Nanonewton Magnetophoretic Microtrap Array for Microsystems.

Here we report on the development of a lab-on-chip that integrates a dense array of micrometer-sized magnetic traps, with each individual trap generating a magnetic force as high as a few nN on standard superparamagnetic beads. The composite materials embedding traps are prepared from the microstructural engineering of a mixture between iron microparticles and polydimethylsiloxane. This approach breaks with standard microfabrication technologies: it is inexpensive, relatively easy to implement, and offers the ability to modulate the magnetic properties of the composites on a customized basis. The magnetic forces acting on the superparamagnetic beads have been measured following two approaches: first, on-chip through the hydrodynamic determination of the holding magnetic force, simultaneously on a large population of traps; and second, ex situ, by atomic force microscopy equipped with a colloidal probe, on individual traps. The experimental results have been compared with calculations from finite element modeling. Despite the geometrical simplification of the modeled system, both experiments and calculations give consistent values of force, ranging from 0.5 to 5 nN. These findings show that in operando determination of forces is a robust method that gives a high throughput overview of the forces acting in the device. It further demonstrates that the use of such functional composite materials can be a relevant alternative to standard microfabrication technologies, as it leads to competitive magnetophoretic performances.

Electrokinetic biomolecule preconcentration using xurography-based micro-nano-micro fluidic devices.

In this paper we introduce a low cost rapid prototyping framework for designing Micro-Nano-Micro (MNM) fluidic preconcentration device based on ion concentration polarization (ICP) phenomenon. Xurography-based microchannels are separated by a strip of ion perm-selective Nafion membrane which plays the role of nanofluidic potential barrier for the negatively charged molecules. As a result, by using this rapid and inexpensive fabrication technique, it is possible to get preconcentration plugs as high as 5000 fold with an original symmetric electroosmotic flow (EOF) condition. Due to its simplicity and performance, this device could be implemented in various bioanalysis systems.

Self-Assembled Permanent Micro-Magnets in a Polymer-Based Microfluidic Device for Magnetic Cell Sorting.

Magnetophoresis-based microfluidic devices offer simple and reliable manipulation of micro-scale objects and provide a large panel of applications, from selective trapping to high-throughput sorting. However, the fabrication and integration of micro-scale magnets in microsystems involve complex and expensive processes. Here we report on an inexpensive and easy-to-handle fabrication process of micrometer-scale permanent magnets, based on the self-organization of NdFeB particles in a polymer matrix (polydimethylsiloxane, PDMS). A study of the inner structure by X-ray tomography revealed a chain-like organization of the particles leading to an array of hard magnetic microstructures with a mean diameter of 4 µm. The magnetic performance of the self-assembled micro-magnets was first estimated by COMSOL simulations. The micro-magnets were then integrated into a microfluidic device where they act as micro-traps. The magnetic forces exerted by the micro-magnets on superparamagnetic beads were measured by colloidal probe atomic force microscopy (AFM) and in operando in the microfluidic system. Forces as high as several nanonewtons were reached. Adding an external millimeter-sized magnet allowed target magnetization and the interaction range to be increased. Then, the integrated micro-magnets were used to study the magnetophoretic trapping efficiency of magnetic beads, providing efficiencies of 100% at 0.5 mL/h and 75% at 1 mL/h. Finally, the micro-magnets were implemented for cell sorting by performing white blood cell depletion.

Amplification of electro-osmotic flows by wall slippage: direct measurements on OTS-surfaces.

The control of water flow in Electrostatic Double Layers (EDL) close to charged surfaces in solution is an important issue with the emergence of nanofluidic devices. We compare here the zeta potential governing the electrokinetic transport properties of surfaces, to the electrostatic potential directly measured from their interaction forces. We show that on smooth hydrophilic silica these quantities are similar, whereas on OTS-silanized hydrophobic surfaces the zeta potential is significantly higher, leading to an enhanced electro-osmotic velocity. The enhancement obtained is consistent with an interfacial water slippage on the silanized surface, characterized by a constant slip length of approximately 8 nm independent of the salt concentration in the range 10(-4)-10(-3)M.

Adhesion between highly rough alumina surfaces: an atomic force microscope study.

The removal of contaminant particles in microelectronics processes now extends not only to the compounds themselves but also to the reactor pieces where they are fabricated. This raises new issues as both the particles and the reactor walls are highly rough and a maximum number of particles per unit area is tolerated. In this work we study the adhesion force of a sapphire particle onto alumina substrates of roughness ranging from 10 nm to 3 microm peak-to-peak on a 5 microm x 5 microm area, in water. A contribution of this work is the prediction of the statistic of the adhesion force by a fast running numerical computation, without any adjustable parameters. The perspective is to be able to predict the best conditions for removing contaminant particles.