Activity monitoring of functional OprM using a biomimetic microfluidic device

This paper describes the fabrication and use of a biomimetic microfluidic device for the monitoring of a functional porin reconstituted within a miniaturized suspended artificial bilayer lipid membrane (BLM). Such a microfluidic device allows for (1) fluidic and electrical access to both sides of the BLM and (2) reproducible membrane protein insertion and long-term electrical monitoring of its conductance (G(i)), thanks to the miniaturization of the BLM. We demonstrate here for the first time the feasibility to insert a large trans-membrane protein through its β-barrel, and monitor its functional activity for more than 1 hour (limited by buffer evaporation). In this paper, we specifically used our device for the monitoring of OprM, a bacterial efflux channel involved in the multidrug resistance of the bacteria Pseudomonas aeruginosa. Sub-steps of the OprM channel conductance were detected during the electrical recordings within our device, which might be due to oscillations between several structural conformations (sub-states) adopted by the protein, as part of its opening mechanism. This work is a first step towards the establishment of a genuine platform dedicated to the investigation of bacterial proteins under reconstituted conditions, a very promising tool for the screening of new inhibitors against bacterial channels involved in drug resistance.     

Characterizing doxorubicin-induced apoptosis in HepG2 cells using an integrated microfluidic device

Apoptosis has now established its importance in numerous areas of biology and is recently receiving great attention as an important topic related to the development of diseases. In this work, an integrated microfluidic device was developed to characterize doxorubicin-induced apoptosis in human hepatocellular carcinoma (HepG2) cells. A continuous concentration gradient of stimulator (doxorubicin) was generated in the upstream network and used to perfuse downstream cultured HepG2 cells. The appropriate fluorescent dyes were introduced into cells from the inlets connected to the cell culture chambers, allowing one to distinguish apoptotic cells from nonapoptotic or necrotic cells. The resultant fluorescence of cellular population was monitored and quantified with single-cell resolution to infer the apoptosis process being studied. The feasibility of studying apoptosis was demonstrated by measuring several apoptotic events, including morphological alterations, plasma membrane phosphatidylserine externalization, and mitochondrial membrane potential collapse. This microfluidic device, integrating the cell culture, stimulation, staining, and washing steps into a single device, can simultaneously generate a number of experimental conditions and investigate multiple parameters relating stimulation to apoptosis. It offers a unique platform to characterize various cellular responses in a high-throughput fashion, which is otherwise impossible with conventional methods.     

Temperature gradient focusing of bio-analyte in a microfluidic channel dealing with non-Newtonian electrolyte considering temperature-dependent zeta potential

Temperature gradient focusing (TGF) relies on establishing a precise balance between the electrophoretic motility of a target analyte and the advective flow of the background electrolyte (BGE) to locally concentrate the analyte in a microfluidic configuration. This paper presents a finite-element-based numerical analysis where the coupled electric field and the transport equations are solved to describe the effects of the shear-dependent apparent viscosity of a non-Newtonian BGE on the localized concentration buildup of a charged bio-sample inside a microchannel by TGF via Joule heating. Effects of the temperature-dependent nature of the wall zeta potential and the flow behavior index (n) of BGE on the flow, thermal, and species concentration profiles inside the microchannel have been investigated. Study using a fluorescein-Na analyte sample shows that the maximum normalized analyte concentration (C_max/C₀) reduces as the zeta potential increases linearly with temperature. The maximum concentration enhancement is achieved when the BGE displays the Newtonian rheology. For example, C_max/C₀ increases 134- to 280-fold when n is increased from 0.8 to 1 (pseudoplastic regime) and again reduces to 190-fold when n increases further from 1 to 1.2 (dilatant regime).     

Dispersion control in microfluidic chips by localized zeta potential variation using the field effect

A new technique to minimize the effects of turn-induced dispersion within U-shaped separation channels by using the field effect within a capacitor to vary the zeta potential along the channel walls in the vicinity of the microchannel is described. The effects of the separation channel geometry, the fluid velocity profile, and the use of the field effect to control the zeta potential on the band distribution in the detection area are extensively discussed. The results for a U-shaped separation channel indicate that varying the zeta potential by controlling the field effect significantly reduces the band dispersion induced by the 90 degrees turns within the channel. Finally, it is shown that the application of the proposed localized zeta potential variation method also results in a correction of the band tilting phenomenon and a reduction in the racetrack effect.     

Stop-flow lithography in a microfluidic device

Polymeric particles in custom designed geometries and with tunable chemical anisotropy are expected to enable a variety of new technologies in diverse areas such as photonics, diagnostics and functional materials. We present a simple, high throughput and high resolution microfluidic method to synthesize such polymeric particles. Building off earlier work that we have done on continuous flow lithography (CFL) (D. Dendukuri, D. C. Pregibon, J. Collins, T. A. Hatton, P. S. Doyle, Nat. Mater., 2006, 5, 365-369; ref. 1), we have devised and implemented a new setup that uses compressed air driven flows in preference to syringe pumps to synthesize particles using a technique that we call stop-flow lithography (SFL). A flowing stream of oligomer is stopped before polymerizing an array of particles into it, providing for much improved resolution over particles synthesized in flow. The formed particles are then flushed out at high flow rates before the cycle of stop-polymerize-flow is repeated. The high flow rates enable orders-of-magnitude improvements in particle throughput over CFL. However, the deformation of the PDMS elastomer due to the imposed pressure restricts how quickly the flow can be stopped before each polymerization event. We have developed a simple model that captures the dependence of the time required to stop the flow on geometric parameters such as the height, length and width of the microchannel, as well as on the externally imposed pressure. Further, we show that SFL proves to be superior to CFL even for the synthesis of chemically anisotropic particles with sharp interfaces between distinct sections.     

High-throughput rheology in a microfluidic device

High-throughput rheological measurements in a microfluidic device are demonstrated. A series of microrheology samples are generated as droplets in an immiscible spacer fluid using a microfluidic T-junction. The compositions of the sample droplets are continuously varied over a wide range. Rheology measurements are made in each droplet using multiple particle tracking microrheology. We review critical design and operating parameters, including the droplet size, flow rates and rapid fabrication methods. Validation experiments are performed by measuring the solution viscosity of glycerine and the biopolymer heparin as a function of concentration. Overall, the combination of microrheology with microfluidics maximizes the number of rheological measurements while simultaneously minimizing the sample preparation time and amount of material, and should be particularly suited to the characterization of scarce or expensive materials.     

Characterizing electroosmotic flow in microfluidic devices

The current-monitoring method was used to measure the electroosmotic flow (EOF) in borosilicate glass capillaries and zeonor plastic microfluidic devices. The surface of the zeonor devices must be oxidized to support EOF and this treatment shows signs of aging within 6 days. Oxidized zeonor devices showed the same response to changes in applied field, pH, and ionic concentration as the capillaries. The effects of several common dynamic surfactant coatings on the walls were also studied (0.1%, v/v solutions of POP-6, POP4, Pluronics L81, and NP-40). These generally significantly suppressed the EOF but required several days to stabilize.     

Optimal MEMS device for mobility and zeta potential measurements using DC electrophoresis

We have developed a novel microchannel geometry that allows us to perform simple DC electrophoresis to measure the electrophoretic mobility and zeta potential of analytes and particles. In standard capillary geometries, mobility measurements using DC fields are difficult to perform. Specifically, measurements in open capillaries require knowledge of the hard to measure and often dynamic wall surface potential. Although measurements in closed capillaries eliminate this requirement, the measurements must be performed at infinitesimally small regions of zero flow where the pressure driven‐flow completely cancels the electroosmotic flow (Komagata Planes). Furthermore, applied DC fields lead to electrode polarization, further questioning the reliability and accuracy of the measurement. In contrast, our geometry expands and moves the Komagata planes to where velocity gradients are at a minimum, and thus knowledge of the precise location of a Komagata plane is not necessary. Additionally, our microfluidic device prevents electrode polarization because of fluid recirculation around the electrodes. We fabricated our device using standard MEMS fabrication techniques and performed electrophoretic mobility measurements on 500 nm fluorescently tagged polystyrene particles at various buffer concentrations. Results are comparable to two different commercial dynamic light scattering based particle sizing instruments. We conclude with guidelines to further develop this robust electrophoretic tool that allows for facile and efficient particle characterization.     

T cell chemotaxis in a simple microfluidic device

This paper describes the use of a simple microfluidic device for studying T cell chemotaxis. The microfluidic device is fabricated in poly(dimethylsiloxane) (PDMS) using soft-lithography and consists of a "Y" type fluidic channel. Solutions are infused into the device by syringe pumps and generate a concentration gradient in the channel by diffusion. We show that the experimentally measured gradient profiles agree nicely with theoretical predictions and the gradient is stable in the observation region for cell migration. Using this device, we demonstrate robust chemotaxis of human T cells in response to single and competing gradients of chemokine CCL19 and CXCL12. Because of the simplicity of the device, it can flexibly control gradient generation in space and time, and would allow generation of multiple gradient conditions in a single chip for highly parallel chemotaxis experimentation. Visualization of T cell chemotaxis has previously been limited to studies in 3D matrices or under agarose assays, which do not allow precise control or variation in conditions. Acknowledging the importance of lymphocyte homing in the adaptive immune response, the ability to study T cell chemotaxis in microfluidic devices offers a new approach for investigating lymphocyte migration and chemotaxis in vitro.     

Functionalized electrospun nanofibers as bioseparators in microfluidic systems

Functionalized electrospun nanofibers were integrated into microfluidic channels to serve as on-chip bioseparators. Specifically, poly(vinyl alcohol) (PVA) nanofiber mats were shown to successfully serve as bioseparators for negatively charged nanoparticles. Nanofibers were electrospun onto gold microelectrodes, which were incorporated into poly(methyl methacrylate) (PMMA) microfluidic devices using UV-assisted thermal bonding. PVA nanofibers functionalized with poly(hexadimethrine bromide) (polybrene) were positively charged and successfully filtered negatively charged liposomes out of a buffer solution, while negatively charged nanofibers functionalized with Poly(methyl vinyl ether-alt-maleic anhydride) (POLY(MVE/MA)) were shown to repel the liposomes. The effect of fiber mat thickness was studied using confocal fluorescence microscopy, determining a quite broad optimal range of thicknesses for specific liposome retention, which simplifies fiber mat production with respect to retention reliability. Finally, it was demonstrated that liposomes bound to positively charged nanofibers could be selectively released using a 4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES)-sucrose-saline (HSS) solution of pH 9, which dramatically changes the nanofiber zeta potential and renders the positively charged nanofibers negatively charged. This is the first demonstration of functional electrospun nanofibers used to enable sample preparation procedures of isolation and concentration in lab-on-a-chip devices. This has far reaching impact on the ability to integrate functional surfaces and materials into microfluidic devices and to significantly expand their ability toward simple lab-on-a-chip devices.     

Hydrodynamic shearing of DNA in a polymeric microfluidic device

With the advent of next-generation sequencing (NGS) systems and the associated high throughput they afford, the input to these machines requires manageable lengths of fragments (~1000 bp) produced from chromosomal DNAs. Therefore, it is critical to develop devices that can shear DNA in a controlled fashion. We report a polymer-based microfluidic device that establishes an efficient and inexpensive platform with performance comparable to a commercially available bench-top system.     

Sample purification on a microfluidic device.

Sample preparation has long been recognized as a significant barrier to the implementation of macroscopic protocols on microfabricated devices. Macroscopically, such tasks as removing salts, primers and other contaminants are performed by methods involving precipitation, specialized membranes and centrifuges, none of which are readily performed in microfluidic structures. Although some microfluidic systems have been developed for performing sample purification, their complexity may hinder the degree to which they can be implemented. We present a method of microchip-based sample purification that can be performed with even the simplest microfluidic designs. The technique is demonstrated by removing primers from a sample of amplified DNA, leaving only the product DNA. This provides a new sample preparation capability for microfluidic systems.     

Flexible generation of gradient electrospinning nanofibers using a microfluidic assisted approach

The nanofiber surface modified with physical or chemical gradients is very useful in a wide range of areas including tissue engineering, regenerative medicine, drug screening, and biomaterial chemistry. In this work, we presented a novel and straightforward microfluidic assisted approach to produce electrospinning nanofibers containing gradients in different compositions, nanoparticles and biomolecule concentrations. The series of gradient nanofibers were mainly produced by using a two inlet microfluidic device in combination with an electrospinning nozzle on a 3-D controllable platform, which exhibited different functions and properties. The controlled nanofibers with incorporated biomolecule gradient were used for guiding the spatial differentiation in mesenchymal stem cells (MSCs). This established approach is very simple, and flexible to operate, which might find enormous potential for biology and tissue engineering applications.     

Particle sorting using a porous membrane in a microfluidic device

Porous membranes have been fabricated based on the development of the perforated membrane mold [Y. Luo and R. N. Zare, Lab Chip, 2008, 8, 1688-1694] to create a single filter that contains multiple pore sizes ranging from 6.4 to 16.6 μm inside a monolithic three-dimensional poly(dimethylsiloxane) microfluidic structure. By overlapping two filters we are able to achieve smaller pore size openings (2.5 to 3.3 μm). This filter operates without any detectable irreversible clogging, which is achieved using a cross-flow placed in front of each filtration section. The utility of a particle-sorting device that contains this filter is demonstrated by separating polystyrene beads of different diameters with an efficiency greater than 99.9%. Additionally, we demonstrate the effectiveness of this particle-sorting device by separating whole blood samples into white blood cells and red blood cells with platelets.     

Endothelial cell polarization and chemotaxis in a microfluidic device

The directed migration of endothelial cells is an early and critical step in angiogenesis, or new blood vessel formation. In this study, the polarization and chemotaxis of human umbilical vein endothelial cells (HUVEC) in response to quantified gradients of vascular endothelial growth factor (VEGF) were examined. To accomplish this, a microfluidic device was designed and fabricated to generate stable concentration gradients of biomolecules in a cell culture chamber while minimizing the fluid shear stress experienced by the cells. Finite element simulation of the device geometry produced excellent agreement with the observed VEGF concentration distribution, which was found to be stable across multiple hours. This device is expected to have wide applicability in the study of shear-sensitive cells such as HUVEC and non-adherent cell types as well as in the study of migration through three-dimensional matrices. HUVEC were observed to chemotax towards higher VEGF concentrations across the entire range of concentrations studied (18-32 ng mL(-1)) when the concentration gradient was 14 ng mL(-1) mm(-1). In contrast, shallow gradients (2 ng mL(-1) mm(-1)) across the same concentration range were unable to induce HUVEC chemotaxis. Furthermore, while all HUVEC exposed to elevated VEGF levels (both in steep and shallow gradients) displayed an increased number of filopodia, only chemotaxing HUVEC displayed an asymmetric distribution of filopodia, with enhanced numbers of protrusions present along the leading edge. These results suggest a two-part requirement to induce VEGF chemotaxis: the VEGF absolute concentration enhances the total number of filopodia extended while the VEGF gradient steepness induces filopodia localization, cell polarization, and subsequent directed migration.     

Electrochemical hematocrit determination in a direct current microfluidic device

Hematocrit (HCT) tests are widely performed to screen blood donors and to diagnose medical conditions. Current HCT test methods include conventional microhematocrit, Coulter counter, CuSO₄ specific gravity, and conductivity‐based point‐of‐care (POC) HCT devices, which can be either expensive, environmentally inadvisable, or complicated. In the present work, we introduce a new and simple microfluidic system for a POC HCT determination. HCT was determined by measuring current responses of blood under 100 V DC for 1 min in a microfluidic device containing a single microchannel with dimensions of 180 μm by 70 μm and 10 mm long. Current responses of red blood cell (RBC) suspensions in PBS or separately plasma at HCT concentrations of 10, 20, 25, 30, 35, 40, 45, 50, 55, 60, and 70 vol% were measured to show feasibility of the microfluidic system for HCT determination. Key parameters affecting current responses included electrolysis bubbles and irreversible RBC adsorption; parameters were optimized via addition of nonionic surfactant Triton X‐100 into sample solution and carbonizing electrode surfaces. The linear trend line of current responses over a range of RBC concentrations were obtained in both PBS and plasma. This work suggested that a simple microfluidic device could be a promising platform for a new POC HCT device.     

Simultaneous generation of multiple aqueous droplets in a microfluidic device

This paper describes a microfluidic platform for the on-demand generation of multiple aqueous droplets, with varying chemical contents or chemical concentrations, for use in droplet based experiments. This generation technique was developed as a complement to existing techniques of continuous-flow (streaming) and discrete-droplet generation by enabling the formation of multiple discrete droplets simultaneously. Here sets of droplets with varying chemical contents can be generated without running the risk of cross-contamination due to the isolated nature of each supply inlet. The use of pressure pulses to generate droplets in parallel is described, and the effect of droplet size is examined in the context of flow rates and surfactant concentrations. To illustrate this technique, an array of different dye-containing droplets was generated, as well as a set of droplets that displayed a concentration gradient of a fluorescent dye.     

Modeling electroosmotic and pressure-driven flows in porous microfluidic devices: zeta potential and porosity changes near the channel walls

This work presents analytical solutions for both pressure-driven and electroosmotic flows in microchannels incorporating porous media. Solutions are based on a volume-averaged flow model using a scaling of the Navier-Stokes equations for fluid flow. The general model allows analysis of fluid flow in channels with porous regions bordering open regions and includes viscous forces, permitting consideration of porosity and zeta potential variations near channel walls. To obtain analytical solutions problems are constrained to the linearized Poisson-Boltzmann equation and a variation of Brinkman's equation [Appl. Sci. Res., Sect. A 1, 27 (1947); 1, 81 (1947)]. Cases include one continuous porous medium, two adjacent regions of different porosities, or one open channel adjacent to a porous region, and the porous material may have a different zeta potential than that of the channel walls. Solutions are described for two geometries, including flow between two parallel plates or in a cylinder. The model illustrates the relative importance of porosity and zeta potential in different regions of each channel.     

An automated fluid-transport device for a microfluidic system

An automated fluid-transport device for a chip-based capillary electrophoresis system has been developed. The device mainly consists of six peristaltic micropumps, two vacuum micropumps, microvalves, multi-way joints, titanium tubes, and a macro-to-micro connector. Various solutions used for the cleaning and activation of chip channels, and electrophoresis separation, are allowed to automatically transport to chip reservoirs by the electric control module. The performance of the whole system was characterized by the analysis of fluorescein sodium using chip electrophoresis with LED-induced fluorescence detection. The peak-height variation (RSD) was 3.8% in six cycles of analyses. Additionally, compared with conventional manual operation, the developed device can spare 60% time for chip pretreatment. This microdevice offers high-efficiency pretreatment for microchips, thereby resulting in a remarkable improvement of analytical capacity for batch samples.