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.     

Electroosmotic flow in poly(dimethylsiloxane) microchannels

This paper reports on the study of electroosmotic flow (EOF) in poly(dimethylsiloxane) (PDMS) microchannels on the basis of indirect amperometric detection method. Gradual increase of EOF rate in freshly prepared PDMS microchannels was observed with the running buffer of phosphate buffer solution (PBS). With the same concentration (10 mM) of PBS containing different cations and the same pH value (7.0) and, the time of the stable EOF in PDMS microchannels under the applied separation voltage of 1000 V was 49.8 s (Li+ -PBS), 57.1 s (Na+ -PBS), 91 s (K+ -PBS), respectively. Meanwhile, the different adsorption of cations (Li+, Na+ and K+) on hydrophobic PDMS wall was observed through their separation in PDMS microchannels. Such experimental results demonstrated that the EOF in PDMS microchannels came from the cations and anions adsorbed on PDMS wall. This study would not only help us understand the surface state of PDMS, but also provide a useful guidance for establishing the effective surface modification methods in PDMS microchip CE.     

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.     

Cross-linked coatings for electrophoretic separations in poly(dimethylsiloxane) microchannels

We have developed a strategy using ultraviolet light to polymerize mixed monomer solutions onto the surface of a poly(dimethylsiloxane) (PDMS) microdevice. By including monomers with different chemical properties, electrophoretic separations were optimized for a test set of analytes. The properties of surfaces grafted with a single neutral monomer, a neutral and a negative monomer, or a neutral, negative, and cross-linking monomer were assessed. The highest quality separations were achieved in channels with cross-linked coatings. The separation efficiency for biologically relevant peptides (kinase substrates) on these surfaces was as high as 18 600 theoretical plates in a 2.5 cm channel. The test peptides were fluorescein-AEEEIYGEFEAKKKK, fluorescein-GRPRAATFAEG, fluorescein-GRPRAA(T-PO(3))FAEG, fluorescein-DLDVPIP GRFDRRVSVAAE, and fluorescein-DLDVPIPGRFDRRV(S-PO(3))VAAE. Separations between two different peptides occurred in as little as 400 ms after injection into the separation channel. The simultaneous separation of five kinase and phosphatase substrates was also demonstrated. By carefully selecting mixtures of monomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different electrophoretic separations.     

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.     

Electrokinetic control of fluid flow in native poly(dimethylsiloxane) capillary electrophoresis devices

Capillary zone electrophoresis (CZE) devices fabricated in poly(dimethylsiloxane) (PDMS) require continuous voltage control of all intersecting channels in the fluidic network in order to avoid catastrophic leakage at the intersections. This contrasts with the behavior of similar flow channel designs fabricated in glass substrates. When the injection plugs are shaped by voltage control and leakage from side channels is controlled by the application of pushback voltages during separation, fluorescein samples give 64 200 theoretical plates (7000 V separation voltage, E = 1340 V/cm). Native PDMS devices exhibit stable retention times (+/- 8.6% RSD) over a period of five days when filled with water. Contact angles were unchanged (+/- 1.9% RSD) over a period of 16 weeks of dry storage, in contrast to the known behavior of plasma-oxidized PDMS surfaces. Electroosmotic flow (EOF) was observed in the direction of the cathode for the buffer systems studied (phosphate, pH 3-10.5), in the presence or absence of hydrophobic ions such as tetrabutylammonium or dodecyl sulfate. Electroosmotic mobilities of 1.49 x 10(-5) and 5.84 x 10(-4) cm2/Vs were observed on average at pH 3 and 10.5, respectively, the variation strongly suggesting that silica fillers in the polymer dominate the zeta potential of the material. Hydrophobic compounds such as dodecyl sulfate and BODIPY 493/503 adsorbed strongly to the PDMS, indicating the hydrophobicity of the channel walls is clearly problematic for CZE analysis of hydrophobic analytes. A method to stack multiple channel layers in PDMS is also described.     

Stabilization of two-phase octanol/water flows inside poly(dimethylsiloxane) microchannels using polymer coatings

In this paper we present our first results on the realization of stable water/octanol, two-phase flows inside poly(dimethylsiloxane) (PDMS) microchannels. Native PDMS microchannels were coated with high molecular weight polymers to change the surface properties of the microchannels and thus stabilize the laminar flow profile. The polymers poly(2-hydroxyethyl methacrylate), poly(vinyl pyrrolidone), poly(ethylene oxide), poly(ethylene glycol), and poly(vinyl alcohol) were assessed for their quality as stabilization coatings after deposition from flowing and stationary solutions. Additionally, the influence of coating the microchannels homogeneously with a single kind of polymer or heterogeneously with two different polymers was investigated. From the experimental observations, it can be concluded that homogeneous polymer coatings with poly(2-hydroxyethyl methacrylate) and poly(vinyl pyrrolidone) led to the effective stabilization of laminar water/octanol flows. Furthermore, heterogeneous coatings led to two-phase flows which had a better-defined and more stable interface over long distances (i.e., 40-mm-long microchannels). Finally, the partitioning of fuchsin dye in the coated microchannels was demonstrated, establishing the feasibility of the use of the polymer-coated PDMS microchannels for determination of logP values in laminar octanol/water flows.     

Preparation and characterization of some unusually transparent poly(dimethylsiloxane) nanocomposites

A technique was developed for preparing poly(dimethylsiloxane) nanocomposites having unusually high transparencies as quantitatively judged by ultraviolet–visible spectroscopy. The method was the in situ generation of silica particles by a two‐step sol–gel procedure in which the required water of hydrolysis was simply absorbed from the air, and the catalyst was generated in situ from a tin salt. Electron microscopy showed that the phase‐separated silica domains were very small (30–50 nm in diameter) and well dispersed, as expected from the transparency of the composites. Stress‐strain measurements in tension indicated that the particles provide very good reinforcement. Ultra‐small‐angle X‐ray scattering data showed that the domain morphology depends strongly on catalyst, but weakly on loading level. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1897–1901, 2003     

Characterization of molecular transport in poly(dimethylsiloxane) microchannels for electrophoresis fabricated with synchrotron radiation-lithography and UV-photolithography

In the present paper, a study was undertaken of molecular transport in ploy(dimethylsiloxane) microchannels that were fabricated by ultraviolet (UV)-photolithography and synchrotron radiation (SR)-lithography characterized and compared for microchip capillary electrophoresis by evaluating in-channel molecular dispersion. A fluorescent tag, sulforhodamine B was used as the probing molecule. It was found that microchannels made by SR-lithography fabrication were superior to those made by UV-photolithography fabrication in terms of molecular transport performance. A deep insight into surface conditions characterized by scanning electron microscopy suggested it was related to the difference in surface roughness. Chromatographic retention in electropherograms further supported such a conclusion, which depended on the phase ratio of the channel surface. The results revealed for PDMS microchannels in this work were in good agreement with the phenomenon found for glass microchannels in the literature.     

Electrokinetic characterization of poly(acrylic acid) and poly(ethylene oxide) brushes in aqueous electrolyte solutions

Surfaces carrying hydrophilic polymer brushes were prepared from poly(styrene)-poly(acrylic acid) and poly(styrene)-poly(ethylene oxide) diblock copolymers, respectively, using a Langmuir-Blodgett technique and employing poly(styrene)-coated planar glass as substrates. The electrical properties of these surfaces in aqueous electrolyte were analyzed as a function of pH and KCl concentration using streaming potential/streaming current measurements. From these data, both the zeta potential and the surface conductivity could be obtained. The poly(acrylic acid) brushes are charged due to the dissociation of carboxylic acid groups and give theoretical surface potentials of -160 mV at full dissociation in 10(-)(3) M solutions. The surface conductivity of these brushes is enormous under these conditions, accounting for more than 93% of the total measured surface conductivity. However, the mobility of the ions within the brush was estimated from the density of the carboxylic acid groups and the surface conductivity data to be only about 14% of that of free ions. The poly(ethylene oxide) (PEO) brushes effectively screen the charge of the underlying substrate, giving a very low zeta potential except when the ionic strength is very low. From the data, a hydrodynamic layer thickness of the PEO brushes could be estimated which is in good agreement with independent experiments (neutron reflectivity) and theoretical estimates. The surface conductivity in this system was slightly lower than that of the polystyren substrate. This also indicates that no significant amount of preferentially, i.e., nonelectrostatically attracted, ions taken up in the brush.     

Synthesis and characterization of semicrystalline cycloaliphatic polyester/poly(dimethylsiloxane) segmented copolymers

High molecular weight poly(dimethylsiloxane)/semicrystalline cycloaliphatic polyester segmented copolymers based on dimethyl-1,4-cyclohexane dicarboxylate were prepared and characterized. The copolymers were synthesized using a high trans content isomer that afforded semicrystalline morphologies. Aminopropyl-terminated poly(dimethylsiloxane) (PDMS) oligomers of controlled molecular weight were synthesized, end capped with excess diester to form a diester-terminated oligomer, and incorporated via melt transesterification step reaction copolymerization. The molecular weight of the polysiloxane and chemical composition of the copolymer were systematically varied. The polysiloxane segment was efficiently incorporated into the copolymers via an amide link and its structure was unaffected by low concentrations of titanate transesterification catalyst, as shown by control melt experiments. The homopolymer and copolymers were characterized by solution, thermal, mechanical, and surface techniques. The segmented copolymers were microphase separated as determined by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and by transmission electron microscopy (TEM). It was demonstrated that relatively short poly(dimethylsiloxane) segment lengths and compositions were required to maintain single phase melt polymerization conditions. This was, in fact, the key to the successful preparation of these materials. The copolymers derived from short poly(dimethylsiloxane) segments demonstrated good mechanical properties, melt viscosities representative of single phase polymer melts, and were easily compression molded into films. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3495–3506, 1997     

Connecting the wetting and rheological behaviors of poly(dimethylsiloxane)-grafted silica spheres in poly(dimethylsiloxane) melts

Using dynamic light scattering, mechanical rheometry, and visual observation, the static wetting behavior of PDMS-grafted silica spheres (PDMS-g-silica) in PDMS melts is related to their rheology. A phase diagram is mapped out for a constant grafted chain length as a function of grafting density and free polymer chain length. The transition between stable and aggregated regions is determined optically and with dynamic light scattering. It is associated with a first-order wetting transition. In the stable region Newtonian behavior is observed for semidilute suspensions. The hydrodynamic brush thicknesses, deduced from viscosity measurements, correspond closely to values obtained from self-consistent field calculations for the various parameter values. At the transition, the brush collapses suddenly and shear-thinning and thixotropy appear. The rheology indicates a degree of aggregation that increases with increasing length of the free polymer, as suggested by the theory.     

Synthesis and characterization of model diblock copolymers of poly(dimethylsiloxane) with poly(1,4‐butadiene) or poly(ethylene)

Model diblock copolymers of poly(1,4‐butadiene) (PB) and poly(dimethylsiloxane) (PDMS), PB‐b‐PDMS, were synthesized by the sequential anionic polymerization (high vacuum techniques) of butadiene and hexamethylciclotrisiloxane (D₃) in the presence of sec‐BuLi. By homogeneous hydrogenation of PB‐b‐PDMS, the corresponding poly(ethylene) and poly(dimethylsiloxane) block copolymers, PE‐b‐PDMS, were obtained. The synthesized block copolymers were characterized by nuclear magnetic resonance (¹H and ¹³C NMR), size‐exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and rheology. SEC combined with ¹H NMR analysis indicates that the polydispersity index of the samples (M_w/M_n) is low, and that the chemical composition of the copolymers varies from low to medium PDMS content. According to DSC and TGA experiments, the thermal stability of these block copolymers depends on the PDMS content, whereas TEM analysis reveals ordered arrangements of the microphases. The morphologies observed vary from spherical and cylindrical to lamellar domains. This ordered state (even at high temperatures) was further confirmed by small‐amplitude oscillatory shear flow tests. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1579–1590, 2006     

A sol-gel immobilization of nano and micron size sorbents in poly(dimethylsiloxane) (PDMS) microchannels for microscale solid phase extraction (SPE)

Sorbent particles consisting of nano and micro silica, and micron size octadecylsilica (ODS) were immobilized using sol-gel chemistry onto poly(dimethylsiloxane) (PDMS) microfluidic channels to serve as μ-chip solid phase extraction (SPE) devices. Extraction, preconcentration and purification of biological and chemical analytes were carried out using these. Micro and nano scale silica-immobilized μ-SPE were used for the extraction/purification of DNA from recombinant Escherichia coli crude lysate. The average DNA recovery was 77 ± 9% (X ± R.S.D.) for the micron size silica particles and 70 ± 5% (X ± R.S.D.) for the nano silica particles. The extracted DNA could be amplified by polymerase chain reaction (PCR) whereas the DNA from the crude lysate solution could not be. This was a testimony to the purification capability of the μ-SPE device. ODS immobilized μ-SPE were used to study the extraction efficiency (EE) and enhancement factor (EF) for three groups of organic compounds, aromatics, phenols and carboxylic acids. They showed poor recovery and low enrichment because the analytes sorbed into the PDMS and were not quantitatively extracted.     

Electrokinetic particle entry into microchannels

The fundamental understanding of particle electrokinetics in microchannels is relevant to many applications. To date, however, the majority of previous studies have been limited to particle motion within the area of microchannels. This work presents the first experimental and numerical investigation of electrokinetic particle entry into a microchannel. We find that the particle entry motion can be significantly deviated from the fluid streamline by particle dielectrophoresis at the reservoir-microchannel junction. This negative dielectrophoretic motion is induced by the inherent non-uniform electric field at the junction and is insensitive to the microchannel length. It slows down the entering particles and pushes them toward the center of the microchannel. The consequence is the demonstrated particle deflection, focusing, and trapping phenomena at the reservoir-microchannel junction. Such rich phenomena are studied by tuning the AC component of a DC-biased AC electric field. They are also utilized to implement a selective concentration and continuous separation of particles by size inside the entry reservoir.     

Synthesis and characterization of multiblock copolyesters containing poly(dimethylsiloxane) in the soft segments

Synthesis and thermal properties of poly(aliphatic/aromatic-ester) copolymers containing additionally poly(dimethylsiloxane) (PDMS) chains in the soft segments are discussed. A two step method of transesterification and polycondensation from the melt was carried out in a presence of magnesium-titanate catalyst. An aliphatic dimer fatty acid was used as a component of the soft segments while poly(butylene terephthalate) (PBT) constituted the hard blocks. Effectiveness of the incorporation of PDMS into polymer chain was confirmed by the Soxhlet extraction and infrared spectroscopy of an excess of 1,4-butane diol destilled off from the polycondensation reaction. Multiblock copolymers showed microphase separation as determined by differential scanning calorimetry. Incorporation of a small amount of PDMS (up to 14.5 wt.-%) into polymer chain containg low concentration of hard segments of PBT lead to decrease in crystallinity of such copolymers. This may indicate that semicrystalline PBT are dissolved in the amorphous matrix of the soft segments.     

Equation of state of poly(dimethylsiloxane) melts

The pressure–volume–temperature (PVT) properties of three commercial samples of poly(dimethylsiloxane) are studied experimentally and theoretically in the temperature range 25–150°C and for pressure to ∼ 3 kbar. The Tait equation is employed to represent the data at elevated pressure. Isothermal compressibilities are computed for the three samples. The melt data are analyzed in terms of the Simha–Somcynsky hole theory, and scaling parameters of pressure, volume, and temperature are obtained. Satisfactory agreement between theory and experiment is found over the entire range of experimental pressures. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 841–850, 1998     

Fabrication of microfluidic systems in poly(dimethylsiloxane)

Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.     

The modulus of randomly-crosslinked poly(dimethylsiloxane) networks

In several published studies, randomly crosslinked networks were prepared from poly(dimethylziloxane) by the selective crosslinking of vinyl side chains with a silicon-hydride crosslinking agent. Stress-strain measurements on these elastomers gave values of the elongation modulus in the limits of small and large deformations which exceeded those predicted by the Flory-Erman theory. Although these unexpectedly large values at the small-strain limit have frequently been attributed to contributions from trapped entanglements, the present analysis interprets them as simply arising from contributions from short chains inadvertently introduced from the silicon-hydride crosslinking agent. In this interpretation there is a bimodal distribution of network chain lengths and, possibly, of crosslink functionalities as well. The present analysis gives results in good agreement with experiment.