Project description:The design and the combination of innovative metamaterials are attracting increasing interest in the scientific community because of their unique properties that go beyond the ones of natural materials. In particular, auxetic materials and phononic crystals are widely studied for their negative Poisson's ratio and their bandgap opening properties, respectively. In this work, auxeticity and phononic crystals bandgap properties are properly combined to obtain a single phase periodic structure with a tridimensional wide tunable bandgap. When an external tensile load is applied to the structure, the auxetic unit cells change their configurations by exploiting the negative Poisson's ratio and this results in the tuning, either hardening or softening, of the frequencies of the modes limiting the 3D bandgap. Moreover, the expansion of the unit cell in all the directions, due to the auxeticity property, guarantees a fully 3D bandgap tunability of the proposed structure. Numerical simulations and analytical models are proposed to prove the claimed properties. The first experimental evidence of the tunability of a wide 3D bandgap is then shown thanks to the fabrication of a prototype by means of additive manufacturing.
Project description:Auxetic materials have been extensively studied for their design, fabrication and mechanical properties. These material systems exhibit unique mechanical characteristics such as high impact resistance, shear strength, and energy absorption capacity. Most existing auxetic materials are two-dimensional (2D) and demonstrate half-auxetic behavior, characterized by a negative Poisson's ratio when subjected to either tensile or compressive forces. Here, we present novel three-dimensional (3D) auxetic mechanical metamaterials, termed coupling chiral cuboids, capable of achieving negative Poisson's ratio under both tension and compression. We perform experiments, theoretical analysis, and numerical simulations to validate the wholly auxetic response of the proposed coupling chiral cuboids. Parametric studies are carried out to investigate the effects of structural parameters on the elastic modulus and Poisson's ratio of the coupling chiral cuboids. The results imply that the Poisson's ratio sign-switching from negative to positive can be implemented by manipulating the thickness of Z-shaped ligaments. Finally, the potential application of the coupling chiral cuboids as inner cores for impact-resistant sandwich panels is envisioned and validated. Test results demonstrate a remarkable 49.3% enhancement in energy absorption compared to conventional solid materials.
Project description:A very low-frequency mode supported within an auxetic structure is presented. We propose a constrained periodic framework with corner-to-corner and edge-to-edge sharing of tetrahedra and develop a kinematic model incorporating two types of linear springs to calculate the momentum term under infinitesimal transformations. The modal analysis shows that the microstructure with its two degrees of freedom has both low- and high-frequency modes under auxetic transformations. The low-frequency mode approaches zero frequency when the corresponding spring constant tends to zero. With regard to coupled eigenmodes, the stress-strain relationship of the uniaxial forced vibration covers a wide range. When excited, a very slow motion is clearly observed along with a structural expansion for almost zero values of the linear elastic modulus.
Project description:By combining the two basic deformation mechanisms for auxetic open-cell metamaterials, re-entrant angle and chirality, new hybrid chiral mechanical metamaterials are designed and fabricated via a multi-material 3D printer. Results from mechanical experiments on the 3D printed prototypes and systematic Finite Element (FE) simulations show that the new designs can achieve subsequential cell-opening mechanism under a very large range of overall strains (2.91%-52.6%). Also, the effective stiffness, the Poisson's ratio and the cell-opening rate of the new designs can be tuned in a wide range by tailoring the two independent geometric parameters: the cell size ratio [Formula: see text], and re-entrant angle ?. As an example application, a sequential particle release mechanism of the new designs was also systematically explored. This mechanism has potential application in drug delivery. The present new design concepts can be used to develop new multi-functional smart composites, sensors and/or actuators which are responsive to external load and/or environmental conditions.
Project description:Tensegrity, or tensional integrity, is a property of a structure indicating a reliance on a balance between components that are either in pure compression or pure tension for stability. Tensegrity structures exhibit extremely high strength-to-weight ratios and great resilience, and are therefore widely used in engineering, robotics and architecture. Here, we report nanoscale, prestressed, three-dimensional tensegrity structures in which rigid bundles of DNA double helices resist compressive forces exerted by segments of single-stranded DNA that act as tension-bearing cables. Our DNA tensegrity structures can self-assemble against forces up to 14 pN, which is twice the stall force of powerful molecular motors such as kinesin or myosin. The forces generated by this molecular prestressing mechanism can be used to bend the DNA bundles or to actuate the entire structure through enzymatic cleavage at specific sites. In addition to being building blocks for nanostructures, tensile structural elements made of single-stranded DNA could be used to study molecular forces, cellular mechanotransduction and other fundamental biological processes.
Project description:We report here structures, constructed with regular polygonal prisms, that exhibit negative Poisson's ratios. In particular, we show how we can construct such a structure with regular n-gonal prism-shaped unit cells that are again built with regular n-gonal component prisms. First, we show that the only three possible values for n are 3, 4 and 6 and then discuss how we construct the unit cell again with regular n-gonal component prisms. Then, we derive Poisson's ratio formula for each of the three structures and show, by analysis and numerical verification, that the structures possess negative Poisson's ratio under certain geometric conditions.
Project description:Supramolecular chirality typically originates from either chiral molecular building blocks or external chiral stimuli. Generating chirality in achiral systems in the absence of a chiral input, however, is non-trivial and necessitates spontaneous mirror symmetry breaking. Achiral nematic lyotropic chromonic liquid crystals have been reported to break mirror symmetry under strong surface or geometric constraints. Here we describe a previously unrecognised mechanism for creating chiral structures by subjecting the material to a pressure-driven flow in a microfluidic cell. The chirality arises from a periodic double-twist configuration of the liquid crystal and manifests as a striking stripe pattern. We show that the mirror symmetry breaking is triggered at regions of flow-induced biaxial-splay configurations of the director field, which are unstable to small perturbations and evolve into lower energy structures. The simplicity of this unique pathway to mirror symmetry breaking can shed light on the requirements for forming macroscopic chiral structures.
Project description:Biomimetic robots yearn for compliant actuators that are comparable to biological muscle in both functions and structural properties. For that, electrostatic actuators have been developed to imitate bio-muscle in features of fast response, high power, energy-efficiency, etc. However, those actuators typically lack impact damping performance, making them vulnerable and unstable in real applications. Here, we present auxetic electrostatic actuators that address this issue and demonstrate muscle-like performance by using elastomer-enhanced auxetics and electrostatic zipping mechanism. The proposed actuators contract linearly on applied voltage, producing large actuation strength (15 N) and contraction ratio (59%). Fabricated from readily available materials, our prototypes can quickly attenuate vibrations caused by impacts and absorb shock energy in 0.3 s. Furthermore, leveraging their 2-dimensional working mode and self-locking mechanism, a stiffness-changing muscle for a robotic arm and an active tensegrity device exemplify the potential applications of auxetic electrostatic actuators to a wide range of bionic robots.
Project description:Cryoablation of slow pathway doesn't usually cause junctional beats. If this occurs, the nearness to AV compact node is supposed. 3d electroanatomical mapping during this unusual finding may help to clarify the relationship between junctional beats (JBs) during cryomapping/cryoablation and Koch's triangle.
Project description:Lattice structures for implants can be printed using metal three-dimensional (3D)-printing and used as a porous microstructures to enhance bone ingrowth as orthopedic implants. However, designs and 3D-printed products can vary. Thus, we aimed to investigate whether targeted pores can be consistently obtained despite printing errors. The cube-shaped specimen was printed with one side 15 mm long and a full lattice with a dode-thin structure of 1.15, 1.5, and 2.0 mm made using selective laser melting. Beam compensation was applied, increasing it until the vector was lost. For each specimen, the actual unit size and strut thickness were measured 50 times. Pore size was calculated from unit size and strut thickness, and porosity was determined from the specimen's weight. The actual average pore sizes for 1.15, 1.5, and 2.0 mm outputs were 257.9, 406.2, and 633.6 μm, and volume porosity was 62, 70, and 80%, respectively. No strut breakage or gross deformation was observed in any 3D-printed specimens, and the pores were uniformly fabricated with < 10% standard deviation. The actual micrometer-scaled printed structures were significantly different to the design, but this error was not random. Although the accuracy was low, precision was high for pore cells, so reproducibility was confirmed.