Microchannel Size Research Articles

In vitro Verification of a 3-D Regenerative Neural Interface Design: Examination of Neurite Growth and Electrical Properties Within a Bifurcating Microchannel Structure

Toward the development of neuroprosthesis, we propose a 3-D regenerative neural interface design for connecting with the peripheral nervous system. This approach relies on bifurcating microstructures to achieve defasciculated ingrowth patterns and, consequently, high selectivity. In vitro studies were performed to validate this design by showing that fasciculation during nerve regeneration can be influenced by providing a scaffold to guide growth appropriately. With this approach, neurites can be separated from one another and guided toward specific electrode sites to create a highly selective interface. The neurite separation characteristics were examined for smaller microchannel structures (2.5 and 5 ?m wide) and larger microchannels (10 and 20 ?m wide), with smaller microchannels shown to be statistically more effective at initiating separation. Electrodes incorporated at different locations within the microchannels allowed for the recording and tracking of action potential propagation. Microchannel size was also found to play an important role in this regard, with smaller microchannels amplifying the recordable extracellular signal; a twofold increase in the signal to noise ratio was found for 5 ?m wide microchannels.

Dielectrophoretic choking phenomenon in a converging-diverging microchannel.

Experiments show that particles smaller than the throat size of converging-diverging microchannels can sometimes be trapped near the throat. This critical phenomenon is associated with the negative dc dielectrophoresis arising from nonuniform electric fields in the microchannels. A finite-element model, accounting for the particle-fluid-electric field interactions, is employed to investigate the conditions for this dielectrophoretic (DEP) choking in a converging-diverging microchannel for the first time. It is shown quantitatively that the DEP choking occurs for high nonuniformity of electric fields, high ratio of particle size to throat size, and high ratio of particle's zeta potential to that of microchannel.

On-chip direct methanol fuel cells of a monolithic design: consideration on validity of active-type system

The performance of an on-chip direct methanol fuel cell of monolithic design with an active-type system was improved more than 6-fold up to >100 µW by using hydrogen peroxide as an oxidant solution instead of dissolved oxygen, but the energy-loss caused by strong capillary forces required a redesign of both microchannel size and the system.

An in vitro study of the relationship between the size of lipid-coated microbubbles and frequency-dependent attenuation coefficient.

Monodisperse populations of lipid‐coated microbubbles were produced using a flow‐focusing microfluidic device. The microbubbles were formed by forcing a lipid solution and perfluorocarbon gas through a narrow orifice simultaneously. The average bubble size depended on the lipid solution flow rate, the gas pressure, and the size of the inlet microchannels and narrow orifice. We have successfully produced monodispersions of microbubbles with average diameters ranging from 4–11 μm. Monodisperse bubble populations were suspended in an exposure vessel, and the attenuation coefficient between 1–7.5 MHz was measured using an acoustic spectroscopy method. The frequency at which the peak attenuation coefficient was measured was dependent on the mean bubble diameter. For example, microbubbles with diameters of 4 and 8 μm had peak attenuation coefficients at approximately 3.0 and 1.5 MHz, respectively. Additionally, it was noted that the bandwidth of the frequency‐dependent attenuation coefficients was wider for polydisperse populations compared to monodisperse populations of lipid‐coated microbubbles. Implications of these results on the acoustic characterization and use of monodispersions of lipid‐coated microbubbles for molecular imaging will be discussed.

Rheological behaviour of low/high density polyethylene melt flowing through micro-channels

AbstractThe determination of the proper rheological behaviour of the polymer melt within micro structured geometry is vital for accurately simulating the micro moulding. The paucity of suitable equipments is one of main hurdles in the investigation of micro melt rheology. In the present study, a measurement system for the melt viscosity of low and high density polyethylene polymer melts flowing through micro-channels was established. The capillary flow model with Rabinowitsch correction was used in the calculations of the viscosity based on the measured pressure drop and volumetric flow rate. The effect of the morphology structure on the viscosity characteristics for both the LDPE and HDPE resins within micro-channels was investigated and discussed. It was found that the measured viscosity values for LDPE and HDPE in the test ranges are significantly lower (about 40~56% and 22~29% for LDPE and HDPE, respectively, flowing through a channel size of 150μm) than those obtained with a traditional capillary rheometer. Moreover, both the percentage reduction in the viscosity value and the ratio of the slip velocity to the mean velocity increase as the micro-channel size decreases. It can be observed that the rheological behaviours of the HDPE and LDPE resins in microscopic scale are different from those in macroscopic scale as a result of the wall slip effect. It also revealed that the wall slip occurs more easily for the LDPE resin within micro channels than HDPE resin due to enlarged effect of morphology structure.

Numerical study on micro-reformer performance and local transport phenomena of the plate methanol steam micro-reformer

The objective of this work is to investigate the transport phenomena and performance of a plate steam methanol micro-reformer. Micro channels of various height and width ratios are numerically analyzed to understand their effects on the reactant gas transport characteristics and micro-reformer performance. In addition, influences of Reynolds number and geometric size of micro channel on methanol conversion of micro-reformer and gas transport phenomena are also explored. The predicted results demonstrated that better performance is noted for a micro channel reformer with lower aspect-ratio micro channel. This is due to the larger the chemical reaction surface area for a lower aspect-ratio channel reformer. It is also found that the methanol conversion decreases with increasing Reynolds number Re. The results also indicate that the smaller micro channel size experiences a better methanol conversion. This is due to the fact that a smaller micro channel has a much more uniform temperature distribution, which in turn, fuel utilization efficiency is improved for a smaller micro channel reformer.

Microneedle mediated delivery of nanoparticles into human skin

The development of novel cutaneous delivery technologies that can produce micron-sized channels within the outermost skin layers has stimulated interest in the skin as an interface for localised and systemic delivery of macromolecular and nanoparticulate therapeutics. This investigation assesses the contribution of physicochemical factors to the rate and extent of nanoparticle delivery through microchannels created in a biological tissue, the skin, by novel delivery technologies such as the microneedle array. The hydrodynamic diameter, zeta potential and surface morphology of a representative fluorescent nanoparticle formulation were characterised. Permeation studies using static Franz-type diffusion cells assessed (i) the diffusion of nanoparticle formulations through a model membrane containing uniform cylindrical microchannels of variable diameter and (ii) nanoparticle penetration across microneedle treated human skin. Wet-etch microneedle array devices can be used to significantly enhance the intra/transdermal delivery of nanoparticle formulations. However the physicochemical factors, microchannel size and particle surface charge, have a significant influence on the permeation and subsequent distribution of a nanoparticle formulation within the skin. Further work is required to understand the behaviour of nanoparticle formulations within the biological environment and their interaction with the skin layers following disruption of the skin barrier with novel delivery devices such as the microneedle array.

Investigation on Methanol Steam Reforming in a Microchannel Reactor by Experiments and Mathematical Model Analysis

A mathematic model of steam reforming of methanol in a microchannel reactor is presented. The effects of two parameters on the performance of microchannel methanol reformers have been investigated by the mathematical model, such as the thickness of catalyst coatings and the size of microchannels. Results indicate that the near-isothermal operation can be achieved due to the high coefficients of mass and heat transfer in microchannels. It has been found that significant volume and weight reduction is possible by using microchannel reactors attributed to the negligible heat and mass transfer limitations in catalyst coatings. The microchannel type reactor with thin catalyst coating is favorable for a low CO concentration level. Based on the calculation results, a microchannel reactor has been designed and developed by the methods of electric spark processing, diffusion bonding, and coating. The experimental results show that the developed microchannel reactor could generate enough hydrogen for power output of 11 W.

Experiment Study and Numerical Analysis of Flow and Heat Transfer in (110) Silicon Base Microchannel

Microchannel heat sink is fabricated on silicon wafer by anisotropic etching, and used Pyrex #7740 as a transparent cover that integrated by anodic bonding. Rectangular microchannel presents the flow phenomena of fluid in micro scale, and this study focus on the boundary conditions which hydraulic diameter (Dh) is from 80m to 350m and Aspect ratio is from 0.24~7.8 of working fluid (DI water). While the size of microchannel is decreasing, laminar flow occurs on the low Reynolds number, which caused by the interaction of viscosity and friction on boundary layer. Sequentially, the influence of dimension decreasing on microchannel that induced transition and turbulent flow in early stage as Reynolds number is still in the range of 600~800. Pressure drop is high (2 bar) when fluid flows through the micro channel, and flux is constrained by the flow resistance during experiment operating. In this study, it takes effect by increasing aspect ratio to reduce pressure drop and enlarge the conductive surface. Geometry of microchannel, hydraulic diameter, and aspect ratio are the key factors in flow phenomena investigation. This research presents the difference between micro scale flow and traditional pipe flow by consideration of Reynolds number. By using computer aided engineering to optimize the aspect ratio of microchannel, which can find the maximum conductive surface under the limitation of pressure drop. The best value of aspect ratio is 0.88~1.22. The simulation result makes good sequence with experiment data. Based on this methodology, numerical analysis can be used to design the optimal microchannel on wafer for cooling hot spot.

Rheological behavior of POM polymer melt flowing through micro-channels

Determination of the rheological behavior of the polymer melt within micro-structured geometry is vital for accurate simulation modeling of micro-molding. The lack of commercial equipment is one of main hurdles in the investigation of micro-melt rheology. In this study, a melt viscosity measurement system for POM melt flowing through micro-channels was established. For measured pressure drop and volumetric flow rate, both capillary and slit flow models were used for the calculation of viscosity. The calculated results were also compared with those of PS resin to discuss the effect of morphology structure on the viscosity characteristics of polymer within micro-channels. It was found that the measured POM viscosity values in the test ranges are significantly lower (about 29–35% for a channel size of 150 μm) than those obtained with a traditional capillary rheometer. Meanwhile, the percentage reduction in the viscosity value and the ratio of slip velocity relative to mean velocity all increase with decreasing micro-channel size, but less significantly when compared with PS resin. In the present study we emphasize that the rheological behavior of the POM resin in microscopic scale is also different from that of macroscopic scale as PS resin but displays a less significant lower. It also revealed that the wall slip occurs more easily for the PS resin within micro-channels than POM resin due to enlarge the effect of molecular weight.

Breakup of Oil-in-Water Emulsion Droplets in Microchannel Array Devices at Low Applied Pressures

The breakup phenomena of large oil-in-water (O/W) emulsion droplets in silicon microchannel (MC) array devices at low applied pressures were investigated by microscopic and high-speed observations. This study used two sizes of MC arrays, each of which consisted of uniformly sized MCs, slit-like terraces, and deeply etched wells. Monodisperse feed O/W emulsion droplets that had entered the MC arrays were broken up into fine droplets primarily by snap-off that occurs due to localized shear inside the MCs and by pinch-off due to interfacial tension effects inside the MCs or on the inlet side of a terrace. The average droplet size of the permeate O/W emulsions depended significantly on the MC size and exceeded the MC size by 1.2 to 1.4 times. Their coefficients of variation were 18% to 21%. The results obtained from preceding work were compared with the results for direct generation of O/W emulsion droplets using the same MC array device.

Novel Bamboo-Like Fibrous, Micro-Channeled and Functional Gradient Microstructure Control of Ceramics

In this paper development of novel ceramic microstructures by the fibrous monolithic process were reported. Attempts were made to tailor the microstructures of these advanced ceramics by introducing fibrous or layered morphology to incorporate multi-toughening mechanisms. Fabrication and characterization of four different kinds of microstructure, termed in this manuscript as network type, double network type, functional gradient microchanneled and functional gradient microchanneled double network type microstructure, were reported. In network and double network type microstructure developments the two phase Al 2 O 3 -(m-ZrO 2 ) cores were embedded within network-type frames of t-ZrO 2 single phase. In the double network type microstructure, an additional thicker network-type t-ZrO 2 enclosure surrounded a network-type micro-assembly. Unidirectionally aligned continuously porous HAp-Al 2 Ο 3 -ZrO 2 tri-phase ceramic composites were fabricated by the same method to incorporated functional gradient for improved functionality and depress processing defects. The microstructure had three layers of HAp/HAp-(Al 2 Ο 3 -(m-ZrΟ 2 ))/Al 2 Ο 3 -(t-ZrΟ 2 ) around the continuous pore with compositional gradient. In the functionally gradient micro-channeled HAp/ HAp-(Al 2 Ο 3 -(m-ZrΟ 2 ))/Al 2 Ο 3 -(t-ZrΟ 2 ) composites, the size and distribution of micro-channels were homogeneously controlled. Detailed morphological analyses of the resulting microstructures were studied.

Continuous Size Classification of Nanoparticles Utilizing Brownian Motion in Microchannel Size Exclusion Chromatography

AbstractThis study presents a new method for classifying the sizes of colloidal nanoparticles of below 100 nm in diameter in liquid dispersion using a microchannel size exclusion chromatography (SEC) chip. This chip can classify polydisperse colloidal nanoparticles containing a mix of two monodisperse nanoparticles into several monodisperse particle populations. The particles classified by the SEC chip are then sequentially analyzed by a photon correlation spectroscopy (PCS) method in combination with a flow cell. Two different pillar patterns of such SEC chips were used in experiments to investigate the effects of these patterns on the nanoparticle classification performance. The results obtained were compared with those from a numerical simulation. Standard polystyrene latex particles with diameters of 20 nm and 100 nm were used in this study. The usefulness of this methodology was verified since the simulation and measurement results were in good agreement with each other.

Optimization design of microchannel cooling heat sink

PurposeThis paper sets out to optimize the shape and size of microchannels cooling heat sink, which has been widely used to cool electronic chip for its high heat transfer coefficient and compact structure.Design/methodology/approachSequential Quadratic Programming (SQP) method is used to optimize the cross‐section sizes of microchannels. Finite volume method is used to numerically simulate the cooling performance of optimal microchannel cooling heat sink.FindingsThe optimized cross‐section shape of microchannel is rectangular, and the width and depth of microchannel is 50 and 1,000 μm, respectively, the number of microchannels is 60, and the corresponding least thermal resistance is 0.115996°C/W. The results show that the heat transfer performance of microchannel cooling heat sink is affected intensively by its cross‐section shape and dimension. The convection heat resistance Rconv between inner surface in microchannels and working fluid has more influence in the total heat resistance. The heat flux of chip is 278 W/cm2 and, through the optimization microchannel cooling heat sink, the highest temperature in the chip can be kept below 42°C, which is about half of that without optimizing heat sink and can ensure the stability and reliability of chip.Research limitations/implicationsThe convection heat transfer coefficient is calculated approximatively here for convenience, and that may induce some errors.Originality/valueThe optimized microchannels cooling heat sink may satisfy the request for removal of high heat flux in new‐generation chips.

Efficient formation of uniform-sized embryoid bodies using a compartmentalized microchannel device

The formation of spherical aggregates of cells called embryoid bodies (EBs) is an indispensable step in many protocols in which embryonic stem (ES) cells are differentiated to other cell types. Appropriate morphology and embryo size are critical for the sequential developmental stages of naturally conceived embryos. Likewise, regulating the size of EBs and the timing of their formation is crucial for controlling the differentiation of ES cells within the EB. Existing methods of formation of EBs, however, are tedious or provide heterogeneously-sized EBs. Here we describe a microfluidic system for straightforward synchronized formation of uniform-sized EBs, the size of which can be controlled by changing the cross-sectional size of microchannels in the microfluidic device. The device consists of two microchannels separated by a semi-porous polycarbonate membrane treated to be resistant to cell adhesion. ES cells introduced into the upper channel self-aggregate to form uniformly-sized EBs. The semi-porous membrane also allows subsequent treatment of the non-attached EBs with different reagents from the lower channel without the need for wash out because of the compartmentalization afforded by the membrane. This method provides a simple yet robust means to control the formation of EBs and the subsequent differentiation of ES cells in a format compatible for ES cell processing on a chip.

Influence of gravity on a laminar flow in a microbioanalysis system

Laminar flow which is formed in a microchannel generally provides specific microfluidic characteristics and is applied to a variety of uses such as micro-biochemical analysis, handling of biological objects and micromixing. Influence of gravity on the laminar flow behaviour has not been much investigated because its influence has been considered to be very small due to the largeness of surface effects. In this work, a quantitative investigation into gravity influence on horizontal two-layer laminar flow was carried out using computational fluid dynamics simulation. Results showed the possibility of representing horizontal two-layer laminar flow behaviour by a dimensionless parameter which is a function of density difference, viscosity, microchannel size and velocity, under the condition that the microchannel cross section is square, and interfacial tension and diffusion are negligible. The dimensionless parameter becomes important especially for microfluidics where laminar flow can be formed easily. This simple parameter enables a rough estimation of the horizontal two-layer laminar flow behaviour. The results in this work are useful for microdevice design, biochemical analysis, mixing and chemical reaction.

Flow characteristics of water in straight and serpentine micro-channels with miter bends

Flow characteristics of pressure-driven de-ionized water were investigated experimentally in straight and serpentine micro-channels with miter bends. The micro-channels had rectangular cross-sections with hydraulic diameters of 0.209 mm, 0.395 mm and 0.549 mm. To evaluate bend loss coefficient in the serpentine micro-channel and micro-scale size effect on it, the additional pressure drop due to the miter bend must be obtained. This additional pressure drop can be achieved by subtracting the frictional pressure drop in the straight micro-channel from the total pressure drop in the serpentine micro-channel. Since currently there still has a debate on the relationship between the friction factor and Re number in the straight micro-channel, the frictional pressure drop had to be obtained experimentally here. Three groups of micro-channels were fabricated to remove the inlet and outlet losses. The experimental results show that after considering the measurement uncertainties the experimental Poiseuille number can be well predicted by the conventional laminar incompressible flow theory when Re number is less than some value around 1500, the discrepancy observed by the former researchers can be attributed to not accounting for the additional pressure drop in the entrance region. The onset of transition to turbulence might be at 1500–1700. For serpentine micro-channels, the additional pressure drop can be divided into two regions. One is Re < 100. It is very small since no circulation exists. The other one is Re larger than some value in 100–200. At this time the circulation appears and develops at the inner and outer wall of the bend. The additional pressure drop increases sharply with Re number. The bend loss coefficient was observed to decrease and tend to be a constant with decreasing Re number. It is found to be larger than the predicted value for macro-channel turbulent flow and related with the channel size when flow separation appears, namely Re > 100–200.

Application of a microchannel to catalytic dehydrogenation of cyclohexane on Pd membrane

A microreactor was constructed by inserting a stainless-steel rod into a Pd membrane tube reactor to investigate the effects of microchannel size on the dehydrogenation of cyclohexane to benzene. The Pd membrane, having selective permeability for hydrogen, was coated onto the surface of an α-alumina porous tube. The dehydrogenation of cyclohexane was carried out at 573 K and atmospheric pressure. The yield of benzene was affected by varying the diameter of the stainless-steel rod, and the yield with the thickest rod was approximately two times higher than that without any rods. The application of the microchannels apparently increased the surface area per volume of the Pd membrane, and thus increased the catalytic reaction on the Pd membrane. The present results indicate that the dehydrogenation of cyclohexane is accelerated by applying microchannels.

Size scale effects on cavitating flows through microorifices entrenched in rectangular microchannels

Cavitating flow of deionized water through various microorifices and microchannels has been investigated. Multifarious cavitating flow patterns, including incipient, choking and supercavitation have been detected. Effects of microorifice and microchannel size on cavitation have been discussed and results indicate the existence of strong size scale effects. Incipient and choking cavitation numbers are observed to increase with the area ratio between the microorifice and the microchannel while the orifice discharge coefficient plummets once cavitation activity erupts. Additionally, for a fixed microchannel width, the incipient and the choking cavitation numbers rise with the ratio between the hydraulic diameters of the microorifice and the microchannel. In addition, velocity and pressure effects on cavitation have been investigated for several microorifices and the observed trends have been compared with established macroscale results. Furthermore, the flow patterns encountered at choking and supercavitation are significantly influenced by the microorifice and microchannel size. Flow rate choking occurs irrespective of the inlet pressures and is a direct consequence of cavitation inside the microorifice. The predicted choked cavitation number is always higher than the experimental data. This discrepancy is suspected to be the result of exceedingly small residence time for nuclei growth and the ability of the liquid to withstand low pressures at such scales. Flow and cavitation hysteresis is observed but its effects are more pronounced for the smallest microorifice.

Incompressible and compressible flows through rectangular microorifices entrenched in silicon microchannels

Incompressible and compressible flows through indispensable configurations such as rectangular microorifices entrenched in microchannels have been experimentally investigated. The current endeavor evaluates the effects of microorifice and microchannel size, estimates the discharge coefficients associated with both compressible and incompressible flows, examines the contraction coefficients, probes subsonic and supercritical gas flows, and explores the presence of any anomalous effects such as those reported for microchannels. The discharge coefficient in incompressible flow, using deionized (DI) water as the working fluid, rises and peaks at a critical Reynolds number, (200/spl les/Re/sub Crit//spl les/500). The reported range of the transitional Reynolds number compares favorably with the values observed in conventional scale studies and suggests the absence of any irregular scaling effects. Furthermore, nitrogen flows through various microorifices suggests that the constriction element rather than the microchannel area determines the flow rate. Additionally, the critical pressure ratio at choking is close to the isentropic value (0.47/spl les/(P/sub 2//P/sub 1/)/sub Crit//spl les/0.64) and no anomalous scale or slip effects have been observed. Unlike macroscale compressible flows through an orifice, the losses seem minimal and the discharge coefficients are close to unity. The geometry acts as a smooth converging-diverging nozzle and the mass flow rate trends appear similar to the data obtained in micronozzle flows.