Electrophoresis cell electroosmosis

The motion of electrically charged particles or molecules in a stationary medium under the influence of an electric field is called electrophoresis. In such transport the electric force is applied through a potential difference between electrodes. Selective use of the Lorentz force by applying a magnetic field can also induce such movement. Electrophoresis and electroosmosis are two key modaUties of electrokinetic transport which are very useful in micro- and nanofluidics for a variety of apphcations including biomedical (bio-NEMS, etc.), fuel cell, and micro total analysis systems (/r-TASs). In electroosmosis the bulk fluid moves due to the existence of a charged double layer at the solid-hquid interface. While one-dimensional electrophoresis is more commonly used, two-dimensional electrophoresis may also become a useful tool for the separation of gel proteins based on isoelectric property. 

Minor et al. [32] have analyzed the time dependence of both the electroosmotic flow and electrophoretic mobility in an electrophoresis cell. They concluded that, for most experimental conditions, the colloidal particle reaches its steady motion after the application of an external field in a much shorter time than electroosmotic flow does. Hence, if electrophoresis measurements are performed in an alternating field with a frequency much larger than the reciprocal of the characteristic time for steady electroosmosis (t 10° sec), but smaller than that of steady electrophoresis (t 10 sec), the electroosmotic flow cannot develop. In such conditions, electroosmosis is suppressed, and the velocity of the particle is independent of the position in the cell. Figure 3.6 is an example we measured the velocity of polystyrene particles in the center of a cylindrical cell using a pulsed field with the frequency indicated when the frequency is above 10 Hz, the velocity (average between the field-on and field-off values) tends to the true electrophoretic velocity measured at the stationary level. Another way to overcome the electroosmosis problem is to place both electrodes providing the external field, inside the cell, completely surrounded by electroneutral solution as no net external field acts on the charged layer close to the cell walls, the associated electroosmotic flow wiU not exist [33]. 

Electrokinetic stimulation may also be achieved by transport of immobilized indigenous or bioaugmented bacteria to biogeochemical niches of suitable chemical and environmental conditions (Fig. 2). Several studies have demonstrated centimeter- to meter-scale electrokinetic transport of bacteria and yeast cells through sand, soU, and aquifer sediments by either electrophoresis or electroosmosis. Transport direction and rates depend on the type of the subsurface matrix, the environmental conditions, and the size and... 

Four different electrokinetic processes are known. Two of them, electroosmosis and electrophoresis, were described in 1809 by Ferdinand Friedrich Renss, a professor at the University of Moscow. The schematic of a cell appropriate for realizing and studying electroosmosis is shown in Fig. 31.1a. An electrolyte solution in a U-shaped cell is divided in two parts by a porous diaphragm. Auxiliary electrodes are placed in each of the half-cells to set up an electric held in the solution. Under the inhuence of this held, the solution starts to how through the diaphragm in the direction of one of the electrodes. The how continues until a hydrostahc pressure differential (height of liquid column) has been built up between the two cell parts which is such as to compensate the electroosmotic force. 

The current was applied in the form of electrical pulses in order to permeabilize the membranes of the tumor cells for the entry of the chemotherapeutic agent bleomycin which is a very potent cytotoxic molecule. Clinical complete responses were achieved in 56.4% of the tumors and partial responses were observed in 28.9% of the tumors. This work is thus not strictly electrochemical treatment in the sense of Nordenstrom18 and Xin32 but is rather chemotherapy aided by electrochemical-driven movement of ions, molecules and drugs etc. (e.g., by electroosmosis, electrophoresis) into the tissue regions targeted for necrosis, as in several studies28,51,93 94 described earlier within this chapter. 

Finally, if heavy beads such as silica (SG = 2.1) are used, they can be assembled into a regular matrix by sedimentation [20]. One particular advantage of silica beads is that, after assembling them in a glass cell, sucrose can be added to the electrophoresis buffer to closely match the index of refraction of silica. This leads to a transparent material that is ideal for optical detection (at the expense of separation time, since sucrose increases the medium s viscosity, and thus reduces electrophoretic mobility). A second advantage is that some of the well-documented surface treatment strategies against electroosmosis developed for capillary electrophoresis can be directly transposed. 

Li and Harrison carried out the first cell assay in microchannels [2]. This seminal work made use of electrokinetically driven flow (electroosmosis and electrophoresis) to transport bacteria, yeast, and mammalian cells in channels and to implement low-volume chemical lysis (cell death). This theme of microfluidics-based cell transport, sorting, and lysis has continued to be a popular application, as well as related work in using microfluidics to culture cells and to pattern them into structures. The utility of these methods is acknowledged (and that they are featured in several good reviews [1] and other entries in the encyclopedia) but focuses here on describing microfluidics-based cell assays that fit the definition described above - application of a stimulus and measurement of a response. 

The most common method employed for particle electrophoresis is the use of closed, free-fluid electrophoresis chambers (5). In this method, particles suspended in a fluid medium are electrophoresed in an enclosed electrode chamber. Particle mobility is determined optically through the chamber wall. However, if the chamber surface is charged, electroosmosis is induced at the chamber walls due to the applied electric field. A hydrodynamic circulatory flow in the chamber thus results. This hydro-dynamic flow complicates mobility determination since the flow will impose upon the particle s mobility. This hydrodynamic flow must be taken into account. Particle mobility must either be measured in regions of the cell where there is no significant fluid flow, or the fluid velocity must be measured and subtracted from the observed particle velocity. 

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