The force-displacement response of a single duplex DNA molecule was measured. The force saturates at a plateau around 70 piconewtons, which ends when the DNA has been stretched about 1.7 times its contour length. This behavior reveals a highly cooperative transition to a state here termed S-DNA. Addition of an intercalator suppresses this transition. Molecular modeling of the process also yields a force plateau and suggests a structure for the extended form. These results may shed light on biological processes involving DNA extension and open the route for mechanical studies on individual molecules in a previously unexplored range.

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Various experimental models are used to study brain development and degeneration. They range from whole animal models, which preserve anatomical structures but strongly limit investigations at the cellular level, to dissociated cell culture systems that allow detailed observation of cell phenotypes but lack the highly ordered physiological neuron connection architecture. We describe here a platform comprising independent cell culture chambers separated by an array of "axonal diodes". This array involves asymmetric micro-channels, imposing unidirectional axon connectivity with 97% selectivity. It allows the construction of complex, oriented neuronal networks not feasible with earlier platforms. Different neuronal subtypes could be co-cultivated for weeks, and sequential seeding of different cell populations reproduced physiological network development. To illustrate possible applications, we created and characterized a cortico-striatal oriented network. Functional synaptic connections were established. The activation of striatal differentiation by cortical axons, and the synchronization of neural activity were demonstrated. Each neuronal population and subcompartment could be chemically addressed individually. The directionality of neural pathways being a key feature of the nervous system organization, the axon diode concept brings in a paradigmatic change in neuronal culture platforms, with potential applications for studying neuronal development, synaptic transmission and neurodegenerative disorder such as Alzheimer and Parkinson diseases at the sub-cellular, cellular and network levels.

The dynamics of a single polymer tethered to a solid surface in a shear flow was observed using fluorescently labeled DNA chains. Dramatic shear enhanced temporal fluctuations in the chain extension were observed. The rate of these fluctuations initially decreased for increasing shear rate gamma; and increased above a critical gamma;. Simulations revealed that these anomalous dynamics arise from a continual recirculating motion of the chain or cyclic dynamics. These dynamics arise from a coupling of the chain velocity in the flow direction to thermally driven fluctuations of the chain in the shear gradient direction.

We present a purely hydrodynamic method for the high-throughput encapsulation of single cells into picoliter droplets, and spontaneous self-sorting of these droplets. Encapsulation uses a cell-triggered Rayleigh-Plateau instability in a flow-focusing geometry, and self-sorting puts to work two extra hydrodynamic mechanisms: lateral drift of deformable objects in a shear flow, and sterically driven dispersion in a compressional flow. Encapsulation and sorting are achieved on-flight in continuous flow at a rate up to 160 cells per second. The whole process is robust and costeffective, involving no optical or electrical discrimination, active sorting, flow switching, or moving parts. Successful encapsulation and sorting of 70 -80% of the injected cell population into drops containing one and only one cell, with <1% contamination by empty droplets, is demonstrated. The system is also applied to the direct encapsulation and sorting of cancerous lymphocytes from a whole blood mixture, yielding individually encapsulated cancer cells with a >10,000-fold enrichment as compared with the initial mix. The method can be implemented in simple ''soft lithography'' chips, allowing for easy downstream coupling with microfluidic cell biology or molecular biology protocols.T he postgenome era is stimulating a strong demand for high-throughput cell assays. Because genetically identical cells may display very heterogeneous behaviors (1, 2), bulk measurements on cell populations provide only partial information on cell metabolism, in particular from a dynamic point of view. Cell-to-cell variability is also of paramount importance for cancer research, developmental biology, drug screening (3), and stem cell research. Recent trends in cell biology thus put strong emphasis on studies at the single-cell level (4).Microtechnologies raise the hope of dramatic breakthroughs in this field (5). For instance, compartmentalization of single cells in microchambers allows the analysis of stochastic protein expression at the single molecule level (6). In another approach, the capture of cells in microdroplets within double emulsions enables the screening of enzyme libraries with an unprecedented resolution and speed (7). In this perspective, combining encapsulation within droplets with microfluidic techniques may allow the observation and analysis of individual cells (e.g., with drugs or reagents) in a fully automated, time-resolved manner. These operations can be achieved using optical traps, but this approach remains complex and is hardly amenable to high throughput (8). In contrast, the use of classical continuous microdroplets generation techniques such as flow-focusing (9) or break-up at a T-junction (10) from a cell suspension offers potential for very high throughput, but could so far not warrant a controlled distribution of the cells in the drops (11). In these methods, the number of cells contained in the formed drops is dictated by the probability that a given volume of the initial cell suspension contains a given number of cells, follow...

We propose a unique method for cell sorting, "Ephesia," using columns of biofunctionalized superparamagnetic beads selfassembled in a microfluidic channel onto an array of magnetic traps prepared by microcontact printing. It combines the advantages of microfluidic cell sorting, notably the application of a well controlled, flow-activated interaction between cells and beads, and those of immunomagnetic sorting, notably the use of batch-prepared, well characterized antibody-bearing beads. On cell lines mixtures, we demonstrated a capture yield better than 94%, and the possibility to cultivate in situ the captured cells. A second series of experiments involved clinical samples-blood, pleural effusion, and fine needle aspirates-issued from healthy donors and patients with B-cell hematological malignant tumors (leukemia and lymphoma). The immunophenotype and morphology of B-lymphocytes were analyzed directly in the microfluidic chamber, and compared with conventional flow cytometry and visual cytology data, in a blind test. Immunophenotyping results using Ephesia were fully consistent with those obtained by flow cytometry. We obtained in situ high resolution confocal three-dimensional images of the cell nuclei, showing intranuclear details consistent with conventional cytological staining. Ephesia thus provides a powerful approach to cell capture and typing allowing fully automated high resolution and quantitative immunophenotyping and morphological analysis. It requires at least 10 times smaller sample volume and cell numbers than cytometry, potentially increasing the range of indications and the success rate of microbiopsy-based diagnosis, and reducing analysis time and cost.lab-on-a-chip | magnetic beads | cell sorting | cancer diagnosis C ell-based screening is a major tool in medicine and pharmaceutical research. In oncology and haematology, the morphological and phenotypic typing of cancer cells is already used routinely for diagnostic and therapeutic purposes. This typing is made all the more relevant by the development of "personalized medicine" approaches, that of new anticancer drugs targeting specific mutations, such as trastuzumab or rituximab, which require specific tumor cell typing regarding HER2 and CD20 expression, respectively

Magnetic tweezers are used to study the mechanical response under torsion of single nucleosome arrays reconstituted on tandem repeats of 5S positioning sequences. Regular arrays are extremely resilient and can reversibly accommodate a large amount of supercoiling without much change in length. This behavior is quantitatively described by a molecular model of the chromatin 3-D architecture. In this model, we assume the existence of a dynamic equilibrium between three conformations of the nucleosome, which are determined by the crossing status of the entry/exit DNAs (positive, null or negative). Torsional strain, in displacing that equilibrium, extensively reorganizes the fiber architecture. The model explains a number of long-standing topological questions regarding DNA in chromatin, and may provide the ground to better understand the dynamic binding of most chromatin-associated proteins.

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