Ultraviolet sealing and poly(dimethylacrylamide) modification for poly(dimethylsiloxane)/glass microchips

Simple sealing methods for poly(dimethylsiloxane) (PDMS)/glass-based capillary electrophoresis (CE) microchips by UV irradiation are described. Further, we examined the possibility to modify the inner surface of separation channels, using polymethylacrylamide (PDMA) as a dynamic coating reagent. The surface properties of native PDMS, UV-irradiated PDMS, and PDMA-coated PDMS were systematically studied by atomic force microscopy (AFM), infrared absorption by attenuated total reflection infrared (ATR-IR) spectroscopy, and contact angle measurement. We found that PDMA forms a stable coating on PDMS and glass surfaces, eliminating the nonhomogeneous electroosmotic flow (EOF) in channels on PDMS/glass microchips, and improving the hydrophilicity of PDMS surfaces. Mixtures of flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and fluorescein were separated in 35 s using PDMA-coated PDMS/glass microchips. A high efficiency of theoretical plates with at least 1365 (105 000 N/m) and a good reproducibility with relative standard deviations (RSD) below 4% in five successive separations were achieved.     

Microwave plasma treatment of polymer surface for irreversible sealing of microfluidic devices

Microwave plasma was generated in a glass bottle containing 2-3 Torr of oxygen for plasma treatment of a polymer surface. A "kitchen microwave oven" and a dedicated microwave digestion oven were used as the power source. Poly(dimethylsiloxane)(PDMS) slabs treated by a 30 W plasma for 30-60 s sealed irreversibly to form microfluidic devices that can sustain solution flow of an applied pressure of 42 psi without leaking. Experimental set up and conditions for the production of a homogeneous plasma to activate the PDMS surface for irreversible sealing are described in detail. The surface of a microwave plasma-treated PDMS slab was characterized using atomic force microscopy (AFM) and attenuated total reflection-Fourier Transform infrared spectroscopy (ATR-FTIR). The plasma-treated surface bears silica characteristics.     

Measurement of the electrophoretic mobility of sheep erythrocytes using microcapillary chips

Cell electrophoretic mobility (EPM) can be used to characterize individual cells. The purpose of this study is to establish reproducible and reliable cell EPM values obtained using microcapillary electrophoresis (microCE) chips. We studied cell electrophoresis on microCE chips through the comprehensive measurement of EPM and zeta potential. The inner wall of microchannels in microCE chips was coated with three kinds of reagents, namely bovine serum albumin (BSA), gelatin, and 2-methacryloyloxyethylphosphorylcholine (MPC) polymer to prevent nonspecific adhesion and interaction between cells and the inner wall. Electrophoresis was conducted in phosphate-buffered saline (pH 4-9) using erythrocytes extracted from sheep whole blood. Electroosmotic flow (EOF) mobility was measured using noncharged particles, and then the true EPM was calculated by subtracting the EOF mobility from the electromigration. MPC polymer coatings in microCE chips reduced the zeta potential of the inner wall and fully prevented nonspecific adhesion. EPM data obtained using microCE chips were almost the same and reproducible over a wide range of pH irrespective of the coating reagent used. In conclusion, reliability in the measurement of cell EPM using microCE chips was realized.     

Water-vapor plasma-based surface activation for trichlorosilane modification of PMMA

Separation rates and resolutions within capillary electrophoretic (CE) systems can be enhanced when surface zeta potentials are uniform with minimum deviations from ideal pluglike flow. Microfluidic CE devices based on poly(methyl methacrylate) (PMMA) are being developed due to the optical clarity, availability, stability, and reproducible electroosmotic flow (EOF) rates displayed by this polymer. Control of EOF in polymer-based CE systems can be achieved by surface zeta potential alteration through chemical modification. Herein, a method will be presented for the surface functionalization of PMMA with chemistry analogous to formation of trichlorosilane self-assembled monolayers on SiO2. The current method involves two separate steps. First, surface activation with water-vapor plasma introduces surface hydroxylation. Second, treatment of the plasma-treated PMMA with a substituted trichlorosilane solution forms the functional surface layer. The modified surfaces were characterized using several analytical techniques, including water contact angle, X-ray photoelectron spectroscopy, Fourier transform infrared-attenuated total reflection, secondary ion mass spectroscopy, and measurement of EOF velocities within PMMA microchannels.     

Electrokinetic flow control in microfluidic chips using a field-effect transistor

A field-effect transistor is developed to control flow in microfluidic chips by modifying the surface charge condition. In this investigation, zeta potential at a particular location is altered locally by applying a gate voltage, while zeta potential at other locations is maintained at its original value. This non-uniform zeta potential results in a secondary electroosmotic flow in the lateral direction, which is used for flow control in microgeometries. Here, microchannel structures and field-effect transistors are formed on polydimethylsiloxane (PDMS) using soft lithography techniques, and a micro particle image velocimetry technique is used to obtain high resolution velocity distribution in the controlled region. The flow control is observed at relatively low gate voltage (less than 50 V), and this local flow control is primarily due to current leakage through the interface between PDMS and glass layers. A leakage capacitance model is introduced to estimate the modified zeta potential for the straight channel case, and excellent agreement is obtained between the predicted and experimental zeta potential results. This leakage-current based field-effect is then applied to a T-channel junction to control flow in the branch channel. Experiments show that the amount of discharge in the branch channel can be controlled by modulating gate voltage.     

Zeta potentials of PDMS surfaces modified with poly(ethylene glycol) by physisorption

Controlling zeta potential of PDMS surface coated with a layer of PEG is important for electroosmosis and electrophoresis in PDMS made microfluidic chips. Here, zeta potentials of PDMS surfaces modified by simple physisorption of PEG of different concentrations in phosphate buffer solutions, pure water, and PEG solution were reported. Coating PEG on PDMS surfaces was achieved by immersing a PDMS layer into the PEG solution for 10 min and then taking it out and placing it in an oven at 80℃ for 10 h. To avoid damaging the PEG layer on the PDMS surface, an induction current method was employed for zeta potential measurement. Zeta potentials of PEG modified PDMS in electrolyte solutions were measured. The results show that 2.5% PEG can effectively modify PDMS surface with positive zeta potential value in phosphate buffer solutions, pure water and 10% PEG solution. Further increase in PEG solution beyond 5% for surface modification has no obvious effect on zeta potential change.     

聚二甲基硅氧烷基质微流控芯片通道的氧气氛改性研究

通过将新制的PDMS微流控芯片置于氧气氛中对通道表面进行处理的简单方法,使电渗流大小及稳定性有了显著的改善。同时研究了氧气处理PDMS通道表面的时间对电渗流的影响,得到氧气处理的最佳时间为3d。讨论了氧气作用于PDMS芯片表面的机理。在氧气处理3d的PDMS微流控芯片上进行氨基酸分离实验,得到较好的分离效果。     

Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection

Glass microdevices for capillary electrophoresis (CE) gained a lot of interest in the development of micrototal analysis systems (microTAS). The fabrication of a microTAS requires integration of sampling, chemical separation and detection systems into a microdevice. The integration of a detection system into a microchannel, however, is hampered by the lack of suitable microfabrication technology. Here, a microfabrication method for integration of insulated microelectrodes inside a leakage-free microchannel in glass is presented. A combination of newly developed technological approaches, such as low-temperature glass-to-glass anodic bonding, channel etching, fabrication of buried metal interconnects, and deposition of thin plasma-enhanced chemical vapour deposition (PECVD) silicon carbide layers, enables the fabrication of a CE microdevice with an integrated contactless conductivity detector. The fabrication method of this CE microdevice with integrated contactless conductivity detector is described in detail. Standard CE separations of three inorganic cations in concentrations down to 5 microM show the viability of the new microCE system.     

微流控芯片毛细管电泳在蛋白质分离分析中的应用研究进展

重点介绍了近年来国内外在微流控芯片毛细管电泳法用于蛋白质分离分析方面的研究进展。按照分离模式的不同,综述了各种应用于蛋白质分离的微流控芯片毛细管电泳系统,讨论了抑制芯片中的蛋白吸附的各种方法,并展望了芯片毛细管电泳系统在蛋白质分离领域的发展前景。引用文献47篇。     

Plasma stencilling methods for cell patterning

In this paper we describe plasma stencilling techniques for patterning 10 mammalian cell lines on hydrophobic and cell repellent poly(dimethylsiloxane) (PDMS), methylated glass and bacterial grade polystyrene surfaces. An air plasma produced with a Tesla generator operating at atmospheric pressure was used with microengineered stencils for patterned surface oxidation, selectively transforming the surface to a hydrophilic state to enable cell adhesion and growth. Plasma stencilling obviates the need for directly patterning cell adhesion molecules. Instead, during cell culture, adhesion proteins from the media assemble in a bioactive form on the hydrophilic regions. Critically, the removal of protein patterning prior to cell culture provides the option to also use PDMS–PDMS plasma bonding to incorporate cell patterns within microfluidic systems. Linear patterns were generated using PDMS microchannel stencils, and polyimide stencils with through holes were used for the production of cellular arrays. For the production of smaller cellular arrays, a novel microcapillary-based dielectric barrier discharge system was developed. A numerical method to characterise the cell patterns is also introduced and was used to demonstrate that plasma stencilling is highly effective, with complete patterns confined during long term cell culture (>10 days). In summary, plasma stencilling is simple, rapid, inexpensive, reproducible and a potentially universal cell line patterning capability.

Protein arrays on high-surface-area plasma-nanotextured poly(dimethylsiloxane)-coated glass slides

Treatment of poly(dimethylsiloxane) (PDMS) surfaces with SF(6) plasma results in the creation of high-surface-area nanotextured surfaces that considerably favour protein adsorption with respect to untreated ones. In order to employ such nanotextured surfaces as substrates for microarrays to be created and analysed using standard instrumentation, we fabricated thin PDMS films on top of standard low-cost microscope glass slides. The properties of both untreated and plasma-treated PDMS-coated slides towards spotting of protein solutions were evaluated in terms of spot signal intensity and homogeneity as well as of spot shape and size. It was found that the plasma-treated PDMS-coated glass slides provided highly homogeneous spots (mean intra-spot variation 7.6%) with spot signal intensity 6-times higher than that obtained using the untreated ones. In addition, comparison with commercially available polystyrene and aminosilanized-glass microarray slides showed that the proposed slides provided 3-times higher spot signal intensity and 2-times lower intra-spot signal variation. In addition, the implementation of long-aged-after-plasma-treatment nanotextured PDMS-coated glass slides provided spots whose shape and size matched those of the spotting tip. As a consequence, denser arrays of variable spot shape can be created using SF(6) plasma-treated PDMS-coated slides instead of standard microarray slides opening new potentials for bioanalytical applications.     

Characterization of PDMS-modified glass from cast-and-peel fabrication

In glass/poly(dimethylsiloxane) (PDMS) hybrid microfluidic chips, two different fabrication approaches are used: photolithographic or solid ink molds, or cast-and-peel methods. In the latter, a thin slab of PDMS is laid down and fluid channels are cut manually or by machine. The cast-and-peel approach has been used successfully for low-shear culture devices, among other applications. The main drawback, not reported to date, of cast-and-peel methods is that removal of PDMS (exposing the glass substrate) results in nanoscopic domains of PDMS still attached to the surface. This residual PDMS is not observable by eye, but affects the hydrophobicity of the device. Using contact angle measurement, atomic force and fluorescence microscopy, the changes in glass surfaces from the cast-and-peel technique were elucidated. This study demonstrates the enhanced protein (NeutrAvidin) adsorption on PDMS treated glass surfaces, and the potential influence of altered glass properties on microfluidic applications has been discussed as well.     

Corrigendum to “Deposition of chemically reactive and repellent sites on biosensor chips for reduced non-specific binding” [Colloids Surf. B 79 (2010) 270–275]

Treatment of poly(dimethylsiloxane) (PDMS) surfaces with SF₆ plasma results in the creation of high-surface-area nanotextured surfaces that considerably favour protein adsorption with respect to untreated ones. In order to employ such nanotextured surfaces as substrates for microarrays to be created and analysed using standard instrumentation, we fabricated thin PDMS films on top of standard low-cost microscope glass slides. The properties of both untreated and plasma-treated PDMS-coated slides towards spotting of protein solutions were evaluated in terms of spot signal intensity and homogeneity as well as of spot shape and size. It was found that the plasma-treated PDMS-coated glass slides provided highly homogeneous spots (mean intra-spot variation 7.6%) with spot signal intensity 6-times higher than that obtained using the untreated ones. In addition, comparison with commercially available polystyrene and aminosilanized-glass microarray slides showed that the proposed slides provided 3-times higher spot signal intensity and 2-times lower intra-spot signal variation. In addition, the implementation of long-aged-after-plasma-treatment nanotextured PDMS-coated glass slides provided spots whose shape and size matched those of the spotting tip. As a consequence, denser arrays of variable spot shape can be created using SF₆ plasma-treated PDMS-coated slides instead of standard microarray slides opening new potentials for bioanalytical applications.     

Modification of the glass surface property in PDMS-glass hybrid microfluidic devices

This paper presents a simple method to change the hydrophilic nature of the glass surface in a poly(dimethylsiloxane) (PDMS)-glass hybrid microfluidic device to hydrophobic by an extra-heating step during the fabrication process. Glass substrates bonded to a native or oxygen plasma-treated PDMS chip having microchambers (12.5 mm diameter, 110 μm height) were heated at 200°C for 3 h, and then the hydrophobicity of the glass surfaces on the substrate was evaluated by measuring the contact angle of water. By the extra-heating process, the glass surfaces became hydrophobic, and its contact angle was around 109°, which is nearly the same as native PDMS surfaces. To demonstrate the usefulness of this surface modification method, a PDMS-glass hybrid microfluidic device equipped with microcapillary vent structures for pneumatic manipulation of droplets was fabricated. The feasibility of the microcapillary vent structures on the device with the hydrophobic glass surfaces are confirmed in practical use through leakage tests of the vent structures and liquid handling for the electrophoretic separation of DNA molecules.     

Silicate glass coated microchannels through a phase conversion process for glass-like electrokinetic performance

The surface modified polydimethylsiloxane (PDMS) microchannels show a much more inferior performance to the durable and reproducible glass chip. In this paper, a facile approach to preparing a silicate glass modified PDMS microchannel for glass-like performance is presented. This glass-like performance is made possible by a phase conversion of a preceramic polymer--allylhydridopolycarbosilane (AHPCS). The, several hundred nanometer thick, polymer that coats the PDMS channel is hydrolyzed to form hydrophilic silicate glass via phase conversion under an aqueous alkali condition. It is characterized by XPS, FTIR-ATR, AFM, and contact angle measurements. The silicate glass coated PDMS channel from AHPCS has an excellent solvent resistance, delivers a high electroosmotic flow (EOF) that is stable in the long-term (4.9±0.1×10(-4) cm(2) V(-1) s(-1)) and a reliable capillary electrophoresis (CE), which are comparable to those of native glass channels. Moreover, the silicate glass PDMS channel allows easy regeneration of the electrokinetic behavior, just as in a glass channel, by a simple treatment with alkali solution. This coating approach can be applied to other polymer substrates such as polyimide (PI).     

Thermoset polyester as an alternative material for microchip electrophoresis/electrochemistry

Microchip CE coupled with electrochemical detection (MCE-EC) is a good method for the direct detection of many small molecule analytes because the technique is sensitive and readily miniaturized. Polymer materials are being increasingly used with MCE due to their affordability and ease of fabrication. While PDMS has become arguably the most widely used material in MCE-EC due to the simplicity of microelectrode incorporation, it suffers from a lack of separation efficiency, lower surface stability, and a tendency for analyte sorption. Other polymers, such as poly(methylmethacrylate) (PMMA) and poly(carbonate) (PC), have higher separation efficiencies but require more difficult fabrication techniques for electrode incorporation. In this report, thermoset polyester (TPE) was characterized as an alternative material for MCE-EC. TPE microchips were characterized in their native and plasma oxidized forms and after coating with polyelectrolyte multilayers (PEMs). TPE provides higher separation efficiencies when compared to PDMS microchips, while still using simple fabrication protocols. In this work, separation efficiencies as high as 295,000 N/m were seen when using TPE MCE-EC devices. Furthermore, the EOF was higher and more consistent as a function of pH for both native and plasma-treated TPE than PDMS. Finally, TPE is amenable to modification using simple PEM coatings as another way to control surface chemistry and surface charge.     

Study on 172‐nm vacuum ultraviolet light surface modifications of polydimethylsiloxane for micro/nanofluidic applications

In this study, we developed a technique for modifying the surface of the silicone elastomer Poly(dimethylsiloxane) (PDMS) by 172‐nm wavelength vacuum ultraviolet (VUV) light exposure. Such materials have high potential for application to micro/nanofluidic devices if their surface properties can be adequately controlled. The hydrophilicity, zeta potential and bonding strength of the VUV‐exposed surfaces were investigated and compared to surfaces exposed to conventional vacuum oxygen plasma. It was found that the proposed technique was effective at modifying the surface conditions from hydrophobic to hydrophilic, increasing the zeta potential, and allowing good bonding to glass. The time required to produce the maximum bonding strength was found to be similar to that for vacuum oxygen plasma exposure. However, since VUV exposure does not require the creation of a vacuum, it offers a faster turnaround, making it suitable for mass production. Copyright © 2010 John Wiley & Sons, Ltd.     

Analytical investigations on the effects of substrate kinetics on macromolecular transport and hybridization through microfluidic channels

In this paper, a generalized surface-kinetics based model is developed to analytically investigate the influences of the substrate types and the buffer compositions on the macromolecular transport and hybridization in microfluidic channels, under electrokinetic influences. For specific illustration, three typical microchannel substrates, namely silanized glass, polycarbonate and PDMS, are considered, in order to obtain analytical expressions for their zeta potentials as a function of the buffer pH and the substrate compositions. The expressions for the zeta potential are subsequently employed to derive the respective velocity distributions, under the application of electric fields of identical strengths in all cases. It is also taken into consideration that the charged macromolecules introduced into these channels are subjected to electrophoretic influences on account of the applied electric fields. Closed form expressions are derived to predict the transport behaviour of the macromolecules and their subsequent hybridization characteristics. From the analysis presented, it is shown that the modification of the channel surface with silane-treatment becomes useful for enhancing the macromolecular transport and surface hybridization, only if the buffer pH permits a large surface charge density. The analytical solutions are also compared with full-scale numerical solutions of the coupled problem of fluid dynamic and macromolecular transport in presence of the pertinent surface reactions, in order to justify the effectiveness of closed-form expressions derived in this study.     

A Glass/PDMS Hybrid Microfluidic Chip Embedded with Integrated Electrodes for Contactless Conductivity Detection

A new glass/PDMS hybrid chip for contactless conductivity detection is reported. This chip consists of a glass substrate with microchannels and a PDMS cover sheet embedded with a small integrated electrode plate. In the region of detection, electrodes are insulated from the microchannel by a formed PDMS membrane about 100 μm in thickness. Without any modification, this glass/PDMS chip is suitable for contactless conductivity detection with good properties, such as excellent heat-dissipation, stable electroosmotic flow, high separation efficiency, satisfactory sensitivity, simple construction and high degree of integration. Its feasibility and performance had been demonstrated by analyzing inorganic ions and amino acids in mixtures, and alkaloids in traditional Chinese medicine. The limits of detection reached micromole per liter (μmol L^?1) levels. This microchip could be promising for mass production due to its stability, reproducibility, ease of fabrication and low cost.     

Surface modification with BSA blocking based on in situ synthesized gold nanoparticles in poly(dimethylsiloxane) microchip

A stable BSA blocking poly(dimethylsiloxane) (PDMS) microchannel was prepared based on in situ synthesized PDMS–gold nanoparticles composite films. The modified microchip could successfully suppress protein adsorption. The assembly was followed by contact angle, charge-coupled device (CCD) imaging, electroosmotic flow (EOF) measurements and electrophoretic separation methods. Contact angle measurements revealed the coated surface was hydrophilic, water contact angle for coated chips was 45.2° compared to a water contact angle for native PDMS chips of 88.5°. The coated microchips exhibited reproducible and stable EOF behavior. With FITC-labeled myoglobin incubation in the coated channel, no fluorescence was observed with CCD image, and the protein exhibited good electrophoretic effect in the modified microchip.