Microfluidic flow system

The most recent variation of FIA is the so-called lab-on-a-valve (LOV) technology introduced by Ruzicka. The LOV idea incorporates an integrated microconduit on top of the selection valve u.sed in SIA. The nii-croconduit is designed to handle all the unit operations needed for a given analytical procedure. Mixing points for analyte and reagents, column reactors, bead reactors, separation columns, and membranes can all be accommodated within the LOV system. In some cases, microfluidic flow systems are used in ways similar to those described in the next section. 

On-chip analyte transport can be realized by integrating microfluidic flow systems or miniaturized gas chambers with the sensor device [69]. Hu et al. demonstrated a microfluidic sensor device monolithically integrated with planar Ge-Sb-S ChG waveguides [70]. Quantitative chemical sensing via evanescent wave absorption spectroscopy was demonstrated using the microfluidic device. 

Miniaturisation of various devices and systems has become a popular trend in many areas of modern nanotechnology such as microelectronics, optics, etc. In particular, this is very important in creating chemical or electrochemical sensors where the amount of sample required for the analysis is a critical parameter and must be minimized. In this work we will focus on a micrometric channel flow system. We will call such miniaturised flow cells microfluidic systems , i.e. cells with one or more dimensions being of the order of a few microns. Such microfluidic channels have kinetic and analytical properties which can be finely tuned as a function of the hydrodynamic flow. However, presently, there is no simple and direct method to monitor the corresponding flows in. situ. 

A novel 24-channel HPLC by Nanostream called Veloce was introduced at PITTCON 04. The column cassette contains 24 parallel microbore columns. The eluted samples are detected by a 24-channel UV filter photometer. The advantage of such a system is that it allows one to work with multiple samples simultaneously. Other interesting systems for parallel HPLC were those introduced by Eksigent, based on microfluidic flow control, and Sepiatec GmbH, which allows the processing of 75 multiple-well plates. 

An alkaloid pain reliever, morphine, is an often abused drug. Chronoampero-metric MIP chemosensors have been devised for its determination [204]. In these chemosensors, a poly(3,4-ethylenedioxythiophene) (PEDOT) film was deposited by electropolymerization in ACN onto an ITO electrode in the presence of the morphine template to serve as the sensing element [204], Electrocatalytic current of morphine oxidation has been measured at 0.75 V vs AglAgCllKClsat (pH = 5.0) as the detection signal. A linear dependence of the measured steady-state current on the morphine concentration extended over the range of 0.1-1 mM with LOD for morphine of 0.2 mM. The chemosensor successfully discriminated morphine and its codeine analogue. Furthermore, a microfluidic MIP system combined with the chronoamperometric transduction has been devised for the determination of morphine [182] with appreciable LOD for morphine of 0.01 mM at a flow rate of 92.3 pL min-1 (Table 6). 

FIGURE 3.33 Geometry and hydrodynamic characteristics of the microfluidic capillary system (CS). (a) top view of a CS. (b) The flow of liquid (arrows) is superimposed on the cross section (not to scale) of the CS. CRV capillary retention valve CP, capillary pump [459]. Reprinted with permission from the American Chemical Society. 

The Lab-on-Chip flow system comprises several miniaturised components, such as imprinted manifolds and electro-osmotic pumps, integrated within a single device. It is within the family of microfluidic techniques further discussion is outside of the scope of this monograph. 

Qualitatively, the operation of the microfluidic flow-focusing system can be described in the following way. Two immiscible phases (e.g. Nitrogen and water, or water and oil) are delivered via their inlet channels to the flow-focusing junction. In this junction, one central inlet channel, that delivers the fluid-to-be-dispersed (e.g. Nitrogen to be dispersed into bubbles) ends upstream of a small constriction (an orifice). From the sides of the central channel, two additional ones terminate upstream of the orifice. These side channels deliver the continuous fluid (e.g. aqueous solution of surfactant). It is important that these continuous phase wets the walls of the microfluidic device preferentially. Otherwise - if the fluid-to-be-dispersed - wets the walls, the resulting flows are erratic [16] and it becomes virtually impossible to form bubbles (droplets) in a reproducible and controllable process. 

The usual way of feeding the microfluidic systems with fluids is to apply either a constant rate of inflow into the chip, or a constant pressirre at the inlet [20]. Formation of droplets or bubbles in systems with such, fixed, boundary conditions for flow is realtively well understood. Two microfluidic geometries are most commonly used a microfluidic T-junction [1] or a microfluidic flow-focusing geometry [6]. 

We first introduce the basic concepts in physics of flow of simple and multiphase fluid in networks of microchannels. We then go on to demonstrate the phenomenology of the flow of droplets through the simplest network - a single loop of channels - and then provide examples of experiments on more complicated systems. The third part of the lecture introduces the subject of modeling of the dynamics of flow of units of resistance through networks of conductors, and show the results of these efforts and their correspondence to microfluidic flows. Finally, we provide an introduction to the subject of automation of flows of droplets in microchannels and demonstrate an example of the droplet-on-demand system constmcted in our laboratory. 

T. Nisisako, T. Torii, T. Takahashi, and Y. Takizawa, Synthesis of monodisperse bicolored janus particles with electrical anisotropy using a microfluidic co-flow system. Advanced Materials, 18, 1152—(-, (2006). 

The front-tracking method is only one example of computational tools that can be used in analysis and design of microfluidic systems. The computational methods for multiphase/fluid flows have been matured enough that they can be safely used as a design tool in microfluidics. In addition, they can be also very useful to discover or understand new flow physics emerging from the miniaturization of flow systems. 

Digital microfluidic architecture is under software-driven electronic control, eliminating the need for mechanical tubes, pumps, and valves that are required for continuous-flow systems. The compatibility of each chemical substance with the electro-wetting platform must be determined initially. Compatibility issues include the following (1) Does the liquid s viscosity and surface tension allow for droplet dispensing and transport by electrowetting ... 

At the same time, much simpler yet very successfiil microfluidic analysis systems based on wettable fleeces emerged First very simple dipsticks for e.g. pH measurement based on a single fleece paved the way for more complex test strips that have been sold as lateral-flow tests in the late 1980s [14]. Examples that are still on the market today are test strips for pregnancy [15], drug abuse [16-18], cardiac markers [19] and also upcoming bio-warfare protection [20]. 

Figure 7.1 Schematic of a microfluidic LC system. (A) Sample loading (B) sample analysis. 1A and IB, pumping channels 2A and 2B, eluent inlet reservoirs 3, eluent outlet reservoir 4, double-T injector that contains the sample plug 5, separation channel 6, sample reservoir 7, sample waste reservoir 8, sample inlet channels 9, sample outlet channels 10, ESI capillary emitter 11, LC waste reservoir. Note arrows indicate the main flow pattern through the system. (Reprinted with permission from ref. 33).

From the vantage point of microfluidics, the structures developed by Petersen et al [33] are the most appropriate. More recently, Baltes and coworkers combined CMOS circuitry with the microfabrication of sensors to construct a thermal mass flow system based on thin-film pyrometers [66]. As free standing mass flow sensors, they have attractive features. However, all of these silicon-based devices operate at relatively high temperatures in the 100-200 °C range. This elevated temperature limits their potential application in more complex microfluidic systems. The ideal flow sensor would be a very-low-temperature element that could be used on the walls of the microchannel. 

Yoshida et al. [55] and Nakano et al. [73] tested ER effects in SU-8 channels with indium tin oxide electrodes, for self-control of ERF in hard substrates. Niu et al. designed a GERF-based microfluidic valve responsive to external DC signals and was able to develop this concept as a system for microfluidic flow control free of any limitations of flow type [74]. 

Renzi et al. demonstrated a hand-held microchip-based analytical instrument for detection of proteins [41]. Recently, a portable microfluidic flow cytometer with simultaneously detection of fluorescence and impedance was reported for cell analysis [42]. This system exploited an LED for excitation and detected fluorescent emission with a solid-state photomultiplier (SSPM). 

Apart from chemical cues, shear stress also plays an important part in the proliferation and differentiation process of stem cells [116]. Continuous microfluidic perfusion systems not only offer control over the cell-media interaction but the mechanical forces applied during flow as well. As such, the flow rate has to be... 

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