Recurrent environmental contamination in a centralized radiopharmacy unit by Achromobacter spp: results of a large microbiological investigation

BACKGROUND

Radiopharmaceuticals preparation unit, such as every aseptic preparation units, are strictly monitored in terms of microbiological contamination. Despite all biocontamination control procedures, our radiopharmacy unit faced repeated environnmental contamination by Achromobacter spp which necessitated a large environmental investigation.

METHOD

Microbiological controls were carried out using Count Tact agars (Biomérieux) for flat surfaces, dry swabbing for hard to reach areas and containers were filled with a sterile water solution (then filtrated on 0.45µm membrane and seeded). Microbiological identification was performed by mass spectrometry (MALDI-TOF-MS, Brucker) on each positive sample.

RESULTS

Achromobacter spp was found in 10% of the 413 samples during the 8 months investigation period. The proportion of positive samples was stable among time but their location was unpredictable. The highest inoculum was finally found in the buckets used for biocleaning.

DISCUSSION

Samples from cleaning buckets taken by dry swabbing were at first negative, but the use of a non-routinely used sampling method allowed to discover the reservoir of this persistent contamination.

CONCLUSION

This investigation alerted us on the high microbiological risk associated with reusable plastic containers and the importance of a sampling method adapted to critical locations.

Efficient deformation mechanisms enable invasive cancer cells to migrate faster in 3D collagen networks

Cancer cell migration is a widely studied topic but has been very often limited to two dimensional motion on various substrates. Indeed, less is known about cancer cell migration in 3D fibrous-extracellular matrix (ECM) including variations of the microenvironment. Here we used 3D time lapse imaging on a confocal microscope and a phase correlation method to follow fiber deformations, as well as cell morphology and live actin distribution during the migration of cancer cells. Different collagen concentrations together with three bladder cancer cell lines were used to investigate the role of the metastatic potential on 3D cell migration characteristics. We found that grade-3 cells (T24 and J82) are characterized by a great diversity of shapes in comparison with grade-2 cells (RT112). Moreover, grade-3 cells with the highest metastatic potential (J82) showed the highest values of migration speeds and diffusivities at low collagen concentration and the greatest sensitivity to collagen concentration. Our results also suggested that the small shape fluctuations of J82 cells are the signature of larger migration velocities. Moreover, the displacement fields generated by J82 cells showed significantly higher fiber displacements as compared to T24 and RT112 cells, regardless of collagen concentration. The analysis of cell movements enhanced the fact that bladder cancer cells were able to exhibit different phenotypes (mesenchymal, amoeboid). Furthermore, the analysis of spatio-temporal migration mechanisms showed that cancer cells are able to push or pull on collagen fibers, therefore producing efficient local collagen deformations in the vicinity of cells. Our results also revealed that dense actin regions are correlated with the largest displacement fields, and this correlation is enhanced for the most invasive J82 cancer cells. Therefore this work opens up new routes to understand cancer cell migration in soft biological networks.

A biomechanical model for the transendothelial migration of cancer cells

We propose a biomechanical model for the extravasation of a tumor cell (TC) through the endothelium of a blood vessel. Based on prior in vitro observations, we assume that the TC extends a protrusion between adjacent endothelial cells (ECs) that adheres to the basement membrane via focal adhesions (FAs). As the protrusion grows in size and branches out, the actomyosin contraction along the stress fibers (SFs) inside the protrusion pulls the relatively rigid nucleus through the endothelial opening. We model the chemo-mechanics of the SFs and the FAs by following the kinetics of the active myosin motors and high-affinity integrins, subject to mechanical feedback. This is incorporated into a finite-element simulation of the extravasation process, with the contractile force pulling the nucleus of the TC against elastic resistance of the ECs. To account for the interaction between the TC nucleus and the endothelium, we consider two scenarios: solid-solid contact and lubrication by cytosol. The former gives a lower bound for the required contractile force to realize transmigration, while the latter provides a more realistic representation of the process. Using physiologically reasonable parameters, our model shows that the SF and FA ensemble can produce a contractile force on the order of 70 nN, which is sufficient to deform the ECs and enable transmigration. Furthermore, we use an atomic force microscope to measure the resistant force on a human bladder cancer cell that is pushed through an endothelium cultured in vitro. The magnitude of the required force turns out to be in the range of 70-100 nN, comparable to the model predictions.

Displacement fields using correlation methods as a tool to investigate cell migration in 3D collagen gels

Living cells embedded in a complex extra-cellular matrix migrate in a sophisticated way thanks to adhesions to matrix fibres and contractility. It is important to know what kind of forces are exerted by the cells. Here, we use reflectance confocal microscopy to locate fibres accurately and determine displacement fields. Correlation techniques are used to this aim, coupled with proper digital image processing. Benchmark tests validate the method in the case of shear and stretching motions. Finally, the method is tested successfully for studying cancer cells migrating in collagen gels of different concentration.

Elastohydrodynamic Lift at a Soft Wall

We study experimentally the motion of nondeformable microbeads in a linear shear flow close to a wall bearing a thin and soft polymer layer. Combining microfluidics and 3D optical tracking, we demonstrate that the steady-state bead-to-surface distance increases with the flow strength. Moreover, such lift is shown to result from flow-induced deformations of the layer, in quantitative agreement with theoretical predictions from elastohydrodynamics. This study thus provides the first experimental evidence of "soft lubrication" at play at small scale, in a system relevant, for example, to the physics of blood microcirculation.

An Information System for Epidemiology based on a Computer-Based Medical Record

A new way is presented to build an information system addressed to problems in epidemiology. Based on our analysis of current and future requirements, a system is proposed which allows for collection, organization and distribution of data within a computer network. In this application, two broad communities of users – physicians and epidemiologists – can be identified, each with their own perspectives and goals. The different requirements of each community lead us to a client-service centered architecture which provides the functionality requirements of the two groups. The resulting physician workstation provides help for recording and querying medical information about patients and from a pharmacological database. All information is classified and coded in order to be retrieved for pharmaco-economic studies. The service center receives information from physician workstations and permits organizations that are in charge of statistical studies to work with “real” data recorded during patient encounters. This leads to a new approach in epidemiology. Studies can be carried out with a more efficient data acquisition. For modelling the information system, we use an object-oriented approach. We have observed that the object-oriented representation, particularly its concepts of generalization, aggregation and encapsulation, are very usable for our problem.

The Rhône-Alpes Health Platform

Objectives: The purpose of this workis to develop a health information platform connecting most health facilities in the Rhône-Alpes region. The health platform called SIS-RA is used through a Web interface. An iconic interface is dedicated to the platform and presents information in a unique way for all users.

Methods: Newtechniques have been used to develop this platform which will be used by a great number of Rhône-Alpes doctors in the future. We chose a user-centered design which takes into account doctors’ requirements (hospital and GP). We also consider that no system has to be rebuilt, but a direct connection to the legacysystems should be provided.

Results: The platform permits fast and more appropriate medical decisions than those made without this information system. The iconic interface presents all medical documents in a uniform way. Currently, 11 healthcare facilities and 15 community health networks are connected to SIS-RA sharing more than 60,000 records with 1.2 million indexed items. 3200 doctors use the system.

Conclusion: The platform is approved by French supervision authorities (regional hospitals association (ARH)), regional practitioners union (URML) and Rhônes-Alpes region administration and is known as the official shared health record.

Unraveling the Receptor-Ligand Interactions between Bladder Cancer Cells and the Endothelium Using AFM

Adhesion of cancer cells to endothelial cells is a key step in cancer metastasis; therefore, identifying the key molecules involved during this process promises to aid in efforts to block the metastatic cascade. We have previously shown that intercellular adhesion molecule-1 (ICAM-1) expressed by endothelial cells is involved in the interactions of bladder cancer cells (BCs) with the endothelium. However, the ICAM-1 ligands have never been investigated. In this study, we combined adhesion assays and atomic force microscopy (AFM) to identify the ligands involved and to quantify the forces relevant in such interactions. We report the expression of MUC1 and CD43 on BCs, and demonstrate that these ligands interact with ICAM-1 to mediate cancer cell-endothelial cell adhesion in the case of the more invasive BCs. This was achieved with the use of adhesion assays, which showed a strong decrease in the attachment of BCs to endothelial cells when MUC1 and CD43 were blocked by antibodies. In addition, AFM measurements showed a similar decrease, by up to 70%, in the number of rupture events that occurred when MUC1 and CD43 were blocked. When we applied a Gaussian mixture model to the AFM data, we observed a distinct force range for receptor-ligand bonds, which allowed us to precisely identify the interactions of ICAM-1 with MUC1 or CD43. Furthermore, a detailed analysis of the rupture events suggested that CD43 is strongly connected to the cytoskeleton and that its interaction with ICAM-1 mainly corresponds to force ramps followed by sudden jumps. In contrast, MUC1 seems to be weakly connected to the cytoskeleton, as its interactions with ICAM-1 are mainly associated with the formation of tethers. This analysis is quite promising and may also be applied to other types of cancer cells.

Lamins and nesprin-1 mediate inside-out mechanical coupling in muscle cell precursors through FHOD1

LINC complexes are crucial for the response of muscle cell precursors to the rigidity of their environment, but the mechanisms explaining this behaviour are not known. Here we show that pathogenic mutations in LMNA or SYNE-1 responsible for severe muscle dystrophies reduced the ability of human muscle cell precursors to adapt to substrates of different stiffness. Plated on muscle-like stiffness matrix, mutant cells exhibited contractile stress fibre accumulation, increased focal adhesions, and higher traction force than controls. Inhibition of Rho-associated kinase (ROCK) prevented cytoskeletal defects, while inhibiting myosin light chain kinase or phosphorylation of focal adhesion kinase was ineffective. Depletion or inactivation of a ROCK-dependent regulator of actin remodelling, the formin FHOD1, largely rescued morphology in mutant cells. The functional integrity of lamin and nesprin-1 is thus required to modulate the FHOD1 activity and the inside-out mechanical coupling that tunes the cell internal stiffness to match that of its soft, physiological-like environment.

Prediction of traction forces of motile cells

When crawling on a flat substrate, living cells exert forces on it via adhesive contacts, enabling them to build up tension within their cytoskeleton and to change shape. The measurement of these forces has been made possible by traction force microscopy (TFM), a technique which has allowed us to obtain time-resolved traction force maps during cell migration. This cell 'footprint' is, however, not sufficient to understand the details of the mechanics of migration, that is how cytoskeletal elements (respectively, adhesion complexes) are put under tension and reinforce or deform (respectively, mature and/or unbind) as a result. In a recent paper, we have validated a rheological model of actomyosin linking tension, deformation and myosin activity. Here, we complement this model with tentative models of the mechanics of adhesion and explore how closely these models can predict the traction forces that we recover from experimental measurements during cell migration. The resulting mathematical problem is a PDE set on the experimentally observed domain, which we solve using a finite-element approach. The four parameters of the model can then be adjusted by comparison with experimental results on a single frame of an experiment, and then used to test the predictive power of the model for following frames and other experiments. It is found that the basic pattern of traction forces is robustly predicted by the model and fixed parameters as a function of current geometry only.

CCM proteins control endothelial β1 integrin dependent response to shear stress

Hemodynamic shear stress from blood flow on the endothelium critically regulates vascular function in many physiological and pathological situations. Endothelial cells adapt to shear stress by remodeling their cytoskeletal components and subsequently by changing their shape and orientation. We demonstrate that β1 integrin activation is critically controlled during the mechanoresponse of endothelial cells to shear stress. Indeed, we show that overexpression of the CCM complex, an inhibitor of β1 integrin activation, blocks endothelial actin rearrangement and cell reorientation in response to shear stress similarly to β1 integrin silencing. Conversely, depletion of CCM2 protein leads to an elongated “shear-stress-like” phenotype even in the absence of flow. Taken together, our findings reveal the existence of a balance between positive extracellular and negative intracellular signals, i.e. shear stress and CCM complex, for the control of β1 integrin activation and subsequent adaptation of vascular endothelial cells to mechanostimulation by fluid shear stress.

Atomic force microscopy reveals a role for endothelial cell ICAM-1 expression in bladder cancer cell adherence

Cancer metastasis is a complex process involving cell-cell interactions mediated by cell adhesive molecules. In this study we determine the adhesion strength between an endothelial cell monolayer and tumor cells of different metastatic potentials using Atomic Force Microscopy. We show that the rupture forces of receptor-ligand bonds increase with retraction speed and range between 20 and 70 pN. It is shown that the most invasive cell lines (T24, J82) form the strongest bonds with endothelial cells. Using ICAM-1 coated substrates and a monoclonal antibody specific for ICAM-1, we demonstrate that ICAM-1 serves as a key receptor on endothelial cells and that its interactions with ligands expressed by tumor cells are correlated with the rupture forces obtained with the most invasive cancer cells (T24, J82). For the less invasive cancer cells (RT112), endothelial ICAM-1 does not seem to play any role in the adhesion process. Moreover, a detailed analysis of the distribution of rupture forces suggests that ICAM-1 interacts preferentially with one ligand on T24 cancer cells and with two ligands on J82 cancer cells. Possible counter receptors for these interactions are CD43 and MUC1, two known ligands for ICAM-1 which are expressed by these cancer cells.

Time-dependent traction force microscopy for cancer cells as a measure of invasiveness

The migration of tumor cells of different degrees of invasivity is studied, on the basis of the traction forces exerted in time on soft substrates (Young modulus∼10 kPa). It is found that the outliers of the traction stresses can be an effective indicator to distinguish cancer cell lines of different invasiveness. Here, we test two different epithelial bladder cancer cell lines, one invasive (T24), and a less invasive one (RT112). Invasive cancer cells move in a nearly periodic motion, with peaks in velocity corresponding to higher traction forces exerted on the substrate, whereas less invasive cells develop traction stresses almost constant in time. The dynamics of focal adhesions (FAs) as well as cytoskeleton features reveals that different mechanisms are activated to migrate: T24 cells show an interconnected cytoskeleton linked to mature adhesion sites, leading to small traction stresses, whereas less invasive cells (RT112) show a less-structured cytoskeleton and unmature adhesions corresponding to higher traction stresses. Migration velocities are smaller in the case of less invasive cells. The mean squared displacement shows super-diffusive motion in both cases with higher exponent for the more invasive cancer cells. Further correlations between traction forces and the actin cytoskeleton reveal an unexpected pattern of a large actin rim at the RT112 cell edge where higher forces are colocalized, whereas a more usual cytoskeleton structure with stress fibers and FAs are found for T24 cancer cells. We conjecture that this kind of analysis can be useful to classify cancer cell invasiveness.

Quantification of depletion-induced adhesion of red blood cells

Red blood cells (RBCs) are known to form aggregates in the form of rouleaux due to the presence of plasma proteins under physiological conditions. The formation of rouleaux can also be induced in vitro by the addition of macromolecules to the RBC suspension. Current data on the adhesion strength between red blood cells in their natural discocyte shapes mostly originate from indirect measurements such as flow chamber experiments, but data is lacking at the single cell level. Here, we present measurements on the dextran-induced aggregation of red blood cells using atomic force microscopy-based single cell force spectroscopy. The effects of dextran concentration and molecular weight on the interaction energy of adhering RBCs were determined. The results on adhesion energy are in excellent agreement with a model based on the depletion effect and previous experimental studies. Furthermore, our method allowed to determine the adhesion force, a quantity that is needed in theoretical investigations on blood flow.

Complex interactions between human myoblasts and the surrounding 3D fibrin-based matrix

Anchorage of muscle cells to the extracellular matrix is crucial for a range of fundamental biological processes including migration, survival and differentiation. Three-dimensional (3D) culture has been proposed to provide a more physiological in vitro model of muscle growth and differentiation than routine 2D cultures. However, muscle cell adhesion and cell-matrix interplay of engineered muscle tissue remain to be determined. We have characterized cell-matrix interactions in 3D muscle culture and analyzed their consequences on cell differentiation. Human myoblasts were embedded in a fibrin matrix cast between two posts, cultured until confluence, and then induced to differentiate. Myoblasts in 3D aligned along the longitudinal axis of the gel. They displayed actin stress fibers evenly distributed around the nucleus and a cortical mesh of thin actin filaments. Adhesion sites in 3D were smaller in size than in rigid 2D culture but expression of adhesion site proteins, including α5 integrin and vinculin, was higher in 3D compared with 2D (p<0.05). Myoblasts and myotubes in 3D exhibited thicker and ellipsoid nuclei instead of the thin disk-like shape of the nuclei in 2D (p<0.001). Differentiation kinetics were faster in 3D as demonstrated by higher mRNA concentrations of α-actinin and myosin. More important, the elastic modulus of engineered muscle tissues increased significantly from 3.5 ± 0.8 to 7.4 ± 4.7 kPa during proliferation (p<0.05) and reached 12.2 ± 6.0 kPa during differentiation (p<0.05), thus attesting the increase of matrix stiffness during proliferation and differentiation of the myocytes. In conclusion, we reported modulations of the adhesion complexes, the actin cytoskeleton and nuclear shape in 3D compared with routine 2D muscle culture. These findings point to complex interactions between muscle cells and the surrounding matrix with dynamic regulation of the cell-matrix stiffness.

Breakdown of cell-collagen networks through collagen remodeling

Collagen model tissues, consisting of cells embedded in a collagen matrix at different concentrations (of cells and collagen) were analyzed. Rheological properties were measured and complementary confocal microscopy analysis carried out. An important feature, corresponding to the breakdown of the collagen network (i.e., decrease in network elasticity) was observed at high collagen concentrations, due to the presence of cells. Thanks to confocal microscopy, we showed that cells elongated within the gel and could remodel it, this being a concentration-dependent feature. A careful analysis of the remodeling process showed that cells can attract collagen in their close neighborhood, this being an irreversible process and that migrating cells create collagen-depleted regions behind them.

Tumor cell/endothelial cell tight contact upregulates endothelial adhesion molecule expression mediated by NFkappaB: differential role of the shear stress

Cancer metastasis is a multistep process involving cell-cell interactions, but little is known about the adhesive interactions and signaling events during extravasation of tumor cells (TCs). In this study, cell adhesion molecule (CAM) expression was investigated using an in vitro assay, in which TCs were seeded onto an endothelial cell (ECs) monolayer and cocultured during 5 h. Flow cytometry, confocal microscopy as well as western blot analysis indicated that endothelial ICAM-1 (Inter Cellular Adhesion Molecule-1), VCAM-1 (Vascular Adhesion Molecule-1) and E-selectin were up-regulated after TC-EC coculture, whereas no change was observed for CAMs expression in tumor cells. This increased CAMs expression required tight contact between TCs and ECs. Incubation of ECs with the pyrrolidine-dithiocarbamate NFkappaB inhibitor prior to coculture, fully prevented coculture-induced expression of endothelial CAMs. Using specific blocking antibodies we showed an implication of ICAM-1 and VCAM-1 for TCs extravasation and VCAM-1 for adhesion. Moreover, fluid flow experiments revealed that high shear stress totally abolished coculture-induced as well as TNFalpha-induced CAMs over-expression. This study suggests that TCs could act as a potent inflammatory stimulus on ECs by inducing CAMs expression via NFkappaB activation, and that this action can be modulated by shear stress.

Traction patterns of tumor cells

The traction exerted by a cell on a planar deformable substrate can be indirectly obtained on the basis of the displacement field of the underlying layer. The usual methodology used to address this inverse problem is based on the exploitation of the Green tensor of the linear elasticity problem in a half space (Boussinesq problem), coupled with a minimization algorithm under force penalization. A possible alternative strategy is to exploit an adjoint equation, obtained on the basis of a suitable minimization requirement. The resulting system of coupled elliptic partial differential equations is applied here to determine the force field per unit surface generated by T24 tumor cells on a polyacrylamide substrate. The shear stress obtained by numerical integration provides quantitative insight of the traction field and is a promising tool to investigate the spatial pattern of force per unit surface generated in cell motion, particularly in the case of such cancer cells.

Fractal approach to the rheology of concentrated cell suspensions

Results on the rheological behavior of Chinese hamster ovary cell suspensions in a large range of concentrations are reported. The concentration-dependent yield stress and elastic plateau modulus are formalized in the context of fractal aggregates under shear, and quite different exponents are found as compared to the case of red blood cell suspensions. This is explained in terms of intrinsic microscopic parameters such as the cell-cell adhesion energy and cell elasticity but also the cell's individual dynamic properties, found to correlate well with viscoelastic data at large concentrations (phi>or=0.5).

Dynamics and rheology of a dilute suspension of vesicles: higher-order theory

Vesicles under shear flow exhibit various dynamics: tank treading (TT), tumbling (TB), and vacillating breathing (VB). The VB mode consists in a motion where the long axis of the vesicle oscillates about the flow direction, while the shape undergoes a breathing dynamics. We extend here the original small deformation theory [C. Misbah, Phys. Rev. Lett. 96, 028104 (2006)] to the next order in a consistent manner. The consistent higher order theory reveals a direct bifurcation from TT to TB if Ca identical with taugamma is small enough-typically below 0.5, but this value is sensitive to the available excess area from a sphere (tau=vesicle relaxation time towards equilibrium shape, gamma=shear rate). At larger Ca the TB is preceded by the VB mode. For Ca1 we recover the leading order original calculation, where the VB mode coexists with TB. The consistent calculation reveals several quantitative discrepancies with recent works, and points to new features. We briefly analyze rheology and find that the effective viscosity exhibits a minimum in the vicinity of the TT-TB and TT-VB bifurcation points. At small Ca the minimum corresponds to a cusp singularity and is at the TT-TB threshold, while at high enough Ca the cusp is smeared out, and is located in the vicinity of the VB mode but in the TT regime.

Modeling breakup and relaxation of Newtonian droplets using the advected phase-field approach

The relaxation and breakup of Newtonian droplets is considered using the advected field approach. This method allows one to follow the deformation of interfaces using an order parameter field [Biben, Europhys. Lett. 63, 623 (2003)] based on a Ginzburg-Landau equation. Using this method, it is possible to follow the breakup of droplets and stability curves can be obtained in both two- and three-dimensional shear and elongational flows. Finally, relaxation of a droplet is considered, following the application of an elongational flow. The results are compared with previous experimental data [Ha and Leal, Phys. Fluids 13, 1568 (2001)], and are found to be in satisfactory agreement. The method is general enough to be applied to other non-Newtonian fluids, such as Oldroyd-B fluids or viscoplastic materials.

The Rhône-Alpes health platform

OBJECTIVES

The purpose of this work is to develop a health information platform connecting most health facilities in the Rhône-Alpes region. The health platform called SIS-RA is used through a Web interface. An iconic interface is dedicated to the platform and presents information in a unique way for all users.

METHODS

New techniques have been used to develop this platform which will be used by a great number of Rhône-Alpes doctors in the future. We chose a usercentered design which takes into account doctors' requirements (hospital and GP). We also consider that no system has to be rebuilt, but a direct connection to the legacy systems should be provided.

RESULTS

The platform permits fast and more appropriate medical decisions than those made without this information system. The iconic interface presents all medical documents in a uniform way. Currently, 11 healthcare facilities and 15 community health networks are connected to SIS-RA sharing more than 60,000 records with 1.2 million indexed items. 3200 doctors use the system.

CONCLUSION

The platform is approved by French supervision authorities (regional hospitals association (ARH)), regional practitioners union (URML) and Rhônes-Alpes region administration and is known as the official shared health record.

Morphological analysis of tumor cell/endothelial cell interactions under shear flow

In the process of hematogenous cancer metastasis, tumor cells (TCs) must shed into the blood stream, survive in the blood circulation, migrate through the vascular endothelium (extravasation) and proliferate in the target organs. However, the precise mechanisms by which TCs penetrate the endothelial cell (EC) junctions remain one of the least understood aspects of TC extravasation. This question has generally been addressed under static conditions, despite the important role of flow induced mechanical stress on the circulating cell-endothelium interactions. Moreover, flow studies were generally focused on transient or firm adhesion steps of TC-EC interactions and did not consider TCs spreading or extravasation. In this paper, we used a parallel-plate flow chamber to investigate TC-EC interactions under flow conditions. An EC monolayer was cultured on the lower plate of the flow chamber to model the endothelial barrier. Circulating TCs were introduced into the flow channel under a well-defined flow field and TC cell shape changes on the EC monolayer were followed in vitro with live phase contrast and fluorescence microscopy. Two spreading patterns were observed: radial spreading which corresponds to TC extravasation, and axial spreading where TCs formed a mosaic TC-EC monolayer. By investigating the changes in area and minor/major aspect ratio, we have established a simple quantitative basis for comparing spreading modes under various shear stresses. Contrary to radial spreading, the extent of axial spreading was increased by shear stress.

Elongation and burst of axisymmetric viscoelastic droplets: a numerical study

The dynamics of elongation and burst of an isolated viscoelastic drop are investigated numerically in the special case of a viscosity ratio lambda = 1 . We show that the burst threshold is not affected by viscoelasticity itself, whereas the stationary drop morphology is. A dimple at the tips of the drop can even be observed when elastic effects are large. The burst dynamics is very sensitive to the presence of viscoelasticity: at low elasticity burst is slowed down while for large elasticity levels it becomes even faster than the Newtonian situation.

Measuring cell viscoelastic properties using a force-spectrometer: influence of protein-cytoplasm interactions

Cell adhesive and rheological properties play a very important role in cell transmigration through the endothelial barrier, in particular in the case of inflammation (leukocytes) or cancer metastasis (cancer cells). In order to characterize cell viscoelastic properties, we have designed a force spectrometer (AFM) which can stretch cells thereby allowing measurement of their rheological properties. This custom-made force spectrometer allows two different visualizations, one lateral and one from below. It allows investigation of the effects of rheology involved during cell stretching. To test the ability of our system to characterize such viscoelastic properties, ICAM-1 transfected CHO cells were analyzed. Two forms of ICAM-1 were tested; wild type ICAM-1, which can interact with the cytoskeleton, and a mutant form which lacks the cytoplasmic domain, and is unable to associate with the cytoskeleton. Stretching experiments carried out on these cells show the formation of long filaments. Using a previous model of filament elongation, we could determine the viscoelastic properties of a single cell. As expected, different viscoelastic components were found between the wild type and the mutant, which reveal that the presence of interactions between ICAM-1 and the cytoskeleton increases the stiffness of the cell.

Rheological Properties of Living Materials. From Cells to Tissues

In this paper, we review the role of the rheological properties at the cellular and macroscopic scale. At the cellular scale, the different components of the cell are described, and comparisons with other similar systems are made in order to state what kind of rheological properties and what constitutive equations can be expected. This is based on expertise collected over many years, dealing with components such as polymers, suspensions, colloids and gels. Various references are considered. Then we review the various methods available in the literature, which can allow one to go from the microscopic to the macroscopic properties of an ensemble of cells, in other words a tissue. One of the questions raised is: can we find different properties at the macroscopic level than the ones that we start with at the cellular level? Finally, we consider different biological materials which have been used and characterized, in order to classify them. Constitutive laws are also proposed and criticized. The most difficult part of modeling is taking into account the active part of cells, which are not just plain materials, but are living objects.

Characterization of wheat-flour-water doughs: a new method using ultrasound

In this paper, an original method of evaluating the physical properties of wheat-flour-water systems using high-frequency low-power ultrasound is presented. Most of the experiments were performed with a reflectance technique measuring the acoustic impedance of doughs. The velocity of propagation, attenuation and viscoelastic moduli have been evaluated for both compressional and shear ultrasonic waves in the interval 2-10 MHz for doughs of different hydrations. The 53% water content was found to be critical with respect to the presence of free water. The influence of the mixing and rest times on the longitudinal ultrasonic parameters is also studied.