van der Waals Forces between Cylinders: I. Nonretarded Forces between Thin Isotropic Rods and Finite Size Corrections

The free energy of interaction of two long thin dielectric cylinders is evaluated in the nonretarded limit by the method of van Kampen. The result includes all nonadditive many body forces and the effects of an intervening medium. In the limit of dilute systems the result reduces to that obtained by summation of r^-6 pair potentials.     

Aspects of skeletal mechanics of the starfish Asterias forbesii

The skeletal mechanics of Astenas forbesii are examined in order to erect a theoretical framework for the evaluation of starfish skeletal systems. Guided by engineering theory, the skeleton is dissected into its functional parts and an approximation made of the stresses sustained by each. All relevant mathematical operations are demonstrated. It is shown that the whole body acts as a unit sustaining compressive forces across its oral surface and tensile stresses across its aboral surface. Two points of maximum stress are identified: the aboral ridge of the ray close to the disk and the inferior transverse amhulacral muscle (ITAM). Structural stability is conferred upon the former by virtue of its position in the body and upon the latter by the inclusion of strong tensile fibres in parallel with the muscle.     

Computational biodynamics of human knee joint in gait: from muscle forces to cartilage stresses

Using a validated finite element model of the intact knee joint we aim to compute muscle forces and joint response in the stance phase of gait. The model is driven by reported in vivo kinematics-kinetics data and ground reaction forces in asymptomatic subjects. Cartilage layers and menisci are simulated as depth-dependent tissues with collagen fibril networks. A simplified model with less refined mesh and isotropic depth-independent cartilage is also considered to investigate the effect of model accuracy on results. Muscle forces and joint detailed response are computed following an iterative procedure yielding results that satisfy kinematics/kinetics constraints while accounting at deformed configurations for muscle forces and passive properties. Predictions confirm that muscle forces and joint response alter substantially during the stance phase and that a simplified joint model may accurately be used to estimate muscle forces but not necessarily contact forces/areas, tissue stresses/strains, and ligament forces. Predictions are in general agreement with results of earlier studies. Performing the analyses at 6 periods from beginning to the end (0%, 5%, 25%, 50%, 75% and 100%), hamstrings forces peaked at 5%, quadriceps forces at 25% whereas gastrocnemius forces at 75%. ACL Force reached its maximum of 343 N at 25% and decreased thereafter. Contact forces reached maximum at 5%, 25% and 75% periods with the medial compartment carrying a major portion of load and experiencing larger relative movements and cartilage strains. Much smaller contact stresses were computed at the patellofemoral joint. This novel iterative kinematics-driven model is promising for the joint analysis in altered conditions.     

Three-dimensional finite element stress analysis of the dentate human mandible.

The biomechanical events which accompany functional loading of the human mandible are not fully understood. The techniques normally used to record them are highly invasive. Computer modelling offers a promising alternative approach in this regard, with the additional ability to predict regional stresses and strains in inaccessible locations. In this study, we built two three-dimensional finite element (FE) models of a human mandible reconstructed from tomographs of a dry dentate jaw. The first model was used for a complete mechanical characterization of physical events. It also provided comparative data for the second model, which had an increased vertical corpus depth. In both cases, boundary conditions included rigid restraints at the first right molar and endosteal cortical surfaces of the articular eminences of temporal bones. Groups of parallel multiple vectors simulated individual masticatory muscle loads. The models were solved for displacements, stresses, strains, and forces. The simulated muscle loads in the first model deformed the mandible helically upward and toward its right (working) side. The highest principal stresses occurred at the bite point, anterior aspects of the coronoid processes, symphyseal region, and right and left sides of the mandibular corpus. In general, the observed principal stresses and strains were highest on the periosteal cortical surface and alveolar bone. At the symphyseal region, maximum principal stresses and strains were highest on the lower lingual mandibular aspect, whereas minimum principal stresses and strains were highest on its upper labial side. Subcondylar principal strains and condylar forces were higher on the left (balancing or nonbiting) side than on the right mandibular side, with condylar forces more concentrated on the anteromedial aspect of the working-side condyle and on the central and lateral aspects of the left. When compared with in vivo strain data from macaques during comparable biting events, the predictive strain values from the first model were qualitatively similar. In the second model, the reduced tensile stress on the working-side, and decreased shear stress bilaterally, confirmed that lower stresses occurred on the lower mandibular border with increased jaw depth. Our results suggested that although the mandible behaved in a beam-like manner, its corpus acted more like a combination of open and closed cross sections due to the presence of tooth sockets, at least for the task modelled.(ABSTRACT TRUNCATED AT 400 WORDS)     

Kinematic analysis of over-determinate biomechanical systems

In this paper, we introduce a new general method for kinematic analysis of rigid multi body systems subject to holonomic constraints. The method extends the standard analysis of kinematically determinate rigid multi body systems to the over-determinate case. This is accomplished by introducing a constrained optimisation problem with the objective function given as a function of the set of system equations that are allowed to be violated while the remaining equations define the feasible set.

We show that exact velocity and acceleration analysis can also be performed by solving linear sets of equations, originating from differentiation of the Karush–Kuhn–Tucker optimality conditions.

The method is applied to the analysis of an 18 degrees-of-freedom gait model where the kinematical drivers are prescribed with data from a motion capture experiment.

The results show that significant differences are obtained between applying standard kinematic analysis or minimising the least-square errors on the two fully equivalent 3D gait models with only the way the experimental data is processed being different.     

Heat and mass exchange processes between the surface of the human body and ambient air at various altitudes

 The rates of convection and evaporation at the interface between the human body and the surrounding air are expressed by the parameters convective heat transfer coefficient h _c, in W m^–2°C^–1 and evaporative heat transfer coefficient h _e, W m^–2 hPa^–1. These parameters are determined by heat transfer equations, which also depend on the velocity of the airstream around the body, that is still air (free convection) and moving air (forced convection). The altitude dependence of the parameters is represented as an exponential function of the atmospheric pressure p: h _c∼p ⁿ and h _e∼p ^1–n, where n is the exponent in the heat transfer equation. The numerical values of n are related to airspeed: n=0.5 for free convection, n=0.618 when airspeed is below 2.0 ms^–1 and n=0.805 when airspeed is above 2.0 ms^–1. This study considers the coefficients h _c and h _e with respect to the similarity of the two processes, convection and evaporation. A framework to explain the basis of established relationships is proposed. It is shown that the thickness of the boundary layer over the body surface increases with altitude. As a medium of the transfer processes, the boundary layer is assumed to be a layer of still air with fixed insulation which causes a reduction in the intensity of heat and mass flux propagating from the human body surface to its surroundings. The degree of reduction is more significant at a higher altitude because of the greater thickness of the boundary layer there. The rate of convective and evaporative heat losses from the human body surface at various altitudes in otherwise identical conditions depends on the following factors: (1) during convection – the thickness of the boundary layer, plus the decrease in air density, (2) during evaporation (mass transfer) – the thickness of the boundary layer, plus the increase with altitude in the diffusion coefficient of water vapour in the air. The warming rate of the air volume due to convection and evaporation is also considered. Expressions for the calculation of altitude dependences h _c (p) and h _e (p) are suggested. Received: 23 June 1998 / Accepted: 10 February 1999     

A finite element model of the lower limb during stance phase of gait cycle including the muscle forces

Background

Results of finite element (FE) analyses can give insight into musculoskeletal diseases if physiological boundary conditions, which include the muscle forces during specific activities of daily life, are considered in the FE modelling. So far, many simplifications of the boundary conditions are currently made. This study presents an approach for FE modelling of the lower limb for which muscle forces were included.

Methods

The stance phase of normal gait was simulated. Muscle forces were calculated using a musculoskeletal rigid body (RB) model of the human body, and were subsequently applied to a FE model of the lower limb. It was shown that the inertial forces are negligible during the stance phase of normal gait. The contact surfaces between the parts within the knee were modelled as bonded. Weak springs were attached to the distal tibia for numerical reasons.

Results

Hip joint reaction forces from the RB model and those from the FE model were similar in magnitude with relative differences less than 16%. The forces of the weak spring were negligible compared to the applied muscle forces. The maximal strain was 0.23% in the proximal region of the femoral diaphysis and 1.7% in the contact zone between the tibia and the fibula.

Conclusions

The presented approach based on FE modelling by including muscle forces from inverse dynamic analysis of musculoskeletal RB model can be used to perform analyses of the lower limb with very realistic boundary conditions. In the present form, this model can be used to better understand the loading, stresses and strains of bones in the knee area and hence to analyse osteotomy fixation devices.

     

Interaction of Horse Heart Cytochrome c with Lipid Bilayer Membranes: Effects on Redox Potentials

Cyclic voltammetry has been used to study the effects of interactions between horse cytochrome c and solid-supported planar lipid membranes, comprised of either egg phosphatidylcholine (PC) or PC plus 20 mol.% cardiolipin (CL), on the redox potential and the electrochemical electron transfer rate between the protein and a semiconductor electrode. Experiments were performed over a wide range of cytochrome c concentrations (0–440 M) at low (20 mM) and medium (160 mM) ionic strengths. Three types of electrochemical behavior were observed, which varied as a function of the experimental conditions. At very low cytochrome c concentration (0.1 M), and under conditions where electrostatic forces dominated the protein–lipid membrane interaction (i.e., low ionic strength with membranes containing CL), a redox potential (265 mV) and an electrochemical electron transfer rate constant (0.09s ^–1)were obtained which compare well with those measured in other laboratories using a variety of different chemical modifications of the working electrode. Two other electrochemical signals (not reported with chemically modified electrodes) were also observed to occur at higher cytochrome c concentrations with this membrane system, as well as with two other systems (membranes containing CL under medium ionic strength conditions, and PC only at low ionic strength). These involved positive shifts of the cytochrome c redox potential (by 40 and 60 mV) and large decreases in the electron transfer rate (to 0.03 and 0.003 s^–1). The observations can be rationalized in terms of a structural model of the cytochrome c–membrane interaction, in which association involves both electrostatic and hydrophobic forces and results in varying degrees of insertion of the protein into the hydrophobic interior of the membrane.     

A survey of unequal crossover systems and their mathematical properties

We present a model of gene duplication by means of unequal crossover (UCO) where the probability of any given pairing between homologous sequences scales as a penalty factor p ^z ≤ 1, with z the number of mismatches due to asymmetric sequence alignment. From this general representation, we derive several limiting case models of UCO, some of which have been treated elsewhere in the literature. One limiting case is random unequal crossover (RUCO), obtained by setting p = 1 (corresponding to equiprobable pairings at each site). Another limiting case scenario (the ‘Krueger-Vogel’ model) proposes an optimal ‘endpoint’ alignment which strongly penalizes both overhang and deviations from endpoint matching positions. For both of these scenarios, we make use of the symmetry properties of the transition operator (together with the more general UCO properties of copy number conservation and equal parent-offspring mean copy number) to derive the stationary distribution of gene copy number generated by UCO. For RUCO, the stationary distribution of genotypes is shown to be a negative binomial, or alternatively, a convolution of geometric distributions on ‘haplotype’ frequencies. A different type of model derived from the general representation only allows recombination without overhang (internal UCO or IntUCO). This process has the special property of converging to a single copy length or a distribution on a pair of copy lengths in the absence of any other evolutionary forces. For UCO systems in general, we also show that selection can readily act on gene copy number in all of the UCO systems we investigate due to the perfect heritability (h ² = 1) imposed by conservation of copy number. Finally, some preliminary work is presented which suggests that the more general models based on misalignment probabilities seem to also converge to stationary distributions, which are most likely functions of parameter value p. An erratum to this article is available at .     

Calcium regulation of skeletal myogenesis. I. Cell content critical to myotube formation

Summary Primary cultures of embryonic chick pectoral skeletal muscle were used to study calcium regulation of myoblast fusion to form multinucleated myotubes. Using atomic absorption spectrometry to measure total cellular calcium and the⁴⁵Ca-exchange method to determine free cellular Ca⁺⁺, our data suggest that only the free cellular calcium changes significantly during development under conditions permissive for myotube formation (0.9 mM external Ca⁺⁺). Increases in calcium uptake occurred before and toward the end of the period of fusion with the amount approximating 2 to 4 pmol per cell in mass cultures. If the medium [Ca⁺⁺] is decreased to 0.04 mM, as determined with a calcium electrode, a fusion-block is produced and free cell Ca⁺⁺ decreased 5- to 10-fold. Removal of the fusion-block by increasing medium [Ca⁺⁺] results in a release of the fusion-block and an increase in cellular Ca⁺⁺ to approximately 1 pmol per cell during fusion, and higher thereafter. Cation ionophore A23187 produced transient increases in cellular calcium and stimulated myoblast fusion and the final extent of myotube formation only when added at the onset of culture. Results suggest that transient increased calcium uptake alone is insufficient for fusion because critical cellular content in conjunction with permissive amounts of medium [Ca⁺⁺] must exist. The latter suggests further that cell surface Ca⁺⁺ was also critical.     

A Mathematical Model of Spatial Self-Organization in a Mechanically Active Cellular Medium

A general continual model of a medium composed of mechanically active cells is proposed. The medium is considered to be formed by three phases: cells, extracellular fluid, and an additional phase that is responsible for active interaction forces between cells and, for instance, may correspond to a system of protrusions that provide the development of active contractile forces. The deformation of the medium, which is identified with the deformation of the cell phase, consists of two components: elastic deformation of individual cells and cell rearrangements. The elastic deformation is associated with stresses in the cell phase. The spherical component of the stress tensor describes the nonlinear resistance of the cellular medium, which leads to the impossibility of its excessive compression. The constitutive equation for pressure in the cell phase is taken in the form of a nonlinear dependence on the volume cell density. The rearrangement of cells is considered as a flow controlled by stresses in the cell phase, active stresses, and fluid pressure. The tensor of active stresses is assumed to be spherical and nonlocally dependent on the cell density. Assuming that the process of biological tissue deformation is slow, we obtained a reduced model that neglects the elastic deformation of cells, compared to the inelastic deformation. A linear stability analysis of a spatially uniform steady-state solution was performed. The hydrostatic pressure of fluid is present among the parameters that are responsible for the loss of stability of the steady-state solution: an increase in it has a destabilizing effect owing to the action of the component of the interphase interaction force that is determined by the fluid pressure. The model we obtained can be used to describe the process of cavity formation in an initially homogeneous cell spheroid. The role of local and nonlocal mechanisms of active stress generation in the formation of cavity is investigated.     

Pinching forces in crayfish and fiddler crabs,and comparisons with the closing forces of other animals

The pinching forces of crustaceans are in many respects analogous to the biting forces of vertebrates. We examined the effects of body size and chelae size and shape, on the closing forces of the fiddler crab, Uca pugilator, and the crayfish, Procambarus clarkii. We hypothesized that the allometric relationships would be similar among species, and comparable to those reported for other decapod crustaceans. We further hypothesized that the scaling of the closing forces of crustaceans, with respect to body size and with the geometry of the pinching or biting structures, would be similar to that of vertebrates. We found that pinching forces increased with body mass, claw dimensions, and claw mass in U. pugilator, but only with claw height and claw mass in P. clarkii. Contraction time increased with body mass for both species combined, whereas contraction speed decreased. Pooled data for these and 17 other species of decapod crustacean revealed a positive correlation between the pinching force and body mass with a scaling exponent of 0.71. These data are remarkably comparable to the values on closing forces of vertebrate jaws, with the pooled data having a scaling exponent of 0.58, slightly below the value of 0.67 predicted for geometric similarity. Maximum closing forces vary tremendously among both crustaceans and animals in general, with body size and food habits being among the most important determining factors.     

The σB signalling activation pathway in the enteropathogen Clostridioides difficile

Clostridium difficile is the main cause of antibiotic-associated diarrhoea. Inside the gut, C. difficile must adapt to the stresses it copes with, by inducing protection, detoxification and repair systems that belong to the general stress response involving σ^B. Following stresses, σ^B activation requires a PP2C phosphatase to dephosphorylate the anti-anti-sigma factor RsbV that allows its interaction with the anti-sigma factor RsbW and the release of σ^B. In this work, we studied the signalling pathway responsible for the activation of σ^B in C. difficile. Contrary to other firmicutes, the expression of sigB in C. difficile is constitutive and not autoregulated. We confirmed the partner switching mechanism that involved RsbV, RsbW and σ^B. We also showed that CD2685, renamed RsbZ, and its phosphatase activity are required for RsbV dephosphorylation triggering σ^B activation. While CD0007 and CD0008, whose genes belong to the sigB operon, are not involved in σ^B activity, depletion of the essential iron–sulphur flavoprotein, CD2684, whose gene forms an operon with rsbZ, prevents σ^B activation. Finally, we observed that σ^B is heterogeneously active in a subpopulation of C. difficile cells from the exponential phase, likely leading to a ‘bet-hedging’ strategy allowing a better chance for the cells to survive adverse conditions.     

Kinematic analysis of over-determinate biomechanical systems

In this paper, we introduce a new general method for kinematic analysis of rigid multi body systems subject to holonomic constraints. The method extends the standard analysis of kinematically determinate rigid multi body systems to the over-determinate case. This is accomplished by introducing a constrained optimisation problem with the objective function given as a function of the set of system equations that are allowed to be violated while the remaining equations define the feasible set. We show that exact velocity and acceleration analysis can also be performed by solving linear sets of equations, originating from differentiation of the Karush-Kuhn-Tucker optimality conditions. The method is applied to the analysis of an 18 degrees-of-freedom gait model where the kinematical drivers are prescribed with data from a motion capture experiment. The results show that significant differences are obtained between applying standard kinematic analysis or minimising the least-square errors on the two fully equivalent 3D gait models with only the way the experimental data is processed being different.     

Effect of culture media and irradiance level on growth and morphology of Persea americana Mill microcuttings

Avocado shoots were multiplied in vitro in two culture media of different consistency, double phase and solid medium, at three different irradiance levels: 35, 60 and 85 μmol m⁻² s⁻¹. Effects of culture and environmental conditions in multiplication rate, rooting capacity, hyperhydricity and leaf surface morphology of microcuttings were evaluated. Double phase medium induced hyperhydricity, producing leaf microcuttings with deformed stomata and low crystalline epicuticular waxes; microcuttings also showed reduced rooting capacity. By contrast, solid medium promoted leaf area development on microcuttings and decreased hyperhydricity. Stomatal index was not affected by these treatments but stomatal density was, interacting with the amount of irradiance applied. Increasing irradiance decreased concentration of chlorophyll a and carotenoids in the leaf but did not affect leaf hyperhydricity. This revised version was published online in June 2006 with corrections to the Cover Date.     

The influence of the oxidation-reduction potential of the medium upon the growth ofAzotobacter chroococcum

Summary 1. Under aerobic conditions the development ofAzotobacter is suppressed by the presence of reducing substances in concentrations lowering the aerobic potential more than ±120 mV.2. The development ofAzotobacter is not only retarded by the presence of these reducing substances, but the cells are killed.3. Under anaerobic conditions no development ofAzotobacter could be observed even when the solution was poised at a high redox potential. The cells were not killed by the anaerobic conditions; as soon as aerobic conditions were restored, a rapid development ofAzotobacter was observed.4. When the anaerobic conditions coincide with a low redox potentiale.g. in culture solutions containing glucose, the cells ofAzotobacter were killed.5. The limits of potential endured byAzotobacter appeared to be dependent on the relations between the ions in the medium.6. The cells ofAzotobacter secrete certain substances enabling them to develop at low potentials.7. The conclusion is drawn that the development ofAzotobacter is in general influenced by the redox potential of the medium. However, this influence may be very complex, as the potential of the medium is only important in so far as the redox potential(s) in the cell are changed. The factors influencing the relation between the redox potential of the medium and the redox potentials of the cell are discussed.8. ForAzotobacter, oxygen cannot be replaced by other redox systems with a suitable redox potential, most probably because these redox systems have to react with the respiratory enzymes of the cell.     

Optimization of selected cultivation parameters for Cordyceps guangdongensis

Aims: To increase the fruit body production of Cordyceps guangdongensis, selected cultivation conditions, especially nutritional parameters were optimized. Methods and Results: Cordyceps guangdongensis was inoculated on potato dextrose agar slants with pH values from 4·5 to 9·0 and cultivated in artificial media with different carbon and nitrogen supplements. Primordium formation in C. guangdongensis was favoured by slightly acidic conditions. Fruit body yields and biological efficiency (BE) recorded were all highest in cultures of C. guangdongensis supplemented with sucrose and KNO₃ as carbon and nitrogen supplements, respectively. Highest fruit body yields and BE values were recorded with C : N ratio of 12 : 1. The optimal medium consisted of (g l^?1) 20·0 sucrose, 4·0 soya bean powder, 5·0 beef extract and 10·0 KNO₃. Cultivation experiments using this medium confirmed its reliability; 18·35% of BE was obtained, compared with a calculated maximum BE of 18·65% based on orthogonal test data. Conclusions: Cordyceps guangdongensis preferred sucrose and potassium nitrate as best carbon and nitrogen supplements. It produced satisfying yield of fruit body with optimized medium. Significance and Impact of the Study: Optimized artificial cultivation conditions could promote the yield of C. guangdongensis and decreased the cost of production.     

Computational biomechanics of human brain with and without the inclusion of the body under different blast orientation

Three different human head models in a free space are exposed to blast waves coming from four different directions. The four head–neck–body models composed of model a, with the neck free in space; model b, with neck fixed at the bottom; and model c, with the neck attached to the body. The results show that the effect of the body can be ignored for the first milliseconds of the head–blast wave interactions. Also one can see that although most biomechanical responses of the brain have similar patterns in all models, the shear stresses are heavily increased after a few milliseconds in model b in which the head motion is obstructed by the fixed-neck boundary conditions. The free-floating head model results are closer to the attached-body model.     

Metabolic diversity of aromatic hydrocarbon-degrading bacteria from a petroleum-contaminated aquifer

We characterized bacteria from contaminated aquifers for their ability to utilize aromatic hydrocarbons under hypoxic (oxygen-limiting) conditions (initial dissolved oxygen concentration about 2 mg/l) with nitrate as an alternate electron acceptor. This is relevant to current intense efforts to establish favorable conditions forin situ bioremediation. Using samples of granular activated carbon slurries from an operating groundwater treatment system, we isolated bacteria that are able to use benzene, toluene, ethylbenzene, orp-xylene as their sole source of carbon under aerobic or hypoxic-denitrifying conditions. Direct isolation on solid medium incubated aerobically or hypoxically with the substrate supplied as vapor yielded 10³ to 10⁵ bacteria ml^–1 of slurry supernatant, with numbers varying little with respect to isolation substrate or conditions. More than sixty bacterial isolates that varied in colony morphology were purified and characterized according to substrate utilization profiles and growth condition (i.e., aerobic vs. hypoxic) specificity. Strains with distinct characteristics were obtained using benzene compared with those isolated on toluene or ethylbenzene. In general, isolates obtained from direct selection on benzene minimal medium grew well under aerobic conditions but poorly under hypoxic conditions, whereas many ethylbenzene isolates grew well under both incubation conditions. We conclude that the conditions of isolation, rather than the substrate used, will influence the apparent characteristic substrate utilization range of the isolates obtained. Also, using an enrichment culture technique, we isolated a strain ofPseudomonas fluorescens, designated CFS215, which exhibited nitrate dependent degradation of aromatic hydrocarbons under hypoxic conditions.Abbreviations BTEX benzene, toluene, ethylbenzene, andp-xylene - HPLC high performance liquid chromatography - GAC granular activated carbon