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.     

Reversible gelation of poly(dimethylsiloxane) with ionic and hydrogen‐bonding substituents

Poly(dimethylsiloxane) copolymers containing a small fraction of carboxylic acid or Zn‐carboxylate groups were prepared and compared regarding reversible gelation by hydrogen‐bonding and ion‐pair interaction. The polymers were synthesized by condensation of a t‐butylcarboxylate functionalized dichlorosilane with an α,ω‐dihydroxy‐poly(dimethylsiloxane), followed by thermal cleavage of the ester bond. Neutralization of the resulting carboxylic acid substituents was achieved by addition of Zn (acac)₂. Reversible crosslinking was investigated by step stress and oscillating shear experiments. The carboxylic acid containing poly(dimethylsiloxane) became rubberlike upon increasing the temperature and liquified again when it was brought back to room temperature. This observation has been explained tentatively by segregation of the carboxylic acid groups into polar domains at high temperatures [i.e., a behavior like it is observed for systems with a lower critical solution temperature (LCST)]. At ambient temperature, the carboxylic acid groups undergo hydrogen bonding to the Si–O–Si backbone. Clustering of the carboxylic acid groups occurs only as these hydrogen bonds break upon raising temperature. Moisture was found to have a strong influence on the reversal of the crosslinking. Addition of zinc acetylacetonate resulted in the formation of an elastic network already at ambient conditions consistent with the concept of ionomers which undergo reversible gelation by formation of ion‐pair multiplets and clusters in the hydrophobic polymer matrix in particularly at low temperatures. At high temperature, both the carboxylic acid and the carboxylate sample exhibited a rather similar viscoelastic behavior consistent with a common structure where transient crosslinks are formed by clusters of the carboxylic acid and the carboxylate groups. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 485–495, 1999     

Stability of filled poly(dimethylsiloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers to cyclic stress at elevated temperature

The response of aluminum oxide-filled poly(dimethyl siloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers, containing 3–24 mol % diphenylsiloxane, to cyclic stress at elevated temperatures (dynamic creep) was evaluated. The materials could be divided into two classes, based on their response to the application of cyclic stress: no or low-diphenylsiloxane content elastomers in which substantial creep and a decrease in crosslink density were observed, and high diphenylsiloxane content (16–24 mol %) elastomers that showed decreased creep with increasing diphenylsiloxane content and an increase in crosslink density. It was suggested that the phenyl groups stabilize the siloxane bond in the polymer backbone, decreasing the rate of chain scission reactions as the diphenylsiloxane content increases and stabilizing the elastomer against creep. The balance of chain scission, chemical crosslinking, and cyclic formation reactions varies depending on diphenylsiloxane content, giving rise to the differences in dynamic creep behavior. An activation energy of 12.9 kcal/mol was measured for dynamic creep of poly(16% diphenylsiloxane/84% dimethyl siloxane), suggesting that a catalyzed degradation mechanism was responsible. The primary catalysts of the degradation reactions are postulated to be the filler particles. © 1996 John Wiley & Sons, Inc.     

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.     

Strain- and temperature-induced polymorphism of poly(dimethylsiloxane)

Phase behavior of silica-filled poly(dimethylsiloxane) (PDMS) network was investigated by wide-angle X-ray diffraction (WAXD) under various strain ratio between room temperature and ?100 °C, and anomalous polymorphic behavior was discovered. At room temperature, when sufficient strain was applied, PDMS network was found to transform into the mesomorphic phase from which only a pair of sharp equatorial reflections and faint meridional scattering were obtained in the WAXD pattern. At low temperature, PDMS network crystallized into one of three different crystal forms according to strain ratio. These crystal forms were denoted as α, transient, and β forms in the descending order of corresponding strain ratio. The mesomorphic phase at room temperature transformed into the crystalline α form by reducing temperature. There was an anomalous feature about the transition of the crystalline forms that the position of reflections in the WAXD pattern changed continuously and reversibly with strain between the α and the β forms through the transient form, while keeping the diffraction angles almost unchanged.     

Electrokinetic characterization of poly(dimethylsiloxane) microchannels

This paper characterizes the basic electrokinetic phenomena occurring within native poly(dimethylsiloxane) (PDMS) microchannels. Using simple buffers and current measurements, current density and electroosmosis data were determined in trapezoidal, reversibly sealed PDMS/PDMS and hybrid PDMS/glass channels with a cross-sectional area of 1035.5 microm(2) and about 6 cm length. This data was then compared to that obtained in an air-thermostated 50 microm inner diameter (1963.5 microm(2) cross-sectional area) fused-silica (FS) capillary of 70 cm length. Having a pH 7.8 buffer with an ionic strength (I) of 90 mM, Ohms's law was observed in the microchannels with electric field strengths of up to about 420 V/cm, which is about twice as high as for the FS capillary. The electroosmotic mobility (micro(EO)) in PDMS and FS is shown to exhibit the same general dependences on I and pH. For all configurations tested, the experimentally determined micro(EO) values were found to correlate well with the relationship micro(EO) = a + b log(I), where a and b are coefficients that are determined via nonlinear regression analysis. Electroosmotic fluid pumping in native PDMS also follows a pH dependence that can be estimated with a model based upon the ionization of silanol. Compared to FS, however, the magnitude of the electroosmotic flow in native PDMS is 50-70% smaller over the entire pH range and is difficult to maintain at acidic pH values. Thus, the origin of the negative charge at the inner wall of PDMS, glass, and FS appears to be similar but the density is lower for PDMS than for glass and FS.     

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.     

Creating patterned poly(dimethylsiloxane) surfaces with amoxicillin and poly(ethylene glycol)

This paper reports a simple microwave plasma patterning of poly(dimethylsiloxane) (PDMS) surfaces, which is accomplished by allowing selective surface areas to microwave plasma exposure in the presence of gaseous monomer. When maleic anhydride is used for microwave plasma reaction in the presence of physical barrier on the PDMS substrate, the resulting patterned surfaces with chemically bonded maleic anhydride and carboxylic acid groups are generated. In this particular study we attached amoxicillin via ammonolysis under weak base conditions in the presence of a catalyst as well as poly(ethyleneglycol) (PEG). A combination of internal reflection IR imaging (IRIRI) and atomic force microscopy (AFM) revealed that amoxicillin and PEG can be readily reacted on the microwave plasma patterned PDMS surfaces. Surface areas directly exposed to microwave plasmons exhibit the highest reactivity due to higher content of functional groups. These studies also show that molecular weight of PEG has also significant effect on kinetics of surface reactions.     

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     

Structural and water-repellent properties of a urea/poly(dimethylsiloxane) sol-gel hybrid and its bonding to cotton fabric

A novel diureapropyltriethoxysilane [bis(aminopropyl)-terminated-poly(dimethylsiloxane) (1000)] (PDMSU) sol-gel hybrid was synthesized and applied on cotton to make it water repellent. Surface-energy values of PDMSU deposited on an aluminum substrate were determined, and the contact angle for water was assessed for impregnated cotton fabrics. The stability of the coatings was determined by repetitive washing, and their degradation was investigated with the help of the infrared attenuated total reflection (ATR) technique. The structure of PDMSU was studied by ATR and 29Si NMR spectroscopy. The results showed the active role of the urea groups in PDMSU/cotton interface bonding, but washing led to the relaxation of the urea-urea associations, as inferred from the appearance of a new amide II band at 1541 cm(-1).     

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.     

Scratching behavior of elastomeric poly(dimethylsiloxane) coatings

Scratch testing has been performed on elastomeric poly(dimethylsiloxane) (PDMS) coatings on stainless steel with a spherical indenter. The friction coefficient (horizontal‐to‐normal force ratio) during scratching decreases with increasing normal load. This result can be explained by assuming that during scratching the contact area is determined by elastic deformation and the horizontal force is proportional to the contact area. With increasing driving speed, the friction coefficient increases, but the rate of increase decreases; this suggests that the scratching of the PDMS coating is a rate process and that the viscoelastic property of the coating influences its frictional behavior. Below a critical normal load, which increases with the coating thickness, the PDMS coating recovers elastically after being scratched so that there are no scratch marks left behind. Above the critical normal load, the coating is damaged by a combination of delamination at the coating/substrate interface and through‐thickness cracking. When the coating is damaged, there is an increase in the friction coefficient, and the friction force displays significant fluctuations. Furthermore, the critical normal load increases with the driving speed; this implies that time is needed to nucleate damage. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1530–1537, 2002     

The behaviour of water in poly(dimethylsiloxane)

Experimentally-determined permeation transients do not support the view that the behaviour of water in PDMS is significantly influenced by statistical-mechanical clustering; rather, they suggest that water behaves in a straightforward way. Simple calculations appear to confirm that the incidence of the statistical clustering of water in the polymer is negligible. A diffusion coefficient derived to include the influence of hydrophilic sites within the polymer is partially successful in mathematically reproducing measured quantities. An entropy calculation appears to suggest that the amount of mobile water in PDMS is solely thermally determined; hence the reduction of supposed hydrophilic impurities would probably not lead to a reduction in water permeation. The apparently large difference between the water solubility in PDMS, and that in siloxane liquids, a point of some interest in separation processes, remains unexplained in this paper.     

Electrosynthesis of poly(o-phenylenediamine) in a room temperature ionic liquid

Poly-o-phenylenediamine (PoPD) thin films were synthesized electrochemically on platinum electrodes in the room temperature ionic liquid (IL) N-butyl-N-methylpyrrolidinium (nonafluorobutanesulfonyl)-(trifluoromethanesulfonyl)imide (PYR₁₄IM₁₄). The polymer films were further characterized by electrochemical analysis and the results are compared with those obtained in conventional H₂SO₄ aqueous solution. The polymer films obtained in the IL-based electrolyte showed a good adherence on Pt and appeared attractive for the realization of biosensors since they showed a good selectivity with respect to the most common interferent compounds. PoPD films deposited from IL-based electrolytes were investigated in solutions containing compounds as ascorbate and acetaminophen, which are common interferents in electrochemical biosensor analysis, and proved satisfying for application in biosensors.     

Elastic behavior of bimodal poly(dimethylsiloxane) networks

The rubberlike elastic behavior of bimodal poly(dimethylsiloxane) (PDMS) networks was investigated by the Monte Carlo simulation method and enumeration calculation method on the basis of the rotational‐isomeric‐state (RIS) model. These bimodal PDMS networks consist of short chains (chain length from 10 to 20) as well as long chains (chain length equal to 150). For long PDMS chains, through generating many PDMS conformations in the equilibrium state using the Monte Carlo simulation method we can obtain the average Helmholtz free energy and the average energy. For short PDMS chains with chain lengths from 10 to 20, as the total number of conformations is only from 6.56 × 10³ to 3.87 × 10⁸, we adopt the enumeration calculation method. The deformation is partitioned nonaffinely between the long and short chains, and this partitioning can be determined by requiring the free energy of the deformed network to be minimized. Chain dimensions and thermodynamic statistical properties of bimodal PDMS networks at various elongation ratios are discussed. We find that elastic force f increases with elongation ratio λ; the energy contribution f_u to elastic force is significant, and the ratio of ranges from 0.15 to 0.36 at T = 343 K. In the meantime, elastic force f increases with the average energy 〈U〉. The energy change in the process of tensile elongation is taken over, which has been ignored in previous theories. Our calculations may provide some insights into the phenomena of rubberlike elasticity of bimodal networks. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 105–114, 2002     

Phase state diagrams of the poly(dimethylsiloxane)—poly(diethylsiloxane) system

Phase equilibria in the poly(dimethylsiloxane)(PDMS)—polydiethylsiloxane (PDES) system in the amorphous and liquid-crystal states were studied by optical interferometry. The findings obtained were compared with the data of calorimetric measurements. The experiments were carried out in a wide range of molecular weights and temperatures, and the phase diagrams were constructed. Thermodynamic analysis of the experimental data was performed in the framework of the Flory—Haggins theory for polymeric solutions. The analytical expressions for calculation of the pair interaction parameter using the binodal and liquidus curves were obtained. The pair interaction parameters of polymers and their dependences on the temperature and molecular weight were determined. The pair interaction parameter was shown to decrease with increasing the molecular weight of the oligomer component, approaching asymptotically a limiting value, which characterizes the interaction of the high molecular-weight PDMS and PDES. It was shown that the phase equilibria in the PDMS—PDES systems can be predicted quantitatively and qualitatively.