Microfluidic devices interfaces

This entry reviews the physicochemical properties of microfluidic devices, mechanisms of biomolecular adsorption and denaturation at microfluidic device interfaces, and methods to minimize or to prevent biomolecular adsorption. The trade-offs between methods to reduce water stiction will also be discussed. 

Fredrickson CK, Fan ZH (2004) Macro-to-micro interfaces for microfluidic devices. Lab Chip 4 526-533... 

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

Plastic microdevices for high-throughput screening with MS detection were also prepared for detection of aflatoxins and barbiturates. These devices incorporated concentration techniques interfaced with electrospray ionization MS (ESI-MS) through capillaries [2], The microfluidic device for aflatoxin detection employed an affinity dialysis technique, in which a poly (vinylidene fluoride) (PVDF) membrane was incorporated in the microchip between two channels. Small molecules were dialyzed from the aflatoxin/antibody complexes, which were then analyzed by MS. A similar device was used for concentrating barbiturate/antibody complexes using an affinity ultrafiltration technique. A barbiturate solution was mixed with antibodies and then flowed into the device, where uncomplexed barbiturates were removed by filtration. The antibody complex was then dissociated and electrokinetically mobilized for MS analysis. In each case, the affinity preconcentration improved the sensitivity by at least one to two orders of magnitude over previously reported detection limits. 

In addition to conventional pressure driven flow, electrokinetic flow is also a commonly used means of transporting liquids in microfluidic devices. One type of electrokinetic flow, electroosmotic flow, relies on the presence of an electrical double layer at the solid-liquid interface. A negatively charged surface in a flow channel will attract cationic species from the fluid to form an electrical double layer at the surface. Application of an external voltage can pull those cationic species through the flow channel inducing bulk flow. The electroosmotic flow velocity can be described... 

Figure 6. Experiments on time-resolved tracking of the shape of the gas-liquid interface during the process of formatiorr of a single bubble in a microfluidic flow-focusing device. From a video recording of the process of break-up, we extract the projection of the interface on the x-y plarre (plane of the microfluidic device). We then extract the minimum width of the neck as a functiorr of time (Adapted Ifom Ref [21]).

Fluidic I/O Loading samples and reagents on chip requires an interface between the microfluidic device and the outside world. Strategies for introducing samples and reagents onto a microfluidic chip are usually not discussed at length by workers in the field. Typically droplets are pipetted onto EWD chips and then the top plate is applied to close the system [14,... 

Mass spectrometry (MS) is one of the most powerful detection techniques used in liquid-phase analyses,1 mainly due to the ease of interfacing with separation techniques such as capillary electrophoresis (CE)2,3 and high-performance liquid chromatography (HPLC).4 Due to its sensitivity and applicability to a wide variety of chemical and biochemical species, MS is also used for the analysis of (bio)chemical molecules processed in microfluidics devices.5,6 Electrospray ionization (ESI)7 10 is often used to transfer samples from microfluidics chips to a mass spectrometer, involving analyte ionization directly from solutions and operating at flow rates typically used in microfluidics devices.11 Due to its effectiveness, the use of chip-MS coupling has rapidly spread in many research areas with bioanalytical applications,12 such as the... 

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