Reversible energy absorption of elasto-plastic auxetic, hexagonal, and AuxHex structures fabricated by FDM 4D printing

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Abstract

The present study aims at introducing reconfigurable mechanical metamaterials by utilising four-dimensional (4D) printing process for recoverable energy dissipation and absorption applications with shape memory effects. The architected mechanical metamaterials are designed as a repeating arrangement of re-entrant auxetic, hexagonal, and AuxHex unit-cells and manufactured using 3D printing fused deposition modelling process. The AuxHex cellular structure is composed of auxetic re-entrant and hexagonal components. Architected cellular metamaterials are developed based on a comprehension of the elasto-plastic features of shape memory polylactic acid materials and cold programming deduced from theory and experiments. Computational models based on ABAQUS/Standard are used to simulate the mechanical properties of the 4D-printed mechanical metamaterials under quasi-static uniaxial compression loading, and the results are validated by experimental data. Research trials show that metamaterial with re-entrant auxetic unit-cells has better energy absorption capability compared to the other structures studied in this paper, mainly because of the unique deformation mechanisms of unit-cells. It is shown that mechanical metamaterials with elasto-plastic behaviors exhibit mechanical hysteresis and energy dissipation when undergoing a loading-unloading cycle. It is experimentally revealed that the residual plastic strain and dissipation processes induced by cold programming are completely reversible through simple heating. The results and concepts presented in this work can potentially be useful towards 4D printing reconfigurable cellular structures for reversible energy absorption and dissipation engineering applications.

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Xiaoyu Zheng, Howon Lee, Todd Weisgraber … (2014)

The mechanical properties of ordinary materials degrade substantially with reduced density because their structural elements bend under applied load. We report a class of microarchitected materials that maintain a nearly constant stiffness per unit mass density, even at ultralow density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with polymers, metals, or ceramics as constituent materials, is made possible by projection microstereolithography (an additive micromanufacturing technique) combined with nanoscale coating and postprocessing. We found that these materials exhibit ultrastiff properties across more than three orders of magnitude in density, regardless of the constituent material.

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Carbon nanotubes are man-made one-dimensional carbon crystals with different diameters and chiralities. Owing to their superb mechanical and electrical properties, many potential applications have been proposed for them. However, polydispersity and poor solubility in both aqueous and non-aqueous solution impose a considerable challenge for their separation and assembly, which is required for many applications. Here we report our finding of DNA-assisted dispersion and separation of carbon nanotubes. Bundled single-walled carbon nanotubes are effectively dispersed in water by their sonication in the presence of single-stranded DNA (ssDNA). Optical absorption and fluorescence spectroscopy and atomic force microscopy measurements provide evidence for individually dispersed carbon nanotubes. Molecular modelling suggests that ssDNA can bind to carbon nanotubes through pi-stacking, resulting in helical wrapping to the surface. The binding free energy of ssDNA to carbon nanotubes rivals that of two nanotubes for each other. We also demonstrate that DNA-coated carbon nanotubes can be separated into fractions with different electronic structures by ion-exchange chromatography. This finding links one of the central molecules in biology to a technologically very important nanomaterial, and opens the door to carbon-nanotube-based applications in biotechnology.

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Ryan Truby, Michael Wehner, Abigail K. Grosskopf … (2018)

Humans possess manual dexterity, motor skills, and other physical abilities that rely on feedback provided by the somatosensory system. Herein, a method is reported for creating soft somatosensitive actuators (SSAs) via embedded 3D printing, which are innervated with multiple conductive features that simultaneously enable haptic, proprioceptive, and thermoceptive sensing. This novel manufacturing approach enables the seamless integration of multiple ionically conductive and fluidic features within elastomeric matrices to produce SSAs with the desired bioinspired sensing and actuation capabilities. Each printed sensor is composed of an ionically conductive gel that exhibits both long-term stability and hysteresis-free performance. As an exemplar, multiple SSAs are combined into a soft robotic gripper that provides proprioceptive and haptic feedback via embedded curvature, inflation, and contact sensors, including deep and fine touch contact sensors. The multimaterial manufacturing platform enables complex sensing motifs to be easily integrated into soft actuating systems, which is a necessary step toward closed-loop feedback control of soft robots, machines, and haptic devices.