Zdeněk Slouka

Advances in three-dimensional rapid prototyping of microfluidic devices for biological applications

The capability of 3D printing technologies for direct production of complex 3D structures in a single step has recently attracted an ever increasing interest within the field of microfluidics. Recently, ultrafast lasers have also allowed developing new methods for production of internal microfluidic channels within the bulk of glass and polymer materials by direct internal 3D laser writing. This review critically summarizes the latest advances in the production of microfluidic 3D structures by using 3D printing technologies and direct internal 3D laser writing fabrication methods. Current applications of these rapid prototyped microfluidic platforms in biology will be also discussed. These include imaging of cells and living organisms, electrochemical detection of viruses and neurotransmitters, and studies in drug transport and induced-release of adenosine triphosphate from erythrocytes.

Parametrical studies of electroosmotic transport characteristics in submicrometer channels.

Spatially two-dimensional nonequilibrium mathematical model describing electroosmotic flow through a submicrometer channel with an electric charge fixed on the channel walls is presented. This system is governed by the hydrodynamic, electrostatic, and mass transport phenomena. The model is based on the coupled mass balances, Poisson, Navier-Stokes, and Nernst-Planck equations. Nonslip boundary conditions are employed. The effect of an imposed electric field on the system behavior is studied by means of a numerical analysis of the model equations. We have obtained the following findings. If the channel width is comparable to the thickness of the electric double layer, the system behaves as an ion-exchange membrane and the dependence of the electric current passing through the channel on the applied voltage is strongly nonlinear. In the case of negatively (positively) charged walls, a narrow region of very low conductivity (so-called ionic gate) is formed in the free electrolyte near the channel entry facing the anode (cathode) side. For a wide channel, the electric current is proportional to the applied voltage and the velocity of electrokinetic flow is linearly proportional to the electric field strength. Complex hydrodynamics (eddy formation and existence of ionic gates) is the most interesting characteristics of the studied system. Hence, current-voltage and velocity-voltage curves and the corresponding spatial distributions of the model variables at selected points are studied and described in detail.

Transient behavior of an electrolytic diode

The transient behavior of an electrolytic diode system was studied. A gel-like electrolytic diode was incorporated in a capillary microfluidic chip. The microfluidic platform guaranteed a constant composition of solutions on the diode boundaries. The current responses of the electrolytic diode to step-like changes of the imposed DC electric voltage were measured. Some of these transients were accompanied by a short-time overshoot of electric current density. In order to explain this phenomenon, a mathematical model of the electrolytic diode system was developed. Dynamical analysis of the model equations confirmed the existence of the electric current overshoots. Because the results of the experimental and the numerical transient studies were quite similar, we have explained the physical meaning of three selected overshoots by means of an analysis of the reaction-transport processes inside the electrolytic diode system. The transient experiments carried out in this study indicate that our physical concept of the electrolytic diode system presented in previous papers is correct.

Dynamical and stationary analysis of an electrolyte diode and comparison with experiments

Abstract Stationary and non-stationary behavior of an electrolyte diode system is studied. The system consists of a microcapillary connecting two reservoirs with a strong acid and strong base. If an external electric field is imposed on the capillary, diode like current-voltage characteristics is observed. A thin layer of electrolyte with extremely large pH gradient is formed in the closed regime of the diode. Transient behavior of the electrolyte diode is studied experimentally and numerically. Observed short-time overshoots of electric current in the current-voltage characteristics are explained based on numerical analysis of the reaction-transport processes in the capillary.

Electrochemical characteristics of ideal polarizable interfaces with limited number of charge carriers.

Recent progress in material chemistry and surface engineering has led to emergence of new electrode materials with unique physical and electrochemical properties. Here, we introduce a physical model describing charging of ideal polarizable electrode-electrolyte interface where the electrode is characterized by a limited capacity to store charge. The analytical model treats the electrode and electrolyte phases as independent nonlinear capacitors that are eventually coupled through the condition of equality of the total stored electrical charge opposite in sign. Gouy-Chapman and condensed layer theories applied to a general 1:n valent electrolyte are used to predict dependencies of differential capacitance of the electrolyte phase and surface concentration of the electrical charge on the applied potential. The model of the nonlinear capacitor for the electrode phase is described by a theory of electron donors and acceptors present in conductive solids as a result of thermal fluctuations. Both the differential capacitance and the surface concentration of the electrical charge in the electrode are evaluated as functions of the applied potential and related to the capacity of the electrode phase to accumulate charge and its ability to form electron donors and acceptors. The knowledge of capacitive properties of both phases allows to predict electrochemical characteristics of ideal polarizable interfaces, e.g., current responses in linear sweep voltammetry. The coupled model also shows significant potential drops in the electrode comparable to those in the electrolyte phase for materials with low charge carrier concentrations.