Surface modification of poly(dimethylsiloxane) microchannels

This review looks at the efforts that are being made to modify the surface of poly(dimethylsiloxane) (PDMS) microchannels, in order to enhance applicability in the field of microfluidics. Many surface modifications of PDMS have been performed for electrophoretic separations, but new modifications are being done for emerging applications such as heterogeneous immunoassays and cell-based bioassays. These new modification techniques are powerful because they impart biospecificity to the microchannel surfaces and reduce protein adsorption. Most of these applications require the use of aqueous or polar solvents, which makes surface modification a very important topic.     

Surface modification of poly(dimethylsiloxane) for retarding swelling in organic solvents

A method of alleviating swelling problems of poly(dimethysiloxane) (PDMS) molds in organic solvents is developed that allows repeated use of the molds without deleterious solvent effects. The method involves surface modification of PDMS surface with poly(urethaneacrylate) that results in a partially modified PDMS surface. This modification leads to a significant reduction in the rate of solvent absorption into PDMS such that the swelling can be controlled.     

Surface modification with well-defined biocompatible triblock copolymers Improvement of biointerfacial phenomena on a poly(dimethylsiloxane) surface

To improve interfacial phenomena of poly(dimethylsiloxane) (PDMS) as biomaterials, well-defined triblock copolymers were prepared as coating materials by reversible addition-fragmentation chain transfer (RAFT) controlled polymerization. Hydroxy-terminated poly(vinylmethylsiloxane-co-dimethylsiloxane) (HO–PV_lD_mMS–OH) was synthesized by ring-opening polymerization. The copolymerization ratio of vinylmethylsiloxane to dimethylsiloxane was 1/9. The molecular weight of HO–PV_lD_mMS–OH ranged from (1.43 to 4.44) × 10⁴, and their molecular weight distribution (M_w/M_n) as determined by size-exclusion chromatography equipped with multiangle laser light scattering (SEC-MALS) was 1.16. 4-Cyanopentanoic acid dithiobenzoate was reacted with HO–PV_lD_mMS–OH to obtain macromolecular chain transfer agents (macro-CTA). 2-Methacryloyloxyethyl phosphorylcholine (MPC) was polymerized with macro-CTAs. The gel-permeation chromatography (GPC) chart of synthesized polymers was a single peak and M_w/M_n was relatively narrow (1.3–1.6). Then the poly(MPC) (PMPC)–PV_lD_mMS–PMPC triblock copolymers were synthesized. The molecular weight of PMPC in a triblock copolymer was easily controllable by changing the polymerization time or the composition of the macro-CTA to a monomer in the feed. The synthesized block copolymers were slightly soluble in water and extremely soluble in ethanol and 2-propanol.

Surface modification was performed via hydrosilylation. The block copolymer was coated on the PDMS film whose surface was pretreated with poly(hydromethylsiloxane). The surface wettability and lubrication of the PDMS film were effectively improved by immobilization with the block copolymers. In addition, the number of adherent platelets from human platelet-rich plasma (PRP) was dramatically reduced by surface modification. Particularly, the triblock copolymer having a high composition ratio of MPC units to silicone units was effective in improving the surface properties of PDMS.

By selective decomposition of the Si–H bond at the surface of the PDMS substrate by irradiation with UV light, the coating region of the triblock copolymer was easily controlled, resulting in the fabrication of micropatterns. On the surface, albumin adsorption was well manipulated.     

Internal modification of poly(dimethylsiloxane) microchannels with a borosilicate glass coating

We report on an original technique for the in situ coating of poly(dimethylsiloxane) (PDMS) microchannels with borosilicate glass, starting from an active nonaqueous and alkali-free precursor solution. By chemical reaction of this active solution inside the microchannel and subsequent thermal annealing, a protective and chemically inert glass borosilicate coating is bonded to the PDMS. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and nuclear magnetic resonance spectroscopy of the active solution show that it is composed of a silicon oxide network with boron connectivity. Thermal gravimetric analysis demonstrates the absence of organic content when curing is done above 150 degrees C. The borosilicate nature of the glass coating covalently bonded to the PDMS is demonstrated using ATR-FTIR spectroscopy and X-ray photoelectron spectroscopy. Atomic force microscopy and scanning electron microscopy show a smooth and crack-free coating. The latter is used as an efficient protective barrier against diffusion in PDMS of fluorescent rhodamine B dye that is dissolved either in water or in toluene. Moreover, the coating prevents swelling and consequent structural damage of the PDMS when the latter is exposed to harsh chemicals such as toluene.     

Surface modification of poly(dimethylsiloxane) microchips using a double-chained cationic surfactant for efficiently resolving fluorescent dye adsorption

This paper described a double-chained cationic surfactant, didodecyldimethylammonium bromide (DDAB), for dynamic surface modification of poly(dimethylsiloxane) (PDMS) microchips to reduce the fluorescent dyes adsorption onto the microchannel. When DDAB with a high concentration was present as the dynamic modification reagent in the running and sample buffer, it not only reversed the direction of electroosmotic flow, but also efficiently suppressed fluorescent dyes pyronine Y (PY) or rhodamine B (RB) adsorption onto the chip surface. In addition, vesicles formed by DDAB in the buffer with higher surface charge density and electrophoretic mobility could provide wider migration window and potential for the separation of compounds with similar hydrophobicity. Factors affecting modification, such as pH and concentrations of the buffer, DDAB concentration in the buffer were investigated. Compared with commonly used single-chained cetyltrimethylammonium bromide, DDAB provided a better modification performance. Furthermore, PY and RB were separated successfully on a PDMS microchip at the appropriate conditions with DDAB.     

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.     

An aqueous-based surface modification of poly(dimethylsiloxane) with poly(ethylene glycol) to prevent biofouling

We report a simple modification of poly(dimethylsiloxane) (PDMS) surfaces with poly(ethylene glycol) (PEG) through the adsorption of a graft copolymer, poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) from aqueous solution. In this approach, the PDMS surface was treated with oxygen plasma, followed by immersion into aqueous solution containing PLL-g-PEG copolymers. Due to the hydroxyl/carboxylic groups generated on the PDMS surface after oxygen plasma, the polycationic PLL backbone is attracted to the negatively charged surface and PEG side chains exhibit an extended structure. The PEG/aqueous interface generated in this way revealed a near-perfect resistance to nonspecific protein adsorption as monitored by means of optical waveguide lightmode spectroscopy (OWLS) and fluorescence microscopy.     

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.     

Comparison of surfactants for dynamic surface modification of poly(dimethylsiloxane) microchips

In the present report, the use of negatively charged surfactants as modifiers of the background electrolyte is reported using poly(dimethylsiloxane) (PDMS) microchips. In particular, the use of anionic surfactants, such as sodium dodecyl sulfate, phosphatidic acid, and deoxycholate, was studied. When surfactants were present in the run buffer, an increase in the electroosmotic flow (EOF) was observed. Two additional effects were also observed: (i) stabilization of the run-to-run EOF, (ii) an improvement in the electrochemical response for several biomolecules. In order to characterize the analysis conditions, the effects of different surfactant, electrolyte, and pH were studied. EOF measurements were performed using either the current monitoring method or by detection of a neutral molecule. The first adsorption/desorption kinetics studies are also reported for different surfactants onto PDMS. The separation of biologically important analytes (glucose, penicillin, phenol, and homovanillic acid) was improved decreasing the analysis time from 200 to 125 s. However, no significant changes in the number of theoretical plates were observed.     

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.     

Simple poly(dimethylsiloxane) surface modification to control cell adhesion

Poly(dimethylsiloxane) (PDMS) has a long history of exploitation in a variety of biological and medical applications. Particularly in the past decade, PDMS has attracted interest as a material for the fabrication of microfluidic biochip. The control of cell adhesion on a PDMS surface is important in many microfluidic applications such as cell culture or cell‐based chemicals/drug testing. Unlike many complicated approaches, this study reports simple methods of PDMS surface modification to effectively inhibit or conversely enhance cell adhesion on a PDMS surface using Pluronic surfactant solution and poly‐L ‐lysine, respectively. This research basically succeeded our prior work to further confirm the long‐term capability of 3% Pluronic F68 surfactant to suppress cell adhesion on a PDMS surface over a 6‐day cell culture. Microscopic observation showed that the treated PDMS surface created an unfavorable interface, where chondrocytes seemed to clump together on day 2 and 6 after chondrocyte seeding, and there was no sign of chondrocyte spreading. On the opposite side, results demonstrated that the poly‐L ‐lysine‐treated surface significantly increased fibroblast adhesion by 32% in contrast to the untreated PDMS, which is comparable to the commercial cell‐culture‐grade microplate. However, fibronectin treatment did not have such an effect. All these fundamental information is found useful for any PDMS‐related application. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface modification of poly(butylene terephthalate) nonwoven by photochemistry and biofunctionalization with peptides for blood filtration

The surface of meltblown poly(butylene terephthalate) (PBT) nonwoven was modified by photochemistry using the photolinker O‐succinimidyl 4‐azido‐2,3,5,6‐tetrafluorobenzoate for the introduction of activated ester functions and then coupling of molecular probes or biomolecules. Approximately 4000 pmol of (L )‐4,5‐[³H]‐lysine was fixed per PBT sample (1.13 cm²) and measured by liquid scintillation counting. The method consisted in a two‐step process: (a) coating of the clip (0.05 mg/sample) on the fibrous surface of the PBT followed by UV irradiation (30 min, 254 nm) and (b) coupling of amine‐terminated molecules (10^?3 M in phosphate buffer–CH₃CN [1/1, v/v], 20 h). Moreover, about 2000 pmol of ³H‐lysine can be immobilized on the PBT surface after UV irradiation (without clip) by aminolysis reactions with the created oxygenated functions. The derivatizations (via the clip and UV irradiation only) were stable after long‐term heating at 100 °C in water or under steam‐sterilization conditions. They induce neither modifications of the nonwoven morphology nor cytotoxicity. This method was applied for the grafting of peptides Gly‐Arg‐Gly‐Asp‐Ser and Gly‐Gly‐Gly‐Gly‐Gly to perform blood filtration experiments and to retain the leukocytes. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The use of poly(dimethylsiloxane) surface modification with gold nanoparticles for the microchip electrophoresis

Poly(dimethylsiloxane) (PDMS) microfluidic channels modified by citrate-stabilized gold nanoparticles after coating a layer of linear polyethylenimine (LPEI) were successfully used to separate dopamine and epinephrine, which were difficult to be separated from baseline in native and hybrid PDMS microchannels. In-channel amperometric detection with a single carbon fibre cylindrical electrode was employed. Experimental parameters of separation and detection processes were optimized in detail. The analytes were well separated within 100 s in a 3.7 cm long separation channel at a separation voltage of +800 V using a 30 mM phosphate buffer solution (PBS, pH 7.0). Linear responses of them were obtained both from 25 to 600 μM with detection limits of 2 μM for dopamine and 5 μM for epinephrine, respectively. The modified PDMS channels have a long-term stability and an excellent reproducibility within 2 weeks.     

Proteins modification of poly(dimethylsiloxane) microfluidic channels for the enhanced microchip electrophoresis

This report described proteins modification of poly(dimethylsiloxane) (PDMS) microfluidic chip based on layer-by-layer (LBL) assembly technique for enhancing separation efficiency. Two kinds of protein-coated films were prepared. One was obtained by successively immobilizing the cationic polyelectrolyte (chitosan, Chit), gold nanoparticles (GNPs), and protein (albumin, Albu) to the PDMS microfluidic channels surface. The other was achieved by sequentially coating lysozyme (Lys) and Albu. Neurotransmitters (dopamine, DA; epinephrine, EP) and environmental pollutants (p-phenylenediamine, p-PDA; 4-aminophenol, 4-AP; hydroquinone, HQ) as two groups of separation models were studied to evaluate the effect of the functional PDMS microfluidic chips. The results clearly showed these analytes were efficiently separated within 140 s in a 3.7 cm long separation channel and successfully detected with in-channel amperometric detection mode. Experimental parameters in two protocols were optimized in detail. The detection limits of DA, EP, p-PDA, 4-AP, and HQ were 2.0, 4.7, 8.1, 12.3, and 14.8 microM (S/N=3) on the Chit-GNPs-Albu coated PDMS/PDMS microchip, and 1.2, 2.7, 7.2, 9.8, and 12.2 microM (S/N=3) on the Lys-Albu coated one, respectively. In addition, through modification, the more homogenous channel surface displayed higher reproducibility and better stability.     

A simple approach to micropatterning and surface modification of poly(dimethylsiloxane)

Ozone treatment is an efficient economical, alternative method for surface activation of poly(dimethylsiloxane) (PDMS). This is illustrated by the derivatization of a PDMS surface with (3-aminopropyl)triethoxysilane (APTES). The apparent surface concentration of amino groups was found to be ca. 10(-8) mol/cm2 using UV/visible spectroscopy of the product from the reaction of the amino groups and fluorescamine. Potential application for micropatterning of biologically active interfaces was demonstrated by the covalent immobilization of oligonucleotides. A simple process for photolithographic patterning on PDMS surfaces has been developed.     

Spatially controlled cell adhesion via micropatterned surface modification of poly(dimethylsiloxane)

Spatial control of cell growth on surfaces can be achieved by the selective deposition of molecules that influence cell adhesion. The fabrication of such substrates often relies upon photolithography and requires complex surface chemistry to anchor adhesive and inhibitory molecules. The production of simple, cost-effective substrates for cell patterning would benefit numerous areas of bioanalytical research including tissue engineering and biosensor development. Poly(dimethylsiloxane) (PDMS) is routinely used as a biomedical implant material and as a substrate for microfluidic device fabrication; however, the low surface energy and hydrophobic nature of PDMS inhibits its bioactivity. We present a method for the surface modification of PDMS to promote localized cell adhesion and proliferation. Thin metal films are deposited onto PDMS through a physical mask in the presence of a gaseous plasma. This treatment generates topographical and chemical modifications of the polymer surface. Removal of the deposited metal exposes roughened PDMS regions enriched with hydrophilic oxygen-containing species. The morphology and chemical composition of the patterned substrates were assessed by optical and atomic force microscopies as well as X-ray photoelectron spectroscopy. We observed a direct correlation between the surface modification of PDMS and the micropatterned adhesion of fibroblast cells. This simple protocol generates inexpensive, single-component substrates capable of directing cell attachment and growth.     

Surface and mechanical analysis of metallized poly(dimethylsiloxane) gel for varifocal micromirrors

Varifocal micromirrors are alternative and miniaturized mirror systems that are used for imaging procedures in biomedical diagnostics, mirror-based tunable lenses, or the correction of spherical aberration. In this study, we demonstrate a new concept for focal length variation by electrostatic mirror deformation of a compact and integrated electroactive polymer actuator based on metallized poly(dimethylsiloxane) (PDMS) gel thin films. The varifocal micromirrors have a lateral size of 70 × 70 μm² and are capable of deforming in either convex or concave direction, depending on a defined surface treatment by O₂-plasma or UV/ozone of the PDMS prior to metallization by physical vapor deposition (PVD). Surface and interface analysis by wetting experiments, sputter depth-profiling X-ray photoelectron spectroscopy (XPS) combined with scanning electron microscopy (SEM) on cross-sections processed with focused ion beam (FIB) as well as polymer and metal surfaces are used to understand and to improve the metal film growth and quality with respect to high reflectivity and conductivity. In addition to the high quality of the metallic mirror layer, the mirror displacement is important and inevitably depends on the gel stiffness of the actuator. Therefore, we investigate the gel mechanics and the performance of the actuator with rheology, confocal microscopy, and image formation on the electrically deformed mirrors. Our concept of varifocal micromirrors offers a wide range of applications for tunable mirror-based optics due to their simple and compact design.     

Surface modification of glass beads with poly(acrylic acid)

Non‐porous P₂ glass beads were etched with sodium hydroxide to increase the number of silanol groups that could be used to modify the surface. The etched glass beads were then functionalized with 3‐aminopropyltriethoxysilane (APS) and/or glycidoxypropyltrimethoxysilane (GPS). The surface of the glass beads were further modified with poly(acrylic acid) (PAA) by reacting the carboxyl groups on PAA with the amino groups of the pregrafted APS. The chemical modifications were characterized by FT‐IR spectroscopy, particle size analyzer and tensiometry for contact angle and porosity measurements. Five different molecular weight PAA polymers ranging from 2000 to 3,000,000 were grafted with less than expected increase of grafted PAA with molecular weight. The amount of APS and PAA on the surface was determined from thermogravimetric analysis and elemental analysis data. The surface properties of the surface modified glass beads were determined by measuring water and hexane penetration rate and contact angle. The surface morphology was examined by scanning electron microscopy. Copyright © 2006 John Wiley & Sons, Ltd.