Hydrophobic Polydimethylsiloxane Surface Research Articles

Ink-based transparent compliant electrode for direct coating on untreated hydrophobic PDMS surface

Polydimethylsiloxane (PDMS) is widely used as the substrate for wearable sensors and elastomeric actuators because of its good dielectric property, stretchability, thermal/chemical resistance, biocompatibility, and transparency. PDMS-based stretchable sensors and electroactive actuators need a coating of thin compliant electrodes. However, PDMS is hydrophobic with water contact angle above 120°. Such low surface energy of PDMS creates issues for uniform coating of ink-based electrodes and/or their adhesion. Similarly, metal thin-film or nanowire-based materials coated on PDMS easily peels off. Surface treatments like UV/plasma exposure can temporarily increase PDMS's surface energy. However, these surfaces return to its original state within hours leaving behind a brittle surface layer that cracks upon stretching. To address these issues, we formulated a viscous composite ink with PEDOT:PSS and PDMS that can easily be coated on untreated PDMS by blade casting, screen printing, and so on. These coatings are highly transparent, stretchable, electrically conductive and work as compliant electrodes. We fabricated transparent dielectric elastomer actuators by coating PEDOT:PSS/PDMS composite ink on Elastosil (PDMS) substrates. The actuation strain and breakdown-fields were slightly lower compared to DEA with conventional graphite electrodes. However, the additional self-clearing capability of PEDOT:PSS/PDMS electrodes enhanced robustness of the DEA in events of localized dielectric breakdown.

Friction and Wear Characteristics of Polydimethylsiloxane under Water-Based Lubrication Conditions.

In this study, the friction and wear characteristics of polydimethylsiloxane (PDMS) were evaluated when using lubricants created by adding surfactants at various ratios to deionized (DI) water. When pure DI water is used as a lubricant, the repulsion of water from the hydrophobic PDMS surface is large and the interfacial affinity is low; thus, the lubrication properties cannot be significantly improved. However, when a lubricant with a surfactant is added to DI water, the interfacial affinity with the PDMS surface increases to form a lubricating film, and the friction coefficient is greatly reduced. In this study, under dry and pure DI water conditions, severe wear tracks were formed on the PDMS surface after 10,000 cycles of reciprocating sliding motion under a vertical load of 100 mN, whereas in the case of the surfactant-based and water-based lubricant, no severe wear tracks occurred. The friction and wear characteristics of the PDMS were evaluated by increasing the normal loads and sliding cycles with a water-based lubricant containing 1 wt % surfactant. Under normal loads of 300 mN and 500 mN, only minor scratches occurred on the PDMS surface up to 10,000 and 100,000 cycles, respectively, but after 300,000 cycles, very severe pit wear tracks occurred.

Creating polysaccharide-protein complexes to control aqueous lubrication

An emerging understanding of natural biolubricants such as saliva and synovial fluid reveals that they rely on synergistic interactions between biomacromolecules in aqueous media, wherein one polymeric component promotes adsorption to biological substrates while another facilitates hydration lubrication. We hypothesize that this phenomenon can be achieved by combining the strong adsorption characteristics of proteins with the hydration of polysaccharides. To confirm this hypothesis, hydrated soluble complexes were formed through electrostatic interactions between positively-charged lysozyme molecules and anionic residues on pectic polysaccharides extracted from Plantago ovata seed mucilage. Boundary friction was measured between smooth hydrophobic polydimethylsiloxane (PDMS) surfaces using a ball-and-disk tribometer, while adsorption was assessed using a combination of the quartz-crystal microbalance with dissipation monitoring technique and ellipsometry. The results indicate that complexation between polysaccharides and proteins leads to improved adsorption on hydrophobic PDMS surfaces, which in turn decreases boundary friction in sliding contact. By comparing the adsorption behaviour of the complexes and individual components, we conclude that pectic polymers maintain the film's hydration while lysozyme improves surface adsorption, thereby providing new evidence that physical complexes can be utilised to design bi-functional surface layers. Our findings open up a range of opportunities for application in food and oral healthcare products, particularly in the use of food-compatible ingredients to control aqueous lubrication.

Precursor-Film-Mediated Thermocapillary Motion of Low-Surface-Tension Microdroplets.

In contrast to microdroplet condensation with high contact angles, the one with low contact angles remains unclear. In this study, we investigated dynamics of microdroplet condensation of low-surface-tension liquids on two flat substrate surfaces by using reflection interference confocal microscopy. Spontaneous migration toward relatively larger droplets was first observed for the microdroplets nucleated on the hydrophilic quartz surface. The moving microdroplets showed a contact angle hysteresis of ∼0.5°, which is much lower than the values observed on typical flat substrates and is within the range observed on slippery lubricant-infused porous surfaces. Because the microdroplets on the hydrophobic polydimethylsiloxane surface did not move, we concluded that the ultrathin precursor film is formed only on the hydrophilic surface, which reduces a resistive force to migration. Also, reduced size of droplets promotes the thermocapillary motion, which is induced by a gradient in local temperature inside a small microdroplet arising due to the difference in size of adjacent droplets.

Heating of a water droplet on inclined transparent polydimethylsiloxane (PDMS) surface

Heat transfer from replicated hydrophobic and transparent PDMS surface to a water droplet is considered. The laser texturing of alumina surface is carried out to obtain hydrophobic surface and later the textured surface is replicated by polydimethylsiloxane (PDMS) via a solvent casting method. An experiment is carried out to assess the water droplet pinning on the replicated inclined PDMS surface while keeping the replicated surface at uniform temperature (308 K). The flow and temperature fields inside the inclined water droplet are simulated in line with the experimental conditions. The flow velocities inside the droplet are validated through the data obtained from the particle imaging velocimetry (PIV). It is found that the velocity predictions agree well with the PIV data. The droplet heating results in a circulation cell inside the droplet due to the Marangoni and the buoyancy currents. Increasing inclination angle of the PDMS replicated surface enhances the maximum velocity inside the droplet, which is more pronounced for the large size droplets. The Nusselt and the Bond numbers increase with the inclination angle of the PDMS surface.

Multifunctional antimicrobial chlorhexidine polymers by remote plasma assisted vacuum deposition

Novel antibacterial materials for implants and medical instruments are essential to develop practical strategies to stop the spread of healthcare associated infections. This study presents the synthesis of multifunctional antibacterial nanocoatings on polydimethylsiloxane (PDMS) by remote plasma assisted deposition of sublimated chlorhexidine powders at low pressure and room temperature. The obtained materials present effective antibacterial activity against Escherichia coli K12, either by contact killing and antibacterial adhesion or by biocide agents release depending on the synthetic parameters. In addition, these multifunctional coatings allow the endure hydrophilization of the hydrophobic PDMS surface, thereby improving their biocompatibility. Importantly, cell-viability tests conducted on these materials also prove their non-cytotoxicity, opening a way for the integration of this type of functional plasma films in biomedical devices.

Single-step fabrication of polydimethylsiloxane microwell arrays with long-lasting hydrophilic inner surfaces

We propose a single-step fabrication method for polydimethylsiloxane (PDMS) cell-adhesive microwell arrays with long-lasting (>10 months in aqueous medium) hydrophilic inner surfaces without the need for any chemical treatment such as development. Irradiation of a PDMS film with a low-energy electron beam (55 kV) in air generated a ∼40-μm-thick hydrophilic silica-like layer on the PDMS surface, which was the key to the prolonged hydrophilicity. Moreover, the concomitant compaction of the irradiated area produced dozens-of-micrometers-deep concave wells. The hydrophilic microwells generated on the hydrophobic non-irradiated PDMS surface easily trapped nano-/picoliter droplets and cells/single-cells. In addition, the surfaces of the microwells offered stable and favorable cell-adherent environments. The method presented here can realize stable and reliable lab-on-chips and cater to the expanding demand in biological and medical applications.

Aqueous Lubrication, Structure and Rheological Properties of Whey Protein Microgel Particles.

Aqueous lubrication has emerged as an active research area in recent years due to its prevalence in nature in biotribological contacts and its enormous technological soft-matter applications. In this study, we designed aqueous dispersions of biocompatible whey-protein microgel particles (WPM) (10-80 vol %) cross-linked via disulfide bonding and focused on understanding their rheological, structural and biotribological properties (smooth polydimethylsiloxane (PDMS) contacts, Ra < 50 nm, ball-on-disk set up). The WPM particles (Dh = 380 nm) displayed shear-thinning behavior and good lubricating performance in the plateau boundary as well as the mixed lubrication regimes. The WPM particles facilitated lubrication between bare hydrophobic PDMS surfaces (water contact angle 108°), leading to a 10-fold reduction in boundary friction force with increased volume fraction (ϕ ≥ 65%), largely attributed to the close packing-mediated layer of particles between the asperity contacts acting as "true surface-separators", hydrophobic moieties of WPM binding to the nonpolar surfaces, and particles employing a rolling mechanism analogous to "ball bearings", the latter supported by negligible change in size and microstructure of the WPM particles after tribology. An ultralow boundary friction coefficient, μ ≤ 0.03 was achieved using WPM between O2 plasma-treated hydrophilic PDMS contacts coated with bovine submaxillary mucin (water contact angle 47°), and electron micrographs revealed that the WPM particles spread effectively as a layer of particles even at low ϕ∼ 10%, forming a lubricating load-bearing film that prevented the two surfaces from true adhesive contact. However, above an optimum volume fraction, μ increased in HL+BSM surfaces due to the interpenetration of particles that possibly impeded effective rolling, explaining the slight increase in friction. These effects are reflected in the highly shear thinning nature of the WPM dispersions themselves plus the tendency for the apparent viscosity to fall as dispersions are forced to very high volume fractions. The present work demonstrates a novel approach for providing ultralow friction in soft polymeric surfaces using proteinaceous microgel particles that satisfy both load bearing and kinematic requirements. These findings hold great potential for designing biocompatible particles for aqueous lubrication in numerous soft matter applications.

Kinematical behaviour of capillary-driven nematic flow in polydimethylsiloxane microchannels: The effects of the dimensions of the channels

In this work we investigate the kinematical behaviour of capillary-driven nematic flows confined in polydimethylsiloxane (PDMS) microchannels. We show that the highly hydrophobic PDMS surfaces have affinity for the cyano-based nematogen 5CB, and are fairly lyophilic with the nematogen. The liquid crystal can flow into the PDMS microchannels without applying any external force. The kinematic behaviour of the nematic flow is sensitive to the geometry of the microchannels. The geometrical effects can be described using an analytic function, also known as the geometrical term. In theory, a scaling-down of the size of the channels will cause a drastic increase in the velocity of the nematic flow. We measured the velocities of the nematic flows in PDMS channels of different sizes. Our preliminary results show that the experimental measurement is highly consistent with the theoretical predictions.

Wrinkling Measurement of the Mechanical Properties of Drying Salt Thin Films

We report a time-resolved approach to probe the mechanical properties of thin films during drying and solidification based on surface wrinkling. The approach is demonstrated by measuring the modulus of a ternary system comprising an inorganic salt (aluminum chlorohydrate), a humectant (glycerol), and water across the glassy film formation pathway. The topography of mechanically induced wrinkling of supported films on polydimethylsiloxane (PDMS) is experimentally monitored during mechanical extension and relaxation cycles. Nontrivial aspects of our method include the need to oxidize the (hydrophobic) PDMS surface prior to solution deposition to enable surface wetting, which simultaneously creates a glassy-layer skin, whose wrinkling can contribute to the overall topography. Film drying is studied as a function of solution concentration and time, and a range of pattern morphologies are found: sinusoidal wrinkling, transient double-wavelength wrinkling accompanying film "crust" formation, ridging associated with stress localization, and cracking. We quantify the evolution of the elastic modulus during the sinusoidal wrinkling stage, employing bi- and trilayer models, which are independently confirmed by nanoindentation. The method provides thus a simple and robust approach for the mechanical characterization of out-of-equilibrium thin films.

Importance of surface modification of a microcontact stamp for pattern fidelity of soluble organic semiconductors

We described the effect of the ultraviolet ozone (UVO) treatment of a polydimethylsiloxane (PDMS) stamp on the fidelity of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) patterns produced from a TIPS-PEN/polymer blend by selective contact evaporation (SCE). During the SCE process, the TIPS-PEN in contact with the nanoporous PDMS was absorbed into the PDMS stamp, leaving out the TIPS-PEN patterns, complementary to the PDMS patterns, in the noncontact regions. For the case of the untreated, hydrophobic PDMS surface, the TIPS-PEN patterns developed initially were shrunken and eventually disappeared after 24 h due to the steady absorption of the TIPS-PEN in time. In contrast, for the UVO-treated case, the TIPS-PEN patterns were found to maintain the initial shapes over the period of 24 h since the absorption of the TIPS-PEN was limited by the hydrophilic nature of the UVO-treated PDMS. The modified PDMS surface by the UVO for 30 min yielded the highest fidelity of the TIPS-PEN patterns in both height and width. The patterned TIPS-PEN layer by the SCE was implemented into an organic field-effect transistor to demonstrate the viability of the SCE combined with the UVO treatment for solution-processed organic electronic devices.

The effects of surface properties on Escherichia coli adhesion are modulated by shear stress

The adhesion of Escherichia coli to glass and polydimethylsiloxane (PDMS) at different flow rates (between 1 and 10mls−1) was monitored in a parallel plate flow chamber in order to understand the effect of surface properties and hydrodynamic conditions on adhesion. Computational fluid dynamics was used to assess the applicability of this flow chamber in the simulation of the hydrodynamics of relevant biomedical systems. Wall shear stresses between 0.005 and 0.07Pa were obtained and these are similar to those found in the circulatory, reproductive and urinary systems. Results demonstrate that E. coli adhesion to hydrophobic PDMS and hydrophilic glass surfaces is modulated by shear stress with surface properties having a stronger effect at the lower and highest flow rates tested and with negligible effects at intermediate flow rates. These findings suggest that when expensive materials or coatings are selected to produce biomedical devices, this choice should take into account the physiological hydrodynamic conditions that will occur during the utilization of those devices.

Adsorption and Aqueous Lubricating Properties of Charged and Neutral Amphiphilic Diblock Copolymers at a Compliant, Hydrophobic Interface

We have investigated the adsorption and lubricating properties of neutral and charged amphiphilic diblock copolymers at a hydrophobic polydimethylsiloxane (PDMS) interface in an aqueous environment. The diblock copolymers consist of a hydrophilic block of either neutral poly(ethylene glycol) (PEG) or negatively charged poly(acrylic acid) (PAA) and of a hydrophobic block of polystyrene (PS) or poly(2-methoxyethyl acrylate) (PMEA), thus generating PEG-b-X or PAA-b-X, where X block is either PS or PMEA. The molecular weight ratios were roughly 1:1 with each block ca. 5 kDa. Comparing the neutral PEG and charged PAA buoyant blocks with all other conditions identical, the former showed superior adsorption onto nonpolar, hydrophobic PDMS surfaces from a neutral aqueous solution. PEG-based copolymers showed substantial adsorption for both PS and PMEA as the anchoring block, whereas PAA-based copolymers showed effective adsorption only when PMEA was employed as the anchoring block. For PAA-b-PS, the poor adsorption properties are chiefly attributed to micellization due to the high interfacial tension between the PS core and water. The poor lubricating properties of PAA-b-PS diblock copolymer for a PDMS-PDMS sliding contact was well correlated with the poor adsorption properties. PAA-b-PMEA copolymers, despite their sizable amount of adsorbed mass, showed insignificant lubricating effects. When the charges of the PAA-b-PMEA diblock copolymers were screened by either adding NaCl to the aqueous solution or by lowering the pH, both the adsorption and lubricity improved. We ascribe the poor adsorption and inferior aqueous lubricating properties of the PAA-based diblock copolymers compared to their PEG-based counterparts mainly to the electrostatic repulsion between charged PAA blocks, hindering the facile formation of the lubricating layer under cyclic tribological stress at the sliding PDMS-PDMS interface.

Enrichment of tumor-initiating breast cancer cells within a mammosphere-culture microdevice

We report for the first time a microdevice that enables the selective enrichment, culture, and identification of tumor-initiating cells on native polydimethylsiloxane (PDMS). For nearly a decade, researchers have identified tumor-initiating breast cancer cells within heterogeneous populations of breast cancer cells by utilizing low-attachment serum-free culture conditions, which lead to the formation of spheroidal colonies (mammospheres) that are enriched for tumor-initiating cells. However, the utility of this assay has been limited by difficulties in combining this culture-plate-based technique with other cellular and molecular analyses. Integrating the mammosphere technique into a microsystem can enable it to be combined directly with a number of functions, such as cell sorting, drug screens, and molecular assays. In this work, we demonstrate mammosphere culture within a PDMS microdevice. We first prove that a native hydrophobic PDMS surface is as effective as commercial low-attachment plates at selectively promoting the formation of mammospheres. We then experimentally assess the PDMS microdevice. Time-lapse images of mammosphere formation within the microdevice show that mammospheres form from single cells or small clusters of cells. Following formation of the mammospheres, it is desirable to evaluate the cells within the spheroids for enrichment of tumor initiating cells. To perform assays such as this (which require the loading and rinsing of reagents) without flushing the cells (which are in suspension) from the device, the culture chamber is separated from a reagent reservoir by a commercially available microporous membrane, and thus reagents are exchanged between the reservoir and the culture chamber by diffusion only. Using this capability, we verify that the mammospheres are enriched for tumor initiating cells by staining aldehyde dehydrogenase activity, a cancer stem cell marker. To the best of our knowledge, this is the first assay that enables the direct observation of tumor-initiating cells within a suspended mammosphere.

Micron Patterning of Polydiacetylene Supramolecules Using Micro-contact Printing in Conjunction with Langmuir-Blodgett Deposition

Since the pioneering discovery by Wagner, polydiacetylenes (PDAs) have been extensively investigated as potential materials for sensor systems as well as active components of optical and electrical devices. Undoubtedly, the most appealing and promising features of PDAs are their stress induced colorimetric and fluorogenic transitions that have been harvested as a response to thermal, chemical, mechanical and biological recognition events. During the last decade, advances in the design, synthesis and application of new PDA derivatives have promoted a demand for methods to reliably fabricate patterned PDA layers on solid substrates. Accordingly, several PDA patterning techniques including macroarray, microcontact printing, replica molding, micromolding in capillaries as well as photolithographic, electrophoretic deposition and probe tip methods have been described. In this work, we demonstrate a simple and straightforward method to pattern PDA supramolecules using LangmuirBlodgett (LB) deposition and micro-contact printing (μ-CP). The general procedure consists of the following steps. First, a surface-patterned polydimethylsiloxane (PDMS) mold is replicated from a photoresist-patterned silicon substrate. Secondly, LB films of the PDA supramolecules are deposited on the surface of the patterned PDMS. Lastly, the PDA LB films are selectively transferred onto silicon substrates using the μ-CP technique. The combination of LB deposition and μ-CP can conveniently fabricate micronsized patterns of PDA supramolecules. Therefore, we believe the patterned PDA supramolecules will find great utility in sensor and optical applications. Figure 1 shows a surface pressure-molecular area (π-A) isotherm of a diacetylene monolayer derived from 10,12pentacosadiynoic acid (PCDA) on the water subphase. The surface pressure starts rising at an area of 100 A and gradually increases upon compression until the onset of a plateau-like region at about 35 mN/m. Upon further compression, the surface pressure rises to nearly 48 mN/m. The LB multilayers were prepared by accumulation of up-strokes and down-strokes at a pressure of 25 mN/m on PDMS substrates. The quality of the deposited monolayer on a substrate can be evaluated by the transfer ratio. For an ideal transfer, the transfer ratio is equal to 1. Transfer ratios for diacetylene supramolecule deposition on PDMS substrates are close to 1 during up-strokes but these are close to zero during down-strokes (data not shown here). This result indicates that diacetylene supramolecules transfer onto the PDMS surface only for up-strokes, which is due to the hydrophobic PDMS surface. Figure 2(a) and 2(c) shows optical images of diacetylene supramolecule patterns on Si substrates fabricated by transferring a diacetylene supramolecule LB layers using the

Surface modification of PDMS by atmospheric-pressure plasma-enhanced chemical vapor deposition and analysis of long-lasting surface hydrophilicity

Atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) offers several benefits such as simplicity, high productivity, and versatility. An AP-PECVD-based method is proposed in this study to modify hydrophobic PDMS (polydimethylsiloxane) surfaces towards a long-lasting hydrophilic character. To enhance the sustainability of the hydrophilicity, two kinds of layers were sequentially deposited by AP-PECVD on the surface of a PDMS block (TEOS-O2/CH4/PDMS). A hydrocarbon layer was first coated on the bare PDMS surface using CH4 as the reactant, and then, a hydrophilic SiOx layer was deposited using tetraethyl orthosilicate and oxygen (TEOS-O2). The highly cross-linked hydrocarbon layer acted as a physical barrier layer (PBL) between the bare PDMS surface and the hydrophilic layer. To confirm that the PBL suppresses the hydrophobic recovery of the modified PDMS surface with double layer, a single-layer-coated PDMS sample (TEOS-O2/PDMS) without the PBL was prepared by AP-PECVD using TEOS-O2. The surface characteristics were determined by static contact angle measurements, surface roughness measurements, and surface chemical composition/chemical bonding determination and compared with those of modified PDMS surface with double layer. The surface morphology of TEOS-O2/PDMS degraded seriously by the diffusion of PDMS oligomers to the hydrophilic layer, but that of TEOS-O2/CH4/PDMS was sustained for a long time. Thus, TEOS-O2/CH4/PDMS had the lowest contact angle, almost 0°, and showed long-lasting surface hydrophilicity, with almost no change in the contact angle for 28 days. Thus, this proposed method is confirmed to be well suited for use in applications that require stable hydrophilic surface property in PDMS.

Electrokinetic secondary-flow behavior in a curved microchannel under dissimilar surface conditions

The curved channel appears to be indispensable for the lab-on-chips systems because it provides a convenient scheme for increasing the channel length per unit chip area in the direction of net flow. A secondary Dean flow in curved rectangular microchannels is examined by applying the finite-volume scheme with a semi-implicit method for pressure-linked equations (SIMPLE) algorithm for the pressure-driven electrokinetic transport. This framework is based on the theoretical model coupled with the full Poisson-Boltzmann, Navier-Stokes, and the Nernst-Planck principles of net charge conservation [Yun et al., Phys. Fluids 22, 052004 (2010)]. The effect of a dissimilar wall condition on the secondary flow at the turn is explored by considering different configurations of channel wall having complementary aspect ratios (i.e., ratio of the channel height to the channel width, H/W = 0.25 and 4.0) with combinations of hydrophilic glass and hydrophobic polydimethylsiloxane surfaces. Simulation results exhibit that, contrary to the case of general narrow-bore channels, the streamwise axial velocity tends to shift toward the inner wall caused by a stronger effect of the spanwise pressure gradient, according to a sufficiently low Dean number. The increasing rate of this shift with increasing curvature ratio is more significant in the shallow (or low-aspect-ratio) channel, due to the effect of greater distance traveled by the fluid along the outer wall. The curvature introduces the presence of pairs of counter-rotating vortices perpendicular to the flow direction. Comparing between shallow and deep (or high-aspect-ratio) channels allows us to identify that the patterns of axial velocity and vorticity are altered by the heterogeneity effect of surfaces occupying a large area. The total magnitude of vorticity at the cross section of the channel increases with increasing slip length, due to the contribution of enhanced axial velocity driven by the slip, while there is no fluid-slip dependency for the slip length of less than about 50 nm.

Oxygen plasma treatment for reducing hydrophobicity of a sealed polydimethylsiloxane microchannel

Rapid prototyping of polydimethylsiloxane (PDMS) is often used to build microfluidic devices. However, the inherent hydrophobic nature of the material limits the use of PDMS in many applications. While different methods have been developed to transform the hydrophobic PDMS surface to a hydrophilic surface, the actual implementation proved to be time consuming due to differences in equipment and the need for characterization. This paper reports a simple and easy protocol combining a second extended oxygen plasma treatments and proper storage to produce usable hydrophilic PDMS devices. The results show that at a plasma power of 70 W, an extended treatment of over 5 min would allow the PDMS surface to remain hydrophilic for more than 6 h. Storing the treated PDMS devices in de-ionized water would allow them to maintain their hydrophilicity for weeks. Atomic force microscopy analysis shows that a longer oxygen plasma time produces a smoother surface.

Low Biofouling Chitosan-Hyaluronic Acid Multilayers with Ultra-Low Friction Coefficients

Resistance to biofouling is an advantageous material property in a variety of biomedical and biofluid processing applications. Protein-resisting surface coatings must also be resistant to wear and degradation and in certain applications good aqueous lubricating properties are required. We show that cross-linked polyelectrolyte multilayers, consisting of chitosan and hyaluronan on polydimethylsiloxane (PDMS) surfaces, form a highly lubricating film that is resistant to wear and protein adsorption. The multilayer film shows much stronger resistance to protein adsorption from human whole saliva than both hydrophobic and hydrophilic PDMS surfaces; the latter two showed identical adsorbed salivary film thicknesses. The boundary friction coefficient under aqueous conditions was extremely low (mu approximately 0.01) between multilayer-coated PDMS substrates and the film is robust against dry rubbing and many hours of tribological experiments in a range of aqueous lubricants. The origins of the assembly's low friction coefficients and robustness are discussed. In addition, we found that the addition of negative phosphate ions to water lowers the boundary lubricating properties of negatively charged hydrophilic PDMS surfaces by 1 order of magnitude to mu approximately 0.01. We consider this to arise from the large hydration sheaths and resulting "ball-bearing" properties of the hydrated phosphate ions, which form a lubricating barrier against asperity contact. These findings offer new insights toward biolubrication processes and suggest that chitosan-hyaluronan polyelectrolyte multilayer films have the potential to be used in (bio-) applications requiring low friction as well as resistance to biofouling and wear.

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.