Effect of microstructure on blood cell clogging in blood separators based on capillary action

In this study, the blood cell clogging phenomenon occurring in blood separators based on capillary action is carefully investigated and how to minimize the reduction in plasma separation speed caused by clogging is discussed. Four different blood separators are fabricated on optically transparent glass substrates to clearly observe the blood plasma separation and the blood cell clogging in the microfluidic devices. Each separation experiment is captured by a high-speed video camera. The captured images are analyzed using a theoretical model proposed in this study to quantify the effect of the microstructure on the degree of blood cell clogging. Finally, design guidelines for the microstructure of the micro blood separator are discussed based on the analysis. D. Kim and J. Y. Yun contributed equally to this work.     

High throughput blood plasma separation using a passive PMMA microfluidic device

Since plasma is rich in many biomarkers used in clinical diagnostic experiments, microscale blood plasma separation is a primitive step in most of microfluidic analytical chips. In this paper, a passive microfluidic device for on-chip blood plasma separation based on Zweifach–Fung effect and plasma skimming was designed and fabricated by hot embossing of microchannels on a PMMA substrate and thermal bonding process. Human blood was diluted in various times and injected into the device. The main novelty of the proposed microfluidic device is the design of diffuser-shaped daughter channels. Our results demonstrated that this design exerted a considerable positive influence on the separation efficiency of the passive separator device, and the separation efficiency of 66.6 % was achieved. The optimum purity efficiency of 70 % was achieved for 1:100 dilution times.     

Capillary flow-driven blood plasma separation and on-chip analyte detection in microfluidic devices

We report a comprehensive review on the capillary flow-driven blood plasma separation and on-chip analyte detection in microfluidic devices. Blood plasma separation is the primary sample preparation step prior to most biochemical assays. Conventionally, centrifugation is used for the sample preparation process. There are numerous works reporting blood plasma separation in microfluidic devices which aim at miniaturizing the sample preparation procedure. Capillary-based blood plasma separation shows promise in actualizing point-of-care diagnostic devices for applications in resource-limited settings including military camps and rural areas. In this review, the devices have been categorized based on active and passive plasma separation techniques used for the separation of plasma from capillary-driven blood sample. A comparison between different techniques used for blood plasma separation is outlined. On-chip detection of analytes present in the separated plasma obtained using some of these reported devices is also presented and discussed.     

Experimental study on self-flowing speed in microchannel related to micro-/nanoscale surface topographies

The microfluidic flowing on chip surface depends on the external load such as centrifugal force, magnetic force and bubbles, but it leads to the complexity of microsystem. Hence, a self-flowing is proposed inside microchannel on chip surface without any external load. The objective is to explore how micro-/nanoscale surface topographies of microchannel influence the microfluidic flowing. First, the microgrinding with a diamond wheel microtip was employed to fabricate the accurate and smooth V-shaped microchannel with the height of 300 µm and less; then, the microfluidic flowing state was modeled by the flowing concave with the parameterization of flow-end height; finally, the microfluidic flowing speed was experimentally investigated with reference to microchannel angle, gradient, surface roughness and chip material. It is shown that the microfluidic self-flowing is mainly induced by the microchannel tip and the nanometer-scale surface cracks around the microchannel tip. Small microchannel angle, large microchannel gradient and smooth microchannel surface may enhance the flowing speed on chip surface. The brittle quartz glass produces the nanometer-scale surface cracks around the microchannel tip, leading to an increase about 40 times in the self-flowing speed compared with the ductile polymer. It is confirmed that the self-flowing speed in dynamic state may be characterized by the proposed concave flow-end height.     

Enhancement of separation efficiency on continuous magnetophoresis by utilizing L/T-shaped microchannels

In this article a novel design of on-chip continuous magnetophoretic separator was proposed by utilizing the magnetic field and L-turning/T-junction effect of the flow field for high throughput applications. The motion of the magnetic bead was simulated based on Lagrangian tracking method and the separation efficiency was calculated according to the trajectories. Impact parameters including geometrical configuration, fluid velocity, magnetic flux density, magnetic bead size, and temperature on separation efficiency were discussed. The results show that both the L- and T-microchannel separators have higher separation efficiency as compared with the conventional straight-microchannel separator because of the L-turning/T-junction effect of the flow field. The separation efficiencies for L- and T-microchannel separators are 63.4 and 100%, respectively, while it is only 43.7% for straight-microchannel separator at the same conditions. Above a critical flow rate the separation efficiency drops drastically from nearly 100% to zero while this decrease is much slower for T-shaped configurations. The separation efficiency increases initially with the increase of the external magnetic flux density and keeps nearly constant at high magnetic flux density owing to saturated magnetization of the beads. It is also found that both the magnetic bead diameter and fluid temperature have great effect on the separation efficiency. The L/T-microchannel separators presented in this article are simple and efficient for magnetophoretic separation at high flow rates and thus useful for the high-efficiency on-chip enrichment of analytes with very low concentrations.     

A microfluidic chip for heterogeneous immunoassay using electrokinetical control

This article presents the development of a novel, automated, electrokinetically controlled heterogeneous immunoassay on a poly(dimethylsiloxane) (PDMS) microfluidic chip. A numerical method has been developed to simulate the electrokinetically driven, time-dependent delivery processes of reagents and washing solutions within the complex microchannel network. Based on the parameters determined from the numerical simulations, fully automated on-chip experiments to detect Helicobacter pylori were accomplished by sequentially changing the applied electric fields. Shortened assay time and much less reagent consumptions are achieved by using this microchannel chip while the detection limit is comparable to the conventional assay. There is a good agreement between the experimental result and numerical prediction, demonstrating the effectiveness of using CFD to assist the experimental studies of microfluidic immunoassay.     

Inertial microfluidics for continuous particle filtration and extraction

In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.     

Surface modification of a glass microchannel for the formation of multiple emulsion droplets

We herein report a method for the preparation of a glass microchannel capable of forming multiple emulsion droplets (i.e., water-in-oil-in-water and oil-in-water-in-oil) by locally controlling the wettability of the glass microchannel. Production of multiple emulsion droplets using a glass microchannel requires partial control of its wettability using a method that consists of two steps: (1) hydrophobization of a whole glass microchannel by filling the microchannel with octadecyltrichlorosilane (OTS) solution, and (2) local hydrophilization of the OTS-treated glass microchannel by exposure to ultraviolet light through a mask. However, conditions for the preparation of OTS-SAMs for controlling microchannel wettability and subsequent multiple emulsion droplet formation have not yet been reported. In this study, we investigated the conditions required to form multiple emulsion droplets and demonstrated formation of multiple emulsion droplets using a treated glass microchannel with multiple junctions. The glass microchannel prepared according to this method was able to form various aqueous and organic droplets due to its resistance to swelling.     

On-chip PCR amplification of genomic and viral templates in unprocessed whole blood

Performing medical diagnosis in microfluidic devices could scale down laboratory functions and reduce the cost for accessible healthcare. The ultimate goal of such devices is to receive a sample of blood, perform genetic amplification (polymerase chain reaction—PCR) and subsequently analyse the amplified products. DNA amplification is generally performed with DNA purified from blood, thus requiring on-chip implementation of DNA extraction steps with consequent increases in the complexity and cost of chip fabrication. Here, we demonstrate the use of unprocessed whole blood as a source of template for genomic or viral targets (human platelet antigen 1 (HPA1), fibroblast growth factor receptor 2 (FGFR2) and BK virus (BKV)) amplified by PCR on a three-layer microfluidic chip that uses a flexible membrane for pumping and valving. The method depends upon the use of a modified DNA polymerase (Phusion™). The volume of the whole blood used in microchip PCR chamber is 30 nl containing less than 1 ng of genomic DNA. For BKV on-chip whole blood PCR, about 3000 copies of BKV DNA were present in the chamber. The DNA detection method, laser-induced fluorescence, used in this article so far is not quantitative but rather qualitative providing a yes/no answer. The ability to perform clinical testing using whole blood, thereby eliminating the need for DNA extraction or sample preparation prior to PCR, will facilitate the development of microfluidic devices for inexpensive and faster clinical diagnostics.     

In-plane spectroscopy of microfluidic systems made in photosensitive glass

This work manifests the use of microstructures made in Apex? photosensitive glass for in-plane investigation of microfluidic systems by demonstrating the ability to detect chemical fluids through etched glass sidewalls. Absorption spectra of liquid ethanol in the near infrared (NIR) region and Raman spectroscopy of dimethyl sulfoxide in microcuvettes made in Apex? photosensitive glass demonstrate the high potential of photosensitive glass processing in microfluidic applications in which stacks of microfluidic systems are analyzed from the sidewalls. This eliminates the need for non-planar observation of microfluidic systems.     

Disposable microfluidic blood cuvette for measuring hemoglobin concentration

This paper presents a new air-bubble free microfluidic blood cuvette for the measurement of hemoglobin concentration. The microfluidic blood cuvette was filled with blood samples by capillary force, and hemoglobin levels in the blood were determined by measuring absorbance at the wavelength of 530 nm. Two different microfluidic blood cuvettes with dual and single sidewall microchannels were investigated. The microfluidic blood cuvette was fabricated using a polymethyl methacrylate substrate and a dry film photoresist. During the blood-filling process, air was trapped in the dual-sided wall-type cuvettes, while no air trapping occurred in the single sidewall-type cuvettes. The sensitivity of the hemoglobin measurements was more linear in a 105 μm deep microchannel than in a 35 μm deep microchannel.     

Pre-programmable polymer transformers as on-chip microfluidic vacuum generators

This paper develops novel polymer transformers using thermally actuated shape memory polymer (SMP) materials. This paper applies SMPs with thermally induced shape memory effect to the proposed novel polymer transformers as on-chip microfluidic vacuum generators. In this type of SMPs, the morphology of the materials changes when the temperature of materials reaches its glass transition temperature (T _g). The structure of the polymer transformer can be pre-programmed to define its functions, which the structure is reset to the temporary shape, using shape memory effects. When subjected to heat, the polymer transformer returns to its pre-memory morphology. The morphological change can produce a vacuum generation function in microfluidic channels. Vacuum pressure is generated to suck liquids into the microfluidic chip from fluidic inlets and drive liquids in the microchannel due to the morphological change of the polymer transformer. This study adopts a new smart polymer with high shape memory effects to achieve fluid movement using an on-chip vacuum generation source. Experimental measurements show that the polymer transformer, which uses SMP with a T _g of 40°C, can deform 310 μm (recover to the permanent shape from the temporary shape) within 40 s at 65°C. The polymer transformer with an effective cavity volume of 155 μl achieved negative pressures of −0.98 psi. The maximum negative up to −1.8 psi can be achieved with an effective cavity volume of 268 μl. A maximum flow rate of 24 μl/min was produced in the microfluidic chip with a 180 mm long channel using this technique. The response times of the polymer transformers presented here are within 36 s for driving liquids to the end of the detection chamber. The proposed design has the advantages of compact size, ease of fabrication and integration, ease of actuation, and on-demand negative pressure generation. Thus, this design is suitable for disposable biochips that need two liquid samples control. The polymer transformer presented in this study is applicable to numerous disposable microfluidic biochips.     

Cell separation in a microfluidic channel using magnetic microspheres

Magnetophoretic isolation of biological cells in a microfluidic environment has strong relevance in biomedicine and biotechnology. A numerical analysis of magnetophoretic cell separation using magnetic microspheres in a straight and a T-shaped microfluidic channel under the influence of a line dipole is presented. The effect of coupled particle–fluid interactions on the fluid flow and particle trajectories are investigated under different particle loading and dipole strengths. Microchannel flow and particle trajectories are simulated for different values of dipole strength and position, particle diameter and magnetic susceptibility, fluid viscosity and flow velocity in both the microchannel configurations. Residence times of the captured particles within the channel are also computed. The capture efficiency is found to be a function of two nondimensional parameters, α and β. The first parameter denotes the ratio of magnetic to viscous forces, while the second one represents the ratio of channel height to the distance of the dipole from the channel wall. Two additional nondimensional parameters γ (representing the inverse of normalized offset distance of the dipole from the line of symmetry) and σ (representing the inverse of normalized width of the outlet limbs) are found to influence the capture efficiency in the T-channel. Results of this investigation can be applied for the selection of a wide range of operating and design parameters for practical microfluidic cell separators.     

Polymer-based fabrication techniques for enclosed microchannels in biomedical applications

Investigations and analyses of body fluids like serum or whole blood are essential tasks in biomedical research in order to understand and diagnose diseases, to conduct pharmacological tests or to culture cells. Therefore, microfluidic systems provide a favorable tool for processing fluid samples as they allow downscaling of sample volumes and handling of single fluid components such as cells or proteins. For this reason, we present simple fabrication techniques for microchannel systems using polymer materials only. The demonstrated fabrication procedures are based on combinations of acrylic glass and the photo resists SU-8 and PerMX3020. On the one hand, these materials are low-priced compared to conventional silicon or glass. On the other hand, they have not shown any interaction with blood or other cell suspensions within the frame of our study. Furthermore, their transparency guarantees an easy observability of all processes within the system. Depending on the channel dimensions, different adhesion bonding techniques for closing of the systems are applied, whereas the fluidic interfaces are included by mechanical drilling. Summing up, we provide complete fabrication processes for fluidic systems which are simpler and more cost-effective than conventional methods and yet cope with all essential requirements for microfluidic applications.     

Femtosecond laser-induced modification of surface wettability of PMMA for fluid separation in microchannels

The modification of polymer surface wettability is receiving increasing interest in recent years. As surface wettability affects the flowing resistance, and thus the separation ratio and/or mixing ratio of samples in different microchannels, the controlled modification of surface wettability is highly desirable. In this study, microfluidic channels with controlled surface wettability were achieved and fabricated using femtosecond (fs) laser direct ablation of polymethyl methacrylate at various fluences. Varied flow velocities and separation ratio of water in microfluidic channels have been successfully obtained through fs laser-induced modification in wetting characteristics of the microchannel surfaces. A concave flow front was observed in a microchannel with hydrophilic surface. Correspondingly, a convex flow front was observed with hydrophobic surface. For an untreated channel, a straight flow front was observed. These results would be attractive for various microfluidic chip applications, such as control of the reagent reaction through controlling liquid medium separation or control of mixing ratio in different channels.     

Fabrication of a Fabry–PÉrot Cavity in a Microfluidic Channel Using Thermocompressive Gold Bonding of Glass Substrates

This paper presents a simple, low-cost, and reliable process for the fabrication of a microfluidic Fabry–PÉrot cavity in a Pyrex glass substrate. The microfluidic channels were etched in HF solution on a glass substrate using a Cr/Au/photoresist etching mask resulting in a channel bottom roughness of 1.309 nm. An effective thermocompressive gold–gold bonding technique was used to bond the photolithographically etched glass substrates inside a 350$^circ C $oven in a$ 10^ - 3~ torr $vacuum. Pressure was applied to the glass pieces by using two aluminum blocks with intermediate copper sheets. This method takes advantage of using Cr/Au layers both as a wet etching mask and as intermediate bonding layers, requiring only one lithography step for the entire process. The fabrication method is also compatible with the incorporation of dielectric mirror coatings in the channels to form a high-finesse Fabry–PÉrot cavity. A parallelism of 0.095 degrees was measured, and a finesse as high as 30 was obtained using an LED. The microfluidic cavity developed here can be used in electrophoresis and intracavity spectroscopy experiments.hfillhbox[1375]     

A power-free deposited microbead plug-based microfluidic chip for whole-blood immunoassay

We present a deposited microbead plug (DMBP)-based microfluidic chip capable of performing plasma extraction and on-chip immunoassay. The DMBP used as a porous blood filter provides pure blood plasma without the contamination of blood cells or beads. Capillary-driven flow eliminates the requirement of external pumps. The human IgG and goat anti-human IgG sample-to-answer assay was performed in this chip within 600 s using only a 10 μl whole-blood sample. This easy-to-use, rapid, inexpensive, and disposable DMBP-based chip holds a great promise for point-of-care application.     

A low molecular weight cut-off polymer–silicate membrane for microfluidic applications

In this article, we report a novel approach to fabricating a low molecular weight cut-off membrane that could readily be employed for several microfluidic applications. The reported structure was created by selectively retaining a precursor solution [5% (w/v) maleic anhydride, 21% (v/v) (37:1) acrylamide/bisacrylamide, and 0.2% (w/v) VA-086 photoinitiator] in a chosen location of a microfluidic network via capillary forces and then photo-polymerizing the mixture. The pores in the resulting membrane were subsequently filled with 3-aminopropyltriethoxysilane, heated, and then treated with sodium silicate solution and heated again, giving a structure having reduced porosity. The composite membrane thus created has been shown to have a molecular weight cut-off that is at least an order of magnitude smaller than other photo-polymerized microfluidic membranes reported in the literature. Moreover, this polymer–silicate structure was observed to be capable of blocking electroosmotic flow, thereby generating a pressure gradient around its interface with an open microchannel upon application of an electric field across the microchannel-membrane junction. In this study, a fraction of the resulting hydrodynamic flow was successfully guided to an electric field free analysis channel to implement a pressure-driven assay. With our current design pressure-driven velocities, up to 1.8 mm/s was generated in the electric field free analysis channel for an applied voltage of 2 kV in the pumping section. Finally, the functionality of this integrated microfluidic device was demonstrated by implementing a reverse phase chromatographic separation using the pressure-driven flow generated on-chip.     

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.     

Improved concentration and separation of particles in a 3D dielectrophoretic chip integrating focusing, aligning and trapping

This article presents a dielectrophoresis (DEP)-based microfluidic device with the three-dimensional (3D) microelectrode configuration for concentrating and separating particles in a continuous throughflow. The 3D electrode structure, where microelectrode array are patterned on both the top and bottom surfaces of the microchannel, is composed of three units: focusing, aligning and trapping. As particles flowing through the microfluidic channel, they are firstly focused and aligned by the funnel-shaped and parallel electrode array, respectively, before being captured at the trapping unit due to negative DEP force. For a mixture of two particle populations of different sizes or dielectric properties, with a careful selection of suspending medium and applied field, the population exhibits stronger negative DEP manipulated by the microelectrode array and, therefore, separated from the other population which is easily carried away toward the outlet due to hydrodynamic force. The functionality of the proposed microdevice was verified by concentrating different-sized polystyrene (PS) microparticles and yeast cells dynamically flowing in the microchannel. Moreover, separation based on size and dielectric properties was achieved by sorting PS microparticles, and isolating 5 μm PS particles from yeast cells, respectively. The performance of the proposed micro-concentrator and separator was also studied, including the threshold voltage at which particles begin to be trapped, variation of cell-trapping efficiency with respect to the applied voltage and flow rate, and the efficiency of separation experiments. The proposed microdevice has various advantages, including multi-functionality, improved manipulation efficiency and throughput, easy fabrication and operation, etc., which shows a great potential for biological, chemical and medical applications.