Spatial Distribution Of Forces Research Articles

CNT/PDMS conductive foam-based piezoresistive sensors with low detection limits, excellent durability, and multifunctional sensing capability

In this paper, a carbon nanotube (CNT)/polydimethylsiloxane (PDMS) conductive composite foam (CCF) was fabricated via a dual-solvent ice template (DSIT) process, whose structure features the conductive filler CNT "embedded" in the cell wall surface, and this CCF was applied to the field of flexible piezoresistive sensing. Benefiting from the sensitive conductive network constructed by the DSIT process, the CNT/PDMS CCF-based piezoresistive sensor can effectively detect compression strains down to 0.1% and exhibits excellent and stable response at compression strains up to 90%. In addition, the CCF shows fast response and recovery times (54 ms and 65 ms), as well as excellent durability and stability (2000 cycles). An electronic skin assembled from the CCF into 5 × 5 pixels was applied to detect the magnitude and spatial distribution of forces and strains. The CCF was also applied for roughness recognition, optical and thermal sensing responses, which shows its potential applications in personalized medical monitoring, electronic smart skin fabrication, external environment monitoring and other fields.

Component decomposition and spatial distribution of unsteady forces on a circular cylinder oscillated under various amplitudes at Re = 2.0×104

Vortex-induced vibrations (VIVs) are of practical significance in the design of engineering structures, and the dynamic loads responsible for VIVs are thus crucial. In this study, the force characteristics of a circular cylinder undergoing forced oscillations at vortex lock-in states are investigated by employing three-dimensional large eddy simulations (LES) at a moderate subcritical Reynolds number of 2.0 × 104. Simulations of the oscillating cylinder are performed under a series of normalized amplitudes ranging from 0.05 to 0.60 at a fixed reduced velocity of 6.0. Moreover, a force spectrum-based approach is proposed to decompose the overall force into motion-induced and vortex-shedding components. Alternative insights into the spatial distribution of the force are also obtained by analysing the spanwise correlation, pressure field, and flow feature. The numerical results show that the motion-induced lifts experience an abrupt jump in both magnitude and phase as the oscillation amplitude increases. Furthermore, the motion-induced forces are almost fully correlated except in the phase-jump regime, although the correlation of the overall forces is relatively weak. Finally, the identified force parameters are successfully applied to predict the VIV amplitudes of the cylinder under various mass-damping conditions.

Environment‐tolerant ionic hydrogel–elastomer hybrids with robust interfaces, high transparence, and biocompatibility for a mechanical–thermal multimode sensor

AbstractThe human skin, an important sensory organ, responds sensitively to external stimuli under various harsh conditions. However, the simultaneous achievement of mechanical/thermal sensitivity and extreme environmental tolerance remains an enormous challenge for skin‐like hydrogel‐based sensors. In this study, a novel skin‐inspired hydrogel–elastomer hybrid with a sandwich structure and strong interfacial bonding for mechanical–thermal multimode sensing applications is developed. An inner‐layered ionic hydrogel with a semi‐interpenetrating network is prepared using sodium carboxymethyl cellulose (CMC) as a nanofiller, lithium chloride (LiCl) as an ionic transport conductor, and polyacrylamide (PAM) as a polymer matrix. The outer‐layered polydimethylsiloxane (PDMS) elastomers fully encapsulating the hydrogel endow the hybrids with improved mechanical properties, intrinsic waterproofness, and long‐term water retention (>98%). The silane modification of the hydrogels and elastomers imparts the hybrids with enhanced interfacial bonding strength and integrity. The hybrids exhibit a high transmittance (~91.2%), fatigue resistance, and biocompatibility. The multifunctional sensors assembled from the hybrids realize real‐time temperature (temperature coefficient of resistance, approximately −1.1% °C−1) responsiveness, wide‐range strain sensing capability (gauge factor, ~3.8) over a wide temperature range (from −20°C to 60°C), and underwater information transmission. Notably, the dual‐parameter sensor can recognize the superimposed signals of temperature and strain. The designed prototype sensor arrays can detect the magnitude and spatial distribution of forces and temperatures. The comprehensive performance of the sensor prepared via a facile method is superior to that of most similar sensors previously reported. Finally, this study develops a new material platform for monitoring human health in extreme environments.image

Electromagnetic force mapping methods on dissimilar finite element meshes: Benchmark, discussion and outcomes

The interpolation of electromagnetic loads to mechanical models, between meshes dissimilar in discretization and/or in topology, should be carefully performed in order to preserve both the global and local mechanical equilibrium of the system and the spatial distribution of forces [1]. The purpose of this work is to test different force mapping methods, determine the validity and limitations of methods and software to be used, and demonstrate the ability to solve practical problems. For these reasons, a benchmark problem has been prepared and proposed to some qualified companies. After a brief introduction of the context in ITER project and the illustration of the proposed benchmark problem, the paper deals with the theoretical background of the methods used by each company, the results presented and the outcome of the benchmark exercise.

Muscle‐Inspired MXene Conductive Hydrogels with Anisotropy and Low‐Temperature Tolerance for Wearable Flexible Sensors and Arrays

AbstractConductive hydrogels as flexible electronic devices, not only have unique attractions but also meet the basic need of mechanical flexibility and intelligent sensing. How to endow anisotropy and a wide application temperature range for traditional homogeneous conductive hydrogels and flexible sensors is still a challenge. Herein, a directional freezing method is used to prepare anisotropic MXene conductive hydrogels that are inspired by ordered structures of muscles. Due to the anisotropy of MXene conductive hydrogels, the mechanical properties and electrical conductivity are enhanced in specific directions. The hydrogels have a wide temperature resistance range of −36 to 25 °C through solvent substitution. Thus, the muscle‐inspired MXene conductive hydrogels with anisotropy and low‐temperature resistance can be used as wearable flexible sensors. The sensing signals are further displayed on the mobile phone as images through wireless technology, and images will change with the collected signals to achieve motion detection. Multiple flexible sensors are also assembled into a 3D sensor array for detecting the magnitude and spatial distribution of forces or strains. The MXene conductive hydrogels with ordered orientation and anisotropy are promising for flexible sensors, which have broad application prospects in human–machine interface compatibility and medical monitoring.

Study of the Spatial Distribution of Forces and Stresses on Wear Surfaces at Optimization of the Excavating Part of an Earthmoving Machine Transverse Profile

The relevance of this research lies in the need to develop a scientifically-grounded methodology for designing the optimal profile of the contact frontal surface of an energy-efficient earthmoving machine operating device, which ensures the redistribution of resistance forces along the cutting elements during the excavation of frozen soil when extracting hydrocarbons. The relevance is confirmed by the need to improve domestic methods for designing the profiles of effective operating devices, ensuring the excavation of frozen soils during the laying of oil and gas pipelines and geological exploration in permafrost areas. Increasing the energy efficiency and service life of highly loaded ripper teeth is done by optimizing the geometric profile and designing a spatial shape that provides minimal resistance to cutting the soil. The object of the research is the process of dynamic interaction of the ripper tooth cutting profile with the frozen soil environment. The research method used was kinematic analysis and force calculation. The cyclic process of dynamic loading of the ripper operating device was investigated. The methodology of system analysis, as well as the method of distribution of resistance forces over the geometric elements of the cutting edge, were used. The mathematical apparatus has been effectively applied to establish the dependences of the change in energy and power properties on performance indicators, soil rheology, and dynamic loads of the ripper tooth. The process of interaction of the ripper tooth tip with frozen soil was investigated. A method for substantiating the design parameters of the operating device by using Legendre polynomials was proposed. Separate calculation results subject to the introduced restrictions were presented for the condition for solving the problem. The optimal parameters of the transverse profile of the contact frontal surface of the earth-moving machine operating device were determined taking into account the coefficient of reducing the fracture resistance of frozen soils.

Cell Junction Mechanics beyond the Bounds of Adhesion and Tension.

Cell-cell junctions, in particular adherens junctions, are major determinants of tissue mechanics during morphogenesis and homeostasis. In attempts to link junctional mechanics to tissue mechanics, many have utilized explicitly or implicitly equilibrium approaches based on adhesion energy, surface energy, and contractility to determine the mechanical equilibrium at junctions. However, it is increasingly clear that they have significant limitations, such as that it remains challenging to link the dynamics of the molecular components to the resulting physical properties of the junction, to its remodeling ability, and to its adhesion strength. In this perspective, we discuss recent attempts to consider the aspect of energy dissipation at junctions to draw contact points with soft matter physics where energy loss plays a critical role in adhesion theories. We set the grounds for a theoretical framework of the junction mechanics that bridges the dynamics at the molecular scale to the mechanics at the tissue scale.

Force localization and reconstruction based on a novel sparse Kalman filter

Most of existing methods for force identification assume the force locations to be known a priori. In this paper, we propose a novel recursive algorithm, named sparse Kalman filter, to simultaneously localize and reconstruct forces in time domain. The spatial distribution of forces at each time step is estimated by Relevance Vector Machine. With its sparsity-inducing ability, sparse Kalman filter can monitor forces at a large number of potential locations with a limited number of sensors, at a speed much higher than traditional batch methods. We also present the application of sparse Kalman filter with a smoothing technique, namely allowing a time delay between the measurement and input estimation step. In this way, the computational dimension is increased in exchange for an improved estimation accuracy. The proposed algorithms are validated on numerical simulations of a fixed-end beam, a truss structure, and an engineering-scale support structure. In each simulation, different sampling frequencies, smoothing delays, measurement noise levels, and force locations are considered to comprehensively understand the performance of sparse Kalman filter and its smoothing version.

Modeling distributed forces within cell adhesions of varying size on continuous substrates.

Cell migration and traction are essential to many biological phenomena, and one of their key features is sensitivity to substrate stiffness, which biophysical models, such as the motor-clutch model and the cell migration simulator can predict and explain. However, these models have not accounted for the finite size of adhesions, the spatial distribution of forces within adhesions. Here, we derive an expression that relates varying adhesion radius ( R) and spatial distribution of force within an adhesion (described by s) to the effective substrate stiffness ( κsub ), as a function of the Young's modulus of the substrate ( E Y ), which yields the relation, , for two-dimensional cell cultures. Experimentally, we found that a cone-shaped force distribution ( s = 1.05) can describe the observed displacements of hydrogels deformed by adherent U251 glioma cells. Also, we found that the experimentally observed adhesion radius increases linearly with the cell protrusion force, consistent with the predictions of the motor-clutch model with spatially distributed clutches. We also found that, theoretically, the influence of one protrusion on another through a continuous elastic environment is negligible. Overall, we conclude cells can potentially control their own interpretation of the mechanics of the environment by controlling adhesion size and spatial distribution of forces within an adhesion.

A Model of Surface Residual Stress Distribution of Cold Rolling Spline

Residual stress is an important parameter in the evaluation of the performance of a cold rolling spline surface. However, research on cold rolling spline is rare. To improve the surface property of a spline, an involute spline is selected as the object of this study. The contour method for determining cold roll-beating residual stress involves measuring the force spatial distribution, performing a statistical analysis of the experimental results, establishing the parameters for the tooth profile for different positions (dedendum, pitch, and addendum) of residual stress, and determining the effect of pressure on the relationship between stress and the depth of the cold roll-beating. A response surface method is used to establish the spline tooth profile of the dedendum, pitch, and addendum of the residual stress and different depths of the stress layer to obtain the parameters of a multiple regression model and perform a comparative analysis of the experimental and prediction results. Research indicates that the prediction results have high reliability. The establishment of this model has important guiding significance to control the residual stress in the cold roll-beating forming process, optimize the cold roll-beating processing parameters, and improve the surface properties of cold rolling spline.

Mechanical coupling of cytoskeletal elasticity and force generation is crucial for understanding the migrating nature of keloid fibroblasts.

One of the key features of keloid is its fibroblasts migrating beyond the original wound border. During migration, cells not only undergo molecular changes but also mechanical modulation. This process is led by actin filaments serving as the backbone of intra-cellular force and transduces external mechanical signal via focal adhesion complex into the cell. Here, we focus on determining the mechanical changes of actin filaments and the spatial distribution of forces in response to changing chemical stimulations and during cell migration. Atomic force microscopy and micropost array detector are used to determine and compare the magnitude and distribution of filament elasticity and force generation in fibroblasts and keloid fibroblasts. We found both filament elasticity and force generation show spatial distribution in a polarized and migrating cell. Such spatial distribution is disrupted when mechano-signalling is perturbed by focal adhesion kinase inhibitor and in keloid fibroblasts. The demonstration of keloid pathology at the nanoscale highlights the coupling of cytoskeletal function with physical characters at the subcellular level and provides new research directions for migration-related disease such as keloid.

High Resolution Force Distribution of Migrating and Spreading Cells

Cell motility is crucial for the immune system and its misregulation is associated with cancer and other diseases. Migration is a complex cellular activity with many proteins coordinating within a subcellular motility machine. The machine's parts conspire to propel the cell forward by exerting forces on the environment with spatial variations down to the nanoscale. In this work we measured and statistically characterized the force distributions with the aim of illuminating underlying mechanisms. We cultured mouse embryonic fibroblast cells expressing GFP-actin on dense arrays of flexible nanoengineered pillars. During cell spreading pillar deflections quantitatively revealed cell traction patterns on the substrate. Simultaneous actin imaging enabled the cellular force map to be visualized within the evolving cell contour. The greatest forces were exerted in a ∼10-20 micron strip behind the leading edge and are directed inward. We analyzed time profiles of forces exerted on single pillars as the cell passed over the pillar location. Forces were relatively short lived and showed more fluctuation for pillars with smaller diameter than for larger ones. When the protruding cell edge reaches a pillar a period of large traction force begins, typically lasting several minutes, with maximum stress on the order of several nN/μm2. Stiffer substrates provoked larger tractions than more compliant substrates. Once the front of the cell has passed by, a drastic reduction in traction occurs. Spatial force distribution and its evolution were characterized by correlation functions relative to the leading edge. By varying the geometric parameters of the pillars, we probed a range of effective substrate rigidities. In summary, we combined high resolution measurements, statistical analysis and quantitative modeling to characterize and interpret signature features of migrating and spreading cell force maps.

Incorporating fractal concepts into equations of attrition for military conflicts

Using an agent-based model as a ‘battlefield laboratory’, we explore equations of combat attrition which extend beyond the conventional Lanchester equations and which endeavour to encapsulate the more complex aspects of warfare. Our approach compares predictions from candidate attrition equations with casualty data generated artificially from an agent-based model. For situations where the initial regimented structure of the fighting forces breaks down, introducing fractal concepts into the attrition equations proves effective at encapsulating complex aspects of the battle; with details in the time dependence of the casualty data able to be reproduced. Furthermore, measuring the fractal dimension of a fighting force's spatial distribution on the battlefield provides a sensitive probe of the combatants’ behaviour. Precise times at which key events occur during a battle can be pinpointed. This study furthers the body of work which considers warfare as a complex adaptive system and where fractal-like structures are expected to emerge.

Comparative study on dynamic analyses of non-classically damped linear systems

In this paper some techniques for the dynamic analysis of non-classically damped linear systems are reviewed and compared. All these methods are based on a transformation of the governing equations using a basis of complex or real vectors. Complex and real vector bases are presented and compared. The complex vector basis is represented by the eigenvectors of the complex eigenproblem obtained considering the non-classical damping matrix of the system. The real vector basis is a set of Ritz vectors derived either as the undamped normal modes of vibration of the system, or by the load dependent vector algorithm (Lanczos vectors). In this latter case the vector basis includes the static correction concept. The rate of convergence of these bases, with reference to a parametric structural system subjected to a fixed spatial distribution of forces, is evaluated. To this aim two error norms are considered, the first based on the spatial distribution of the load and the second on the shear force at the base due to impulsive loading. It is shown that both error norms point out that the rate of convergence is strongly influenced by the spatial distribution of the applied forces.

Calculation of Forces at Focal Adhesions from Elastic Substrate Data: The Effect of Localized Force and the Need for Regularization

Forces exerted by stationary cells have been investigated on the level of single focal adhesions by combining elastic substrates, fluorescence labeling of focal adhesions, and the assumption of localized force when solving the inverse problem of linear elasticity theory. Data simulation confirms that the inverse problem is ill-posed in the presence of noise and shows that in general a regularization scheme is needed to arrive at a reliable force estimate. Spatial and force resolution are restricted by the smoothing action of the elastic kernel, depend on the details of the force and displacement patterns, and are estimated by data simulation. Corrections arising from the spatial distribution of force and from finite substrate size are treated in the framework of a force multipolar expansion. Our method is computationally cheap and could be used to study mechanical activity of cells in real time.

A Method to Improve the Modal Convergence for Structures With External Forcing

The traditional approach of using free vibration modes in the assumed mode method often leads to an extremely slow convergence rate, especially when discete interactive forces are involved. By introducing a number of forced modes, significant improvements can be achieved. These forced modes are intrinsic to the structure and the spatial distribution of forces. The motion of the structure can be described exactly by these forced modes and a few free vibration modes provided that certain conditions are satisfied. The forced modes can be viewed as an extension of static modes. The development of a forced mode formulation is outlined and a numerical example is presented.