Microfluidic systems, beads

Oleschuk, R. D., L. L. Shultz-Lockyear, Y. Ning, and D. J. Harrison. Trapping of bead-based reagents within microfluidic systems On-chip solid-phase extraction and electrochromatography. Anal. Chem. 72, 585-590 (2000). 

A microfluidic system was made consisting of a Y-type channel structure and a packed bed of micro beads in a zone in the center of the outlet channel [162]. At the end of the micro-bead bed a weir is placed. The outlet channel then reopens to a larger flow zone which has several parallel micro channels, splitting the main flow into many sub-streams. These micro channels serve as spatially addressable detection lanes. 

M 86] [P 76] Flow-pattern analysis without micro beads was performed to ensure that laminar-flow properties are maintained throughout the whole microfluidic system, especially concerning the weir conduit [162], Fluorescence images taken at the Y-contact, at the empty channel and at the parallel detection lanes prove that bi-... 

Beads in microfluidic systems are an interesting evolution of the homogeneous phase bead biochips. Indeed, examples of microfluidic platforms in which biospecific molecules are immobilized on an internal channel surface are still rare [26]. 

Besides the synthesis of bulk polymers, microreactor technology is also used for more specialized polymerization applications such as the formation of polymer membranes or particles [119, 141-146] Bouqey et al. [142] synthesized monodisperse and size-controlled polymer particles from emulsions polymerization under UV irradiation in a microfluidic system. By incorporating a functional comonomer, polymer microparticles bearing reactive groups on their surface were obtained, which could be linked together to form polymer beads necklaces. The ability to confine and position the boundary between immiscible liquids inside microchannels was utilized by Beebe and coworkers [145] and Kitamori and coworkers [146] for the fabrication of semipermeable polyamide membranes in a microfluidic chip via interfacial polycondensation. 

Another unique silica-based approach to microscale DNA extraction currently underdevelopment utilizes a serpentine channel design, combined with an immobilized silica-bead solid phase and fluidic oscillation. This method, developed by Chung et al., relies on silica beads immobilized on the plasma-oxidized surface of the polymethylmethacrylate (PMMA) channels, instead of a packed-silica solid phase, as depicted in Figure 43.1c. Following bead immobilization, the solutions required for DNA binding, purification, and release are flowed back-and-forth through the device. This fluidic oscillation over the immobilized phase results in marked improvement of recovery and extraction efficiency over the same extraction methods with free beads. This method represents yet another variation of silica-based purifications that has been accomplished in microfluidic systems, exploiting previously optimized chemistries. In summary, the development of macroscale, commercial, silica SPE protocols has enabled the facile translation of DNA, and now RNA, extraction into microfluidic systems for a variety of applications. 

The intention of the device presented by Lilliehorn et al. was to utilize the miniaturized transducers in combination with the laminar flow conditions in microfluidic systems to create a dynamic arraying device holding several individually controlled transducers as envisioned in Figure 44.21. Here, an array of 16 transducers are loaded with different functionalized beads in a first step. The beads are acoustically trapped in the positions given by the transducers. In a second step, samples are supplied through the orthogonal channels and the response is, for example, read by fluorescence. The transducers are then deactivated and the beads are flushed out of the device and the whole procedure is repeated again for a new set of beads and samples. 

Emulsions and microfluidic structures have been used for many purposes, including fusion of reactants present in two droplets, preparation of gel beads, preparation of multiple emulsions, etc. for a comprehensive overview, please consult the review paper of Vladisavljevic and colleagues [1]. Besides this, the microfluidic systems discussed in this entry can be used as analytical tools in various ways. To illustrate this, the use of Y-shaped junctions for dynamic interfacial tension measurement [14] and the use of T-junctions in combination with a coalescence chamber for emulsion stability research [15] are discussed next. 

Filters with pore size 1 pm located within microfluidic systems/channels, typically used to separate blood cells from plasma and to cmifine micrometer-sized beads within a microfluidic system. 

Piezoelectric Actuation in Multiphase Microfluidics, Fig. 7 (a) A microscope image of a lateral cavity acoustic pump with the main channel filled with DI water and polystyrene beads, (b) The pumping pressure generated by a 1-, 5-, and 10-cavity pair device with a maximum input voltage of 25 Vpp. The pressure output is calculated using the measured average flow velocity and the calculated hydrodynamic resistance of the microfluidic system. 

One solution to the challenges of mixing proteins in microfluidic systems is to immobilize proteins in microreactors [5]. These systems typically consist of chambers of enzymes immobilized on beads, micropillars [6], or porous polymer monoliths [7] (Fig. 2a and b). Such systems have large surface area-to-volume ratios, which minimize diffusion time for reactions with solution-phase reagents. Microreactors can be used either for the conversion of an analyte to another form that is more easily detected or for direct studies of the properties of enzymes and substrates. One of the most common uses is for the digestion of proteins for proteome profiling, but such systems can also be used for the removal of amino acid residues from peptides or proteins or for enzyme kinetic studies. 

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