Microfluidics: a few good tricks.
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Cell Separation↗
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Results for “Microfluidics”
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A microfluidic chemostat for experiments with bacterial and yeast cells.
Bacteria and yeast frequently exist as populations capable of reaching extremely high cell densities. With conventional culturing techniques, however, cell proliferation and ultimate density are limited by depletion of nutrients and accumulation of metabolites in the medium. Here we describe design and operation of microfabricated elastomer chips, in which chemostatic conditions are maintained for bacterial and yeast colonies growing in an array of shallow microscopic chambers. Walls of the chambers are impassable for the cells, but allow diffusion of chemicals. Thus, the chemical contents of the chambers are maintained virtually identical to those of the nearby channels with continuous flowthrough of a dynamically defined medium. We demonstrate growth of cell cultures to densely packed ensembles that proceeds exponentially in a temperature-dependent fashion, and we use the devices to monitor colony growth from a single cell and to analyze the cell response to an exogenously added autoinducer.
Bioreactors↗
Differential measurement with a microfluidic device for the highly selective continuous measurement of histamine released from rat basophilic leukemia cells (RBL-2H3).
We fabricated a micro-fluidic device for the highly selective detection of the histamine released from rat basophilic leukemia (RBL) 2H3 cells. The device has two thin layer flow channels, each with one working electrode. One electrode was modified with Os-polyvinylpyridine based mediator containing horseradish peroxidase (Os-gel-HRP) and histamine oxidase (HAOx), the other was modified with Os-gel-HRP without any HAOx. We employed the device for differential measurement by using the HAOx modified electrode for detection and the unmodified electrode as a reference. The detection limit was greatly improved from 190 to 25 nM since the baseline noise level was suppressed. We used differential measurement to observe the histamine released from RBL-2H3 cells when stimulated with dinitrophenylated bovine serum albumin (DNP-BSA) as an antigen. We injected 5 microM of histamine solution into our device and it remained stable for more than 8 h.
Animals↗
PDMS 2D optical lens integrated with microfluidic channels: principle and characterization.
In this paper, the fabrication and characterization of PDMS 2D-optical lenses are reported. These lenses are designed in order to improve the performance of fluorescent spectroscopy detection performed on a portable chip using optical fibers. The fabrication process of the PDMS layer is first detailed, and the patterns are then checked with a SEM. By comparing various interfacial structures, it is shown that the beam properties of the light coming out from the fiber can be modified depending on the lens curvature radius. As a consequence, for a constant dye concentration, the use of such lenses can increase the intensity of fluorescent response close to the fiber or far from the fiber, compared to the same design with a flat interface. This excitation improvement corresponding to a stronger response from the dye then consequently leads to around three times higher sensitivity of the on-chip detection method for fluorescent spectroscopy.
Dimethylpolysiloxanes↗
The development of integrated microfluidic systems at GeSiM.
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Equipment Design↗
Dealing with real samples: sample pre-treatment in microfluidic systems.
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Equipment Design↗
Microfluidic valve with cored glass microneedle for microinjection.
In this paper, a new microinjection device was constructed by fusing a glass microneedle and a PDMS-based microvalve. The microneedle was fabricated via traditional micropipette pulling. The PDMS-based microvalve regulates the fluid flow in the microchannel and microneedle. The 'ON/OFF' operation of the valve was controlled by manually supplied pneumatic pressure. The valve membrane utilized a two level geometry to improve control at low flow rates. The relation between pressure and flow was measured and the results showed that very small volumes of fluid (>1 nl) could be controlled. The valve operation was investigated by monitoring the tip of the needle and pneumatic pressure simultaneously and it demonstrated very stable 'ON/OFF' operation to the pressure change.
Dimethylpolysiloxanes↗
Capacitive sensing of droplets for microfluidic devices based on thermocapillary actuation.
The design and performance of a miniaturized coplanar capacitive sensor is presented whose electrode arrays can also function as resistive microheaters for thermocapillary actuation of liquid films and droplets. Optimal compromise between large capacitive signal and high spatial resolution is obtained for electrode widths comparable to the liquid film thickness measured, in agreement with supporting numerical simulations which include mutual capacitance effects. An interdigitated, variable width design, allowing for wider central electrodes, increases the capacitive signal for liquid structures with non-uniform height profiles. The capacitive resolution and time response of the current design is approximately 0.03 pF and 10 ms, respectively, which makes possible a number of sensing functions for nanoliter droplets. These include detection of droplet position, size, composition or percentage water uptake for hygroscopic liquids. Its rapid response time allows measurements of the rate of mass loss in evaporating droplets.
Capillary Action↗
Proliferation of microfluidics in literature and intellectual property.
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Humans↗
A microfluidic electroporation device for cell lysis.
We demonstrate a micro-electroporation device for cell lysis prior to subcellular analysis. Simple circuit models show that electrical lysis method is advantageous because it is selective towards plasma membrane while leaving organelle membrane undamaged. In addition, miniaturization of this concept leads to negligible heat generation and bubble formation. The designed microdevices were fabricated using a combination of photolithography, metal-film deposition, and electroplating. We demonstrate the electro-lysis of human carcinoma cells in these devices to release the subcellular materials.
Cell Fractionation↗
Continuous cell washing and mixing driven by an ultrasound standing wave within a microfluidic channel.
Ultrasound standing wave radiation force and laminar flow have been used to transfer yeast cells from one liquid medium to another (washing) by a continuous field-flow fractionation (FFF) approach. Two co-flowing streams, a cell-free suspending phase (flow rate > 50% of the total flow-through volume) and a yeast suspension, were introduced parallel to the nodal plane of a 3 MHz standing wave resonator. The resonator was fabricated to have a single pressure nodal plane at the centre line of the chamber. Laminar flow ensured a stable interface was maintained as the two suspending phases flowed through the sound field. Initiation of the ultrasound transferred cells to the cell-free phase within 0.5 s. This particle transfer procedure circumvents the pellet formation and re-suspension steps of centrifuge based washing procedures. In addition, fluid mixing was demonstrated in the same chamber at higher sound pressures. The channel operates under negligible back-pressure (cross-section, 0.25 [times] 10 mm) and with only one flow convergence and one flow division step, the channel cannot be easily blocked. The force acting on the cells is small; less than that experienced in a centrifuge generating 100g. The acoustically-driven cell transfer and mixing procedures described may be particularly appropriate for the increasingly complex operations required in molecular biology and microbiology and especially for their conversion to continuous flow processes.
Cell Count↗
Glass microfluidic devices with thin membrane voltage junctions for electrospray mass spectrometry.
In this study a novel glass membrane was prepared for conducting high voltage (HV) to solution in the channel of a microfabricated device for generation of liquid electrospray. Taylor cone formation and mass spectra obtained from this microdevice confirmed the utility of the glass membrane, but voltage conduction through the membrane could not be successfully explained based solely on the conductivity of the glass itself. This novel method for developing a high-voltage interface for microdevices avoids direct metal/liquid contact eliminating bubble formation in the channel due to water hydrolysis on the surface of the metal. Further, this arrangement produces no dead volume as is often found with traditional liquid junctions. At the same time, preliminary investigations into the outlet design of glass microdevices for interfacing with electrospray mass spectrometry, was explored. Both the exit shape and the use of hydrophobic coatings at the channel exit of the microdevice electrospray interface were evaluated using standard proteins with results indicating the utility of this type of design after further optimization.
Equipment Design↗
Shear force induced monodisperse droplet formation in a microfluidic device by controlling wetting properties.
Perpendicular flow is used to induce oil droplet breakup by using a capillary as water phase flow channel. It is a new route to produce monodisperse emulsions. The wetting properties of the fluids on the walls are exceedingly important parameters. Depending on the oil and water flow rates, different spatial distributions of the two phases as laminar, plugs, cobbles and drops, are obtained. The effects of two-phase flow rates on plugs and drop size are studied, and the different droplet formation mechanisms of plug flow and drop flow are discussed. Two quantitative equations utilized to predict the droplet size are developed.
Emulsions↗
Microfluidic diffusive filter for apheresis (leukapheresis).
Apheresis is a procedure used to fractionate whole blood into its individual components. Following fractionation, the desired component is isolated and the remaining blood in many cases is returned to the donor. Leukapheresis is one type of apheresis where leukocytes (white blood cells) are selectively removed. This procedure is commonly used for blood transfusions to remove donor leukocytes from being transferred to the recipient. Apheresis also has several therapeutic applications. In this manuscript we discuss the design, fabrication and testing of a continuous flow diffusive filter, fabricated using simple soft lithographic techniques for depletion of leukocytes. This device employs micro sieves that exploit the size and shape difference between the different cell types to obtain depletion of leukocytes from whole blood. Currently, conventional apheresis methods like centrifugation or fiber mesh filtration are commonly used. A theoretical model was developed to determine the optimal shape of the diffuser to ensure that the volumetric flow through individual sieve elements is equal. This device was designed to serve as a passive device that does not require any external manipulation. Results show that for the given device design, isolation of approximately 50% of the inlet erythrocytes (red blood cells), along with depletion of >97% of the inlet leukocytes is possible at a flow rate of 5 microl min(-1). Simple modifications to the geometry and dimensions of the sieves can be made to obtain isolation of plasma.
Blood Component Removal↗
Fabrication and characterization of poly(methylmethacrylate) microfluidic devices bonded using surface modifications and solvents.
The fabrication of polymer microchips allows inexpensive, durable, high-throughput and disposable devices to be made. Poly(methylmethacrylate) (PMMA) microchips have been fabricated by hot embossing microstructures into the substrate followed by bonding a cover plate. Different surface modifications have been examined to enhance substrate and cover plate adhesion, including: air plasma treatment, and both acid catalyzed hydrolysis and aminolysis of the acrylate to yield carboxyl and amine-terminated PMMA surfaces. Unmodified PMMA surfaces were also studied. The substrate and cover plate adhesion strengths were found to increase with the hydrophilicity of the PMMA surface and reached a peak at 600 kN m(-2) for plasma treated PMMA. A solvent assisted system has also been designed to soften less than 50 nm of the surface of PMMA during bonding, while still maintaining microchannel integrity. The extent to which both surface modifications and solvent treatment affected the adhesion of the substrate to the cover plate was examined using nanoindentation methods. The solvent bonding system greatly increased the adhesion strengths for both unmodified and modified PMMA, with a maximum adhesion force of 5500 kN m(-2) achieved for unmodified PMMA substrates. The bond strength decreased with increasing surface hydrophilicity after solvent bonding, a trend that was opposite to what was observed for non-solvent thermal bonding.
Equipment Design↗
Flow-induced deformation of shallow microfluidic channels.
We study the elastic deformation of poly(dimethylsiloxane) (PDMS) microchannels under imposed flow rates and the effect of this deformation on the laminar flow profile and pressure distribution within the channels. Deformation is demonstrated to be an important consideration in low aspect ratio (height to width) channels and the effect becomes increasingly pronounced for very shallow channels. Bulging channels are imaged under varying flow conditions by confocal microscopy. The deformation is related to the pressure and is thus non-uniform throughout the channel, with tapering occurring along the stream-wise axis. The measured pressure drop is monitored as a function of the imposed flow rate. For a given pressure drop, the corresponding flow rate in a deforming channel is found to be several times higher than expected in a non-deforming channel. The experimental results are supported by scaling analysis and computational fluid dynamics simulations coupled to materials deformation models.
Computer Simulation↗
Switchable surface traps for injectable bead-based chromatography in PDMS microfluidic channels.
We report here a reversible microchannel surface capture system for stimuli-responsive grafted bioanalytical beads. Poly(N-isopropylacrylamide) (PNIPAAm) was grafted onto polydimethylsiloxane (PDMS) surfaces by a UV-mediated graft polymerization from a photoinitiator that was preadsorbed in the channel wall. The surface grafting density and resulting switchable hydrophilic/hydrophobic properties were controlled by varying the photo-illumination times and/or the initiator concentration. At limiting PNIPAAm-graft densities, the surfaces demonstrated minimal contact angles of 35 degrees below the lower critical solution temperature (LCST) and maximal contact angles of 82 degrees above it. These contact angles could be varied depending on the graft density. The surface grafts are spatially limited to the photo-illuminated region to define where the trap is constructed. The surface traps capture PNIPAAm-grafted nanobeads uniformly above the LCST and facilitate their rapid release as the temperature is reversed to below the LCST. This dual surface trap and injectable chromatography system could be useful in many applications, such as affinity separations, immunoassays, and enzyme bioprocesses, by providing for the controlled capture and release of chromatography beads.
Acrylic Resins↗
A one-step protocol for the chemical derivatisation of glass microfluidic devices.
A simple and robust derivatisation system for glass and silica microdevices is described. The device surface is coated in a one-step treatment with a highly cross-linked polystyrene/divinylbenzene/allylsiloxane copolymer. The surface derivatisation is highly resistant to solvents, acids, bases and oxidising or reducing agents.
Glass↗