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.     

Hydrophobization of glass surface by adsorption of poly(dimethylsiloxane)

Silica or glass particles are introduced in a poly(dimethylsiloxane) (PDMS) matrix for various applications. A particular feature of these systems is that PDMS adsorbs on the surface of the dispersed particles, thus rendering them more hydrophobic with time. The mechanism of this process of in situ hydrophobization is still poorly understood. The major aims of the present study are (1) to quantify the rate of surface hydrophobization by PDMS and, on this basis, to discuss the mechanism of the process; (2) to compare the contact angles of surfaces that are hydrophobized by different procedures and are placed in contact with different fluid interfaces-PDMS-water, hexadecane-water, and air-water; and (3) to check how the type of surfactant affects the contact angles, viz., the effective hydrophobicity of the surface. We present experimental results for the kinetics of hydrophobization of glass surfaces, which are characterized by measuring the three-phase contact angle of glass-surfactant solution-PDMS. The data reveal two consecutive stages in the hydrophobization process: The first stage is relatively fast and the contact angle increases from 0 degrees to about 90 degrees within several minutes. This stage is explained with the physical adsorption of the PDMS chains, as a result of hydrogen-bond formation with the surface silanol groups. The second stage is much slower and hours or days are required at room temperature to reach the final contact angle (typically, 150-160 degrees). This stage is explained as grafting of the PDMS molecules on the surface by chemical reaction with the surface silanol groups. If the glass surface had been pretreated by hexamethyldisilazane (HMDS), so that CH(3) groups had blocked most of the surface silanol groups, the first stage in the hydrophobization process is almost missing-the contact angle slowly changes at room temperature from about 90 degrees up to 120 degrees. The experiments aimed to compare several hydrophobization procedures showed that PDMS ensures larger contact angle (more hydrophobic surface) than grafted alkyl chains. The contact angles at the PDMS-water and hexadecane-water interfaces were found to be very similar to each other, and much larger than that at the air-water interface. Interestingly, we found that the ionic surfactants practically do not affect the contact angle of PDMS-hydrophobized surface, whereas the nonionic surfactants reduce this angle. Similar trends are expected with silica surfaces, as well.     

The stability of radio-frequency plasma-treated polydimethylsiloxane surfaces

Polydimethylsiloxane (PDMS) is a widely used material for manufacturing lab-on-chip devices. However, the hydrophobic nature of PDMS is a disadvantage in microfluidic systems. To transform the hydrophobic PDMS surface to hydrophilic, it was treated with radio-frequency (RF) air plasma at 150, 300, and 500 mTorr pressures for up to 30 min. Following the surface treatment, the PDMS specimens were stored in air, deionized water, or 0.14 M NaCl solution at 4 degrees C, 20 degrees C, and 70 degrees C. The change in the hydrophilicity (wettability) of the PDMS surfaces was followed by contact angle measurements and Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy as a function of time. As an effect of the RF plasma treatment, the contact angles measured on PDMS surfaces dropped from 113 +/- 4 degrees to 9 +/- 3 degrees . The chamber pressure and the treatment time had no or negligible effect on the results. However, the PDMS surface gradually lost its hydrophilic properties in time. The rate of this process is influenced by the difference in the dielectric constants of the PDMS and its ambient environment. It was the smallest at low temperatures in deionized water and largest at high temperatures in air. Apparently, the OH groups generated on the PDMS surface during the plasma treatment tended toward a more hydrophilic/less hydrophobic environment during the relaxation processes. The correlation between the FTIR-ATR spectral information and the contact angle data supports this interpretation.     

Interaction of chitosan and mucin in a biomembrane model environment

Thermoplastics have been increasingly used for fabricating microfluidic devices because of their low cost, mechanical/biocompatible attributes, and well-established manufacturing processes. However, there is sometimes a need to integrate such a device with components made from other materials such as polydimethylsiloxane (PDMS). Bonding thermoplastics with PDMS to produce hybrid devices is not straightforward. We have reported our method to modify the surface property of a cyclic olefin copolymer (COC) substrate by using corona discharge and grafting polymerization of 3-(trimethoxysilyl)propyl methacrylate; the modified surface enabled strong bonding of COC with PDMS. In this paper, we report our studies on the surface modification mechanism using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurement. Using this bonding method, we fabricated a three-layer (COC/PDMS/COC) hybrid device consisting of elastomer-based valve arrays. The microvalve operation was confirmed through the displacement of a dye solution in a fluidic channel when the elastomer membrane was pneumatically actuated. Valve-enabled microfluidic handling was demonstrated.     

Poly(oxyethylene) based surface coatings for poly(dimethylsiloxane) microchannels

Control of surface properties in microfluidic systems is an indispensable prerequisite for successful bioanalytical applications. Poly(dimethylsiloxane) (PDMS) microfluidic devices are hampered from unwanted adsorption of biomolecules and lack of methods to control electroosmotic flow (EOF). In this paper, we propose different strategies to coat PDMS surfaces with poly(oxyethylene) (POE) molecules of varying chain lengths. The native PDMS surface is pretreated by exposure to UV irradiation or to an oxygen plasma, and the covalent linkage of POE-silanes as well as physical adsorption of a triblock-copolymer (F108) are studied. Contact angle measurements and atomic force microscopy (AFM) imaging revealed homogeneous attachment of POE-silanes and F108 to the PDMS surfaces. In the case of F108, different adsorption mechanisms to hydrophilic and hydrophobic PDMS are discussed. Determination of the electroosmotic mobilities of these coatings in PDMS microchannels prove their use for electrokinetic applications in which EOF reduction is inevitable and protein adsorption has to be suppressed.     

Modification of micro-channel filling flow by poly(dimethylsiloxane) surface functionalization with fluorine—Substituted aminonaphthols

Microfluidics based on the capillarity-induced filling of elastomeric channels by a suitable liquid or solution represents a useful route for realizing portable diagnostic devices designed without additional mechanical or electrical micropumps. In this study, an elastomeric mold made of poly(dimethylsiloxane) (PDMS), containing relief patterns placed in intimate contact with a silicon substrate, is utilized to create a continuous network of rectangular micro-channels for the motion of water fluid. The immobilization on activated PDMS surface of suitable functional molecules such as hydrophilic and hydrophobic fluorine-containing aminonaphthols, obtained through a straightforward and versatile synthetic procedure, allowed us to modulate PDMS surface properties depending on the structural characteristics of the employed derivative. In this context, the incorporation of fluorine groups is important for improving biocompatibility of the resulting device, providing surfaces that could be chemically and biologically inert as well as resistant to surface adhesion phenomena. The functionalization from liquid phase of PDMS replicas, involving a covalent derivatization via silanization reaction of the above mentioned compounds to an oxidized PDMS surface, resulted in a successful modification of microfluidic motion of water in rectangular capillaries, moreover contact angle values evidence also how wettability of PDMS films could be modulated, with the fluorinated aminonaphthols fuctionalized PDMS exhibiting higher contact angles.     

Surface characterization using chemical force microscopy and the flow performance of modified polydimethylsiloxane for microfluidic device applications

The widespread interest in micro total analysis systems has resulted in efforts to develop devices in cheaper polymer materials such as polydimethylsiloxane (PDMS) as an alternative to expensive glass and silicon devices. We describe the oxidation of the PDMS surface to form ionizable groups using a discharge from a Tesla coil and subsequent chemical modification to augment electroosmotic flow (EOF) within the microfluidic devices. The flow performance of oxidized, amine-modified and unmodified PDMS materials has been determined and directly compared to conventional glass devices. Exact PDMS replicas of glass substrates were prepared using a novel two step micromolding protocol. Chemical force microscopy has been utilized to monitor and measure the efficacy of surface modification yielding information about the acid/base properties of the modified and unmodified surfaces. Results with different substrate materials correlates well with expected flow modifications as a result of surface modification. Oxidized PDMS devices were found to support faster EOF (twice that of native PDMS) similar to glass while those derivatized with 3-aminopropyl triethoxysilane (APTES) showed slower flow rates compared to native PDMS substrates as a result of masking surface charge. Results demonstrate that the surface of PDMS microdevices can be manipulated to control EOF characteristics using a facile surface derivatization methodology allowing surfaces to be tailored for specific microfluidic applications and characterized with chemical force microscopy.     

Construction of microfluidic chips using polydimethylsiloxane for adhesive bonding

A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 degrees C for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 microm, a approximately 200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.     

Surface-directed boundary flow in microfluidic channels

Channel geometry combined with surface chemistry enables a stable liquid boundary flow to be attained along the surfaces of a 12 microm diameter hydrophilic glass fiber in a closed semi-elliptical channel. Surface free energies and triangular corners formed by PDMS/glass fiber or OTS/glass fiber surfaces are shown to be responsible for the experimentally observed wetting phenomena and formation of liquid boundary layers that are 20-50 microm wide and 12 microm high. Viewing this stream through a 20 microm slit results in a virtual optical window with a 5 pL liquid volume suitable for cell counting and pathogen detection. The geometry that leads to the boundary layer is a closed channel that forms triangular corners where glass fiber and the OTS coated glass slide or PDMS touch. The contact angles and surfaces direct positioning of the fluid next to the fiber. Preferential wetting of corner regions initiates the boundary flow, while the elliptical cross-section of the channel stabilizes the microfluidic flow. The Young-Laplace equation, solved using fluid dynamic simulation software, shows contact angles that exceed 105 degrees will direct the aqueous fluid to a boundary layer next to a hydrophilic fiber with a contact angle of 5 degrees. We believe this is the first time that an explanation has been offered for the case of a boundary layer formation in a closed channel directed by a triangular geometry with two hydrophobic wetting edges adjacent to a hydrophilic surface.     

Teflon films for chemically-inert microfluidic valves and pumps

We present a simple method for fabricating chemically-inert Teflon microfluidic valves and pumps in glass microfluidic devices. These structures are modeled after monolithic membrane valves and pumps that utilize a featureless polydimethylsiloxane (PDMS) membrane bonded between two etched glass wafers. The limited chemical compatibility of PDMS has necessitated research into alternative materials for microfluidic devices. Previous work has shown that spin-coated amorphous fluoropolymers and Teflon-fluoropolymer laminates can be fabricated and substituted for PDMS in monolithic membrane valves and pumps for space flight applications. However, the complex process for fabricating these spin-coated Teflon films and laminates may preclude their use in many research and manufacturing contexts. As an alternative, we show that commercially-available fluorinated ethylene-propylene (FEP) Teflon films can be used to fabricate chemically-inert monolithic membrane valves and pumps in glass microfluidic devices. The FEP Teflon valves and pumps presented here are simple to fabricate, function similarly to their PDMS counterparts, maintain their performance over extended use, and are resistant to virtually all chemicals. These structures should facilitate lab-on-a-chip research involving a vast array of chemistries that are incompatible with native PDMS microfluidic devices.     

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.     

Surface modification of poly(dimethylsiloxane) microfluidic devices and its application in simultaneous analysis of uric acid and ascorbic acid in human urine

A novel and simple method based on layer-by-layer (LBL) technique has been developed for the modification of the channel in PDMS electrophoresis microchip to create a hydrophilic surface with a stable EOF. The functional surface was obtained by sequentially immobilizing chitosan and deoxyribonucleic acid (DNA) onto the microfluidic channel surface using the LBL assembly technique. Compared to the native PDMS microchips, the contact angle of the chitosan-DNA modified PDMS microchips decreased and the EOF increased. Experimental conditions were optimized in detail. The chitosan-DNA modified PDMS microchips exhibited good reproducibility and long-term stability. Separation of uric acid (UA) and ascorbic acid (AA) performed on the modified PDMS microchip generated 43,450 and 46,790 N/m theoretical plates compared with 4048 and 19,847 N/m with the native PDMS microchip. In addition, this method has been successfully applied to real human urine samples, without SPE, with recoveries of 97-105% for UA and AA.     

Surface modification of microfluidic channels by UV-mediated graft polymerization of non-fouling and ‘smart’ polymers

Microfluidic channels prepared from polydimethylsiloxane (PDMS) have been modified by UV-mediated graft polymerization of temperature-responsive polymers (poly[N-isopropyl acrylamide] or pNIPAAm), temperature- and pH-responsive copolymers (P[NIPAAm-co-acrylic acid (AAc)]), and a non-fouling hydrogel (polyethyleneglycol diacrylate, or PEGDA). This was done by presorbing a photosensitizer (PS) within the PDMS channel surface regions, contacting the different monomer solutions with the PS-containing surface under nitrogen, and irradiating with UV. The pNIPAAm-grafted surface was hydrophilic below its lower critical solution temperature (LCST), resisting non-specific adsorption, while it was hydrophobic above its LCST, now binding pNIPAAm-coated nanoparticles. Combined temperature- and pH-responsive surfaces were also prepared by UV radiation grafting a monomer mixture of pNIPAAm with AAc. The surfaces have been characterized by advancing water contact angle measurements. These smart microfluidic channels should be useful for many applications such as affinity separations and diagnostic assays.     

Tailoring the surface properties of poly(dimethylsiloxane) microfluidic devices

Poly(dimethylsiloxane) (PDMS) is an attractive material for microelectrophoretic applications because of its ease of fabrication, low cost, and optical transparency. However, its use remains limited compared to that of glass. A major reason is the difficulty of tailoring the surface properties of PDMS. We demonstrate UV grafting of co-mixed monomers to customize the surface properties of PDMS microfluidic channels in a simple one-step process. By co-mixing a neutral monomer with a charged monomer in different ratios, properties between those of the neutral monomer and those of the charged monomer could be selected. Mixtures of four different neutral monomers and two different charged monomers were grafted onto PDMS surfaces. Functional microchannels were fabricated from PDMS halves grafted with each of the different mixtures. By varying the concentration of the charged monomer, microchannels with electrophoretic mobilities between +4 x 10(-4) cm2/(V s) and -2 x 10(-4) cm2/(V s) were attainable. In addition, both the contact angle of the coated surfaces and the electrophoretic mobility of the coated microchannels were stable over time and upon exposure to air. By carefully selecting mixtures ofmonomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different applications.     

Bioactive surface modification of mica and poly(dimethylsiloxane) with hydrophobins for protein immobilization

Bioactive surfaces with appropriate hydrophilicity for protein immobilization can be achieved by hydrophobin II (HFBI) self-assembly on mica and polydimethylsiloxane (PDMS) surfaces. X-ray photoelectron spectroscopy and water contact angle measurements illustrated that the surface wettability can be changed from superhydrophobic (PDMS) or superhydrophilic (mica) to moderately hydrophilic, which is suitable for protein (chicken IgG) immobilization on both substrate surfaces. The results suggest that HFBI assembly, one kind of hydrophobin from Trichoderma reesei, may be a versatile and convenient method for the immobilization of biomolecules on diverse substrates, which may have potential applications in biosensors, immunoassays, and microfluidic networks.     

Pneumatic handling of droplets on-demand on a microfluidic device for seamless processing of reaction and electrophoretic separation

Sequential operations of pre-separation reaction process by picoliter droplets and following electrophoretic separation process were realized in a single microfluidic device with pneumatic handling of liquid. The developed device consists of a fluidic chip made of PDMS, an electrode substrate, and a temperature control substrate on which thin film heater/sensor structures are fabricated. Liquid handling, including introduction of liquid samples, droplet generation, and merging of droplets, was implemented by pneumatic manipulation through microcapillary vent structures, allowing air to pass and stop liquid flow. Since the pneumatic manipulations are conducted in a fully automated manner by using a programmable air pressure control system, the user simply has to load liquid samples on each liquid port of the device. Droplets of 420 pL were generated with an accuracy of ± 2 pL by applying droplet generation pressure in the range of 40-100 kPa. As a demonstration, a binding reaction of a 15 mer ssDNA with a peptide nucleic acid oligomer used as an oligoprobe followed by denaturing electrophoresis to discriminate a single-base substitution was performed within 1.5 min. By exploiting the droplet-on-demand capability of the device, the influence of various factors, such as reaction time, mixing ratio and droplet configurations on the ssDNA-peptide nucleic acid binding reaction in the droplet-based process, was studied toward realization of a rapid detection method to discriminate rapid single-base substitution.     

Three-dimensional interconnected microporous poly(dimethylsiloxane) microfluidic devices

This technical note presents a fabrication method and applications of three-dimensional (3D) interconnected microporous poly(dimethylsiloxane) (PDMS) microfluidic devices. Based on soft lithography, the microporous PDMS microfluidic devices were fabricated by molding a mixture of PDMS pre-polymer and sugar particles in a microstructured mold. After curing and demolding, the sugar particles were dissolved and washed away from the microstructured PDMS replica revealing 3D interconnected microporous structures. Other than introducing microporous structures into the PDMS replica, different sizes of sugar particles can be used to alter the surface wettability of the microporous PDMS replica. Oxygen plasma assisted bonding was used to enclose the microstructured microporous PDMS replica using a non-porous PDMS with inlet and outlet holes. A gas absorption reaction using carbon dioxide (CO(2)) gas acidified water was used to demonstrate the advantages and potential applications of the microporous PDMS microfluidic devices. We demonstrated that the acidification rate in the microporous PDMS microfluidic device was approximately 10 times faster than the non-porous PDMS microfluidic device under similar experimental conditions. The microporous PDMS microfluidic devices can also be used in cell culture applications where gas perfusion can improve cell survival and functions.     

Surface modification of poly(dimethylsiloxane) with a perfluorinated alkoxysilane for selectivity toward fluorous tagged peptides

Poly(dimethylsiloxane) (PDMS) and similar polymers have proved to be of widespread interest for use in microfluidic and similar microanalytical devices. Surface modification of PDMS is required to extend the range of applications for devices made of this polymer, however. Here we report on the grafting of perfluorooctyltriethoxysilane via hydrolysis onto an oxidized PDMS substrate in order to form a fluorinated microchannel. Such a fluorinated device could be used for separating fluorous tagged proteins or peptides, similar to that which has been recently demonstrated in a capillary electrophoresis system or in an open tubular capillary column. The modified polymer is characterized using chemical force titrations, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). We also report on a novel means of performing electroosmotic measurements on this material to determine the surface zeta potential. As might be expected, contact angle and chemical force titration measurements indicate the fluorinated surface to be highly hydrophobic. XPS indicates that fluorocarbon groups segregate to the surface of the polymer over a period of days following the initial surface modification, presumably driven by a lower surface free energy. One of the most interesting results is the zeta potential measurements, which show that significant surface charge can be maintained across a wide range of pH on this modified polymer, sufficient to promote electroosmotic flow in a microfluidic chip. Matrix-assisted time-of-flight mass spectrometry (MALDI-TOF MS) measurements show that a fluorous-tagged peptide will selectively adsorb on the fluorinated PDMS in aqueous solution, demonstrating that the fluorinated polymer could be used in devices designed for the enrichment or enhanced detection of fluorous-labeled proteins and peptides.     

Comparison of monodisperse droplet generation in flow-focusing devices with hydrophilic and hydrophobic surfaces

A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.     

Fabrication of biomimetic superhydrophobic surfaces by a simple flame treatment method

A simple flame treatment method was explored to construct micro/nanostructures on a surface and then fabricate a biomimetic superhydrophobic surface at a relatively low cost. SiO₂‐containing polydimethylsiloxane (PDMS) was used as a substrate. The PDMS replicas with various micropatterned surfaces were fabricated using grass leaf, sand paper, and PET sheet with parallel groove geometry as templates via PDMS replica molding. The PDMS replica surfaces with micron structures and the surface of a flat PDMS sheet as a control sample were further treated by flame. The fabricated surfaces were characterized by scanning electron microscopy and water contact angle measurements. The effect of surface microstructures on the transparency of PDMS was also investigated. The studies indicate that the fine nanoscale structures can be produced on the surfaces of PDMS replicas and a flat PDMS sheet by a flame treatment method, and that the hierarchical surface roughness can be adjusted and controlled by varying the flame treatment time. The flame‐treated surfaces of PDMS replicas and a flat PDMS sheet possess superhydrophobicity and an ultra‐low sliding angle reaching a limiting value of 1°, and the anisotropic wettability of the PDMS replica surface with oriented microgroove structures can be greatly suppressed via flame treatment. The visible light transmittance of the flame‐treated flat PDMS surface decreases with prolonged flame treatment times. Copyright © 2016 John Wiley & Sons, Ltd.