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Biomedical subjects

A Colin

Publications and source records attributed to A Colin.

At least 19 recordsLinked to original sources
PubMed2006

Selection of two-phase flow patterns at a simple junction in microfluidic devices.

We study the behavior of a confined stream made of two immiscible fluids when it reaches a T junction. Two flow patterns are witnessed: the stream is either directed in only one sidearm, yielding a preferential flow pathway for the dispersed phase, or splits between both. We show that the selection of these patterns is not triggered by the shape of the junction nor by capillary effects, but results from confinement. It can be anticipated in terms of the hydrodynamic properties of the flow. A simple model yielding universal behavior in terms of the relevant adimensional parameters of the problem is presented and discussed.

Journal Article↗
PubMed2004

Thixotropic organogels based on a simple N-hydroxyalkyl amide: rheological and aging properties.

The thixotropic properties ofthermoreversible organogels composed ofN-3-hydroxypropyl dodecanamide and various apolar fluids have been investigated by X-ray scattering, light microscopy, and rheo-optics experiments. This revealed that gel formation occurs via a precipitation process. Depending upon the cooling rate, large interconnected aggregates are formed and induce an elastic behavior. When submitted to a shear flow, these aggregates disentangled and became aligned in the direction of the velocity. Nevertheless, shear does not alter the structure of the individual aggregate and connections between the aggregates are quickly rebuilt due to gravity and thermal fluctuations when the applied flow is stopped. The alignment under flow and the reformation of the connections after the cessation of the shear induces the thixotropic behavior.

Journal Article↗
PubMed2004

Serum melatonin and urinary 6-sulfatoxymelatonin in major depression.

In this study, serum melatonin and urinary 6-sulfatoxymelatonin (aMT6s) were measured in 14 major depressive inpatients, compared to 14 matched controls according to age, gender, season and hormonal treatment in women. Moreover, the relationship between serum melatonin and urinary aMT6s levels was analysed in the two groups. Results indicated that the two groups of subjects showed a clear melatonin rhythm without significant difference in the mean level of melatonin or aMT6s, in the area under the curve of melatonin or in the melatonin peak. However, the time of the nocturnal melatonin peak secretion was significantly delayed in depressive subjects as compared to healthy controls. Moreover, the depressed patients showed urinary aMT6s concentrations enhanced in the morning compared to night time levels, while these concentrations were lowered from the night to the morning in the control group. These results suggest that the melatonin production is phase-shifted in major depression.

Adult↗
PubMed2004

A spatio-temporal study of rheo-oscillations in a sheared lamellar phase using ultrasound.

We present an experimental study of the flow dynamics of a lamellar phase sheared in the Couette geometry. High-frequency ultrasonic pulses at 36 MHz are used to measure time-resolved velocity profiles. Oscillations of the viscosity occur in the vicinity of a shear-induced transition between a high-viscosity disordered fluid and a low-viscosity ordered fluid. The phase coexistence shows up as shear bands on the velocity profiles. We show that the dynamics of the rheological data result from two different processes: (i) fluctuations of slip velocities at the two walls and (ii) flow dynamics in the bulk of the lamellar phase. The bulk dynamics are shown to be related to the displacement of the interface between the two differently sheared regions in the gap of the Couette cell. Two different dynamical regimes are investigated under applied shear stress: one of small amplitude oscillations of the viscosity delta eta/eta approximately equal to 3%) and one of large oscillations (delta eta/eta approximately equal to 25%). A phenomenological model is proposed that may account for the observed spatio-temporal dynamics.

Acoustics↗
PubMed2003

Towards local rheology of emulsions under Couette flow using Dynamic Light Scattering.

We present local velocity measurements in emulsions under shear using heterodyne Dynamic Light Scattering. Two emulsions are studied: a dilute system of volume fraction phi = 20% and a concentrated system with phi = 75%. Velocity profiles in both systems clearly show the presence of wall slip. We investigate the evolution of slip velocities as a function of shear stress and discuss the validity of the corrections for wall slip classically used in rheology. Focussing on the bulk flow, we show that the dilute system is Newtonian and that the concentrated emulsion is shear-thinning. In the latter case, the curvature of the velocity profiles is compatible with a shear-thinning exponent of 0.4 consistent with global rheological data. However, even if individual profiles can be accounted for by a power law fluid (with or without a yield stress), we could not find a fixed set of parameters that would fit the whole range of applied shear rates. Our data, thus, raise the question of the definition of a global flow curve for such a concentrated system. These results show that local measurements are a crucial complement to standard rheological tools. They are discussed in the light of recent works on soft glassy materials.

Elasticity↗
PubMed2002

Foam drainage: a film contribution?

We have measured the steady drainage of foams of various surfactants, varying the size of the parietal plateau borders from 0.24 to 2.4 mm. We report an evolution of the power laws of drainage with the bubble size, which cannot be explained by the most recent developments of the drainage theory. We believe that one reason for the disagreement between the existing theory and our data is the passage of liquid through the films, from nodes to nodes. Film thickness measurements in situ show that the films are swollen, up to 2 microm and more. Films, usually neglected, may then have an important contribution to drainage by increasing noticeably the volume of the regions where the liquid flows.

Journal Article↗
PubMed2002

Incorporating DNA in a lamellar phase: a Flory model.

Following an experimental work demonstrating that DNA pieces can be incorporated in large quantity in a neutral lamellar phase, we propose a theoretical model showing that finite-size rod-like particles can be incorporated in a lyotropic smectic-A phase with an interlamellar spacing smaller than the rod length. This model based on a Flory-type description on a lattice allowed us to calculate a ternary (rods/water/surfactant) phase diagram. We demonstrate that a particle-enriched lamellar phase and a particle-depleted lamellar phase can coexist. In the latter case the rods exhibit a long-range (nematic-like) orientational order. The predictions are in qualitative agreement with the experiments.

Journal Article↗
PubMed2001

Oscillating viscosity in a lyotropic lamellar phase under shear flow.

We report some time-dependent behavior of lyotropic lamellar phase under shear flow. At fixed stress, near a layering instability, the system presents an oscillating shear rate. We build up a new stress versus shear rate diagram that includes temporal behavior. This diagram is made of two distinct branches of stationary states which correspond, respectively, to disordered and ordered multilamellar vesicle phases. When increasing the shear stress, prior to the transition to the ordered structural state, sustained oscillations of the viscosity are recorded. They correspond to periodic structural change of the entire sample between a disordered and a ordered state of multilamellar vesicles.

Journal Article↗
PubMed2000

Shear-induced structural transitions in newtonian non-newtonian two-phase flow

We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or "onions") immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.

Journal Article↗
PubMed1999

MRI-SPECT fusion for the synthesis of high resolution 3D functional brain images: a preliminary study.

Medical imaging being a fast-expanding field, multimodal data fusion appears more and more as a key element for the optimal use of images. By fusion, we mean the combination of several information sources (in particular images), with the aim of providing either more condensed or more pertinent information. The long term scope of this work would be to improve the interpretation of 3D brain images, providing extra elements for the diagnosis and patient follow up. This preliminary study is part of a wider context: the medical follow up of patients suffering from probable Alzheimer disease observed in single photon emission tomography by fusion after registration with magnetic resonance images. Several information combination techniques based on the possibility theory are presented. A new operator, more specifically adapted to the fusion of anatomical and functional images, as well as a high resolution functional image synthesis technique are proposed. A first comparative study of fusion techniques is then proposed. Although no thorough test protocol has been defined, these preliminary results are encouraging, giving access to a wide field of potential clinical applications.

Aged↗
PubMed1997

A novel tool for rapid prototyping and development of simple 3D medical image processing applications on PCs.

A 32-bit PC-based 3D medical image processing software package is presented. Its basic functions are the display and manipulation of medical images and the inclusion of user-written processing routines in C language. This software runs on inexpensive hardware and is easy to learn. It means the current needs of many research teams working in medical image processing.

Diagnostic Imaging↗
PubMed1996

Application of textural features to the detection of ocular melanomas in scintigraphy.

A method for automated detection of ocular melanoma in scintigraphic images is described. The algorithm first performs an automatic segmentation of the eyes on specific reference images. The images of the eyes are then analyzed using textural parameters computed in several directions and averaged to damp directional information. Assuming that only one eye per patient will be pathological, the ratio of the textural parameters of the two eyes is computed. A statistical analysis is performed over these ratios to select the most highly discriminating textural parameters and detect the pathological patients. The method has been tested successfully on a population of 23 individuals and we found significant differences between pathological and normal patients.

Algorithms↗
PubMed1995

[Clinical heterogeneity of Townes-Brocks syndrome].

BACKGROUND: Townes-Brocks syndrome (TBS) is a rare autosomal dominant entity mainly characterized by ano-rectal, ear and extremities abnormalities with variable clinical expression. CASE REPORTS: The first case had ear and extremities, but not anorectal, abnormalities; a Pierre-Robin sequence with esophageal atresia was also observed. The second case had the classical triad of abnormalities also associated with tetralogy of Fallot which has been only once reported in the literature. CONCLUSIONS: Both cases are other examples of the frequent clinical variability observed in this syndrome explaining diagnostic difficulties in the absence of a specific marker.

Abnormalities, Multiple↗
PubMed1989

[Congenital atrioventricular block and maternal lupus erythematosus. Histologic discovery of tumor of the atrioventricular node].

A case of congenital atrioventricular block in a newborn whose mother presented with systemic lupus erythematosus (SLE) is reported. Despite intensive care the child died a few hours after birth. Serial sections of the heart could be examined. Histology provided information on the appearance and distribution of the lesions. In particular, the sinus node was small for the child's age, and its supplying artery was found to have a hyperplastic media with adventitial sclerosis; the interatrial and interventricular septa showed subendocardial fibrosis invading the adjacent myocardium. Owing to the scarcity of systematic histopathological examinations, such lesions have seldom been described. In addition, a tumour of the atrioventricular node, known as mesothelioma or hemolymphangioma, was discovered. This case is exceptional in that histopathological findings similar to those described in SLE, though rarely as numerous, were associated with a very rare tumour never hitherto described in such a young patient. The relationship between the two categories of lesions is discussed.

Adult↗
PubMed1989

[Fetal supraventricular tachycardias and their treatment. Apropos of 23 cases].

The management of fetal heterotopic tachycardias is reviewed from a cooperative study involving 23 cases treated by French Pediatric Cardiology Centers at Angers, Brest, Nantes, Rennes and Tours. There were 17 cases of supraventricular tachycardia (SVT), 5 of flutter and 1 of atrial tachycardia. Seventeen mothers were given an antiarrythmic treatment. They all received Digoxin by mouth, with either Propranolol in 2 cases, Sotalol and Verapamil in one case each. In 5 cases 2 or more other drugs were associated. There was a good in utero response in 12 cases. Among the 23 patients, 5 died, all with hydrops fetalis at the time of diagnosis of SVT. Finally 2 newborns were successfully treated at the time of delivery by a direct puncture of the umbilical cord.

Anti-Arrhythmia Agents↗