Magnetically activated 3D printable polylactic acid/polycaprolactone/magnetite composites for magnetic induction heating generation

Additive manufacturing technology has attracted the attention of industrial and technological sectors due to the versatility of the design and the easy manufacture of structural and functional elements based on composite materials. The embedding of magnetic nanoparticles in the polymeric matrix enables the development of an easy manufacturing process of low-cost magnetically active novel polymeric composites. In this work, we report a series of magnetic composites prepared by solution casting method combining 5 to 60 wt.% of 140 ±\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document} 50 nm commercial Fe3O4 nanoparticles, with a semi-crystalline, biocompatible, and biodegradable polymeric blend made of polylactic acid (PLA) and polycaprolactone (PCL). The composites were extruded, obtaining 1.5 ±\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pm$$\end{document} 0.2 mm diameter continuous and flexible filaments for fused deposition modelling 3D printing. The chemical, magnetic, and calorimetric properties of the obtained filaments were investigated by differential scanning calorimetry, thermogravimetric analysis, magnetometry, and scanning electron microscopy. Furthermore, taking advantage of the magnetic character of the filaments, their capability to generate heat under the application of low-frequency alternating magnetic fields (magnetic induction heating) was analyzed. The obtained results expose the versatility of these easy manufacturing and low-cost filaments, where selecting a desired composition, the heating capacity can be properly adjusted for those applications where magnetic induction plays a key role (i.e., magnetic hyperthermia, drug release, heterogeneous catalysis, water electrolysis, gas capture, or materials synthesis). This paper presents the manufacture of novel magnetically activated 3D printable filaments, low cost and easy to manufacture, whereby selecting the desired composition, the heating capacity (ρ) generated under the application of an alternating magnetic field (Happ) can be adjusted to the needs of the user.


Introduction
In recent years, additive manufacturing has attracted the attention of industrial and technological sectors due to the possibility of fabricating objects without shape and size restrictions in an accessible, low-cost, swift, and very precise way [1,2]. Additive manufacturing processes involve models, patterns, prototypes, tools, and three-dimensional end products through the automated deposition of materials layer-by-layer. Nowadays, it is possible to manufacture elements composed of polymers [3], metals [4], and/or ceramics [5] with the required geometry according to a simple digital design. Thus, no special tools or pre-formed molds are required for the construction of the final component, which significantly reduces the cost and speeds up the production process. In this regard, fused deposition modelling (FDM) is a well-known 3D printing technique based on low-temperature range (373-523 K) melt extrusion, where thermoplastic materials in the form of filament are used [6]. Due to its low cost, the wide selection of materials, and high precision, it has become one of the most widely used printing techniques worldwide [7]. In addition, FDM 3D printers do not require large or complex equipment and can be configured to operate under low power consumption such as renewable solar energy, so they are easily transportable and adaptable to any place in the world [8].
The current exponential growth of 3D printing new technologies has entailed the necessity to develop novel cuttingedge materials with advanced or combined physicochemical properties. In this sense, the need for novel materials with new and improved properties has led to the use of composite materials (CMs) [9,10]. CMs are formed by mixing two or more materials and are characterized by having superior properties to those of their single components [11]. CMs consist of a matrix, which ensures the cohesion of the material, and a filler, which modifies the properties of the matrix [12]. In this framework, the common base 3D printable raw materials such as polycaprolactone (PCL), polylactic acid (PLA), and acrylonitrile butadiene styrene (ABS) among other biodegradable thermoplastics have been used as matrix due to their low-cost and easy manufacturing properties. Consequently, the nature of the matrix and the filler, their proportions, adhesion, and the production process will mainly determine the final composite characteristics. Specifically, by changing the constituent elements (polymer type, particles, and/or additives), it is possible to modify their physicochemical properties and create new multifunctional materials [13][14][15][16][17].
Specifically, magnetic CMs have acquired an important role due to their diversity of applications in fields such as electronics, aeronautics, environmental remediation, and biomedicine [2,[18][19][20]. So far, the most common structures based on magnetic CMs fabricated by FDM have been prepared using ferromagnets and/or ferrimagnets [21][22][23], classified into two main groups: hard and soft magnetic materials. While the former has the ability to remain permanently magnetized and generate magnetic fields in the absence of external stimulus [24], the composites prepared with soft magnetic materials can be easily magnetized in the direction of an external magnetic field and are widely employed as magnetic flux concentrators. In addition, novel magnetic CMs have been recently proposed using superparamagnetic nanoparticles (NPs). These systems display negligible interparticle magnetic interactions since magnetization can randomly flip direction due to thermal contribution. In general, the most commonly used superparamagnetic NPs for the preparation of magnetic CMs are those based on iron oxides [2]. Interestingly, these superparamagnetic nanostructures have been extensively applied in several technological areas with special emphasis on biomedical applications [25][26][27]. In this aim, superparamagnetic NPs are proposed as contrast agents, image tracers, or local drug carriers due to their high biocompatibility, high magnetic response, and low-cost [28][29][30]. However, many of these potential uses rely on their ability to absorb energy from an externally applied alternating (AC) magnetic field [31]. This energy eventually heats the nearby tissues (magnetic hyperthermia) or/and triggers the liberation of drugs carried by the NPs (drug delivery) [32]. On the other hand, this capability has been also proposed in diverse applications such as heterogeneous catalysis [33], water electrolysis [34], gas capture [35], or materials synthesis [36].
Therefore, in this work, we present the fabrication of a low-cost and easy-manufacturing novel magnetic CMs raw material for FDM 3D printing based on PLA and PCL polymeric blend and commercial Fe 3 O 4 NPs. Although a few works have studied the possibility to manufacture composites for heat generation, they are based on much more expensive polymers or ceramic materials [16,37,[37][38][39]. On the other hand, studies of PLA + Fe 3 O 4 [27], PCL + Fe 3 O 4 [28], and PLA + PCL [29] can be found in the literature, but the PLA + PCL + Fe 3 O 4 ternary composite for heat generation is the first time reported in literature.
A series of different composites (5 to 60 wt.%) have been prepared by solution casting method combining commercial 140 ± 50 nm Fe 3 O 4 NPs embedded in a semi-crystalline, biocompatible, and biodegradable polymeric PLA/PCL blend. Furthermore, the composites were extruded, obtaining continuous and flexible filaments (1.5 ± 0.2 mm diameter) and printed with different shapes demonstrating their capabilities to be used in FDM 3D printing technology. A deep characterization has revealed that the NPs concentration affects the properties of the filaments. Taking advantage of the magnetic character of the filaments, their capability to generate heat under the application of low-frequency AC magnetic fields (magnetic induction) was analyzed. Interestingly, it revealed the versatility of the herein proposed material, where selecting a desired filling factor (i.e., the mass fraction of magnetic NPs per mass of composite) in the filaments and correct applied magnetic field frequency and intensity, the heating capacity can be properly controlled.

Preparation of PLA/PCL/Fe 3 O 4 filaments
In order to obtain the 3D printable magnetic filaments, six composites with different amounts of PLA, PCL, and Fe 3 O 4 NPs were prepared (Table 1).

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To carry out the preparation of the composites, the following procedure was developed (see Fig. 1): first, 10 g of PLA was dissolved in 300 mL of DCM with mechanical stirring at 303 K. As the PLA was dissolved, the colorless solution become more viscous, and 90 g of PCL was added. To promote a homogeneous mixture between both polymers, the solution was left under constant stirring and temperature for 2 h. After this time, the corresponding quantity of Fe 3 O 4 NPs ( Table 1) was added and left in constant mechanical stirring for 1 h. Finally, the temperature was increased to 318 K to facilitate the evaporation of the solvent. The NPs percentage in mass (i.e., filling factor) of the prepared composites was adjusted by changing the mass proportions of the polymers and the Fe 3 O 4 NPs.
The dried magnetic composite was mechanically extruded to obtain continuous and flexible filaments of 1.5 ± 0.2 mm in diameter for their subsequent use in 3D printing (see Fig. 1b, c). The fabrication of magnetic filaments was carried out using an extruder (FelFil Evo) with a 1.75-mm circular cross-section nozzle. The extrusion temperature was adjusted between 363 and 423 K depending on the mass percent of NPs and the polymer mixture in the final product composition. However, similar speed extrusion (10 cm/min) was adopted to assemble the magnetic filaments.

Characterization
Employed commercial iron oxide (Fe 3 O 4 ) particle average size estimation was made by transmission electron microscopy (TEM) measurements in an "FEI TECNAI T20" microscope. The morphologies of the prepared filaments, including the NPs distribution, were investigated by scanning electron microscopy (SEM, Zeiss EVO), high-resolution SEM (HR-SEM, Zeiss GEMINI), and energy dispersive X-ray spectroscopy (EDX).  The thermal stability, thermogravimetric analysis (TGA), of the extruded filaments, was studied using a TGA/DSC 3 + from Mettler Toledo in the range of 193-473 K at a 10 K/min rate in nitrogen flow. Thermal properties were determined by differential scanning calorimetry (DSC) (Q-100 DSC, TA Instruments). The samples were pre-sealed into an aluminum pan, and the analysis was performed in four steps: (i) 1st cooling from 303 to 193 K at 10 K/min, (ii) 1st heating from 193 to 473 K at 10 K/min, (iii) 2nd cooling again to 193 K, and (iv) finally the 2nd heating to 473 K at 10 K/min. The crystallization ( T c PCL ) temperature and crystallization enthalpy per mass ( ΔH c PCL ) of PCL was determined from the 2nd cooling curve while the melting temperature ( T m PCL ) and melting enthalpy per mass ( ΔH m PCL ) were determined from the 2nd heating curve. The equivalent characteristic parameters of PLA were determined from the 2nd heating curve: cold crystallization ( T cc PLA ) temperature, cold crystallization enthalpy per mass ( ΔH cc PLA ), melting temperature ( T m PLA ), and melting enthalpy per mass ( ΔH m PLA ). Crystallization and melting temperatures were taken at the maximum of the crystallization and the melting peaks, respectively. To average out sample statistical fluctuations, 3 different samples were extracted from the same filament, and the mean value of the characteristic thermal parameters was estimated. Since PCL crystallizes on cooling and PLA on heating, two different equations have been used to calculate the crystallinity degree (χ c ) of both polymers at room temperature. First, the crystallinity degree of PLA was obtained by the following [40,41]: The crystallinity degree of PCL was calculated by the following [42,43]: where ΔH m0 = 93,6 J/g (or 135 J/g) is the melting enthalpy of the 100% PLA (or PCL) [42,44].
The magnetic characterization of the commercial NPs and the extruded magnetic filaments were analyzed by the magnetic field (H)-dependent magnetization measurements, M(H), on a Quantum Design SQUID magnetometer (QD MPMS XL-7), with a superconducting magnet up to 70 kOe and equipped with a cryostat that can measure from 2 to 400 K. The values of coercive field (H c ), remanence (M r ), magnetization measured at 10 and 300 K at a maximum applied magnetic field of 60 kOe (normalized to the measured composite sample mass), M s , and reduced remanence (R = M r /M s ) were obtained from the hysteresis loops. The magnetic hyperthermia experiments were performed using a commercial G3 D5 series Multi-mode 3000W Drive from nanoscale Biomagnetics setup. During these experiments, water and air, surrounding composites were heated by magnetic induction. The increase in temperature was measured by a portable IR Thermal Imager (PCE-TC 33 N) [45] (18-40 mg) for filaments on air whereas a fiber optic thermometer (Neoptix Inc., Canada) in contact with aqueous media (1 mL of deionized water) was used in the case of water surrounded samples. The measurements of dynamic magnetization were carried out in aqueous media (1 mL of deionized water) using a previously tested lab-made AC magnetometer [46]. The heat-generating capability of the filaments, the specific absorption rate (SAR), and generated power density (ρ) were obtained from dynamic magnetization measurements at 311 kHz [47]: where f is the AC magnetic field frequency, M t is the dynamic volumetric magnetization (SI units A.m −1 ), and H app is the applied AC magnetic field intensity. A is the area enclosed by the hysteresis loop measured at frequency f (see Fig. S1). Because SAR is normalized by the mass of NPs, ρ is related with the SAR according to the following [48]: where c is the total mass of magnetic NPs in the filaments divided by the NPs volume. The absorbed power density defines the maximum temperature ( T max ) that a composite and the surrounding material (e.g., biological tissues or specific medium) can reach [49]. In the case of a material absorbing energy from an AC magnetic field, T max corresponds to the equilibrium temperature, reached when total power absorbed by the material equals the dissipated power flowing out of the material by thermal conduction, convection, or irradiation.

3D printability of magnetic filaments
The printing of different shapes and sizes of objects was carried out using an Artillery Sidewinder X1 3D printer based on the FDM technology. In all cases, Open Source-Software CURA 4.12.1 (Ultimaker B.V. Utrecht, Netherlands) was used to format the printing algorithm (layer thickness of 0.3 mm and 100% infill density). A printing temperature of 573 K was employed to print the filaments using a 0.4-mm extruding head. The layer height was 0.3 mm, the speed in the printing movements was 12.5 mm/min, and the bed temperature was 313 K.
Absorbed Power mass NPs

Results and discussion
A series of five magnetic composites were prepared by solution casting method combining nanometric 140 ± 50 nm commercial magnetic Fe 3 O 4 NPs (Fig. S2), with a polymeric blend made of a 10% PLA and 90% PCL (see Fig. 1a).
As previously reported [50,51], this polymer mixture enhances the structural strength with respect to neat PCL, while keeping the melting temperature around 328 K, enabling the manufacture of filaments flexible enough to be used as raw material for FDM 3D printing. The samples were labeled as P#%, where # represents the percentage of NPs in the composites (from 5 to 60 wt.%). Figure 2a-c depicts the cross-section SEM images for selected P5%, P20%, and P60% samples. The cross-section SEM images for P5% and P20% samples are represented in Fig. S3a, b. It can be seen that while at low NPs concentrations the filaments show a regular morphology and dense structure, the increase in the magnetic component fosters the deterioration of the morphology. As NPs concentration increases, the filament becomes rougher with a consequent enhancement of the porosity, i.e., porous in the micrometric range [39,[52][53][54]. Interestingly, it must be noted that the increased porosity does not affect the macroscopic aspect of the extruded filament, displaying, all the series of composites, a smooth surface (see Fig. 1c). To study the NPs' dispersion into the polymeric matrix, EDX mapping was carried out (see Fig. 2d-f and Fig. S3c, d).
It can be observed that the NPs are well distributed along the whole filament, and, at the same time, for certain series of compositions, the nanostructures tend to form micrometric agglomerates. The homogeneous NPs dispersity was also confirmed by HR-SEM images, which show that most of the particles were individually distributed in the polymeric matrix (see Fig. 2g, h and Fig. S4). In addition, in Fig. 2i, a part of the micrometric porosity, smaller cavities with diameters ranging from 200 to 600 nm were observed in the filament with the highest NPs loading.
To investigate the effect of the NPs concentration on the thermal properties of the magnetic filaments, TGA curves were obtained for the whole series of composites; two distinct weight losses corresponding to each polymer decomposition can be observed (see Figs. 3 and S5). Since the PLA decomposes at lower temperatures than PCL (i.e., 641 and 688 K, respectively), the first weight loss was employed to determine the amount of PLA in the sample, while the second one enabled the determination of the mass of PCL. Fe 3 O 4 NPs content can be determined as the remnant mass percentage at the end of the thermal scan. Table 1 and Fig. S5 depict the estimated polymer content and the magnetic NPs percentage for each sample. The obtained results are in good agreement with the expected values considering the employed percentages of each component in the composites' preparation. Moreover, it should be noticed that the introduction of magnetic NPs causes an overall reduction of the decomposition temperature of the polymeric blend, enhanced by the increase in the NP filling factor. Specifically, it is observed that the decomposition of both polymers is shifted towards lower temperatures with increasing Fe 3 O 4 wt.%. This is in good agreement with previously reported works [55], where the increase in the pyrolysis of PCL chains is linked to the incorporation of magnetic NPs due to the limitation of the movement of polymeric chains [56,57]. At the same time, water that might be adsorbed on the surface of the NPs could act as depolymerization catalysts and accelerate the degradation of both polymers [58]. Despite this decrease in the decomposition temperatures, it is worth noting that the degradation onset for all the filaments is above the usually used printing temperature (473 K), which ensures the viability of the composites for FDM 3D printing. Furthermore, as previously described in the experimental procedure, the filaments were subjected to four thermal scans. The 1st cooling and heating scans (303 K to 353 K and 353 K to 473 K, respectively) were mainly employed to erase the previous thermal history of the prepared filaments. Figure 4a, b show the following 2nd cooling and heating scans.
The exothermic peak in the cooling curve of the PCL displayed in Fig. 4a can be associated with the PCL crystallization ( T c PCL ) . The following heating scan depicted in Fig. 4b allows the determination of the melting temperature of PCL, T m PCL , and the subsequent crystallization, T cc PLA , and melting temperature, T m PLA , of PLA [59]. Due to PLC/PLA immiscibility, the thermal transitions in each polymer are clearly visible independently [60,61]. Table 2 summarizes the estimated characteristic temperatures from the DSC scans as a function of the NPs percentage.
As can be seen in Table 2, T m PCL and T c PCL do not seem to be affected by the presence of PLA polymer or NPs, while T m PLA and T cc PLA decrease as the filling factor increases. As previously reported [62], this variation in T cc PLA could be due to the presence of PCL in the sample which acts as nucleating agent. However, since the mass ratio between PCL and PLA polymers in all the prepared filaments is constant (mass PLA/mass PCL = 0.13(3), see Table S1), this decrease in T cc PLA should be linked to the presence of the magnetic particles. In fact, the presence of NPs in the polymers is also shown as nucleation centers for the polymer crystallization [63]. Consequently, the interaction between NPs and PLA would minimize the polymer chain mobility, causing the crystallization process at lower temperatures. Although the used PLA has a higher molecular mass, it does not have the characteristic aliphatic chain of PCL that provides higher thermal stability to this last polymer. Thus, PCL is more stable to the change of environment of the polymer chains in the presence of other polymers or inorganic compounds and particles, while PLA is more unstable.  The DSC scans also allow the determination of the corresponding crystallization and melting enthalpies of both polymers. Table 2 includes ΔH c and ΔH m together with the crystalline degree c obtained for the PLA and PLC from Eqs. 1 and 2, respectively. No clear variations can be detected in the enthalpy values and c for both polymers with the NP concentrations, despite the decreasing trend in the crystallization and melting temperatures found for the PLA. It should be noted that the low amount of PLA in the samples could affect the calculation of both enthalpy values, and consequently the possible variations with the NPs filling factor masked by experimental error. Anyway, it can be concluded that the introduction of the magnetic NPs in the polymeric matrix influences its thermal properties, particularly the decomposition process of both polymers and the crystallization temperature of PLA.
The magnetic properties of the extruded filaments and the commercial particles were analyzed via the magnetic field (H)-dependent magnetization, M(H), curves (hysteresis loops) at room (RT) and low (10 K) temperatures (see Fig. 5).
It can be observed that while the commercial particles present the expected saturation magnetization (magnetization at 60 kOe, M 60kOe ) at RT (M 60kOe = 80 emu/g NPs ) for bulk magnetite Fe 3 O 4 [64], M 60kOe decreases as the NPs loading is increased in the magnetic composites (from 41 emu/g composite for sample P60% to 3 emu/g composite for P5%). This reduction is a direct consequence of the negligible magnetic response of both polymers (M ≈ 0) and the fact that the displayed magnetization values are normalized to the total mass of the composite. Therefore, taking into account the experimental M 60kOe , the filling factor of each filament can be estimated through the ratio M 60kOe (composite)/M 60kOe (Fe 3 O 4 ) [61]. These values are in good agreement with the previously estimated NPs filling factor through TGA (see Table 1). On the other hand, nearly zero values of the reduced remanence, R, and coercive field, H C , are obtained at RT for all the series of composites. Interestingly, this behavior depicts slight differences in comparison with the commercial Fe 3 O 4 NPs, rising R at 10 K as the NPs filling factor increases (see Table 3). This increment in R can be ascribed to the formation of preferential magnetic orientations of the particles along the filament axis into the polymer composite with respect to the random distribution in the powder sample [65,66]. Indeed, considering that commercial particles were measured on tightly randomly packed powder samples, the chain formation into the polymer composite could lead to an increase in R as a consequence of the occurrence of interparticle magnetostatics interactions [67][68][69]. Table 2 Characteristic thermal parameters obtained from the DSC scans: cold crystallization temperature (T cc ), crystallization enthalpy (ΔH cc ), melting temperature (T m ), melting enthalpy (ΔH m ), and crystallinity degree (χ c )

Composite
PLA PCL  Due to their magnetic response, the as-prepared 3D printable filaments have the potential capability to act as localized heat sources activated by an externally applied AC magnetic field [70][71][72]. In order to test their heating capacity, the temperature of the different filaments was measured (magnetic hyperthermia experiments) under an AC magnetic field of 400 Oe (amplitude) and 311 kHz (frequency). Figure 6 shows the temperature evolution over time under the AC magnetic field when the filaments were in air (a) and (b) in water.
First, as can be observed in Fig. 6a, the temperature increases in all the filaments for very short periods of AC magnetic field exposure, i.e., 60 s. Afterwards, the filaments reach the thermal equilibrium, i.e., a maximum nearly constant temperature (T max ), when the generated heat by the NPs equals the outgoing heat flow to the environment. It is clearly shown that the heating capability under AC magnetic field arises from the presence of magnetic NPs in the composite as both the initial slope and T max increase with the NPs filling factor. Interestingly, it should be noted that for high NPs loadings, i.e., 60%, the temperature reached at a very short exposition time increases above the decomposition temperature of the polymeric matrix and, thus, completely deteriorates the composite material. In addition, to prove the capability as heat generators in different environments, similar experiments were performed in aqueous media (see Fig. 6b). Although a comparable trend between both experiments is observed, that is, larger heating with the increasing load of magnetic NPs, T max is noticeably lower for water media due to the much higher specific heat capacity of water. In fact, only the sample P60% is able to reach the boiling point of water (373 K at 1 atm).
Finally, in order to fully characterize the heat-generating capability of the filaments, the specific absorption rate (SAR) and generated power density (ρ) values were obtained from the dynamic magnetization (from 13 to 553 Oe) measured at 311 kHz (Eqs. 3 and 4, respectively) in aqueous media. Figure 7 displays the estimated SAR as a function of the amplitude, H app , of the appliedmagnetic field for the series of different filaments.
As it can be seen, the SAR values, normalized with the mass of NPs in the filament, are practically identical and independent of the NPs filling factor. Indeed, this is the expected behavior considering that the heat generated by the composites arises only from the magnetic NPs. However, sample P60% displays a slightly higher SAR value which can be linked to the higher remanence R detected in the previous magnetometry characterization. Although there are not many studies in the literature similar to our current study, the obtained SAR values are comparable to those previously published by other authors measured under similar conditions (see Table 4): for composites with similar [73] or smaller [74] NPs sizes. Indeed, to corroborate the flexibility of the as-prepared magnetic filament, the maximum temperature increment ( ΔT max ) achieved during the magnetic hyperthermia experiments ( Fig. 7) is shown in Fig. 8a as a function of the generated power density (ρ). Moreover, in order to compare the individual heating capabilities for each polymeric composition, the generated ρ as a function of H app is depicted in Fig. 8b. As expected, ρ increases with H app [75]. Figure 8 can be employed in the design of efficient heating filament elements. For instance, according to Fig. 8a, to produce a temperature increase of, i.e., 100 K, it is necessary to generate a ρ of 8.5 kW m −3 . This can be achieved as shown in Fig. 8b, employing 3 samples, P5%, P20%, and P60%, depending on the H app applied in the experiment (100, 200, or 380 Oe, respectively). Similarly, for a fixed H app , the power generated for each sample depends on the NPs filling factor, being higher as NPs present in the sample increase.
This clearly reveals the versatility of these materials, where selecting a desired filling factor in the filaments and suitable applied magnetic field frequency and amplitude, the heating capacity can be adjusted to the needs of the user's application. For example, in medical applications, the H app and f must not exceed a maximum limit ( H app < 200Oe for f = 311 kHz) [76]. As it can be deduced from Fig. 8, in the safe region, a remarkable heating capacity can be achieved by selecting a filling factor above 5%, where the effective temperature could be also controlled through the exposure time (see Fig. 6). However, for other types of applications, such as controlled drug release [27], heterogeneous catalysis [33], water electrolysis [34], gas capture [35], or materials synthesis [77], the inductive heating generated falls on the necessity to overcome these limitations. Therefore, in these cases, filaments with high ρ as those as the magnetic composites herein presented can be envisaged as novel heat generators based on easy and low-cost manufacturing for FDM 3D printing technology.
Finally, the study is not complete if the printability of the prepared filaments is not clearly demonstrated. For this purpose, two different objects were printed: cubes of 10 × 10 × 10 mm with a circular section hole of 5 mm employing all the set of composites (P5%-P60%) and a single 15 × 40 mm rectangular mesh with a 0.7 × 0.8 mm pattern using 10% nanoparticle loading (P10%) filament (see Fig. 9). In both cases, the printed objects show homogeneous and welldefined morphology corroborating the capability of the developed composites as novel easy manufacturing and economic magnetic raw material for FDM 3D printing technology.

Conclusions
In this study, we present the preparation of easy-manufacturing and low-cost magnetically active composites for raw materials for 3D printing technologies. The magnetic composite has been prepared by solution casting methodology mixing commercial Fe 3 O 4 magnetic NPs embedded in a polymer mixture of PLA and PCL. The results reveal the possibility to extrude continuous and flexible magnetic filaments to be used in commercial 3D printers with variable NP-to-polymers compositions in the range of 5 to 60% in wt. While at low NPs concentrations, the filaments show a regular and dense structure, as the filling factor increases the filament becomes rougher with a consequent enhancement of the porosity. However, the NPs are well distributed along the whole structure independently of the NPs filling factor, and the increased porosity does not affect the macroscopic aspect of the extruded filament. Similarly, the thermal stability of the composite depends on the NPs loading, showing an overall reduction of the decomposition temperature of the polymeric blend as NPs filling increases. While the decomposition temperature decreases with the filling factor in both PLA and PCL polymers, only detectable changes in the melting and crystallization temperatures are found for the PLA. On the other hand, the inclusion of the magnetic NPs into the polymeric blend confers to the composite a magnetic response with saturation magnetization proportional to the NPs concentration. Taking advantage of the filament's magnetic character, their capability to generate heat under the application of low-frequency AC magnetic fields was analyzed. The composite can increase the temperature for very short periods (60 s) of AC magnetic field exposure (311 kHz and 400 Oe) in different environments. Lastly, the specific abortion rate (SAR) and absorbed power density (ρ) values were measured, obtaining 182 W g −1 and 45 kW m −3 maximum values at 311 kHz. These results expose the versatility of these easy manufacturing and low-cost filaments, where selecting a desired composition, the heating capacity can be adjusted to the needs of the user in applications where magnetic heat induction plays a key role such as magnetic hyperthermia, drug release, heterogeneous catalysis, water electrolysis, gas capture, or materials synthesis.