Homogeneous and fast mixing of samples at microscale is a critical requirement for successful applications of microfluidics in biochemical analysis, chemical synthesis, drug delivery and nanomaterial synthesis. This paper reports the optimisation of bubble-induced mixing in a microfluidic device in terms of voltage, driving frequency, piezo transducer position and PDMS thickness. The microfluidic device consists of a microwell (with the diameter of 1mm and volume of ~95 nL) with two rectangular bubble traps (400×400μm) on both sides of the well. After the injection of liquid, air bubbles were spontaneously trapped in two rectangular traps. When the frequency of a piezo was equal to the resonance frequency of air bubbles, strong liquid recirculation formed (so called acoustic microstreaming) in the vicinity of the interface of air bubbles and water. The acoustic induced flow of microbeads and mixing of water and fluorescence dye were imaged to study the mixing efficiency. For a given voltage and PDMS thickness, when the piezo was placed on top of the well, the mixing was most vigorous. For a given frequency, the mixing efficiency was directly proportional to the voltage (4-20V) and inversely proportional to the PDMS thickness (0.3-2mm). When the frequency driving the piezo was approaching the resonance frequency of air bubbles, the mixing efficiency was maximal, while when it was far away from the resonance frequency of air bubbles, the mixing efficiency was much lower. This work provides guidance to the design and the application of bubble-induced acoustic mixing in microfluidics.
Estimation and prediction of noise power are very important for communication anti-jamming and efficient allocation of spectrum resources in adaptive wireless communication and cognitive radio. In order to estimate and predict the time-varying noise power caused by natural factors and jamming in the high frequency channel, Variational Bayesian algorithm and adaptive ARMA time series are proposed. Through establishing the time-varying noise power model, which controlled by the noise variance rate, the noise power can be estimated with Variational Bayesian algorithm, and the results show that the estimation error is related to observation interval. What’s more, through the analysis of the correlation characteristics of the estimation power, noise power can be predicted based on adaptive ARMA time series, and the results show that it will be available to predict the noise power in next 5 intervals with the proportional error less than 0.2.
KEYWORDS: Cameras, Power supplies, Proteins, Microfluidics, Control systems, System integration, In vitro testing, Prototyping, Photomultipliers, Signal processing
Microdroplet-based microfluidic devices are emerging as powerful tools for a wide range of biochemical screenings and analyses. Monodispersed aqueous microdroplets from picoliters to nanoliters in volume are generated inside microfluidic channels within an immiscible oil phase. This results in the formation of emulsions which can contain various reagents for chemical reactions and can be considered as discrete bioreactors. In this paper an integrated microfluidic platform for the synthesis, screening and sorting of libraries of an organophosphate degrading enzyme is presented. The variants of the selected enzyme are synthesized from a DNA source using in-vitro transcription and translation method. The synthesis occurs inside water-in-oil emulsion droplets, acting as bioreactors. Through a fluorescence based detection system, only the most efficient enzymes are selected. All the necessary steps from the enzyme synthesis to selection of the best genes (producing the highest enzyme activity) are thus integrated inside a single and unique device. In the second part of the paper, an innovative design of the microfluidic platform is presented, integrating an electronic prototyping board for ensuring the communication between the various components of the platform (camera, syringe pumps and high voltage power supply), resulting in a future handheld, user-friendly, fully automated device for enzyme synthesis, screening and selection. An overview on the capabilities as well as future perspectives of this new microfluidic platform is provided.
Since the past decade, the interest towards microfluidic devices has sensibly grown due to the wide variety of multidisciplinary applications. One branch of the microfluidic domain consists in the synthesis of various types of emulsions requested by cosmetic, food and biotechnological industries In particular, monodisperse water-in-oil microemulsion synthetised in microfluidic devices are quickly becoming the new generation of emulsions for precise bead control and high surface area. These microemulsions are generally aqueous bioreactors in the form of droplets from 500 nm to 10 μm in diameter, enclosed in an oil environment. An increasing demand for bigger emulsions has led us to investigate new techniques for fabricating fluidic devices allowing a better control over the final size of the droplets. An easy, cheap, reproducible and fast technology for generating emulsions in the range of 100s μm with high throughout (up to mL/h) is reported. Simply using pipette tips and tubing, an innovative microfluidic device was fabricated, able to synthetise water-in-oil emulsions within the range 50 – 500 _μm and double emulsions. These new emulsions are currently used for the synthesis of highly porous polymers beads from High Internal Phase Emulsion (HIPE). These beads will find high potential in 3D cell culture due to their high porosity (up to 90%) and pore size (from 5 to 30μm).
Capillary flow in microchannels has received substantial attention of investigation recently due to its potential
applications in microfluidics. This paper will report the new findings of capillary flow behavior in microfluidic Hele-
Shaw flow cells. Flow cells with a rectangular cross section of 50×50, 50×500, 20×200, and 50×1000μm were used. It
was observed that the shape of the meniscus varied with cell size and aspect ratio. The shape was also strongly affected
by the contact lines on both sidewalls and the gas phase flow in front of the meniscus. For cells with same height, the
meniscus was stretched more in the flow direction for wider cells. The measured averaged speed of the interface was 5.7,
8.3, 8.3 and ~8 mm/s for the flow cells with a cross section of 50×50, 50×500 and 50×1000μm, respectively. The speed
of interface movement was not affected significantly by the aspect ratio for the values used in the current study. The
average speed for the flow cells agree reasonably well with the value from the theoretical analysis.
Analysis of chemical warfare agents (CWAs) and their degradation products is an important verification
component in support of the Chemical Weapons Convention and urgently demanding rapid and reliable
analytical methods. A portable microchip electrophoresis (ME) device with contactless conductivity (CCD)
detection was developed for the in situ identification of CWA and their degradation products. A 10mM
MES/His, 0.4mM CTAB - based separation electrolyte accomplished the analysis of Sarin (GB), Tabun( GA)
and Soman (GD) in less than 1 min, which is the fastest screening of nerve agents achieved with portable ME
and CCD based detection methods to date. Reproducibility of detection was successfully demonstrated on
simultaneous detection of GB (200ppm) and GA (278ppm). Reasonable agreement for the four consecutive runs
was achieved with the mean peak time for Sarin of 29.15s, and the standard error of 0.58s or 2%. GD and GA
were simultaneously detected with their degradation products methylphosphonic acid (MPA), pinacolyl
methylphosphonic acid (PMPA) and O-Ethyl Phosphorocyanidate (GAHP and GAHP1) respectively. The
detection limit for Sarin was around 35ppb. To the best of our knowledge this is the best result achieved in
microchip electrophoresis and contactless conductivity based detection to date.
KEYWORDS: Microfluidics, Capillaries, Liquids, Data modeling, Polymethylmethacrylate, Nickel, Nanotechnology, System on a chip, Chemical analysis, Electrophoretic light scattering
Droplet generation in microfluidics has attracted a great deal of attention due to the potential applications in many areas
of science and technology. The understanding of the generation mechanism is still unsatisfactory and proposed models
lack generality for different microchips with flow conditions and channel geometries. In this paper, we present new
results of droplet generation in a PMMA microchip using flow-focusing technique. The current data are compared with
existing published data obtained with similar generation microchip method. The dependence of the droplet size/slug
length on the capillary number and ratio of the continuous phase and dispersed phase flow rates is investigated. A model
has been proposed which explains well the data from several similar studies.
The combined use of film transparency masks and dry film resist has allowed a rapid prototyping of designs and
structures in chips for droplet generation. Patterning of the film resist has produced channels with smooth vertical
sidewalls. The minimum feature dimension, δ, was reduced by increasing the resolution and spacing of the pattern
geometries in the film mask. For a single layer of resist (~35 μm thick), a minimum feature width of ~60 μm was
obtained using 2040 dpi transparency masks, 40 µm for 5800 dpi transparency masks and 25 μm using a Cr mask of
equal size/ spacing of features. A doubling of the spacing between features in a 2400 dpi masks resulted in an attainable
feature size of ~40 μm. The minimum feature dimension increased exponentially with thickness of 5038 resist.
Microfluidic chips which were fabricated in PMMA by this method have demonstrated controlled characteristics in the
generation of oil droplets in water.
Capillary Electrophoresis (CE) is a separation technique that can be used as a sample pre-treatment step in
chemical analysis. When coupled with a detection technique, identification of chemical species can be performed
on the basis of the elution signals. However, the sensor signals are often complicated by high signal noise,
varying baseline and overlapping peaks. There is thus a need for a signal processing technique capable of
robustly detecting peaks in acquired sensor data. Here, we report on an algorithm that utilises the Continuous
Wavelet Transform (CWT) for the detection of analyte peaks.
The algorithm that has been developed makes use of a wavelet equal to the first derivative of a Gaussian function
and has been successfully applied to data obtained from a CCD sensor fabricated on a polymer microfluidic
separation chip. The algorithm operates by taking the CWT of the sensor response. It then analyses patterns in
the local maximum and minimum points evident across scales in the CWT coefficients to find the peaks in the
time series data. The performance of two versions of the algorithm have been compared for synthetic data sets
each with known baseline, peaks and noise. The improved algorithm has been shown to successfully find peaks
with a high sensitivity and low False Discovery Rate within a range of sensitivities.
This paper reports on the development of a hand-held device for on-site detection of
organophosphonate nerve agent degradation products. This field-deployable analyzer relies on
efficient microchip electrophoresis separation of alkyl methylphosphonic acids and their sensitive
contactless conductivity detection. Miniaturized, low-powered design is coupled with promising
analytical performance for separating the breakdown products of chemical warfare agents such as
Soman, Sarin and VX . The detector has a detection limit of about 10 μg/mL and has a good linear
response in the range 10-300 μg/mL concentration range. Applicability to environmental samples is
demonstrated .The new hand-held analyzer offers great promise for converting conventional ion
chromatography or capillary electrophoresis sophisticated systems into a portable forensic laboratory
for faster, simpler and more reliable on-site screening.
Microbubbles have been used as ultrasound contrast agents in medical applications such as imaging, and also for
drug/gene delivery, target destruction and so on. Microbubbles are normally made by sonication techniques and the
resulting size distribution is very large. Microfluidics provides an alternative way of microbubble fabrication due to
recent advances in microfabrication and microfluidics development. The current techniques are capable of making
bubbles with a size of several micrometers. However, the throughput for such a size range is very limited. In this study, a
new microfluidic bubble generation chip was developed, which incorporates a T-junction PDMS microchannel network
with an inserted glass capillary. The flow rate of liquid, gas pressure and the inserted capillary inner diameter are crucial
for control of the bubble size. A series of capillaries with different inner diameters have been used. With co-flow
focusing liquids and a fine-drawn glass capillary, bubble size could be decreased and bubbles with a size of 13 μm in
diameter were generated reliably after the optimizing of liquid flow rate and gas pressure. It was found that a 5 μm
capillary inserted microchip produced 11 μm diameter bubbles with a cross-flow rupturing method.
Droplet-based in vitro compartmentalization (IVC) platform is a powerful tool in protein analysis. Reliable formation of microdroplets is important for the development of the microfluidic chip. In this study, we will examine the effect of surfactants on the formation of microdroplets in the flow focusing microfluidic device which is needed for enzyme evolution. Surfactants of Span 80 and Tween 20 are used in the continuous and dispersed phases, respectively. The droplet formation was affected distinguishably with the presence of Span 80 and Tween 20. The size of droplets decreased as the concentration of Span 80 increased. And the decrease was more pronounced with the combination of Span 80 and Tween 20.
Microchip-based electrophoretic separation systems are essential components in the development of fully integrated
micro total analysis systems. In this paper, a miniaturized analytical system for separating and detecting inorganic ions
is described. The system was based on a polycarbonate (PC) capillary electrophoresis (CE) chip and a contactless
conductivity detector, both being developed at CSIRO Microfluidics and Microfabrication Laboratories, Melbourne,
Australia. The PC chip was fabricated using the soft lithography technique in conjunction with nickel plating and hot
embossing. The detector electrodes were fabricated from a PCB board and attached on the separation chip bottom
surface. The thin capping layer (20 micron) of the chip allowed for sensitive detection of conductivity change. The
system was demonstrated to separate reliably the potassium, sodium and lithium ions in a 20mM MES/His buffer within
a minute at an electrical field of 28.5kV/m. The detection limit for the current design is around 100μM. Such a system
offers great promise to be integrated into robust hand-held devices for in-situ monitoring of chemical and biological
samples with high speed, reliability and low costs.
Disposable polymer microfluidic chips have been used more and more in miniaturized analytical devices. The surface of
the polymers often needs to be treated to acquire specific properties. This study investigates the characteristics of
capillary flow in three microfluidic chips under different surface conditions and the aim is to understand how the surface
property could affect the capillary flow over the shelf life of the chips. The channel surfaces of polymer chips were
treated using air plasma. The interface pattern and velocity were measured by a photographic technique and a micron
Particle Imaging Velocimetry (MicroPIV) method. The glass chip could maintain a capillary flow velocity of around 3.0
mm/s and showed little reduction with time. The velocity agreed well with theory by Washburn. The PDMS chip
surfaces could be easily modified and the capillary flow rate could reach 4 mm/s. However, the hydrophilicity decreased
rapidly over time and was lost completely within a few hours. The polycarbonate chips need more powerful surface
treatment. Once modified, the surface could sustain for much longer time. It took one month for the capillary flow
velocity to decrease by 50%.
Integration of microheaters in microfluidic systems has a wide range of applications such as sensing, actuating and biomedical devices. This study focused on the investigation of the thermal performance of a nickel microheater fabricated on a printed circuit board using a LIGA process. Both experimental and computational methods were used in conjunction with preliminary theoretical analysis. The microheater was tested both in air and water. The temperature distributions of the microheater were measured by an advanced thermography system. The numerical study was carried out using the multiphysics CFD package CFD-Ace+. The temperature of the microheater increased approximately linearly with the input power for the experimental conditions. The microheater heated up exponentially at the start of power supply. At a supply of 120mA electrical current, the microheater could heat up at a rate of around 90°C/s. During the cooling stage, the rate was much higher and could reach 800°C/s. When placed in the microchannel with air flow, the heater could heat the flow effectively without causing significant increase in the chip temperature. The CFD results were validated by comparing temperature distributions on the chip surface and on the heater surface. The validated CFD results allowed more detailed investigations of thermal and fluid flow within the microchannel and the whole chip.
Experiments are presented in which acoustic microstreaming is investigated and applied to a batch micromixing case appropriate to a point-of-care pathology screening test. The flows presented can be created without complex engineering of contacts or surfaces in the microdevice, which could thus be made disposable. Fundamental flow patterns are measured with a micro-Particle-Image Velocimetry (micro-PIV) system, enabling a quantification of the fluiddynamical processes causing the flows. The design of micromixers based on this principle requires a quantification of the mixing. A simple technique based on digital image processing is presented that enables an assessment of the improvement in mixing due to acoustic microstreaming. The digital image processing technique developed was shown to be non-intrusive, convenient and able to generate useful quantitative data. Preliminary indications are that microstreaming can at least halve the time required to mix quantities of liquid typical of a point-of-care test, and significantly greater improvements seem feasible.
Joule heating is a significant problem for microfluidic chips with electrokinetically driven flows. In this paper, we will present the modeling results of the Joule heating of a Polymethylmethacrylate (PMMA) polymer separation chip using both experimental and computational methods. The temperature distributions on the surface of the chip were measured by an advanced thermograph system. The numerical study was carried out using the multiphysics computational fluid dynamics (CFD) package CFD-Ace+. Different solutions and operating conditions were studied. Both the measurements and CFD data revealed that the heat generation was approximately uniform and the subsequent temperature increase was also uniform along the channel except for regions near the liquid ports. The highest temperature increase was observed along the centerline of the channel and the temperature reduced significantly away from the channel. At an electrical field of 45kV/m, the Joule heating effect was negligible for the solution used, even though at such a high electric field significant heating effect has been observed for micro capillary flows in literature. At a higher electrical field (68-120kV/m), the Joule heating could cause an increase of temperature of up to 40°C.
Micro particle image velocimetry (PIV) measurements of the velocity fields around oscillating gas bubbles in microfluidic geometries were undertaken. Two sets of experiments were performed. The first measured the acoustic microstreaming around a gas bubble with a radius of 195 μm attached to a wall in a chamber of 30 mm× 30 mm× 0.66 mm. Under acoustic excitation, vigorous streaming in the form of a circulation around on the bubble was observed. The streaming flow was highest near the surface of the bubble with velocities around 1mm/s measured. The velocity magnitude decreased rapidly with increasing distance from the bubble. The velocity field determined by micro-PIV matched the streaklines of the fluorescent particles very well. The second set of experiments measured the streaming at the interface between a trapped air bubble and water inside a microchannel of cross section 100 μm × 90 μm. The streaming flow was limited to within a short distance from the interface and was observed as a looping flow, moving towards the interface from the top and being circulated back from the bottom of the channel. The characteristic streaming velocity was in the order of 100 μm/s.
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