Synthesis of self-assembled nacre-like materials with 3D periodic layers from nanochitin via hydrogelation and mineralization

Here introduced a route for the synthesis of 3D structures that display a mechanical strength that competes with that of the toughest materials found in nature. Following the “brick-and-mortar” biomineralization typical of nacre, self-stratified, periodic materials are obtained by one-step ion diffusion gradient and hydrogelation of nanochitin with simultaneous mineral coprecipitation. Specifically, under appropriate electrolyte conditions, hydroxyapatite (HA) microspheres grow in an organic network formed from partially deacetylated chitin nanofibers (NCh), resulting in periodic stacking of mineralized (HA) and non-mineralized (NCh) layers. By directional diffusion, customizable 3D shapes are self-assembled and demonstrated to function as optical waveguides with selective light transmission at interfaces. Upon hot pressing, the resulting solid structures exhibit a superb mechanical performance while being biocompatible (tested with chondrogenic ATDC5 cells as a model for physiological mineralization). Overall, a shape-controlled, one-pot biomineralization method was proposed that achieves hierarchical, periodic and strong “nacre-like” structures suitable as biomedical material.

Dynamics of Piezo-Embedded Negative Stiffness Mechanical Metamaterials: A Study on Electromechanical Bandgaps

Dynamics of periodic materials and structures have a profound historic background starting from Newton’s first effort to find sound propagation in the air to Rayleigh’s exploration of continuous periodic structures. This field of interest has received another surge from the early 21st century. Elastic mechanical metamaterials are the exemplars of periodic structures that exhibit interesting frequency-dependent properties like negative Young’s modulus, negative mass and negative Poisson’s ratio in a specific frequency band due to additional feature of local resonance. In this research, we present the modeling of piezo-embedded negative stiffness metamaterials by considering a shunted inductor energy harvesting circuit. For a chain of a finite number of metamaterial units, the coupled equation of motion of the system is deduced using generalized Bloch’s theorem. Successively, the backward substitution method is applied to compute harvested power and the transmissibility of the system. Additionally, through the extensive non-dimensional study of this system, the proposed metamaterial band structure is investigated to perceive locally resonant mechanical and electromechanical bandgaps. The results explicate that the insertion of the piezoelectric material in the resonating unit provides better tun-ability for vibration attenuation and harvested energy.

Finite Volume-Based Asymptotic Homogenization of Periodic Materials Under In-Plane Loading

The previously developed finite volume-based asymptotic homogenization theory (FVBAHT) for anti-plane shear loading (He, Z., and Pindera, M.-J., “Finite-Volume Based Asymptotic Homogenization Theory for Periodic Materials Under Anti-Plane Shear,” Eur. J. Mech. A Solids (in revision)) is further extended to in-plane loading of unidirectional fiber reinforced periodic structures. Like the anti-plane FVBAHT, the present extension builds upon the previously developed finite volume direct averaging micromechanics theory applicable under uniform strain fields and further accounts for strain gradients and non-vanishing microstructural scale relative to structural dimensions, albeit with multidimensional in-plane loadings incorporated. The unit cell problems at different orders of the asymptotic field expansion are solved by satisfying local equilibrium equations and displacement and traction continuity in a surface-averaged sense which is unique among the existing asymptotic homogenization schemes, leading to microfluctuation functions that yield homogenized stiffness tensors at each order for use in macroscale problems. The newly extended multiscale theory is employed in the analysis of a structural boundary-value problem under in-plane loading, illustrating pronounced boundary effects. A combination approach proposed in the literature is subsequently employed to mitigate the boundary layer effects by explicitly accounting for the microstructural details in the boundary region. This combination approach produces accurate recovery of the local fields in both regions. The extension to in-plane problem marks FVBAHT as an alternative, self-contained asymptotic homogenization tool, with documented advantages relative to current numerical techniques, for the analysis of periodic materials in the presence of strain gradients produced by three-dimensional loading regardless of microstructural scale.