Biomedical subjects

J Dual

Publications and source records attributed to J Dual.

12 recordsLinked to original sources

PubMed2006

Oscillating viscometer--evaluation of a new bedside test.

A viscometer for bedside blood measurements was developed, consisting of an oscillating resonator probe mounted directly into a disposable vacutainer tube for blood withdrawal. It was tested in vitro on blood samples with variable hematocrits (20-60%), increasing fibrinogen concentrations (0-20 g/l), increasing concentrations of an admixed radiographic contrast medium and erythrocyte suspensions in dextran 40 and dextran 70. Results were compared with those obtained with a conventional Couette viscometer. Oscillating viscometry yielded generally higher values than Couette viscometry, and had a good sensitivity for changes in hematocrit with a good correlation between the two methods (r=0.96, p<0.0001). Oscillating viscosity depended on the resonator frequency, it was higher at 3900 Hz than at 215 Hz, suggesting a viscoelastic behavior of blood. Erythrocyte aggregation, induced by increasing fibrinogen concentrations or dextran 70, affected oscillating viscometry. At a high frequency, i.e. a smaller penetration depth of the shear wave, oscillating viscosity tended to decrease, which suggests a depletion of the boundary layer from erythrocytes when they aggregate. At low frequency with a deeper shear wave penetration (about 50 microm), erythrocyte aggregation increased oscillating viscosity. Bedside tests in 17 patients with coronary heart disease and 10 controls confirmed the easy practicability of the test and showed lower oscillating viscosity in these patients despite higher fibrinogen concentrations presumably due to increased erythrocyte aggregation. We conclude that oscillating viscometry is an interesting bedside test, which is capable of providing new information on the biorheology of the erythrocyte-poor boundary layer near the vessel wall.

Blood Circulation↗

PubMed2004

Determination of the material properties of microstructures by laser based ultrasound.

In most applications of MEMS the mechanical properties of the used materials are key parameters for the perfect working of the microsystems. Measuring bulk acoustic waves excited in MEMS structures with ultra-short laser pulses is a powerful method for the accurate and non-destructive evaluation as well as for the characterization of material properties. The pump-probe laser-based acoustic method generates bulk acoustic waves in a thermo-elastic way by absorbing the pump laser pulses. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. At the surface of the specimen the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors, activators and bulk acoustic wave (BAW) filters for example. The described non-destructive and non-contact method is the right approach for testing such thin and brittle structures like membranes. Results of measurements on freestanding aluminium-silicon nitride multi-layer membranes with total thicknesses in the order of several hundred nanometers are presented and compared with thermo-elastic models and with measurements of the supported case. The measured results are used for the determination of the moduli of the membranes.

Journal Article↗

PubMed2004

Positioning of small particles by an ultrasound field excited by surface waves.

A method for the controlled positioning of small particles in one or two dimensions by an ultrasound field excited by a surface wave is presented. Particles of a diameter between 10 and 100 microm placed on a surface can be concentrated at certain locations and moved over the surface. In other approaches it is possible to let the particle levitate freely in the fluid. However for the use of ultrasonic positioning in for example microassembling it is necessary to move particles over a surface as well as to let them levitate over the surface. Physical principle: A two- or three-dimensional ultrasound field is excited in a fluid filled gap between a rigid surface at the bottom and a vibrating surface of a solid at the top. The height of the gap varies between 0.1 and 2 mm. A one-dimensional sinusoidal vibration of the upper surface excites a two-dimensional ultrasound field in the fluid. Particles that are arbitrarily distributed on the lower surface will be concentrated in lines by the ultrasound field. First the calculation of the field of forces on particles in the fluid layer is presented. Then the dispersion relation of a vibrating plate which is in contact with a fluid on one side is derived. The technical setup will be introduced. Finally the experiments are shown and compared to the theoretical results.

Journal Article↗

PubMed2002

Micro-manipulation of small particles by node position control of an ultrasonic standing wave.

For the controlled positioning of small particles with ultrasound a standing wave in a fluid is used. The standing wave is implemented in a resonator, that consists of a fluid filled tube and two piezoelectric transducers on each end. A one-dimensional model of a piezo-device including the fluid-loading on one side and a backside support is introduced. This model allows the calculation of the transmitted wave as a function of the applied electric voltage and the incident wave. In addition, when an electrical impedance is connected to the piezo-device, the reflection coefficient can be varied in amplitude and phase, so that the parameters of the reflected wave can be controlled completely. The resonator itself, consisting of a piezo-device on each end and the fluid between, is included in the model. Several methods to shift the nodes of the standing wave in the resonator are investigated and the ability to position particles is discussed.

Journal Article↗

PubMed2002

Simulation of elastic wave propagation in cylindrical structures including excitation by piezoelectric transducers.

The development and optimization of non-destructive testing procedures usually needs experimental data. As experiments are time-consuming and expensive to conduct, we would like to use numerical data instead. This is admissible, if the simulation describes the physical experiments accurately. A three-dimensional displacement-stress finite-difference model is presented for a piezoelectric transducer coupled to an anisotropic tube. The allocation of the displacement and stress components on a staggered grid leads to a stable scheme. A full piezoelectric model of the transducer is used, including transverse isotropy in the elastic, dielectric, and piezoelectric constants. Similar to an experiment, elastic waves are excited in the corresponding simulation by applying a voltage signal to the electrodes of the piezoelectric transducer. Predictions of the simulation model for a piezoelectric ring transducer coupled to a carbon-fibre-reinforced shell are compared to experimental results to test the validity of the numerical data.

Journal Article↗

PubMed2002

Ultrasonic wave propagation in focussing tips with arbitrary geometries.

Pulsed laser acoustic experiments have the advantage of very high temporal resolution. However, the lateral resolution amounts to several wavelengths of light. To improve the lateral resolution a focussing tip in which the mechanical waves are focussed is introduced. The combination of high resolution in time and space domain leads to a new potential time resolved scanning probe method. Therefore several axi-symmetric structures are investigated numerically using a finite difference method. The ultrasonic wave propagation in different tips is discussed. By varying the geometry of the tip, the displacement at the sharp end is maximized. The numerically calculated results are verified experimentally on structures having macroscopic dimensions. Scaling effects are considered in order to translate the results into the microscopic scale where arbitrary geometries are much more challenging to implement.

Journal Article↗

PubMed2002

Inverse finite element characterization of soft tissues.

In this work a tissue aspiration method for the in vivo determination of biological soft tissue material parameters is presented. An explicit axisymmetric finite element simulation of the aspiration experiment is used together with a Levenberg-Marquardt algorithm to estimate the material model parameters in an inverse parameter determination process. An optimal fit of the simulated experiment and the real experiment is sought with the parameter estimation algorithm. Soft biological tissue is modelled as a viscoelastic, non-linear, nearly incompressible, isotropic continuum. Viscoelasticity is accounted for by a quasi-linear formulation. The aspiration method is validated experimentally with a synthetic material. In vivo (intra-operatively during surgical interventions) and ex vivo experiments were performed on human uteri.

Algorithms↗

PubMed2000

Measurement of the dispersion relation of guided non-axisymmetric waves in filament-wound cylindrical structures

To determine the dispersion relation, guided waves are excited in specimens over a broad frequency range. The surface displacements are measured over time and space. The recorded data are analysed using a quasi-three-dimensional spectrum estimation algorithm. In the time domain a fast Fourier transform is used to extract the frequencies. To obtain the wave numbers, in space a two-dimensional matrix-pencil approach is applied to the data set. Using a suitable constitutive model (transversely isotropic or orthotropic) dispersion curves are calculated. Good agreement was found between the experimental and the numerically calculated dispersion relations after adjusting the material parameters. Since the dispersion relation of a structure depends on the mechanical material properties frequency-dependent material parameters can be extracted from the above-mentioned relation between frequency and wave number.

Journal Article↗

PubMed1999

Virtual reality based surgery simulation for endoscopic gynaecology.

Virtual reality (VR) based surgical simulator systems offer very elegant possibilities to both enrich and enhance traditional education in endoscopic surgery. However, while a wide range of VR simulator systems have been proposed and realized in the past few years, most of these systems are far from able to provide a reasonably realistic surgical environment. We explore the basic approaches to the current limits of realism and ultimately seek to extend these based on our description and analysis of the most important components of a VR-based endoscopic simulator. The feasibility of the proposed techniques is demonstrated on a first modular prototype system implementing the basic algorithms for VR-training in gynaecologic laparoscopy.

Computer Graphics↗

PubMed1998

Rheological properties of blood as assessed with a newly designed oscillating viscometer.

A newly designed type of oscillating viscometer was tested for blood viscosity measurements. The viscometer consists of a probe (either a tube or a rod) oscillating at a resonance frequency with amplitudes in the micro- and nanometer range. The torsional oscillations are dampened by fluids flowing through the tube or surrounding the rod. The degree of damping depends on the viscosity of the fluid, which allows to measure viscosity. Data obtained with these instruments were compared with those obtained with a conventional Couette viscometer. An increase of erythrocyte aggregation by the addition of dextran 70 in vitro led to the expected increase of viscosity in the Couette viscometer; in the oscillating tube viscometer, however, it remained unchanged, which may be explained by a decreased erythrocyte concentration near the tube wall due to increased aggregation and flow of erythrocytes in the tube center. In ex vivo experiments on blood flowing without anticoagulant directly through the tube viscometer an inverse correlation between viscosity and fibrinogen concentration was found. This is in contrast to actual knowledge and may indicate that high fibrinogen levels have a beneficial rheological effect at the tube or vessel wall. Our data suggest that the new oscillating tube viscometer is an interesting tool, which may contribute to a more comprehensive understanding of blood flow.

Adult↗

PubMed1996

Application of neural networks to quantitative nondestructive evaluation.

This paper concentrates on the quantitative analysis of reflection and transmission characteristics of structural waves in notched beams using neural networks. At one end of a rod a piezoelectric transducer excites a flexural guided wave, which propagates through the structure and is reflected from the notch as well as from the ends of the rod. At another point, a laser interferometer measures the resulting displacement versus time, which is recorded and serves as the digital fingerprint of the experiment. Our aim is to obtain quantitative information about the notch characteristics, i.e. its location and depth, on the basis of these digital recordings. This requires an inverse problem to be solved using neural networks. For the training of the network, synthetically produced wave patterns are used. The results of the corresponding wave experiment simulations compare well with experimental data. For fixed location, the calculated wave patterns could be quantitatively analysed to yield the depth of the notch if it is greater than a twentieth of the diameter. Another network could be trained for the localization with an accuracy of 2 cm for 93% of the training patterns. On the basis of these results a strategy is formulated on how neural networks could be trained to evaluate quantitatively both of the notch characteristics.

Algorithms↗

PubMedDate not supplied

A newly designed oscillating viscometer for blood viscosity measurements.

A newly designed type of oscillating viscometer is described. The viscometer consists of either a tube or a rod oscillating at a resonance frequency with amplitudes in the micro- and nanometer range. A fluid flowing through the tube or surrounding the rod damps the torsional oscillations. The increase in the damping depends on the viscosity of the fluid and is used to determine viscosity. It was found that viscosity measurements are feasible during blood flow. This new type of viscometer may be useful to the study of biophysical properties of blood at the wall surface during flow and give new insights into blood flow. The device allows direct viscosity measurement on blood directly as it is drawn from the vein through the tube without any anticoagulant.

Blood Viscosity↗