Marie Frenea-Robin

Microfluidic immunomagnetic cell separation using integrated permanent micromagnets

In this paper, we demonstrate the possibility to trap and sort labeled cells under flow conditions using a microfluidic device with an integrated flat micro-patterned hard magnetic film. The proposed technique is illustrated using a cell suspension containing a mixture of Jurkat cells and HEK (Human Embryonic Kidney) 293 cells. Prior to sorting experiments, the Jurkat cells were specifically labeled with immunomagnetic nanoparticles, while the HEK 293 cells were unlabeled. Droplet-based experiments demonstrated that the Jurkat cells were attracted to regions of maximum stray field flux density while the HEK 293 cells settled in random positions. When the mixture was passed through a polydimethylsiloxane (PDMS) microfluidic channel containing integrated micromagnets, the labeled Jurkat cells were selectively trapped under fluid flow, while the HEK cells were eluted towards the device outlet. Increasing the flow rate produced a second eluate much enriched in Jurkat cells, as revealed by flow cytometry. The separation efficiency of this biocompatible, compact micro-fluidic separation chamber was compared with that obtained using two commercial magnetic cell separation kits.

Micromagnet structures for magnetic positioning and alignment

High performance hard magnetic films (NdFeB, SmCo) have been patterned at the micron scale using thermo-magnetic patterning. Both out-of-plane and in-plane magnetized structures have been prepared. These micromagnet arrays have been used for the precise positioning and alignment of superparamagnetic nano- and microparticles. The specific spatial arrangement achieved is shown to depend on both the particle size and the size and orientation of the micromagnets. These micromagnet arrays were used to trap cells magnetically functionalized by endocytosis of 100 nm superparamagnetic particles. These simple, compact, and autonomous structures, which need neither an external magnetic field source nor a power supply, have much potential for use in a wide range of biological applications.

Contactless diamagnetic trapping of living cells onto a micromagnet array

this paper focuses on the application of magnetophoresis to a new cell patterning method. The principle was demonstrated by using a CoPt micromagnet array, producing regularly spaced magnetic traps where cells were confined without any contact under the effect of negative magnetophoresis. To obtain this effect, yeast cells (Saccharomyces cerevisiae), which are diamagnetic, were placed in an aqueous solution enriched in paramagnetic ions. Unlabeled (non-magnetic) cell manipulation by magnetophoresis requires the production of high magnetic field gradients, ensuring significant forces. Therefore, micromagnets are particularly interesting for our application, since the field gradient increases as magnet dimensions are reduced.

From Bipolar to Quadrupolar Electrode Structures: An Application of Bond-Detach Lithography for Dielectrophoretic Particle Assembly

We describe a new, simple process for fabricating transparent quadrupolar electrode arrays enabling large-scale particle assembly by means of dielectrophoresis. In the first step, interdigitated electrode arrays are made by chemical wet etching of indium tin oxide (ITO). Then, the transition from a bipolar to a quadrupolar electrode arrangement is obtained by covering the electrode surface with a thin poly(dimethylsiloxane) (PDMS) film acting as an electrical insulation layer in which selective openings are formed using bond-detach lithography. The PDMS insulating layer thickness was optimized and controlled by adjusting experimental parameters such as the PDMS viscosity (modulated by the addition of heptane) and the PDMS spin-coating velocity. The insulating character of the PDMS membrane was successfully demonstrated by performing a dielectrophoretic assembly of polystyrene particles using interdigitated electrodes with and without a PDMS layer. The results show that the patterned PDMS film functions properly as an electrical insulation layer and allows the reconfiguration of the electric field cartography. Electric field simulations were performed in both configurations to predict the dielectrophoretic behavior of the particles. The simulation results are in perfect agreement with experiments, in which we demonstrated the formation of concentrated clusters of polystyrene particles and living cells of regular size and shape.

Magnetic nanoparticle DNA labeling for individual bacterial cell detection and recovery

A culture independent approach was developed for recovering individual bacterial cells out of communities from complex environments including soils and sediments where autofluorescent contaminants hinder the use of fluorescence based techniques. For that purpose fifty nanometer sized streptavidin-coated superparamagnetic nanoparticles were used to chemically bond biotin-functionalized plasmid DNA molecules. We show that micromagnets can efficiently trap magnetically labeled transformed Escherichia coli cells after these bacteria were subjected to electro-transformation by these nanoparticle-labeled plasmids. Among other applications, this method could extend the range of approaches developed to study DNA dissemination among environmental bacteria without requiring cultivability of recombinant strains or expression of heterologous genes in the new hosts.

Tunable and label-free bacteria alignment using standing surface acoustic waves

This paper describes a new technique for focusing bacteria in a microfluidic channel and subsequently controlling their trajectory. Bacteria alignment is obtained using standing surface acoustic waves (SSAW) generated by two interdigitated transducer electrodes (IDTs) patterned on a piezoelectric wafer. The bacteria are focused in the standing wave pressure nodes, separated by half a wavelength, the electrode geometry and applied voltage frequency being chosen accordingly. Interestingly, the position of a pressure node can be modulated by introducing a phase shift between the electrical signals applied to both IDTs. The bacteria, trapped in this node, follow it and can therefore be deflected. This technique works with label-free bacteria in their culture medium and induces low power consumption, which is very interesting for portable devices.

A reproducible method for μm precision alignment of PDMS microchannels with on-chip electrodes using a mask aligner

This paper describes a reproducible method for μm precision alignment of polydimethylsiloxane (PDMS) microchannels with coplanar electrodes using a conventional mask aligner for lab-on-a-chip applications. It is based on the use of a silicon mold in combination with a PMMA sarcophagus for precise control of the parallelism between the top and bottom surfaces of molded PDMS. The alignment of the fabricated PDMS slab with electrodes patterned on a glass chip is then performed using a conventional mask aligner with a custom-made steel chuck and magnets. This technique allows to bond and align chips with a resolution of less than 2 μm.

Towards improved finite element modeling of SSAW-based acoustic tweezers

We present a simulation of acoustic tweezers that will improve our understanding of standing surface acoustic wave (SSAW) generation in microfluidic chips made of soft polymers such as polydimethysiloxane (PDMS). This simulation aims at providing a tool to improve acoustofluidic chip design. In this paper, we describe a finite element method (FEM) modeling approach of a complete acoustofluidic chip including two interdigitated transducers (IDTs) and a PDMS microchannel in between. The acoustic radiation force exerted on particles in the microchannel is computed for different phase shifts between the two input signals.

A new microfluidic device for electric lysis and separation of cells

This paper demonstrates the potential use of a new microfluidic device embedding thick electrodes for cell lysis and cell separation applications. The system consists of a microfluidic channel featuring conductive walls made of a polydimethylsiloxane (PDMS) matrix mixed with carbon nanoparticles. Cell lysis was performed electrically by applying square pulses across the channel width, which was monitored by fluorimetry. Lysed and unlysed cells showed different dielectrophoretic behavior under appropriate experimental conditions, which suggests that the developed device is suitable to perform both cell lysis and subsequent sorting of viable and dead cells.

Electromagnetic characterization of biological cells

This paper presents the most commonly used method to characterize individual biological cells on a dielectric point of view. It is a force based technique which lays on dielectrophoresis and/or electrorotation. First the principle of these phenomena are described and analyzed with an extension to magnetic forces at the micrometric scale level. Secondly we present an experimental setup which permits to acquire the dielectrophoretic spectrum which is a dielectric signature of a cell. The main dielectric parameters can be deduced by fitting the theoretical response of the cell issued from a dielectric model and the experimental data. At the end we present an improved fitting method which takes advantage of a sensitivity analysis based on a probabilistic approach.