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Soft piezoresistive wearable conductors have led to a paradigm shift in the monitoring of human bodily motions. Cellular additively manufactured conductors are promising piezoresistive components as they offer mechanical tunability and provide controllable percolation pathways. In the present study, we engineer high surface-area cellular structures with the triply periodic minimal surface (TPMS)-based architectures to tailor their piezoresistive response for use in wearable devices. A simple and economical fabrication process is proposed, wherein a fused deposition modeling (FDM) 3D printing technique is utilized to fabricate flexible thermoplastic polyurethane (TPU) cellular structures. Interconnectivity of TPMS designs enables the coating of a continuous graphene layer over the TPU internal surfaces via a facile dip-coating process. The effects of pore shape on piezoresistivity are studied in four different TPMS structures (i.e., Primitive, Diamond, Gyroid, and I-WP). Mechanical properties of sensors are evaluated through experimental procedures and computation methods using finite element analysis of the Mooney-Rivlin hyperelastic model. The piezoresistive performance of sensors exhibits durability under cyclic compression loading. Finally, we conclude that the Primitive structure offers suitable piezoresistive characteristics for sensing of walking, whereas the Diamond structure presents favorable results for respiration monitoring.

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In this research, the size effect in the micro-upsetting process of oxygen-free high conductivity copper has been investigated numerically and experimentally. For the numerical part of this study, the crystal plasticity finite element (CPFE) method was employed and grains were produced using the Voronoi algorithm. The homogenization method was used for curve fitting and to obtain the material-hardening parameters. Then, simulation of the micro-upsetting process was carried out using a VUMAT subroutine which was written to implement crystal plasticity formulation. The micro-upsetting process was carried out at compression deformation of 50% for parts with diameters 0.75, 1 and 2 mm and height ratio of 1.5. Comparison of the data obtained from the CPFE and experimental procedures indicated a remarkable agreement. The results showed that, by increasing grain size from 30 to 60 μm, the forming forces for the diameters 0.75, 1 and 2 mm reduce by 12.3%, 6.8% and 6.7%, respectively, admitting the Hall–Petch equation. Also, according to the results obtained for constant grain sizes, when sample dimensions decrease, the stress–strain curve of the micro-upsetting process shows a downward shift which happens due to the increase in the grain size ratio and size effect. For the first time, investigation of the barrel shape and end surface geometry obtained from simulations and experimental tests shows distortion in the boundaries of both the modeled and fabricated specimens. Therefore, in micro-sized parts, the grain size has a greater effect on behavior of the material and geometrical accuracy.

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