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PubMed2003

Measurement of enzyme kinetics using a continuous-flow microfluidic system.

This paper describes a microanalytical method for determining enzyme kinetics using a continuous-flow microfluidic system. The analysis is carried out by immobilizing the enzyme on microbeads, packing the microbeads into a chip-based microreactor (volume approximately 1.0 nL), and flowing the substrate over the packed bed. Data were analyzed using the Lilly-Hornby equation and compared to values obtained from conventional measurements based on the Michaelis-Menten equation. The two different enzyme-catalyzed reactions studied were chosen so that the substrate would be nonfluorescent and the product fluorescent. The first reaction involved the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and N-acetyl-3,7-dihydroxyphenoxazine (amplex red) to yield fluorescent resorufin, and the second the beta-galactosidase-catalyzed reaction of nonfluorescent resorufin-beta-D-galactopyranoside to yield D-galactose and fluorescent resorufin. In both cases, the microfluidics-based method yielded the same result obtained from the standard Michaelis-Menten treatment. The continuous-flow method required approximately 10 microL of substrate solution and 10(9) enzyme molecules. This approach provides a new means for rapid determination of enzyme kinetics in microfluidic systems, which may be useful for clinical diagnostics, and drug discovery and screening.

Catalysis↗

PubMed2005

Interfacial scattering at electrochemically fabricated atom-scale junctions between thin gold film electrodes in a microfluidic channel.

Atom-scale junctions were formed between two Au thin-film electrodes by a combination of lithography, microfluidics, and electrochemistry. Two Au thin-film electrodes with a small (0.25-25 microm) gap between them were lithographically defined such that the gap fell in the center of a 100-microm-wide microfluidic channel in poly(dimethylsiloxane). Directional electrodeposition between the Au thin-film electrodes, accomplished by applying a potential between the thin-film electrodes, caused Au to etch from the anode and deposit on the cathode, thereby closing the gap. Current through the gap was monitored continuously, and the directional electrodeposition was terminated when a current near that corresponding to the conductance quantum, G(0) = 2e(2)/h, was reached. To regenerate the device, the atom-scale junction was broken with a potential sweep, the microfluidic channel was rinsed, and the junction was re-formed with a subsequent comparator-terminated directional electrodeposition. Alternating current impedance was measured while hexadecanethiol (HDT) was chemisorbed onto the atom-scale junction. The interfacial scattering from chemisorption of the Lewis base HDT on the atom-scale junction caused a normalized impedance change of 71 +/- 1%, the noise level being equivalent to a population fluctuation of five HDT molecules.

Journal Article↗

PubMed2005

On-chip microfluidic transport and mixing using electrowetting and incorporation of sensing functions.

An integrated system was developed that performs microfluidic transport, mixing, and sensing on a single chip. The operation principle for the microfluidic transport was based on electrowetting. A solution to be transported was confined in a space between a row of gold working electrodes and a protruding poly(dimethylsiloxane) (PDMS) structure. When a negative potential was applied to one of the gold working electrodes, it became hydrophilic, and the solution was transported through the flow channel. The solution could be transported in any desired direction in a network of flow channels by switching on necessary electrodes one by one. Furthermore, two solutions transported through two flow channels could be mixed using a mixing electrode based on the same principle. To demonstrate the applicability of a lab-on-a-chip, an air gap ammonia electrode was integrated by taking advantage of the open structure of the flow channel. Gaseous ammonia that was produced after pH adjustment and diffused through an air gap caused a pH change in the electrolyte layer, which was measured with an iridium oxide pH indicator electrode. The 90% response time was less than 1 min for the millimolar order of ammonia. The calibration curve was linear down to 10 microM. The ammonia-sensing system was also applied to construct biosensing systems for urea and creatinine. A linear relationship was observed between the potential and the logarithm of the concentration of the analytes down to 50 microM for both urea and creatinine. The developed microfluidic system can be a basic building block for future systems.

Journal Article↗

PubMed2006

Nanofluidic injection and heterogeneous kinetics of organomercaptan surface displacement reactions on colloidal gold in a microfluidic stream.

Colloidal gold is developed as a molecular capture reagent in hybrid nanofluidic-microfluidic devices for mass-limited sample analysis. Two fluorescent organomercaptans are injected through a nanocapillary array membrane and subsequently captured at the surface of 19-nm-diameter colloidal Au nanoparticles. The surface displacement kinetics are monitored via quenching of the organomercaptan fluorescence by the metallic particles coupled to a distance-time conversion based on fluid velocity in the microfluidic channel using the point of mixing as the zero of time. The adsorbate concentration, colloid concentration, and fluid velocity are varied to determine the surface displacement rate constants for these heterogeneous reactions in the microfluidic device. Surface displacement rate constants are approximately 10(4) M(-1) s(-1) for a small organic molecule and for an octapeptide. These values are similar to values determined in macroscale measurements made with a traditional fluorometer and are 1 order of magnitude larger than values reported for adsorption of organomercaptans on planar Au, indicating faster kinetics in the colloid-adsorbate system. These results highlight the utility of colloidal Au nanoparticles as molecular carriers for the sequestration of analytes, allowing the manipulation of mass-limited samples and ultimately the capture and delivery of selected analytes from a microfabricated device to an off-line detector.

Journal Article↗

PubMed2002

A supramolecular microfluidic optical chemosensor.

A supramolecular microfluidic optical chemosensor (muFOC) has been fabricated. A serpentine channel has been patterned with a sol-gel film that incorporates a cyclodextrin supramolecule modified with a Tb(3+) macrocycle. Bright emission from the Tb(3+) ion is observed upon exposure of the (mu)FOC to biphenyl in aqueous solution. The signal transduction mechanism was elucidated by undertaking steady-state and time-resolved spectroscopic measurements directly on the optical chemosensor patterned within the microfluidic network. The presence of biphenyl in the cyclodextrin receptor site triggers Tb(3+) emission by an absorption-energy transfer-emission process. These results demonstrate that the intricate signal transduction mechanisms of supramolecular optical chemosensors are successfully preserved in microfluidic environments.

Cyclodextrins↗

PubMed1999

Uniaxial compression of thermal gels based on microfluidized blends of WPI and heat-denatured WPI.

The effect of heat-denatured whey protein isolate (dWPI)/whey protein isolate (WPI) ratio (0-0.6), microfluidization pressure (0-1000 bar), and number of passes (1-10) on the uniaxial shear stress at 10% (sigma(10)) and 80% (sigma(80)) relative deformation of dWPI/WPI heat-induced gels (14% total protein, w/w) was studied. No correlation between the average diameter of aggregates and the dWPI/WPI ratio, microfluidization pressure, or number of passes was found. However, increasing the microfluidization pressure or the number of passes resulted in a narrower size distribution of aggregates. Increasing the dWPI/WPI ratio and the number of passes resulted in a decrease and an increase of gel hardness, respectively. The results were interpreted in terms of more random aggregation/gelation of proteins in the presence of aggregates that could result in localized heterogeneities into gels and more dissipation of the deformation energy during compression. The positive effect of the number of passes on the gel hardness was also considered to be due to a more homogeneous aggregation/gelation of proteins in the presence of smaller aggregates.

Food Handling↗

PubMed2006

Control of the ZnO nanowires nucleation site using microfluidic channels.

We report on the growth of uniquely shaped ZnO nanowires with high surface area and patterned over large areas by using a poly(dimethylsiloxane) (PDMS) microfluidic channel technique. The synthesis uses first a patterned seed template fabricated by zinc acetate solution flowing though a microfluidic channel and then growth of ZnO nanowire at the seed using thermal chemical vapor deposition on a silicon substrate. Variations the ZnO nanowire by seed pattern formed within the microfluidic channel were also observed for different substrates and concentrations of the zinc acetate solution. The photocurrent properties of the patterned ZnO nanowires with high surface area, due to their unique shape, were also investigated. These specialized shapes and patterning technique increase the possibility of realizing one-dimensional nanostructure devices such as sensors and optoelectric devices.

Journal Article↗

PubMed2005

Microfluidic platform for the generation of organic-phase microreactors.

Rapid prototyping photolithography of a thiolene-based resin was used to fabricate microfluidic devices stable to aliphatic and aromatic organic solvents. The swelling of the cross-linked polymer matrix in various organic solvents was quantified, and the solvent resistance properties of these microfluidic devices are described. Discrete droplets of hexanes and toluene of uniform size were generated in microfluidic devices inside a water matrix containing SDS surfactant (SDS = sodium dodecyl sulfate). Variation of water and organic flow rates in the fluidic channels was used to control droplet size and separation. Droplet composition could be controlled by varying flow rates of two joined organic streams. Organic-phase synthetic reactions within the droplets were demonstrated with the bromination of alkenes inside benzene droplets.

Journal Article↗

PubMed2005

Selective filling for patterning in microfluidic channels.

The ability to pattern different polymers in microfluidic channels is indispensable for the development of multifunctional, "lab-on-a-chip" devices. A simple method, based on the concept of selective filling, is described for introducing different polymers at defined locations in microfluidic channels. Selective filling is based on the difference in the free energy of filling between an open and a covered part of the channel. It is implemented by covering part of the channel, along its length, with a temporary poly(dimethylsiloxane) (PDMS) slab. Preferential filling is related to the contact angle of the liquid solution on the chip surface. An expression for the critical contact angle is derived, and its dependence on the geometry of the channel is established. It is further shown that a trapezoidal geometry of the cross-section of the channel is optimal for selective filling. Experimental verification of the applicability of the critical contact angle in predicting selective filling is demonstrated by introducing liquid prepolymer solutions of different contact angles in the glass channel that was etched using photolithography and wet etching. Finally, patterning of two different polymers along the axial direction of the microfluidic channel is demonstrated using this selective filling technique.

Journal Article↗

PubMed2002

Electrowetting-based actuation of droplets for integrated microfluidics.

The serviceability of microfluidics-based instrumentation including 'lab-on-a-chip' systems critically depends on control of fluid motion. We are reporting here an alternative approach to microfluidics based upon the micromanipulation of discrete droplets of aqueous electrolyte by electrowetting. Using a simple open structure, consisting of two sets of opposing planar electrodes fabricated on glass substrates, positional and formational control of microdroplets ranging in size from several nanoliters to several microliters has been demonstrated at voltages between 15-100 V. Since there are no permanent channels or structures between the plates, the system is highly flexible and reconfigurable. Droplet transport is rapid and efficient with average velocities exceeding 10 cm s(-1) having been observed. The dependence of the velocity on voltage is roughly independent of the droplet size within certain limits, thus the smallest droplets studied (approximately 3 nl) could be transported over 1000 times their length per second. Formation, mixing, and splitting of microdroplets was also demonstrated using the same microactuator structures. Thus, electrowetting provides a means to achieve high levels of functional integration and flexibility for microfluidic systems.

Journal Article↗

PubMed2002

An evaporation-based microfluidic sample concentration method.

We present a method for sample concentration within microfluidic devices using evaporation-induced flow. Evaporation-induced flow is easy to incorporate into microfluidic designs and can be used to concentrate a wide variety of molecules. The practicality of this method was demonstrated with 0.2 microm fluorescent spheres and FITC-labeled BSA. Thirty two percent of the 0.6 microL fluorescent sphere suspension was concentrated into a well within a microfluidic device. In the same amount of time, 93% of the 0.6 microL FITC-labeled BSA solution was concentrated.

Journal Article↗

PubMed2002

CO(2)-laser micromachining and back-end processing for rapid production of PMMA-based microfluidic systems.

In this article, we focus on the enormous potential of a CO(2)-laser system for rapidly producing polymer microfluidic structures. The dependence was assessed of the depth and width of laser-cut channels on the laser beam power and on the number of passes of the beam along the same channel. In the experiments the laser beam power was varied between 0 and 40 W and the passes were varied in the range of 1 to 7 times. Typical channel depths were between 100 and 300 microm, while the channels were typically 250 microm wide. The narrowest produced channel was 85 microm wide. Several bonding methods for microstructured PMMA [poly(methyl methacrylate)] parts were investigated, such as solvent-assisted glueing, melting, laminating and surface activation using a plasma asher. A solvent-assisted thermal bonding method proved to be the most time-efficient one. Using laser micromachining together with bonding, a three-layer polymer microstructure with included optical fibers was fabricated within two days. The use of CO(2)-laser systems to produce microfluidic systems has not been published before. These systems provide a cost effective alternative to UV-laser systems and they are especially useful in microfluidic prototyping due to the very short cycle time of production.

Journal Article↗

PubMed2002

Rapid prototyping for injection moulded integrated microfluidic devices and diffractive element arrays.

This paper describes two fabrication procedures that makes it possible to design, fabricate and injection mold a microfluidic system with an on board coupling element or an optical array platform in less than four hours. Epoxy masters for the array and a single diffractive element were produced using conventional soft lithography techniques and a commercially available UV curable epoxy. The fabrication of the master for the integrated microfluidic device utilized the surface chemistry of polyester and its interaction with the anionic surfactant sodium dodecyl sulfate (SDS), to selectively inhibit the adhesion between the epoxy and the polyester film during the curing reaction. The transfer of a microfluidic design and the required coupling element (632 nm holographic grating) along the base of the channel was completed in a single step. The turnaround time from design to injection molded device whether a microchannel or array was 3.5 h.

Journal Article↗

PubMed2003

Microfluidic chip for fast nucleic acid hybridization.

The design and experimental verification of a fast nucleic acid hybridization microchip using the fluidic velocity and strain rate effects was conducted. This hybridization chip was able to increase the hybridization signal 6-fold, reduce non-specific target-probe binding and background noise within 30 min, as compared to conventional hybridization methods, which may take from 4 h to overnight. Excellent correlation between experimental results and simulation analysis was obtained in this study. A detailed study of a newly designed microfluidic chip for enhancing hybridization was conducted. Three different designs of devices were fabricated and tested. Two different lengths of targets, 25-mer oligonucleotide and 1.4 kb ssDNA, were tested in this study. The hybridization efficiency can be improved by introducing velocity and extensional strain rate to the sample. This study demonstrates that the signal in the proposed method exhibits intensities 6-fold higher than those in static conditions. The necessary time for the hybridization process can be reduced from overnight to 30 min using the methods developed in this study. Experimental results also show that the strain rate provides stronger effect on hybridization than that of velocity. Combining hybridization with microfluidic concepts of velocity and strain rate effects may provide additional specificity and efficiency in nucleic acid detection and genomic study. This microfluidic hybridization chip can provide potential application in genomic study.

Algorithms↗

PubMed2004

Electrokinetic microfluidic phenomena by a lattice Boltzmann model using a modified Poisson-Boltzmann equation with an excluded volume effect.

A lattice Boltzmann model (LBM) for electrokinetic microfluidics recently proposed by us [J. Colloid Interface Sci. 263, 144 (2003); Langmuir 19, 3041 (2003)] is employed by consideration of a modified Poisson-Boltzmann equation including an excluded volume effect. In our study, pressure is considered as the only external force for liquid flow. As commonly used, the Poisson-Boltzmann equation assumes only point charge, the predicted microfluidic phenomena of KCl and LiCl electrolyte solutions are theoretically the same; these phenomena, however, have been found to be experimentally different. Our LBM in conjunction with the modified Poisson-Boltzmann equation are capable of explaining this discrepancy and the results are in good agreement with recent experimental data for KCl and LiCl electrolyte solutions in pressure-driven microchannel flow, suggesting that our proposed LBM can be employed to predict the more complex microfluidic systems that might be problematic using conventional methods and electrokinetic models.

Journal Article↗

PubMed2001

Using three-dimensional microfluidic networks for solving computationally hard problems.

This paper describes the design of a parallel algorithm that uses moving fluids in a three-dimensional microfluidic system to solve a nondeterministically polynomial complete problem (the maximal clique problem) in polynomial time. This algorithm relies on (i) parallel fabrication of the microfluidic system, (ii) parallel searching of all potential solutions by using fluid flow, and (iii) parallel optical readout of all solutions. This algorithm was implemented to solve the maximal clique problem for a simple graph with six vertices. The successful implementation of this algorithm to compute solutions for small-size graphs with fluids in microchannels is not useful, per se, but does suggest broader application for microfluidics in computation and control.

Algorithms↗

PubMed1999

Performance of sonication and microfluidization for liquid-liquid emulsification.

The purpose of this research was to evaluate and compare liquid-liquid emulsions (water-in-oil and oil-in-water) prepared using sonication and microfluidization. Liquid-liquid emulsions were characterized on the basis of emulsion droplet size determined using a laser-based particle size analyzer. An ultrasonic-driven benchtop sonicator and an air-driven microfluidizer were used for emulsification. Sonication generated emulsions through ultrasound-driven mechanical vibrations, which caused cavitation. The force associated with implosion of vapor bubbles caused emulsion size reduction and the flow of the bubbles resulted in mixing. An increase in viscosity of the dispersion phase improved the sonicator's emulsification capability, but an increase in the viscosity of the dispersed phase decreased the sonicator's emulsification capability. Although sonication might be comparable to homogenization in terms of emulsification efficiency, homogenization was relatively more effective in emulsifying more viscous solutions. Microfluidization, which used a high pressure to force the fluid into microchannels of a special configuration and initiated emulsification via a combined mechanism of cavitation, shear, and impact, exhibited excellent emulsification efficiency. Of the three methodologies, sonication generated more heat and might be less suitable for emulsion systems involving heat-sensitive materials. Homogenization is in general a more effective liquid-liquid emulsification method. The results derived from this study can serve as a basis for the evaluation of large-scale liquid-liquid emulsification in the microencapsulation process.

Emulsions↗

PubMed2004

A microfluidic rectifier: anisotropic flow resistance at low Reynolds numbers.

It is one of the basic concepts of Newtonian fluid dynamics that at low Reynolds number (Re) the Navier-Stokes equation is linear and flows are reversible. In microfluidic devices, where Re is essentially always low, this implies that flow resistance in microchannels is isotropic. Here we present a microfluidic rectifier: a microscopic channel of a special shape whose flow resistance is strongly anisotropic, differing by up to a factor of 2 for opposite flow directions. Its nonlinear operation at arbitrary small Re is due to non-Newtonian elastic properties of the working fluid, which is a 0.01% aqueous solution of a high molecular weight polymer. The rectifier works as a dynamic valve and may find applications in microfluidic pumps and other integrated devices.

Journal Article↗