Simple Microchannels Research Articles

In-line Raman imaging of mixing by herringbone grooves in microfluidic channels.

The control over fluid flow achievable in microfluidic devices creates opportunities for applications in many fields. In simple microchannels, flow is purely laminar when one solvent is used, and hence, achieving reliable mixing is an important design consideration. Integration of structures, such as grooves, into the channels to act as static mixers is a commonly used approach. The mixing induced by these structures can be validated by determining concentration profiles in microfluidic channels following convergence of solvent streams from separate inlets. Spatially resolved characterisation is therefore necessary and requires in-line analysis methods. Here we report a line-focused illumination approach to provide operando, spatially resolved Raman spectra across the width of channels in the analysis of single- and multi-phase liquid systems and chemical reactions. A scientific complementary metal oxide semiconductor (sCMOS) sensor is used to overcome smearing encountered during spectral readout of images with CCD sensors. Isotopically labelled probes, in otherwise identical flow streams, show that z-confocality limits the spatial resolution and certainty as to the extent of mixing that can be achieved. These limitations are overcome using fast chemical reactions between reagents entering a microchannel in separate solvent streams. We show here that the progression of a chemical reaction, for which only the product is observable, is a powerful approach to determine the extent of mixing in a microchannel. Specifically resonance enhancement of Raman scattering from a product formed allows for determination of the true efficiency of mixing over the length and width of microchannels. Raman spectral images obtained by line-focused illumination show onset of mixing by observing the product of reagents entering from the separate inlets. Mixing is initially off-centre and immediately before the apex of the first groove of the static mixer, and then evolves along the entire width of the channel after a full cycle of grooves.

Enhancement of concentrator photovoltaic system through convergent-divergent microchannels

Concentrated photovoltaic technology is one of the evolving technologies that converts solar to electrical energy with a high efficiency percentage. Increasing concentrated photovoltaic cell’s temperature significantly reduces the performance of the system. Thus, in order to properly functioning of system, thermal management will be a rudimentary issue. This article focuses on the cooling performance of high-concentration photovoltaic system using microchannels attached to the bottom plate of solar cell. To this end, the effect of convergent-divergent microchannels at three amplitudes and three wave lengths at different Reynolds numbers is investigated, and results are compared with simple microchannels. The output results are presented via temperature, pressure, velocity contours as well as Nusselt number, friction factor, and effectiveness ratio. Compared to the simple microchannels, convergent-divergent microchannel lowers the wall temperature about 0.4–2 Kelvin by making velocity gradients along the channel. This leads to boosting the heat transfer capability up to 3 times in the best case and up to 1.5 times in the worst case. Increasing the wave length enhances the performance of system, while the opposite trend has been seen for greater wave amplitudes. For case A with lowest wave length, Nu increases 1.92 and 2.4 times at lowest and highest Re numbers while friction factor increases due to the higher contact surface area along the fluid stream. Moreover, increasing the wave amplitude leads to higher Nu number, higher friction factor and consequently lower effectiveness ratio. Finally, effectiveness ratio as a criterion to choose best case has shown that a higher wave length alongside with lower wave amplitude offers better heat transfer enhancement and lower pressure drop compared to simple channel. Case C-1 as a best case offers 0.8K lower wall temperature, 1.6 times better Nu number with only 1.3-time higher friction factor.

Freeform Etching of Microchannels in Hydrogels by Ultrasonic Cavitation

With structural similarities to biological tissues, hydrogels offer many potential applications in biomedicine. To improve hydrogel perfusion, simple microchannels can be fabricated using a variety of templating and printing approaches, but the formation of interconnected, winding, and branching channels remains a significant challenge. The cavitation‐mediated etching of microchannels in agarose hydrogels is demonstrated. An ultrasonic cavitation transducer coupled with a motorized control system is used to enable the formation of consistent microchannels within the agarose hydrogels with ellipsoid cross‐sectional areas and uniform widths on the order of 244 ± 19.5 μm. With increasing transducer voltage, the average microchannel width increases, while higher positional translation speed results in shorter dwell times and, therefore, smaller microchannels. Infusion of fluorescent dyes indicates little turbulence within the microchannels formed by the cavitation etching process. This technique can fabricate branched and complex microchannel paths. Furthermore, the mechanical and swelling properties of hydrogels with internal microchannels formed by cavitation at varying pH support future development in diverse applications including tissue engineering, drug delivery, and biomimetic lab‐on‐a‐chip systems.

An Integrated Pulsation-Free, Backflow-Free Micropump Using the Analog Waveform-Driven Braille Actuator.

The widespread adoption of long-term organs-on-a-chip culture necessitates both active perfusions that mimic physiological flow conditions and minimization of the complexity of microfluidic system and fluid handling. In particular, flow in microtissue such as microvascular is free of pulsation and backflow. The refreshable Braille actuator-based integrated microfluidic system can be employed with simple microchannels and setups. However, due to high pulsatile flow and backflow, ordinary Braille-driven micropumps generate non-physiological flow conditions. We have described a simple method for creating steady flow employing Braille actuators driven with a high-voltage analog waveform, called “constant flow waveform”, without incorporating complicated structures into the microchannel or actuator. We determined the constant flow waveform by measuring volume change of microchannel caused by actuated Braille pins using a conventional fluorescent dye and microscope. Using the constant flow waveform, we demonstrated that a Braille-driven pump reduced pulsating flow by 79% and backflow by 63% compared to conventional Braille-driven pump. Furthermore, we demonstrated that a parallel pair of three-stranded pin pumps effectively eliminated backflow by driving two pumps with the constant flow waveform half-cycle shifted to each other. Moreover, by raising the driving frequency, we could increase the average flow rate to ~2× higher than previously reported flow rate of a typical Braille-driven micropump.

Computational and Experimental Model to Study Immunobead-Based Assays in Microfluidic Mixing Platforms.

In immunobead-based assays, micro/nanobeads are functionalized with antibodies to capture the target analytes, which can significantly improve the assay's performance. The immunobead-based assays have been recently combined with microfluidic mixing devices and customized for a variety of applications. However, device design and process optimization to achieve the best performance remain a substantial technological challenge. Here, we introduce a computational model that enables the rational design and optimization of the immunobead-based assay in a microfluidic mixing channel. We use numerical methods to examine the effect of the flow rates, channel geometry, bead's trajectory, and the analyte and reagent characteristics on the efficiency of analyte capture on the surface of microbeads. This model accounts for different bead movements inside the microchannel, with the goal of simulating an actual active binding environment. The model is further validated experimentally where different microfluidic channels are tested to capture the target analytes. Our experimental results are shown to meet theoretical predictions. While the model is demonstrated here for the analysis of IgG capture in simple and herringbone-structured microchannels, it can be readily adapted to a broad range of target molecules and different device designs.

Poly(acrylic acid) as an adhesion promoter for UV-assisted thermoplastic bonding: Application for the in vitro construction of human blood vessels

In this study, we introduced a novel adhesion bonding method for fabricating thermoplastic microdevices using poly(acrylic acid) (PAA) as a UV-assisted adhesion promoter. The bonding mechanism was based on the covalent cross-links between poly(methyl methacrylate) (PMMA) and PAA via the free radicals in their carbon backbone generated under UV irradiation. The water contact angle and Fourier-transformed infrared (FTIR) analysis were performed to analyze the surface characteristics of the PAA-coated PMMA. PMMAs were bonded under UV treatment for 60 s with the highest bond strength of around 1.18 MPa. The PMMA microdevice was leak-proof for over 200 h. Besides, clog-free PMMA microdevices with various-sizes microchannels were performed to demonstrate such a high applicable bonding method for microdevice fabrication. Moreover, PMMAs were bonded with other thermoplastics with a bond strength of around 0.5 MPa. Notably, collagen was easily coated inside the PMMA microchannels via electrostatic interaction between PAA and collagen which is beneficial for on-device cell culture. As a result, a layered co-culture model of smooth muscle cells (SMCs) and human umbilical vein endothelial cells (HUVECs) was realized inside simple straight microchannels mimicking human blood vessel wall. Therefore, the introduced bonding method could pave the way for fabricating microdevice for cell-related applications.

Chitosan/tripolyphosphate nanoparticles in active and passive microchannels.

Background and purpose:In recent years, the interest in chitosan nanoparticles has increased due to their application, especially in drug delivery. The main aim of this work was to find a suitable method for simulating pharmaceutical nanoparticles with computational fluid dynamics (CFD) modeling and use it for understanding the process of nanoparticle formation in different types of microchannels.Experimental approach:Active and passive microchannels were compared to find the advantages and disadvantages of each system. Twenty-eight experiments were done on microchannels to quantify the effect of 4 parameters and their interactions on the size and polydispersity index (PDI) of nanoparticles. CFD was implemented by coupling reactive kinetics and the population balance method to simulate the synthesis of chitosan/tripolyphosphate nanoparticles in the microchannel.Findings/Results:The passive microchannel had the best performance for nanoparticle production. The most uniform microspheres and the narrowest standard deviation (124.3 nm, PDI = 0.112) were achieved using passive microchannel. Compared to the active microchannel, the size and PDI of the nanoparticles were 28.7% and 70.5% higher for active microchannels, and 55.43% and 105.3% higher for simple microchannels, respectively. Experimental results confirmed the validity of CFD modeling. The growth and nucleation rates were determined using the reaction equation of chitosan and tripolyphosphate.Conclusion and implications:CFD modeling by the proposed method can play an important role in the prediction of the size and PDI of chitosan/tripolyphosphate nanoparticles in the same condition and provide a new perspective for studying the production of nanoparticles by numerical methods.

Thermohydrodynamic Performance Evaluation of Recharging, Interrupted and Simple Microchannels: A Comparative Study

Abstract In this study, a three-dimensional numerical investigation on the thermohydrodynamic performance of a recently proposed recharging microchannel (RMC) is carried out. In this design, a straight microchannel is split into more than one smaller length channels (having individual inlet and outlet) placed end to end. This design enhances overall heat transfer and maintains temperature uniformity across the substrate length. The comparison of fluid flow and heat transfer performance of RMC, interrupted microchannel (IMC) and straight microchannel (SMC) with the same hydraulic diameter and substrate length are presented to explore the effect of geometrical configuration on heat transfer enhancement. The parametric variations include the number of channels (n), transverse wall length (Ltw), channel aspect ratio (α), and flow Reynolds number. The results reveal that recharging microchannel shows better thermal performance compared to simple and interrupted microchannel with a maximum performance factor of 1.80. The results also indicate that the performance factor of RMC increases with an increase in the number of small channels, transverse wall length, and channel aspect ratio. The outcome of this study indicates the possible use of recharging microchannel heat sinks for high heat flux removal applications such as electronic cooling.

In vitro blood flow visualizations and cell-free layer (CFL) measurements in a microchannel network

Microvascular networks are not simple straight microchannels but rather complex geometries composed by successive asymmetric divergent and convergent bifurcations. Despite the extensive research work in this field, still lack of knowledge about the blood flow behavior in microvascular networks. The current study applies the most current advanced visualization and microfabrication techniques to provide further insights into to the blood flow in network geometries. Hence, by using a high-speed video microscopy system, blood flow measurements and visualizations of the cell-free layer (CFL) were performed along a microchannel network composed by several divergent and convergent bifurcations. The inlet flow rate was kept constant whereas the hematocrit (Hct) and the depth of the geometry was changed in order to evaluate their effects into the CFL thickness. The results, show clearly that the Hct has a significant impact on the CFL thickness whereas the effect of reducing the depth did not contribute to a noticeable change on the CFL. In addition, the in vitro blood flow results reported here provide for the first time that in microfluidic devices having several asymmetric confluences it is likely to have the formation of several CFLs not only around the walls but also in middle of the main channels just downstream of the last confluence apex. Although, to best of our knowledge there is no evidence that this kind of flow phenomenon also happens in vivo, we believe that for microvascular networks with similar geometries and under similar flow conditions tested in this work, this kind of phenomenon may also happen in vivo. Furthermore, the results from this study could be extremely helpful to validate current numerical microvascular network models and to develop more realistic multiphase numerical models of blood flow in microcirculation.

Rapid and inexpensive method for the simple fabrication of PDMS-based electrochemical sensors for detection in microfluidic devices.

The fabrication of PDMS microfluidic structures through soft lithography is widely reported. While this well-established method gives high precision microstructures and has been successfully used for many researchers, it often requires sophisticated instrumentation and expensive materials such as clean room facilities and photoresists. Thus, we present here a simple protocol that allows the rapid molding of simple linear microchannels in PDMS substrates aiming microfluidics-based applications. It might serve as an alternative to researchers that do not have access to sophisticated facilities such as clean rooms. The method developed here consists on the use of pencil graphite leads as template for the molding of PDMS channels. It yields structures that can be used for several applications, such as housing support for electrochemical sensors or channels for flow devices. Here, the microdevices produced through this protocol were employed for the accommodation of carbon black paste, which was utilized for the first time as amperometric sensor in microchip electrophoresis. This platform was successfully used for the separation and detection of model analytes. Ascorbic acid and iodide were separated within 45 s with peak resolution of 1.2 and sensitivities of 198 and 492 pA/μM, respectively. The background noise was ca. 84 pA. The analytical usefulness of the system developed was successfully tested through the quantification of iodide in commercial pharmaceutical formulations. It demonstrates good efficiency of the microfabrication protocol developed and enables its use for the easy and rapid prototyping of PDMS structures over a low fabrication cost.

Fabrication of Metallic Micromixers Using WEDM and EDM for Application in Microfluidic Devices and Circuitries

Background: Micromixers having various simple and complex microchannels is an inevitable part of almost all the microfluidic devices and circuitries nowadays. Particularly the metallic micromixers are very important for many chemical and process industries. Fabrication of metallic microchannels still remains a challenging task. Objective: The work focuses on the fabrication of microchannels of different geometries and intrinsic features on metals. Here, an alternate approach for fabricating microchannel in metals is demonstrated which is easy and doesn’t demand a very high level of expertise. Method: The fabrication is carried out using Wire-Cut Electric Discharge Machine (WEDM) and Electric Discharge Machine (EDM) and accomplished in two stages: 1) Micro-tool fabrication in WEDM; 2) Microchannel machining in EDM. Parametric investigation of the processes is also carried out for understanding the influence of different parameters in microfabrication. Results: Different serpentine micromixer configurations are fabricated on copper and aluminum. Open channel configuration of metallic micromixers of average microchannel width 400 μm is fabricated as per the design. During micro-tool fabrication in WEDM, parametric conditions: pulse on time 25 µs, pulse off time 6 μs, input current 1 A and wire feed rate 75 mm/min gave optimal results among the investigated parametric combinations. During the second stage of microfabrication using EDM, the effect of input parameters, viz. input current, pulse on time and pulse off time on the output parameters viz. machining time, overcut, edge deviation and tool depletion are investigated. Results highlight the importance of careful selection and control of all these process parameters for manufacturing metallic microchannels by EDM. Conclusion: The proposed method is able to give rise to micro-features like microchannel on metals with reasonable ease and accuracy. The fabricated micromixers can be used for Lab on Chip (LOC), Point of Care (POC) devices or/and microreactors. The metallic micromixers can be used to handle various corrosive species and reagents which the conventional polymeric substrate can’t handle. Thus it can lead to further advancement in the field of microfluidics.

Liquid-liquid extraction for the separation of Co(II) from Ni(II) with Cyanex 272 using a pilot scale Re-entrance flow microreactor

A novel pilot scale microreactor in continuous re-entrance flow was evaluated in selective extraction of Co from Ni sulfate solution with Cyanex 272. The microreactor had three-dimensional (3D) conical sections symmetrically connected in 6 series with an internal volume of 12ml and was capable of handling flow rate of 9l/h. Using this extraction system correlation relating pressure drop and energy dissipation to flow rate was studied. Effects of variation of flow rate/residence time and extractant concentration on extraction and separation efficiency were investigated. The results indicated that high extraction ratio of Co, good selectivity for separation of Co from Ni at low Cyanex 272 concentration were obtained in single-stage microextraction system. Above 99.9% of Co was separated from aqueous phase in two-stage counter-current extraction processes. Moreover, mass transfer performance between the two immiscible phases was investigated and compared to other unstructured microchannels. The overall volumetric mass transfer coefficient kLα of the target Co in the special 3D conical microstructure were superior to other simple microchannels due to the high extraction ratio at short residence time caused by re-entrance flow effect.

Particle Migration due to Viscoelasticity of the Suspending Liquid and Its Relevance in Microfluidic Devices

The fast growth of microfluidic applications based on complex fluids is a result of the unique fluid dynamics of these systems, enabling the creation of devices for health care or biological and chemical analysis. Microchannels designed to focus, concentrate, or separate particles suspended in viscoelastic liquids are becoming common. The key fluid dynamical issue on which such devices work is viscoelasticity-induced lateral migration. This phenomenon was discovered in the 1960s in macroscopic channels and has received great attention within the microfluidic community in the past decade. This review presents the current understanding, both from experiments and theoretical analysis, of viscoelasticity-driven cross-flow migration. Examples of promising microfluidic applications show the unprecedented capabilities offered by such technology based on geometrically simple microchannels and rheologically complex liquids.

The application of microbeads to microfluidic systems for enhanced detection and purification of biomolecules.

This paper describes microbead-based microfluidic systems. Several aspects of bead assays in microfluidics make them advantageous for bioassays in simple microchannels, including enhanced surface-to-volume ratio, improved molecular recognition reaction efficiency, and the wide range of surface functionalization available with commercial microbeads. Two-level SU-8 molds are used to fabricate PDMS microchannels that can hydrodynamically trap different types of microbeads, with characteristic dimensions of tens of microns. The use of these microbead-based microfluidic systems in the biosensing of antibodies, toxins and nucleic acids, as well as in antibody purification will be presented and discussed in this paper.

Intensification of liquid-liquid mass transfer in micromixer assisted by ultrasound irradiation and Fe3O4 nanoparticles

In the present study, Cu (II) removal from aqueous solution was performed in a micromixer by using D2EHPA solvent. The impact of nanoparticles addition, ultrasound wave irradiation and combination of them for improving mixing and mass transfer was investigated. The experiments were carried out in three different ultrasound input electrical powers (3.5–7W). A constant concentration of nanoparticles (0.01w/v) was employed in a reactor consists of a circular pit at its junction, and the results were compared with the Y-type microchannel (without ultrasonic source). Depending on the mixing approach type and flow rate of the phases (1.875–6mL/min), several separate flow patterns were observed, mainly slug, droplet, dispersed and parallel flow. The mass transfer coefficient (KLa) and removal efficiency (E) enhanced up to 251% and 53%, respectively. One of the benefits of the proposed techniques is a significant reduction in pressure drop compared with simple microchannels with the same volume. The highest amount of performance ratio (1.56) was observed in the presence of Fe3O4 nanoparticles under 7W ultrasonic irradiation power.

Size-Based Separation of Particles and Cells Utilizing Viscoelastic Effects in Straight Microchannels.

Viscoelasticity-induced particle migration has recently received increasing attention due to its ability to obtain high-quality focusing over a wide range of flow rates. However, its application is limited to low throughput regime since the particles can defocus as flow rate increases. Using an engineered carrier medium with constant and low viscosity and strong elasticity, the sample flow rates are improved to be 1 order of magnitude higher than those in existing studies. Utilizing differential focusing of particles of different sizes, here, we present sheathless particle/cell separation in simple straight microchannels that possess excellent parallelizability for further throughput enhancement. The present method can be implemented over a wide range of particle/cell sizes and flow rates. We successfully separate small particles from larger particles, MCF-7 cells from red blood cells (RBCs), and Escherichia coli (E. coli) bacteria from RBCs in different straight microchannels. The proposed method could broaden the applications of viscoelastic microfluidic devices to particle/cell separation due to the enhanced sample throughput and simple channel design.

Characterization and Theoretical Analysis of Rapidly Prototyped Capillary Action Autonomous Microfluidic Systems

Capillary action arising from surface tension-based forces has been demonstrated to be an effective means of passively actuating various fluids through simple and sophisticated microfluidic channel networks. Systems based on this technique are advantageous compared with standard pumping strategies, as they have zero power requirements, are readily integrated into the overall fluidic chip design, and can be fabricated rapidly in a single manufacturing step. This paper comprehensively investigates time lapsed average velocity profiles of various capillary action microfluidic systems, including channels and pumping structures, and compares experimental data against prominent, competing, and flow-based theoretical models. We demonstrate that the average meniscus flow velocity of CO 2 laser ablated capillary systems can be adequately predicted and characterize smooth fluidic velocity profiles in simple microchannels and complex interlinking channel pump/filling structures. Such systems offer a useful, rapid, and low cost alternative to traditional fluidic actuation and flow control systems such as those found in on-chip based biological and chemical analysis.

Understanding microchannel culture: parameters involved in soluble factor signaling

While the importance of autocrine-paracrine signaling in vivo is clear, the ability to study the effects of secreted endogenous factors in vitro is hampered by canonical culture platforms. In multi-well plates, the large air-liquid interface gives rise to convective flows that continually mix the fluid disrupting the local diffusion-based accumulation. Simple microchannels provide a more controlled microenvironment that can be used to study secreted factor effects. Here, we utilize microchannel culture to examine basic culture parameters and their interactions using normal mammary gland epithelial cells (NMuMG). The following parameters were studied: (1) cell density (80 vs. 240 cells mm(-2)), (2) exogenous growth factors (epidermal growth factor [EGF] vs. fetal bovine serum), (3) medium change frequency (1 h, 4 h, 12 h), and (4) culture platform (microchannels vs. 96-well plates). The cells exhibited increased growth rates in microchannels as compared to 96-well plates. Cell proliferation increased as the frequency of media change decreased. For the microchannel geometries used, important threshold concentrations were reached in a few hours. In aggregate, the results indicate that the function of the four factors and their interactions on NMuMG growth are spatially and temporally related by molecular diffusion in the controlled microchannel space. The convective-free microchannel environment may prove useful for studying soluble factor signaling in vitro, and to test models and predictions of autocrine-paracrine signaling.

High-speed particles and biomolecules sorting microsystem using thermosensitive hydrogel

Two types of high-speed particle and biomolecule sheath flow sorters were realized using thermosensitive hydrogel. One is all hydrogel sheath flow and the other is two-phase sheath flow that consists of a water flow including samples and two hydrogel carrier flows; these have been utilized instead of the air and water droplet two-phase flow used in the conventional commercialized cell sorting system. Flow switching is performed by the sol–gel transfer of the thermosensitive hydrogel generated by focused infrared (IR) laser irradiation. High-speed sorting less than 5.0 ms and no error sorting (120 000 counts) in the all hydrogel sheath flow system while 20 ms in the two-phase sheath flow was realized. Since sorting was performed in simple PDMS-glass microchannels without any electric stimulation and mechanical valve structures, the proposed system is suitable for many biochemical applications.

Three-dimensional sheath flow sorting microsystem using thermosensitive hydrogel

This paper details a three-dimensional sheath flow sorting microsystem for separation and collection of biomolecules. High-speed particle and biomolecule 3D sheath flow sorting system was realized using reversible thermosensitive hydrogel. The sol–gel transfer of the thermosensitive hydrogel generated by focused IR laser irradiation performs flow switching, the response time of the sol–gel transformation using Mebiol Gel™ was less than 1 ms. For actual bio-applications, accurate and high-speed micro sorter is realized by optimizing the channel structure and by utilizing 3D sheath flow. High-speed sorting less than 3.0 ms from detection to separation of targets and no error sorting for 45 min (100,000 counts) in 3D sheath flow sorting system was realized. 3D sheath flow can be focused to 1.2 μm of transverse distribution and 6.0 μm of longitudinal distribution in regard to microbeads of sample flow. The same flow velocity and flow line of the focused sample flow are kept at constant laser irradiation time and power to sort the target. Since sorting was performed in simple and disposable PDMS-glass microchannels without any electric stimulation and mechanical valve structures, the proposed system is suitable for many biochemical applications.