Pigment pattern formation in zebrafish during late larval stages: a model based on local interactions.

We present a model to explain pigment cell pattern formation in the late larval stages of zebrafish, Danio rerio, and its mutants, based on new time sequence experimental data. The model stresses the importance of differential intercellular adhesion and the mechanisms of stem cell regulation to explain pigment pattern formation in zebrafish. All interactions included in the model are local in contrast to the previous models for pattern formation, which are based on long range interactions. The hypotheses of the model are tested through a mathematical (cellular automaton) model. Simulations show that differential cellular adhesion together with an appropriate mechanism of stem cell regulation are able to reproduce the main characteristics of pattern formation in the wild-type zebrafish and mutants. The results obtained with the simulations are compared with experimental data and specific experimental tests to the predictions of the model are provided.     

微型植入式医疗电子设备的信号传导机制

微型植入式医疗电子设备是一种长时间埋于人体,用于监测生理特征的微型医疗器械。为了研究微型植入式医疗电子设备如何利用电信号实现植入物与体表检测设备的通信,并分析信号传导机制,本研究利用数学建模的方法,通过设定合理的边界条件和假设,建立微型植入式医疗电子设备体导通信信道模型。为了验证体导通信信道模型的解的准确性,建立了等效的数值解模型,通过分析体导通信信道模型和数值解模型的结果,发现两种模型的误差小于1%。为了验证体导通信信道模型与实验结果的一致性,选用文献中的实验数据来进行验证,通过对比实验结果与体导通信信道模型计算结果,发现体导通信信道模型的解和实验结果误差小于4 dB,具有良好的一致性。因此,体导通信信道模型的解是准确的,体导通信信道模型与实验也有较好的一致性。     

Normal and osteoarthritic hip joint mechanical behaviour: a comparison study

The assessment of contact areas within the hip joint during activities of daily living is of critical importance to understand why degeneration mechanisms are sometimes initiated. A generic finite element model is developed and constrained with experimental personalized conditions to locate contact areas and determine pressure distribution, both during walking and stair climbing. Bony structures are positioned in relation to each other by using experimental kinematical data. Implemented loading conditions are computed from an inverse dynamic approach coupled with an optimization method. The mechanical behaviour of a healthy hip joint is first simulated. This model is then used as a reference for the evaluation of a pathological mechanical behaviour. Thus, experimental data are collected for a patient presenting a coxarthrosis. The comparison of the pathological and normal behaviours emphasizes that the contact area swept within the osteoarthritic hip joint is limited both during walking and stair climbing.     

人在摔倒时股骨上端承载能力的有限元分析——(I)有限元分析模型及股骨失效准则的建立

摔跤后,骨折已成为老年人最严重的损伤之一。有限元分析被证实在股骨的结构分析中是一项非常有用的工具。我们建立了股骨上端的有限元模型,并基于有关文献中关于股骨密质骨和松质骨的强度实验数据建立了Hoffm an失效准则。有限元模型和失效准则用前人的实验结果进行了验证。结果表明:预测的松质骨失效载荷仅比实验值低0.5%,而密质骨失效载荷仅比实验值高4.2%。这说明我们建立的有限元模型结合Hoffm an失效准则将很好地预测人摔倒时股骨的承载能力。     

Constitutive modelling of inelastic behaviour of cortical bone

A visco-elasto-plastic constitutive model is formulated for investigating the mechanics of cortical bone tissue, accounting for an anisotropic configuration and post-elastic and time-dependent phenomena. The constitutive model is developed with reference to experimental data obtained from literature on the behaviour of cortical bone taken from multiple samples. Regarding the constitutive model, a specific procedure based on a coupled deterministic and stochastic method is applied in order to determine the values of the constitutive parameters with regard to human samples. The procedure entails processing of data deduced from mechanical tests to achieve relationships between permanent and total strain, elastic modulus and strain rate, and creep elastic modulus and time. Numerical results obtained by using a finite element model are compared with tensile experimental data on cortical bone including the post-elastic range and creep phenomena. The model shows an excellent capability to describe the tensile behaviour of the cortical bone for the specific mechanical condition analysed.     

Evidence in favor of a role of idiotypic network in autoimmune hemolytic anemia induction: theoretical and experimental studies

Formation dynamics of antibodies to rat erythrocytes (REs) and auto-antibodies to mouse erythrocytes were studied in an experimental model of autoimmune hemolytic anemia (AHA) in mice. The experimental conditions of AHA were simulated in a mathematical model of an immune network. It was found that maximal production of auto-antibodies and antibodies to REs do not synchronize. Antiserum, obtained at the peak of auto-antibodies formation, competed with REs for bounding with antibodies. This represents proof that auto-antibodies to erythrocytes and antibodies to REs are an idiotype-anti-idiotypic pair. In the autoimmune reaction, the autoreactive clone, being anti-idiotypic, responded earlier than the clone reacting to the injected antigen. Comparison of autoimmune reaction kinetics in the mathematical model of an immune network with experimental dynamics of AHA shows them to be similar. So activation of the autoreactive clone to erythrocytes during experimental AHA in mice is mediated by idiotype-anti-idiotypic interactions with the clone reacting to REs.     

Experimental design optimisation: theory and application to estimation of receptor model parameters using dynamic positron emission tomography

The general framework and various criteria for experimental design optimisation are presented. The methodology is applied to the estimation of receptor-ligand reaction model parameters with dynamic positron emission tomography data. The possibility of improving parameter estimation using a new experimental design combining an injection of the beta+-labelled ligand and an injection of the cold ligand is investigated. Numerical simulations predict a remarkable improvement in the accuracy of the parameter estimates with this new experimental design and particularly the possibility of separate estimations of the association constant (k+1) and of the receptor density (B'max) in a single experiment. Simulation predictions are validated using experimental PET data in which parameter uncertainties are reduced by factors ranging from 17 to 1000.     

Laplace reconstruction of experimental diagnostic x-ray spectra

This paper displays the results of a blind study used to determine the capability of a Laplace transform pair model to accurately reconstruct diagnostic x-ray spectra from experimental attenuation data. Spectra reconstructed from attenuation measurements are compared to experimental spectra obtained on the same unit using an intrinsic germanium spectrometer system. The results show that when pure attenuation materials are used, good agreement is obtained between the experimental and computed spectra. If an alloy attenuator like 1100 aluminum is used, the proportion of contaminants must be included in the Laplace formulation for accurate reconstruction.     

Modeling of muscle fatigue using Hill's model

A new model incorporating muscle fatigue has been developed to predict the effect of muscle fatigue on the force-time relationship of skeletal muscle by using the PAK-program. Differential equations in the incremental form have been implemented into Hill's muscle model. In order to describe the effect of muscle fatigue and recovery on skeletal muscle behaviors, a set of equations in terms of three phenomenological parameters which are a fatigue curve under sustained maximal activation, a recovery curve and an endurance function were developed. With reference to existing models and experimental results, the input parameters for fatigue curve under sustained maximal activation and endurance function were determined. The model has been investigated under an isometric condition. The effects of different shapes of the recovery curves have also been considered in this model. Validation of the model has been performed by comparing the predicted results with the experimental data from an existing literature.     

Mathematical model of static platelet adhesion on a solid surface

The scheme of platelet/surface interaction and a kinetic model of platelet adhesion on a solid surface are suggested. The elaborated approach takes into account the platelet activation by the surface and accumulation of free activated cells in the bulk of the liquid phase. This effect has an especially important role in static experimental conditions. The suggested model explains three types of adhesion kinetic curves, obtained in experiments in vitro: sigmoid curves with or without saturation and an exponential curve with saturation. According to the model, the curve shape is determined by material surface properties, platelet functionality, and experimental conditions of the platelet/surface interaction. The data of static platelet adhesion from platelet rich plasma on glass, siliconized glass, hexadecyltrichlorosilane monolayers, and low-density polyethylene are described mathematically with the proposed model. Numerical parameters are calculated from approximation of experimental data by the model. These parameters allow quantitative characterization of platelet interaction with the material surface.     

Biomechanical behaviour of oesophageal tissues: Material and structural configuration,experimental data and constitutive analysis

The aim of the present work is to propose an approach to the biomechanical analysis of oesophagus by defining an appropriate constitutive model and the associated constitutive parameters. The configuration of the different tissues and layers that compose the oesophagus shows very complicated internal anatomy, geometry and mechanical properties. The coupling of these tissues adds to the complexity. The constitutive models must be capable of interpreting the highly non-linear mechanical response. This is done adopting a specific hyperelastic anisotropic formulation. Experimental data are essential for the investigation of the tissues’ biomechanical behaviour and also represent the basis for the definition of constitutive parameters to be adopted within the constitutive formulation developed. This action is provided by using a specific stochastic optimization procedure, addressed to the minimization of a cost function that interprets the discrepancy between experimental data and results from the analytical models developed. Unfortunately, experimental data at disposal do not satisfy all requested information and a particular solution must be provided with regard to definition of the lateral contraction of soft tissues. The anisotropic properties of the tissues are investigated considering the configuration of embedded fibres, according to their mechanical characteristics, quantity and distribution. Collagen and muscular fibres must be considered. The formulation provided on the basis of axiomatic theory of constitutive relationships and the procedure for constitutive parameters identification are described. The evaluation of constitutive parameters requires the analysis of data from experimental tests, that are extracted from the literature. Result validation is performed by comparing the experimental data and model results. In this way a valid basis is provided for the investigation of biomechanical behaviour of oesophagus, looking at deeper information from the experimental point of view that should offer data to be implemented in the procedure for a more detailed and accurate problem definition.     

Clinical ion beams: semi-analytical calculation of their quality

The aim of this work is to define a simplified semi-analytical beam transportation code that can calculate the spatial distribution of projectile fragments which are widely distributed in a patient's body during heavy-ion beam radiotherapy. In this code, we employed an elemental pencil beam model where the spatial distribution of radiation quality for an elemental beam is calculated and superposed according to the emittance ellipse of the narrow heavy-ion beam determined at the entrance of the target. The radiation quality for an elemental beam was calculated using Goldhaber's model of fragment distribution. The calculation results were compared with the experimental observations for a mono-energetic narrow (12)C beam measured at the secondary beam line in HIMAC. Despite its simplicity, the developed code could reproduce the experimental results well.     

Stiffening by fiber reinforcement in soft materials: a hyperelastic theory at large strains and its application

This work defines an incompressible, hyperelastic theory of anisotropic soft materials at finite strains, which is tested by application to the experimental response of fiber-reinforced rubber materials. The experimental characterization is performed using a uniaxial testing device with optical measures of the deformation, using two different reinforcing materials on a ground rubber matrix. In order to avoid non-physical responses of the underlying structural components of the material, the kinematics of the deformation are described using a novel deformation tensor, which ensures physical consistency at large strains. A constitutive relation for incompressible fiber-reinforced materials is presented, while issues of stability and ellipticity for the hyperelastic solution are considered to impose necessary restrictions on the constitutive parameters. The theoretical predictions of the proposed model are compared with the anisotropic experimental responses, showing high fitting accuracy in determining the mechanical parameters of the model. The constitutive theory is suitable to account for the anisotropic response at large compressive strains, opening perspectives for many applications in tissue engineering and biomechanics.     

Thermal stress analysis for polyimide thin film and a substrate layer system

We propose an analytical model based on Timoshenko's theory which describes chemical and physical properties of a polymer and a substrate layer system. This temperature dependent model is developed to account for the thermal stress caused by a mismatch of isobaric expansivity, due to the solvent evaporation and imidization. The stress‐temperature profiles calculated from the proposed model are compared with experimental data for the given systems. The model gives a qualitative information for the stress development during the heating‐cooling process.     

A mechanism for sensory re-weighting in postural control

A key finding of human balance experiments has been that the integration of sensory information utilized for postural control appears to be dynamically regulated to adapt to changing environmental conditions and the available sensory information, a process referred to as “sensory re-weighting.” We propose a postural control model that includes automatic sensory re-weighting. This model is an adaptation of a previously reported model of sensory feedback that included manual sensory re-weighting. The new model achieves sensory re-weighting that is physiologically plausible and readily implemented. Model simulations are compared to previously reported experimental results to demonstrate the automated sensory re-weighting strategy of the modified model. On the whole, the postural sway time series generated by the model with automatic sensory re-weighting show good agreement with experimental data, and are capable of producing patterns similar to those observed experimentally.     

Polarized light propagation in multiply scattering media exhibiting both linear birefringence and optical activity: Monte Carlo model and experimental methodology

A Monte Carlo model for polarized light propagation in birefringent, optically active, multiply scattering media is developed in an effort to accurately represent the propagation of polarized light in biological tissue. The model employs the Jones N-matrix formalism to combine both linear birefringence and optical activity into a single effect that can be applied to photons as they propagate between scattering events. Polyacrylamide phantoms with strain-induced birefringence, sucrose-induced optical activity, and polystyrene microspheres as scattering particles are used for experimental validation. Measurements are made using a Stokes polarimeter that detects scattered light in different geometries, and compared to the results of Monte Carlo simulations run with similar parameters. The results show close agreement between the experimental measurements and Monte Carlo calculations for phantoms exhibiting turbidity and birefringence, as well as for phantoms exhibiting turbidity, birefringence, and optical activity. Other scattering-independent polarization properties can be incorporated into the developed Jones N-matrix formalism, enabling quantification of the polarization effects via an accurate polarization-sensitive Monte Carlo model.     

Modeling event‐related heart period responses

Cardiac rhythm is generated locally in the sinoatrial node, but modulated by central neural input. This may provide a possibility to infer central processes from observed phasic heart period responses (HPR). Currently, operational methods are used for HPR analysis. These methods embody implicit assumptions on how central states influence heart period. Here, we build an explicit psychophysiological model (PsPM) for event‐related HPR. This phenomenological PsPM is based on three experiments involving white noise sounds, an auditory oddball task, and emotional picture viewing. The model is optimized with respect to predictive validity—the ability to separate experimental conditions from each other. To validate the PsPM, an independent sample of participants is presented with auditory stimuli of varying intensity and emotional pictures of negative and positive valence, at short intertrial intervals. Our model discriminates these experimental conditions from each other better than operational approaches. We conclude that our PsPM is more sensitive to distinguish experimental manipulations based on heart period data than operational methods, and furnishes a principled approach to analysis of HPR.     

Modeling and identification of an intra-aorta pump

The intra-aorta pump is a novel left ventricular assist device (LVAD) that assists the heart without the need for percutaneous wires and conduits. It is implanted between the radix aortae and the aortic arch to avoid damage to the aortic valve. To predict the mean pressure head and blood flow, a nonlinear lumped parameter model, which does not need the parameters of the circulatory system, is established. The model includes a speed-controlled current source, an internal resistor, and an inductance for simulating the pressure-flow rate relationship. The speed-controlled current source is used to represent the blood flow caused by the kinetic energy from the impeller, the internal resistor is used to stimulate the resistance character of the radial clearance of the intra-aorta pump, and the inductance is used to model the inertia of the blood that passes through the radial clearance. Each part of the model has clear physical significance, which is helpful for extending the model to other blood pumps. It can generate all status of the pump from suction to pulmonary congestion. The model is summarized as a function of the pressure head, the blood flow, and rotational speed of which the values of parameters in the model are determined by experiment. The model and prediction method are tested experimentally on an in vitro mock loop. A comparison of the predicted pressure head obtained from our model with experimental data shows that our model can predict the differential pressure accurately with error <5% for all experimental conditions over the entire range of intended use of the intra-aorta pump.