The biological function of filopodia has been extensively studied while only little work has been done on their mechanical properties. In the present study, we apply magnetic microbeads to explore the capturing and initial step of phagocytosis of pathogens by macrophages through filopodia. Microbeads were covered by the bacterial coat protein invasin which is known to trigger the invasion of the intestine by the bacteria Yersinia enterocolitica. These mimetics of bacteria were placed in the vicinity of J774 mouse macrophages exhibiting long filopodia. The specific adhesion of beads to the tip of a filopodium induced the retraction of the protrusion resulting in the dragging of the bead towards the cell body. The dynamics of the retraction process was analyzed by following the in-plane motion of the bead. We estimated the minimal force developed by filopodia and compared the results with previous magnetic tweezer studies of mechanical force induced growth of protrusions (Vonna et al. 2003). We show that very thin filopodia can generate astonishingly large retraction forces over large distances (>10 microm) and can act as an efficient mechanical tool to detach pathogens adhering on surfaces.

Mesoporous SBA-15 silica materials were grafted with trialkylsilyl compounds having short (C1) and long (C8) carbon chain and characterized by XRD, N2 physisorption analysis, 29Si MAS-NMR and contact angle (CA) measurements. A drastic enhancement of the hydrophobic property after grafting was observed by forced intrusion water; it occurred in two steps and with quite high intrusion pressures (mean values - 10 and - 15 MPa). The hydrophobic nature of both internal and external surface area was confirmed by 29Si MAS-NMR and CA measurements, respectively. After contact with water, materials displayed a partial hydrophobic behaviour with uncompleted spontaneous extrusion. The energies absorbed during water intrusion correspond to 4.3 and 6.1 J x g(-1) for C1 and C8 grafted species, respectively.

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We present an experimental investigation of the mechanical stability of silica nanoparticle-based coatings as a function of the size of the nanoparticles. The coatings are built following a layer-by-layer procedure, alternating positive and negative surface charges. The mechanical stability of the multilayers is studied in water, on the basis of an ultrasonic cavitation test. The resistance of the coating to cavitation is found to remarkably increase with decreasing the size of the nanoparticles, indicating an increase of the cohesive energy density. The relative contribution of van der Waals and electrical double-layer interactions to the stability of the multilayer is discussed toward their size dependence.

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Sodium alginate is a natural, water-soluble polysaccharide polymer which is not known so far to crystallize spontaneously. Here we show that the controlled drying of wetting films of the aqueous solution of this biopolymer can lead to the formation of large-scale self-assembly and crystallization structures. We show that the structures which grow exclusively within the final drying spot arise from the ordered assembly of the concentrated polysaccharide chains mediated by the condensation of trace cations present in the system. Moreover, we show that these trace cations, which are dragged with the polysaccharide chains and concentrated by the drying process inside the residual spot, come mainly from the ultrapure water used for the experiment. This study does not bring only new insights on the behavior and structural properties of polysaccharide polymers. It also shows that the use of water, even of high purity (conductivity e1 μS 3 cm -1 ), can lead to singular and unexpected effects when dealing with sensitive materials and experiments (polysaccharide/DNA, drying drops).

Magnetic tweezers were used to study the passive and active response of macrophages to local centripetal nanonewton forces on β1 integrins. Superparamagnetic beads coated with the β1-integrin-binding protein invasin were attached to J774 murine macrophages to mimic phagocytosis of bacterial pathogens. Forces exceeding ∼0.5 nN induce the active formation of trumpet-like protrusions resembling pseudopodia after an initial elastic deflection and a response time of ∼30 seconds. The speed of advancement of the protrusion is <v>=0.065±0.020 μm second-1 and is force independent. After saturation (after about 100 seconds) the protrusion stops abruptly and is completely retracted again against forces exceeding 5 nN with an effective relaxation time of ∼30 seconds. The active protrusion is tentatively attributed to the growth of the actin cortex in the direction of the force, and evidence for the involvement of actin is provided by the finding that Latrunculin A abolishes the activated cone growth. The growth is assumed to be activated by cell signaling mediated by the invasin-specific integrins (exhibiting β1 chains) and could play a role in phagocytic and protrusive events during immune response by macrophages.

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