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Brain tissue is considered to be biphasic, with approximately 80% liquid and 20% solid matrix, thus exhibiting viscoelasticity due to rearrangement of the solid matrix and poroelasticity due to fluid migration within the solid matrix. However, how to distinguish poroelastic and viscoelastic effects in brain tissue remains challenging. In this study, we proposed a method of unconfined compression-isometric hold to measure the force versus time relaxation curves of porcine brain tissue samples with systematically varied sample lengths. Upon scaling the measured relaxation force and relaxation time with different length-dependent physical quantities, we successfully distinguished the poroelasticity and viscoelasticity of the brain tissue. We demonstrated that during isometric hold, viscoelastic relaxation dominated the mechanical behavior of brain tissue in the short-time regime, while poroelastic relaxation dominated in the long-time regime. Furthermore, compared with poroelastic relaxation, viscoelastic relaxation was found to play a more dominant role in the mechanical response of porcine brain tissue. We then evaluated the differences between poroelastic and viscoelastic effects for both porcine and human brain tissue. Because of the draining of pore fluid, the Young's moduli in poroelastic relaxation were lower than those in viscoelastic relaxation; brain tissue changed from incompressible during viscoelastic relaxation to compressible during poroelastic relaxation, resulting in reduced Poisson ratios. This study provides new insights into the physical mechanisms underlying the roles of viscoelasticity and poroelasticity in brain tissue. STATEMENT OF SIGNIFICANCE: Although the poroviscoelastic model had been proposed to characterize brain tissue mechanical behavior, it is difficult to distinguish the poroelastic and viscoelastic behaviors of brain tissue. The study distinguished viscoelasticity and poroelasticity of brain tissue with time scales and then evaluated the differences between poroelastic and viscoelastic effects for both porcine and human brain tissue, which helps to accurate selection of constitutive models suitable for application in certain situations (e.g., pore-dominant and viscoelastic-dominant deformation).

Acoustic transduction by plants has been proposed as a mechanism to enable just-in-time up-regulation of metabolically expensive defensive compounds. Although the mechanisms by which this "hearing" occurs are unknown, mechanosensation by elongated plant hair cells known as trichomes is suspected. To evaluate this possibility, we developed a theoretical model to evaluate the acoustic radiation force that an elongated cylinder can receive in response to sounds emitted by animals, including insect herbivores, and applied it to the long, cylindrical stem trichomes of the tomato plant Solanum lycopersicum. Based on perturbation theory and validated by finite element simulations, the model quantifies the effects of viscosity and frequency on this acoustic radiation force. Results suggest that acoustic emissions from certain animals, including insect herbivores, may produce acoustic radiation force sufficient to trigger stretch-activated ion channels.

How cells sense tissue stiffness to guide cell migration is a fundamental question in development, fibrosis and cancer. Although durotaxis-cell migration towards increasing substrate stiffness-is well established, it remains unknown whether individual cells can migrate towards softer environments. Here, using microfabricated stiffness gradients, we describe the directed migration of U-251MG glioma cells towards less stiff regions. This 'negative durotaxis' does not coincide with changes in canonical mechanosensitive signalling or actomyosin contractility. Instead, as predicted by the motor-clutch-based model, migration occurs towards areas of 'optimal stiffness', where cells can generate maximal traction. In agreement with this model, negative durotaxis is selectively disrupted and even reversed by the partial inhibition of actomyosin contractility. Conversely, positive durotaxis can be switched to negative by lowering the optimal stiffness by the downregulation of talin-a key clutch component. Our results identify the molecular mechanism driving context-dependent positive or negative durotaxis, determined by a cell's contractile and adhesive machinery.

The ballistic performance of protective structures under multiple projectile impacts attracts increasing attention due to its practical importance, and existing studies were seldomly devoted to exploring how the structure would deform and fail when subjected to such loads. This study aimed to characterize the multi-hit ballistic resistance of fully-clamped thin plates made of 304 stainless steel using finite element method, with the equivalent plastic strain employed to define material damage and failure/fracture. The numerical model was validated against existing experimental results of double impacts at the same location, with good agreement achieved. The model was subsequently employed to quantify the effects of impact position, interval time between successive hits, projectile nose shape (e.g., spherical, flat, and conical), and boundary condition of target plate on ballistic limit and deformation/failure modes. Further, ballistic limit boundaries were constructed for both double and triple impacts of projectiles. Obtained results are helpful for designing high-performance protective structures against multiple projectile impacts.

Pain sensation induced by kidney stone (renal calculi) in ureter, a kind of visceral ducts connecting the kidneys and bladder, critically depends upon the relative size of stone to ureter. To quantify such pain sensation, we draw a parallel analogy between the mechanisms underlying skin pain (which can be quantified with a holistic pain model consisting of a modified Hodgkin–Huxley model and gate control theory) and mechanism of ureteral pain to extend the holistic pain model to the stone-blocked ureter. We then perform finite element simulations to obtain key mechanical stresses on the ureter wall exerted by a kidney stone having varying sizes. These stresses are subsequently adopted to calculate the voltage potential of neuron membrane in the holistic pain model and eventually a theoretical framework to quantify the dependence of ureteral pain sensation on stone size is established, for the first time. We demonstrate that ureter pain sensation increases sharply when the diameter of the kidney stone becomes 7.5% to 20% larger than the inner diameter of ureter, peaking at ∼20% larger; however, increasing further the stone diameter leads only to marginally exacerbated pain sensation. Other related effects on ureter pain sensation, such as ureter wall thickness, ureter stiffness, and intra-abdominal pressure (IAP), are evaluated. The results of the present study provide insightful information for urologists to diagnose and treat patients with renal calculi in a more personalized way.

SummaryGlnR activates nitrogen metabolism genes under nitrogen-limited conditions whereas MtrA represses these genes under nutrient-rich conditions in Streptomyces. In this study, we compared the transcription patterns of nitrogen metabolism genes in a double deletion mutant (ΔmtrA-glnR) lacking both mtrA and glnR and in mutants lacking either mtrA (ΔmtrA) or glnR (ΔglnR). The nitrogen metabolism genes were expressed similarly in ΔmtrA-glnR and ΔglnR under both nitrogen-limited and nutrient-rich conditions, with patterns distinctly different from that of ΔmtrA, suggesting a decisive role for GlnR in the control of nitrogen metabolism genes and further suggesting that regulation of these genes by MtrA is GlnR-dependent. MtrA and GlnR utilize the same binding sites upstream of nitrogen metabolism genes, and we showed stronger in vivo binding of MtrA to these sites under nutrient-rich conditions and of GlnR under nitrogen-limited conditions, consistent with the higher levels of MtrA or GlnR under those respective conditions. In addition, we showed that both mtrA and glnR are auto-regulatory. Our study provides new insights into the regulation of nitrogen metabolism genes in Streptomyces.

Although coupling between cardiomyocytes and myofibroblasts is well known to affect the physiology and pathophysiology of cardiac tissues across species, relating these observations to humans is challenging because the effect of this coupling varies across species and because the sources of these effects are not known. To identify the sources of cross-species variation, we built upon previous mathematical models of myofibroblast electrophysiology and developed a mechanoelectrical model of cardiomyocyte-myofibroblast interactions as mediated by electrotonic coupling and transforming growth factor-β1. The model, as verified by experimental data from the literature, predicted that both electrotonic coupling and transforming growth factor-β1 interaction between myocytes and myofibroblast prolonged action potential in rat myocytes but shortened action potential in human myocytes. This variance could be explained by differences in the transient outward K(+) current associated with differential Kv4.2 gene expression across species. Results are useful for efforts to extrapolate the results of animal models to the predicted effects in humans and point to potential therapeutic targets for fibrotic cardiomyopathy.

Liquid-filled porous materials exist widely in nature and engineering fields, with the diffusion of substances in them playing an important role in system functions. Although surface evaporation is often inevitable in practical scenarios, the evaporation effects on diffusion behavior in liquid-filled porous materials have not been well explored yet. In this work, we performed noninvasive diffusion imaging experiments to observe the diffusion process of erioglaucine disodium salt dye in a liquid-filled nitrocellulose membrane under a wide range of relative humidities (RHs). We found that evaporation can significantly accelerate the diffusion rate and alter concentration distribution compared with the case without evaporation. We explained the accelerated diffusion phenomenon by the mechanism that evaporation would induce a weak flow in liquid-filled porous materials, which leads to convective diffusion, i.e., evaporation-induced flow and diffusion (EIFD). Based on the EIFD mechanism, we proposed a convective diffusion model to quantitatively predict the diffusion process in liquid-filled porous materials under evaporation and experimentally validated the model. Introducing the dimensionless Peclet (Pe) number to measure the relative contribution of the evaporation effect to pure molecular diffusion, we demonstrated that even at a high RH of 95%, where the evaporation effect is usually assumed negligible in common sense, the evaporation-induced diffusion still overwhelms the molecular diffusion. The flow velocity induced by evaporation in liquid-filled porous materials was found to be 0.4–5 μm/s, comparable to flow in many biological and biomedical systems. The present analysis may help to explain the driving mechanism of tissue perfusion and provide quantitative analysis or inspire new control methods of flow and material exchange in numerous cutting-edge technologies, such as paper-based diagnostics, hydrogel-based flexible electronics, evaporation-induced electricity generation, and seawater purification.

The underbody of a vehicle system, either military or civil, is typically made of a relatively thin metallic plate, thus vulnerable to mine blast attacks. To improve the blast resistance, a multitude of protective structures have been proposed as attachments to the thin plate. In the present study, a novel ultralight all-metallic sandwich panel with three-dimensional (3D) tube cellular cores mounted to the vehicle underbody was envisioned as such a protective system. A metallic substrate (mimicking vehicle bottom) was placed above the proposed sandwich panel to construct a sandwich-substrate combinative structure. A series of sandwich panels having 3D tube cellular cores were fabricated via argon protected welding and laser welding. Mechanical responses of the combinative structure subjected to the denotation of 6 kg TNT explosives shallow-buried in dry sand were experimentally measured. Full numerical simulations with the method of finite elements (FE) were subsequently carried out to explore the physical mechanisms underlying the observed dynamic performance and quantify the effects of key geometrical parameters and connection conditions of the protective system. The performance of the proposed sandwich panel under shallow-buried explosives was also compared with competing sandwich constructions having equal mass. Finally, a preliminary optimal design of the 3D tube cellular core was carried out.

Contractile stress generated by a cell itself is crucial in sensing its surrounding microenvironment and regulating cell adhesion, differentiation and cytokinesis. However, the precise mechanisms underlying mechanotransduction remain unknown. In this paper, based on the Eshelby inclusion problem, we develop a theoretical model to characterize quantitatively the mechanical environment around a self-contractile cell experiencing stress fibers reorganization. We divide the contractile stress into two parts: the constant contractile stress and the perturbed contractile stress due to stress fibers reorganization, for internal stress fibers have enough time to reorganize actively during long-term deformation, leading to changes of contractile stress in both magnitude and direction. Obtained results suggest that stress fibers reorganization may cause significant changes in the mechanical environment of the cell, helpful for exploring the mechanisms behind cell mechanotransduction.

During descent, a single-winged maple seed (samara) can naturally reach a delicate equilibrium state, stable autorotation, before landing. This article reveals the intrinsic equilibrium of a particular type of samaras in terms of measurable aerodynamic and geometric parameters. To this end, we conducted a series of in situ measurements for the rate of vertical descent (exclusive of crosswind) of an autorotating samara in a natural range of samara sizes and masses. We then extended the range of size and mass by introducing artificial samaras, with discrete mass elements purposely designed to approximate the asymmetrical and nonuniform distribution of mass found with natural samaras. Based on the widened range, a fundamental nondimensional correlation of dynamic pressure and disc loading was generalized, where all stable autorotation descent profiles collapse to a single descent characteristic curve, irrespective of the size and mass of the natural and artificial samara's specimens. Results reveal that for stably autorotating (both natural and artificial) samaras, their terminal descent velocity (expressed as dynamic pressure) and disc loading attained equilibrium at a value that is inversely proportional to the coefficient of lift.

A theoretical model is developed to study the superior sound absorption performance of ultralight mirco-perforated sandwich panels with double-layer hierarchical honeycomb core. Numerical simulations are performed to validate theoretical model predictions and explore physical mechanisms underlying the sound absorption. Systematic parametric study is implemented to investigate the influence of specific structural parameters on sound absorption. To maximize sound absorption, optimal structural parameters of the hierarchical sandwich are obtained using the method of simulated annealing. It is demonstrated that viscous dissipation of the air inside micro-perforations and around inlet/outlet regions dominates sound absorption. Compared to micro-perforated sandwich panels with regular honeycomb core, not only the proposed hierarchical construction has much improved load-bearing capacity, but also significantly enhanced sound absorption covers a wide range of frequency.

Poroelasticity of cytoplasm is a rate- and size-dependent biphasic material behavior that reflects the normal activities and pathological states of cells, mainly caused by the migration of fluid molecules and the deformation of porous solid skeleton (protein scaffold). While micro/nano-indentation tests have been extensively used to characterize the poroelasticity of a cell, characterizing the in situ poroelasticity of cytoplasm remains elusive. In this study, based on the theory of the translation of a rigid spherical inclusion, we proposed a new method to characterize the in situ poroelasticity of cytoplasm. Based on data from optical/magnetic tweezers tests, we estimated three key poroelasticity parameters—shear modulus, Poisson ratio and diffusion coefficient—of cytoplasm for a variety of cells, including cardiomyocytes, endothelial cells of bovine capillary, and fibroblasts. The proposed method provides a powerful tool for in situ measurement of poroelastic properties of cytoplasm via optical/magnetic tweezers.

Cells are compressible and can be regarded as a kind of coated liquid inclusion embedded in a three-dimensional elastic matrix. In the presence of far-field loading, how the coating influences the mechanical response (e.g., volume change) of the liquid inclusion remains elusive, especially when surface tension effects become significant at cell size level. We developed a theoretical model to characterize the mechanical amplification or attenuation role of coating on spherical liquid inclusions, with surface tension and liquid compressibility accounted for. We found that surface tension could increase the volumetric strain of the inclusion through decreasing its effective bulk modulus. We further found that, when there is a monotonic stiffness variation (either decreasing or increasing) from matrix via coating to inclusion, the presence of coating amplified the volumetric strain compared with the case without coating; in the opposite, when there is a non-monotonic stiffness change from matrix via coating to inclusion, the volumetric strain is attenuated by the coating. The results are useful for understanding and exploring the mechanobiological sensation of certain types of cell, e.g., osteocytes and cancer cells.

Ceramic materials have been extensively used as armour materials for nearly 50 years and continue to attract great interest in the field of defense technology. As confinement is crucial for ceramic...

Three different alumina/aluminum bi-layer armors having different striking faces, i.e., monolithic alumina, mosaic alumina, and mosaic alumina enhanced by aluminum honeycomb, were fabricated and tested under the impact of the flat projectile. The ballistic performance of each armor type was also investigated using three-dimensional (3D) finite element (FE) simulations. Upon validating the FE simulation results with experimental measurements, the ballistic limit velocity and failure mechanisms for each type of armor, as well as the influence of ceramic tile size, impact position, border effect, and inter tile gap width were quantified. For the enhanced mosaic armor, the metallic honeycomb lattice performed as a cellular skeleton to confine the ceramic tiles and fragments, leading to enhanced ballistic resistance. Besides, the honeycomb enhanced mosaic armor was also found to have much improved multi-hit ballistic resistance in comparison with monolithic and mosaic alumina. The honeycomb preserved the structural integrity of the mosaic armor so that a high level of residual ballistic resistance remained even after impact. With the extended reliability calculation method, single shot ballistic data were used to estimate the performance of the honeycomb enhanced mosaic armor under multiple projectile impacts.

During drought, plant functions at multi-levels (i.e., tissue, cellular and molecular) are adjustable with the change of water condition, which is known as drought resistance. Various biological, chemical and physical mechanisms have been found in plant drought resistance, among which the role of physical cues (especially mechanics) has attracted significantly increasing attention. Recent studies have shown that mechanics is one of the fundamental factors that control the responses and self-adaptation from tissue to molecular levels in plant when the external conditions changes. In the review, we examine how the factor of mechanics acts on the multi-level plant functions under drought stress, including water transport, tissue deformation, cell growth, cell movements, molecules interaction and signal pathway.

Micro-perforated panel (MPP) absorbers with circular perforations are used in many noise control applications due to their attractive wide-brand sound absorption performance. Different from a common MPP with circular perforations, a unique type of MPP absorber with petal-shaped perforations is proposed. The sound absorption theory for the MPP with petal-shaped perforations is developed by accurately considering the fluid velocity in the petal-shaped perforation hole. This theory can account for the effect of altered perforation morphology (from circular to petal) on sound absorption. Finite element simulations are performed to validate the proposed theory, with good agreement achieved. The sound absorption of MPP with petal-shaped perforations is compared with that of the traditional MPP with the same porosity. It is demonstrated that the change in hole shape significantly modifies the fluid velocity field and the flow resistivity in/of the hole, and hence the sound absorption of the proposed MPP with petal-shaped perforations can outperform that of the traditional MPP in the considered case. This work proposes a general MPP theory that not only contains the classical Maa's theory for circular MPP, but also accounts for the MPP with petal-shaped perforations.

All-metallic truncated conical sandwich shells with corrugated cores (TCSS) were investigated for crashworthiness under quasi-static oblique compression using a combined experimental and numerical ...

All-metallic sandwich-walled cylindrical and conical structures for aerospace applications often require simultaneous excellent vibration and load-carrying capacities. In the present study, the free vibration and axial compression behaviors of cylindrical and truncated conical sandwich shells with corrugated cores are investigated using a combined experimental and numerical approach. Excellent agreement between experimental measurements and finite element simulations for representative vibration and axial compression characteristics is achieved. Parametric studies based on the response surface model are subsequently performed to quantify the influence of key geometrical parameters on vibration and axial compression performance. A multi-functional collaborative design to meet the requirement of high load-bearing capacity subjected to the constraint of low natural frequency is also carried out, which demonstrates the unique advantages of the novel sandwich-walled shells for aeronautical and astronautical applications.

Thin structural elements such as large-scale covering plates of aerospace protection structures and vertical stabilizers of aircraft are strongly influenced by gravity (and/or acceleration); thus, exploring how the mechanical behaviors of such structures are affected by gravitational field is necessary. Built upon a zigzag displacement model, this study establishes a three-dimensional vibration theory for ultralight cellular-cored sandwich plates subjected to linearly varying in-plane distributed loads (due to, e.g., hyper gravity or acceleration), with the cross-section rotation angle induced by face sheet shearing accounted for. For selected boundary conditions, the theory enables quantifying the influence of core type (e.g., close-celled metal foams, triangular corrugated metal plates, and metal hexagonal honeycombs) on fundamental frequencies of the sandwich plates. For validation, three-dimensional finite element simulations are carried out, with good agreement achieved between theoretical predictions and simulation results. The validated theory is subsequently employed to evaluate how the geometric parameters of metal sandwich core and the mixture of metal cores and composite face sheets influence the fundamental frequencies. Triangular corrugated sandwich plate possesses the highest fundamental frequency, irrespective of boundary conditions. For each type of sandwich plate considered, the presence of in-plane distributed loads significantly affects its fundamental frequencies and modal shapes.