Traversing granular regolith, especially in reduced gravity environments, remains a potential challenge for wheeled rovers. Mitigating hazards for planetary exploration rovers requires testing in representative environments, but direct Earth-based testing fails to account for the effect of reduced gravity on the soil itself. Granular scaling laws (GSL) have been proposed in the literature to predict performance of a larger wheel based on tests with a smaller wheel, or to predict performance in one gravity level based on tests in another gravity level. However, this is the first work to experimentally validate GSL in reduced gravity. Here, an expanded version of existing GSL was evaluated experimentally by measuring performance of a single wheel driving through cohesionless lunar soil simulant GRC-1 aboard parabolic flights that reproduce the effects of lunar gravity, and comparing those results to scaled tests performed on the ground. This scaled-wheel testing achieved less than 10% prediction error on three measured output metrics: drawbar pull (i.e. net traction), sinkage, and power draw. Predictions also erred on the conservative side. Subsurface soil imaging revealed similar soil behavior between scaled tests. GSL thus offers an accurate and conservative method for predicting wheel performance in reduced gravity based on 1-g experiments, at least in cohesionless soil.

Traversing granular regolith in reduced gravity remains a challenge for wheeled rovers. Earth-based testing cannot fully predict rover mobility as it precludes gravity's effects on the soil. The simulant GRC-1 was designed to account for this by matching cone penetrometer readings from Apollo; the assumption is that replicating the cone penetrometer response of lunar soil will also replicate its response to vehicle loading. Cone penetrations were performed in GRC-1 at three densities in 1-g and 1/6-g aboard parabolic flights producing effective lunar-g. A fourfold decrease in cone index gradient (G) was seen in 1/6-g, indicating significantly reduced shear strength. Wheel experiments in GRC-1 in 1/6-g at one density and two slip values were compared to 1-g experiments at a lower density producing the same G value. In 1-g, drawbar pull was 44% higher and sinkage was 13.5% lower, indicating that the assumption made during the creation of GRC-1 was not quite correct, and that caution should be exercised when interpreting results obtained with this simulant. At the loosest possible density in 1-g, drawbar pull was still 24% higher than in lunar-g; sinkage was 76% higher. Future experiments utilizing a stronger simulant in lunar-g are outlined to elaborate upon these findings. (c) 2021 ISTVS. Published by Elsevier Ltd. All rights reserved.

This work reviews wheel load application methods in single-wheel testbeds (SWTBs), used widely to study terramechanics. Normal force oscillations have been reported in SWTBs, and visible in time-series data when provided. This has usually been attributed to grousers, but is also visible for wheels without grousers suggesting other system dynamics (e.g. friction in the vertical axis) are the cause. Furthermore, accelerations related to periodic vertical displacements of the wheel (whether caused by grousers or otherwise) cannot explain more than a small fraction of the amplitude of normal force vari-ations. In this work, a 4-bar mechanism was developed that balances the normal load and ensures that a constant (not oscillating) normal force is achieved throughout terramechanics experiments. System behaviour with and without the 4-bar mechanism was assessed, highlighting the need for a 4-bar mech-anism or other method of balancing normal loads if a constant normal force is desired. No significant effects on average experimental results were observed due to normal force oscillation, but artifacts were observed in the time-series data. Comparisons of results with a system where wheel load is applied pneu-matically and characterization of friction in each testbed provide further support to the conclusion that normal force oscillations are mainly caused by friction in the vertical axis. (c) 2021 ISTVS. Published by Elsevier Ltd. All rights reserved.

One of the major challenges faced by planetary exploration rovers today is the negotiation of difficult terrain, such as fine granular regolith commonly found on the Moon and Mars. Typical Earth-based testing methods cannot accurately predict rover mobility in reduced gravity environments (i.e. the Moon and Mars) as they fail to account for the effect of reduced gravity on the soil itself. Preliminary efforts have been made to account for effects of gravity on granular materials, at least indirectly, through simulant design. The soil simulant GRC-1 is designed to produce cone penetrometer readings comparable to those collected on the Moon (i.e. in lunar gravity) during Apollo. The assumption is that replicating the mechanical properties of lunar soil in terms of cone penetration resistance will also replicate the response to vehicle loading in terms of traction. The present research was designed to thoroughly characterize this assumption by exploring the relationship between rover mobility and cone penetration resistance experimentally in lunar gravity (1/6-g) and terrestrial gravity (1-g). The cone penetrometer response of GRC-1 was measured at relative densities (DR'S) of 46%,63%, and 69 % in both 1-g and 1/6-g aboard parabolic flights producing effective lunar gravitational accelerations. Cone index gradient (G) values measured in 1/6-g were 4 x lower on average than those measured in 1-g, indicating a significant decrease in shear strength in lunar-g. Wheel-soil interactions between a smooth rigid wheel and G R C-1 were also characterized at 69% DR in 1/6-g. The wheel experiments measured wheel-soil interaction data at controlled slip values of 20 % and 70 %. Wheel-soil interaction data collected include subsurface soil imaging, force/torque sensor data, wheel sinkage, and motor current, with drawbar pull (DP) and sinkage data reported here. Average DP/W values observed at 20% slip and 70% slip were 0.11 ± 0.02 and 0.32 ± 0.03, respectively, and maximum observed sinkage was 9.0 ± 1.9 mm and 16.2 ± 2.1 mm at 20% and 70 % slip, respectively. These results will be compared to 1-g experiments in a lower-density soil that produces an equivalent cone penetrometer response in order to test the hypothesis that equivalent wheel performance will be observed in soil with equal G values across differing gravity levels.

This research investigates the development and validation of state-of-the-art high-fidelity m odels o f s oil cutting operations. The accurate and efficient modeling of complex tool-soil interactions is an open problem in the literature. Modeling options that provide more flexibility in trading off accuracy and computational efficiency than current state-ofthe-art continuum or discrete element methods are sought. In this work, two modern numerical methods, the material point method (MPM) and a hybrid approach, are presented with the goal to simulate excavation maneuvers efficiently and with high accuracy. MPM, as an accurate, continuum-based and meshfree method, uses a constitutive model (here, nonlocal granular fluidity model) for computing internal forces to update particle velocities and positions. The hybrid approach, a combination of particle and grid-based methods, avoids explicit integration scheme difficulties and unnecessary computations in the static regime. Visual and quantitative data, including forces on the excavation tool, are collected experimentally to evaluate these two simulation methods with respect to geometry of the soil deformation as well as interaction forces, both as a function of time.

Computational cellular image analysis routines are methods that quantify objects, distances, concentrations, velocities of cells, and subcellular structures. In traction force microscopy (TFM) technique, cells are cultured on flexible polymer substrates in which nanoscale fluorescent beads have been embedded as fiduciary markers. Many microscopic technologies provide vast cytoarchitectural information that needs to be quantitatively analyzed through computational image processing tools in order to extract fruitful information. Some of the major image analysis techniques include visualization, tracking, and statistical analysis. The image processing toolboxes in MATLAB and custom-built software or scripts can be used to quantify the static and dynamic images in order to quantify the structural dynamics within the living cell. This chapter describes important architectural structures in the cell, experimental approaches to studying cell mechanics, and the image analysis routines, which are employed to analyze the data from these experiments. It provides examples of computational algorithms and post-processing techniques to understand the dynamical behavior of living cells.

Living cells possess an exquisite ability to sense and respond to physical information in their microenvironment. This ability plays a key role in many fundamentally important physiological and pathological processes. We will describe our work utilizing a variety of biophysical tools to investigate the dynamic responses of cells to mechanical stimuli and how physical cues can be employed to re-purpose and manipulate biological processes. These responses to physical cues are not simply a side-product of biology but are key components of biological and physical feedback loops that govern the life of a cell.

A multi-layered polydimethylsiloxane microfluidic device with an integrated suspended membrane has been fabricated that allows dynamic and multi-axial mechanical deformation and simultaneous live-cell microscopy imaging. The transparent membrane’s strain field can be controlled independently along two orthogonal directions. Human foreskin fibroblasts were immobilized on the membrane’s surface and stretched along two orthogonal directions sequentially while performing live-cell imaging. Cyclic deformation of the cells induced a reversible reorientation perpendicular to the direction of the applied strain. Cells remained viable in the microdevice for several days. As opposed to existing microfluidic or macroscale stretching devices, this device can impose changing, anisotropic and time-varying strain fields in order to more closely mimic the complexities of strains occurring in vivo.

Tools that allow the application of mechanical forces to cells and tissues or that can quantify the mechanical properties of biological tissues have contributed dramatically to the understanding of basic mechanobiology. These techniques have been extensively used to demonstrate how the onset and progression of various diseases are heavily influenced by mechanical cues. This article presents a multi-functional biaxial stretching (BAXS) platform that can either mechanically stimulate single cells or quantify the mechanical stiffness of tissues. The BAXS platform consists of four voice coil motors that can be controlled independently. Single cells can be cultured on a flexible substrate that can be attached to the motors allowing one to expose the cells to complex, dynamic, and spatially varying strain fields. Conversely, by incorporating a force load cell, one can also quantify the mechanical properties of primary tissues as they are exposed to deformation cycles. In both cases, a proper set of clamps must be designed and mounted to the BAXS platform motors in order to firmly hold the flexible substrate or the tissue of interest. The BAXS platform can be mounted on an inverted microscope to perform simultaneous transmitted light and/or fluorescence imaging to examine the structural or biochemical response of the sample during stretching experiments. This article provides experimental details of the design and usage of the BAXS platform and presents results for single cell and whole tissue studies. The BAXS platform was used to measure the deformation of nuclei in single mouse myoblast cells in response to substrate strain and to measure the stiffness of isolated mouse aortas. The BAXS platform is a versatile tool that can be combined with various optical microscopies in order to provide novel mechanobiological insights at the sub-cellular, cellular and whole tissue levels.

Physical forces arising in the cellular microenvironment have been hypothesized to play a major role in governing cell function. Moreover, it is thought that gene regulation may be sensitive to nuclear deformations taking place in response to extracellular forces over short and long timescales. Although nuclear responses to mechanical stimuli over long timescales are relatively well studied, the short‐term responses are poorly understood. Therefore, to characterize the short‐term instantaneous deformation of the nucleus in a mechanically dynamic environment, we exposed MDCK epithelial monolayers to varying mechanical strain fields. The results reveal that nuclei deform anisotropically in response to substrate strain, specifically, the minor nuclear axis is significantly more deformable than the major axis. We show that upon microtubule depolymerization, nuclear deformation anisotropy completely disappears. Moreover, the removal of actin causes a significant increase in nuclear deformation along the minor axis and a corresponding increase in mechanical anisotropy. The results demonstrate that the nucleus deforms in a manner that is very much dependent on the direction of strain and the characteristics of the strain field. Actin and microtubules also appear to play distinct roles in controlling the anisotropic deformation of the nucleus in response to mechanical forces that arise in the cellular microenvironment. © 2013 Wiley Periodicals, Inc.

AIMS:Expression of Heat Shock Protein-27 (HSP27) is reduced in human coronary atherosclerosis. Over-expression of HSP27 is protective against the early formation of lesions in atherosclerosis-prone apoE(-/-) mice (apoE(-/-)HSP27(o/e)) - however, only in females. We now seek to determine if chronic HSP27 over-expression is protective in a model of advanced atherosclerosis in both male and female apoE(-/-) mice. METHODS AND RESULTS:After 12 weeks on a high fat diet, serum HSP27 levels rose more than 16-fold in male and female apoE(-/-)HSP27(o/e) mice, although females had higher levels than males. Relative to apoE(-/-) mice, female apoE(-/-)HSP27(o/e) mice showed reductions in aortic lesion area of 35% for en face and 30% for cross-sectional sinus tissue sections - with the same parameters reduced by 21% and 24% in male cohorts; respectively. Aortic plaques from apoE(-/-)HSP27(o/e) mice showed almost 50% reductions in the area occupied by cholesterol clefts and free cholesterol, with fewer macrophages and reduced apoptosis but greater intimal smooth muscle cell and collagen content. The analysis of the aortic mechanical properties showed increased vessel stiffness in apoE(-/-)HSP27(o/e) mice (41% in female, 34% in male) compare to apoE(-/-) counterparts. CONCLUSIONS:Chronic over-expression of HSP27 is atheroprotective in both sexes and coincides with reductions in lesion cholesterol accumulation as well as favorable plaque remodeling. These data provide new clues as to how HSP27 may improve not only the composition of atherosclerotic lesions but potentially their stability and resilience to plaque rupture.

Integrins, focal adhesions, the cytoskeleton and the extracellular matrix, form a structural continuum between the external and internal environment of the cell and mediate the pathways associated with cellular mechanosensitivity and mechanotransduction. This continuum is important for the onset of muscle tissue generation, as muscle precursor cells (myoblasts) require a mechanical stimulus to initiate myogenesis. The ability to sense a mechanical cue requires an intact cytoskeleton and strong physical contact and adhesion to the microenvironment. Importantly, myoblasts also undergo reorientation, alignment and large scale remodeling of the cytoskeleton when they experience mechanical stretch and compression in muscle tissue. It remains unclear if such dramatic changes in cell architecture also inhibit physical contact and adhesion with the tissue microenvironment that are clearly important to myoblast physiology. In this study, we employed interference reflection microscopy to examine changes in the close physical contact of myoblasts with a substrate during induced remodeling of the cytoarchitecture (de-stabilization of the actin and microtubule cytoskeleton and inhibition of acto-myosin contractility). Our results demonstrate that while each remodeling pathway caused distinct effects on myoblast morphology and sub-cellular structure, we only observed a ~13% decrease in close physical contact with the substrate, regardless of the pathway inhibited. However, this decrease did not correlate well with changes in cell adhesion strength. On the other hand, there was a close correlation between cell adhesion and β1-integrin expression and the presence of cell-secreted fibronectin, but not with the presence of intact focal adhesions. In this study, we have shown that myoblasts are able to maintain a large degree of physical contact and adhesion to the microenvironment, even during shot periods (<60 min) of large scale remodeling and physiological stress, which is essential to their in-vivo functionality.

Cellular organization, migration and proliferation in three-dimensions play a critical role in numerous physiological and pathological processes. Nano- and micro-fabrication approaches have demonstrated that nano- and micro-scale topographies of the cellular microenvironment directly impact organization, migration and proliferation. In this study, we investigated these dynamics of two cell types (NIH3T3 fibroblast and MDCK epithelial cells) in response to microscale grooves whose dimensions exceed typical cell sizes. Our results demonstrate that fibroblasts display a clear preference for proliferating along groove ridges whereas epithelial cells preferentially proliferate in the grooves. Importantly, these cell-type dependent behaviours were also maintained when in co-culture. We show that it is possible to spatially separate a mixed suspension of two cell types by allowing them to migrate and proliferate on a substrate with engineered microtopographies. This ability may have important implications for investigating the mechanisms that facilitate cellular topographic sensing. Moreover, our results may provide insights towards the controlled development of complex three-dimensional multi-cellular constructs.

INTRO Extracellular forces can result in significant nuclear deformations and may modulate transcription activity. However, the precise role of the cytoskeleton network in regulating nuclear mechanics remains poorly understood. The goal of the present study is to investigate the nuclear deformability under complex substrate strain fields to clarify the role of the microtubule and actin network on the mechanical behavior of the nucleus. METHOD A custom-built cell bi-axial stretching device allowing for real time imaging of mouse myoblast (C2C12) nuclei was used. Cells were seeded on a cross-shaped silicone membrane coated with collagen I and cultured in growth medium. A live cell fluorescent nuclear stain, Hoechst-33342, was used to image the nucleus during stretching. Nocodazole and cytochalasin-D were used to depolymerize the microtubule or actin network, respectively. All cells were exposed to a stretching strain field of 23% parallel to major or minor nuclear axis. RESULTS For all cells, substrate stretching along one nuclear axis induced stretching along this axis and compression along the orthogonal axis. Control cells displayed a higher nuclear stretching along the minor axis than the major axis with a deformation of 5% and 2% respectively. This anisotropy vanished completely in microtubule- and actin- deprived cells with a deformation of about 4% and 3.5% respectively for both axis. In concert with stretching, increased nuclear compression was observed along the minor axis compared to the major axis for all cells. CONCLUSION The anisotropic deformation of C2C12 cell nuclei are driven by actin and microtubule networks. It is important to understand how the cytoskeleton regulates force-induced deformation of the nucleus since they are known to alter gene expression and cell polarity. Such intrinsic mechanical behavior could also be exploited as a readout mechanism of substrate strain fields.

Background: Previously, we discovered that over-expression of Heat Shock Protein 27 (HSP27) in atherosclerosis-prone apoE -/- mice (apoE -/- HSP27 o/e ) maintained on a high fat diet (HFD) for 4 weeks reduces aortic lesion area - however, only in females. Purpose/Objective: To determine if chronic HSP27 over-expression modulates plaque lesion characteristics including aortic mechanical properties. Methods/Results: Male and female apoE -/- and apoE -/- HSP27 o/e mice fed a high fat diet for 12 weeks were studied. Relative to apoE -/- mice, female apoE -/- HSP27 o/e mice had aortic lesion area reductions of 35% for en face and 30% for cross-sectional sinus tissue sections. Interestingly, the same parameters were also reduced by 21% and 24%, respectively, in male cohorts. Aortic plaques from apoE -/- HSP27 o/e mice showed almost 50% reductions in the area occupied by cholesterol clefts and free cholesterol, with fewer macrophages and reduced macrophage apoptosis. The mechanical properties of the aorta were studied ex vivo using 2-mm vessel segments (n=14-20/group). We developed an apparatus to determine vessel stiffness during radial stretching at a constant rate, recording tension via a force transducer. There was an increase in vessel stiffness in apoE -/- HSP27 o/e mice compare to apoE -/- (female: 628±30Pa vs 444±38Pa, p<0.001; male: 679±19Pa vs 505±22Pa; p<0.001). The vasoactive response of aortic segments (n=12/group) to phenylephrine (PE), carbachol (CCh) and sodium nitroprusside (SNP) were also determined. While the vasorelaxation response induced by CCh and SNP were similar between the two animal groups, PE (10μM) produced a greater increase in vasoconstriction in apoE -/- HSP27 o/e mice compared to apoE -/- (female: 205±9% vs 162±10%, p<0.001; male: 190±9% vs 139±12%, p<0.01). Conclusion: Chronic over-expression of HSP27 provides a persistent atheroprotective effect that benefits both sexes and coincides with remarkable reductions in lesion cholesterol accumulation as well as macrophage apoptosis. In addition, it increases the aortic stiffness and the vasoactive response to PE in apoE -/- mice model. These data provide new clues as to how HSP27 may improve atherosclerotic lesion stability.

BACKGROUND/INTRODUCTION:Diseases of the aorta can alter the local mechanical properties of the tissue, leading to aneurysms and plaque instability. Local tissue properties are best evaluated from surgical samples or autopsy tissue using mechanical testing ex vivo. We examined whether formalin-fixed tissues preserve regional and local variations in tissue properties when compared to fresh tissues in order to determine if fixed tissue can be used to infer mechanical changes associated with tissue remodeling. METHODS:Equibiaxial mechanical tests were performed on canine descending thoracic aorta to quantify the regional and local tissue stiffness. Samples were taken from four locations along the aorta and from the lateral and medial side at each location. Half of the samples were randomly formalin fixed and used to measure the effect of fixation on local thickness, stiffness, and anisotropy. RESULTS:In fresh tissue, regional differences in tissue stiffness and thickness are present. Aortic tissue stiffens and thins along the aorta. Fixation did not alter thickness, significantly increased tissue stiffness, and altered the directional dependency of the mechanical properties (anisotropy) at low strain. Formalin fixation altered local stiffness of the aorta near the aortic arch. CONCLUSION:The changes in mechanical properties along the aorta were preserved in formalin-fixed samples. However, our results show that formalin fixation can change the variation in tissue stiffness and significantly affects the anisotropic properties of vascular tissues. Formalin fixation introduces spurious changes in mechanical properties, and we therefore strongly recommend the use of fresh aortic tissues for biomechanical analysis.

The human ascending aorta (AA) is exposed to very high shear and pressure stresses exerted by the blood flow ejected from the left ventricle outflow tract. This vessel has a unique structural behaviour which adequately redistributes the energy captured from the blood flow ejection to sustain a more continuous blood flow through the entire vascular system. Unfortunately, this vessel is prone to a pathological dilation process involving significant structural changes that can lead to fundamental modification of its mechanical behaviour and functions. Genetic and/or environmental factors have been implicated in the disease process. It is believed that in particular the forces created by blood flow (hemodynamics) can be a stimulus for vessel remodelling. For patients suffering from this deadly condition, surgical replacement or repair is the best solution to increase life expectancy. However, the replacement materials available as a treatment have a significant impact on the blood flow, the biomechanics of the aortic arch, and the entire vascular system. In this review we summarise the current understanding of the pathogenesis mechanisms involved in the dilation of the AA from a mechanical and biochemical point of view. We will also underline the needs for better replacement materials in surgical repair to improve graft patency.

Background: Dilation and dissection of aneurysmal ascending aortic tissues occur preferentially at the outer curvature of the vessel. In this study we hypothesize that the density and contractile properties of the vascular smooth muscle cells (VSMCs) of the pig ascending aorta (AA) are heterogeneous and could explain the non-uniform remodeling and weakening of the AA during aneurysm formation.Methods: Eleven pig AA rings were collected. Two square samples of 15 x 15 mm were taken from each ring from the inner and outer curvature of the AA. Each sample was subjected to equi-biaxial tensile testing in Krebs-Ringer solution maintained at 37 degrees C. Each test consisted of 8 cycles of preconditioning followed by one experimental run from 0% to 30% strain. Phenylephrine (10(-5) M) was added to contract VSMCs. After biaxial testing, samples were paraffin-embedded and stained with hematoxylin-phloxine-saffron (HPS) to quantify VSMC density.Results: Significant differences in cell density, maximum contractile stress resultant magnitude (MCSRM) and orientation (theta(MCSR)) were found between the inner and outer curvature. The inner curvature had the greatest contraction. The outer curvature had the highest VSMC density with the maximum contraction stress resultant oriented towards the axial direction.Conclusion: VSMC activation with phenylephrine had a significant effect on the stiffness of the pig AA. This effect was independent of location and direction. However, cell orientation, density and contractile properties were dependent on location and suggest variations in the remodeling capabilities, tissue strain and cell phenotype between locations. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.