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...

Light induced three dimensional (3D) printing techniques generally use printable formulations that are based on acrylic monomers because of their fast reactivity, which is balanced with their good final properties. However, the possibility to enlarge the palette of 3D printable materials is a challenging target. In this work, hybrid printable formulations that are based on acrylic and epoxy resins are presented and their printability on DLP (Digital Light Processing) machines is demonstrated. Hexanediol diacrylate (HDDA) and an epoxy resin-3,4-Epoxycylohexylmethyl-3 ,4epoxycyxlohexane carboxylate (CE)-in different ratios are used and the influence of a bridging agent, Glycidyl methacrylate (GMA), is also investigated. The reactivity of the different active species during irradiation is evaluated and the mechanical properties, including the impact toughness, the thermo-mechanical properties, and the volumetric shrinkage, are studied on printed samples.

No abstract

The revolution of 4D printing allows combining smart materials to additive processes to create behavioral objects able to respond to external stimuli, such as temperature, light, electrical, or magnetic fields. Here, a modified commercial digital light processing (DLP) 3D printer is used to obtain complex macroscopic remotely controlled gear‐based devices. The fabrication process is based on the printing of magnetoresponsive polymers containing in situ self‐assembled microstructures, i.e., composed of oriented chains of Fe3O4 nanoparticles (NPs). First, it is demonstrated that magnetoresponsive hammer‐like actuators with different stiffness can be printed allowing both pure rotation or/and bending motions. Then, the microstructure to create a magnetoresponsive gear is exploited. In particular, this work shows that they can be successfully used to transfer torque to other gears, thereby converting a rotation movement into linear translation. Finally, it is demonstrated that magnetoresponsive gears can also be combined with other nonmagnetic elements to create complex assemblies, such as gear‐trains, linear actuators, and grippers that can be remotely controlled.

Soft robots are an emerging class of robots that, differently from their conventional and rigid counterparts, are able to perform precise and delicate tasks, e.g., they can comply with external surfaces through large deformation, squeeze and navigate in unknown and small spaces, recognize shapes and textures, grasp and move delicate objects, interact safely with humans. [1,2] They are made of "soft" materials, including any gel, colloid, foam, or polymer highly prone to deformation. The soft bodies of these innovative robots are inspired by living beings that can perform smooth actions and rapidly adapt to their surroundings.In this regard, an efficient soft robot must include sensors that provide the perception of the environment and of the robot itself. Mechanical sensors are used to convert the deformations caused by mechanical stimuli into an electrical signal, which can be exploited for the robot control to grant effective and safe interactions between the soft robot and the surrounding. Due to the importance of sensing deformations, scientists researched various materials and technologies to develop soft mechanical sensors, i.e., mechanical sensors made of soft materials. [3][4][5] Generally, soft sensors are fabricated via conventional manufacturing techniques such as casting, [6] tapering and pasting, and combining different parts (i.e., dielectricconductive, substrate-electrode, etc.) in a step-by-step procedure. However, the soproduced devices suffer from poor adhesion due to the lack of mechanical and/or chemical compatibility between the various elements; moreover, the fabrication processes are excessively laborious, time-consuming, and with intrinsic low reproducibility and scalability. 3D printing, or additive manufacturing (AM), is a key technology to overcome these issues and move towards a new class of reliable and scalable soft sensors. It is one of the most disruptive technologies of the last decades that enables the fabrication of complex shapes by adding sub-units of material starting from a digital model, [7] in contrast to conventional, subtractive technologies. Due to the intrinsic design freedom and the possibility of using deformable materials, AM allows the implementation of complex soft robotic designs, [8] reaching applications in several fields such as biomedical engineering, healthcare, food, fashion, automotive, aerospace, etc. [9][10][11][12][13][14] Besides the seamless fabrication procedure, AM guarantees the opportunity to build actual 3D geometries, unlike previous 2D and 2.5D fabrication technologies. [15,16] This aspect is crucial and opens technological possibilities such as: 1) inspiration from natural sensory receptors to guide new morphological designs, 2) enhancing the deformation of the bulk material by incorporating voids through lattice-like geometries, and 3) investigating designs that enhance deformations in specific directions. These new design principles can pave the way to develop soft mechanical sensors able to discriminate different types of mechanical stimuli...

Soft robots must embody mechanosensing capabilities to merge and act in the environment. Stretchable waveguides are making the mark in soft mechanical sensing since they are built from pristine elastomers....

Thermal conductivity is a key property in many applications from electronic to informatics. The interaction of fillers with Sylgard 184 was studied; this study explores new composites and the influence of metal particles (copper and nickel), carbon-based materials (carbon nanotubes and carbon black), and ceramic nanoparticles (boron nitride) as fillers to enhance thermal properties of silicon-based composites. The effect of the fillers on the final performances of the composite materials was evaluated. The influence of filler volume, dimension, morphology, and chemical nature is studied. Specifically, FT-IR analysis was used to evaluate curing of the polymer matrix. DSC was used to confirm the data and to further characterize the composites. Thermo-mechanical properties were studied by DMTA. The filler morphology was analyzed by SEM. Finally, thermal conductivity was studied and compared, enlightening the correlation with the features of the fillers. The results demonstrate a remarkable dependence among the type, size, and shape of the filler, and thermal properties of the composite materials. Underlining a that the volume filler influenced the thermal conductivity obtaining the best results with the highest added volume filler and higher positive impact on the k of the composites is reached with large particles and with irregular shapes. In contrast, the increase of filler amount affects the rigidity of the silicon-matrix, increasing the rigidity of the silicon-based composites.