Intermediate Stiffness Research Articles

Variable Stiffness Fiber with Self-Healing Capability.

A variable stiffness fiber made of silicone and low melting point alloys quickly becomes >700 times softer and >400 times more deformable when heated above 62 °C. It shows remarkable self-healing properties and can be clamped, knitted, and bonded, as shown in a foldable multi-purpose drone, a wearable cast for bone injuries, and a soft multi-directional actuator.

Differential Behavior of Normal and Fibrotic Fibroblasts under the Synergistic Influence of Micropillar Topography and the Rigidity of Honey/Silk-Fibroin Substrates.

We report differential proliferation behavior of normal and fibrosis associated human oral fibroblasts on micropillar honey embedded silk fibroin substrates (HSF). Oral fibroblasts of different origins manifest differences in proliferation rate, morphology, and the cytoskeletal arrangement on HSF substrates with distinct topography (H, D, and S), stiffness, and honey concentration. It is observed that the proliferation rate is maximized for normal and inhibited for fibrosis associated fibroblasts on a HSF substrate surface with moderate height of ∼8.5 μm and 2% honey concentration. Molecular expression analysis reveals decrease in c-myc and p53 expression in later cells validating the inhibition of their proliferation rate, which is further correlated with the decreased Col I and Col III expression on this substrate. A substrate with enhanced interspacing and intermediate mechanical stiffness (0.57 ± 0.32 μN/nm) favors strong adhesion and stable cell-matrix interaction for normal cells, while exhibiting negative influence on fibrotic fibroblasts with poor adhesion and spreading capability. Decrease in vimentin, fibronectin expression, and cytoskeleton reorganization justify the poor stability of later cells on the optimized substrate, thereby allowing selective modulation of normal and fibrosis associated fibroblasts under the synergistic influence of honey concentration, topography, and rigidity of HSF substrates. The work highlights the possible therapeutic efficacy of honey based micropatterned substrates as smart patches for fast wound healing and in minimizing the chances of recurrence of precancer post oral tumor resection surgeries.

Role of the polymer phase in the mechanics of nacre-like composites

Although strength and toughness are often mutually exclusive properties in man-made structural materials, nature is full of examples of composite materials that combine these properties in a remarkable way through sophisticated multiscale architectures. Understanding the contributions of the different constituents to the energy dissipating toughening mechanisms active in these natural materials is crucial for the development of strong artificial composites with a high resistance to fracture. Here, we systematically study the influence of the polymer properties on the mechanics of nacre-like composites containing an intermediate fraction of mineral phase (57vol%). To this end, we infiltrate ceramic scaffolds prepared by magnetically assisted slip casting (MASC) with monomers that are subsequently cured to yield three drastically different polymers: (i) poly(lauryl methacrylate) (PLMA), a soft and weak elastomer; (ii) poly(methyl methacrylate) (PMMA), a strong, stiff and brittle thermoplastic; and (iii) polyether urethane diacrylate-co-poly(2-hydroxyethyl methacrylate) (PUA-PHEMA), a tough polymer of intermediate strength and stiffness. By combining our experimental data with finite element modeling, we find that stiffer polymers can increase the strength of the composite by reducing stress concentrations in the inorganic scaffold. Moreover, infiltrating the scaffolds with tough polymers leads to composites with high crack initiation toughness KIC. An organic phase with a minimum strength and toughness is also required to fully activate the mechanisms programmed within the ceramic structure for a rising R-curve behavior. Our results indicate that a high modulus of toughness is a key parameter for the selection of polymers leading to strong and tough bioinspired nacre-like composites.

Graded effects of metallic foam cores for spherical sandwich shells subjected to close-in underwater explosion

A fluid model is developed and used in combination with Abaqus/Explicit to investigate the effects of graded foam cores on the loading of a sandwich spherical shell subject to underwater explosion from the inner side, after having validated the modeling technique by reproducing results by other authors. Based on the relation between the core strength and the stiffness of the outer face sheet (OFS), four different situations are considered to discuss the graded effects. It is demonstrated that for the case of relatively strong cores and the OFS with low stiffness or soft cores and the OFS with high stiffness, the core arrangement of low/medium/high (relative density from the inside to the outside) has the best performance to shock loadings which is a consequence of the effects of the fluid–structure interaction and the energy absorption capability; on the other hand, for the case of intermediate core strengths and stiffness of the OFS where the pulling-back force due to the stretching of the OFS is close to the core strength, the configuration of high/medium/low has the best performance due to its higher energy absorption efficiency of the foam and lower transmitted stress.

Endothelial directed collective migration depends on substrate stiffness via localized myosin contractility and cell-matrix interactions

Macrovascular endothelial injury, which may be caused by percutaneous intervention, requires endothelial cell directed collective migration to restore an intact endothelial monolayer. While interventions are often performed in arteries stiffened by cardiovascular disease, the effect of substrate stiffness on endothelial cell collective migration has not been examined. We studied porcine aortic endothelial cell directed collective migration using a modified cage assay on 4, 14, and 50kPa collagen-coated polyacrylamide gels. Total cell migration distance was measured over time, as were nuclear alignment and nuclear:total β-catenin as measures of cell directedness and cell–cell junction integrity, respectively. In addition, fibronectin fibers were examined as a measure of extracellular matrix deposition and remodeling. We now show that endothelial cells collectively migrate farther on stiffer substrates by 24h. Cells were more directed in the migration direction on intermediate stiffness substrates from 12 to 24h, with an alignment peak 400–700µm back from the migratory interface. However, cells on the softest substrates had the highest cell–cell junction integrity. Cells on all substrates deposited fibronectin, however fibronectin fibers were most linear and aligned on the stiffer substrates. When Rho kinase (ROCK) was inhibited with Y27632, cells on soft substrates migrated farther and cells on both soft and stiff substrates were more directed. When α5 integrin was knocked down with siRNA, cells on stiffer substrates did not migrate as far and were less directed. These data suggest that ROCK-mediated myosin contractility inhibits endothelial cell collective migration on soft substrates, while cell-matrix interactions are critical to endothelial cell collective migration on stiff substrates.

Effects of the transmission shaft on the main reducer vibration based on ADAMS and experimental demonstration

A variable axial transmission system which is composed of universal joint transmission shafts and a gear pair plays an important role in many engineering fields. However, researchers have done a lot of work in the area of gear dynamic characteristics, seldom considering the effects of universal joint shafts on the modal characteristics of gear pair system. This project takes a rear-wheel drive transmission system as a research subject. A transmission shaft-main reducer assembly virtual prototype simulation model is established by the dynamics software ADAMS; and the effects of the angle between shafts and the intermediate support stiffness on the noise and vibration characteristics of a main reducer in a rear axle are analysed. At the same time, an experiment is conducted to demonstrate the authenticity of the simulation. The result provides reference for parameters design of a drive shaft and a main reducer in a rear axle.

A Simple Model to Estimate Plantarflexor Muscle-Tendon Mechanics and Energetics During Walking With Elastic Ankle Exoskeletons.

A recent experiment demonstrated that when humans wear unpowered elastic ankle exoskeletons with intermediate spring stiffness, they can reduce their metabolic energy cost to walk by ∼7%. Springs that are too compliant or too stiff have little benefit. The purpose of this study was to use modeling and simulation to explore the muscle-level mechanisms for the "sweet spot" in stiffness during exoskeleton assisted walking. We developed a simple lumped uniarticular musculoskeletal model of the plantarflexors operating in parallel with an elastic "exo-tendon." Using an inverse approach with constrained kinematics and kinetics, we rapidly simulated human walking over a range of exoskeleton stiffness values and examined the underlying neuromechanics and energetics of the biological plantarflexors. Stiffer ankle exoskeleton springs resulted in larger decreases in plantarflexor muscle forces, activations, and metabolic energy consumption. However, in the process of unloading the compliant biological muscle-tendon unit, the muscle fascicles experienced larger excursions that negatively impacted series elastic element recoil that is characteristic of a tuned "catapult mechanism." The combination of disrupted muscle-tendon dynamics and the need to produce compensatory forces/moments to maintain overall net ankle moment invariance could explain the "sweet spot" in metabolic performance at intermediate ankle exoskeleton stiffness. Future work will aim to provide experimental evidence to support the model predictions presented here using ultrasound imaging of muscle-level dynamics during walking with elastic ankle exoskeletons. Engineers must account for the muscle-level effects of exoskeleton designs in order to achieve maximal performance objectives.

Strength and stiffness requirements for intermediate ring stiffeners on discretely supported cylindrical shells

Silos in the form of a cylindrical metal shell are often supported on a ring beam which rests on discrete column supports. This support condition produces a circumferential non-uniformity in the axial membrane stresses in the silo shell. One way of reducing the non-uniformity of these stresses is to use a very stiff ring beam which partially or fully redistributes the stresses from the local support into uniform stresses in the shell. A better alternative is to use a combination of a flexible ring beam and an intermediate ring stiffener. Recent research by the authors has identified the ideal location of the intermediate ring stiffener to provide circumferentially uniform axial membrane stresses above the stiffener. To be fully effective, this intermediate ring should locally prevent both radial and circumferential displacements in the shell. This paper explores the strength and stiffness requirements for this intermediate ring stiffener. Pursuant to this goal, the cylindrical shell below the intermediate ring stiffener is analysed using the membrane theory of shells and the reactions produced by the stiffener on the shell are identified. These reactions are then applied to the intermediate ring stiffener. Vlasov's curved beam theory is used to derive closed form expressions for the variation of the stress resultants around the circumference to obtain a strength design criterion for the stiffener. A stiffness criterion is then developed by considering the ratio of the circumferential stiffness of the cylindrical shell to that of the intermediate ring stiffener. The circumferential displacements of the ring and the shell are found for the loading condition previously obtained to determine the required strength. A simple algebraic expression is developed for this intermediate ring stiffness criterion. These analytical studies are then compared with complementary finite element analyses that are used to identify a suitable value for the intermediate ring stiffness ratio for practical design.

Enhancement of mobility in an interacting colloidal system under feedback control.

Feedback control schemes are a promising way to manipulate transport properties of driven colloidal suspensions. In the present article, we suggest a feedback scheme to enhance the collective transport of colloidal particles with repulsive interactions through a one-dimensional tilted washboard potential. The control is modeled by a harmonic confining potential, mimicking an optical "trap," with the center of this trap moving with the (instantaneous) mean particle position. Our theoretical analysis is based on the Smoluchowski equation combined with dynamical density functional theory for systems with hard-core or ultrasoft (Gaussian) interactions. For either type of interaction, we find that the feedback control can lead to an enhancement of the mobility by several orders of magnitude relative to the uncontrolled case. The largest effects occur for intermediate stiffness of the trap and large particle numbers. Moreover, in some regions of the parameter space the feedback control induces oscillations of the mean velocity. Finally, we show that the enhancement of mobility is robust against a small time delay in implementing the feedback control.

Tunable, Photoreactive Hydrogel System To Probe Synergies between Mechanical and Biomolecular Cues on Adipose-Derived Mesenchymal Stem Cell Differentiation.

The incorporation of the photoreactive molecule benzophenone into polyacrylamide hydrogels, allowing for orthogonal control over spatial incorporation of biomolecules and selective modulation of matrix stiffness, is described. Adipose-derived mesenchymal stem cells were cultured on matrices whose elastic moduli are tuned to ∼5, ∼14, and ∼37 kPa combined with an immobilized growth factor, bone morphogenic protein 2 (BMP-2), and their lineage differentiation was determined. BMP-2 was observed to have the most pronounced effect at the intermediate stiffness, while the lowest and highest stiffness hydrogels are directed by elasticity alone. Together, this approach describes a facile platform for fundamental studies of cell fate decisions in the context of both mechanical and biochemical cues and may lead to improved insight and effectiveness of stem cell therapies.

Using non-local Timoshenko beam theories for prediction of micro- and macro-structural responses

This paper presents the developments of the non-local sandwich beam theories that can be used to assess the micro- and macro-structural responses. The paper utilizes homogenization–localization, modified couple stress, and thick-faces beam theories. The homogenization derives prevailing differential equations from displacements through strains and stresses to external loading. This enables accurate localization process that recovers the microstructural effects from the homogenized solution and couples them to the global response. Some case studies are supported by computational experiments: the shortest beams have only four unit cell along their length. The present solution converges to the physically correct solution in the cases of infinite and zero shear stiffness. The limit of zero shear stiffness is important because there the traditional Timoshenko beam theory fails to predict the response correctly. Some difference in the solutions is observed on very short beams at intermediate rotation stiffness between the faces and the core, where the wavelengths of the global and local responses heavily interact. Present theory can be extended to different microstructures and to plates. The benefit of the present approach is that it is analytical which enables identification of the physical parameters.

Comparative Analysis of the Energy Dissipation of Steel Buildings with Concentric and Eccentric Braces

A comparative study has been done to analyze the behavior of regular steel building structures of 4, 6, 8 and 10 stories, located in seismic zone 5 and soil type S1. The structures were upgraded with different brace configurations according to current Venezuelan codes. A total number of 24 numerical models were analyzed considering non-linear static and incremental dynamic analysis (IDA). The buildings were initially designed as moment resisting frames, and upgraded with six different bracing configurations: concentric braces in “X” and inverted “V”; eccentric braces inverted "V" with horizontal links, inverted “Y” and “X” with vertical links. Short length links were used to ensure a shear failure. The used methodology is based on obtaining the capacity, IDA curves, and bilinear approximations of these curves that allow the determination of yield and ultimate capacity points, in order to estimate important parameters of seismic response: overstrength and ductility; and considering these areas under the curves to estimate elastic deformation energy, energy dissipated by hysteretic damping and equivalent damping. According to the results, the cases with no brace enhancement showed the lowest lateral strength and lateral stiffness and high deformation capacity. On the other hand, the concentric bracing cases, resulted with the highest stiffness and strength and the lowest deformation capacity, therefore they have low ductility and energy dissipation capacity under seismic loading. Structures with links showed intermediate stiffness and strengths, resulting in the best performance in terms of ductility and energy dissipation capacity. The present study provides a better understanding of the benefits of eccentrically braced systems.

Mineralogy and geotechnical properties of Singapore marine clay at Changi

An engineering geological study was undertaken to determine the geotechnical properties and mineralogy of Singapore marine clay at Changi in the Republic of Singapore. This soft soil is a quartenary deposit that lies within submarine valleys cut in an old alluvium formation. The marine clay comprises a soft upper marine clay layer overlying a stiffer lower marine clay layer. An intermediate stiff clay layer is sandwiched between these two marine clay layers; it is believed to be the dessicated crust of the lower marine clay layer. In the present study, morphological and mineralogical observations of Singapore clay were taken by scanning electron microscopy, X-ray diffraction and photographic identification. The geotechnical investigation included physical, compression, permeability and field vane shear tests. The upper marine clay was found to be soft with undrained shear strength values ranging from 10 to 30kPa, while the lower marine clay was found to have undrained shear strength values from 30 to 60kPa. The sensitivity of the marine clay at Changi varied from 3 to 8 and is described as highly sensitive marine clay. The upper clay layer had a coefficient of consolidation of 0.47–0.6m2/year due to the vertical flow (cv) and a coefficient of consolidation of 2–3m2/year due to the horizontal flow (ch). The lower marine clay had a cv of 0.8–1.5m2/year and a ch of 3–5m2/year, while the intermediate stiff clay had a cv of 1–4.5m2/year and a ch of 5–10m2/year. The primary clay mineral was kaolinite, followed by smectite and mica. With this high kaolinite content, the activity of the Singapore marine clay was found to be low at approximately 0.5–1.3 and classified as inactive to active clay. With reference to the intrinsic state line, natural Singapore clay (upper, intermediate and lower layers) has been classified as structured clay and is in a meta-stable state. The compression behaviour can be modelled by the Intrinsic State Line and the destructuring framework. The predicted and the measured compression curves were comparable for various depths. The successful modelling of compression curves will be useful for settlement calculations and for further research on the development of a constitutive model for Singapore marine clay.

Reversal of Fibrosis in Chronic Rejection in Mouse Airways through Combined Relaxin and Lysyl Oxidase Inhibitor Therapy

The leading cause of death in lung transplant recipients is the development of chronic rejection, signified by obliterative bronchiolitis, which is histologically characterized by a fibrotic process that leads to slow narrowing of the airways. The fibrosis occurs when activated fibroblasts deposit excess extracellular matrix (ECM), which compromises gas exchange. Accumulation of ECM proteins in chronic rejected allograft in lung transplant may increase the stiffness of tissue and result in further unbalance of protein turnover and fibrous obliteration of the airway. We hypothesized that relaxin, a hormone that helps pelvic and cervical expansion and relaxation during pregnancy, could ameliorate fibrosis in chronic rejection. Relaxin might prohibit fibroblast-to-myofibroblast differentiation and fibroblast contraction, thus inhibiting fibrosis. These effects can be amplified by combining with lysyl oxidase inhibitor, β-aminoproprilnitrile (BAPN), which functions by inhibiting the cross-linking of fiber-forming stiff collagen, which decreases the ECM stiffness. We test in vitro by transplanting tracheas from major histocompatibility complex-mismatched mice to C57BL/6 mice treated with recombinant relaxin combined with BAPN after chronic rejection. The combined treatment of relaxin and BAPN attenuates airway fibrosis, decreases cross-linked collagen deposition, and promotes re-epithelialization. Relaxin inhibits fibroblast-to-myofibroblast differentiation and procollagen deposition in fibroblasts plated on matrix with intermediate stiffness, but not high stiffness. Relaxin also decreases fibroblast contractility, evidenced by decreased myosin light chain protein phosphorylation, through up-regulating prostaglandin E2. The beneficial effects of combined treatment might result from the regulation of cell contractility by relaxin and the decreased ECM stiffness caused by inhibition of collagen cross-linking by BAPN.

Rheology evaluations of WMA binders using ultraviolet and PAV aging procedures

Oxidation of an asphalt binder affects the long term performance of an asphalt pavement, especially under thermal and/or ultraviolet (UV) radiation conditions. The objective of this study is to investigate the rheological behaviors of various WMA binders after long term UV aging and conventional PAV aging procedures. The following rheological properties were tested and evaluated in this study: complex shear modulus, phase angle, elastic modulus, viscous modulus at an intermediate service temperature, and deflection, stiffness, and m-value at a low temperature. The experimental design included four asphalt binder sources (two grades); four WMA additives and control. Two aging methods of UV and PAV aging were utilized and a total of 40binders were tested and evaluated. The test results indicated that UV and PAV aging procedures are significantly affected by asphalt binder source and grade. In addition, WMA additives generally have no influences on rheological properties of UV and PAV aged binders. Moreover, the differences in deflection, creep stiffness, and m-values of UV and PAV aged binders are generally not affected by their grade, source and WMA additives. Therefore, UV aging procedure used in this study may be an effective method in investigating rheological characteristics of WMA binder after a long term aging process.

Numerical Analysis and Demonstration: Transmission Shaft Influence on Meshing Vibration in Driving and Driven Gears

The variable axial transmission system composed of universal joint transmission shafts and a gear pair has been applied in many engineering fields. In the design of a drive system, the dynamics of the gear pair have been studied in detail. However few have paid attention to the effect on the system modal characteristics of the gear pair, arising from the universal joint transmission shafts. This work establishes a torsional vibration mathematical model of the transmission shaft, driving gear, and driven gear based on lumped masses and the main reducer system assembly of a CDV (car-based delivery vehicle) car. The model is solved by the state space method. The influence of the angle between transmission shafts and intermediate support stiffness on the vibration and noise of the main reducer is obtained and verified experimentally. A reference for the main reducer and transmission shaft design and the allied parameter matching are provided.

From amorphous aggregates to polymer bundles: The role of stiffness on structural phases in polymer aggregation

We study the aggregation transition of a finite theta-polymer system in dependence on the bending stiffness κ with the help of parallel multicanonical simulations. In order to distinguish amorphous aggregates from polymer bundles we introduce an order parameter, measuring the correlation of the end-to-end vectors. With the help of this order parameter, we construct generic phase diagrams for systems with 2 and 8 polymers and discuss the occurring phases from amorphous aggregates to bundle structures. For an intermediate stiffness range we find multiple aggregated phases which change with increasing number of polymers and discuss their nature with the help of microcanonical analyses. We show that the stiffness of semiflexible theta polymers is the distinguishing parameter for the emergent structural motifs.

The effects of successful percutaneous mitral balloon valvuloplasty on acute and intermediate term aortic stiffness.

The aim of this study was to evaluate the effects of percutaneous mitral balloon valvuloplasty (PMBV) on short- and intermediate-term aortic stiffness in patients with mitral stenosis. This prospective study included 56 patients with critical mitral stenosis in normal sinus rhythm (68% female; mean age: 42 ± 11 years) and 37 healthy controls. Aortic stiffness was measured using transthoracic echocardiography before PMBV, and 24-48 hours and 1 year post procedure. Aortic strain and distensibility were significantly higher in the patients with mitral stenosis, both after PMBV and 1 year post procedure, whereas the aortic stiffness index (ASI) was significantly lower. There was also a significant decrease in mitral mean gradient (MMG) and systolic pulmonary artery pressure (sPAP) after PMBV, based on echocardiography and catheterization. Mitral valve area (MVA) significantly increased after PMBV. There was a significant correlation between change in ∆MVA and ∆MMG and aortic elastic properties. There was a significantly negative correlation between the ∆MVA and ∆∆ aortic stiffness (r = -0.62, P < 0.001), and a significantly positive correlation between ∆MMG and ∆ASI (r = 0.60, P < 0.001). Mitral valve stenosis was associated with impaired aortic stiffness and following PMBV, aortic stiffness decreased during both the acute period and the intermediate period.

Edgewise connectivity: an approach to improving segmented primary mirror performance

As future astrophysics missions require space telescopes with greater sensitivity and angular resolution, the corresponding increase in the primary mirror diameter presents numerous challenges. Since fairing restrictions limit the maximum diameter of monolithic and deployable segmented mirrors that can be launched, there is a need for on-orbit assembly methods that decouple the mirror diameter from the choice of launch vehicle. In addition, larger mirrors are more susceptible to vibrations and are typically so lightly damped that vibrations could persist for some time if uncontrolled. To address these challenges, we present a segmented mirror architecture in which the segments are connected edgewise by mechanisms analogous to damped springs. These mechanisms can be damped springs, flux-pinning mechanisms, virtual mechanisms, or any other device with the same basic behavior. Using a parametric finite-element model, we show that for low to intermediate stiffnesses, the stiffness and damping contributions from the mechanisms improve both the natural frequency and disturbance response of the segmented mirror. At higher stiffnesses, the mechanisms structurally connect the segments, leading to a segmented mirror that performs comparably to a monolith—or better, depending on the mechanism damping—with the modular design enabling on-orbit assembly and scalability.

The bricycle: a bicycle in zero gravity can be balanced or steered but not both

A bicycle or inverted pendulum can be balanced, that is kept nearly upright, by accelerating the base. This balance is achieved by steering on a bicycle. Simultaneously one can also control the lateral position of the base: changing of the track line of a bike or the position of hand under a balanced stick. We show here with theory and experiment that if the balance problem is removed, by making the system neutrally stable for balance, one cannot simultaneously maintain balance and control the position of the base. We made a bricycle, essentially a bicycle with springy training wheels. The stiffness of the training wheel suspension can be varied from near infinite, making the bricycle into a tricycle, to zero, making it effectively a bicycle. The springy training wheels effectively reduce or even negate gravity, at least for balance purposes. One might expect a smooth transition from tricycle to bicycle as the stiffness is varied, in terms of handling, balance and feel. Not so. At an intermediate stiffness, when gravity is effectively zeroed, riders can balance easily but no longer turn. Small turns cause an intolerable leaning. Thus there is a qualitative difference between bicycles and tricycles, a difference that cannot be met halfway.