Publications by authors named "Abdul I Barakat"

55 Publications

A compact integrated microfluidic oxygenator with high gas exchange efficiency and compatibility for long-lasting endothelialization.

Lab Chip 2021 Jul 26. Epub 2021 Jul 26.

Université Paris-Saclay, CNRS, Centre de Nanosciences et Nanotechnologies C2N, UMR9001, Palaiseau 91120, France.

We have developed and tested a novel microfluidic device for blood oxygenation, which exhibits a large surface area of gas exchange and can support long-term sustainable endothelialization of blood microcapillaries, enhancing its hemocompatibility for clinical applications. The architecture of the parallel stacking of the trilayers is based on a central injection for blood and a lateral injection/output for gas which allows significant reduction in shear stress, promoting sustainable endothelialization since cells can be maintained viable for up to 2 weeks after initial seeding in the blood microchannel network. The circular design of curved blood capillaries allows covering a maximal surface area at 4 inch wafer scale, producing high oxygen uptake and carbon dioxide release in each single unit. Since the conventional bonding process based on oxygen plasma cannot be used for surface areas larger than several cm2, a new "wet bonding" process based on soft microprinting has been developed and patented. Using this new protocol, each 4 inch trilayer unit can be sealed without a collapsed membrane even at reduced 15 μm thickness and can support a high blood flow rate. The height of the blood channels has been optimized to reduce pressure drop and enhance gas exchange at a high volumetric blood flow rate up to 15 ml min-1. The simplicity of connecting different units in the stacked architecture is demonstrated for 3- or 5-unit stacked devices that exhibit remarkable performance with low primary volume, high oxygen uptake and carbon dioxide release and high flow rate of up to 80 ml min-1.
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http://dx.doi.org/10.1039/d1lc00356aDOI Listing
July 2021

Integration of substrate- and flow-derived stresses in endothelial cell mechanobiology.

Commun Biol 2021 06 21;4(1):764. Epub 2021 Jun 21.

LadHyX, CNRS, Ecole polytechnique, Institut polytechnique de Paris, Palaiseau, France.

Endothelial cells (ECs) lining all blood vessels are subjected to large mechanical stresses that regulate their structure and function in health and disease. Here, we review EC responses to substrate-derived biophysical cues, namely topography, curvature, and stiffness, as well as to flow-derived stresses, notably shear stress, pressure, and tensile stresses. Because these mechanical cues in vivo are coupled and are exerted simultaneously on ECs, we also review the effects of multiple cues and describe burgeoning in vitro approaches for elucidating how ECs integrate and interpret various mechanical stimuli. We conclude by highlighting key open questions and upcoming challenges in the field of EC mechanobiology.
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http://dx.doi.org/10.1038/s42003-021-02285-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8217569PMC
June 2021

eG Coated Stents Exhibit Enhanced Endothelial Wound Healing Characteristics.

Cardiovasc Eng Technol 2021 May 18. Epub 2021 May 18.

Hydrodynamics Laboratory, CNRS UMR7646, Ecole Polytechnique, Institut Polytechnique de Paris, Palaiseau, France.

Purpose: Despite their widespread use, a significant fraction of coronary stents suffer from in-stent restenosis and stent thrombosis. Stent deployment induces extensive injury to the vascular endothelium. Rapid endothelial wound closure is essential for the success of a stenting procedure. A recent study has demonstrated that the BuMA Supreme® sirolimus-eluting stent exhibits particularly attractive strut coverage characteristics. A unique feature of this stent is the presence of a thin brush layer of poly-butyl methacrylate (PBMA), covalently bonded to the stent's cobalt-chromium frame via electro-grafting (eG™). The present study aimed to determine whether the PBMA coating has an effect on endothelial cell wound healing and stent strut coverage.

Methods: We used an in vitro coronary artery model whose wall consisted of an annular collagen hydrogel and whose luminal surface was lined with a monolayer of endothelial cells. Mechanical wounding of the endothelial lining was preformed prior to deployment of a bare cobalt-chromium stent either with or without the PBMA layer. The migration of fluorescently labeled endothelial cells was monitored automatically over a period of 48 h to determine endothelial wound healing rates.

Results: Quantitative assessment of endothelial wound healing rates within the simulated arterial model is achievable using automated image analysis. Wound healing is significantly faster (44% faster at 48 h) for stents with the PBMA eG Coating™ compared to bare metal stents.

Conclusion: The PBMA eG Coating™ has the effect of promoting endothelial wound healing. Future studies will focus on elucidating the mechanistic basis of this observation.
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http://dx.doi.org/10.1007/s13239-021-00542-xDOI Listing
May 2021

3D Printing for Cardiovascular Applications: From End-to-End Processes to Emerging Developments.

Ann Biomed Eng 2021 Jul 17;49(7):1598-1618. Epub 2021 May 17.

Faculty of Engineering, School of Mechanical and Manufacturing, UNSW, Sydney, Australia.

3D printing as a means of fabrication has seen increasing applications in medicine in the last decade, becoming invaluable for cardiovascular applications. This rapidly developing technology has had a significant impact on cardiovascular research, its clinical translation and education. It has expanded our understanding of the cardiovascular system resulting in better devices, tools and consequently improved patient outcomes. This review discusses the latest developments and future directions of generating medical replicas ('phantoms') for use in the cardiovascular field, detailing the end-to-end process from medical imaging to capture structures of interest, to production and use of 3D printed models. We provide comparisons of available imaging modalities and overview of segmentation and post-processing techniques to process images for printing, detailed exploration of latest 3D printing methods and materials, and a comprehensive, up-to-date review of milestone applications and their impact within the cardiovascular domain across research, clinical use and education. We then provide an in-depth exploration of future technologies and innovations around these methods, capturing opportunities and emerging directions across increasingly realistic representations, bioprinting and tissue engineering, and complementary virtual and mixed reality solutions. The next generation of 3D printing techniques allow patient-specific models that are increasingly realistic, replicating properties, anatomy and function.
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http://dx.doi.org/10.1007/s10439-021-02784-1DOI Listing
July 2021

Is there a universal mechanism of cell alignment in response to substrate topography?

Cytoskeleton (Hoboken) 2021 Apr 11. Epub 2021 Apr 11.

LadHyX, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, Palaiseau, France.

Cell alignment and elongation in the direction of anisotropic and aligned topographies are key manifestations of cellular contact guidance and are observed in many cell types. Whether this observation occurs through a universal mechanism remains to be established. In this Views article, we begin by presenting the most widely accepted model of topography-driven cell alignment which posits that anisotropic topographies impose lateral constraints on the growth of focal adhesions and actin stress fibers, thereby driving anisotropic force generation and cellular elongation and alignment. We then discuss particular scenarios where alternative or complementary mechanisms of cell alignment appear to be at play. These include the cases of specific cell types such as amoeboid-like cells and neurons as well as certain topography sizes. Finally, we review the role of the actin cytoskeleton in modulating topography-driven cell alignment and underscore the need for elucidating the role that other cytoskeletal elements play. We close by identifying key open questions the responses to which will significantly enhance our understanding of the role of cellular contact guidance in health and disease.
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http://dx.doi.org/10.1002/cm.21661DOI Listing
April 2021

Pericyte mechanics and mechanobiology.

J Cell Sci 2021 03 22;134(6). Epub 2021 Mar 22.

LadHyX, CNRS, Ecole polytechnique, Institut polytechnique de Paris, 91120, Palaiseau, France.

Pericytes are mural cells of the microvasculature, recognized by their thin processes and protruding cell body. Pericytes wrap around endothelial cells and play a central role in regulating various endothelial functions, including angiogenesis and inflammation. They also serve as a vascular support and regulate blood flow by contraction. Prior reviews have examined pericyte biological functions and biochemical signaling pathways. In this Review, we focus on the role of mechanics and mechanobiology in regulating pericyte function. After an overview of the morphology and structure of pericytes, we describe their interactions with both the basement membrane and endothelial cells. We then turn our attention to biophysical considerations, and describe contractile forces generated by pericytes, mechanical forces exerted on pericytes, and pericyte responses to these forces. Finally, we discuss 2D and 3D engineered models for studying pericyte mechano-responsiveness and underscore the need for more evolved models that provide improved understanding of pericyte function and dysfunction.
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http://dx.doi.org/10.1242/jcs.240226DOI Listing
March 2021

Rapid viscoelastic changes are a hallmark of early leukocyte activation.

Biophys J 2021 05 17;120(9):1692-1704. Epub 2021 Mar 17.

LadHyX, CNRS, Ecole polytechnique, Institut Polytechnique de Paris, Palaiseau, France. Electronic address:

To accomplish their critical task of removing infected cells and fighting pathogens, leukocytes activate by forming specialized interfaces with other cells. The physics of this key immunological process are poorly understood, but it is important to understand them because leukocytes have been shown to react to their mechanical environment. Using an innovative micropipette rheometer, we show in three different types of leukocytes that, when stimulated by microbeads mimicking target cells, leukocytes become up to 10 times stiffer and more viscous. These mechanical changes start within seconds after contact and evolve rapidly over minutes. Remarkably, leukocyte elastic and viscous properties evolve in parallel, preserving a well-defined ratio that constitutes a mechanical signature specific to each cell type. Our results indicate that simultaneously tracking both elastic and viscous properties during an active cell process provides a new, to our knowledge, way to investigate cell mechanical processes. Our findings also suggest that dynamic immunomechanical measurements can help discriminate between leukocyte subtypes during activation.
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http://dx.doi.org/10.1016/j.bpj.2021.02.042DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8204340PMC
May 2021

Mueller polarimetric imaging for fast macroscopic mapping of microscopic collagen matrix remodeling by smooth muscle cells.

Sci Rep 2021 Mar 15;11(1):5901. Epub 2021 Mar 15.

LPICM (CNRS UMR 7647), Ecole Polytechnique, IP Paris, Paris, France.

Smooth muscle cells (SMCs) are critical players in cardiovascular disease development and undergo complex phenotype switching during disease progression. However, SMC phenotype is difficult to assess and track in co-culture studies. To determine the contractility of SMCs embedded within collagen hydrogels, we performed polarized light imaging and subsequent analysis based on Mueller matrices. Measurements were made both in the absence and presence of endothelial cells (ECs) in order to establish the impact of EC-SMC communication on SMC contractility. The results demonstrated that Mueller polarimetric imaging is indeed an appropriate tool for assessing SMC activity which significantly modifies the hydrogel retardance in the presence of ECs. These findings are consistent with the idea that EC-SMC communication promotes a more contractile SMC phenotype. More broadly, our findings suggest that Mueller polarimetry can be a useful tool for studies of spatial heterogeneities in hydrogel remodeling by SMCs.
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http://dx.doi.org/10.1038/s41598-021-85164-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7960740PMC
March 2021

The basement membrane as a structured surface - role in vascular health and disease.

J Cell Sci 2020 09 16;133(18). Epub 2020 Sep 16.

Hydrodynamics Laboratory, CNRS UMR7646, Ecole Polytechnique, Palaiseau, France.

The basement membrane (BM) is a thin specialized extracellular matrix that functions as a cellular anchorage site, a physical barrier and a signaling hub. While the literature on the biochemical composition and biological activity of the BM is extensive, the central importance of the physical properties of the BM, most notably its mechanical stiffness and topographical features, in regulating cellular function has only recently been recognized. In this Review, we focus on the biophysical attributes of the BM and their influence on cellular behavior. After a brief overview of the biochemical composition, assembly and function of the BM, we describe the mechanical properties and topographical structure of various BMs. We then focus specifically on the vascular BM as a nano- and micro-scale structured surface and review how its architecture can modulate endothelial cell structure and function. Finally, we discuss the pathological ramifications of the biophysical properties of the vascular BM and highlight the potential of mimicking BM topography to improve the design of implantable endovascular devices and advance the burgeoning field of vascular tissue engineering.
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http://dx.doi.org/10.1242/jcs.239889DOI Listing
September 2020

Shear stress in the microvasculature: influence of red blood cell morphology and endothelial wall undulation.

Biomech Model Mechanobiol 2019 Aug 6;18(4):1095-1109. Epub 2019 Mar 6.

Hydrodynamics Laboratory (LadHyX), École Polytechnique, Palaiseau, France.

The effect of red blood cells and the undulation of the endothelium on the shear stress in the microvasculature is studied numerically using the lattice Boltzmann-immersed boundary method. The results demonstrate a significant effect of both the undulation of the endothelium and red blood cells on wall shear stress. Our results also reveal that morphological alterations of red blood cells, as occur in certain pathologies, can significantly affect the values of wall shear stress. The resulting fluctuations in wall shear stress greatly exceed the nominal values, emphasizing the importance of the particulate nature of blood as well as a more realistic description of vessel wall geometry in the study of hemodynamic forces. We find that within the channel widths investigated, which correspond to those found in the microvasculature, the inverse minimum distance normalized to the channel width between the red blood cell and the wall is predictive of the maximum wall shear stress observed in straight channels with a flowing red blood cell. We find that the maximum wall shear stress varies several factors more over a range of capillary numbers (dimensionless number relating strength of flow to membrane elasticity) and reduced areas (measure of deflation of the red blood cell) than the minimum wall shear stress. We see that waviness reduces variation in minimum and maximum shear stresses among different capillary and reduced areas.
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http://dx.doi.org/10.1007/s10237-019-01130-8DOI Listing
August 2019

ATP Release by Red Blood Cells under Flow: Model and Simulations.

Biophys J 2018 12 25;115(11):2218-2229. Epub 2018 Oct 25.

University Grenoble Alpes, LIPHY, Grenoble, France; CNRS, LIPHY, Grenoble, France. Electronic address:

ATP is a major player as a signaling molecule in blood microcirculation. It is released by red blood cells (RBCs) when they are subjected to shear stresses large enough to induce a sufficient shape deformation. This prominent feature of chemical response to shear stress and RBC deformation constitutes an important link between vessel geometry, flow conditions, and the mechanical properties of RBCs, which are all contributing factors affecting the chemical signals in the process of vasomotor modulation of the precapillary vessel networks. Several in vitro experiments have reported on ATP release by RBCs due to mechanical stress. These studies have considered both intact RBCs as well as cells within which suspected pathways of ATP release have been inhibited. This has provided profound insights to help elucidate the basic governing key elements, yet how the ATP release process takes place in the (intermediate) microcirculation zone is not well understood. We propose here an analytical model of ATP release. The ATP concentration is coupled in a consistent way to RBC dynamics. The release of ATP, or the lack thereof, is assumed to depend on both the local shear stress and the shape change of the membrane. The full chemo-mechanical coupling problem is written in a lattice-Boltzmann formulation and solved numerically in different geometries (straight channels and bifurcations mimicking vessel networks) and under two kinds of imposed flows (shear and Poiseuille flows). Our model remarkably reproduces existing experimental results. It also pinpoints the major contribution of ATP release when cells traverse network bifurcations. This study may aid in further identifying the interplay between mechanical properties and chemical signaling processes involved in blood microcirculation.
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http://dx.doi.org/10.1016/j.bpj.2018.09.033DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6289826PMC
December 2018

Endothelial autophagic flux hampers atherosclerotic lesion development.

Autophagy 2018 29;14(1):173-175. Epub 2018 Jan 29.

a INSERM , U970, Paris Cardiovascular Research Center - PARCC.

Blood flowing in arteries generates shear forces at the surface of the vascular endothelium that control its anti-atherogenic properties. However, due to the architecture of the vascular tree, these shear forces are heterogeneous and atherosclerotic plaques develop preferentially in areas where shear is low or disturbed. Here we review our recent study showing that elevated shear forces stimulate endothelial autophagic flux and that inactivating the endothelial macroautophagy/autophagy pathway promotes a proinflammatory, prosenescent and proapoptotic cell phenotype despite the presence of atheroprotective shear forces. Specific deficiency in endothelial autophagy in a murine model of atherosclerosis stimulates the development of atherosclerotic lesions exclusively in areas of the vasculature that are normally resistant to atherosclerosis. Our findings demonstrate that adequate endothelial autophagic flux limits atherosclerotic plaque formation by preventing endothelial apoptosis, senescence and inflammation.
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http://dx.doi.org/10.1080/15548627.2017.1395114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5846556PMC
February 2019

Autophagy is required for endothelial cell alignment and atheroprotection under physiological blood flow.

Proc Natl Acad Sci U S A 2017 10 25;114(41):E8675-E8684. Epub 2017 Sep 25.

INSERM, U970, Paris Cardiovascular Research Center, 75015 Paris, France.

It has been known for some time that atherosclerotic lesions preferentially develop in areas exposed to low SS and are characterized by a proinflammatory, apoptotic, and senescent endothelial phenotype. Conversely, areas exposed to high SS are protected from plaque development, but the mechanisms have remained elusive. Autophagy is a protective mechanism that allows recycling of defective organelles and proteins to maintain cellular homeostasis. We aimed to understand the role of endothelial autophagy in the atheroprotective effect of high SS. Atheroprotective high SS stimulated endothelial autophagic flux in human and murine arteries. On the contrary, endothelial cells exposed to atheroprone low SS were characterized by inefficient autophagy as a result of mammalian target of rapamycin (mTOR) activation, AMPKα inhibition, and blockade of the autophagic flux. In hypercholesterolemic mice, deficiency in endothelial autophagy increased plaque burden only in the atheroresistant areas exposed to high SS; plaque size was unchanged in atheroprone areas, in which endothelial autophagy flux is already blocked. In cultured cells and in transgenic mice, deficiency in endothelial autophagy was characterized by defects in endothelial alignment with flow direction, a hallmark of endothelial cell health. This effect was associated with an increase in endothelial apoptosis and senescence in high-SS regions. Deficiency in endothelial autophagy also increased TNF-α-induced inflammation under high-SS conditions and decreased expression of the antiinflammatory factor KLF-2. Altogether, these results show that adequate endothelial autophagic flux under high SS limits atherosclerotic plaque formation by preventing endothelial apoptosis, senescence, and inflammation.
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http://dx.doi.org/10.1073/pnas.1702223114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5642679PMC
October 2017

Micropipette force probe to quantify single-cell force generation: application to T-cell activation.

Mol Biol Cell 2017 Nov 20;28(23):3229-3239. Epub 2017 Sep 20.

Laboratoire d'Hydrodynamique (LadHyX), Department of Mechanics, Ecole polytechnique-CNRS UMR7646, 91128 Palaiseau, France

In response to engagement of surface molecules, cells generate active forces that regulate many cellular processes. Developing tools that permit gathering mechanical and morphological information on these forces is of the utmost importance. Here we describe a new technique, the micropipette force probe, that uses a micropipette as a flexible cantilever that can aspirate at its tip a bead that is coated with molecules of interest and is brought in contact with the cell. This technique simultaneously allows tracking the resulting changes in cell morphology and mechanics as well as measuring the forces generated by the cell. To illustrate the power of this technique, we applied it to the study of human primary T lymphocytes (T-cells). It allowed the fine monitoring of pushing and pulling forces generated by T-cells in response to various activating antibodies and bending stiffness of the micropipette. We further dissected the sequence of mechanical and morphological events occurring during T-cell activation to model force generation and to reveal heterogeneity in the cell population studied. We also report the first measurement of the changes in Young's modulus of T-cells during their activation, showing that T-cells stiffen within the first minutes of the activation process.
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http://dx.doi.org/10.1091/mbc.E17-06-0385DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5687025PMC
November 2017

The stentable in vitro artery: an instrumented platform for endovascular device development and optimization.

J R Soc Interface 2016 12;13(125)

Hydrodynamics Laboratory (LadHyX), Ecole Polytechnique, Route de Saclay, 91128 Palaiseau, France

Although vascular disease is a leading cause of mortality, in vitro tools for controlled, quantitative studies of vascular biological processes in an environment that reflects physiological complexity remain limited. We developed a novel in vitro artery that exhibits a number of unique features distinguishing it from tissue-engineered or organ-on-a-chip constructs, most notably that it allows deployment of endovascular devices including stents, quantitative real-time tracking of cellular responses and detailed measurement of flow velocity and lumenal shear stress using particle image velocimetry. The wall of the stentable in vitro artery consists of an annular collagen hydrogel containing smooth muscle cells (SMCs) and whose lumenal surface is lined with a monolayer of endothelial cells (ECs). The system has in vivo dimensions and physiological flow conditions and allows automated high-resolution live imaging of both SMCs and ECs. To demonstrate proof-of-concept, we imaged and quantified EC wound healing, SMC motility and altered shear stresses on the endothelium after deployment of a coronary stent. The stentable in vitro artery provides a unique platform suited for a broad array of research applications. Wide-scale adoption of this system promises to enhance our understanding of important biological events affecting endovascular device performance and to reduce dependence on animal studies.
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http://dx.doi.org/10.1098/rsif.2016.0834DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5221533PMC
December 2016

Mechanical Criterion for the Rupture of a Cell Membrane under Compression.

Biophys J 2016 Dec;111(12):2711-2721

Hydrodynamics Laboratory, CNRS UMR 7646, Department of Mechanics, École Polytechnique, Palaiseau, France. Electronic address:

We investigate the mechanical conditions leading to the rupture of the plasma membrane of an endothelial cell subjected to a local, compressive force. Membrane rupture is induced by tilted microindentation, a technique used to perform mechanical measurements on adherent cells. In this technique, the applied force can be deduced from the measured horizontal displacement of a microindenter's tip, as imaged with an inverted microscope and without the need for optical sensors to measure the microindenter's deflection. We show that plasma membrane rupture of endothelial cells occurs at a well-defined value of the applied compressive stress. As a point of reference, we use numerical simulations to estimate the magnitude of the compressive stresses exerted on endothelial cells during the deployment of a stent.
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http://dx.doi.org/10.1016/j.bpj.2016.11.001DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5192693PMC
December 2016

T-lymphocyte passive deformation is controlled by unfolding of membrane surface reservoirs.

Mol Biol Cell 2016 11 7;27(22):3574-3582. Epub 2016 Sep 7.

Laboratoire d'Hydrodynamique, Ecole Polytechnique, CNRS UMR 7646, 91128 Palaiseau, France

T-lymphocytes in the human body routinely undergo large deformations, both passively, when going through narrow capillaries, and actively, when transmigrating across endothelial cells or squeezing through tissue. We investigate physical factors that enable and limit such deformations and explore how passive and active deformations may differ. Employing micropipette aspiration to mimic squeezing through narrow capillaries, we find that T-lymphocytes maintain a constant volume while they increase their apparent membrane surface area upon aspiration. Human resting T-lymphocytes, T-lymphoblasts, and the leukemic Jurkat T-cells all exhibit membrane rupture above a critical membrane area expansion that is independent of either micropipette size or aspiration pressure. The unfolded membrane matches the excess membrane contained in microvilli and membrane folds, as determined using scanning electron microscopy. In contrast, during transendothelial migration, a form of active deformation, we find that the membrane surface exceeds by a factor of two the amount of membrane stored in microvilli and folds. These results suggest that internal membrane reservoirs need to be recruited, possibly through exocytosis, for large active deformations to occur.
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http://dx.doi.org/10.1091/mbc.E16-06-0414DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5221589PMC
November 2016

A simple microfluidic device to study cell-scale endothelial mechanotransduction.

Biomed Microdevices 2016 08;18(4):63

Laboratoire d'Hydrodynamique de l'École polytechnique, CNRS-EP UMR 7646, Palaiseau, France.

Atherosclerosis is triggered by chronic inflammation of arterial endothelial cells (ECs). Because atherosclerosis develops preferentially in regions where blood flow is disturbed and where ECs have a cuboidal morphology, the interplay between EC shape and mechanotransduction events is of primary interest. In this work we present a simple microfluidic device to study relationships between cell shape and EC response to fluid shear stress. Adhesive micropatterns are used to non-invasively control EC elongation and orientation at both the monolayer and single cell levels. The micropatterned substrate is coupled to a microfluidic chamber that allows precise control of the flow field, high-resolution live-cell imaging during flow experiments, and in situ immunostaining. Using micro particle image velocimetry, we show that cells within the chamber alter the local flow field so that the shear stress on the cell surface is significantly higher than the wall shear stress in regions containing no cells. In response to flow, we observe the formation of lamellipodia in the downstream portion of the EC and cell retraction in the upstream portion. We quantify flow-induced calcium mobilization at the single cell level for cells cultured on unpatterned surfaces or on adhesive lines oriented either parallel or orthogonal to the flow. Finally, we demonstrate flow-induced intracellular calcium waves and show that the direction of propagation of these waves is determined by cell polarization rather than by the flow direction. The combined versatility and simplicity of this microfluidic device renders it very useful for studying relationships between EC shape and mechanosensitivity.
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http://dx.doi.org/10.1007/s10544-016-0090-yDOI Listing
August 2016

Drug-Eluting Stent Design is a Determinant of Drug Concentration at the Endothelial Cell Surface.

Ann Biomed Eng 2016 Feb 28;44(2):302-14. Epub 2016 Jan 28.

Hydrodynamics Laboratory (LadHyX), CNRS UMR7646, Ecole Polytechnique, Route de Saclay, 91128, Palaiseau Cedex, France.

Although drug-eluting stents (DES) have greatly reduced arterial restenosis, there are persistent concerns about stent thrombosis. DES thrombosis is attributable to retarded vascular re-endothelialization due to both stent-induced flow disturbance and the inhibition by the eluted drug of endothelial cell proliferation and migration. The present computational study aims to determine the effect of DES design on both stent-induced flow disturbance and the concentration of eluted drug at the arterial luminal surface. To this end, we consider three closed-cell stent designs that resemble certain commercial stents as well as three "idealized" stents that provide insight into the impact of specific characteristics of stent design. To objectively compare the different stents, we introduce the Stent Penalty Index (SPI), a dimensionless quantity whose value increases with both the extent of flow disturbance and luminal drug concentration. Our results show that among the three closed-cell designs studied, wide cell designs lead to lower SPI and are thus expected to have a less adverse effect on vascular re-endothelialization. For the idealized stent designs, a spiral stent provides favorable SPI values, whereas an intertwined ring stent leads to an elevated SPI. The present findings shed light onto the effect of stent design on the concentration of the eluted drug at the arterial luminal surface, an important consideration in the assessment of DES performance.
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http://dx.doi.org/10.1007/s10439-015-1531-0DOI Listing
February 2016

Elastocapillary Instability in Mitochondrial Fission.

Phys Rev Lett 2015 Aug 20;115(8):088102. Epub 2015 Aug 20.

Laboratoire d'Hydrodynamique, Ecole Polytechnique, CNRS UMR 7646, 91128 Palaiseau, France.

Mitochondria are dynamic cell organelles that constantly undergo fission and fusion events. These dynamical processes, which tightly regulate mitochondrial morphology, are essential for cell physiology. Here we propose an elastocapillary mechanical instability as a mechanism for mitochondrial fission. We experimentally induce mitochondrial fission by rupturing the cell's plasma membrane. We present a stability analysis that successfully explains the observed fission wavelength and the role of mitochondrial morphology in the occurrence of fission events. Our results show that the laws of fluid mechanics can describe mitochondrial morphology and dynamics.
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http://dx.doi.org/10.1103/PhysRevLett.115.088102DOI Listing
August 2015

Characterizing cell adhesion by using micropipette aspiration.

Biophys J 2015 Jul;109(2):209-19

Department of Mechanics, Hydrodynamics Laboratory (LadHyX), École Polytechnique, Palaiseau, France. Electronic address:

We have developed a technique to directly quantify cell-substrate adhesion force using micropipette aspiration. The micropipette is positioned perpendicular to the surface of an adherent cell and a constant-rate aspiration pressure is applied. Since the micropipette diameter and the aspiration pressure are our control parameters, we have direct knowledge of the aspiration force, whereas the cell behavior is monitored either in brightfield or interference reflection microscopy. This setup thus allows us to explore a range of geometric parameters, such as projected cell area, adhesion area, or pipette size, as well as dynamical parameters such as the loading rate. We find that cell detachment is a well-defined event occurring at a critical aspiration pressure, and that the detachment force scales with the cell adhesion area (for a given micropipette diameter and loading rate), which defines a critical stress. Taking into account the cell adhesion area, intrinsic parameters of the adhesion bonds, and the loading rate, a minimal model provides an expression for the critical stress that helps rationalize our experimental results.
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http://dx.doi.org/10.1016/j.bpj.2015.06.015DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4621874PMC
July 2015

Optimization of Drug Delivery by Drug-Eluting Stents.

PLoS One 2015 17;10(6):e0130182. Epub 2015 Jun 17.

Laboratoire d'Hydrodynamique (LadHyX), École Polytechnique-CNRS, Palaiseau cedex, France.

Drug-eluting stents (DES), which release anti-proliferative drugs into the arterial wall in a controlled manner, have drastically reduced the rate of in-stent restenosis and revolutionized the treatment of atherosclerosis. However, late stent thrombosis remains a safety concern in DES, mainly due to delayed healing of the endothelial wound inflicted during DES implantation. We present a framework to optimize DES design such that restenosis is inhibited without affecting the endothelial healing process. To this end, we have developed a computational model of fluid flow and drug transport in stented arteries and have used this model to establish a metric for quantifying DES performance. The model takes into account the multi-layered structure of the arterial wall and incorporates a reversible binding model to describe drug interaction with the cells of the arterial wall. The model is coupled to a novel optimization algorithm that allows identification of optimal DES designs. We show that optimizing the period of drug release from DES and the initial drug concentration within the coating has a drastic effect on DES performance. Paclitaxel-eluting stents perform optimally by releasing their drug either very rapidly (within a few hours) or very slowly (over periods of several months up to one year) at concentrations considerably lower than current DES. In contrast, sirolimus-eluting stents perform optimally only when drug release is slow. The results offer explanations for recent trends in the development of DES and demonstrate the potential for large improvements in DES design relative to the current state of commercial devices.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0130182PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4470631PMC
April 2016

Model of cellular mechanotransduction via actin stress fibers.

Biomech Model Mechanobiol 2016 Apr 17;15(2):331-44. Epub 2015 Jun 17.

Hydrodynamics Laboratory (LadHyX), Department of Mechanics, Ecole Polytechnique, CNRS UMR7646, Palaiseau, France.

Mechanical stresses due to blood flow regulate vascular endothelial cell structure and function and play a key role in arterial physiology and pathology. In particular, the development of atherosclerosis has been shown to correlate with regions of disturbed blood flow where endothelial cells are round and have a randomly organized cytoskeleton. Thus, deciphering the relation between the mechanical environment, cell structure, and cell function is a key step toward understanding the early development of atherosclerosis. Recent experiments have demonstrated very rapid (∼100 ms) and long-distance (∼10 μm) cellular mechanotransduction in which prestressed actin stress fibers play a critical role. Here, we develop a model of mechanical signal transmission within a cell by describing strains in a network of prestressed viscoelastic stress fibers following the application of a force to the cell surface. We find force transmission dynamics that are consistent with experimental results. We also show that the extent of stress fiber alignment and the direction of the applied force relative to this alignment are key determinants of the efficiency of mechanical signal transmission. These results are consistent with the link observed experimentally between cytoskeletal organization, mechanical stress, and cellular responsiveness to stress. Based on these results, we suggest that mechanical strain of actin stress fibers under force constitutes a key link in the mechanotransduction chain.
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http://dx.doi.org/10.1007/s10237-015-0691-zDOI Listing
April 2016

Dynamics of receptor-mediated nanoparticle internalization into endothelial cells.

PLoS One 2015 22;10(4):e0122097. Epub 2015 Apr 22.

Laboratoire d'Hydrodynamique (LadHyX), École Polytechnique, CNRS UMR 7646, Palaiseau, France.

Nanoparticles offer a promising medical tool for targeted drug delivery, for example to treat inflamed endothelial cells during the development of atherosclerosis. To inform the design of such therapeutic strategies, we develop a computational model of nanoparticle internalization into endothelial cells, where internalization is driven by receptor-ligand binding and limited by the deformation of the cell membrane and cytoplasm. We specifically consider the case of nanoparticles targeted against ICAM-1 receptors, of relevance for treating atherosclerosis. The model computes the kinetics of the internalization process, the dynamics of binding, and the distribution of stresses exerted between the nanoparticle and the cell membrane. The model predicts the existence of an optimal nanoparticle size for fastest internalization, consistent with experimental observations, as well as the role of bond characteristics, local cell mechanical properties, and external forces in the nanoparticle internalization process.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0122097PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4406860PMC
January 2016

New approach to investigate the cytotoxicity of nanomaterials using single cell mechanics.

J Phys Chem B 2014 Feb 23;118(5):1246-55. Epub 2014 Jan 23.

Department of Chemistry, University of California, Davis , One Shields Avenue, Davis, California 95616, United States.

Current in vitro methods to assess nanomaterial cytotoxicity involve various assays to monitor specific cellular dysfunction, such as metabolic imbalance or inflammation. Although high throughput, fast, and animal-free, these in vitro methods suffer from unreliability and lack of relevance to in vivo situations. New approaches, especially with the potential to reliably relate to in vivo studies directly, are in critical need. This work introduces a new approach, single cell mechanics, derived from atomic force microscopy-based single cell compression. The single cell based approach is intrinsically advantageous in terms of being able to directly correlate to in vivo investigations. Its reliability and potential to measure cytotoxicity is evaluated using known systems: zinc oxide (ZnO) and silicon dioxide (SiO2) nanoparticles (NP) on human aortic endothelial cells (HAECs). This investigation clearly indicates the reliability of single cell compression. For example, ZnO NPs cause significant changes in force vs relative deformation profiles, whereas SiO2 NPs do not. New insights into NPs-cell interactions pertaining to cytotoxicity are also revealed from this single cell mechanics approach, in addition to a qualitative cytotoxicity conclusion. The advantages and disadvantages of this approach are also compared with conventional cytotoxicity assays.
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http://dx.doi.org/10.1021/jp410764fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3980960PMC
February 2014

Modeling the transport of drugs eluted from stents: physical phenomena driving drug distribution in the arterial wall.

Biomech Model Mechanobiol 2014 Apr 7;13(2):327-47. Epub 2014 Jan 7.

Laboratoire d'Hydrodynamique (LadHyX), CNRS, École Polytechnique, 91128 , Palaiseau Cedex, France.

Despite recent data that suggest that the overall performance of drug-eluting stents (DES) is superior to that of bare-metal stents, the long-term safety and efficacy of DES remain controversial. The risk of late stent thrombosis associated with the use of DES has also motivated the development of a new and promising treatment option in recent years, namely drug-coated balloons (DCB). Contrary to DES where the drug of choice is typically sirolimus and its derivatives, DCB use paclitaxel since the use of sirolimus does not appear to lead to satisfactory results. Since both sirolimus and paclitaxel are highly lipophilic drugs with similar transport properties, the reason for the success of paclitaxel but not sirolimus in DCB remains unclear. Computational models of the transport of drugs eluted from DES or DCB within the arterial wall promise to enhance our understanding of the performance of these devices. The present study develops a computational model of the transport of the two drugs paclitaxel and sirolimus eluted from DES in the arterial wall. The model takes into account the multilayered structure of the arterial wall and incorporates a reversible binding model to describe drug interactions with the constituents of the arterial wall. The present results demonstrate that the transport of paclitaxel in the arterial wall is dominated by convection while the transport of sirolimus is dominated by the binding process. These marked differences suggest that drug release kinetics of DES should be tailored to the type of drug used.
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http://dx.doi.org/10.1007/s10237-013-0546-4DOI Listing
April 2014

Intracellular regulation of cell signaling cascades: how location makes a difference.

J Math Biol 2014 Jul 18;69(1):213-42. Epub 2013 Jun 18.

Department of Applied Mathematics and Theoretical Physics (DAMTP), University of Cambridge, Cambridge, UK,

Organelles such as endosomes and the Golgi apparatus play a critical role in regulating signal transmission to the nucleus. Recent experiments have shown that appropriate positioning of these organelles within the intracellular space is critical for effective signal regulation. To understand the mechanism behind this observation, we consider a reaction-diffusion model of an intracellular signaling cascade and investigate the effect on the signaling of intracellular regulation in the form of a small release of phosphorylated signaling protein, kinase, and/or phosphatase. Variational analysis is applied to characterize the most effective regions for the localization of this intracellular regulation. The results demonstrate that signals reaching the nucleus are most effectively regulated by localizing the release of phosphorylated substrate protein and kinase near the nucleus. Phosphatase release, on the other hand, is nearly equally effective throughout the intracellular space. The effectiveness of the intracellular regulation is affected strongly by the characteristics of signal propagation through the cascade. For signals that are amplified as they propagate through the cascade, reactions in the upstream levels of the cascade exhibit much larger sensitivities to regulation by release of phosphorylated substrate protein and kinase than downstream reactions. On the other hand, for signals that decay through the cascade, downstream reactions exhibit larger sensitivity than upstream reactions. For regulation by phosphatase release, all reactions within the cascade show large sensitivity for amplified signals but lose this sensitivity for decaying signals. We use the analysis to develop a simple model of endosome-mediated regulation of cell signaling. The results demonstrate that signal regulation by the modeled endosome is most effective when the endosome is positioned in the vicinity of the nucleus. The present findings may explain at least in part why endosomes in many cell types localize near the nucleus.
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http://dx.doi.org/10.1007/s00285-013-0701-7DOI Listing
July 2014

Mechanisms of cytoskeleton-mediated mechanical signal transmission in cells.

Commun Integr Biol 2012 Nov;5(6):538-42

Hydrodynamics Laboratory (LadHyX); Ecole Polytechnique; CNRS UMR7646; Palaiseau, France.

Recent experiments have demonstrated very rapid long-distance transmission of mechanical forces within cells. Because the speed of this transmission greatly exceeds that of reaction-diffusion signaling, it has been conjectured that it occurs via the propagation of elastic waves through the actin stress fiber network. To explore the plausibility of this conjecture, we recently developed a model of small amplitude stress fiber deformations in prestressed viscoelastic stress fibers subjected to external forces. The model results demonstrated that rapid mechanical signal transmission is only possible when the external force is applied orthogonal to the stress fiber axis and that the dynamics of this transmission are governed by a balance between the prestress in the stress fiber and the stress fiber's material viscosity. The present study, which is a follow-up on our previous model, uses dimensional analysis to: (1) further evaluate the plausibility of the elastic wave conjecture and (2) obtain insight into mechanical signal transmission dynamics in simple stress fiber networks. We show that the elastic wave scenario is likely not the mechanism of rapid mechanical signal transmission in actin stress fibers due to the highly viscoelastic character of these fibers. Our analysis also demonstrates that the time constant characterizing mechanical stimulus transmission is strongly dependent on the topology of the stress fiber network, implying that network organization plays an important role in determining the dynamics of cellular responsiveness to mechanical stimulation.
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http://dx.doi.org/10.4161/cib.21633DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3541317PMC
November 2012

Dynamics of mechanical signal transmission through prestressed stress fibers.

PLoS One 2012 13;7(4):e35343. Epub 2012 Apr 13.

Hydrodynamics Laboratory, Department of Mechanics, École Polytechnique, CNRS UMR7646, Palaiseau, France.

Transmission of mechanical stimuli through the actin cytoskeleton has been proposed as a mechanism for rapid long-distance mechanotransduction in cells; however, a quantitative understanding of the dynamics of this transmission and the physical factors governing it remains lacking. Two key features of the actin cytoskeleton are its viscoelastic nature and the presence of prestress due to actomyosin motor activity. We develop a model of mechanical signal transmission through prestressed viscoelastic actin stress fibers that directly connect the cell surface to the nucleus. The analysis considers both temporally stationary and oscillatory mechanical signals and accounts for cytosolic drag on the stress fibers. To elucidate the physical parameters that govern mechanical signal transmission, we initially focus on the highly simplified case of a single stress fiber. The results demonstrate that the dynamics of mechanical signal transmission depend on whether the applied force leads to transverse or axial motion of the stress fiber. For transverse motion, mechanical signal transmission is dominated by prestress while fiber elasticity has a negligible effect. Conversely, signal transmission for axial motion is mediated uniquely by elasticity due to the absence of a prestress restoring force. Mechanical signal transmission is significantly delayed by stress fiber material viscosity, while cytosolic damping becomes important only for longer stress fibers. Only transverse motion yields the rapid and long-distance mechanical signal transmission dynamics observed experimentally. For simple networks of stress fibers, mechanical signals are transmitted rapidly to the nucleus when the fibers are oriented largely orthogonal to the applied force, whereas the presence of fibers parallel to the applied force slows down mechanical signal transmission significantly. The present results suggest that cytoskeletal prestress mediates rapid mechanical signal transmission and allows temporally oscillatory signals in the physiological frequency range to travel a long distance without significant decay due to material viscosity and/or cytosolic drag.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0035343PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3325979PMC
August 2012

Integration of basal topographic cues and apical shear stress in vascular endothelial cells.

Biomaterials 2012 Jun 13;33(16):4126-35. Epub 2012 Mar 13.

Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.

In vivo, vascular endothelial cells (VECs) are anchored to the underlying stroma through a specialization of the extracellular matrix, the basement membrane (BM) which provides a variety of substratum associated biophysical cues that have been shown to regulate fundamental VEC behaviors. VEC function and homeostasis are also influenced by hemodynamic cues applied to their apical surface. How the combination of these biophysical cues impacts fundamental VEC behavior remains poorly studied. In the present study, we investigated the impact of providing biophysical cues simultaneously to the basal and apical surfaces of human aortic endothelial cells (HAECs). Anisotropically ordered patterned surfaces of alternating ridges and grooves and isotropic holed surfaces of varying pitch (pitch = ridge or hole width + intervening groove or planar regions) were fabricated and seeded with HAECs. The cells were then subjected to a steady shear stress of 20 dyne/cm(2) applied either parallel or perpendicular to the direction of the ridge/groove topography. HAECs subjected to flow parallel to the ridge/groove topography exhibited protagonistic effects of the two stimuli on cellular orientation and elongation. In contrast, flow perpendicular to the substrate topography resulted in largely antagonistic effects. Interestingly, the behavior depended on the shape and size of the topographic features. HAECs exhibited a response that was less influenced by the substratum and primarily driven by flow on isotropically ordered holed surfaces of identical pitch to the anistropically ordered surfaces of alternating ridges and grooves. Simultaneous presentation of biophysical cues to the basal and apical aspects of cells also influenced nuclear orientation and elongation; however, the extent of nuclear realignment was more modest in comparison to cellular realignment regardless of the surface order of topographic features. Flow-induced HAEC migration was also influenced by the ridge/groove surface topographic features with significantly altered migration direction and increased migration tortuosity when flow was oriented perpendicular to the topography; this effect was also pitch-dependent. The present findings provide valuable insight into the interaction of biologically relevant apical and basal biophysical cues in regulating cellular behavior and promise to inform improved prosthetic design.
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http://dx.doi.org/10.1016/j.biomaterials.2012.02.047DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3633103PMC
June 2012
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