I mmunoassays have long been widely used in a variety of applications, such as for medical diagnostics, pharmaceutical analysis, environmental, food safety testing, and for basic scientific investigations because of its simplicity, sensitivity, and specificity. Microfluidic systems, also well known as a ''lab-on-a-chip'' or a ''micro-total-analysis-system'' have attracted a lot of attention in the past two decades because of advantages associated with miniaturization, integration, and automation. A promising platform for the combination of these two technologies, microfluidic immunoassays, has been extensively explored in recent years. The aim of this article is to review recent advancements in microfluidic immunoassays. A brief introduction to immunoassays and microfluidic devices will include a literature review, followed by an in-depth discussion of essential techniques in designing a microfluidic-based immunoassay from different perspectives, including device substrates, sample/reagent transportation, surface modification, immobilization, and detection schemes. Finally, future perspectives on microfluidic immunoassays will be provided. These developments with microfluidic immunoassays may provide a promising tool for automatic, sensitive, and selective measurements in practical applications.
A new method for actively controlling the number of internal droplets of water-in-oil-in-water (W/O/ W) double-emulsion droplets was demonstrated. A new microfluidic platform for double-emulsion applications has been developed, which integrates T-junction channels, moving-wall structures, and a flow-focusing structure. Inner water-in-oil (W/O) single-emulsion droplets were first formed at a major T-junction. Then the droplets were sub-divided into smaller uniform droplets by passing through a series of secondary T-junctions (branches). The moving-wall structures beside the secondary T-junctions were used to control the number of the sub-divided droplets by selectively blocking the branches. Finally, doubleemulsion droplets were formed by using a flow-focusing structure downstream. Experimental data demonstrate that the inner and outer droplets have narrow size distributions with coefficient of variation (CV) of less than 3.5% and 5.7%, respectively. Double-emulsion droplets with 1, 2, 3, and up to 10 inner droplets have been successfully formed using this approach. The size of the inner droplets and outer droplets could be also fine-tuned with this device. The development of this new platform was promising for drug delivery applications involving double emulsions. Keywords Double emulsion Á T-junction Á Moving wall Á Microfluidics Á MEMS Abbreviations CCD Charge-coupled device CV Coefficient of variation DGI Dodecyl glyceryl itaconate DI Deionized EMV Electromagnetic valve HLB Hydrophilic-lipophilic balance MEMS Micro-electro-mechanical-systems O/W Oil-in-water PDMS Polydimethylsiloxane R 1 Volumetric flow rate of inner phase R 2Volumetric flow rate of medium phase R 3
A simple, sequential DNA pre-concentration and separation method by using a micro-CE chip integrated with a normally closed valve, which is activated by pneumatic suction, has been developed. The CE chip is fabricated using PDMS. A surface treatment technique for coating a polymer bilayer with an anionic charge is applied to modify the surface of the microchannel. A normally closed valve with anionic surface charges forms a nanoscale channel that only allows the passage of electric current but traps the DNA samples so that they are pre-concentrated. After the pre-concentration step, a pneumatic suction force is applied to the normally closed valve. This presses down the valve membrane, which reconnects the channels. The DNA samples are then moved into a separation channel for further separation and analysis. Successful DNA pre-concentration and separation has been achieved. Fluorescent intensity at the pre-concentration area is increased by approximately 3570 times within 1.9 min of operation. The signals from the separation of phiX174 DNA/HaeIII markers are enhanced approximately 41 times after 100 s of pre-concentration time, as compared with the results using a traditional cross-shaped micro-CE chip. These results clearly demonstrate that successful DNA pre-concentration for signal enhancement and separation analysis can be performed by using this new micro-CE chip.
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