Fluidic Access Research Articles

Gradient Elution Moving Boundary Electrophoresis for High-Throughput Multiplexed Microfluidic Devices

A novel method for performing electrophoretic separations is described-gradient elution moving boundary electrophoresis (GEMBE). The technique utilizes the electrophoretic migration of chemical species in combination with variable hydrodynamic bulk counterflow of the solution through a separation capillary or microfluidic channel. Continuous sample introduction is used, eliminating the need for a sample injection mechanism. Only analytes with an electrophoretic velocity greater than the counterflow velocity enter the separation channel. The counterflow velocity is varied over time so that each analyte is brought into the separation column at different times, allowing for high-resolution separations in very short channels. The new variable of bulk flow acceleration affords a new selectivity parameter to electrophoresis analogous to gradient elution compositions in chromatography. Because it does not require extra channels or access ports to form an injection zone and because separations can be performed in very short channels, GEMBE separations can be implemented in much smaller areas on a micro-fluidic chip as compared to conventional capillary electrophoresis. Demonstrations of GEMBE separations of small dye molecules, amino acids, DNA, and immunoassay products are presented. A low-cost, polymeric, eight-channel multiplexed microfluidic device was fabricated to demonstrate the reduced area requirements of GEMBE; the device was less than 1 in.2 in area and required only n + 1 fluidic access ports per n analyses (in this instance, nine ports for eight analyses). Parallel separations of fluorescein and carboxyfluorescein yielded less than 3% relative standard deviation (RSD) in interchannel migration times and less than 5% RSD in both peak and height measurements. The device was also used to generate a calibration curve for a homogeneous insulin immunoassay using each of the eight channels as a calibration point with less than 5% RSD at each point with replicate analyses.

The application of polycrystalline diamond in a thin film packaging process for MEMS resonators

This paper reports the design, fabrication and testing of a polycrystalline diamond (poly-C) thin film packaging process for a MEMS cantilever type resonator using a 4-mask fabrication process, which integrates chemical vapor deposition (CVD) diamond thin film technology with an encapsulation packaging process. After poly-C cantilever beam resonators were fabricated using the first two masks, a sacrificial PECVD SiO 2 layer with a thickness in the range of 4–5 μm was deposited at 350 °C and patterned to create the package anchor. Then, a 4-μm-thick poly-C film was grown and patterned to create the thin film packaging structure containing fluidic access ports for the removal of the sacrificial layer. The fluidic access ports were finally sealed with an additional poly-C growth. To evaluate the efficacy of the poly-C encapsulation process, poly-C cantilever beam resonators were tested using a piezoelectric actuation and laser detection method before and after the poly-C packaging process. Resonance frequencies measured before and after are in the range of 240–320 kHz, which is consistent with predicted calculations. A modified fabrication process was designed to test the fluidic hermiticity of the thin film package.

Microfluidic Serial Dilution Circuit

In vitro evolution of RNA molecules requires a method for executing many consecutive serial dilutions. To solve this problem, a microfluidic circuit has been fabricated in a three-layer glass-PDMS-glass device. The 400-nL serial dilution circuit contains five integrated membrane valves: three two-way valves arranged in a loop to drive cyclic mixing of the diluent and carryover, and two bus valves to control fluidic access to the circuit through input and output channels. By varying the valve placement in the circuit, carryover fractions from 0.04 to 0.2 were obtained. Each dilution process, which is composed of a diluent flush cycle followed by a mixing cycle, is carried out with no pipeting, and a sample volume of 400 nL is sufficient for conducting an arbitrary number of serial dilutions. Mixing is precisely controlled by changing the cyclic pumping rate, with a minimum mixing time of 22 s. This microfluidic circuit is generally applicable for integrating automated serial dilution and sample preparation in almost any microfluidic architecture.

Explosive vaporization in microenclosures

The explosive vaporization of a liquid above planar microheaters induces a fast increase of pressure that is exploited in many thermally driven actuators in MEMS components such as ink jet printer cartridges, pumps, valves and optical switches. Some of these components need to enclose the working fluid as it is the case of valves in which the heated liquid is separated from the flow that it regulates by a flexible membrane. To achieve a better insight into the thermodynamic processes involved, the present work investigates experimentally an enclosed microsystem designed and fabricated for this purpose, composed of a small liquid volume (8 nL) heated by a electric pulse for 2 μs supplied to a planar microfabricated heater. During the heating, the temperature-induced change in resistance can be determined by imposing a defined current and measuring the voltage drop over the heater. While the chip is based on a silicon substrate with integrated platinum heaters and sensors, the structure enclosing the fluid (cavity and fluidic access to it) is made of a silicone elastomer, poly(dimethylsiloxane) (PDMS). This transparent material is widely used in microfluidics and allows for flexible and transparent walls that can be deflected by increasing the pressure inside the cavity. To seal the system the inlet and the outlet were closed by blocking them with a metallic stab. In the present work we visualize vaporization of isopropanol in contact with a suddenly heated planar resistor for two different cavity heights, 150 μm and 16 μm. The rate of temperature rise of the thin liquid layer in contact with the heater is of the order of 10 7 K s −1 for a pulse duration of 2 μs. We compare bubble growth and collapse for the open and closed systems. Compared to the open system, the bubble growth in the closed system is considerably damped.

Microfluidic Electrophoresis Chip Coupled to Microdialysis for in Vivo Monitoring of Amino Acid Neurotransmitters

Microfluidic electrophoresis devices were coupled on-line to microdialysis for in vivo monitoring of primary amine neurotransmitters in rat brain. The devices contained a sample introduction channel for dialysate, a precolumn reactor for derivatization with o-phthaldialdehyde, a flow-gated injector, and a separation channel. Detection was performed using confocal laser-induced fluorescence. In vitro testing revealed that the initial device design had detection limits for amino acids of approximately 200 nM, relative standard deviation of peak heights of 2%, and separations within 95 s with up to 30,200 theoretical plates when applying an electric field of 370 V/cm. A second device design that allowed electric fields of 1320 V/cm to be applied while preserving the reaction time allowed separations within 20 s with up to 156,000 theoretical plates. Flow splitting into the electrokinetic network from hydrodynamic flow in the sample introduction channel was made negligible for sampling flow rates from 0.3 to 1.2 microL/min by placing a 360-microm-diameter fluidic access hole that had flow resistance (0.15-7.2) x 10(8)-fold lower than that of the electrokinetic network at the junction of the sample introduction channel and the electrokinetic network. Using serial injections, the device allowed the dialysate stream to be analyzed at 130-s intervals. In vivo monitoring was demonstrated by using the microdialysis/microfluidic device to record glutamate concentrations in the striatum of an anesthetized rat during infusion of the glutamate uptake inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid. These results prove the feasibility of using a microfabricated fluidic system coupled to sampling probes for chemical monitoring of complex media such as mammalian brain.

An “all-diamond” inkjet realized in sacrificial layer technology

Recently, we have developed a universal fabrication technology for diamond surface microfluidics with high aspect ratio [1] [R. Müller, P. Schmid, A. Munding, R. Gronmaier, E. Kohn, Diamond Relat. Mater. 13 (2004) 780]. The key detail is a high aspect ratio sacrificial layer of copper, which is overgrown with diamond, followed by a dry-etch step to open a fluidic access to remove the sacrificial metal. In this approach, using the above mentioned sacrificial layer technology, an inkjet element is designed and fabricated where only diamond is in contact with the liquid. The result is a monolithic structure, i.e. no assembly of a nozzle-plate to the system involving alignment, gluing or bonding is necessary. The monolithic nature of the fabrication process also may allow to scale the system down to micron-size droplets.

Behaviour and design considerations for continuous flow closed-open-closed liquid microchannels

This paper introduces a method of combining open and closed microchannels in a single component in a novel way which couples the benefits of both open and closed microfluidic systems and introduces interesting on-chip microfluidic behaviour. Fluid behaviour in such a component, based on continuous pressure driven flow and surface tension, is discussed in terms of cross sectional flow behaviour, robustness, flow-pressure performance, and its application to microfluidic interfacing. The closed-open-closed microchannel possesses the versatility of upstream and downstream closed microfluidics along with open fluidic direct access. The device has the advantage of eliminating gas bubbles present upstream when these enter the open channel section. The unique behaviour of this device opens the door to applications including direct liquid sample interfacing without the need for additional and bulky sample tubing.

A Low-Temperature Thin-Film Electroplated Metal Vacuum Package

This paper presents a packaging technology that employs an electroplated nickel film to vacuum seal a MEMS structure at the wafer level. The package is fabricated in a low-temperature (<250/spl deg/C) 3-mask process by electroplating a 40-/spl mu/m-thick nickel film over an 8-/spl mu/m sacrificial photoresist that is removed prior to package sealing. A large fluidic access port enables an 800/spl times/800 /spl mu/m package to be released in less than three hours. MEMS device release is performed after the formation of the first level package. The maximum fabrication temperature of 250/spl deg/C represents the lowest temperature ever reported for thin film packages (previous low /spl sim/400/spl deg/C). Implementation of electrical feedthroughs in this process requires no planarization. Several mechanisms, based upon localized melting and Pb/Sn solder bumping, for sealing low fluidic resistance feedthroughs have been investigated. This package has been fabricated with an integrated Pirani gauge to further characterize the different sealing technologies. These gauges have been used to establish the hermeticity of the different sealing technologies and have measured a sealing pressure of /spl sim/1.5 torr. Short-term (/spl sim/several weeks) reliability data is also presented.

Continuous perfusion microfluidic cell culture array for high-throughput cell-based assays

We present for the first time a microfluidic cell culture array for long-term cellular monitoring. The 10 x 10 array could potentially assay 100 different cell-based experiments in parallel. The device was designed to integrate the processes used in typical cell culture experiments on a single self-contained microfluidic system. Major functions include repeated cell growth/passage cycles, reagent introduction, and real-time optical analysis. The single unit of the array consists of a circular microfluidic chamber, multiple narrow perfusion channels surrounding the main chamber, and four ports for fluidic access. Human carcinoma (HeLa) cells were cultured inside the device with continuous perfusion of medium at 37 degrees C. The observed doubling time was 1.4 +/- 0.1 days with a peak cell density of approximately 2.5*10(5) cells/cm(2). Cell assay was demonstrated by monitoring the fluorescence localization of calcein AM from 1 min to 10 days after reagent introduction. Confluent cell cultures were passaged within the microfluidic chambers using trypsin and successfully regrown, suggesting a stable culture environment suitable for continuous operation. The cell culture array could offer a platform for a wide range of assays with applications in drug screening, bioinformatics, and quantitative cell biology.

Preparation of Diffuser Type Micropump Using Screen-Printed PZT-PCW Thick Films

This paper describes the preparation and experimental results of the piezoelectric actuating diaphragm and the diffuser type micropump using screen printing thick-film technologies and Si micro-machining. The micropump has a pumping chamber with an oscillating diaphragm which compresses the pumping chamber. Inlet and outlet planar diffusers direct the liquid flow. The micropump is a stack of two layers bonded together: one silicon piece with Si diaphragm actuator using PZT-PCW thick films, other piece having pumping chamber, planar diffuser and fluidic access holes. Si diaphragm of about 15-25 w m thickness are prepared on Si(100) wafers by TMAH wet etching. The PZT-PCW thick films are prepared by hybrid method of screen printing modified with PZT sol-gel application. The results from this study show that it is good to perform combination of screen printed PZT thick-film onto micromachined Si structure.

Preparation of Diffuser Type Micropump Using Screen-Printed PZT-PCW Thick Films

This paper describes the preparation and experimental results of the piezoelectric actuating diaphragm and the diffuser type micropump using screen printing thick-film technologies and Si micro-machining. The micropump has a pumping chamber with an oscillating diaphragm which compresses the pumping chamber. Inlet and outlet planar diffusers direct the liquid flow. The micropump is a stack of two layers bonded together: one silicon piece with Si diaphragm actuator using PZT-PCW thick films, other piece having pumping chamber, planar diffuser and fluidic access holes. Si diaphragm of about 15-25 w m thickness are prepared on Si(100) wafers by TMAH wet etching. The PZT-PCW thick films are prepared by hybrid method of screen printing modified with PZT sol-gel application. The results from this study show that it is good to perform combination of screen printed PZT thick-film onto micromachined Si structure.