The detection and counting of micro particles having sizes comparable to biological entities can provide a tremendous impetus to rapid diagnostics and clinical applications. MEMS technology has already been used in capture and detection of such micron size entities in miniscule concentrations. For this purpose a concentration step is normally added prior to the detection process. A variety of methodologies are used for quantization of such micron size particles/entities including change in permittivity, medium impedance, magnetic permeability and other means. Although optical studies have been extensively performed prior to this, it has not been used for quantization of the micro particles. We have designed, developed and characterized a MEMS counter which captures micron size fluorescent beads using delectrophoresis (DEP) and monitors their accumulation in a 12 μm x 230 μm size channel and monitors this accumulation as growth of overall fluorescence. The field is generated by a set of finely placed interdigitated microelectrodes. As we apply an alternating voltage at 10 V(pp) for a range of different frequencies we are able to capture the flowing beads and concentrate them by several orders of magnitude. This is also followed by their quantification in terms of growing fluorescence signal. For quantitating the fluorescence values a CCD (charge couple device) module fitted over an inverted fluorescence microscope is used that visualizes the whole capture process and a Labview based image acquisition software simultaneously calculates the signal intensity over these frames and arranges it temporally. Our work will have tremendous utility in developing a rapid bacterial counting procedure and will be a valuable tool in microbiological laboratories.

The extremely low limit of detection (LOD) posed by global food and water safety standards necessitates the need to perform a rapid process of integrated detection with high specificity, sensitivity and repeatability. The work reported in this article shows a microchip platform which carries out an ensemble of protocols which are otherwise carried in a molecular biology laboratory to achieve the global safety standards. The various steps in the microchip include pre-concentration of specific microorganisms from samples and a highly specific real time molecular identification utilizing a q-PCR process. The microchip process utilizes a high sensitivity antibody based recognition and an electric field mediated capture enabling an overall low LOD. The whole process of counting, sorting and molecular identification is performed in less than 4 hours for highly dilute samples.

Micro-mixing is an important research area for various applications in sensing and diagnostics. In this paper, we present a performance comparison of several different passive micromixer designs based on the idea of staggered herringbone mixers (SHM). The working principle in such designs includes the formation of centers of flow rotation thus leading to multiple laminations with decreasing sizes of the lamellae as the flow passes over staggered structures. We have realized different layout designs of staggered herringbones inside micro-channels and compared their mixing performance. An overall reduction in mixing time and length has been observed as the degree of asymmetry within these structures is increased. The layouts of these staggered structures are based on herringbone bilayers wherein these layers are positioned on the top and bottom walls of a micro-channel. Fluorescence microscopy and computational fluid dynamics (CFD) based modeling have been used to observe the extent of mixing and understand the reasons behind the enhanced mixing effects. We have further varied the degree of asymmetry of the herringbone bilayers and investigated mixing as a function of the asymmetry. We have developed a novel microfabrication strategy to realize these micro-devices using an inexpensive non-photolithographic technique which we call micro-replication by double inversion (MRDI). The paper basically attempts to develop an overall understanding of the mixing process by letting two fluids flow pass over a variety of asymmetric structures.Keywords Micromixer Á Micromixing Á Staggered herringbones Á Microfabrication Á Micro-replication by double inversion (MRDI) Á Bilayer Á Asymmetry

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