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Highly sensitive wearable strain sensors based on graphene and its derivatives have shown great potential to be applied in home monitoring and electronic skin. To date, it still remains a technical challenge to achieve a reliable performance with high responsiveness at a small elastic strain for graphene strain sensors. Here we demonstrate the fabrication of micro/nanocrack-based strain sensors by using a porous thin film composed of graphene nanowalls (GNWs), which showed ultrahigh sensitivity with a gauge factor up to ≈8.6 × 104 at 4% strain. The ultrahigh gauge factor at such a small deformation can be attributed to the synergistic effect of micro/nanocrack evolution of GNW building blocks derived from the substrate's Poisson's ratio effect. This working mechanism is distinct from the general case where the cracks were created in an initially continuous film. The wearable GNW strain sensors were functionally presented as audio analog-to-digital converters for acoustic signature recognition and electronic skin devices to monitor physiological signals from the human body at high resolution. [ABSTRACT FROM AUTHOR]

Vacuum filtration is a highly effective and easy scale-up approach and has been widely used to fabricate graphene monoliths, such as graphene paper and graphene frameworks for various applications. In general, the microstructure of filtrated monoliths exhibits layer-by-layer stacking of graphene sheets due to the directional flow-induced assembly process. In this work, we found that the horizontally oriented structure of filtrated graphene frameworks can be modulated to an approximately isotropic arrangement by lowering the lateral size of graphene sheets. This size-dependent microstructure transition from anisotropic to isotropic was further confirmed by measuring the in- and through-plane thermal conductivity of the graphene/epoxy composites with different arrangements of graphene frameworks as a filler. Optimally, we obtained an epoxy composite embedded with a quasi-isotropic graphene framework (QIGF) by a simple two-step process: vacuum filtration of small graphene sheets to obtain the framework followed by the infiltration of epoxy resin. Based on the interconnected graphene sheets with an approximately isotropic arrangement, QIGF provides heat channels of graphene–graphene along both the in- and through-plane directions within epoxy. With a low graphene loading of 5.5 wt%, QIGF/epoxy (QIGF/EP) presents in- and through-plane thermal conductivities of 10.0 and 5.4 W mK−1, respectively, which are equivalent to ∼55 and 29 times higher than those of neat epoxy. As compared to the current graphene/epoxy composites prepared by various methods, our QIGF/EP has the highest thermal conductivity value with this level of filler loading. Our findings provide an insight for the development of polymer composites for thermal management applications in industry. [ABSTRACT FROM AUTHOR]