Mechanical Fourier transform for programmable metamaterials.

Proactively programming materials toward target nonlinear mechanical behaviors is crucial to realize customizable functions for advanced devices and systems, which arouses persistent explorations for rapid and efficient inverse design strategies. Herein, we propose a "mechanical Fourier transform" strategy to program mechanical behaviors of materials by mimicking the concept of Fourier transform. In this strategy, an arbitrary target force-displacement curve is decomposed into multiple cosine curves and a constant curve, each of which is realized by a rationally designed multistable module in an array-structured metamaterial. Various target curves with distinct shapes can be rapidly programmed and reprogrammed through only amplitude modulation on the modules. Two exemplary metamaterials are demonstrated to validate the strategy with a macroscale prototype based on magnet lattice and a microscale prototype based on an etched silicon wafer. This strategy applies to a variety of scales, constituents, and structures, and paves a way for the property programming of materials.

Inverse finite element characterization of the human thigh soft tissue in the seated position.

Pressure ulcers are localized damage to the skin and underlying tissues caused by sitting or lying in one position for a long time. Stresses within the soft tissue of the thigh and buttocks area play a crucial role in the initiating mechanism of these wounds. Therefore, it is crucial to develop reliable finite element models to evaluate the stresses caused by physiological loadings. In this study, we compared how the choice of material model and modeling area dimension affect prediction accuracy of a model of the thigh. We showed that the first-order Ogden and Fung orthotropic material models could approximate the mechanical behavior of soft tissue significantly better than neo-Hookean and Mooney-Rivlin. We also showed that, significant error results from using a semi-3D model versus a 3D model. We then developed full 3D models for 20 participants employing Ogden and Fung material models and compared the estimated material parameters between different sexes and locations along the thigh. We showed that males tissues are less deformable overall when compared to females and the material parameters are highly dependent on location, with tissues getting softer moving distally for both men and women.

A fully nonlinear viscohyperelastic model for the brain tissue applicable to dynamic rates.

Understanding the mechanical response of the brain to external loadings is of critical importance in investigating the pathological conditions of this tissue during injurious conditions. Such injurious loadings may occur at high rates, for example among others, during road traffic or sport accidents, falls, or due to explosions. Hence, investigating the injury mechanism and design of protective devices for the brain requires constitutive modeling of this tissue at such rates. Accordingly, this paper is aimed at critically investigating the physical background for viscohyperelastic modeling of the brain tissue with scrutinizing the elastic fields pertinent to large, time dependent deformations, and developing a fully nonlinear multimode Maxwell model that can mathematically explain such deformations. The proposed model can be calibrated using the simple monotonic uniaxial deformation of the sample extracted from the tissue, and does not require additional information from relaxation or creep experiments. The performance of the proposed model is examined using the experimental results of two different studies, which reveals a desirable agreement. The usefulness, limitations, and future developments of the proposed model are discussed in this paper.

Towards intervertebral disc engineering: Bio-mimetics of form and function of the annulus fibrosus lamellae.

The aging western society is heavily afflicted with intervertebral disc (IVD) degeneration. Replacement or repair of the degenerated IVD with an artificial bio-mimetic construct is one of the challenges of future research due to its complex structure and unique biomechanical function. Herein, biocomposite laminates made of long collagen fibers in unidirectional (-1.3 ±â€¯2.1°) and angle-plied ±â€¯30° orientations (30.4 ±â€¯6.4 and -29.8 ±â€¯4.5), embedded in alginate hydrogel, were fabricated to mimic the form of single annulus fibrosus (AF) lamella and the circumferential AF, respectively. The mechanical behavior of the composites was measured and compared with in vitro existing data of the human native AF as well as with new data obtained from ovine and bovine specimens. The mechanical behavior was found to reproduce the full stress- strain behavior of the human AF single lamella in several regions of the AF and the Young's modulus was 28.3 ±â€¯8.6 MPa. Moreover, the modulus of the angle-plied laminates was 16.8 ±â€¯2.9 MPa, which is approximately 5% less than the in vitro data. The full stress-strain behavior was also compared with bovine and ovine circumferential AF samples and found to be very similar, with a difference in the modulus of 4.1% and 19.7%, respectively. Moreover, an FE model of the L3-L4 functional spinal unit (FSU) was developed and calibrated to evaluate the mechanical ability of the biocomposite to be used as an AF substitute under physiological IVD loading modes. The biocomposite demonstrated a good ability to mimic the stiffness of the native tissue under physiologic loading modes as flexion, extension, lateral bending and compression, but was too flexible under torsion. It was found that the proposed biomimetics AF design resulted in a compatible function in several mechanical levels, which holds great potential to be used as a viable AF replacement towards full IVD engineering.