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The direct and indirect (by changing mechanical properties) effects of hydration at interfaces on cellular processes and tissue diseases are reviewed. The essential effect of substrate stiffness on cellular processes was demonstrated in the last decade. The combined effect of surface stiffness and hydration at interfaces has garnered much less attention, though hydration and dehydration play important roles in biological processes. This review focuses on the studies that demonstrate how hydration affects biological processes at interfaces. Elevated sodium and dehydration stimulate inflammatory signaling in endothelial cells and promote atherosclerosis. Various types of implant and blood contacting device coatings with varied surface stiffness and hydration have been reported. Effect of hydration on polymer modulus of elasticity and viscoelasticity was discussed taking into account cells adhesion, migration, proliferation, differentiation on surfaces with various degree of hydration. Future directions of research were considered, including the use of nanotechnology to regulate the hydration degree.

Willwacher, S, Fischer, KM, Rohr, E, Trudeau, MB, Hamill, J, and Bruggemann, G-P. Surface stiffness and footwear affect the loading stimulus for lower extremity muscles when running. J Strength Cond Res XX(X): 000-000, 2020-Running in minimal footwear or barefoot can improve foot muscle strength. Muscles spanning the foot and ankle joints have the potential to improve performance and to reduce overuse injury risk. Surface stiffness or footwear use could modify the intensity of training stimuli acting on lower extremity joints during running. The purpose of this study was to systematically investigate external ankle, knee, and hip joint moments during shod and barefoot running while considering the stiffness of the running surface. Two footwear conditions (barefoot and neutral running shoe) and 4 surface conditions (Tartan, Tartan + Ethylene Vinyl Acetate [EVA] foam, Tartan + artificial turf, Tartan + EVA foam + artificial turf) were tested at 3.5 m·s. Repeated measures analysis of variance revealed that barefoot running in general and running barefoot on harder surfaces increased and decreased ankle (between +5 and +26%) and knee (between 0 and -11%) joint moments, respectively. Averaged over all surfaces, running barefoot was characterized by a 6.8° more plantarflexed foot strike pattern compared with running shod. Foot strike patterns were more plantarflexed on harder surfaces; the effects, however, were less than 3°. Most surface effects were stronger in barefoot compared with shod running. Surface stiffness may be used to modulate the loading intensity of lower extremity muscles (in particular extrinsic and intrinsic foot muscles) during running. These results need to be considered when coaches advise barefoot running as a method to improve the strength of extrinsic and intrinsic foot muscles or when trying to reduce knee joint loading.

Background While mini-trampolines have been used among a variety of groups including children as an intervention tool, the motor behavior children adopt while hopping on this soft, elastic surface is unknown. Identifying coordinative structures and their stability for hopping on a mini-trampoline is imperative for recommending future interventions and determining appropriateness to populations with motor dysfunctions. Research question Do children demonstrate similar biomechanical and coordination patterns as adults while hopping on a mini-trampoline? Methods Fifteen adults aged 18–35 years and 14 children aged 7–12 years completed bouts of continuous two-legged hopping in-place on a stiff surface for 10 s at a time and on a mini-trampoline for 30 s at a time. 3-D motion capture tracked whole-body movement. We evaluated whole-body vertical stiffness as a ratio of peak vertical force and peak vertical displacement, as well as spatiotemporal parameters of hopping. Coordinative structures were evaluated as continuous relative phase angles of the foot, shank, thigh, and pelvis segments. Results and significance Adults did not modify whole-body vertical stiffness on a mini-trampoline, while children increased whole-body vertical stiffness to compensate for the reduced surface stiffness. Both groups conserved the coordinative structure for hopping on a mini-trampoline by modulating hopping cycle timing. Moreover, children hopped with an adult-like coordinative structure, but required greater shank-thigh and thigh-pelvis out-of-phase motion. However, the consistency of their coordination was diminished compared to adults. Children aged 7–12 years old have formed a stable coordinative structure for spring-mass center-of-mass dynamics that is preserved on this soft, elastic surface. However, children might be developing control strategies for preferred whole-body vertical stiffness, particularly when required to dampen peak vertical forces. These results highlight the importance of evaluating the emerging motor behavior to manipulated environmental constraints, particularly when considering the utility and appropriateness of mini-trampoline interventions for children with motor dysfunctions.

International audience; Purpose: Muscle–tendon units are forcefully stretched during rapid deceleration events such as landing. Consequently, tendons act as shock absorbers by buffering the negative work produced by muscle fascicles likely to prevent muscle damage. Landing surface properties can also modulate the amount of energy dissipated by the body, potentially effecting injury risk. This study aimed to evaluate the influence of three different surfaces on the muscle–tendon interactions of gastrocnemius medialis (GM), and vastus lateralis (VL) during single- and double-leg landings from 50 cm.Methods: Ultrasound images, muscle activity and joint kinematics were collected for 12 participants. Surface testing was also performed, revealing large differences in mechanical behavior.Results: During single-leg landing, stiffer surfaces increased VL fascicle lengthening and velocity, and muscle activity independent of joint kinematics while GM length changes showed no difference between surfaces. Double-leg landing resulted in similar fascicle and tendon behavior despite greater knee flexion angles on stiffer surfaces.Conclusion: This demonstrates that VL fascicle lengthening is greater when the surface stiffness increases, when performing single-leg landing. This is due to the combination of limited knee joint flexion and lower surface absorption ability which resulted in greater mechanical demand mainly withstood by fascicles. GM muscle–tendon interactions remain similar between landing surfaces and types. Together, this suggests that surface damping properties primarily affect the VL muscle–tendon unit with a potentially higher risk of injury as a result of increased surface stiffness when performing single-leg landing tasks.

Sports surface properties can substantially alter the overall performance and risk of injury. Surface mechanical properties influence the loading of the human musculoskeletal system by modulating the amount of foot-impact energy transmitted to the athlete. Natural grass and synthetic turf are commonly used pitches in football and rugby. More recently, reinforced natural grass technology has been used at the elite-level facilities, but its influence on player is not well defined. This thesis aimed at evaluating the influence of three different surfaces (reinforced natural grass, synthetic turf and athletic track) on the muscle-tendon interactions and neuromuscular coordination of gastrocnemius medialis (GM) and vastus lateralis (VL) muscles during landings and jumping tasks. Analysis of dynamic ultrasound imaging, 2D kinematics and electromyographic data showed that: i) surface mechanical properties influenced muscle-tendon interactions as well as the level of muscle activity; ii) the reinforced natural grass surface seems to optimize the muscular response during the movement and iii) GM and VL muscles displayed specific behaviors relative to the type of movement, its intensity and the type of surface. This emphasizes that the human response cannot be predicted by only analyzing the mechanical surface properties and highlights the important role of in vivo ecological testing to better understand player-surface interaction.; Les propriétés des surfaces sportives peuvent impacter directement la performance et le risque de blessure en modulant la part d’énergie transmise à l'athlète lors de l'impact du pied sur la surface. Les pelouses naturelle et synthétique sont couramment utilisées sur les terrains de football et de rugby. Depuis quelques années, une nouvelle génération de pelouse dite naturelle renforcée a fait son apparition dans les clubs professionnels mais son influence sur la biomécanique du geste sportif est encore mal connue. Cette thèse vise à évaluer l'influence de trois types de surfaces (gazon naturel renforcée, gazon synthétique et tartan) sur les interactions muscle-tendon et les coordinations neuromusculaires des muscles gastrocnemius medialis (GM) et vastus lateralis (VL) lors de mouvements de réception uni et bilatérale ainsi que de saut. L’analyse des données échographiques dynamiques, de cinématique 2D et d’activité musculaire nous a permis de montrer que : i) les propriétés mécaniques des surfaces peuvent altérer les interactions entre les faisceaux musculaires et les tissus tendineux ainsi que l’amplitude d’activation musculaire ; ii) la pelouse naturelle renforcée semble avoir des propriétés plus optimales que la pelouse synthétique lors de sauts et réceptions ; iii) il existe des différences de comportement marquées entre le GM et VL qui dépendent du type de surface, du type de mouvement et de son intensité. Cela souligne l’importance de ne pas se limiter à l’étude des propriétés mécaniques des surface pour comprendre leur influence sur le mouvement sportif. Par ailleurs, l’étude des comportements musculo-tendineux in vivo en condition écologique permet de mieux comprendre les interactions complexes entre l’homme et la surface.

Sports surface properties can substantially alter the overall performance and risk of injury. Surface mechanical properties influence the loading of the human musculoskeletal system by modulating the amount of foot-impact energy transmitted to the athlete. Natural grass and synthetic turf are commonly used pitches in football and rugby. More recently, reinforced natural grass technology has been used at the elite-level facilities, but its influence on player is not well defined. This thesis aimed at evaluating the influence of three different surfaces (reinforced natural grass, synthetic turf and athletic track) on the muscle-tendon interactions and neuromuscular coordination of gastrocnemius medialis (GM) and vastus lateralis (VL) muscles during landings and jumping tasks. Analysis of dynamic ultrasound imaging, 2D kinematics and electromyographic data showed that: i) surface mechanical properties influenced muscle-tendon interactions as well as the level of muscle activity; ii) the reinforced natural grass surface seems to optimize the muscular response during the movement and iii) GM and VL muscles displayed specific behaviors relative to the type of movement, its intensity and the type of surface. This emphasizes that the human response cannot be predicted by only analyzing the mechanical surface properties and highlights the important role of in vivo ecological testing to better understand player-surface interaction.; Les propriétés des surfaces sportives peuvent impacter directement la performance et le risque de blessure en modulant la part d’énergie transmise à l'athlète lors de l'impact du pied sur la surface. Les pelouses naturelle et synthétique sont couramment utilisées sur les terrains de football et de rugby. Depuis quelques années, une nouvelle génération de pelouse dite naturelle renforcée a fait son apparition dans les clubs professionnels mais son influence sur la biomécanique du geste sportif est encore mal connue. Cette thèse vise à évaluer l'influence de trois types de surfaces (gazon naturel renforcée, gazon synthétique et tartan) sur les interactions muscle-tendon et les coordinations neuromusculaires des muscles gastrocnemius medialis (GM) et vastus lateralis (VL) lors de mouvements de réception uni et bilatérale ainsi que de saut. L’analyse des données échographiques dynamiques, de cinématique 2D et d’activité musculaire nous a permis de montrer que : i) les propriétés mécaniques des surfaces peuvent altérer les interactions entre les faisceaux musculaires et les tissus tendineux ainsi que l’amplitude d’activation musculaire ; ii) la pelouse naturelle renforcée semble avoir des propriétés plus optimales que la pelouse synthétique lors de sauts et réceptions ; iii) il existe des différences de comportement marquées entre le GM et VL qui dépendent du type de surface, du type de mouvement et de son intensité. Cela souligne l’importance de ne pas se limiter à l’étude des propriétés mécaniques des surface pour comprendre leur influence sur le mouvement sportif. Par ailleurs, l’étude des comportements musculo-tendineux in vivo en condition écologique permet de mieux comprendre les interactions complexes entre l’homme et la surface.

Spheroids can be used as building-blocks for bottom-up generation of artificial vascular beds, but current biofabrication strategies are often time-consuming and complex. Also, pre-optimization of single spheroid properties is often neglected. Here, we report a simple setup for rapid biomanufacturing of spheroid-based patch-like vascular beds. Prior to patch assembly, spheroids combining mesenchymal stem/stromal cells (MSCs) and outgrowth endothelial cells (OECs) at different ratios (10:1; 5:1; 1:1; 1:5) were formed in non-adhesive microwells and monitored along 7 d. Optimal OEC retention and organization was observed at 1:1 MSC/OEC ratio. Dynamic remodelling of spheroids led to changes in both cellular and extracellular matrix components (ECMs) over time. Some OEC formed internal clusters, while others organized into a peripheral monolayer, stabilized by ECM and pericyte-like cells, with concomitant increase in surface stiffness. Along spheroid culture, OEC switched from an active to a quiescent state, and their endothelial sprouting potential was significantly abrogated, suggesting that immature spheroids may be more therapeutically relevant. Non-adhesive moulds were subsequently used for triggering rapid, one-step, spheroid formation/fusion into square-shaped patches, with spheroids uniformly interspaced via a thin cell layer. The high surface area, endothelial sprouting potential, and scalability of the developed spheroid-based patches make them stand out as artificial vascular beds for modular engineering of large tissue constructs.

Maxwell lattices are mechanical networks on the verge of instability and feature convenient mechanisms of transformation. They exhibit topologically protected floppy modes and states of self-stress, offering an excellent platform for topologically robust and yet transformable mechanical and acoustic properties. In this talk, we will discuss recent progress in developing topological mechanical metamaterials based on Maxwell lattices, from dramatic changes of surface stiffness upon a small stimuli, to programmable stress distribution for fracturing protection, and nonreciprocal propagation for nonlinear waves. These results reveal great potential for Maxwell lattices as a new class of topological mechanical metamaterials with emerging applications.

Polysiloxane is one of the most favorite polymeric materials used in the emerging field of passive surface icephobicity; This is due to its tailorable softness, hydrophobicity, and relatively high durability. Given the state-of-the-art ice adhesion strength of polysiloxane surfaces has reached a threshold below 1 kPa, a timely survey on the published polysiloxane icephobic surfaces can serve as a valuable reference concerning how far the research field has already progressed and how much potential remains to be exploited for the future development of polymeric icephobic materials. This review categorizes the use of polysiloxane materials for icephobic strategies into three classes according to their surface stiffness. The advantages and shortcomings of each polysiloxane material group are assessed. By scrutinizing the current ice adhesion strength theory, a reference coating thickness is identified, which can be used for optimizing icephobic coating design. A surface icephobicity diagram is also presented, where a lower bound of ice adhesion on a smooth surface is derived, depicting the needs of incorporating different mechanisms to break the theoretical low ice adhesion limit. Finally, the challenges in applying the polysiloxane icephobic materials are discussed, and the possible key research directions are highlighted. 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

The gloss transition defect of injection-molded surfaces should be mitigated because it creates a poor impression of product quality. Conventional approaches for the suppression of the gloss transition defect employ a trial-and-error approach and additional equipment. The causes of the generation of a low-gloss polymer surface and the surface change during the molding process have not been systematically analyzed. This article proposes the causes of the generation of a low-gloss polymer surface and the occurrence of gloss transition according to the molding condition. The changes in the polymer surface and gloss were analyzed using gloss and topography measurements. The shrinkage of the polymer surface generates a rough topography and low glossiness. Replication to the smooth mold surface compensates for the effect of surface shrinkage and increases the surface gloss. The surface stiffness and melt pressure influence the degree of mold surface replication. The flow front speed and mold temperature are the main factors influencing the surface gloss because they affect the development rate of the melt pressure and the recovery rate of the surface stiffness. Therefore, the mold design and process condition should be optimized to enhance the uniformity of the flow front speed and mold temperature.

The goal of this study was to develop a pragmatic approach to build patient-specific models of the peripheral artery that are aware of plaque inhomogeneity. Patient-specific models using element-specific material definition (to understand the role of plaque composition) and homogeneous material definition (to understand the role of artery diameter and thickness) were automatically built from intravascular ultrasound images of three artery segments classified with low, average, and high calcification. The element-specific material models had average surface stiffness values of 0.0735, 0.0826, and 0.0973 MPa /mm, whereas the homogeneous material models had average surface stiffness values of 0.1392, 0.1276, and 0.1922 MPa/mm for low, average, and high calcification, respectively. Localization of peak lumen stiffness and differences in patient-specific average surface stiffness for homogeneous and element-specific models suggest the role of plaque composition on surface stiffness in addition to local arterial diameter and thickness.

The sap of urushi tree (Toxicodendron vernicifluum (Stokes) F.A. Barkley) has been used for coating materials and is known as urushi or oriental lacquer in East Asia. The potential of termite attacks against wood samples coated with four types of urushi: Ki-urushi, Sugurome-urushi, Kuro-Sugurome-urushi, and Bengara-urushi, in correlation with their chemical and mechanical properties was investigated in this study. Mortalities of the subterranean Formosan termite (Coptotermes formosanus Shiraki) after a 3-week no-choice feeding tests in samples coated with all types of urushi showed no significant difference from those of control samples. However, mass loss of the sample coated with urushi was lower than that of the control sample especially for Sugurome-urushi (Tukey’s test: p

Introduction: One of the major factors related to overuse injuries in runners is impact force resulting from the runner hitting the ground at heel strike. (Hreljac et al., 2000). This impact will increase and decrease based on the construction of the running shoe used as well as the type of surface being run on. The purpose of this research was to determine the effects that both midsole cushioning and surface stiffness have on this impact force. Methods: Six recreation runners running at least 10 miles per week ran a 2.2 km course 3 times at a self selected pace. A variable cushioning running shoe (Adidas One) was used and set in a low cushioning, high cushioning, or self adjusting cushioning mode for each run. Accelerometers were attached at the leg and forehead to measure impacts and impact attenuation. Accelerometer data were sampled at 512 Hz. The course consisted of the following surfaces: cement, asphalt, dirt, gravel, woodchips, sand, grass, and a wooden suspension bridge. Cohen’s D formula was used to calculate effect sizes comparing each shoe and each surface. The values 0.5 and 0.8 were used to determine medium and large effect sizes respectively. Results: The soft shoe setting had a medium effect (ES=0.66) compared to the hard shoe setting. No medium or large effects were found in head acceleration or impact attenuation between shoes. Sand had the largest leg accelerations of 7.8 g’s while the wooden bridge had the smallest (6.3 g’s). Large effects were found in leg acceleration with sand and 3 different surfaces. Gravel had the largest head acceleration of 1.58 g’s. Only medium effects were present between surfaces when comparing head accelerations. Sand had the highest impact attenuation of 83.8% while gravel had the smallest impact attenuation of 77.2%.

In-stent restenosis and thrombosis are the main severe problems that occur after the percutaneous vascular intervention. The competition between endothelial cells (ECs) and smooth muscle cells (SMCs) plays a key role during these pathological changes. The regulation of this competition offers new opportunities to design biomaterials in the cardiovascular fields. Bioactive molecules have been typically employed to increase EC adhesion and thereafter to enhance EC competitiveness; however, this method is associated with limitations from the point of view of practical and industrial applications. Herein, we present an approach to enhance EC competitiveness over that of SMC through the selective EC adhesion, which is achieved by modulating a complex surface stiffness based on the technique of layer-by-layer (LbL) assembly. This complex stiffness can be achieved by regulating the thickness of multilayer films coordinating with a rigid underlying substrate. The selective cell adhesion is attributed to changes in the complex surface stiffness and a different intrinsic property between ECs and SMCs. This study provides a facile and broadly applicable approach for the purpose of the enhancement of EC competitiveness over that of SMC, which has great potential for the development of cell-based functional biomaterials in the cardiovascular field.

It is very difficult to deform flexible objects in computer animation. This paper presents a novel approach to address this problem. A detail-sensitive and deformation-sensitive simplification is first conducted on the original mesh. The simplified mesh is then deformed, and this deformation is transferred to the original mesh to produce an initial result. Because of the discontinuity between some vertices, an as-rigid-as-possible optimization is employed to prevent the shape distortion and control the surface stiffness. Various experimental data demonstrate that our algorithm is intuitive, efficient, and effective in deforming large meshes. Copyright © 2014 John Wiley & Sons, Ltd.

Leg geometry at touchdown has a critical effect on joint loading in the initial contact phase in running. The purpose of this study was to systematically investigate the effects of footwear, surface conditions and sex on the kinematic striking configuration of the lower extremity when running at a constant speed (3.5 m/s). Three-dimensional touchdown kinematics were captured from 20 male and 19 female participants when running barefoot and with a neutral running shoe on four different surfaces. On harder surfaces and when running barefoot, subjects tended to land with a more plantarflexed foot position and ankle angle as well as a more vertical shank alignment. Different adaptation strategies to running surface stiffness were observed between barefoot and shod conditions. It seems that touchdown behaviour is adapted to compensate for the force distributing and energy absorption potentials of distinct surface x shoe combinations. If the combined compliance of the shoe plus surface combination exceeds a cer...

Surface stiffness of bulk soft tissue in musculoskeletal extremities is important to consider in the design of prosthetics, exoskeletons, and protective gear. This knowledge is also foundational for surgical simulation and clinical interventions leveraging manipulation of the musculoskeletal surfaces. Injuries to musculoskeletal extremities are common and surgical and preventive interventions require interactions between various objects such as surgical tools and support surfaces with tissue boundaries. While a handful of investigations examined the variations in indentation mechanics due to pathology or injury specific sites, a comprehensive analysis across the surfaces of musculoskeletal extremities has not been completed. In this study we examine variations of surface stiffness across 8 sites of the upper and lower arms and legs for 95 subjects using an instrumented ultrasound device. Differences in surface stiffness were observed between gender, activity level, and indentation location groups. The lower arm posterior location had the highest average stiffness (3.89 × 10−3 MPa/mm), while the lowest stiffness was observed at the upper leg posterior location (0.98 × 10−3 MPa/mm). The differences between indentation sites were larger in magnitude when compared to differences due to demographics (gender and activity level). However the large ranges of the 95% confidence intervals suggest that an aggregated metric based on population or sub-group may not capture individual variations. This study implicates the motivation to explore tissue composition variations within the indentation sites as well as the potential importance to include variations in surface stiffness during surgical simulations.

The sensitivity of the resonance frequencies and the amplitudes of the first four flexural vibration modes of atomic force microscopy with a rectangular cross-section cantilever in an air environment to variation of a sample's stiffness have been analysed. The cantilever has been modelled using Timoshenko beam theory, and the vertical and tangential forces between the tip and the sample in this simulation have been considered. The effect of tip position and the angle of the tip relative to the sample in changes of these sensitivities have been studied. Results indicate that for soft materials, the first mode compared with other modes is more sensitive to changes in the sample's elasticity, so the first mode is the best mode for supplying high-contrast images but by increasing the sample's stiffness, the higher mode will be sensitive, respectively. In addition, by increasing the angle between the cantilever and the sample's surface, the first mode is less sensitive and higher modes will be sensitive faster because of changes in material stiffness but in contrast, by increasing the distance between the tip's position and the free end of the cantilever, the sensitivity of the first mode increases and the higher mode will be sensitive for more stiff material.

Orientated focal cross-linking can be used to generate surface anisotropy, improve material stiffness and layer integration for the production of a stable 3D construct. Riboflavin (0.25 mM) diffusion into plastically compressed (PC) collagen gel was assessed by measuring the diffusion depth of riboflavin with time. The dynamic force analyser was used for peel force testing for interlayer cross-linking and material stiffness in perpendicular axis after orientated/topical cross-linking. One minute riboflavin diffusion time on either surface will saturate >12% of the collagen gel. Bonding strength doubled between PC collagen gel layers with a 5 min increase in cross-linking time (between 4 and 9 min) and break stress was increased significantly after cross-linking. Importantly, mechanical anisotropy was introduced in the break stress using orientated stripes of riboflavin in cross-linking, almost doubling the break stress parallel to the stripes. Limited riboflavin penetration in 1 min means that surface photo-dynamic cross-linking will enhance deep cell survival within the gel. Riboflavin mediated focal/orientated cross-linking generated new predictable anisotropy at the construct. The increase in bonding strength between layers after cross-linking enhances layer integration and graded surface stiffness will impact on cellular/mechanical properties of compressed gels.

Stem cell-based therapies are emerging as a potent new approach to treat a variety of diseases. This chapter is focused on the application and potential of plasma-assisted material modification in stem cells culture substrates and therapies. The effect of substrate chemical and physical properties achieved using plasma-coating methods on the fate of stem cells is reviewed. In particular, plasma-assisted methods to produce tailored surface chemistries, controlled topography and stiffness, surface gradient substrates, and modify 3D structures are explained together with their significance in addressing challenges faced in conventional stem cell culture. Refereed/Peer-reviewed

In this paper, the effect of nanosilica (NS) on the physical properties as well as aging on the morphology of asphalt binder was investigated. Asphalt binder penetration grade 60/70 (PEN 60/70) was modified with NS at concentrations of 1% to 5% by weight of binder. Scanning electron microscopy (SEM) was used to have a visual evaluation of Nanosilica dispersion. The physical properties tested include is softening point, penetration, ductility, viscosity and storage stability. Temperature susceptibility was evaluated using penetration index (PI) and penetration viscosity number (PVN). Nanosilica modified binder (NSMB) were aged using the rolling thin film oven test (RTFO) and pressure aging vessel (PAV). The morphology of the virgin asphalt binder and NSMB before and after aging was characterized by tapping mode atomic force microscopy (AFM). From the physical properties test, the addition of NS was found to improve the asphalt binder properties and can resist high temperature susceptibility. The results of the AFM imaging showed that the addition of nanosilica in asphalt binder improved its surface stiffness. The overall surface stiffness of the asphalt binder after aging also increased. It can be concluded from this study that 2% to 4% of NS gave the optimum performance for the binder.

Gait impairment due to neurological disorders has become an important problem of the 21st century. Stroke is a leading cause of long-term disability with approximately 90% of stroke survivors having some functional disability, with mobility being a major impairment. Despite the growing interest in using robotic devices for rehabilitation of sensorimotor function, their widespread use remains somewhat limited, as results so far in gait rehabilitation do not generally show improved outcomes over traditional treadmill-based therapy. This work focuses on understanding the mechanisms of inter-leg coordination, and based on that, proposing novel methods for gait rehabilitation. Using a novel robotic device, the Variable Stiffness Treadmill (VST), we apply walking surface stiffness perturbations to one leg, and analyze the response of the human nervous system in both low- (muscle) and high- (brain) levels, focusing on the mechanisms involved in the response of the other (unperturbed) leg. We show that the unperturbed leg uniquely responds to unilateral stiffness perturbations, while we provide solid evidence that the brain is involved in this observed inter-leg coordination. From a clinical prospective, the results of this study can be disruptive since they suggest that supraspinal neural activity can be evoked by altering the stiffness of the walking surface. Moreover, our methods provide a safe and targeted way to provide gait rehabilitation in hemiparesis since direct manipulation of the paretic side is not required. The present work provides for the first time evidence that specific robotic intervention in gait rehabilitation can have direct and predictable effects on the brain, opening a new avenue of research on targeted robot-assisted gait rehabilitation.

Preventing brake squeal is a significant task in brake development because brake squeal bothers users, thereby reducing a vehicle's commercial value. Many researchers have tried to find the mechanisms that cause brake squeal, so the number of complaint to brake squeal is declining. However, brake squeal sometimes appears after a vehicle has been in use for years even though it was not generated early in the vehicle's life. In order to enable people to use vehicles for a long time with a high satisfaction level, it is important to find the factor that causes brake squeal to arise gradually because of specific changes; so we focused on the stiffness of brake pads, which are always wearing during braking, because the pads' stiffness correlates with brake squeal. This study presents the influence of pads' surface texture on disk brake squeal. Pads with different surface texture were prepared by changing the braking time and machining. We measured the pads' stiffness, which we evaluated by exciting a pad at the frequency and vibration amplitude that cause brake squeal. The measurement results show that pads' stiffness increases as their surface becomes smoother, which means that the surface stiffness of pad influences the pads' stiffness. Finally, we used a numerical model to analyze the generation of squeal, which is caused by pads' surface texture.

Effect of aging on the morphology of bitumen was investigated. Two bitumens were aged according to the thin film oven test (TFOT), pressure aging vessel (PAV) test and ultraviolet (UV) radiation, respectively. The morphology of the binders before and after aging was characterized by atomic force microscopy. The physical properties and chemical compositions of the binders were also measured. The results showed that aging affected the bitumen morphology significantly. Aging increased the overall surface stiffness of the bitumen and made the bitumen surface more solid-like. The extent of these changes was dependent on aging conditions. TFOT decreased the contrast between the dispersed domains and the matrix, which contributed to the single-phase trend of the binders. The effect of PAV aging on morphology of the binders was dependent on the base bitumen. In one case, it further accelerated the single-phase trend of bitumen in comparison with that after TFOT. In the other case, it caused the phase separation of bitumen. In both cases, PAV aging increased the surface roughness of the binders obviously. As a result of UV aging, the contrast between the matrix phase and dispersed phase was increased due to the difference in sensitivity to UV radiation of the bitumen molecules, which caused or further promoted the phase separation in the binders. Regardless of the aging procedure carried out, a strong correlation was observed between the changes in morphology and physical properties as well as chemical compositions of the binders before and after aging.

Implantation of autologous chondrogenic cells has become the mainstay strategy for repairing articular cartilage defects. Because the availability of autologous chondrocytes is extremely limited, many recent studies have used artificially induced mesenchymal stem cells (iMSCs) as substitutes for chondrocytes. In this study, we analyzed the differences between the iMSCs and chondrocytes, including their molecular biological and mechanical properties. Human bone marrow-derived MSCs were collected and induced to exhibit the chondrogenic phenotype by culturing the pelleted MSCs in a chemically defined culture medium supplemented with transforming growth factor-beta 1. The molecular biological properties of iMSCs and culture-expanded chondrocytes, including their mRNA profiles and surface proteomics, were analyzed using reverse transcription-polymerase chain reaction (RT-PCR) and flow cytometry, respectively. The biomechanical properties of iMSCs and native chondrocytes, including their surface topology, adhesion force, and membrane stiffness, were analyzed using atomic force microscopy (AFM). Both iMSCs and chondrocytes presented type II collagen and glycosaminoglycan, whereas only chondrocytes presented type X collagen. Flow cytometric assays showed that the expression of type II collagen and integrin-1 was higher in the chondrocytes than in the iMSCs. AFM revealed that the MSCs, iMSCs, and chondrocytes greatly differed in their shape. The MSCs were spindle shaped and easily distinguishable from the spherical chondrocytes. The iMSCs appeared round and resembled the spherical chondrocytes; however, the iMSCs were flatter with a central hump of condensed mass and a surrounding thin and broad pleat. The mean adhesion force and mean surface stiffness were significantly lower for the iMSCs (4.54 nN and 0.109 N/m, respectively) than for the chondrocytes (6.86 nN and 0.134 N/m, respectively). To conclude, although the iMSCs exhibited the chondrogenic phenotype, they differed from the chondrocytes in their molecular biological and mechanical properties.

A surface free energy polar plot contains two possible sources of singularity: the cusps that give facets on an equilibrium crystal, and the circular arcs connecting the cusps that can lead to missing orientations. Instead of specifying surface free energy, we model the surface stiffness so that both singularities can be treated precisely. Here, we apply our model to three-dimensional axisymmetric crystals and obtain for the first time an analytic solution to the nonlinear differential equation governing the crystal shape.

The elastic properties of ZnO nanofilms with different film thickness, surface orientations and loading directions are investigated by using molecular mechanics (MM) method. The size dependence of elastic properties is relevant to both the film surface crystallographic orientation and loading direction. Both atomic structure analysis and energy calculation are employed to identify the mechanisms of the size-dependent elastic properties, under different loading directions and surface orientations. Upon small axial deformation, the relationship between intralayer and interlayer bond length variation and film elastic stiffness is established; it is found that the atomic layers with larger bond length variation have higher elastic stiffness. The strain energies of atomic layers of ZnO nanofilm and bulk are decoupled, from which the stiffness of film surface, intralayers, and interlayers are derived and compared with their bulk counterparts. The surface stiffness is found to be much lower than that of the interior layers and bulk counterpart, and with the decrease of film thickness, the residual tension-stiffened interior atomic layers are the main contributions of the increased elastic modulus of ZnO nanofilms.

Material surface characteristics are important to cell–material interactions at the biological interface, which are essential for the long-term success of biomedical implants. In this chapter, I will mainly focus on surface coatings for orthopaedic and dental implants. For implanted materials, both mammalian cell–material interactions and bacterial–material interactions should be considered. It is desirable for surface coatings to be able to support the biological functions of mammalian cells and subsequent tissue–implant integration, as well as discriminate bacterial colonization at the biological interface. Mammalian cell–material interactions and bacterial adhesion are affected by various chemical and physical factors of the material’s surface. Primary factors such as surface biochemistry, surface charge, surface hydrophobicity, surface roughness/topography, surface porosity, surface crystallinity and surface stiffness have been discussed in this chapter. In addition, key challenges and future trends in coatings and surface design for biomedical implants are explored.

A surface energy polar plot contains two possible singularities: the cusps that give facets on an equilibrium crystal, and the circular arcs connecting the cusps that can lead to missing orientations. The common approach of specifying the surface energy usually cannot handle both singularities simultaneously. We model the surface stiffness to avoid missing orientations. Furthermore, a facet is represented by the Dirac delta function. Thus, both singularities are treated precisely. We demonstrate our approach by modeling two axially symmetric crystals.

SUMMARY During hopping in place and running, humans maintain similar center of mass dynamics by precisely adjusting leg mechanics to compensate for moderate changes in surface stiffness. We investigated the limits of this precise control by asking humans to hop in place on extremely soft elastic surfaces. We found that hoppers drastically altered leg mechanics and maintained similar center of mass dynamics despite a sevenfold change in surface stiffness(11–81 kN m-1). On the stiffest surfaces, the legs compressed in early stance and then extended in late stance in the pattern that is typical for normal bouncing gaits. On the softest surfaces, however, subjects reversed this pattern so that the legs extended up to 8 cm in early stance and then compressed by a similar distance in late stance. Consequently, the center of mass moved downward during stance by 5–7 cm less than the surface compressed and by a similar distance as on the stiffest surfaces. This unique leg action probably reduced extensor muscle pre-stretch because the joints first extended and then flexed during stance. This interpretation is supported by the observation that hoppers increased muscle activation by 50% on the softest surface despite similar joint moments and mechanical leg work as on the stiffest surface. Thus, the extreme adjustment to leg mechanics for very soft surfaces helps maintain normal center of mass dynamics but requires high muscle activation levels due to the loss of the normal extensor muscle stretch–shorten cycle.

Humans running and hopping maintain similar center-of-mass motions, despite large changes in surface stiffness and damping. The goal of this study was to determine the contributions of anticipation and reaction when human hoppers encounter surprise, expected, and random changes from a soft elastic surface (27 kN/m) to a hard surface (411 kN/m). Subjects encountered the expected hard surface on every fourth hop and the random hard surface on an average of 25% of the hops in a trial. When hoppers on a soft surface were surprised by a hard surface, the ankle and knee joints were forced into greater flexion by passive interaction with the hard surface. Within 52 ms after subjects landed on the surprise hard surface, joint flexion increased, and the legs became less stiff than on the soft surface. These mechanical changes occurred before electromyography (EMG) first changed 68–188 ms after landing. Due to the fast mechanical reaction to the surprise hard surface, center-of-mass displacement and average leg stiffness were the same as on expected and random hard surfaces. This similarity is striking because subjects anticipated the expected and random hard surfaces by landing with their knees more flexed. Subjects also anticipated the expected hard surface by increasing the level of EMG by 24–76% during the 50 ms before landing. These results show that passive mechanisms alter leg stiffness for unexpected surface changes before muscle EMG changes and may be critical for adjustments to variable terrain encountered during locomotion in the natural world.

This work deals with the approximation of the contact problem for a viscoplastic material with the Signorini contact conditions by the problem with normal compliance, when the surface deformability coefficient converges to zero, i.e., when the surface stiffness tends to infinity, which represents a perfectly rigid obstacle. The possible damage of the material caused by compression or tension is taken into account. The approximate problem is formulated as a variational inequality and its convergence to the Signorin problem is proved. Then, the fully discrete scheme for the two problems is described and its convergence established. Results of numerical simulations, based on these schemes, are presented in one and two dimensions which show the convergence.

This paper explores a novel stiffness sensor which is mounted on the tip of a laparoscopic camera. The proposed device is able to compute stiffness when interacting with soft surfaces. The sensor can be used in Minimally Invasive Surgery, for instance, to localise tumor tissue which commonly has a higher stiffness when compared to healthy tissue. The purely mechanical sensor structure utilizes the functionality of an endoscopic camera to the maximum by visually analyzing the behavior of trackers within the field of view. Two pairs of spheres (used as easily identifiable features in the camera images) are connected to two springs with known but different spring constants. Four individual indenters attached to the spheres are used to palpate the surface. During palpation, the spheres move linearly towards the objective lens (i.e. the distance between lens and spheres is changing) resulting in variations of their diameters in the camera images. Relating the measured diameters to the different spring constants, a developed mathematical model is able to determine the surface stiffness in real-time. Tests were performed using a surgical endoscope to palpate silicon phantoms presenting different stiffness. Results show that the accuracy of the sensing system developed increases with the softness of the examined tissue.

Background: Running shoe compliance and track surface stiffness can reduce peak vertical foot forces. It is therefore of interest to measure directly the force-deflection curve for running shoes in the heel and forefoot areas. This study compares these measurements with similar work on track and field surfaces, and derives a mathematical stress-strain model useful over the entire force range. Methods: Six different running shoes from 4 popular brands are measured to determine vertical spring stiffness. The heel and ball areas are tested with 3.8 and 5.1 cm (1.5 and 2.0 in.) diameter heels in the force range of 0 to 0.13 kN and 2.0 to 2.6 kN (0 to 30 lbf. and 450 to 600 lbf.). The results show a factor of 2 difference from one shoe to the next, holding test area, heel diameter and force range constant. Load increments are applied on a time scale of 0.1 seconds, comparable to typical foot contact times during running. Results: The measured spring constants are essentially independent of plunger area, a useful simplification. For a given shoe, the ball area can be three times less compliant than the heel. Conclusion: Heel spring constants at the high force levels fall in the range from 290 kN/m to 600 kN/m (20,000 to 42,000 lbf/ft.), and thus approximate optimal track stiffness. In terms of theory, an exponential function derives from the observation that the data fall along a straight line on semi-log co-ordinates. This mathematical model enables calculation of running shoe compressive response and spring constant at physiological force levels.

Changes in surface stiffness, mass, volume and density of fresh as well as stored eggplants were studied. Storage period was varied between 0 and 168 h at 15–30 °C temperature and 90% relative humidity. Mass and volume were measured using standard methods and density was computed from them. Surface stiffness was determined using force–deformation curve obtained on uniaxial compression testing machine. Surface stiffness and volume of eggplant decreased nonlinearly till certain time of storage whereas mass decreased linearly throughout the storage period; and the density showed a reverse trend. Rate of reduction of these properties was observed maximum at 25 °C storage temperature. The storage period of 96 and 120 h were found to be critical for the eggplant stored at 30 and 20–25 °C, respectively, after which it may start rotting and may not be good enough to use them for vegetable purposes. Eggplant stored at 15 °C did not show any such sign of detoriation in the ranges of variables studied.

The aging behavior of partially and completely crosslinked RTM6 epoxy resin samples in the glassy state is investigated by thermophysical and mechanical analyses. Curing degree, glass transition temperature, density, and micromechanical modulus are investigated as function of aging period, initial curing degree or depth below the sample surface. A clear increase of density and modulus with aging period as well as an enhanced surface stiffness is detected for all curing degrees. Also, the aging period necessary to achieve the steady state modulus is independent from the curing degree. In contrast, the degree of physical aging induced modulus changes shows a significant dependence on the curing degree. A discontinuity is detected in the so-called transition region, which is related to the transition from rubber to glassy state during crosslinking. This emphasizes the importance of curing history for physical aging processes and the high potential of partial curing for the development of new processing routes, in particular for production of samples with low sensitivity to physical aging. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41121.

Aim It is accepted that leg stiffness (Kleg) increases when surface stiffness decreases, and vice versa. However, little is known how the central nervous system fulfils this task. To understand the effect of surface stiffness on the neural control of stretch-shortening cycle movements, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases after ground contact during two-legged hopping when changing from solid to elastic ground. Methods Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)- and long (LLR)-latency responses of the soleus muscle (SOL) during two-legged hopping on different stiffness surfaces, elastic and stiff. Results Soleus H-reflexes during two-legged hopping on the elastic surface were lower at SLR and larger at LLR than on the stiff surface (P

Molecular dynamics simulations were performed to determine how interfacial properties and friction forces of two model hydroxylated α-alumina surfaces are affected by softening or stiffening the surface potential, without changing the surface−surface intermolecular potential Vinter. The surface−surface vibrational spectrum for the soft surface, in absence of applied load, is influenced by coupling between Vinter and the surface vibrations. For the stiff surface, this coupling becomes important as the applied load is increased. Friction forces for the stiff and soft surfaces are similar at low loads, but at high loads the stiff surface has higher friction forces. This result mirrors the surface−surface vibrational spectra for the soft and stiff surfaces versus load. Increasing the load forces both systems into potential energy wells, requiring additional energy (or external force) to surmount potential energy barriers for sliding, leading to enhanced friction. At high sliding velocity, the instantaneous ba...

Treated and untreated wool fibers are analyzed by methodologies based on atomic force microscopy. Untreated fibers are known to have an intact native surface lipid layer, while KOH treatment has the effect of removing this layer as well as disrupting the molecular substructure. The effective bulk Young's modulus of the untreated fibers is measured in air and in situ during exposure to water. There is a nearly instantaneous drop by a factor of two from approximately 1.2 GPa. after onset of aqueous exposure. The surface stiffness of both untreated and KOH treated fibers is measured in situ during aqueous exposure for durations up to 12 X 103seconds. A decrease by a factor of two is observed for the untreated surface, while there is a more rapid decrease by a factor of ten for the KOH treated surface. Measurements of an equivalent Young's modulus for the near-surface layers obtained by transverse compression of less than 200 nm by the tip result in values of less than 1 MPa. Because wool is an inhomogeneous and anisotropic medium, the outcomes of the two different measurements of the modulus are complementary but not comparable. Tip-to-surface adhesion is also monitored during aqueous exposure, with the KOH treated surface exhibiting the lower value by a factor of approximately ten. There are also trends showing lower adhesion with duration of exposure. Rapid ingress of water into the bulk appears to be associated with decreased bulk stiffness, but has little immediate effect on surface and near-surface properties. The decrements in surface stiffness and observed effects on adhesion from aqueous exposure are then due to a much slower radial ingress of water, which is additionally rate-limited by the native lipid layer.

Synopsis Surface structure and surface mechanical properties of human hair have been characterized by atomic force microscopy in the imaging mode and by force vs. distance, F-d, analysis. The effects of treatment by commercial conditioner/shampoo or by aqueous exposure have been investigated. The cuticular structure has been imaged at medium resolution; longitudinal striations with lateral spacings of 150–350 nm and vertical corrugations in the range 2–8 nm were observed at higher resolution. The features are similar to those observed for untreated wool fibre. Both adventitious debris/contamination and residues from cosmetic treatment can be imaged with resolution in the low-nanometre range. Removal of the cuticular surface layer from treatment with alkali solution, and subsequent imaging, revealed a fibrous substructure. F-d analysis of the surface is a rich source of spatially resolved mechanical and chemical information. Surface stiffness, and an equivalent Young's Modulus, E, can be inferred from the shape of the ‘approach’ tip-to-surface contact curve. A value of E of ≈ 10 MPa was obtained for untreated hair. During aqueous exposure for 1 h the stiffness and modulus decreased by approximately a factor of 10. The discontinuity seen at ‘lift-off’ during the retract half-cycle of F-d analysis represents a measure of tip-to-surface adhesion. Adhesion decreased during aqueous exposure and was below the level of detectability after 1 h. Likewise, treatment by conditioner had the effect of lowering adhesion. High resolution F-d data revealed features that are consistent with the presence of a thin and readily compressible surface layer, probably analogous to the surface lipid layer on untreated wool fibre. Resume La structure de surface et les proprietes mecaniques de surface du cheveu humain ont ete caracterisees par microscopie atomique en mode image et par analyse de la force en fonction de la distance, F-d. Les effets d'un traitement par un shampoing/apres shampoing commercial ou d'une exposition a l'eau ont eteetudies. La structure cuticulaire a ete representee a resolution moyenne; a resolution superieure on observe des stries longitudinales avec des espacements lateraux de 150-350 nm et des ondulations verticales dans la fourchette 2-8 nm . Les caracteristiques sont similaires a celles observees pour des fibres de laine non traitees. Aussi bien les debris adventifs et la contamination que les residus du traitement cosmetique peuvent etre representes avec une resolution de l'ordre de quelques nanometres. Le retrait de la couche de surface cuticulaire par un traitement avec une solution alcaline et la representation qui en resulte revelent une sous structure fibreuse. L'analyse F-d de la surface est une source riche d'information chimique et mecanique resolue dans l'espace. La raideur de surface, et le module d'Young E equivalent, peuvent etre deduits d'apres la forme de la courbe de contact entre pointe et surface “par approche”. On obtient une valeur de E d'environ 10 MPa pour les cheveux non traites. Apres une exposition aqueuse de 1 heure la raideur et le module diminuent d'un facteur environ 10. La discontinuite observee au “decollage” durant le demi-cycle de retrait de l'analyse F-d represente une mesure de l'adhesion entre pointe et surface. L'adhesion diminue lors de l'exposition aqueuse et se trouve en dessous du seuil de detection apres 1 heure. De la meme facon, le traitement par un apres shampoing a pour effet une diminution de l'adhesion. Les donnees de F-d a haute resolution revelent des caracteristiques qui sont coherentes avec la presence d'une couche de surface fine et facilement compressible, probablement analogue a la couche lipidique de surface des fibres de laine non traitee.