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This in-depth review deals with the subject of how mechanical reinforcement takes place in polymer nanocomposites containing graphene and carbon nanotubes and offers guidelines for the maximization of the performance of such systems.

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Edible electronics will facilitate point-of-care testing through safe devices digested/degraded in the body/environment after performing a specific function. This technology, to thrive, requires a library of materials that are the basic building blocks for eatable platforms. Edible electrical conductors fabricated with green methods and at a large scale and composed of food derivatives, ingestible in large amounts without risk for human health are needed. Here, conductive pastes made with materials with a high tolerable upper intake limit (≥mg kg −1 body weight per day) are proposed. Conductive oleogel composites, made with biodegradable and food-grade materials like natural waxes, oils, and activated carbon conductive fillers, are presented. The proposed pastes are compatible with manufacturing processes such as direct ink writing and thus are suitable for an industrial scale-up. These conductors are built without using solvents and with tunable electromechanical features and adhesion depending on the composition. They have antibacterial and hydrophobic properties so that they can be used in contact with food preventing contamination and preserving its organoleptic properties. As a proof-of-principle application, the edible conductive pastes are demonstrated to be effective edible contacts for food impedance analysis, to be integrated, for example, in smart fruit labels for ripening monitoring.

Graphene is considered an ideal filler for the production of multifunctional nanocomposites; as a result, considerable efforts have been focused on the evaluation and modeling of its reinforcement characteristics. In this work, we modelled successfully the mechanical percolation phenomenon, observed on a thermoplastic elastomer (TPE) reinforced by graphene nanoplatelets (GNPs), by designing a new set of equations for filler contents below and above the percolation threshold volume fraction (Vp). The proposed micromechanical model is based on a combination of the well-established shear-lag theory and the rule-of-mixtures and was introduced to analyse the different stages and mechanisms of mechanical reinforcement. It was found that when the GNPs content is below Vp, reinforcement originates from the inherent ability of individual GNPs flakes to transfer stress efficiently. Furthermore, at higher filler contents and above Vp, the nanocomposite materials displayed accelerated stiffening due to the reduction of the distance between adjacent flakes. The model derived herein, was consistent with the experimental data and the reasons why the superlative properties of graphene cannot be fully utilized in this type of composites, were discussed in depth.2

A comprehensive study has been undertaken on the dimensional swelling of graphene-reinforced elastomers in liquids. Anisotropic swelling was observed for samples reinforced with graphene nanoplatelets (GNPs), induced by the in-plane orientation of the GNPs. Upon the addition of the GNPs, the diameter swelling ratio of the nanocomposites was significantly reduced, whereas the thickness swelling ratio increased and was even greater than that of the unfilled elastomers. The swelling phenomenon has been analyzed in terms of a modification of the Flory–Rehner theory. The newly-derived equations proposed herein, can accurately predict the dependence of dimensional swelling (diameter and thickness) on volume swelling, independent of the type of elastomer and solvent. The anisotropic swelling of the samples was also studied in combination with the evaluation of the tensile properties of the filled elastomers. A novel theory that enables the assessment of volume swelling for GNP-reinforced elastomers, based on the filler geometry and volume fraction has been developed. It was found that the swelling of rubber nanocomposites induces a biaxial constraint from the graphene flakes.

3D printing is revolutionizing the manufacturing sector due to the myriad of materials and techniques available. Furthermore, it allows decentralized production sites for both industry and the public. However, it is restricted to static structures that cannot react to external stimuli or adapt to the environment and are, therefore, not suitable for functional and motile parts. Recently, two approaches are proposed to give dynamism to 3D printed structures: the printing of “stimulus‐responsive” (a.k.a. smart) materials and “4D printing,” the first implying features change due to a stimulus while the second indicating the time evolution of properties after a stimulus activation. Nanomaterials, particularly 2D nanomaterials, exhibit a broad and distinctive combination of features. Thus, they are highly effective at enabling this dynamism due to their morphological, optoelectrical, and mechanical properties. This review summarizes recent advances in 3D/4D printing of smart deformable and stimuli‐responsive materials which utilize 2D nanomaterials. The benefits of 2D materials in this framework are summarized, and how to translate their potential into 3D/4D printing is also discussed. The most promising achievements to date are deformable piezoresistive materials for strain sensing, Joule heaters, and actuators. Future advancements and possible upcoming application areas are finally proposed.