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This paper presents a fast method for determining the conductance of gas microfluidic devices with low flow rates and very small pressure drops starting from 30 Pa, corresponding to Re = 0.3. This method is based on discharging a gas-pressurized chamber through the microfluidic device under test. The microfluidic device's conductance can be estimated as a function of inlet pressure and the Reynolds number of the flow by recording the upstream pressure during the discharging process and calculating the time derivative of the gas pressure. The pressurized chamber is considered as an isothermal chamber. Experimental results show that a sufficiently accurate isothermal discharging process up to an upstream-to-downstream pressure ratio of 0.8 can be achieved by immersing the chamber in a thermal bath. The method presented here is very fast, requiring only a few seconds for the acquisition procedure and computerized data processing. [DOI: 10.1115/1.4003089]

A three-dimensional structure was constructed to demonstrate contact-free liquid addition and distribution to a conventional centrifugal microfluidic platform. The structure allowed a continuous stream of liquid to be added to the rotating platform where it was dispensed and metered to a series of reservoirs with 9% relative standard deviation of the volume. An off-platform generated gradient was used to demonstrate this design's potential to make gradient centrifugal microfluidic chromatography possible without the use of platform space for liquid storage. The system demonstrates a convenient and versatile 'World-to-Chip' or more appropriately, World-to-Disk, macro-to-micro interface. 10.1021/ac1024478

Despite the increasing industrial and domestic demand for gas analyzers, the unmanageable size and cost of the available devices prevent them from fulfilling this pervasive need. In this paper, we demonstrate that, monitored by a generic gas sensor, the progress rate of a gaseous analyte's free diffusion through an air-filled centimeter-long microfluidic channel yields sufficient information for gas recognition. The installation of additional channels made from different materials provides more uncorrelated information, enabling more detailed gas analyses. This device classifies gases based on their physical features at room temperature, namely, diffusivity and interaction (physisorption/deserption) with channel walls. This process does not degrade the gas discriminating element of the instrument. Our prototype, featuring a 50 mm long 50 [micro]m bore borosilicate glass channel, successfully differentiated between 24 analytes, including four butanol isomers, and estimated the compositions of their binary and ternary gas mixtures dispersed in air at different concentration levels. The findings presented here are directly applicable to the production of a new inexpensive, compact, portable, and durable generation of artificial olfaction system whose performance is almost entirely independent of the utilized gas sensor's drift. 10.1021/ac101767r

Cell concentration via centrifugation is a ubiquitous step in many cell culture procedures. At the macroscale, centrifugation suffers from a number of limitations, particularly when dealing with small numbers of cells (e.g., less than 50 000). On the other hand, typical microscale methods for cell concentration can affect cell physiology and bias readouts of cell behavior and function. In this paper, we present a microfluidic concentrator device that utilizes the effects of gravity to allow cells to gently settle out of a suspension into a collection region without the use of specific adhesion ligands. Dimensional analysis was performed to compare different device designs and was verified with flow modeling to optimize operational parameters. We are able to concentrate low-density cell suspensions in a microfluidic chamber, achieving a cell loss of only 1.1 [+ or -] 0.6% (SD, n = 7) with no observed loss during a subsequent cell staining protocol which incorporates ~36 complete device volume replacements. This method provides a much needed interface between rare cell samples and microfluidic culture assays. 10.1021/ac101866p

Polymeric substrates have significant advantages over silicon and glass for use in microfluidics. However, before polymer microfluidic devices can be mass produced, it must be shown that the manufacturing method used to create these devices is robust and repeatable. For this paper, a polymer manufacturing process, hot embossing, was used to produce microsized features in polymethylmethacrylate (PMMA) chips. A design of experiments that varied two factors during the hot embossing process (temperature and pressure), was conducted to determine the robustness of hot embossing microsized channels in PMMA. The channel height and width were measured at three sites on each chip, and the results were analyzed in two ways: response surface modeling (RSM) and nested variance analysis. For the RSM analysis, two separate ANOVA tests and regressions were performed on both channel width and channel height to obtain the response surface models between temperature, pressure and the channel width and height. Furthermore, the variance of channel width and height at each design point was determined and then two ANOVA tests and two separate regressions were performed to obtain the response surface models between temperature, pressure and the variance of channel height and channel width. This analysis was used to determine if hot embossing microfluidic devices is a robust process capable of producing quality parts at different operating conditions. The nested variance analysis was used to determine the primary source of the variation in channel height and width. For the nested variance analysis, two separate calculations were performed in order to determine whether the variance of channel width and height is mostly caused by within-chip variance or chip-to-chip variance. The analysis showed that the channel widths and heights were statistically equal across the four different operating points used (the low-temperature, low-pressure point was omitted). The variance of channel width and the variance of channel height remained constant in the desired operating region. Based on this analysis, it was concluded that hot embossing is a robust process for features on the order of 50 [micro]m. Furthermore, the nested variance analysis showed that the variance of channel width and height is mostly caused by site-to-site measurements on a chip rather than between-chip variance. Therefore, it was determined that hot embossing microfluidic devices are repeatable and consistent from chip-to-chip. [DOI: 10.1115/1.4001421] Keywords: hot embossing, microfluidics, process robustness, polymer manufacturing

The development of a photothermal absorbance detector for use with microfluidic devices is described. Unlike thermo-optical techniques that rely on measuring refractive index changes, the solution viscosity is probed by continuously monitoring solution conductivity. Platinum electrodes microfabricated on a quartz substrate and bonded to a substrate containing the microchannels enable contact conductivity measurements. The effects of excitation frequency and voltage, electrode spacing, laser power, and laser modulation (chopping) frequency were evaluated experimentally. In the current configuration, a limit of detection of 5 nM for DABSYL-tagged glucosamine was obtained using long injections (to give fat-topped peaks). This corresponds to an absorbance of 4.4 x [10.sup.-7] AU. Separation and detection of DABSYL-tagged glycine, proline, and tryptophan are also shown to demonstrate the feasibility of the method. In addition, simulations were used to investigate the applicability of the technique to small volume platforms. 10.1021/ac902975r

This article shows that the added mass due to fluid-structure interaction significantly affects the vibrational dynamics of cilia-based (vibrating cantilever-type) devices for handling microscale fluid flows. Commonly, the hydrodynamic interaction between the cilia-based actuators and fluid is modeled as a drag force that results in damping of the cilia motion. Our main contribution is to show that such damping effects cannot explain the substantial reduction in the resonant-vibrational frequency of the cilia actuator operating in liquid when compared with the natural frequency of the cilia in air. It is shown that an added-mass approach (that accounts for the inertial loading of the fluid) can explain this reduction in the resonant-vibrational frequency when operating cantilever-type devices in liquids. Additionally, it is shown that the added-mass effect can explain why the cilia-vibration amplitude is not substantially reduced in a liquid by the hydrodynamic drag force. Thus, this article shows the need to model the added-mass effect, both theoretically and by using experimental results. [DOI: 10.1115/1.4000766]

Measuring buried, undercut microstructures is a challenging task in metrology. These structures are usually characterized by measuring their cross sections after physically cutting the samples. This method is destructive and the obtained information is incomplete. The distortion due to cutting also affects the measurement accuracy. In this paper, we first apply the laser fluorescent confocal microscopy and intensity differentiation algorithm to obtain the complete three-dimensional profile of the buried, undercut structures in microfluidic devices, which are made by the soft lithography technique and bonded by the oxygen plasma method. The impact of material wettability and the refractive index (n) mismatch among the liquid, samples, cover layer, and objective on the measurement accuracy are experimentally investigated. OCIS codes: 100.5010, 100.6890, 120.0120, 120.2830, 120.6650.

This article describes a method for fabricating 3D microfluidic devices by stacking layers of patterned paper and double-sided adhesive tape. Paper-based 3D microfluidic devices have capabilities in microfluidics that are difficult to achieve using conventional open-channel microsystems made from glass or polymers. In particular, 3D paper-based devices wick fluids and distribute microliter volumes of samples from single inlet points into arrays of detection zones (with numbers up to thousands). This capability makes it possible to carry out a range of new analytical protocols simply and inexpensively (all on a piece of paper) without external pumps. We demonstrate a prototype 3D device that tests 4 different samples for up to 4 different analytes and displays the results of the assays in a side-by-side configuration for easy comparison. Three-dimensional paper-based microfluidic devices are especially appropriate for use in distributed healthcare in the developing world and in environmental monitoring and water analysis. diagnostics | patterned paper

This paper describes a pumping technique applicable to centrifugal microfluidic platforms, involving the use of a regulated stream of compressed gas to pump liquid radially inward and toward the center of the platform while spinning. This technique provides a noncontact method for pumping fluids and is highly efficient, requiring only approximately 60 s to reach completion. This pumping operation can be attained with an applied gas flow rate of 58.8 L [min.sup.-1], while the platform is rotated at frequencies less than 180 rpm (3.0 Hz). 10.1021/ac102071b

Digital microfluidic systems (DMSs) are emerging with high potential to serve as true labs-on-a-chip (LOCs) systems for various chemical and biochemical analyses. Significant research efforts have been devoted toward the design and fabrication of improved LOC systems. Due to design, fabrication, and also runtime uncertainties, feedback control schemes must be incorporated to ensure reliability and accuracy of LOCs for practical use. This paper introduces the basics of the control analysis and design of the DMS, which is a relatively unexplored area in digital microfluidics. Based on the semi-empirical models of the droplet dynamic system, simulation results of the closed-loop position control of a droplet between two adjacent cells of a DMS are presented. The dynamic response is analyzed and discussed in relation to the operational parameters of a DMS. [DOI: 10.1115/1.4000077] Keywords: digital microfluidics, droplet, feedback control

A microfluidic device could assist with the challenge of detecting infectious agents such as HIV in remote areas of the world. Samuel Sia at Columbia University in New York and [...]