Walking microdroplets collect viruses and bacteria

Miniaturised devices designed to carry out reactions and chemical analyses are often heralded as the future of chemistry. The fabrication of such systems, popularly called "labs on chip," is seriously challenging engineers. "You can make an excellent microfabricated analytical system, but to make it work you still have to prepare the sample in a proper way," says Dr Martin Jönsson-Niedziółka from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS), adding: "Assume we have viruses and bacteria on a surface. Would we try to simply wash the surface with water, and then introduce a droplet of that water into a microanalyser, the result would be poor. Just because the concentration of the pollutants will be too low."

To ensure the highest possible concentration of bioparticles, it is better to use a small water droplet, just a microliter in volume. Such a droplet is introduced between two plates: a lower one with pollutant particles earlier deposited electrostatically, and a top one, coated with a system of small electrodes and an insulating layer (protecting against current flow through the droplet, which could lead to electrolysis). By making use of a phenomenon called electrowetting and applying voltage in a proper way, the droplet can be precisely displaced over the surface and so collect bioparticles from the entire plate. An additional advantage of the method is that the sample collected is already in the liquid state as required by many measurement methods. Moreover, as the displacement of the microdroplet is easy to control, the problem of how to deliver the sample to further lab-on-chip components disappears.

The systems with microdroplets have been fabricated and tested for a couple of years. A group of researchers from the Institute of Physical Chemistry of the PAS (Dr Martin Jönsson-Niedziółka) and the French Institut d'Electronique, de Microélectronique et de Nanotechnologie (Dr Vincent Thomy) and the Institut de Recherche Interdisciplinaire (Dr Rabah Boukherroub) had doubts about the fact that all earlier reports on microdroplets described the collection of latex microspheres from the surface. These particles are used because they are easily available in various sizes and safe in tests. It was not clear if the microdroplets would equally efficiently collect real bioparticles, such as bacterial spores or viruses.

The study made use of a microdevice fabricated by the French group. The particles investigated in tests included inactive bacteriophage MS2 (virus that infects bacteria), Bacillus atrophaeus spores and OA (ovalbumin) proteins. The bioparticles were deposited on two different surfaces. One of them was a hydrophobic surface, coated with a substance that resembles Teflon. The other plate was fabricated using nanowires (1 micrometer long) and its hydrophobicity was close to that characteristic for the famous lotus leaves. Until now, such superhydrophobic surfaces were not studied in any microdevices making use of electrowetting.

In experiments with viruses, the type of surface they were deposited on did not have any significant effect on the microdroplet cleaning efficiency. The cleaning efficiency was 98-99% and was higher than that typical for latex particles (92-93%). Different results have been found, however, for spores and protein molecules. High cleaning efficiency was found here for superhydrophobic surface only (99 and 92%, respectively), whereas the corresponding figures for hydrophobic surface were distinctly lower (46% and 71%, respectively).

The findings published in the journal Lab on a Chip demonstrate that the surface cleaning efficiency using microdroplets strongly depends on both the type of collected particles and the hydrophobic properties of the surface. "Everyone who wants to collect efficiently various bioparticles using microdroplets should use superhydrophobic surfaces," sums up Dr Jönsson-Niedziółka.