Precision moulding of biomimetic disposable chips for droplet-based applications

In this study, we present a biomimetic approach to improve the stability and reproducibility of droplet generation processes and to reduce the adhesion of aqueous droplets to channel surfaces of microfluidic polymer chips. The hierarchical structure of the lotus leaf was used as a template for a partial laser structuring of the moulds that were used for casting the polymer chips. The hydrophobic wax layer of the lotus leaf was technologically replicated by coating the polymer chips using a plasma deposition process. The resulting microfluidic polymer chip surfaces reveal a topography and a surface free energy similar to those of the lotus leaf. Subsequent droplet-based microfluidic experiments were performed using a 2D flow focussing set-up. Droplets from both, serum-supplemented cell culture medium and anticoagulated human whole blood, could be generated stably and reproducibly using a fluorocarbon as continuous phase. The presented results illustrate the application potential of the lotus-leaf-like polymer chips in life sciences, e.g. in the field of personalised medicine.     

Micro/nanoporous membrane based gas–water separation in microchannel

In this paper, a method coupling micro/nanoporous membranes with microchannels for the gas/water separation in microscale has been demonstrated. The principle of separation is based on the basic properties of membranes—wetted micro/nanoporous hydrophilic membranes can block gas due to a large water surface tension in the entrance of micro/nano pores to withstand the gas pressure, while it allows water to permeate with overcoming the flow resistance of pores; in contrast, hydrophobic membranes permit gas to penetrate but prevent water flow. These two kinds of membranes coupled with microchannels in cross-flow mode or dead-end mode can separate gas/water in microscale. The separation in cross-flow mode can achieve relative high water permeate flow rate, while the separation in dead-end mode can completely separate gas/water segments in a single operation step. The membrane-based separation method has low cost, easy integration into microfludics channels, design flexibility with two configuration modes (cross-flow and dead-end).     

Use of electrochemical microsensors for hydrodynamics study in crossing microchannels

The study of microfluidic systems is an important research challenge related to the design of microdevices for chemical processes. The understanding of physical phenomena, such as flow behaviour and heat and mass transfer performance is needed in order to develop these microsystems for industrial applications as mixers, reactors or heat exchangers. This work aims at characterizing two flow pattern behaviours, by using an electrochemical method, in a microdevice composed of crossing microchannels. A nonintrusive electrodiffusion method involving an electrochemical reaction of active species on an electrode flush-mounted into a wall is used to investigate wall shear stress. The measured limiting diffusion current is related to the wall shear rate in the vicinity of the electrode. The experimental cell consists of two crossing microchannels intersecting at right angle. Two channels sections are investigated, respectively 500 and 833 μm in hydraulic diameter. In each case, the influence of the crossing on the flow behaviour and on the mixing performance are characterized locally by using microelectrodes implemented at several positions on the wall of the channels located after the crossing. The experimental results are analyzed and a comparison with the results of CFD simulations using Fluent is performed.     

Modular component design for portable microfluidic devices

A new modular design concept for microfluidic devices is proposed and demonstrated in this study. We designed three key modular microfluidic components: pumps, valves, and reservoirs, and demonstrated that a microfluidic device with specific functions can be easily assembled with those key modular components. Our pumps are man-powerable so that the assembled microfluidic devices require no any other power sources like expensive syringe pumps or air compressors. This feature makes the assembled microfluidic devices completely portable. We also combined our assembled device with other existing mixing microchannels to serve as the mixing and loading system in polymerase chain reaction experiment to amplify DNA successfully. This result shows that those modular components can be integrated into other microchannels, implying great potential applications of the modular design.     

Design optimization of micromixer with square-wave microchannel on compact disk microfluidic platform

This paper presents a passive micromixer on a compact disk (CD) microfluidic platform that performs plasma mixing function. The driving force of CD microfluidic platform including, the centrifugal force due to the system rotation, the Coriolis force as a function of the rotation angular frequency and velocity of liquid. Numerical simulations are performed to investigate the flow characteristics and mixing performance of three CD microfluidic mixers with square-wave, curved and zig-zag microchannels, respectively. Of the three microchannels, the square-wave microchannel is found to yield the best mixing performance, and is therefore selected for design optimization. Four CD microfluidic micromixers incorporating square-wave PDMS microchannels with different widths in the x- and y-directions are fabricated using conventional photolithography techniques. The mixing performance of the four microchannels is investigated both numerically and experimentally. The results show that given an appropriate specification of the microchannel geometry and a CD rotation speed of 2,000 rpm, a mixing efficiency of more than 93 % can be obtained within 5 s.     

The effect of asymmetry on micromixing in curvilinear microchannels

The necessity of microscale mixing processes has been tremendously increasing in most of the microsize chemical and biochemical devices during recent years, particularly in the design of lab-on-a-chip and micrototal analysis systems. Different approaches were implemented in the available micromixers in the literature for improving the mixing performance. Due to the absence of any external source, mixing by utilizing passive mixing techniques is more economical. In curvilinear microchannels, which offer effective passive mixing, chaotic advection results in continuous radial perforation of inter-diffusion layer between the fluid streams due to the transverse secondary flows. In this study, the effects of Dean vortices and secondary flows were investigated in asymmetrical polydimethylsiloxane curvilinear rectangular microchannels, which were fabricated by one-step lithography process and had repeated S-shape patterns with a curvature of 280° along the channel. Moreover, the effect of asymmetry was assessed by comparing the mixing results with symmetrical microchannels. Mixing performance was analyzed by using NaOH and phenolphthalein solutions as mixing fluids, which entered from the channel inlets. According to the results, the significant effects of stretching and contracting motion of Dean vortices revealed themselves above a certain Dean number value, thereby making the asymmetrical microchannel outperform the symmetrical channel in the mixing performance. Below this threshold, the symmetrical microchannel was observed to be superior to the asymmetrical microchannel.     

Multilamination of flows in planar networks of rotating microchannels

We describe a new multilamination technique to accelerated mixing of centrifugally pumped flows through a simple network of preferentially radial, low-aspect-ratio microchannels. Mixing by multilamination is enforced by planar split-and-recombine structures, consisting of a common inlet for two concurrent centrifugal flows, and a transient region of parallel microchannels which merge again into one common outlet. A repatterning of flow is observed in each parallel channel which is induced by the Coriolis pseudo force. In a distinct regime of the parameter space spanned by the speed of rotation, the channel geometry as the viscosity (and density) of the liquids, a multilamination of flow is achieved at the entrance of the common outlet channel. We also present parallelization and cascading strategies to further enhance the homogeneity and throughput of mixing by multilamination.     

An analytical model for mixing based on time-interleaved sequential segmentation

Due to constrains caused by the laminar flow in microscale, effective and fast mixing is important for many microfluidic applications. From the scaling law, decreasing the mixing path can shorten the mixing time and enhance the mixing quality. One of the techniques for reducing mixing path is time-interleaved sequential segmentation. This technique divides solvent and solute into segments in axial direction. The mixing path can be controlled by the switching frequency and the mean velocity of the flow. In this brief communication, we present a simple time-dependent one-dimensional analytical model for time-interleaved sequential segmentation. The model considers an arbitrary mixing ratio between solute and solvent as well as the axial Taylor–Aris dispersion. The analytical solution indicates that the Peclet number is the key parameter for this mixing concept.     

Effect of wall roughness on performance of microchannel applied in microfluidic device

The influence of wall roughness on flow and heat transfer performance in microchannels at low Reynolds number is investigated in this paper. Two sizes of PMMA microchannels are fabricated by microinjection molding and the width is 20 μm and 800 μm respectively. The surface profile of bottom wall is described by the two-dimensional fractal geometry method and it is found there is error within 5% between surface roughness obtained by the fractal geometry method and actual roughness. Then, the effects of dimensionless relative roughness (5–7.5%), fractal dimension (1.5–1.8), aspect ratio (0.025–4) on the flow resistance and heat transfer performance are analyzed by numerical and experimental method respectively. Reynolds number considered here are 10–60. The results show that the better flow performance and heat transfer performance can be obtained with high aspect ratio of rectangular microchannel. However, increasing surface roughness not only increases the heat transfer performance, but also introduces a large flow resistance, which makes the friction coefficient rise sharply. As a result, surface roughness has great influence on the flow and heat transfer performance, and the most suitable surface morphology should be obtained according to the specific application.

     

Design optimization of capillary-driven micromixer with square-wave microchannel for blood plasma mixing

A numerical and experimental investigation is performed into the flow characteristics and mixing performance of three microfluidic polydimethylsiloxane blood plasma mixing devices incorporating square-wave, curved and zigzag microchannels, respectively. For each device, the plasma is introduced into the microfluidic channel under the effects of capillary action alone. Of the three devices, that with the square-wave microchannel is found to yield the best mixing performance, and is therefore selected for design optimization. Four microfluidic micromixers incorporating square-wave microchannels with different widths in the x- and y-directions are fabricated using conventional photolithography techniques. The mixing performance of the four microchannels is investigated both numerically and experimentally. The results show that given an appropriate specification of the microchannel geometry, a mixing efficiency of approximately 76 % can be obtained within 4 s. The practical feasibility of the micromixer is demonstrated by performing prothrombin time (PT) tests using a total liquid volume of 4.0 μL (2.0 μL of plasma and 2.0 μL of PT reagent). It is shown that the mean time required to complete the entire PT test (including loading, mixing and coagulation) is less than 30 s.

     

Pressure-driven flow through PDMS-based flexible microchannels and their applications in microfluidics

Flexible microchannels have soft walls which undergo deformation under the influence of fluid flow. The dimensional and flexural similarity of flexible microchannels make them ideal candidates for mimicking biological structures such as blood vessels and air pathway in lungs. The analysis of fluid flow and the dynamics of interaction of cells through flexible arteries provide valuable insights about cardiovascular-related diseases. Flexible microchannels can be instrumental in the in vitro investigation of such diseases. This review discusses the recent developments in pressure-driven flow through flexible microchannels and their applications. Here we present the existing theoretical models that predict the deformation and pressure-flow characteristics of flexible microchannels and the corresponding experimental validations. We compare the models for laminar flow of Newtonian fluids through flexible microchannels with their corresponding experimental validation and enlist their limitations. We discuss in detail the various applications of flexible microchannels and their relevance in cell mechanophenotyping, micropumps, microflow stabilizers, and organ-on-chip devices. The insight into the flow dynamics provided herein will extend using flexible microchannels to develop organs-on-chip and other microfluidic applications.     

Acoustically generated flows in microchannel flexural plate wave sensors: Effects of compressibility

Acoustically generated flowfields in flexural plate wave sensors filled with a Newtonian liquid (water) are considered. A computational model based on compressible flow is developed for the sensor with a moving wall for pumping and mixing applications in microchannels. For the compressible flow formulation, an isothermal equation of state for water is employed. The velocity and pressure profiles for different parameters including flexural wall frequency, channel height, amplitude of the wave and wave length are investigated for four microchannel height/length geometries. It is found that the flowfield becomes pseudo-steady after sufficient number of flexural cycles. Both instantaneous and time averaged results show that an evanescent wave is generated in the microchannel. The predicted flows generated by the FPWs are compared with results available in the literature. The proposed device can be exploited to integrate micropumps with complex microfluidic chips improving the portability of micro-total-analysis systems.     

Mass production of highly monodisperse polymeric nanoparticles by parallel flow focusing system

Nanoparticles can be prepared through nanoprecipitation by mixing polymers dissolved in organic solvents with anti-solvents. However, due to the inability to precisely control the mixing processes during the synthesis of polymeric nanoparticles, its application is limited by a lack of homogeneous physicochemical properties. Here, we report that this obstacle can be overcome through rapid and controlled mixing by parallel flow focusing outside the microfluidic channels. Using the nanoprecipitation of methoxyl poly-(ethylene glycol)–poly-(lactic-co-glycolic acid) (MPEG–PLGA) block copolymers as an example, we prove that our parallel flow focusing method is a robust and predictable approach to synthesize highly monodisperse polymeric nanoparticles, and demonstrate that it improves the production speed of nanoparticles by an order of magnitude or more compared with previous microfluidic systems. Possible aggregation on the surface of PDMS wall and clogging of microchannels reported previously were avoided in the synthesis process of our method. This work is a typical application combining the advantages of microfluidics with nanoparticle technologies, suggesting that microfluidics may find applications in the development and mass production of polymeric nanoparticles with high monodispersity in large-scale industrial production field.     

A numerical study of the Coriolis effect in centrifugal microfluidics with different channel arrangements

The Coriolis force has been of great interest to control the transversal flow that is critical for mixing or switching fluids in centrifugal microfluidics. Therefore, the variation in the Coriolis effect has been extensively investigated by varying the rotational speeds and the cross-sectional geometry of microchannels. However, the subject of such investigations has been limited to radially positioned microchannels even though channels can lie everywhere on the rotating platform with different arrangements. In this study, we use numerical methods to investigate the Coriolis effect resulting from the angular alignment (AA) and radial displacement (RD) of rotating microchannels. Our analysis focuses on determining the contribution that different channel arrangements have in the deviation of parabolic velocity profiles, which is generally produced by the effective Coriolis force. We found that the flow can deviate even at a low rotational speed, where the Coriolis force is negligible, with an AA of up to 33 % which is a sufficient amount for flow switching. Once the rotational speed reaches to the critical RPM, the flow deviates by an effective Coriolis force, but the deviation systematically varies with AA or RD. As the Coriolis force becomes more dominant with a high rotational speed, the deviation reaches a saturation point, while flow rate is regulated by AA or RD, enabling the flow rate to remain low even at very high RPM, without reducing the deviation. The variation in the Coriolis effect due to the different channel arrangements investigated in this study is believed to provide an essential basis to design and develop centrifugal microfluidic systems.     

Lattice Boltzmann simulations in microfluidics: probing the no-slip boundary condition in hydrophobic, rough, and surface nanobubble laden microchannels

In this contribution, we review recent efforts on investigations of the effect of (apparent) boundary slip by utilizing lattice Boltzmann simulations. We demonstrate the applicability of the method to treat fundamental questions in microfluidics by investigating fluid flow in hydrophobic and rough microchannels as well as over surfaces covered by nano- or microscale gas bubbles.     

Optically monitored segmented flow for controlled ultra-fast mixing and nanoparticle precipitation

Nanoparticles of drugs or colloidal carrier systems are capable of providing substantial advantages for drug bioavailability, but manufacturing nanoparticulate drugs or drug carriers remains a challenge because traditional mechanical or chemical batch mode processes might lack precise control of nanoparticle sizes. Microfluidic approaches are believed to give advantages but often do not provide chemically inert environments and lack controllable operation. Here, segmented flow devices with symmetrical design for centered organic phase injection and for nanoparticle precipitation in transparent and chemically inert glass microchannels are presented. Femtosecond laser fabrication was used to structure borosilicate glass wafers with hydrophilic microchannels of nearly circular cross section. They allow for ultra-fast mixing of solvents with aqueous fluids and subsequent precipitation of poorly water soluble drug nanoparticles or colloidal carrier particles. The best results for mixing and controlled precipitation were obtained with flow focusing and gas segmentation occurring at the same channel intersection point. In such systems, early interdiffusion of the solvent and aqueous solution before ultra-fast convective mixing in the plug is suppressed. A novel optical analysis technique revealed that the speed of mixing can be well controlled by simply adjusting the volume flow rate of the gas phase where changes in the liquid flow rate have practically no influence. In a controlled and stable Taylor flow, smallest plug volumes of 3.8 nl can be generated, which allows complete mixing in 9 ms. The production of lipid nanoparticles down to a diameter of 74 nm could already be demonstrated.     

Evaporation in microchannels: influence of the channel diameter on heat transfer

This paper discusses the effect of diameter on both flow boiling heat transfer and transition from macro to microchannel evaporation. A recently proposed three-zone flow boiling model based on evaporation of elongated bubbles in microchannels is briefly described and used for the present analysis. In the microscale range, the model predicts an increase in the two-phase heat transfer coefficient with a decrease of diameter for low values of vapor quality and a decrease of the heat transfer coefficient for larger values of vapor quality. This behavior is explained by the influence of the liquid film thickness, deposited periodically behind passing liquid slugs.     

A microreactor for hydrogen production in micro fuel cell applications

A silicon-chip based microreactor has been successfully fabricated and tested for carrying out the reaction of methanol reforming for microscale hydrogen production. The developed microreactor in combination with a micro fuel cell is proposed as an alternative to conventional portable sources of electricity such as batteries due to its ability to provide an uninterrupted supply of electricity as long as a supply of methanol and water can be provided. The microreformer-fuel cell combination has the advantage of not requiring the tedious recharging cycles needed by conventional rechargeable lithium-ion batteries. It also offers significantly higher energy storage densities, which translates into less frequent "recharging" through the refilling of methanol fuel. The microreactor consists of a network of catalyst-packed parallel microchannels of depths ranging from 200 to 400 /spl mu/m with a catalyst particle filter near the outlet fabricated using photolithography and deep-reactive ion etching (DRIE) on a silicon substrate. Issues related to microchannel and filter capping, on-chip heating and temperature sensing, introduction and trapping of catalyst particles in the microchannels, flow distribution, microfluidic interfacing, and thermal insulation have been addressed. Experimental runs have demonstrated a methanol to hydrogen molar conversion of at least 85% to 90% at flow rates enough to supply hydrogen to an 8- to 10-W fuel cell.     

Investigation of surface roughness effects on fluid flow in passive micromixer

Surface roughness effects are dominant at microscale. In this study, microchannels are fabricated on Silicon substrate. The roughness morphology is modeled for the fabricated structure using Weierstrass-Mandelbrot function for self-similar fractals. A two dimensional model of hexagonal passive micromixer is analyzed with surface roughness present on inner walls of channels using parallel Lattice Boltzmann method, implemented on sixteen node cluster. The results are compared by simulating this micromixer structure using Navier–Stokes equations. The experimental results on the fabricated micromixers are also presented. The effects of relative roughness, fractal dimension and Reynolds number are discussed on laminar flow in hexagonal passive micromixers. The study concludes the importance of modeling surface roughness effect for better mixing efficiency.     

Convective–diffusive transport in parallel lamination micromixers

Effective mixing and a controllable concentration gradient are important in microfluidic applications. From the scaling law, decreasing the mixing length can shorten the mixing time and enhance the mixing quality. The small sizes lead to small Reynolds numbers and a laminar flow in microfluidic devices. Under these conditions, molecular diffusion is the main transport effect during the mixing process. In this paper, we present complete 2D analytical models of convective–diffusive transport in parallel lamination micromixers for a binary system. An arbitrary mixing ratio between solute and solvent is considered. The analytical solution indicates the two important parameters for convective–diffusive transport in microchannels: the Peclet number and the dimensionless mixing length. Furthermore, the model can also be extended to the mixing of multiple streams—a common and effective concept of parallel mixing in microchannels. Using laser machining and adhesive bonding, polymeric micromixers were fabricated and tested to verify the analytical results. The experimental results agree well with the analytical models.This revised version was published online in March 2005 with corrections to Eq. 12.