid and favourable magnetic response of FASMC sample 1 (marked as ①) occurred since it is close to the magnet compared with sample 2 and 3.The uneven distribution of the magnetic field was simulated as well as the gradients of the magnetic field (Figure 6j,k), which could cause the rotating sequence.The gradient of magnetic field increases from position 3-2 to 1, leading to a faster rotating response.In a word, the fabricated FASMC could achieve a series of complex actuation while maintaining high repeatability and stability.Figure s10 shows the multi-stage or sequential use of FASMC.

Conclusions
In this paper, we developed a soft magnetic resin and successfully fabricated strong anisotropic magnetic soft parts using a lab-built MF-DLP 4D printer, achieving both high flexibility and stretchability.The CIPs were formed into chains along the magnetic lines during printing, forming high anisotropy within the FASMC.Several complex structures were fabricated to demonstrate excellent printability even with high CIP concentration (up to 45 wt.%).Cyclic tensile tests show excellent mechanical properties and durability of the fabricated FASMC, as well as good magnetic performance and robustness.Moreover, the anisotropic magnetic properties were demonstrated by a series of external magnetic field stimuli-response tests.The behaviours of flipping, controlled deflection and rotation were studied against their anisotropic properties.This work provides insights into external field-assisted additive manufacturing of anisotropic soft magnetic materials and could be regarded as valuable guidance for the application of FASMC parts in sensors, actuators, soft robots, and drug delivery.

Figure 1 .
Figure 1.(a) Schematic of the lab-built magnetic field-assisted DLP 4D printer.(b) Top view of the resin tank showing magnetic induction lines in the printing area.(c) Experimental measurement and finite element simulation of magnetic field distribution in the printing area.(d) Three main compositions of magnetic photosensitive resin are 4-HBA as the flexible matrix, CIP as magnetic fillers, and fumed silica as rheological modifiers, respectively.(e) The viscosity changes with the shear ratio of magnetic resins with CIP concentration of 5, 15, 25, 35, and 45 wt.%, respectively, showing typical non-Newtonian fluid behaviour.(f) Chain-like aligned CIPs in liquid resin and cured parts.(g) A 10 mm*10 mm* 2 mm sample with CIP concentration of 45 wt.% was fabricated using the MF-DLP technique.(h)-(k) SEM images of the top and side surfaces of the sample, demonstrating uniform distribution of the CIPs in the cured parts.(l) EDS results show the main elements inside the FASMC sample.

Figure 2 .
Figure 2. The printability and mechanical properties of FASMC composites.(a) Printability with different complex structures.(b) Zoomed optical image of dragon showing fine features of the printing techniques.(c) Demonstration of ∼5000 times load capacity of FASMC composites.(d) Inflated balloons showing good stretchability of FASMC composites.(e) Uniaxial tensile tests of FASMC composites with different CIPs concentration.(f) Tensile strength and broken strain with different CIP concentration.(g) Stress-strain curve with progressive strain increment (20% to 50% to 80%).(h) Stress-strain curves of cyclic tensile tests under a maximum strain of 50% for 100 cycles.(i) The residual strains increase with CIP content.Scale bar in (a): 5 mm.

Figure 3 .
Figure 3. Stimuli-responsive jumping behaviours of FASMC samples.(a) Schematic of parallel and perpendicular chain configurations with respect to the external magnetic field.(b) A starting reference point is set for magnets moving down until complete attraction of the FASMC sample.(c) Displacement curve for the FASMC sample with 45 wt.% CIP content, chain parallel to the external magnetic field.(d) and (e) The process of stimuli-responsive jumping for FASMC samples under parallel and perpendicular configurations, and the perpendicular samples present flipping behaviour.(f) Moving distance h for different CIP concentration samples.(g) Magnetic hysteresis loops at room temperature.(h) Saturation magnetisation for different CIP concentration samples.Scale bars in (d) and (e): 5 mm.

Figure 4 .
Figure 4. Deformation of the FASMC samples during magnetic stimuli-response.(a) Schematic of magnetic bending stimuli-response.(b) Two mounting configurations of FASMC samples with different CIP chain alignments.(c) Bending process of FASMC cantilever beams for the two configurations with CIP concentration of 45 wt.%.(d) and (e) Experimental and simulation results for FASMC cantilever beams for the two configurations with CIP concentration of 25 wt.%.(f) and (g) bending distance and angles with CIP concentration.(h) Magnetic field with magnet moving distance showing magnetic shielding effects of both configurations.Scale bar in (c): 10 mm.

Figure 5 .
Figure 5. (a) The process of standing up of a FASMC solid vase with a high CIP concentration of 45 wt.%.(b) Schematic of magnet manipulation to control the motion and tilting of the solid vase (c) Moving trajectory of the magnet at a constant speed of 120 mm/ min.(d) The solid vase was forced to tilt in forward, backward, left, and right directions using a stimuli magnet.(e) Tilting angle with moving distance from the origin to 10 mm in different directions.(f) MF-DLP printed three-period minimum curved micro-lattice with a size of 20*20*20 mm.(g) Force-strain curves of the micro-lattice under compression in two configurations.(h) Stiffness of the microlattice under compression in different directions under vertical magnetic field.Scale bar in (a): 10 mm.

Figure 6 .
Figure 6.Rotating actuation for MF-DLP fabricated FASMC (a) Arrows, inspired by the compass, sitting in fluid and can rotate freely.(b) Schematic of CIP chain directions inside the FASMC.(c) Final alignment states of the arrow samples with 5 wt.%CIP concentration driven by a magnet.(d) The difference angle and magnetic field when the magnet approaches.(e) Three MF-DLP printed letters 'H', 'I' and 'T' forming an array in (f), where the alignment direction of CIP chains in the 1st, 2nd and 3rd row are 90, 45 o and 0 o , respectively.(f) Randomly letter arrays driven by magnetic field.(g,h) Robustness testing during rotation.(i) Rotation sequence in a letter array driven by magnetic field gradient.(j) and (k) Simulated magnetic field and its gradient.Scale bars in (a,f) and (g) are 10, 20 and 5 mm, respectively.