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Microfluidic devices are increasingly used to perform biological experiments on a single-cell basis. However, long-term stability of cell positions is still an issue. A novel biocompatible method for cell entrapment and release on a microchip is presented. It is based on the controlled formation of an alginate hydrogel by bringing two laminar flows of alginate and calcium ions in the range of 2 mM to 40 mM into contact. The resulting growth of a gel bar is used to enclose and immobilize yeast cells. Adding ethylenediaminetetraacetic acid (EDTA) to the alginate solution allows for control of the hydrogel growth, and by varying the ratio of Ca(2+) to EDTA concentrations gel growth or gel shrinkage can be induced at will. Trapped cells are released during shrinkage of the gel. The trapping efficiency for different cell speeds is investigated and the properties of gel growth are discussed using a diffusion model. Precise positioning of a single cell is demonstrated. The technique presented allows not only the reversible immobilization of cells under gentle conditions but also offers the potential of long-term cell cultures as shown by on-chip incubation of yeast cells. The procedure may provide a simple and fully biocompatible technique for a multitude of innovative experiments on cells in microsystems.

This work reports the demonstration of a thermopneumatic microfluidic actuator used to determine blood coagulation time. In this new system, the detection principle is the obstruction of a microfluidic channel by clotted blood. The sample actuation is done by aspiration due to air cooling in a cavity. We made a study of the position of the blood plug depending on the presence in the channel of coagulation reagent and observe a slowdown of the plug on a coated channel compared to a blank one.

A contactless microfluidic dosing system based on actuation by an electrochemical reaction has been designed and tested. The device is composed of two chambers separated by a polydimethylsiloxane (PDMS) plate. One chamber is loaded with the liquid sample to be dosed while the electrochemical reaction occurs in the second chamber. Due to the strict separation between the reaction responsible of the actuation and the sample, the latter is preserved from contamination and/or electrochemical modification. Silicon master microstructures were fabricated using clean-room technologies and subsequently replicated into the PDMS. A full microsystem has been fabricated and tested. Microliter sample solutions can be easily and reproducibly dispensed in a time frame of a few tens of seconds.