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Monolayers of simple fatty acids exhibit a rich polymorphism with at least seven solid phases. The reason for such a variety of phases is not known yet and therefore is the object of the current investigation. In the present work the pH of the subphase is varied between 2 and 10.5 and the influence of the degree of dissociation on phase transition pressures and temperatures of arachidic acid monolayers is investigated in detail in the range from 2 to 45 °C by means of Brewster-angle microscopy and surface pressure−area isotherms. The shift of phase boundaries is ascribed to the change in the steric, electrostatic, and bonding interactions between the lipid headgroups. The impressive correlation between the L2/Ov nearest neighbor to next nearest neighbor (NN/NNN) transition pressure and the position correlation length as functions of the pH value suggests that the L2/Ov NN/NNN transition in fatty acid monolayers is strongly influenced or even originates in headgroup bonding interactions. The influence of headgroup dissociation on the phase behavior of fatty acid monolayers is investigated in the absence of solutes that interact with the lipids. The comparison of the phase behavior of this system with that of a system, in which solutes are present that interact with the charged fatty acids, e.g., polymers or polyvalent metallic ions, should provide information on specific solute−lipid interactions separately from the effect of headgroup dissociation.

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.

Combining deflective dielectrophoretic barriers with controlled pressure driven liquid flows in microfluidic devices allows accurate handling of particles such as biological cells in suspensions. Working towards cell-based lab-on-a-chip applications, a platform permitting rapid testing of devices having different dielectrophoretic and fluidic subunits was developed. The performance of such a system is shown in the cases of (A) flooding a small number of immobilised cells with a dye and (B) transient buffer swapping of a large number of cells in flow. The transition times for moving cells from one reagent to the other are below 0.5 s in the case of flow-through cell dipping.