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Lab-on-a-chip News: A self-organizing nanoparticle-based molecular sieve is developed to identify and separate DNAs or cells

March 21, 2002

Because living organisms contain millions of different molecules, identifying or separating any single one of these from their natural environment in order to carry out research work or perform diagnoses is quite like looking for a needle in a haystack. A number of molecular separation technologies are of course available, and are used by laboratories on a daily basis, but they are often unwieldy and costly. Scientists the world over are therefore attempting to develop a new generation of analytic devices, known as labs-on-a-chip, where all the technological phases of laboratory work are integrated into speedy automated procedures, in what can be deemed to be a single sample to diagnosis step.

CNRS scientists (1) working at the Institut Curie, together with an ESPCI team, have broken new ground in this field, coming closer to such systems with a technology they have called Ephesia (2) . Combining knowledge and tools developed in physics, chemistry, and biology, they have developed an original approach based on the use of self-organizing nanospheres, which handle the key molecule sorting phase within these chips. This new technology paves the way to a whole field of applications both in genetics and in biochemistry, ranging from the study of molecules to medical diagnostics, in particular in oncology with a view to detecting mutations or micrometastases. These new results are to be published by Science magazine on March 22.

A wide range of labs-on-a-chip using very different concepts and materials are currently being developed the world over. The basic idea which they all share is that the various component phases involved in the analysis of given samples are conducted within microchannels (ranging from one tenth to one hundredth of a millimeter) etched onto a microchip. The samples and the substances used to process them, with a view to extracting specific molecules, are injected into these channels and moved about using micropumps, ultra-small pneumatic systems, and electric fields. The device developed by the Institut Curie team is based on a silicone rubber wafer with a 4 cm diameter, within which fine channels have been moulded. This medium was initially developed by G.M. Whitesides at Harvard University in the United States and is well suited for mass production because of its low cost. One of the major issues in developing a lab-on-a-chip involves building molecule-sorting sieves that will operate within these microchannels. This is the problem to which the Institut Curie and ESPCI teams have provided an original solution, interfacing physics, chemistry, and biology.

Ephesia, a novel molecular sieve

Teams headed by Jean-Louis Viovy at the Institut Curie (joint CNRS/Institut Curie research unit UMR 168 "Physico-chimie Curie") and Jérôme Bibette at the ESPCI (Colloids and Nanostructures Laboratory) decided to use magnetic nanospheres (under one thousandth of a millimeter small) which work as tiny magnets in a water-based suspension. When no magnetic field is applied, the nanoparticles do not have any special orientation, and float about in a liquid medium. They are therefore easy to introduce into a channel, however small. When a magnetic field is applied to this suspension, the particles line up following the field lines, and form rigid columns barring the microchannel with a regular array of obstacles. This new device, a product of magnetic nanoparticle chemistry and the corresponding physics, can therefore effectively and spontaneously self-organize a sieve within the chip, in order to separate biological molecules. The molecules are simply introduced into the device, using a very low voltage electric field, and the ancillary channels etched onto the chip. An electric field is also used to push the molecules through the sieve.

A nanoscopic hurdle race

The hurdle race then begins and as in all races, not all runners will cross the finish line at the same time. As larger molecules are the slowest because of the trouble they have making their way around the hurdles, sorting can be performed according to size. In order to see the molecules, an optical system has been devised which catches them when they cross a light beam barring the channel just after the obstacle array. This original approach has three major advantages as compared to all other technologies traditionally used in laboratories (see "Electrophoresis Technologies"). On the one hand, Ephesia is reversible: as mentioned earlier, the obstacle array spontaneously appears when a magnetic field is applied, but it returns to a liquid suspension immediately upon field switch-off. The sieve can therefore easily and automatically be replaced in between runs, thus avoiding deterioration or contamination. On the other hand, Ephesia is adjustable: sieve size can be adapted to that of the molecules or objects to be separated. Finally, Ephesia is very convenient for implementation on miniature devices as the sieve can be set up remotely and without direct contact, through the application of a magnetic field.

From gene analysis to the detection of micrometastases

Ephesia technology developed by Jean-Louis Viovy?s team is currently relevant to large-size DNAs. Up till now, separating these very large molecules involved using a slow and tedious technology known as pulsed-field electrophoresis (which would take about 24 hours). The new technology is a considerable leap forward, insofar as separation time has been brought down to 20 minutes for DNAs with 10 to 200 kilobase pairs, or kbp.

The team?s goal is now to reach the 500 kpb level as soon as possible, in order to allow for the speedy and convenient analysis of all DNAs contained in gene banks, the contents of which are growing ever faster in the framework of major genomics projects. These large DNAs are inter alia involved in research into the large-scale genomic changes occurring in cancer cells.

Other future application of Ephesia are also being contemplated. Jean-Louis Viovy?s team is thus attempting to graft antibodies onto the nanospheres so that they can lock on to specific cell types. Another very promising project was recently initiated in Jean Paul Thiery?s laboratory at the Institut Curie (UMR 144 CNRS/Institut Curie). It involves using the new technology to identify residual cancer cells known as micrometastases that may survive in some patients following initial treatment, in order to help physicians eliminate such cells before they trigger a possible recurrence of the disease.

Scientists at the Institut Curie are also looking into integrating this new concept into a genuine, entirely automated lab-on-a-chip. They hope to develop small, easy to use, economical and speedy devices for biologists and physicians alike, which will have better analytic and diagnostic capabilities than the most sophisticated laboratory apparatus currently available.

(1) CNRS Department of Physics and Mathematics, and CNRS Department of Chemistry. (2) After the Temple to Artemis with numerous columns, which was repeatedly destroyed and rebuilt during Antiquity.

Institut Curie