Frédéric Dumas-Bouchiat

Evolutionary conservation of early mesoderm specification by mechanotransduction in Bilateria

The modulation of developmental biochemical pathways by mechanical cues is an emerging feature of animal development, but its evolutionary origins have not been explored. Here we show that a common mechanosensitive pathway involving β-catenin specifies early mesodermal identity at gastrulation in zebrafish and Drosophila. Mechanical strains developed by zebrafish epiboly and Drosophila mesoderm invagination trigger the phosphorylation of β-catenin–tyrosine-667. This leads to the release of β-catenin into the cytoplasm and nucleus, where it triggers and maintains, respectively, the expression of zebrafish brachyury orthologue notail and of Drosophila Twist, both crucial transcription factors for early mesoderm identity. The role of the β-catenin mechanosensitive pathway in mesoderm identity has been conserved over the large evolutionary distance separating zebrafish and Drosophila. This suggests mesoderm mechanical induction dating back to at least the last bilaterian common ancestor more than 570 million years ago, the period during which mesoderm is thought to have emerged.

Sub-Microsecond RF MEMS Switched Capacitors

This paper presents fast switching RF microelectromechanical systems (MEMS) capacitors with measured switching times between 150-400 ns. By introducing bent sides on a planar microbeam, it is shown experimentally that the resonance frequency of aluminum bridges is increased by a factor of 25 compared to standard RF MEMS components. In addition, this original shape can be implemented easily in post-processing of complementary metal-oxide-semiconductor circuits. Several designs are presented with measured mechanical resonance frequencies between 1-3 MHz and measured switching times under 400 ns. Using this original approach, sub-microsecond RF MEMS switched capacitors have been designed and fabricated on quartz substrate. Their resulting RF performance is presented with a measured capacitance ratio of 2.3. Reliability tests have also been performed and have demonstrated no significant mechanical behavior variation over 14 billion cycles and a moderate sensitivity to temperature variation.

Magnetic characterization of micropatterned Nd–Fe–B hard magnetic films using scanning Hall probe microscopy

Scanning Hall probe microscopy has been used for the quantitative measurement of the z-component (out-of-plane) of the stray magnetic fields produced by Nd–Fe–B hard magnetic films patterned at the micron scale using both topographic and thermomagnetic methods. Peak-to-peak field values in the range 20–120 mT have been measured at scan heights of 25–30 μm above the samples. Quantitative comparison between calculated and measured field profiles gives nondestructive access to the micromagnets’ internal magnetic structure. In the case of topographically patterned films the average value of remanent magnetization is extracted; in the case of thermomagnetically patterned films the depth of magnetization reversal is estimated. The measured field profiles are used to derive the spatial variation in the field and field gradient values at distances in the range 0.1–10 μm above the micromagnet arrays. These length-scales are relevant to the application of the micromagnet arrays for lab-on-chip applications (trappin...

Autonomous micro-magnet based systems for highly efficient magnetic separation

The various forces experienced by magnetic particles pumped through microfluidic channels placed above a chessboard array of micromagnets were calculated as a function of particle size and device dimensions. A device incorporating magnetically microstructured hard magnetic NdFeB films was fabricated. Good agreement was achieved between the calculated and observed distance over which magnetic particles travel before they are trapped. Using this simple and autonomous device, mixed solutions of magnetic and non-magnetic micro-particles were separated into two distinct solutions containing a concentration of up to 99.9% and 94.5% of non-magnetic and magnetic particles, respectively.

Microfluidic immunomagnetic cell separation using integrated permanent micromagnets

In this paper, we demonstrate the possibility to trap and sort labeled cells under flow conditions using a microfluidic device with an integrated flat micro-patterned hard magnetic film. The proposed technique is illustrated using a cell suspension containing a mixture of Jurkat cells and HEK (Human Embryonic Kidney) 293 cells. Prior to sorting experiments, the Jurkat cells were specifically labeled with immunomagnetic nanoparticles, while the HEK 293 cells were unlabeled. Droplet-based experiments demonstrated that the Jurkat cells were attracted to regions of maximum stray field flux density while the HEK 293 cells settled in random positions. When the mixture was passed through a polydimethylsiloxane (PDMS) microfluidic channel containing integrated micromagnets, the labeled Jurkat cells were selectively trapped under fluid flow, while the HEK cells were eluted towards the device outlet. Increasing the flow rate produced a second eluate much enriched in Jurkat cells, as revealed by flow cytometry. The separation efficiency of this biocompatible, compact micro-fluidic separation chamber was compared with that obtained using two commercial magnetic cell separation kits.

Micro-magnetic imprinting of high field gradient magnetic flux sources

We report here on the fabrication of hard magnetic powder based micro-flux sources using micro-patterned hard magnetic films as templates or master structures. The micro-magnetic imprinting (μMI) process is simple and the constituent materials of the final structures, commercial hard magnetic powders and polymer, are inexpensive. The structures may be transparent, and either flexible or rigid, depending on the choice of polymer matrix used. The peak-to-peak intensity of the z-component of the stray magnetic field measured above a test μMI structure made with spherical NdFeB particles of average particle size 16 μm is in good agreement with simulated field values (150 mT at 5 μm). Simulations indicate magnetic field gradients of up to 5 × 105 T/m at the surface of such μMI structures. The trapping of cells functionalised with superparamagnetic beads by these structures has been demonstrated. The μMI fabrication technique has much potential for the development of high field gradient magnetic flux sources for applications in biology and beyond.

Micromagnet structures for magnetic positioning and alignment

High performance hard magnetic films (NdFeB, SmCo) have been patterned at the micron scale using thermo-magnetic patterning. Both out-of-plane and in-plane magnetized structures have been prepared. These micromagnet arrays have been used for the precise positioning and alignment of superparamagnetic nano- and microparticles. The specific spatial arrangement achieved is shown to depend on both the particle size and the size and orientation of the micromagnets. These micromagnet arrays were used to trap cells magnetically functionalized by endocytosis of 100 nm superparamagnetic particles. These simple, compact, and autonomous structures, which need neither an external magnetic field source nor a power supply, have much potential for use in a wide range of biological applications.

Cobalt cluster-assembled thin films deposited by low energy cluster beam deposition: Structural and magnetic investigations of deposited layers

Cobalt cluster-assembled thin films were deposited on amorphous-carbon-coated copper grids and on silicon substrates at room temperature by low energy cluster beam deposition. Characterizations using high-resolution transmission electronic microscopy and atomic force microscopy reveal randomly stacked agglomerates of 9–11nm diameter, which are themselves composed of small 3.6nm diameter fcc cobalt clusters. The films are ferromagnetic up to room temperature and above, which implies that the clusters are exchange coupled. The approach to saturation is analyzed within the random anisotropy model. The values of the exchange coefficient A and the anisotropy constant K then derived are discussed. The temperature dependence of the coercivity below 100K is discussed in terms of thermal activation effects. All results indicate that the fundamental entity governing the magnetic behaviors is constituted by the 9–11nm diameter agglomerates rather than by the clusters themselves.

Mechanotransductive cascade of Myo-II-dependent mesoderm and endoderm invaginations in embryo gastrulation

Animal development consists of a cascade of tissue differentiation and shape change. Associated mechanical signals regulate tissue differentiation. Here we demonstrate that endogenous mechanical cues also trigger biochemical pathways, generating the active morphogenetic movements shaping animal development through a mechanotransductive cascade of Myo-II medio-apical stabilization. To mimic physiological tissue deformation with a cell scale resolution, liposomes containing magnetic nanoparticles are injected into embryonic epithelia and submitted to time-variable forces generated by a linear array of micrometric soft magnets. Periodic magnetically induced deformations quantitatively phenocopy the soft mechanical endogenous snail-dependent apex pulsations, rescue the medio-apical accumulation of Rok, Myo-II and subsequent mesoderm invagination lacking in sna mutants, in a Fog-dependent mechanotransductive process. Mesoderm invagination then activates Myo-II apical accumulation, in a similar Fog-dependent mechanotransductive process, which in turn initiates endoderm invagination. This reveals the existence of a highly dynamic self-inductive cascade of mesoderm and endoderm invaginations, regulated by mechano-induced medio-apical stabilization of Myo-II.

Life on magnets: stem cell networking on micro-magnet arrays.

Interactions between a micro-magnet array and living cells may guide the establishment of cell networks due to the cellular response to a magnetic field. To manipulate mesenchymal stem cells free of magnetic nanoparticles by a high magnetic field gradient, we used high quality micro-patterned NdFeB films around which the stray field’s value and direction drastically change across the cell body. Such micro-magnet arrays coated with parylene produce high magnetic field gradients that affect the cells in two main ways: i) causing cell migration and adherence to a covered magnetic surface and ii) elongating the cells in the directions parallel to the edges of the micro-magnet. To explain these effects, three putative mechanisms that incorporate both physical and biological factors influencing the cells are suggested. It is shown that the static high magnetic field gradient generated by the micro-magnet arrays are capable of assisting cell migration to those areas with the strongest magnetic field gradient, thereby allowing the build up of tunable interconnected stem cell networks, which is an elegant route for tissue engineering and regenerative medicine.