Strain-dependent Behavior Research Articles

The nitrogen source impacts major volatile compounds released by Saccharomyces cerevisiae during alcoholic fermentation

Sulphur-containing amino acids, cysteine and methionine, are generally found in very low concentrations in grape-juice. The objective of this study was to identify the effects of methionine on aroma compounds formation. Nitrogen source effects on growth, fermentative behaviour and aroma compounds formation were evaluated in three strains of Saccharomyces cerevisiae cultivated in batch under moderate nitrogen concentration, 267mg YAN/L, supplied as di-ammonium phosphate (DAP), a mixture of amino acids with (AA) or without methionine (AAwMet), and a mixture of AA plus DAP. Fermentative vigour and final biomass yields were dependent on the nitrogen source, for each of the strains tested, in particular for EC1118. Additionally, despite the strain-dependent behaviour with respect to the basal level of H2S produced, the comparison of treatments AA and AAwMet showed that presence of methionine suppressed H2S production in all strains tested, and altered aroma compound formation, particularly some of those associated with fruity and floral characters which were consistently more produced in AAwMet. Moreover, DAP supplementation resulted in a remarkable increase in H2S formation, but no correlation between sulphide produced and yeast fermentative vigour was observed.Results suggest that the use of different nitrogen sources results in the production of wines with divergent aroma profiles, most notably when EC1118 strain is used. Methionine determination and its management prior to fermentation are crucial for suppressing H2S and to endowing beverages with diverse sensory traits.

In vitro susceptibility of Staphylococcus aureus and Staphylococcus epidermidis isolated from prosthetic joint infections

Prosthetic joint infections (PJI) are severe complications in Orthopedics, with Staphylococcus aureus and Staphylococcus epidermidis being the most commonly isolated pathogens. The variable antimicrobial susceptibility found in these microorganisms, along with the increasing number of methicillin-resistant strains, increases the difficulty of antibiotic selection and makes it necessary to perform individual susceptibility studies to select the optimal antibiotic treatment. The aim of this study was to evaluate the in vitro susceptibility pattern of 35 clinical strains isolated from PJI (17 S. aureus and 18 S. epidermidis) against rifampin, vancomycin, tygecicline, clindamycin, cotrimoxazole, cloxacillin, ciprofloxacin, daptomycin and fosfomycin. In vitro susceptibility assays were performed using the broth microdilution method and agar dilution for fosfomycin. MBC was also determined. Tygecicline and daptomycin showed the highest antimicrobial activity with low MIC(90) values, and no resistant strains were detected. On the other hand, ciprofloxacin and cloxacillin exhibited a poor antimicrobial effect with a high percentage of nonsusceptible strains in both species. Bactericidal activity rates revealed the bacteriostatic behavior of rifampin, tygecicline, cotrimoxazole, fosfomycin and clindamycin, whereas vancomycin and cloxacillin showed species- and strain-dependent behavior. Daptomycin and ciprofloxacin were observed to be efficient bactericidal agents against the tested strains. According to our data, rifampin, tigecycline, daptomycin and fosfomycin showed high in vitro activity against most staphylococcal strains isolated from the PJIs tested, although daptomycin seems to be the best alternative to vancomycin therapy.

Cartilage-like mechanical properties of poly (ethylene glycol)-diacrylate hydrogels

Hydrogels prepared from poly-(ethylene glycol) (PEG) have been used in a variety of studies of cartilage tissue engineering. Such hydrogels may also be useful as a tunable mechanical material for cartilage repair. Previous studies have characterized the chemical and mechanical properties of PEG-based hydrogels, as modulated by precursor molecular weight and concentration. Cartilage mechanical properties vary substantially, with maturation, with depth from the articular surface, in health and disease, and in compression and tension. We hypothesized that PEG hydrogels could mimic a broad range of the compressive and tensile mechanical properties of articular cartilage. The objective of this study was to characterize the mechanical properties of PEG hydrogels over a broad range and with reference to articular cartilage. In particular, we assessed the effects of PEG precursor molecular weight (508 Da, 3.4 kDa, 6 kDa, and 10 kDa) and concentration (10–40%) on swelling property, equilibrium confined compressive modulus (HA0), compressive dynamic stiffness, and hydraulic permeability (kp0) of PEG hydrogels in static/dynamic confined compression tests, and equilibrium tensile modulus (Eten) in tension tests. As molecular weight of PEG decreased and concentration increased, hydrogels exhibited a decrease in swelling ratio (31.5–2.2), an increase in HA0 (0.01–2.46 MPa) and Eten (0.02–3.5 MPa), an increase in dynamic compressive stiffness (0.055–42.9 MPa), and a decrease in kp0 (1.2 × 10−15 to 8.5 × 10−15 m2/(Pa s)). The frequency-dependence of dynamic compressive stiffness amplitude and phase, as well as the strain-dependence of permeability, were typical of the time- and strain-dependent mechanical behavior of articular cartilage. HA0 and Eten were positively correlated with the final PEG concentration, accounting for swelling. These results indicate that PEG hydrogels can be prepared to mimic many of the static and dynamic mechanical properties of articular cartilage.

A non-linear viscoelastic material constitutive model for polyurea

A non-linear viscoelastic constitutive model for polyurea by assuming a separable time and strain dependent material behaviour was proposed in this study. The strain dependent function was described by a nine-parameter Mooney–Rivlin model whereas the time function was assumed to follow a Prony series. A method based on the finite time-increment formulation was used to calculate the material parameters using a simple fitting routine. The effectiveness and accuracy of the model was validated using existing compression and tension data from the literature. The proposed model was found to be very efficient in capturing the behaviour of polyurea under both compressive and tensile loadings for a wide range of strain rates.

A Nonlinear Constitutive Model for Stress Relaxation in Ligaments and Tendons

A novel constitutive model that describes stress relaxation in transversely isotropic soft collagenous tissues such as ligaments and tendons is presented. The model is formulated within the nonlinear integral representation framework proposed by Pipkin and Rogers (J. Mech. Phys. Solids. 16:59-72, 1968). It represents a departure from existing models in biomechanics since it describes not only the strain dependent stress relaxation behavior of collagenous tissues but also their finite strains and transverse isotropy. Axial stress-stretch data and stress relaxation data at different axial stretches are collected on rat tail tendon fascicles in order to compute the model parameters. Toward this end, the rat tail tendon fascicles are assumed to be incompressible and undergo an isochoric axisymmetric deformation. A comparison with the experimental data proves that, unlike the quasi-linear viscoelastic model (Fung, Biomechanics: Mechanics of Living Tissues. Springer, New York, 1993) the constitutive law can capture the observed nonlinearities in the stress relaxation response of rat tail tendon fascicles.

Strain dependent energy dissipation in multi-scale carbon fiber composites containing carbon nanofibers

This study investigates strain dependent energy dissipation characteristics in carbon nanofiber (CNF) reinforced carbon fiber epoxy composites (multi-scale composites) by characterizing their viscoelastic properties and vibrational damping response. The air damping effect on the energy dissipation characteristics is also examined. The viscoelastic properties of epoxy containing two weight fractions (3 and 5wt%) of added CNFs were characterized using dynamic mechanical analysis. Carbon fiber layers were then infiltrated with the two epoxy resins containing the CNFs to form multi-scale composites. A strain dependent loss factor behavior of the multi-scale composites was observed in the dynamic cyclic testing due to CNF’s stick–slip friction, showing a 53% increase in loss factor for the composites containing 5wt% CNFs. The beam vibration test results also indicated an improvement in loss factor for the multi-scale composite beams relative to those without the CNF addition in the first two resonant frequencies. The multi-scale composite beams exhibit an increase in loss factor, up to 43%, at high amplitude excitation, while a reduction in loss factor was seen at low amplitude. These observed strain dependent damping characteristics seem to result from both the stick–slip friction and the air damping effect.

Damage Mechanics of Porcine Flexor Tendon: Mechanical Evaluation and Modeling

Porcine flexor tendons underwent cyclic and stress relaxation testing before and after strain exceeding elastic limit ("overstretch") to examine which mechanical parameters undergo changes following subfailure damage. From these data, we developed an "effective strain" damage model (in which the tendon is modeled as if being pulled to a lower strain). Damage was induced at three strain levels to determine the extent to which post-damage parameter changes were affected by overstretch strain level. We found that diffuse damage induced by overstretch decreased elastic and viscoelastic parameters obtained during testing. The stress response of tendon to strain is therefore altered following damage. We next compared the strain-dependent parameter behavior to damage-dependent behavior to determine the effective strain for each parameter. Effects of damage became more pronounced as strain during overstretch increased; following overstretch to 6.5, 9, or 13% strain, effective strain was 2.43 ± 0.33, 1.98 ± 0.3, or 0.88 ± 0.43% strain, respectively. By determining the effective strain and using it to calculate predicted values of post-damage mechanical parameters, it was possible to predict the stress relaxation behavior of tendons with Schapery's nonlinear viscoelastic model. Using this approach, a single parameter predicts both elastic and viscoelastic compromise from known, strain-dependent behaviors.

Nonlinear viscoelasticty plays an essential role in the functional behavior of spinal ligaments

Despite the significant role ligament viscoelasticity plays in functional spinal biomechanics, relatively few studies have been performed to develop constitutive models that explicitly characterize this complex behavior. Unfortunately, the application and interpretation of these previous models are limited due to the use of simplified (quasi-linear) viscoelastic formulations or characterization techniques that have been shown to affect the predictive accuracy of the fitted coefficients. In order to surmount these previous limitations, the current study presents the application of a novel fitting technique (applied to stress relaxation experiments) and nonlinear viscoelastic constitutive formulation to human cervical spine anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL) and ligamentum flavum (LF). The fitted coefficients were validated by quantifying the ability of the constitutive equation to predict an independent cyclic data set across multiple physiologic strain amplitudes and frequencies. The resulting validated constitutive formulation indicated that the strain-dependent viscoelastic behavior of the longitudinal ligaments (ALL and PLL) was dominated by both the short-term (t=0.1s) and the steady-state (as t→∞) behavior. Conversely, the LF exhibited consistent relaxation behavior across the investigated temporal spectrum. From these data, it can be hypothesized that the unique strain-dependent temporal behavior of these spinal ligaments may be a functional adaptation that minimizes muscular expenditure during quasi-static postures while maximizing structural stability of the spine during transient loading events.

The role of filler network in nonlinear viscoelastic behavior of vapor grown carbon nanofiber filled polystyrene: A strain dependent rheological behavior and electrical conductivity study

AbstractInfluence of filler network on Payne effect and modulus recovery for vapor grown carbon nanofiber (VGCF)/polystyrene (PS) composites with VGCF content above electrical percolation threshold was studied by using simultaneous measurements of viscoelasticity and electrical conductivity. The strain softening seems to be closely related to breakdown of filler network. Recovery tests of modulus and electrical conductivity by means of time sweep indicate that the reformation of deformed VGCF network structure could not be completed in several hours. Compared with recovery behavior of carbon black (CB) and silica (SiO2) network, the reformation of VGCF network appears more difficult. Moreover, solidification of composites exerts some effect on modulus recovery. The filler network disrupted by small strain can be perfectly recovered by matrix solidification while the initial filler structure collapsing at large strain is only partially restored. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers

The impact of hepatic pressurization on liver shear wave speed estimates in constrained versus unconstrained conditions

Increased hepatic venous pressure can be observed in patients with advanced liver disease and congestive heart failure. This elevated portal pressure also leads to variation in acoustic radiation-force-derived shear wave-based liver stiffness estimates. These changes in stiffness metrics with hepatic interstitial pressure may confound stiffness-based predictions of liver fibrosis stage. The underlying mechanism for this observed stiffening behavior with pressurization is not well understood and is not explained with commonly used linear elastic mechanical models. An experiment was designed to determine whether the stiffness increase exhibited with hepatic pressurization results from a strain-dependent hyperelastic behavior. Six excised canine livers were subjected to variations in interstitial pressure through cannulation of the portal vein and closure of the hepatic artery and hepatic vein under constrained conditions (in which the liver was not free to expand) and unconstrained conditions. Radiation-force-derived shear wave speed estimates were obtained and correlated with pressure. Estimates of hepatic shear stiffness increased with changes in interstitial pressure over a physiologically relevant range of pressures (0–35 mmHg) from 1.5 to 3.5 m s−1. These increases were observed only under conditions in which the liver was free to expand while pressurized. This behavior is consistent with hyperelastic nonlinear material models that could be used in the future to explore methods for estimating hepatic interstitial pressure noninvasively.

Quasi-linear viscoelastic modeling of arterial wall for surgical simulation

Realistic soft tissue deformation modeling and haptic rendering for surgical simulation require accurate knowledge of tissue material characteristics. Biomechanical experiments on porcine tissue were performed, and a reduced quasi-linear viscoelastic model was developed to describe the strain-dependent relaxation behavior of the arterial wall. This information is used in surgical simulation to provide a realistic sensation of reduction in strength when the user holds a virtual blood vessel strained at different levels. Twelve pieces of porcine abdominal artery were tested with uniaxial elongation and relaxation test in both circumferential and longitudinal directions. The mechanical property testing system consists of automated environment control, testing, and data collection mechanism. A combined logarithm and polynomial strain energy equation was applied to model the elastic response of the specimens. The reduced relaxation function was modified by integrating a rational equation as a corrective factor to precisely describe the strain-dependent relaxation effects. The experiments revealed that (1) stress is insensitive to strain rate in arterial tissue when the loading rate is low, and (2) the rate of stress relaxation of arterial wall is highly strain dependent. The proposed model can accurately represent the experimental data. Stress-strain function derived from the combined strain energy function is able to fit the tensile experimental data with R(2) equals to 0.9995 in circumferential direction and 0.999 in longitudinal direction. Modified reduced relaxation function is able to model the strain-dependent relaxation with R(2) equals to 0.9686 in circumferential direction and 0.988 in longitudinal direction. The proposed model, based on extensive biomechanical experiments, can be used for accurate simulation of arterial deformation and haptic rendering in surgical simulation. The resultant model enables stress relaxation status to be determined when subjected to different strain levels.

Strain-induced magnetic transitions in half-fluorinated single layers of BN, GaN and graphene

Recently, extensive experimental and theoretical studies on single layers of BN, GaN and graphene have stimulated enormous interest in exploring the properties of these sheets by decorating their surfaces. In the present work we discuss half-fluorinated single layers of BN, GaN and graphene, in the context of intercoupling between strain and magnetic property. First-principles calculations reveal that the energy difference between ferromagnetic and antiferromagnetic couplings increases significantly with strain increasing for half-fluorinated BN, GaN and graphene sheets. More surprisingly, the half-fluorinated BN and GaN sheets exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain, even giving rise to half-metal when the sheets are under compression of 6%. It is found that the magnetic coupling as well as the strain-dependent magnetic transition behavior arise from the combined effects of both through-bond and p-p direct interactions. Our work offers a new avenue to facilitate the design of controllable and tunable spin devices.

Influence of applied in-plane strain on transverse thermal conductivity of 0°/90° and plain weave ceramic matrix composites

A computationally economic finite-element-based stress analysis model, developed previously by the authors, has been extended to predict the thermal behaviour of ceramic matrix composites with strain-induced damage. The finite element analysis utilises a solid element to represent a homogenised orthotropic medium of a heterogeneous uni-directional tow. The non-linear multi-axial strain dependent thermal behaviour has been discretised by multi-linear curves, which have been implemented by a user defined subroutine, USDFLD, in the commercial finite element package, ABAQUS. The model has been used to study the performance of two CMC composites: a SiC (Nicalon) fibre-calcium aluminosilicate (CAS) matrix, 0°/90° cross-ply laminate Nicalon-CAS; and, carbon fibre-dual carbon-SiC matrix (C/C-SiC), plain weave laminate DLR-XT. The global through-thickness thermal conductivity degradation with composite uni-axial strain has been predicted. Comparisons have been made between the predictions and experimental data for both materials, and good agreement has been achieved. For the Nicalon-CAS 0°/90° cross-ply the dominant mechanism of thermal conductivity degradation is combined fibre failure and associated wake debonding; and, for the DLR-XT plain weave the same mechanisms act in combination with out-of-plane shear failure.

Parametric analysis of the strain-dependent behavior of a metamaterial electric resonator

In this paper, we describe the strain-dependent behavior of an electric-LC (ELC) resonator unit cell, commonly used in metamaterial designs. We leverage analytic expression to understand the way strain manifests itself in a change in electromagnetic (EM) response. We verify the simplified physical models using full-wave simulations and generalize the trends to accommodate the strain profile for any arbitrary plane-stress loading scenario.

Flow of non‐newtonian fluids in porous media

AbstractThe study of flow of non‐Newtonian fluids in porous media is very important and serves a wide variety of practical applications in processes such as enhanced oil recovery from underground reservoirs, filtration of polymer solutions and soil remediation through the removal of liquid pollutants. These fluids occur in diverse natural and synthetic forms and can be regarded as the rule rather than the exception. They show very complex strain and time dependent behavior and may have initial yield‐stress. Their common feature is that they do not obey the simple Newtonian relation of proportionality between stress and rate of deformation. Non‐Newtonian fluids are generally classified into three main categories: time‐independent whose strain rate solely depends on the instantaneous stress, time‐dependent whose strain rate is a function of both magnitude and duration of the applied stress and viscoelastic which shows partial elastic recovery on removal of the deforming stress and usually demonstrates both time and strain dependency. In this article, the key aspects of these fluids are reviewed with particular emphasis on single‐phase flow through porous media. The four main approaches for describing the flow in porous media are examined and assessed. These are: continuum models, bundle of tubes models, numerical methods and pore‐scale network modeling. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Stress relaxation and recovery in tendon and ligament: Experiment and modeling

Accurate joint models require the ability to predict soft tissue behavior. This study evaluates the ability of constitutive equations to predict the nonlinear and viscoelastic behavior of tendon and ligament during stress relaxation testing in a porcine model. Three constitutive equations are compared in their ability to model relaxation, recovery and reloading of tissues. Quasi-linear viscoelasticity (QLV) can fit a single stress relaxation curve, but fails to account for the strain-dependence in relaxation. Nonlinear superposition can fit the single relaxation curve and will account for the strain-dependent relaxation behavior, but fails to accurately predict recovery behavior. Schapery's nonlinear viscoelastic model successfully fits a single relaxation curve, accounts for strain-dependent relaxation behavior, and accurately predicts recovery and reloading behavior. Comparing Schapery's model to QLV and nonlinear superposition, Schapery's method was uniquely capable of fitting the different nonlinearities that arise in stress relaxation curves from different tissues, e.g. the porcine digital flexor tendon and the porcine medial collateral ligament (MCL), as well as predicting subsequent recovery and relaxation curves after initial loads.

Strain-Dependent Photoluminescence Behavior of CdSe/CdS Nanocrystals with Spherical, Linear, and Branched Topologies

The photoluminescence of CdSe/CdS core/shell quantum dots, nanorods, and tetrapods is investigated as a function of applied hydrostatic and non-hydrostatic pressure. The optoelectronic properties of all three nanocrystal morphologies are affected by strain. Furthermore, it is demonstrated that the unique morphology of seeded tetrapods is highly sensitive to non-isotropic stress environments. Seeded tetrapods can thereby serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications including sensitive optomechanical devices and biological force investigations.

Strain-Dependent Oxygen Diffusivity in Bovine Annulus Fibrosus

The intervertebral disk (IVD) is the largest avascular structure in the human body. Transport of small molecules in IVD is mainly through diffusion from the endplates and the peripheral blood vessels surrounding IVD. Studies have investigated the structure, chemical components, and water content in IVD, but to our knowledge no study has investigated the effect of mechanical loading on oxygen transport in IVD. The objective of this study was to determine the strain-dependent behavior of oxygen diffusivity in IVD tissue. A one-dimensional steady-state diffusion experiment was designed and performed to determine the oxygen diffusivity in bovine annulus fibrosus (AF). The oxygen diffusivity was calculated using equation derived from Fick's law. A total of 20 AF specimens (d=6 mm, h approximately 0.5 mm) from bovine coccygeal IVD were used to determine oxygen diffusivity at three levels of compressive strain. The average oxygen diffusivity (mean+/-SD) of bovine AF in the axial direction was 1.43+/-0.242 x 10(-5) cm(2)/s (n=20) at 4.68+/-1.67% compressive strain level, 1.05+/-0.282 x 10(-5) cm(2)/s (n=20) at 14.2+/-1.50% strain level, and 7.71+/-1.63 x 10(-6) cm(2)/s (n=20) at 23.7+/-1.34% strain level. There was a significant decrease in oxygen diffusivity with increasing level of compressive strain (ANOVA, p<0.05). Oxygen diffusivity of bovine AF in the axial direction has been determined. The mechanical loading has a significant effect on oxygen transport in IVD tissues. This study is important in understanding nutritional transport in IVD tissues and related disk degeneration.

Strain-dependent viscoelastic behaviour and rupture force of single chondrocytes and chondrons under compression

The chondron in articular cartilage includes the chondrocyte and its surrounding pericellular matrix (PCM). Single chondrocytes and chondrons were compressed between two parallel surfaces by a micromanipulation technique to investigate their biomechanical properties and to discover the mechanical significance of the PCM. The force imposed on the cells was measured directly during deformation at various compression speeds and deformations up to cell rupture. When the deformation at the end of compression was 50%, relaxation showed that the cells were viscoelastic, but this viscoelasticity was generally insignificant at 30% deformation or lower. When the deformation was 70%, the cells had deformed plastically. Chondrons ruptured at a mean deformation of 85 +/- 1%, whilst chondrocytes ruptured at a mean deformation of 78 +/- 1%. Chondrons were generally stiffer than chondrocytes and showed less viscoelastic behaviour than chondrocytes. Thus, the PCM significantly influences the mechanical properties of the cells.

Application of modified superposition model to viscoelastic behavior of periodontal ligament

The periodontal ligament (PDL) is a soft bio-logical tissue which shows a strongly nonlinear and time dependent mechanical behavior. Re-cent experiments on rabbit PDL revealed that the rate of stress relaxation is strain dependent. This nonlinear behavior of PDL cannot be de-scribed well by the separable quasi linear vis-coelasticity theory which is usually used in tis-sue biomechanics. Therefore, PDL requires a more general description which considers this nonlinearity and time dependency. The purpose of this study was to model strain dependent stress relaxation behavior of PDL using modi-fied superposition method. It is shown herein that modified superposition method describes viscoelastic nonlinearties well and shows a good compatibility with available experimental PDL data. Hence, the modified superposition model is suggested to describe periodontal ligament data, because it can suitably demon-strate both elastic nonlinearity and strain-dependent stress relaxation behavior of PDL.