PDMS Microfluidic Device Research Articles

High‐sensitivity detection of oxytetracycline using light scattering agglutination assay with aptasensor

We present an aptamer-based biosensor (aptasensor) for rapid and high-sensitive detection of oxytetracycline (OTC) antibiotic in PBS inside a Y-channel PDMS microfluidic device. The detection was made by real-time monitoring of the agglutination assay of ssDNA aptamer-conjugated polystyrene latex microspheres with proximity optical fibers. The agglutination assay was performed with serially diluted OTC antibiotic solutions using highly carboxylated polystyrene particles of 920 nm diameter conjugated with OTC-binding ssDNA aptamer. Proximity optical fibers were used to measure the increase in 45° forward light scattering of the aggregated particles by fixing them around the viewing cell of the device with stable angle and distance to the detector. The detection limit was around 100 ppb for the current aptasensor system with the detection time less than 3 min.

Micro-macro hybrid soft-lithography master (MMHSM) fabrication for lab-on-a-chip applications

We present a novel micro-macro hybrid soft-lithography master (MMHSM) fabrication technique where microdevices having both microscale and macroscale features can be replicated with a single soft-lithography step. A poly(methyl methacrylate) (PMMA) master having macroscale structures was first created by a bench-top milling machine. An imprinting master mold having microscale structures was then imprinted on the PMMA surface through a hot-embossing process to obtain a PMMA master mold. A poly(dimethylsiloxane) (PDMS) master was then replicated from this PMMA master through a standard soft-lithography process. This process allowed both microscale (height: 3-20 microm, width: 20-500 microm) and macroscale (height: 3.5 mm, width: 1.2-7 mm) structures to co-exist on the PDMS master mold, from which final PDMS devices could be easily stamped out in large quantities. Microfluidic structures requiring macroscale dimensions in height, such as reservoirs or fluidic tubing interconnects, could be directly built into PDMS microfluidic devices without the typically used manual punching process. This significantly reduced alignment errors and time required for such manual fabrication steps. In this paper, we successfully demonstrated the utility of this novel hybrid fabrication method by fabricating a PDMS microfluidic device with 40 built-in fluidic interfaces and a PDMS multi-compartment neuron co-culture platform, where millimeter-scale compartments are connected via arrays of 20 microm wide and 200 microm long microfluidic channels. The resulting structures were characterized for the integrity of the transferred pattern sizes and the surface roughness using scanning electron microscopy and optical profilometry.

Image-based feedback control for real-time sorting of microspheres in a microfluidic device.

We describe a control system to automatically distribute antibody-functionalized beads to addressable assay chambers within a PDMS microfluidic device. The system used real-time image acquisition and processing to manage the valve states required to sort beads with unit precision. The image processing component of the control system correctly counted the number of beads in 99.81% of images (2689 of 2694), with only four instances of an incorrect number of beads being sorted to an assay chamber, and one instance of inaccurately counted beads being improperly delivered to waste. Post-experimental refinement of the counting script resulted in one counting error in 2694 images of beads (99.96% accuracy). We analyzed a range of operational variables (flow pressure, bead concentration, etc.) using a statistical model to characterize those that yielded optimal sorting speed and efficiency. The integrated device was able to capture, count, and deliver beads at a rate of approximately four per minute so that bead arrays could be assembled in 32 individually addressable assay chambers for eight analytical measurements in duplicate (512 beads total) within 2.5 hours. This functionality demonstrates the successful integration of a robust control system with precision bead handling that is the enabling technology for future development of a highly multiplexed bead-based analytical device.

Microfluidic contact printing: a versatile printing platform for patterning biomolecules on hydrogel substrates

This work describes a printing platform that utilizes components of two complementary techniques—microcontact printing and microfluidic patterning—to provide a high-throughput, reusable tool for patterning biomolecularly responsive chemical features on soft materials. Hydrogel substrates imprinted with biomolecular targets (e.g., proteins or peptides) are of considerable interest because of their utility in diverse bioanalytical applications such as diagnostics and studies of cells in culture. In the system described here, a track-etched polycarbonate membrane is used to seal a PDMS microfluidic channel device to form a print head. Model ‘inks’—solutions of biotin labeled biomolecular targets—are constantly replenished via perfusion of solution through the membrane and captured on a streptavidin-incorporating polyacrylamide hydrogel-coated substrate placed in conformal contact with the print head. The patterns obtained can be controlled through modifications of channel design and secondary programming via selective membrane wetting. Multiple channel designs have been used to pattern three model classes of biomolecular inks (i.e., peptides, polysaccharides, and proteins). Hydrogels patterned with polylysine are used to illustrate a specific biological application for this soft-patterning method, in this case directing in vitro primary mammalian hippocampal neuronal growth. The short cycle time and reproducibility of this soft-patterning technique are highlighted.

Solvent‐Resistant PDMS Microfluidic Devices with Hybrid Inorganic/Organic Polymer Coatings

AbstractThis study presents a method for the fabrication of solvent‐resistant poly(dimethylsiloxane) (PDMS) microfluidic devices by coating the microfluidic channel with a hybrid inorganic/organic polymer (HR4). This modification dramatically increases the resistance of PDMS microfluidic channels to various solvents, because it leads to a significant reduction in the rate of solvent absorption and consequent swelling. The compatibility of modified PDMS with a wide range of solvents is investigated by evaluating the swelling ratio measured through weight changes in a standard block. The HR4‐modified PDMS microfluidic device can be applied to the formation of water‐in‐oil (W/O) and oil‐in‐water (O/W) emulsions. The generation of organic solvent droplets with high monodispersity in the microfluidic device without swelling problems is demonstrated. The advantage of this proposed method is that it can be used to rapidly fabricate microfluidic devices using the bulk properties of PDMS, while also increasing their resistance to various organic solvents. This high compatibility with a variety of solvents of HR4‐modified PDMS can expand the application of microfluidic systems to many research fields.

Microchip CE analysis of amino acids on a titanium dioxide nanoparticles‐coated PDMS microfluidic device with in‐channel indirect amperometric detection

In this paper, titanium dioxide nanoparticles (TiO(2) NPs) were employed to construct a functional film on PDMS microfluidic channel surface, which was formed by sequentially immobilizing poly(diallyldimethylammonium chloride) and TiO(2) NPs on PDMS surface by layer-by-layer assembly technique. The modified PDMS microchip exhibited a decreased and stable EOF, which was favorable for the separation of biomolecules with similar migration times. Arginine, phenylalanine, serine and threonine were used as model analytes to evaluate the performance of the modified microchip. The four amino acids were efficiently separated within 100 s in a 3.7 cm long separation channel and successfully detected on the carbon fiber electrode in conjunction with in-channel indirect amperometry. Resolutions and theoretical plate numbers of the analytes were considerably enhanced in the presence of TiO(2) NPs. The modified microchip demonstrated excellent stability and reproducibility with improved RSDs of migration times and peak currents for run-to-run, day-to-day and chip-to-chip analyses, respectively. Variables influencing the separation efficiency and amperometric response, including injection and separation voltage, the working electrode position and buffer concentration, were optimized in detail.

Targeted cell adhesion on selectively micropatterned polymer arrays on a poly(dimethylsiloxane) surface

Herein, we introduce the fabrication of polymer micropattern arrays on a chemically inert poly(dimethylsiloxane) (PDMS) surface and employ them for the selective adhesion of cells. To fabricate the micropattern arrays, a mercapto-ester-based photocurable adhesive was coated onto a mercaptosilane-coated PDMS surface and photopolymerized using a photomask to obtain patterned arrays at the microscale level. Robust polymer patterns, 380 microm in diameter, were successfully fabricated onto a PDMS surface, and cells were selectively targeted toward the patterned regions. Next, the performance of the cell adhesion was observed by anchoring cell adhesive linker, an RGD oligopeptide, on the surface of the mercapto-ester-based adhesive-cured layer. The successful anchoring of the RGD linker was confirmed through various surface characterizations such as water contact angle measurement, XPS analysis, FT-IR analysis, and AFM measurement. The micropatterning of a photocurable adhesive onto a PDMS surface can provide high structural rigidity, a highly-adhesive surface, and a physical pathway for selective cell adhesion, while the incorporated polymer micropattern arrays inside a PDMS microfluidic device can serve as a microfluidic platform for disease diagnoses and high-throughput drug screening.

Microfluidic hydrogel layers with multiple gradients to stimulate and perfuse three-dimensional neuronal cell cultures

We present a simple and easy to handle PDMS microfluidic device for neuronal cell culture studies in three-dimensional hydrogel scaffolds. The hydrogel is structured in parallel layers to reconstruct cell layers close to the natural environment. Dissociated cortical neurons of embryonic rats have been cultured in 0.5% w/v agarose including 0.2% w/v alginate. The cells formed neurite networks through neighboring cell free hydrogel layers. The cell culture showed neurite outgrowth in the microfluidic channel over more than seven days in vitro without perfusion. Culturing neurons in hydrogel layers surrounded by a liquid phase containing culture medium resulted in denser neuronal networks.

Rapid detection of Mycoplasma pneumonia in a microfluidic device using immunoagglutination assay and static light scattering

We present real-time, rapid detection of Mycoplasma pneumonia in PBS inside a Y-channel PDMS microfluidic device via optical fiber monitoring of latex immunoagglutination. The latex immunoagglutination assay was performed with serially diluted M. pneumonia solutions using highly carboxylated polystyrene particles of 390 and 500 nm diameter conjugated with monoclonal anti-M. pneumonia. Proximity optical fibers were located around the viewing cell of the device, which were used to measure the increase in 45 degrees forward light scattering of the aggregated particles. The detection limit are slightly less than 50 pg mL(-1) both for 390 and 500 nm microspheres and the detection time do not exceed 90 s.

Flow-through functionalized PDMS microfluidic channels with dextran derivative for ELISAs

In this work, a dextran modified PDMS microfluidic ELISA device was fabricated. The dextran functionalization was conducted with a simple, economic and fast flow-through process in a fabricated PDMS microfluidic device, and demonstrated significant enhancement of hydrophilicity and efficient covalent immobilization of proteins on the PDMS microchannel surface. The device was used to simultaneously detect multiple important biomarker IL-5, HBsAg, and IgG, showing a limit of detection of 100 pg mL(-1) and a dynamic range of 5 orders of magnitude, which significantly improved the performance of the reported hydrophobic and plasma-treated hydrophilic PDMS flow-through immunoassay devices. The fabricated PDMS device demonstrated its capability for colorimetric detection of proteins through direct observation by human eyes. Thus, this work not only demonstrates great potential to fabricate an economical and sensitive lab-on-chip system for high throughput screening of various infectious diseases, but also provides an opportunity to develop a portable microfluidic ELISA device via human eye examination for heath point-of-care services.

Surface Treated PDMS by UV-Vis Light for Microfluidic Device

AbstractHydrophobic property of PDMS surface was improved by the 400 W UV-Vis lamp light irradiation in the atmospheric condition for several ten minutes. As a result of this surface treatment, the surface became to hydrophilic character for one month long. This surface treatment technique applied to PDMS micro-fluidic device and verified valve-less switching. The UV-Vis light irradiated to PDMS micro fluidic pattern with partly covered by aluminum foil. Finally inlet and outlets were connected 0.5 mm diameter tubes. The syringe pumps injected the distilled water into the inlet of the PDMS micro channel at the flow rates of 0.5, 5.0, and 50 μl/min for the both width channel. As results of water injection water flowed only the UV-Vis treated channel at the flow rates of 0.5 and 5.0 μl/min. On the other hand, the water flowed for all channels at the flow rate of 50 μl/min. This result was observed from 5.0 μl/min flow again for both width devices which dried by air. These results were occurred by the difference of the flow conductance and wettability. The mechanism of this hyrophilicity of PDMS was reported to form Si-O in the surface by means of glassy surface. From the IR spectra, the Si-O-Si peak shifted to higher wave number for UV-Vis irradiated PDMS than the untreated PDMS comparing with the other IR peaks. This result showed that the Si-O-Si network bonding of PDMS changed to the O-Si-O bonding around the surface.

Multiplexed proteomic sample preconcentration device using surface-patterned ion-selective membrane

In this paper, we report a new method of fabricating a high-throughput protein preconcentrator in poly(dimethylsiloxane) (PDMS) microfluidic chip format. We print a submicron thick ion-selective membrane on the glass substrate by using standard patterning techniques. By simply plasma-bonding a PDMS microfluidic device on top of the printed glass substrate, we can integrate the ion-selective membrane into the device and rapidly prototype a PDMS preconcentrator without complicated microfabrication and cumbersome integration processes. The PDMS preconcentrator shows a concentration factor as high as approximately 10(4) in 5 min. This printing method even allows fabricating a parallel array of preconcentrators to increase the concentrated sample volume, which can facilitate an integration of our microfluidic preconcentrator chip as a signal enhancing tool to various detectors such as a mass spectrometer.

A PDMS viscometer for microliter Newtonian fluid

This paper describes a poly(dimethylsiloxane) (PDMS) microfluidic device for measuring the viscosity of Newtonian fluids. The viscometer utilized the high solubility and permeability of air in PDMS to generate Poiseuille flow in the degassed PDMS microfluidic device. By measuring the distance the fluids traveled and the flow velocity in the PDMS microchannel, the ratio of the viscosity of the sample fluid to the viscosity of a reference fluid was determined, and the viscosity of the sample fluid was then obtained. Only 5 µL or less volume was consumed for the viscosity measurement. For most of the tested fluids, the results were in good agreement with the results from Ubbelohde viscometers, and the coefficients of variance were 3% or better. The wettability of the fluids on PDMS did not affect the measurement as the capillary forces were cancelled out in the data analysis. The PDMS viscometer was found applicable to a broad range of fluids, including aqueous solutions, non-PDMS-swelling organic solvents, fluorinated oil and blood plasma.

Electrohydrodynamic generation and delivery of monodisperse picoliter droplets using a poly(dimethylsiloxane) microchip.

We developed a drop-on-demand microdroplet generator for the discrete dispensing of biosamples into a bioanalytical unit. This disposable PDMS microfluidic device can generate monodisperse droplets of picoliter volume directly out of a plane sidewall of the microfluidic chip by an electrohydrodynamic mechanism. The droplet generation was accomplished without using either an inserted capillary or a monolithically built-in tip. The minimum droplet volume was approximately 4 pL, and the droplet generation was repeatable and stable for at least 30 min, with a typical variation of less than 2.0% of drop size. The Taylor cone, which is usually observed in electrospray, was suppressed by controlling the surface wetting property of the PDMS device as well as the surface tension of the sample liquids. A modification of the channel geometry right before the opening of the microchannel also enhanced the continuous droplet generation without applying any external pumping. A simple numerical simulation of the droplet generation verified the importance of controlling the surface wetting conditions for the droplet formation. Our microdroplet generator can be effectively applied to a direct interface of a microfluidic chip to a biosensing unit, such as AMS, MALDI-MS or protein microarray-type biochips.

A microfluidic flow distributor generating stepwise concentrations for high-throughput biochemical processing

In this paper, we describe a microfluidic device in which solutions with stepwise concentrations can be accurately generated by continuously introducing two kinds of miscible liquids from each inlet, and biochemical processing can be conducted at the various conditions. Introduced liquid flows are geometrically divided into a number of downstream flows through multiple distribution channels, and each divided flow is then mixed with the divided flow of another liquid at a confluent point. The lengths of the precisely designed distribution channels determine the mixing ratio of the two liquids, without the influence of flow rate. In this study, a PDMS microfluidic device able to generate nine different concentrations was fabricated, and the performance of this device was estimated via colorimetric assay. As a biological application of this device, cell cultivation was performed under different concentration conditions. Due to its simplicity of operation, this microfluidic flow distributor will be applied to various kinds of biological analysis and screening systems.

On-Chip Transformation of Bacteria

On-chip transformation of Escherichia coli cells was accomplished for the first time using a microbial array chip. The continuous E. coli transformation procedures were performed on a chip in which the microcompartment was composed of PDMS microfluidic channels and a silicon substrate predeposited with different plasmid DNAs. The PDMS microfluidic device enabled the parallel transformation of E. coli cells with various plasmid DNAs by separating each transformation area. The phenotypic differences reflecting different plasmid DNAs were identified by various approaches such as colorimetry, fluorometry, and electrochemical methods. This microbial array chip could become a versatile tool for many cell biological applications.

Heterogeneous integration of CdS filters with GaN LEDs for fluorescence detection microsystems

Microassembly of a hybrid fluorescence detection microsystem by heterogeneous integration of a CdS thin-film filter, an (In, Ga)N thin-film blue LED, and a disposable PDMS microfluidic device onto a Si PIN photodetector substrate is described. The CdS thin film filter was deposited directly onto a photodetector by pulsed-laser deposition. A thin-film (In, Ga)N LED was then transferred by a novel “pixel-to-point” laser lift-off process from the sapphire growth substrate to the silicon photodetector substrate. The final integration step was achieved by positioning a disposable polymer microfludic device onto the excitation/detection subsystem. Pixel-to-point transfer is potentially an enabling microassembly process for the fabrication of multicolor fluorescence-based bioassays and chemical detection microsystems.

Development and characterization of an ELISA assay in PDMS microfluidic channels

This report describes the first preliminary evaluation of a heterogeneous sandwich enzyme-linked immunoassay in a PDMS microfluidic device. The PDMS devices were fabricated using replica molding against a patterned photoresist generated by photolithographic techniques. With this experimental setup, the microfluidic sensor chip was successfully used to quantify a model analyte (sheep IgM) with sensitivity down to 17 nM. The conventional blocking cocktail used in nearly all polystyrene microtiter plate-based ELISA assays failed to block the nonspecific adsorption of the analyte and secondary antibody. This report describes the successful use of surface modification chemistries and a modified blocking cocktail to reduce background due to nonspecific adsorption and to thereby increase the sensitivity of the assay. These protocols can be extended to the detection of a variety of analytes by immunoassay in PDMS microchannels.