Force Relaxation Time Research Articles

Translucency and negative temperature-dependence for the slip length of water on graphene.

Carbonous materials, such as graphene and carbon nanotubes, have attracted tremendous attention in the fields of nanofluidics due to the slip at the interface between solid and liquid. The dependence of slip length for water on the types of supporting substrates and thickness of the carbonous layer, which is critical for applications such as sustainable cooling of electronic devices, remains unknown. In this paper, using colloidal probe atomic force microscopy, we measured the slip length ls of water on graphene supported by hydrophilic and hydrophobic substrates, i.e., SiO2 and octadecyltrimethoxysilane (OTS). The ls on single-layer graphene supported by SiO2 is found to be 1.6 ± 1.9 nm, and that of OTS is 8.5 ± 0.9 nm. When the thickness of few-layer graphene increases to 3-4 layers, both ls values gradually converge to the value of graphite (4.3 ± 3.5 nm). Such a thickness dependence is termed slip length translucency. Further, ls is found to decrease by about 70% when temperature increases from 300 K to 350 K for 2-layer graphene supported by SiO2. These observations are explained by analysis based on the Green-Kubo relation and McLachlan theory. Our results provide the first set of reference values for the slip length of water on supported few-layer graphene. They can not only serve as a direct experimental reference for solid-liquid interaction, but also provide a guideline for the design of nanofluidics-based devices, for example thermo-mechanical nanofluidic devices.

Mechanical characterization of compressible chromatographic particles

During operation of packed-bed chromatography columns, the mechanical properties of the porous chromatographic particles determine the overall macroscopic compression behavior. The porous particles are biphasic materials consisting of a solid matrix and interstitial fluid. Mechanical characterization of selected arrays of two types of polymeric chromatographic particles made of agarose and methacrylate with a mean particle diameter of 60μm and 83μm, respectively, was carried out by applying a high-resolution micromanipulation technique. The effect of compression speed and displacement level was investigated. Both particle types showed pronounced time-dependent compression behavior and force relaxation. Long-time compression load relaxation experiments revealed that two different time-dependent mechanisms were involved. It was shown that the characteristic force relaxation times describing either mechanism differed by a factor of 13–31. Short-time quasistatic compression experiments were excellently described by a Maxwell-type standard linear solid model in which the force relaxation time matched the shorter relaxation time. It was assumed, that force relaxation occurring at short time is ascribed to poroelastic material behavior and at longer times to viscoelastic material behavior. This enabled the determination of fundamental particle properties, i.e. instantaneous and relaxed particle shear and Young's moduli, Poisson's ratio, and coefficient of restitution.

Improved Energy and Sensory Properties of Instant Porridge Made from a Roasted Mixture of Grated Orange-Fleshed Sweet Potatoes and Flour Made from Shredded Sun Dried Cassava

Mozambican diets are often lacking in vitamin A. Orange-fleshed sweet potatoes (OFSP) are rich in beta-carotene, a precursor of vitamin A, but are not easily introduced into the diet as the consistency of the porridge made from OFSP is unappetising. Flour made from roasted cassava (garri) produces a palatable porridge with a high energy density. We propose a simple procedure for producing an instant porridge by roasting grated OFSP with flour from shredded sun-dried cassava (G-OFSP). This is an easily adopted variant of the traditional garri-making process. The consistency of this porridge made from milled G-OFSP (G-OFSPf) was compared to the consistency of porridges made from maize flour (Mf), cassava flour (Cf), OFSP flour (OFSPf) and garri flour (Gf) at 43°C, and then correlated to the perceived characteristics assessed by a sensory panel. The dry matter concentration of the porridges was adjusted to give similar consistency, measured by the maximum force of back extrusion, using traditional maize porridge as a reference. The porridges were additionally characterized by back extrusion force relaxation time. Short relaxation times were obtained for G-OFSPf, Mf and Gf porridges and longer times for Cf and OFSPf porridges, corresponding roughly to the sensory preference. In the sensory test of consistency, G-OFSPf porridge was ranked first, followed by Mf, Gf, OFSPf and Cf. In overall sensory tests comparing G-OFSPf porridge to Gf porridge, G-OFSPf porridge was significantly preferred with regard to all sensory attributes evaluated (p < 0.01). The energy density of the G-OFSPf porridge was 5.6 kJ/ml; the highest of the three most preferred porridges with regard to texture. The preparation of flour from a mixture of grated OFSP and shredded sun-dried cassava improved the perceived porridge consistency, increased its energy density, and its high acceptability would increase the intake of betacarotene.

Quantitative myotonia assessment using force relaxation curve modelling

The lack of a robust quantitative measure of myotonia has been underlined in previous studies. Recent publications have proposed methods to quantify myotonia based on the measurement of force relaxation times during maximal contractions. However, they present several drawbacks mainly due to unstable force, odd peaks or digital noise. A possible solution to this issue consists in fitting the force curve with a convenient regression model. The aim of this study was, therefore, to provide a regression model in order to fit the force relaxation time curve automatically and to provide a robust index for quantitative assessment of myotonia in clinical settings. Force curves were fitted by an asymmetric sigmoidal function. The inverse function was then used to compute various absolute and relative relaxation times automatically. These variables were calculated for 16 controls and 16 patients with myotonic dystrophy type 1 (DM1). All variables were significantly increased in DM1 patients compared to controls. For instance, the relaxation time between 40 and 60% of the initial contraction level was 18.2 (SD: 3.3) ms in controls and 40.1 (SD: 17.7) ms in DM1 patients. All relaxation variables were highly discriminant. Force curve modelling provides an objective and effective quantification of myotonia.

Effects of Chlorpromazine on Mechanical Properties of the Outer Hair Cell Plasma Membrane

An optical tweezers system was used to characterize the effects of chlorpromazine (CPZ) on the mechanical properties of the mammalian outer hair cell (OHC) through the formation of plasma membrane tethers. Such tethers exhibited force relaxation when held at a constant length for several minutes. We used a second-order generalized Kelvin body to model tether-force behavior from which several mechanical parameters were then calculated including stiffness, viscosity-associated measures, and force relaxation time constants. The results of the analysis portray a two-part relaxation process characterized by significantly different rates of force decay, which we propose is due to the local reorganization of lipids within the tether and the flow of external lipid into the tether. We found that CPZ’s effect was limited to the latter phenomenon since only the second phase of relaxation was significantly affected by the drug. This finding coupled with an observed large reduction in overall tether forces implies a common basis for the drug’s effects, the plasma membrane-cytoskeleton interaction. The CPZ-induced changes in tether viscoelastic behavior suggest that alterations in the mechanical properties of the OHC lateral wall could play a role in the modulation of OHC electromotility by CPZ.

Young MLP deficient mice show diastolic dysfunction before the onset of dilated cardiomyopathy

Targeted deletion of cytoskeletal muscle LIM protein (MLP) in mice consistently leads to dilated cardiomyopathy (DCM) after one or more months. However, next to nothing is known at present about the mechanisms of this process. We investigated whether diastolic performance including passive mechanics and systolic behavior are altered in 2-week-old MLP knockout (MLPKO) mice, in which heart size, fractional shortening and ejection fraction are still normal. Right ventricular trabeculae were isolated from 2-week-old MLPKO and wildtype mice and placed in an apparatus that allowed force measurements and sarcomere length measurements using laser diffraction. During a twitch from the unloaded state at 1 Hz, MLPKO muscles relengthened to slack length more slowly than controls, although the corresponding force relaxation time was unchanged. Active developed stress at a diastolic sarcomere length of 2.00 μm was preserved in MLPKO trabeculae over a wide range of pacing frequencies. Force relaxation under the same conditions was consistently prolonged compared with wildtype controls, whereas time to peak and maximum rate of force generation were not significantly altered. Ca 2+ content of the sarcoplasmic reticulum (SR) and the quantities of Ca 2+ handling proteins were similar in both genotypes. In summary, young MLPKO mice revealed substantial alterations in passive myocardial properties and relaxation time, but not in most systolic characteristics. These results indicate that the progression to heart failure in the MLPKO model may be driven by diastolic myocardial dysfunction and abnormal passive properties rather than systolic dysfunction.

PKC-dependent Preconditioning with Norepinephrine Protects Sarcoplasmic Reticulum Function in Rat Trabeculae Following Metabolic Inhibition

The authors have previously shown that norepinephrine (NE) pretreatment attenuates Ca2+overloading in cardiac rat trabeculae during metabolic inhibition, and improves contractile function during a subsequent recovery period. The present study investigated: (i) whether protection of sarcoplasmic reticulum (SR) function during metabolic inhibition (MI) is involved in the preconditioning-like effect of NE-pretreatment, and (ii) whether or not this process is PKC-dependent. A 15 min preincubation period was used with 1μmol/l exogenous NE to precondition isolated, superfused rat trabeculae against contractile dysfunctioning following 40 min of MI in 2 mmol/l NaCN containing Tyrode (gassed with 95% O2/5% CO2; pH 7.4, 24°C) without glucose at 1-Hz stimulation frequency. Contractile recovery was studied during a subsequent 60 min recovery period (RP) in glucose containing Tyrode at 0.2 Hz. Force and intracellular free calcium ([Ca2+]ii) were monitored throughout the experimental protocol. Pretreatment of trabeculae with NE (group NE) substantially diminished the Ca2+rise from the onset of rigor development during MI, compared to preparations which were pretreated with NE, in the presence of specific PKC blocker chelerythrine (2μmol/l; group NE+CHEL). After 40 min of MI, resting [Ca2+]iin group NE and NE+CHEL was increased to 0.50±0.03 and 2.08±0.20μmol/l, respectively (P<0.05), whereas total intracellular ATP levels were similar in both groups (approximately 0.20μmol/g dry wt). This corresponded with an increase in active force development (119%) and a decrease in twitch force relaxation time (77%) during subsequent RP in group NE, compared to pre-MI values of the same group. In contrast, a significant decrease in force recovery (54%) and an increase in twitch force relaxation time (123%) was observed in group NE+CHEL. Values for [Ca2+]i, contractile recovery, and twitch force relaxation time in untreated controls as well as CHEL preparations corresponded to those measured in the NE+CHEL group. Rapid cooling contractures (RCCs), which provide information on both SR-Ca2+loading and Ca2+re-uptake activity, revealed a 2-fold higher SR Ca2+content during RP in group NE compared to controls and group NE+CHEL. In addition, kinetic analysis of the RCC rewarming spike (RWS) showed that this was accompanied by greater than a 28% increase in the maximum rate of RWS relaxation (−dF/dt/rws) in group NE compared to group NE+CHEL. The change of −dF/dt/rws in the NE group during RP following MI persisted after SR Ca2+-release channel blockade by ryanodine treatment (100μmol/l), which suggests involvement of NE-induced, PKC-dependent protection of SR Ca2+-ATPase activity. The results of the present study point to an inverse relationship between the Ca2+rise during MI and SR functioning, in which PKC appears to play a key role. It is concluded that the preconditioning-like effect of NE-pretreatment on contractile recovery is at least partly mediated by protection of SR function.

Neck flexor muscle strength, efficiency, and relaxation times in normal subjects and subjects with unilateral neck pain and headache.

To determine the test-retest reliability of a new method for measuring muscular strength, efficiency, and relaxation times of the neck flexor musculature of healthy adults, and to compare these neck flexor muscle properties in subjects who have unilateral neck pain and headache with those in controls. Subjects lay supine and isometrically flexed their necks against a force transducer attached to the back of a webbing and velcro helmet. Electromyograms (EMGs) were recorded from surface electrodes on the sternocleidomastoid (SCM) muscles. Two consecutive sessions of five contractions of varying levels of effort from minimal through moderate and maximal effort were analyzed. Ambulatory referral center. Volunteer control subjects (n = 10, 3 men and 7 women) were recruited from hospital and university personnel. Volunteer neck pain subjects (n = 10, 3 men and 7 women) were recruited from a physiatric chronic pain practice and a hospital outpatient physical therapy practice. In the controls, the intraclass correlation coefficients (ICCs) for the first two maximum neck flexion contractions were; peak force ICC = .81; peak force/body weight ICC = .75; average force ICC = .75; force relaxation time ICC = .73; SCM EMG relaxation times: right ICC = .60 and left ICC = .67. Comparing sessions 1 and 2 the intraclass correlations for SCM efficiencies were right ICC = .58 and left ICC = .97. The peak force in controls (mean = 45.3 +/- 17.6N) was reduced by 50% in the neck pain subjects (mean = 22.4 +/- 13.1N) (p = .004). Similarly, peak force/body weight in the neck pain subjects (X = 0.3 +/- 0.2N/kg) was 46% of control (mean = 0.7 +/- 0.2N/kg) (p = .001), and average force in the neck pain subjects (X = 12.1 +/- 7.5N) was 43% of controls (mean = 28.5 +/- 11.0N) (p = .001). In two neck pain subjects. SCM, EMG and force relaxation times were abnormally long in both the affected and the unaffected SCM muscles, exceeding the control values by greater than 3 standard deviations. The difference between the right SCM efficiency of the control subjects (mean = 0.3 +/- 0.2N/ microV) and the affected SCM efficiency of the neck pain subjects (mean = 0.1 +/- 0.1 N/microV) approached the p < .05 criterion for significance (p = .055). The technique was found to be highly reliable for the measurement of neck flexor peak force, peak force/body weight, average force, and force relaxation time, and moderately reliable for the quantitation of SCM EMG relaxation times and SCM efficiency. All force values were significantly lower in the neck pain population compared with the controls. In the neck pain population, force and SCM EMG relaxation times, as well as efficiencies, suggested abnormalities. Neck pain subjects showed no significant differences in SCM EMG relaxation time or SCM efficiency between affected and unaffected SCM muscles.