Bead microchannel

Adding additional surface. Whilst a high ratio of surface to volume is inherent in an empty/open micro-channel, this characteristic may be further pronounced by packing micro-beads in the channel (He et al. 2004). The introduction of beads within microchannels can introduce some practical difficulties, but a number of successful applications using this type of methodology have been described (He et al. 2004 Nikbin and Watts 2004). 

Efforts toward integrating SPE onto a lab-on-a-chip device are currently being investigated by the Collins group. Two complementary approaches are being pursued. One approach is to use small-diameter, Cl8 functionalized silica beads that are packed into a microchannel to form an extraction bed [46], A sample solution containing trace levels of explosives is electrokinetically directed across the microcolumn bed, causing the hydrophobic explosive molecules to adsorb onto the stationary phase with nearly 100% efficiency. Subsequently,... 

Hydrostatic flow (based on liquid level difference) has been used to introduce beads into microchannels. This method has been found to be superior to the use of HDF [959],... 

Affinity chromatography of streptavidin was performed on a PET chip. The microchannel was first filled with the dual-modified latex beads (as shown in Figure 6.3). The biotinylated beads were surface-modified with a temperature-sensitive polymer, poly(N-isopropylacrylamide (PNIPAAm, 11 kDa). When the temperature was raised above the lower critical solution temperature (LCST) of PNIPAAm, the beads aggregated and adhered to the channel wall, because of a hydrophilic-to-hydrophobic phase transition. Then streptavidin from a sample solution was captured by these adhered biotinylated beads. Thereafter, when the temperature was reduced below the LCST, the beads dissociated and eluted from the channel wall together with the captured streptavidin [203],... 

DNA extraction has been achieved using silica beads (5 pm), which were packed in a glass microchannel and held by a sol-gel [913]. This method provided more reproducible extraction than a previous method in which extraction was performed without physically fixing the beads in microchambers [639]. 

FIGURE 9.14 Optical image of the xPEG-DA hydrogel microstructures photopolymer-ized within the microchannels for sealing off microchambers (B, C), which also contained weirs to retain the beads. Scale bar, 100 pm [960]. Reprinted with permission from the American Chemical Society. 

State two reasons why the microchannel bead-based immunoassay shortened the reaction time to 1% of the conventional microplate-based immunoassay method. [1021] (2 marks)... 

Micro-SMB separators have only been studied numerically (Subramani and Kurup, 2006), but one can think of ways to implement a real moving bed in a miniaturized version by applying a shifting magnetic field on, for example, magnetic resin beads or applying DEP on adsorbent particles, in a microchannel. 

Scheme 4.95 Schematic view of the microreactor in which two enzymes (glucose oxidase and horseradish peroxidase) immobilized on beads are located in different parts of the microchannel. (Reprinted with permission from [406], Copyright 2002 American Chemical Society.)...

With the help of this method, His-tagged L-lactate dehydrogenase was immobilized. By pumping pyruvic acid as substrate with NADH as cofactor, it was demonstrated that the enzyme was still active in the microchannel. In this case, cofactor was used up. Srinivasan et al. [433] incorporated PikC hydroxylase from Streptomyces venezuelae into a PDMS-based microfluidic channel with a similar approach. The enzyme was immobilized to Ni-NTA agarose beads with an in situ attachment, following the addition of the beads to the microchannel. This enabled the rapid hydroxylation of the macrolide YC-17 to methymycin and neomethymycin (Scheme 4.104) in about equal amounts with a conversion of >90% at a flow rate of 70nl/min. 

A newer and less expensive alternative to the microchannel plate is the microsphere plate (MSP). As illustrated in Figure 3.6, this electron multiplier consists of glass beads with diameters from 20 to 100 pm that are sintered to form a thin plate with a thickness of 0.7 mm. This plate is porous with irregularly shaped channels between the planar faces. The surfaces of the beads are covered with an electron emissive material and the two sides of the plate are coated to make them conductive. The operating principle of this electron multiplier is similar to that of the microchannel plate. A potential difference of between 1.5 and 3.5 kV is applied across the plate, with the output side of the plate at the more positive potential. When particles hit the input side of the microsphere plate, they produce secondary electrons. These electrons are then accelerated by the electric field through the porous plate and collide with other beads. Secondary electron multiplication in the gaps occurs and finally a large number of secondary electrons are emitted from the output side of the plate. 

In a conventional microtiter plate assay, a 1.5-mm movement would be necessary for the most distandy located antibody molecule to react with the antigen fixed on the surface of the well, since the liquid depth was 1.5 mm. On the other hand, the liquid phase of the microchannel filled with polystyrene beads was much smaller. The longest distance from an antibody molecule to the reaction-solid surface may be less than 20 pm. Diffusion time is proportional to the squares of the diffusion distance, so the diffusion time of the antibody molecule to the antigen in the microchip would be more than 5600 times shorter than the conventional method. 

As a flnal example, it should be pointed out that it is also possible to circumvent the need of immobilization of the biorecognition element in the microchannel system. Instead, it can be immobilized on superparamagnetic beads, silica beads, latex particles, etc. [79-81]. These beads are applied together with the sample into the microchannel system and can be collected on or near the transducer via magnetic or membrane separation. 

Xie et al. (1994) manufactured a microbiosensor with an integrated thermopile on a quartz chip because of the lower heat conductivity of quartz compared to silicon. The size of the whole sensor was 25.2 x 14.8 x 0.6 mm, and immobilized enzymes (CPG beads) were were placed in the chips microchannels. The sensor was applied to glucose analysis in the 2 to 25 mmol/1 range, using only 1 pi samples. Due to clinical analysis requirements, the sensor might be interesting for blood glucose measurements. 

The microchannel emulsion technique has been extended to the formation of multiple emulsions [158-163], encapsulation [123, 158, 164—166], polymer bead formation [123, 125, 167-169], demulsification [116, 158, 170], and even microbubble formation [171]. New methods of stabilizing emulsions have also been investigated in this realm, including particle-stabilized [172] and protein-stabilized emulsions [173], with some work in emulsification without surfactants [135,146]. In the case of multiple emulsions, microchannel architecture can enable the formation of W/O/W emulsions in which two water droplets of different compositions can be encased in the same oil droplet [163]. 

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