Surface biopassivation of replicated poly(dimethylsiloxane) microfluidic channels and application to heterogeneous immunoreaction with on-chip fluorescence detection

Exploring Sensitive Label-Free Multiplex Analysis with Raman-Coded Microbeads and SERS-Coded Reporters

Suspension microsphere immunoassays are rapidly gaining attention in multiplex bioassays. Accurate detection of multiple analytes from a single measurement is critical in modern bioanalysis, which always requires complex encoding systems. In this study, a novel bioassay with Raman-coded antibody supports (polymer microbeads with different Raman signatures) and surface-enhanced Raman scattering (SERS)-coded nanotags (organic thiols on a gold nanoparticle surface with different SERS signatures) was developed as a model fluorescent, label-free, bead-based multiplex immunoassay system. The developed homogeneous immunoassays included two surface-functionalized monodisperse Raman-coded microbeads of polystyrene and poly(4-tert-butylstyrene) as the immune solid supports, and two epitope modified nanotags (self-assembled 4-mercaptobenzoic acid or 3-mercaptopropionic acid on gold nanoparticles) as the SERS-coded reporters. Such multiplex Raman/SERS-based microsphere immunoassays could selectively identify specific paratope–epitope interactions from one mixture sample solution under a single laser illumination, and thus hold great promise in future suspension multiplex analysis for diverse biomedical applications.

Sorption of Neuropsychopharmaca in Microfluidic Materials for In Vitro Studies

Sorption (i.e., adsorption and absorption) of small-molecule compounds to polydimethylsiloxane (PDMS) is a widely acknowledged phenomenon. However, studies to date have largely been conducted under atypical conditions for microfluidic applications (lack of perfusion, lack of biological fluids, etc.), especially considering biological studies such as organs-on-chips where small-molecule sorption poses the largest concern. Here, we present an in-depth study of small-molecule sorption under relevant conditions for microphysiological systems, focusing on a standard geometry for biological barrier studies that find application in pharmacokinetics. We specifically assess the sorption of a broad compound panel including 15 neuropsychopharmaca at in vivo concentration levels. We consider devices constructed from PDMS as well as two material alternatives (off-stoichiometry thiol-ene-epoxy, or tape/polycarbonate laminates). Moreover, we study the much neglected impact of peristaltic pump tubing, an essential component of the recirculating systems required to achieve in vivo-like perfusion shear stresses. We find that the choice of the device material does not have a significant impact on the sorption behavior in our barrier-on-chip-type system. Our PDMS observations in particular suggest that excessive compound sorption observed in prior studies is not sufficiently described by compound hydrophobicity or other suggested predictors. Critically, we show that sorption by peristaltic tubing, including the commonly utilized PharMed BPT, dominates over device sorption even on an area-normalized basis, let alone at the typically much larger tubing surface areas. Our findings highlight the importance of validating compound dosages in organ-on-chip studies, as well as the need for considering tubing materials with equal or higher care than device materials.

A vascular-liver chip for sensitive detection of nutraceutical metabolites from human pluripotent stem cell derivatives

Human pluripotent stem cell (hPSC) is a great resource for generating cell derivatives for drug efficiency testing. Metabolites of nutraceuticals can exert anti-inflammatory effects on blood vessels. However, the concentration of nutraceutical metabolites produced in hPSC-derived hepatocytes (hPSC-HEPs) is usually low. To enable the detection of these metabolites under the in vitro environment, we have developed a co-culture model consisting of parallel co-culture chambers and a recirculating microfluidic system with minimum fluid volume, optimal cell culture environment. The model allows cells to be exposed continuously to nutraceutical metabolites. In this perfused culturing model, hPSC-derived endothelial cells and hPSC-HEPs are co-cultured without physical contact. When an anti-inflammatory nutraceutical, quercetin, was administrated to the co-culture, higher levels of quercetin metabolites were detected on-chip compared with static control. We further induced inflammation with Interleukin-1β in the co-culture model and measured interleukin 8 (IL-8) generation. The IL-8 level was suppressed more significantly by quercetin metabolites in the perfusion co-culture, as compared to static culture. This is due to enhanced metabolites production on-chip. This microfluidic co-culture model enables in vitro screening of nutraceuticals using hPSC-derived cells.

A review of electronic skin: soft electronics and sensors for human health

This article reviews several categories of electronic skins (e-skins) for monitoring signals involved in human health. It covers advanced candidate materials, compositions, structures, and integrate strategies of e-skin, focusing on stretchable and wearable electronics. In addition, this article further discusses the potential applications and expected development of e-skins. It is possible to provide a new generation of sensors which are able to introduce artificial intelligence to the clinic and daily healthcare.

Replica molding-based nanopatterning of tribocharge on elastomer with application to electrohydrodynamic nanolithography

Replica molding often induces tribocharge on elastomers. To date, this phenomenon has been studied only on untextured elastomer surfaces even though replica molding is an effective method for their nanotexturing. Here we show that on elastomer surfaces nanotextured through replica molding the induced tribocharge also becomes patterned at nanoscale in close correlation with the nanotexture. By applying Kelvin probe microscopy, electrohydrodynamic lithography, and electrostatic analysis to our model nanostructure, poly(dimethylsiloxane) nanocup arrays replicated from a polycarbonate nanocone array, we reveal that the induced tribocharge is highly localized within the nanocup, especially around its rim. Through finite element analysis, we also find that the rim sustains the strongest friction during the demolding process. From these findings, we identify the demolding-induced friction as the main factor governing the tribocharge's nanoscale distribution pattern. By incorporating the resulting annular tribocharge into electrohydrodynamic lithography, we also accomplish facile realization of nanovolcanos with 10 nm-scale craters.

Protein immobilization techniques for microfluidic assays

Microfluidic systems have shown unequivocal performance improvements over conventional bench-top assays across a range of performance metrics. For example, specific advances have been made in reagent consumption, throughput, integration of multiple assay steps, assay automation, and multiplexing capability. For heterogeneous systems, controlled immobilization of reactants is essential for reliable, sensitive detection of analytes. In most cases, protein immobilization densities are maximized, while native activity and conformation are maintained. Immobilization methods and chemistries vary significantly depending on immobilization surface, protein properties, and specific assay goals. In this review, we present trade-offs considerations for common immobilization surface materials. We overview immobilization methods and chemistries, and discuss studies exemplar of key approaches-here with a specific emphasis on immunoassays and enzymatic reactors. Recent "smart immobilization" methods including the use of light, electrochemical, thermal, and chemical stimuli to attach and detach proteins on demand with precise spatial control are highlighted. Spatially encoded protein immobilization using DNA hybridization for multiplexed assays and reversible protein immobilization surfaces for repeatable assay are introduced as immobilization methods. We also describe multifunctional surface coatings that can perform tasks that were, until recently, relegated to multiple functional coatings. We consider the microfluidics literature from 1997 to present and close with a perspective on future approaches to protein immobilization.

Elastomeric microparticles for acoustic mediated bioseparations

BACKGROUND

Acoustophoresis has been utilized successfully in applications including cell trapping, focusing, and purification. One current limitation of acoustophoresis for cell sorting is the reliance on the inherent physical properties of cells (e.g., compressibility, density) instead of selecting cells based upon biologically relevant surface-presenting antigens. Introducing an acoustophoretic cell sorting approach that allows biochemical specificity may overcome this limitation, thus advancing the value of acoustophoresis approaches for both the basic research and clinical fields.

RESULTS

The results presented herein demonstrate the ability for negative acoustic contrast particles (NACPs) to specifically capture and transport positive acoustic contrast particles (PACPs) to the antinode of an ultrasound standing wave. Emulsification and post curing of pre-polymers, either polydimethylsiloxane (PDMS) or polyvinylmethylsiloxane (PVMS), within aqueous surfactant solution results in the formation of stable NACPs that focus onto pressure antinodes. We used either photochemical reactions with biotin-tetrafluorophenyl azide (biotin-TFPA) or end-functionalization of Pluronic F108 surfactant to biofunctionalize NACPs. These biotinylated NACPs bind specifically to streptavidin polystyrene microparticles (as cell surrogates) and transport them to the pressure antinode within an acoustofluidic chip.

CONCLUSION

To the best of our knowledge, this is the first demonstration of using NACPs as carriers for transport of PACPs in an ultrasound standing wave. By using different silicones (i.e., PDMS, PVMS) and curing chemistries, we demonstrate versatility of silicone materials for NACPs and advance the understanding of useful approaches for preparing NACPs. This bioseparation scheme holds potential for applications requiring rapid, continuous separations such as sorting and analysis of cells and biomolecules.

Reductive surface synthesis of gold nanoparticles on silicate glass and their biochemical sensor applications

Gold nanoparticles (Au NPs) were directly synthesized on the surface of polyvinylsilazane (PVSZ, -[(vinyl)SiH-NH2]-) without use of extra reductive additives. The reductive Si-H functional groups on the surface of cured PVSZ acted as surface bound reducing agents to form gold metal when contacted with an aqueous Au precursor (HAuCl4) solution, leading to formation of Au NPs adhered to silicate glass surface. The Au NPs-silicate platforms were preliminarily tested to detect Rhodamine B (1 μM) by surface enhanced Raman scattering. Furthermore, gold microelectrode obtained by post-chemical plating was used as an integrated amperometric detection element in the polydimethylsilane-glass hybrid microfluidic chip.

A new soft lithographic route for the facile fabrication of hydrophilic sandwich microchips

Manufacturing materials are an essential element for the fabrication of microfluidic chips. PDMS, the most widely used polymeric material, is associated with apparent disadvantages such as hydrophobic nature, while other materials also suffer from some limitations. In this paper, a new soft lithographic route was proposed for the facile manufacturing of hydrophilic sandwich microchips, using bisphenol A based epoxy acrylate (BABEA) as a new patterning material. The BABEA copolymers are hydrophilic, highly transparent in visible range while highly untransparent when the wavelength is less than 290 nm, and of high replication fidelity. By combining with appropriate monomers, including glycidyl methacrylate, methylmethacrylate, and acrylic acid, the copolymers contain active functional groups, which allows for easy postmodification for desirable functional units. A fabrication procedure was proposed for manufacturing hybrid quartz/BABEA copolymer/quartz microchips. In the procedure, no micromachining equipments, wet etching, or imprinting techniques were involved, making the fabrication approach applicable in ordinary chemistry laboratories. The performance of the prepared microchips was demonstrated in terms of CIEF with UV-whole channel imaging detection. The hydrophilic microchannel ensures stable focusing while the polymeric middle layer acts as a perfectly aligned optical slit for whole channel UV absorbance detection.

A new angle on pluronic additives: advancing droplets and understanding in digital microfluidics

Biofouling in microfluidic devices limits the type of samples which can be handled and the duration for which samples can be manipulated. Despite the cost of disposing fouled devices, relatively few strategies have been developed to tackle this problem. Here, we have analyzed a series of eight amphiphilic droplet additives, Pluronic coblock polymers of poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO), as a solution to biofouling in digital microfluidics using serum-containing cell culture media as a model fluid. Our analysis shows that species with longer PPO chains are superior for enabling droplet motion and reducing biofouling. Two of the tested species, L92 and P105, were found to lengthen device lifetimes by 2-3 times relative to additives used previously when used at optimal concentrations. Pluronics with low PEO content such as L92 were found to be cytotoxic to an immortalized mammalian cell line, and therefore we recommend that Pluronic additives with greater or equal to 50% PEO composition, such as P105, be used for digital microfluidic applications involving cells. Finally, contact angle measurements were used to probe the interaction between Pluronic-containing droplets and device surfaces. Strong correlations were found between various types of contact angle measurements and the capacity of additives to reduce biofouling, which suggests that contact angle measurements may be useful as a tool for rapidly screening new candidates for the potential to reduce biofouling. We propose that this study will be useful for scientists and engineers who are developing digital microfluidic platforms for a wide range of applications involving protein-containing solutions, and in particular, for applications involving cells.

Microfluidic chips for point-of-care immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.

Integrated electrokinetic sample focusing and surface plasmon resonance imaging system for measuring biomolecular interactions

Label-free biomolecular binding measurement methods, such as surface plasmon resonance (SPR), are becoming increasingly more important for the estimation of real-time binding kinetics. Recent advances in surface plasmon resonance imaging (iSPR) are emerging for label-free microarray-based assay applications, where multiple biomolecular interactions can be measured simultaneously. However, conventional iSPR microarray systems rely on protein printing techniques for ligand immobilization to the gold imaging surface and external pumps for analyte transport. In this article, we present an integrated microfluidics and iSPR platform that uses only electrokinetic transport and guiding of ligands and analytes and, therefore, requires only electrical inputs for sample transport. An important advantage of this new approach, compared to conventional systems, is the ability to direct a single analyte to a specific ligand location in the microarray, which can facilitate analysis parallelization. Additionally, this simple approach does not require complicated microfluidic channel arrangements, external pumps, or valves. As a demonstration, kinetics and affinity have been extracted from measured binding responses of human IgG and goat antihuman IgG using a simple 1:1 model and compared to responses measured with conventional pressure driven analyte transport. The measured results indicate similar binding kinetics and affinity between the electrokinetic and pressure-driven sample manipulation methods and no cross contamination to adjacent measurement locations has been observed.

Immunoassays in microfluidic systems

Immunoassays have greatly benefited from miniaturization in microfluidic systems. This review, which summarizes developments in microfluidics-based immunoassays since 2000, includes four sections, focusing on the configurations of immunoassays that have been implemented in microfluidics, the main fluid handling modalities that have been used for microfluidic immunoassays, multiplexed immunoassays in microfluidic platforms, and the emergence of label-free detection techniques. The field of microfluidic immunoassays is continuously improving and has great promise for the future.

RNA-protein binding kinetics in an automated microfluidic reactor

Microfluidic chips can automate biochemical assays on the nanoliter scale, which is of considerable utility for RNA-protein binding reactions that would otherwise require large quantities of proteins. Unfortunately, complex reactions involving multiple reactants cannot be prepared in current microfluidic mixer designs, nor is investigation of long-time scale reactions possible. Here, a microfluidic 'Riboreactor' has been designed and constructed to facilitate the study of kinetics of RNA-protein complex formation over long time scales. With computer automation, the reactor can prepare binding reactions from any combination of eight reagents, and is optimized to monitor long reaction times. By integrating a two-photon microscope into the microfluidic platform, 5-nl reactions can be observed for longer than 1000 s with single-molecule sensitivity and negligible photobleaching. Using the Riboreactor, RNA-protein binding reactions with a fragment of the bacterial 30S ribosome were prepared in a fully automated fashion and binding rates were consistent with rates obtained from conventional assays. The microfluidic chip successfully combines automation, low sample consumption, ultra-sensitive fluorescence detection and a high degree of reproducibility. The chip should be able to probe complex reaction networks describing the assembly of large multicomponent RNPs such as the ribosome.

Rapid protein depletion from complex samples using a bead-based microfluidic device for the point of care

Translation of sample preparation methods to point-of-care formats has remained a challenge. We present a plastic laminate microfluidic device for protein depletion from human plasma using ligand immobilized porous beads stored dry within a novel, pneumatically-driven mixer. The card design accelerated the protein depletion process from hours to minutes. Using immunoglobulin G as a model protein, we have successfully shown protein removal efficiency from spiked buffer between 70-80% and from diluted human plasma samples between 66-77%. Low non-specific binding of our downstream target ligand, immunoglobulin M, was observed with the spiked buffer and diluted human plasma samples. For future device optimization, the physical limitations to rapid protein removal on card were also explored. Bench-top experiments with improved mixing efficiency and a lower sample dilution factor achieved 99% IgG removal using the same amount of mixing time. This design can easily be adapted for depletion of other high abundance or interfering proteins by inclusion of other ligand immobilized beads.

Environmentally friendly surface modification of PDMS using PEG polymer brush

A PEG-NH2-based environmentally friendly surface modification strategy was developed for PDMS microchips to prevent protein adsorption and to enhance separation performance. PEG-NH2 was synthesized using a modified synthesis procedure. A two-step grafting method was used for PDMS modification. FTIR absorption by attenuated total reflection and contact angle measurements verified the successful grafting of PEG-NH2 onto the PDMS surface. Subsequent EOF Measurements and protein adsorption studies of PEG-modified PDMS microchips revealed noticeable EOF suppression and resistance to nonspecific protein adsorption for more than 30 days. Separation of four FITC-labeled amino acids was further demonstrated with high repeatability and reproducibility. Comparison of electrophoresis of 3-(2-furoyl)quinoline-2-carboxaldehyde-labeled BSA using PDMS microchips before and after surface modification resulted in significantly improved electrophoretic performance of the PEG-modified PDMS microchips, suggesting that our PEG grafting method successfully modified PDMS surface property and prevented adsorption of proteins. We expect that this environmentally friendly surface modification method will be useful for future protein separations with long-term surface stability.

Enabling organosilicon chemistries on inert polymer surfaces with a vapor-deposited silica layer

Given the large surface area-to-volume ratios commonly encountered in microfluidics applications, the ability to engineer the chemical properties of surfaces encountered in these applications is critically important. However, as various polymers are rapidly replacing glass and silicon as the chosen materials for microfluidics devices, the ability to easily modify the surface chemistry has been diminished by the relatively inert nature of some commonly employed polymer surfaces, such as poly(methyl methacrylate) (PMMA), polystyrene, and polydimethylsiloxane (PDMS). This paper describes the low-temperature, vapor-phase deposition of robust silica layers to PMMA, polystyrene, and PDMS surfaces, which enables the functionalization of these surfaces by standard organosilane chemistries. Attenuated total reflection infrared spectroscopy, contact angle goniometry, ellipsometry, and atomic force microscopy are used to characterize the silica layers that form on these surfaces. Aqueous immersion experiments indicate that the silica layer has excellent stability in aqueous environments, which is a prerequisite for microfluidics applications, but for PMMA surfaces, low adhesion of the silica layer to the underlying substrate is problematic. For PDMS substrates, the presence of the silica layer helps to slow the process of hydrophobic recovery, which is an additional advantage.

Development of microfluidic chips for heterogeneous receptor-ligand interaction studies

A simple microfluidic-based technique to quantitate the binding affinity between the glycopeptide antibiotics teicoplanin from Actinoplanes teicomyceticus and vancomycin from Streptomyces orientalis and 5-carboxyfluorescein-D-Ala-D-Ala-D-Ala (5-FAM-(DA)(3)) is described. In this work, (3-aminopropyl)triethoxysilane is used to modify the surfaces of a series of microchannels, and each channel is subsequently exposed to a solution of antibiotic for a few minutes. The antibiotic is retained after washing through electrostatic interactions, and the series of channels are subsequently exposed to an increasing concentration of 5-FAM-(DA)(3) followed by washing to exclude any nonspecific binding. The extent of fluorescence is quantified using a microscope fitted with a CCD camera. The binding constants for the interaction of teicoplanin and vancomycin with the fluorescent peptide were determined to be 6.03 +/- 0.97 x 10(4) and 4.93 +/- 1.13 x 10(4) M(-1), respectively, in good agreement with previous data. The ease of quantifying the extent of interaction in this microchip technique may prove powerful for exploration of a myriad of receptor-ligand pairs.

Application of poly(dimethylsiloxane)/glass microchip for fast electrophoretic separation of serum small, dense low-density lipoprotein

Due to the mounting evidence of altered low-density lipoprotein (LDL) size in several disease states, there has been an increasing interest in developing new analytical methods for small, dense low-density lipoprotein (sdLDL) for diagnosis. The present report demonstrates that sdLDL analysis can be performed in a poly(dimethylsiloxane) (PDMS/glass) microchannel. n-Dodecyl beta-D-maltoside (DDM) was utilized to alter channel surface to make it become hydrophilic and nonionic, thus reducing the interaction between the protein and the surface. Moreover, hydroxypropylcellulose (HPC) was added into the running buffer to suppress the adsorption of analytes and also to serve as a sieving matrix. Under optimal conditions, two baseline separations of lipoproteins including high-density lipoprotein (HDL), sdLDL, and lLDL were achieved with different selectivity. LDL particles shown on the electropherogram were also identified by several procedures. This method affords high separation speed and high reproducibility. The intraassay and interassay RSDs of lipoprotein migration times were in the range of 2.01-2.45%. The variation of serum sdLDL of a patient between prior treatment and post-treatment was assessed by this method. This system has the potential for rapid and sensitive detection of different LDL forms, and thus will be applicable to clinical diagnosis.

AFM study of adsorption of protein A on a poly(dimethylsiloxane) surface

In this paper, the morphology and kinetics of adsorption of protein A on a PDMS surface is studied by AFM. The results of effects of pH, protein concentration and contact time of the adsorption reveal that the morphology of adsorbed protein A is significantly affected by pH and adsorbed surface concentration, in which the pH away from the isoelectric point (IEP) of protein A could produce electrical repulsion to change the protein conformation, while the high adsorbed surface protein volume results in molecular networks. Protein A can form an adsorbed protein film on PDMS with a maximum volume of 2.45 x 10(-3) microm(3). This work enhances our fundamental understanding of protein A adsorption on PDMS, a frequently used substrate component in miniaturized immunoassay devices.

On the role of oxygen in fabricating microfluidic channels with ultraviolet curable materials

We present the effects of oxygen on the irreversible bonding of a microchannel using an ultraviolet (UV) curable material of polyurethane acrylate (PUA). Microchannels were fabricated by bonding a top layer with impressions of a microfluidic channel and a bottom layer consisting of a PUA coating on a glass or a polyethylene terephthalate (PET) film substrate. The resulting channel is a homogeneous conduit of the PUA material. To find optimal bonding conditions, the bottom layer was cured under different oxygen concentration and UV exposure time at a constant UV intensity (10 mW cm(-2)). Our experimental and theoretical studies revealed that the channel bonding is severely affected by the concentration of oxygen either in the form of trapped air or permeated air out of the channel. In addition, an optimal UV exposure time is needed to prevent clogging or non-bonding of the channel.

Engineering of PDMS surfaces for use in microsystems for capture and isolation of complex and biomedically important proteins: epidermal growth factor receptor as a model system

Elastomers based on poly(dimethylsiloxane) (PDMS) are promising materials for fabrication of a wide range of microanalytical systems due to their mechanical and optical properties and ease of processing. To date, however, quantitative studies that demonstrate reliable and reproducible methods for attachment of binding groups that capture complex receptor proteins of relevance to biomedical applications of PDMS microsystems have not been reported. Herein we describe methods that lead to the reproducible capture of a transmembrane protein, the human epidermal growth factor (EGF) receptor, onto PDMS surfaces presenting covalently immobilized antibodies for EGF receptor, and subsequent isolation of the captured receptor by mechanical transfer of the receptor onto a chemically functionalized surface of a gold film for detection. This result is particularly significant because the physical properties of transmembrane proteins make this class of proteins a difficult one to analyze. We benchmark the performance of antibodies to the human EGF receptor covalently immobilized on PDMS against the performance of the same antibodies physisorbed to conventional surfaces utilized in ELISA assays through the use of EGF receptor that was (32)P-radiolabeled in its autophosphorylation domain. These results reveal that two pan-reactive antibodies for the EGF receptor (clones H11 and 111.6) and one phosphospecific EGF receptor antibody (clone pY1068) capture the receptor on both PDMS and ELISA plates. When using H11 antibody to capture EGF receptor and subsequent treatment with a stripping buffer (NaOH and sodium dodecylsulfate) to isolate the receptor, the signal-to-background obtained using the PDMS surface was 82 : 1, exceeding the signal-to-background measured on the ELISA plate (<48 : 1). We also characterized the isolation of captured EGF receptor by mechanical contact of the PDMS surface with a chemically functionalized gold film. The efficiency of mechanical transfer of the transmembrane protein from the PDMS surface was found to be 75-81%. However, the transfer of non-specifically bound protein was substantially less than 75%, thus leading to the important finding that mechanical transfer of the EGF receptor leads to an approximately four-fold increase in signal-to-background from 20 : 1 to 88 : 1. The signal-to-background obtained following mechanical transfer is also better than that obtained using ELISA plates and stripping buffer (<48 : 1). The EGF receptor is a clinically important protein and the target of numerous anticancer agents and thus these results, when combined, provide guidance for the design of PDMS-based microanalytical systems for the capture and isolation of complex and clinically important transmembrane proteins.

Influence of different amount of Au on the wetting behavior of PDMS membrane

Polydimethylsiloxane (PDMS) has been widely used as a base material for bio-MEMS/NEMS devices. It is difficult for PDMS to transfer and spread aqueous solution as a kind of highly hydrophobic material. Therefore, surface modification is necessary for PDMS to make it hydrophilic. In this paper, a method of hydrophilization of PDMS surface is proposed. Gold is sputtered to the PDMS substrate by sputter coater in different average thicknesses. Relationship between the average thickness of gold on the PDMS substrate and the contact angle of the surface was studied. It was found that even gold of average thickness less than 1 nm can result in about 25 degrees change of contact angle. AFM is also used to get topographic information of PDMS surface coated with gold. Three cases are classified with different amount of Au: (1) Heterogeneous zone; (2) Transition zone; (3) Film zone. For heterogeneous zone, a simple model about heterogeneous phase wetting is put forward to interpret this phenomenon.

Virtual walls in microchannels

Microfluidic channels were studied, in which the surface is modified from a solid/liquid interface into solid/liquid and air/liquid alternating interface, creating the equivalent of a superhydrophobic surface on the interior of the channel. The composite microchannel can be easily fabricated using embossing or cast molding of PDMS. The channels are stable under typical microfluidic conditions. For the most part, fluid flow behavior is not significantly changed; however, interesting mass transport effects can be observed in such channels under appropriate conditions. An application example of a microvalve based on the mass transport effect is demonstrated, showing advantages of simple design, fabrication, no moving part and zero dead volume.

Microfluidic biochip for chemiluminescent detection of allergen-specific antibodies

Protein microarrays for allergen-specific antibodies detection were integrated in microfluidic chips, with imaging chemiluminescence as the analytical technique. This paper demonstrates the feasibility of miniaturized chemiluminescent ELISA by presenting rapid, reproducible and sensitive detection of protein antibodies using microfluidics. Three different proteins, beta-lactoglobulin, peanut lectin and human IgG were immobilized via a "macromolecules to polydimethylsiloxane elastomer (PDMS) transfer" protocol and used as capturing agent for the detection of specific antibodies. A convenient and reversible procedure was used to bond the PDMS microarray substrate to complimentary SU-8/glass microfluidic reaction chambers. The hydrodynamic behaviours of the three proteins interactions within the micro-chambers were investigated to select the most efficient flowing parameters (come to terms with the assay time and performances). The use of optimized conditions led to the concomitant detection of three specific antibodies at pM level in 300 microL and using 6 min sample incubation time. Finally, sera from allergic patients were assayed using the microfluidic device modified with apple hazelnut and pollen allergen. The results obtained compared favourably with those obtained with the classical Pharmacia CAP system.

Long-term affinity modification on poly(dimethylsiloxane) substrate and its application for ELISA analysis

Poly(dimethylsiloxane) (PDMS) possesses many advantages, such as biocompatibility and high oxygen permeability, which makes it an attractive material for fabricating biodevices. Creating an affinity surface with long-term stability and reactivity for biomolecular interactions on a PDMS substrate, however, is difficult due to its inherent hydrophobicity. In this study, an affinity surface on a PDMS substrate with long-term hydrophilicity and affinity reactivity is reported. This modification is composed of two parts. The bottom part is made of polyelectrolyte multilayers and is capable of providing long-term hydrophilic stability. The top part consists of three protein layers, bovine serum albumin (BSA), anti-BSA, and protein G, and offers an affinity surface for antibody binding and, more importantly, provides favorable orientation and minimum nonspecific binding. The chemical modification for the different stages was monitored by atomic force microscopy (AFM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FT-IR), and contact angle and fluorescence measurements. A long-term PDMS immunodevice (LPID) based on polyelectrolyte multilayers and protein layers was fabricated and applied to the detection of transforming growth factor beta (TGF-beta) protein in mouse serum by the enzyme-linked immunosorbent assay (ELISA) method. Results show that a linear calibration curve was obtained in the concentration range from 500 to 15.125 pg/mL, and the relative standard deviation was less than 3%. Also, the amount of TGF-beta spiked in mouse serum was precisely determined. Results indicate that the modified surface was hydrophilic and reactive to biospecies up to more than 7 days in its dry form. Moreover, the blocking reagent used to reduce nonspecific binding was found to be not necessary for the LPID. Thus, the reported method is expected to hold a great potential for fabricating PDMS-based affinity devices such as protein chips.

Electrophoretic Effects of the Adsorption of Anionic Surfactants to Poly(dimethylsiloxane)-Coated Capillaries

Poly(dimethylsiloxane) (PDMS) is one of the most convenient materials to construct capillary electrophoresis microchips. Even though PDMS has many advantages, its use is often limited by its hydrophobicity. Although it is well-known that the surface properties of PDMS can be modified by anionic surfactants, very little is known regarding the driving forces or the electrophoretic consequences of the adsorption of anionic surfactants. In this work, the adsorption of alkyl surfactants on PDMS was studied by performing electroosmotic flow (microEOF) measurements. In order to mimic the behavior of PDMS microchannels, fused-silica capillaries were coated with PDMS and used for the microEOF measurements. This approach allowed using standard CE instrumentation and provided significant advantages over similar experiments performed on microchips. The change in the microEOF in the presence of surfactants was correlated to the surfactant adsorbed amount which, plotted versus surfactant concentration, gives an adsorption isotherm. The adsorption isotherms were obtained using alkyl surfactants with different chain lengths and head groups. According to our results, the interaction of alkyl surfactants with the PDMS surface is determined by a combination of hydrophobic and electrostatic interactions, where the former is more significant than the latter. The affinity of each surfactant for the PDMS surface was calculated by fitting the adsorption profiles with a Langmuir equation and, in the case of single-charged surfactants, correlated to the corresponding cmc value.