Applications In Microfluidic Systems Research Articles

Electroporation of Mammalian Cells in a Microfluidic Channel with Geometric Variation

Electroporation has been widely used to load impermeant exogenous molecules into cells. Rapid electrical lysis based on electroporation has also been applied to analyze intracellular materials at single-cell level. There has been increasing demand to implement electroporation in a microfluidic format as a basic tool for applications ranging from screening of drugs and genes to studies of intracellular dynamics. In this report, we have developed a simple technique to electroporate mammalian cells with high throughput on a microfluidic platform. In our design, electroporation only happened in a defined section of a microfluidic channel due to the local field amplification by geometric variation. The time of exposure of the cells to this high field was determined by the velocity of the cells and the length of the section. The change in the cell morphology during electroporation was observed in real time. We determined that electroporation of Chinese hamster ovary cells occurred when the local field strength was increased to approximately 400 V/cm. The internalization of membrane-impermeant molecules (SYTOX green) with cell viability preserved was also carried out to demonstrate transient electropermeabilization. The influence of the operational parameters of the device on cell viability was determined. A large percentage of cells remained viable after electroporation when the parameters were tuned. We also studied rapid cell lysis when the field intensity was in the range of 600-1200 V/cm. The rupture of cell membrane happened within 30 ms when the field strength was 1200 V/cm. Given the simplicity, high throughput, and high compatibility with other devices, this microfluidic electroporation technique may increase the application of microfluidic systems in screening of drugs and biomolecules and chemical cytometry.

Polymers for Microfluidic Chips

Microfluidic systems have attracted much research attention recently in the areas of genomics, proteomics, pharmaceutics, clinical diagnostics, and analytical biochemistry, as they provide miniaturized platforms for conventional analysis techniques. The microfluidic systems allow faster and cheaper analysis using much smaller amounts of sample and reagent than conventional methods. Polymers have recently found useful applications in microfluidic systems due to the wide range of available polymeric materials and the relative ease of chemical modification. This paper discusses the fundamentals of microfluidic systems and the roles, essential properties and various forms of polymers used as solid supports in microfluidic systems, based on the recent advances in the use of polymers for microfluidic chips.

Thermal ablation of PMMA for water release using a microheater

A new approach was developed in this work to ablate a micropore through a polymethylmethacrylate (PMMA) layer using a microheater for the on-demand release of water stored in a microreservoir. The size of the ablated micropore in the PMMA capping layer is mainly governed by the heater temperature profile and ablation time. Furthermore, the molten PMMA resulting from the heating energy tends to retract away from the micropore location, taking away the gold heater lines from the pore area. This prevents the possibility of any gold traces blocking the flow of water released from within the microreservoir. Simulation was conducted to find temperature profiles on the surface of the microheater, and the results were used to interpret the phenomena observed in the ablation process. Simulation was also conducted for the case when the microreservoir was filled with water. In addition, two alternative ablation materials, unexposed SU-8 and polydimethylsiloxane (PDMS), were examined as possible microreservoir capping layers. The approach developed has potential applications in microfluidic systems to release encapsulated fluids in a controllable manner.

Ultrasonic standing wave manipulation technology integrated into a dielectrophoretic chip

Several cell-based biological applications in microfluidic systems require simultaneous high-throughput and individual handling of cells or other bioparticles. Available chip-based tools for contactless manipulation are designed for either high-precision handling of individual particles, or high-throughput handling of ensembles of particles. In order to simultaneously perform both, we have combined two manipulation technologies based on ultrasonic standing waves (USWs) and dielectrophoresis (DEP) in a microfluidic chip. The principle is based on the competition between long-range ultrasonic forces, short-range dielectrophoretic forces and viscous drag forces from the fluid flow. The ultrasound is coupled into the microchannel resonator by an external transducer with a refractive element placed on top of the chip, thereby allowing transmission light microscopy to continuously monitor the biological process. The DEP manipulation is generated by an electric field between co-planar microelectrodes placed on the bottom surface of the fluid channel. We demonstrate flexible and gentle elementary manipulation functions by the use of USWs and linear or curved DEP deflector elements that can be used in high-throughput biotechnology applications of individual cells.

Controlled pore structure modification of diatoms by atomic layer deposition of TiO2

Diatoms produce diverse three-dimensional, regular silica structures with nanometer to micrometer dimensions and hold considerable promise for biological or biomimetic fabrication of nanostructured materials and devices. The unique hierarchical porous structure of diatom frustules is in particular attractive for membrane applications in microfluidic systems. In this paper, a procedure for pore size modifications of two centric diatom species, Coscinodiscus sp. and Thalassiosira eccentrica (T. eccentrica) using the atomic layer deposition (ALD) of ultrathin films of titanium oxide (TiO2) is described. TiO2 is deposited by sequential exposures to titanium chloride (TiCl4) and water. The modified diatom membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDAX), and secondary ion mass spectrometry (SIMS). These techniques confirmed the controlled reduction of pore sizes while preserving the shape of the diatom membrane pores. Pore diameters of diatom membranes can be further tailored for specific applications by varying the number of cycles and by changing their surface functionality.

Study of the electroosmotic flow as a means to propel the mobile phase in capillary electrochromatography in view of further miniaturization of capillary electrochromatography systems

In this paper, we investigate the phenomenon of electroosmosis as a means to propel a mobile phase, in particular in view of an application in microfluidic systems, which are characterized by significantly smaller volumes of the reservoirs and the separation channels compared to conventional instrumentation. In the microfluidic chip, pH changes due to water electrolysis quickly showed an effect on the electroosmotic flow (EOF), which could be counteracted by either regularly exchanging or buffering the mobile phase. Surface treatment was of no effect in regard to EOF stabilization in empty channels but may have an influence in channels filled with a charged monolith. In fused-silica capillaries the EOF was generally found to decrease from 'naked' to surface-treated to monolith-filled capillaries. The EOF tended to be higher when an organic solvent (acetonitrile) was added to the mobile phase and could be further increased by substituting the water with equal amounts of methanol. In addition, the hydrostatic pressure exerted by the EOF was investigated. In a microfluidic chip with empty (cross-)channels such an effect could be responsible for a redirection of the flow. In capillaries partially filled with a noncharged (non-EOF-generating) monolith, a linear relationship could be established between the EOF created in the empty section of the capillary (apparent mobility) and the length of the monolith (backpressure). In capillaries partially filled with a charged (EOF-producing) monolith, flow inhomogeneities must be expected as a consequence of a superimposition of hydrodynamic pressure and EOF as mobile phase driving force.

Non-invasive nano-flow sensor for application in micro-fluidic systems

A novel, non-invasive fluid flow sensor principle is proposed, which enables measuring flow rates in the nano- to microliter per minute range. In the chosen sensor set-up, the liquid flows from an open reservoir, a typical design in micro-fluidic systems. The sensor is based on a two to one optical fiber coupler, which illuminates the liquid surface in the reservoir and collects the light reflected from the surface. The intensity of the captured light is proportional to the distance between the optical probe and the liquid surface. As a consequence, the liquid flow can be derived in quasi real time (<0.4 s in the case of 0.3 nl/s) from the decline of the liquid surface, an excellent basis for adaptive flow control in micro-fluidics. The sensor can be easily adapted to any kind of micro-fluidic system as long as the liquid flows from an open reservoir. Moreover, there is no need for complex integrated sensor fabrication under clean room conditions. The proposed flow sensor has potentially a higher sensitivity and a wider dynamic range than the previously existing solutions.

Thermal challenges in MEMS applications: phase change phenomena and thermal bonding processes

Two thermal challenges for current and next generation microelectromechanical systems (MEMS) applications are discussed. The first topic is the fundamental investigations of phase change phenomena in the microscale. It has been demonstrated that microresistive heaters can generate single, spherical and controllable thermal bubbles with diameters between 2 and 500μm. Both simplified steady state and transient analyses that provide the scientific foundation of bubble nucleation in the microscale have been established but require further investigations. Several device demonstrations are briefed including microbubble-powered actuators, microbubble-powered nozzle-diffuser pumps and microbubble-powered micromixers for applications in microfluidic systems. The second topic addresses key heat transfer issues during the thermal bonding processes for MEMS fabrication and packaging applications. Basic thermal analyses on the microscale bonding processes have been developed while in-depth study is required to advance the understandings of the thermal bonding processes in the microscale. Successful new thermal bonding processes are introduced, including localized eutectic bonding, localized fusion bonding, localized chemical vapor deposition (CVD) bonding, localized solder bonding and nanosecond laser bonding for encapsulation of MEMS devices.

Generating high-pressure sub-microliter flow rate in packed microchannel by electroosmotic force: potential application in microfluidic systems

We have developed a liquid delivery pump, known as an electroosmotic pump (EOP), based on the electrically induced osmosis principle, which is mainly made of one or several microchannels packed with porous fine dielectric material and connected in parallel. The EOP is tested with methanol, phosphate sodium buffer and their mixture, which can generate pressures from 0.1 to 15 MPa and flow rates of tens of nanoliters per minute to several microliters per minute. Constant and pulsation-free flow from the EOP adapts well to microfluidic systems.

Surface modification of polymer-based microfluidic devices

We report the chemical modification of poly(methyl methacrylate) (PMMA), and poly(carbonate) (PC) surfaces for applications in microfluidic systems. For PMMA, a reaction of the surface methyl ester groups with a monoanion of α,ω-diaminoalkanes (aminolysis reaction) to yield amine-terminated PMMA surfaces will be described. Furthermore, it was found that the amine functionalities were tethered to the PMMA backbone through an alkane bridge to amide bonds formed during the aminolysis of the surface ester functionalities. The electro-osmotic flow (EOF) in aminated-PMMA microchannels was reversed when compared to that in unmodified channels. Finally, the availability of the surface amine groups was further demonstrated by their reaction with n-octadecane–1-isocyanate to form PMMA surfaces terminated with well ordered and highly crystalline octadecane chains, appropriate for performing reverse-phase separations. Examples of reverse-phase separations of ion-paired double-stranded DNAs in electric fields (capillary electrochromatography (CEC)) will be demonstrated using a PMMA-based fluidic chip. For PC, sulfonation of the surface with SO 3 will be described; this sulfonation makes the surface very hydrophilic. EOF studies of the sulfonated-PC surfaces indicated changes in the pH-dependent profile when compared to unmodified PC.

Control and applications of immiscible liquids in microchannels.

Photolithography was used in combination with photocleavable self-assembled monolayers to pattern surface free energies inside microchannels enabling the control of the boundary between immiscible liquids. While aqueous solutions are confined to the hydrophilic pathways by surface forces alone, organic liquids are confined to the hydrophobic region only if the aqueous liquid first occupies the hydrophilic region. In this way, stable liquid boundaries between immiscible liquids are possible as long as the pressures are maintained below critical values. The maximum pressures are determined by the interfacial tension of the aqueous solution and organic liquid, channel depth, and advancing contact angle (theta;(a)). Experimental results on maximum pressures are in good agreement with the analytical values. The ability to confine and position the boundary between immiscible liquids inside microchannels leads to a broad range of applications in microfluidic systems, which is exemplified by fabrication of a semipermeable membrane in a surface-patterned channel via interfacial polymerization.

Polymerization optimization of SU-8 photoresist and its applicationsin microfluidic systems and MEMS

In this paper, SU-8 EPON-based photoresist (PR) polymerizationoptimization and its possible microfluidic and MEMS applications arereported. First, the optimization results of SU-8 under UV lithography arereported. The parameters which could have an influence on the lithographyquality were chosen and optimized by a three-level, L9 orthogonal array ofthe Taguchi method. By optimization, the optimal parameter range and theweighted per cent of a parameter on the final results were determined. ForSU-8-5 and SU-8-50, many microstructures with thicknesses of more than 100and 500 µm and aspect ratios of more than 20 and 50 were obtained withhigh resolution. The optimization results show that the prebake time playsthe key role in the quality, which is different from the previouslypublished results. With the optimization results obtained, some possibleapplications of SU-8 were developed and demonstrated. These applicationsincluded using SU-8 as a structural material for a microfluidic system, as amicromold for electroplating, as a master for plastic hot-embossing, and evenas a mask for some wet-etching processes.