nanocomposites [12][13][14][15] -have been investigated for a broad variety of applications, from micro-and soft-robotics [10,[16][17][18] to biomedicine. [19][20][21][22][23][24] Among the different strategies, an accessible pathway to fabricate stimuli-responsive (4D) printed objects consists in magnetizing a soft-polymer by loading the polymeric matrix with magnetic fillers, such as particles of magnetite (Fe 3 O 4 ) or neodymium-iron-boron (NdFeB). [25][26][27][28][29][30][31] Direct ink writing (DIW) and fused filament fabrication (FFF) have been used to fabricate fast responding actuators, [32][33][34][35][36][37][38][39][40] inks containing high loads of magnetic fillers [41] and 2D planar structures that exploit folding and unfolding processes. [42] Additionally, 3D printed permanent magnets were developed. [43][44][45][46][47][48] However, both DIW and FFF present some drawbacks: first in terms of resolution; second in terms of the dispersion of the fillers, that may lead to nonhomogeneous magnetic response; and third in terms of temperature of processing, which could be not compatible with the fillers. [48,49] For the last one, the temperature can be decreased using some additives, however this approach may affect the mechanical performances of devices. [41] An alternative to DIW and FFF is digital light processing (DLP). This vat polymerization 3D printing technology involves the use of photosensitive (liquid) resins which are able to cure (i.e., to solidify) upon irradiation with a suitable light source. In DLP, a digital light projector (digital micromirror device) illuminates a photocurable resin with a 2D pixel pattern allowing the curing of single slices of the 3D object. [50][51][52] The aforementioned drawbacks associated to DIW and FFF can be overcome by the use of DLP. Indeed: (i) the printing resolution in DLP belongs to the pixel dimensions and it is generally higher than DIW and FFF, [53,54] (ii) in DLP the dispersion of the fillers is easier to control since liquid formulations are used; and (iii) the fabrication process generally occurs at room temperature. Nevertheless, two precautions must be taken into account: first, the increase of the content of nanoparticles may affect the photopolymerization process since they compete with the photoinitiator in absorbing the incident radiation; and second, the dispersion of the fillers must be stable for the whole printing procedure in order to print an object whose response is homogeneous to an external input. For the latter, macroscopic sedimentation, segregation, and spatial inhomogeneity must be avoided.Digital light processing is used for printing magnetoresponsive polymeric materials with tunable mechanical and magnetic properties. Mechanical properties are tailored, from stiff to soft, by combining urethane-acrylate resins with butyl acrylate as the reactive diluent. The magnetic response of the printed samples is tuned by changing the Fe 3 O 4 nanoparticle loading up to 6 wt%. Following this strategy, magnetoresponsive active components ar...

The power released by magnetic nanoparticles submitted to an alternating driving field is temperature dependent owing to the variation of the fundamental magnetic properties.

No abstract

The magnetic properties of various families of nanocomposite materials containing nanoparticles of transition metals or transition-metal compounds are reviewed here. The investigated magnetic nanocomposites include materials produced either by dissolving a ferrofluid containing pre-formed nanoparticles of desired composition and size in a fluid resin submitted to subsequent curing treatment, or by generating the nanoparticles during the very synthesis of the embedding matrix. Two typical examples of these production methods are polymer nanocomposites and ceramic nanocomposites. The resulting magnetic properties turn out to be markedly different in these two classes of nanomaterials. The control of nanoparticle size, distribution, and aggregation degree is easier in polymer nanocomposites, where the interparticle interactions can either be minimized or exploited to create magnetic mesostructures characterized by anisotropic magnetic properties; the ensuing applications of polymer nanocomposites as sensors and in devices for Information and Communication Technologies (ICT) are highlighted. On the other hand, ceramic nanocomposites obtained from transition-metal loaded zeolite precursors exhibit a remarkably complex magnetic behavior originating from the simultaneous presence of zerovalent transition-metal nanoparticles and transition-metal ions dissolved in the matrix; the applications of these nanocomposites in biomedicine and for pollutant remediation are briefly discussed.