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A new theoretical model is presented for the mechanical response of a thin-walled bio-mimetic microcapsule upon indentation using an atomic force microscope. The membrane is strained by both bending moments and membrane stress in the presence of a tensile pre-stress. When the intrinsic membrane stress is large compared to the local stress around the indentation dimple, the constitutive relation is linear; and when it is small, a cubic relation is deduced. The new theory is rigorously compared to the existing models of: (i) membrane under pure bending by Boulbitch; and (ii) membrane under pure stretching by Yao et al., which are shown to be two limiting cases of our general solution. Published experimental data in literature show that AFM measurement always involves deformation mode of mixed bending and stretching. [Copyright &y& Elsevier]

Adhesion of cells on biomaterial surface is resulted from the complex interplay of specific recognitions and colloidal interactions. Thus understanding the role of electrostatic interactions in bioadhesion may help to elucidate the physiochemical basis of cell signaling pathway on therapeutic devices. In this report, high-resolution reflection interference contrast microscopy, cross-polarized light microscopy and contact mechanics modeling are applied to probe the equilibrium adhesion of giant phospholipid vesicles on 3-amino-propyl-triethoxy-silane coated glass. Simultaneously, the effects of vesicle wall thickness, pH, osmotic stress and surface chemistry on the electrostatic interactions at the membrane–substrate interface are evaluated. The results show that both unilamellar vesicles (ULV) and multilamellar vesicles (MLV) strongly adhere on the cationic substrates at neutral pH. In the presence of electrostatic interactions, ULV is slightly deformed on the substrate as the dimension of its adhesive–cohesive zone is only 6–10% higher than the theoretical value of a rigid sphere with the same mid-plane diameter. The variances of contact angle and capillary length at different locations surrounding MLV are ten times higher than those of ULV. The adhesion energy of ULV with mid-plane diameter of 45 and 20 μm is determined as 3.8×10−12 and 8.6×10−12 J/m2, respectively, from the truncated sphere model. Moreover, the increase of osmotic stress induces irregular pattern in ULV''s adhesion disc and raises the adhesion energy by 10-fold. Finally, the reduction of pH further enhances the electrostatic attractions/repulsions between vesicle surface and cationic or anionic substrates and leads to an increase of adhesion strength. [Copyright &y& Elsevier]

High-resolution reflection interference contrast microscopy (HR-RICM) was developed for probing the deformation and adhesion of phospholipid vesicles induced by colloidal forces on solid surfaces. The new technique raised the upper limit of the measured membrane–substrate separation from 1 to 4.5 μm and improved the spatial resolution of the heterogeneous contact zones. It was applied to elucidate the effects of wall thickness, pH and osmotic stress on the non-specific adhesion of giant unilamellar vesicles (ULV) and multilamellar vesicles (MLV) on fused silica substrates. By simultaneous cross-polarization light microscopy and HR-RICM measurements, it was observed that ULV with the wall thickness of a single bilayer would be significantly deformed in its equilibrium state on the substrate as the dimension of its adhesive–cohesive zone was 29% higher than the theoretical value of a rigid sphere with the same diameter. Besides, electrostatic interaction was shown as a significant driving force for vesicle adhesions since the reduction in pH significantly increased the degree of deformation of adhering ULV and heterogeneity of the adhesion discs. The degree of MLV deformation on the solid surfaces was significantly less than that of ULV. When the wall thickness of vesicle increased, the dimension of contact zone was reduced dramatically due to the increase of membrane bending modulus. Most important, the adhesion strength of colloidal adhesion approached that of specific adhesion. Finally, the increase of osmotic stress led to the collapse of adhering vesicles on the non-deformable substrate and raised the area of adhesive contact zone. To interpret these results better, the equilibrium deformation of adhering vesicle was modeled as a truncated sphere and the adhesion energy was calculated with a new theory. [Copyright &y& Elsevier]

A microfluidic device was designed to investigate filtration of particles in an electrolyte in the presence of liquid flow. Polystyrene spheres in potassium chloride solution at concentrations of 3–100 mM were allowed to settle and adhere to a glass substrate. A particle free solution at the same concentration was then flushed through the microfluidic channel at 0.03–4.0 mL/h. As the hydrodynamic drag on the adhered particles exceeded the intersurface interaction with the substrate, “pull-off” occurred and the particles detached. Filtration efficiency, α, was shown to a function of both ionic concentration of the liquid medium and flow speed, consistent with a phenomenological model based on the classical DLVO theory. The results elucidates the underlying physics of filtration. [ABSTRACT FROM AUTHOR]