Chaotic mixing using source-sink microfluidic flows in a PDMS chip

We present an active fixed-volume mixer based on the creation of multiple source-sink microfluidic flows in a polydimethylsiloxane (PDMS) chip without the need of external or internal pumps. To do so, four different pressure-controlled actuation chambers are arranged on top of the 5μl volume of the mixing chamber. After the mixing volume is sealed/fixed by microfluidic valves made using ‘microplumbing technology', a virtual source-sink pair is created by pressurizing one of the membranes and, at the same time, releasing the pressure of a neighboring one. The pressurized air deforms the thin membrane between the mixing and control chambers and creates microfluidic flows from the squeezed region (source) to the released region (sink) where the PDMS membrane is turned into the initial state. Several schemes of operation of virtual source-sink pairs are studied. In the optimized protocol, mixing is realized in just a sub-second time interval, thanks to the implementation of chaotic advectio

Chaotic mixing using source-sink microfluidic flows in a PDMS chip

We present an active fixed-volume mixer based on the creation of multiple source–sink microfluidic flows in a polydimethylsiloxane (PDMS) chip without the need of external or internal pumps. To do so, four different pressure-controlled actuation chambers are arranged on top of the 5 μl volume of the mixing chamber. After the mixing volume is sealed/fixed by microfluidic valves made using ‘microplumbing technology’, a virtual source–sink pair is created by pressurizing one of the membranes and, at the same time, releasing the pressure of a neighboring one. The pressurized air deforms the thin membrane between the mixing and control chambers and creates microfluidic flows from the squeezed region (source) to the released region (sink) where the PDMS membrane is turned into the initial state. Several schemes of operation of virtual source–sink pairs are studied. In the optimized protocol, mixing is realized in just a sub-second time interval, thanks to the implementation of chaotic advection

Optical Microscopy in the Nano-World

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland

Understanding the mixing process in 3D microfluidic nozzle/diffuser systems: simulations and experiments

We characterise computationally and experimentally a three-dimensional (3D) microfluidic passive mixer for various Reynolds numbers ranging from 1 to 100, corresponding to primary flow rates of 10-870 mu l min(-1). The 3D mixing channel is composed of multiple curved segments: circular arcs situated in the substrate plane and curved nozzle/diffuser elements normal to the substrate plane. Numerical simulation provides a detailed understanding of the mixing mechanism resulting from the geometrical topology of the mixer. These Comsol software-based simulations reveal the development of two secondary flows perpendicular to the primary flow: a swirling flow resulting from tangential injection of the flow into the nozzle holes and Dean vortices present in the circular arcs. These phenomena are particularly important at a Reynolds number larger than 30, where mixing occurs by chaotic advection. Experimentally, the 3D mixer is fabricated in a monolithic glass substrate by powder blasting machining, exploiting eroding powder beams at various angles of impact with respect to the substrate plane. Experimental mixing was characterised using two coloured dyes, showing nearly perfect mixing for a microfluidic footprint of the order of a few mm(2), in good agreement with the simulations

Bead-based protein microarrays realized through electrostatic self-assembly of carboxylated beads

We report here a simple method, based on electrostatic self-assembly, for realizing bead-based single protein microarrays. We generate positively-charged surface micro-patterns on a glass substrate and used them as the template for adsorbing negatively-charged carboxylated beads. The, positively-charged, template for bead adsorption is generated by patterning a aminosilane layer using conventional lift-off technique. The –COOH groups which are present on the patterned beads’ surface is activated using carbodiimide chemistry and then a protein of interest is covalently coupled to the bead surface. The patterned beads bind strongly to the aminosilane patterns and they are able to even withstand agitated rinse in de-ionized water. We also show here that the same method can be extended for bead patterning with in sealed microfluidic channels

Bead-based single protein micro-array realized through electrostatic self-assembly of carboxylated beads

We report here a simple method, based on electrostatic self-assembly, for realizing bead-based single protein micro-arrays. We generate positively charged surface micro-patterns on a glass substrate and use them as template for adsorbing negatively-charged carboxylated beads. The template for bead adsorption is generated by patterning an aminosilane layer using conventional lift-off technique. The –COOH moiety which is present on the patterned bead surface is activated using carbodiimide chemistry and then a protein of interest is covalently coupled to the bead surface. The patterned beads bind strongly to the aminosilane patterns and they are able to withstand agitated rinse in de-ionized water. We also show here that the same method can be extended for bead patterning within sealed microfluidic channels

Micropatterning of protein-functionalized magnetic beads on glass using electrostatic self-assembly

We demonstrate a simple and fast method for self-assembly-driven micropatterning of protein functionalized magnetic beads on glass. We use positively charged aminosilane micro-patterns as template for selective immobilization of streptavidin-coated beads by electrostatic interactions. We show that addition of a non-ionic surfactant to the bead solution increases the selectivity of the micropatterning process. Streptavidin-coated magnetic beads (1 micron and 2.8 microns in size) are immobilized, with high reproducibility and in a very short processing time (~ 30 minutes), in the form of stripes and dots, both on bare substrates and in situ in microfluidic channels. The arrangement and the number of immobilized beads can be controlled by tuning the aminosilane template size

Self-assembled melamine microlens arrays for immunofluorescence enhancement

Self-assembled dielectric microlenses are used for focusing of the fluorescent signal emitted from a surface-based immunoassay, performed on silane micropatterns as assay substrates, to enhance the detection signal. In our model system, a fluorescent immunocomplex is formed on (3-aminopropyl)triethoxysilane (APTES) microstructures, and then carboxyl-functionalized melamine microparticles with two different sizes are electrostatically self-assembled on the microstructures. Mouse IgG diluted in phosphate buffered saline (PBS) is used as model target antigen and easily detected down to a concentration of 2 ng/mL. We also present a detailed two-dimensional numerical study using the Finite Element Method (FEM)

Ultra-thick micro-optical components using the PRISM photosensitive flexopolymer

We present the photosensitive flexopolymer PRISM as a new promising material for the realization of thick optical components. The PRISM flexopolymer can be directly polymerized using conventional UV exposure and is simply developed in a water-based solution. A casting method is used to realize flexopolymer layers of a few millimetres thickness in a single application step. Optical components as thick as 2 mm have been fabricated using an exposure time of less than 1 min and a development time below 3 min. No baking process is required, making the process very fast and avoiding any temperature-induced stress problems. Due to its elastomeric nature, the material can be easily applied either on rigid or flexible supports. The good optical transmission of the PRISM flexopolymer in the 400–800 nm spectral range makes it a promising material for optical applications. Refractive index measurements are performed at different wavelengths in the UV–visible range and the flexopolymer refractive index dispersion behaviour is determined. Optical components such as right-angle prisms, penta-prisms and cylindrical lenses with thicknesses up to a few mm have been successfully fabricated