Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)

Cell culture models in microfluidic systems

Microfluidic technology holds great promise for the creation of advanced cell culture models. In this review, we discuss the characterization of cell culture in microfluidic systems, describe important biochemical and physical features of the cell microenvironment, and review studies of microfluidic cell manipulation in the context of these features. Finally, we consider the integration of analytical elements, ways to achieve high throughput, and the design constraints imposed by cell biology applications.

Microfluidic pump powered by self-organizing bacteria

Results are presented that demonstrate the successful use of live bacteria as mechanical actuators in microfabricated fluid systems. The flow deposition of bacteria is used to create a motile bacterial carpet that can generate local fluid motion inside a microfabricated system. By tracking the motion of tracer particles, we demonstrate that the bacterial cells that comprise the carpet self-organize, generating a collective fluid motion that can pump fluid autonomously through a microfabricated channel at speeds as high as 25 microm s(-1). The pumping performance of the system can also be augmented by changing the chemical environment. The addition of glucose to the working buffer raises the metabolic activity of the bacterial carpet, resulting in increased pumping performance. The performance of the bacterial pump is also shown to be strongly influenced by the global geometry of the pump, with narrower channels achieving a higher pumping velocity with a faster rise time.

Remotely powered distributed microfluidic pumps and mixers based on miniature diodes

We demonstrate new principles of microfluidic pumping and mixing by electronic components integrated into a microfluidic chip. The miniature diodes embedded into the microchannel walls rectify the voltage induced between their electrodes from an external alternating electric field. The resulting electroosmotic flows, developed in the vicinity of the diode surfaces, were utilized for pumping or mixing of the fluid in the microfluidic channel. The flow velocity of liquid pumped by the diodes facing in the same direction linearly increased with the magnitude of the applied voltage and the pumping direction could be controlled by the pH of the solutions. The transverse flow driven by the localized electroosmotic flux between diodes oriented oppositely on the microchannel was used in microfluidic mixers. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows. The techniques may be used in novel actively controlled microfluidic-electronic chips.

Flexible microarray construction and fast DNA hybridization conducted on a microfluidic chip for greenhouse plant fungal pathogen detection

This study employed a microfluidic method in which probe creation does not require pin-spotting and fast hybridization is conducted on the same microarray chip for the detection of three greenhouse pathogens ( Botrytis cinerea, Didymella bryoniae, and Botrytis squamosa). In this method, 16 oligonucleotide probe line arrays were created on a glass substrate by a microfluidic printing method. Then, low amounts of the DNA samples (1 fmol of oligonucelotides or 1.4 ng of PCR products) were introduced into the microchannels that were orthogonal to these probe lines. The hybridizations of 16 samples (21-mer complementary oligonuleotides and approximately 260 bp PCR products) were fulfilled at the channel-probe line intersections and in a short time (minutes). The optimization of probe immobilization and sample hybridization are described in detail. The method successfully detected and discriminated between two 260 bp PCR products with a one-base-pair difference from closely related greenhouse plant fungal pathogens (B. cinerea and B. squamosa).

Detection of kinase translocation using microfluidic electroporative flow cytometry

Directed localization of kinases within cells is important for their activation and involvement in signal transduction. Detection of these events has been largely carried out based on imaging of a low number of cells and subcellular fractionation/Western blotting. These conventional techniques either lack the high throughput desired for probing an entire cell population or provide only the average behaviors of cell populations without information from single cells. Here we demonstrate a new tool, referred to as microfluidic electroporative flow cytometry, to detect the translocation of an EGFP-tagged tyrosine kinase, Syk, to the plasma membrane in B cells at the level of the cell population. We combine electroporation with flow cytometry and observe the release of intracellular kinase out of the cells during electroporation. We found that the release of the kinase was strongly influenced by its subcellular localization. Cells stimulated through the antigen receptor have a fraction of the kinase at the plasma membrane and retain more kinase after electroporation than do cells without stimulation and translocation. We are able to differentiate a cell population with translocation from one without it with the information collected from individual cells of the entire population. This technique potentially allows detection of protein translocation at the single-cell level. Due to the frequent involvement of kinase translocations in disease processes such as oncogenesis, our approach will have utility for kinase-related drug discovery and tumor diagnosis and staging.

Cascaded free-flow isoelectric focusing for improved focusing speed and resolution

This work presents the first implementation of cascaded stages for a microfabricated free-flow isoelectric focusing (FF-IEF) device. Both analytical and computational models for IEF suggest device performance will be improved by utilizing multiple stages to reduce device residence time. These models are shown to be applicable by using focusing of small IEF markers as a demonstration. We also show focusing of fluorescently tagged proteins under different channel geometries, with the most efficient focusing occurring in the cascaded design, as predicted by theory. An additional aim of this work is to demonstrate the compatibility of cascaded FF-IEF with common bioanalytical tools. As an example, outlet fractions from cascaded FF-IEF were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Processing of whole cell lysate followed by immunoblotting for cell signaling markers demonstrates the reduction of albumin from samples, as well as the enrichment of apoptotic markers.

Sickle cell vasoocclusion and rescue in a microfluidic device

The pathophysiology of sickle cell disease is complicated by the multiscale processes that link the molecular genotype to the organismal phenotype: hemoglobin polymerization occurring in milliseconds, microscopic cellular sickling in a few seconds or less [Eaton WA, Hofrichter J (1990) Adv Protein Chem 40:63-279], and macroscopic vessel occlusion over a time scale of minutes, the last of which is necessary for a crisis [Bunn HF (1997) N Engl J Med 337:762-769]. Using a minimal but robust artificial microfluidic environment, we show that it is possible to evoke, control, and inhibit the collective vasoocclusive or jamming event in sickle cell disease. We use a combination of geometric, physical, chemical, and biological means to quantify the phase space for the onset of a jamming event, as well as its dissolution, and find that oxygen-dependent sickle hemoglobin polymerization and melting alone are sufficient to recreate jamming and rescue. We further show that a key source of the heterogeneity in occlusion arises from the slow collective jamming of a confined, flowing suspension of soft cells that change their morphology and rheology relatively quickly. Finally, we quantify and investigate the effects of small-molecule inhibitors of polymerization and therapeutic red blood cell exchange on this dynamical process. Our experimental study integrates the dynamics of collective processes associated with occlusion at the molecular, polymer, cellular, and tissue level; lays the foundation for a quantitative understanding of the rate-limiting processes; and provides a potential tool for optimizing and individualizing treatment, and identifying new therapies.

Matrix-dependent adhesion of vascular and valvular endothelial cells in microfluidic channels

The interactions between endothelial cells and the underlying extracellular matrix regulate adhesion and cellular responses to microenvironmental stimuli, including flow-induced shear stress. In this study, we investigated the adhesion properties of primary porcine aortic endothelial cells (PAECs) and valve endothelial cells (PAVECs) in a microfluidic network. Taking advantage of the parallel arrangement of the microchannels, we compared adhesion of PAECs and PAVECs to fibronectin and type I collagen, two prominent extracellular matrix proteins, over a broad range of concentrations. Cell spreading was measured morphologically, based on cytoplasmic staining with a vital dye, while adhesion strength was characterized by the number of cells attached after application of shear stresses of 11, 110, and 220 dyn cm(-2). Results showed that PAVECs were more well spread on fibronectin than on type I collagen (P < 0.0001), particularly for coating concentrations of 100, 200, and 500 microg mL(-1). PAVECs also withstood shear significantly better on fibronectin than on collagen for 500 microg mL(-1). PAECs were more well spread on collagen compared to PAVECs (P < 0.0001), but did not have significantly better adhesion strength. These results demonstrate that cell adhesion is both cell-type and matrix dependent. Furthermore, they reveal important phenotypic differences between vascular and valvular endothelium, with implications for endothelial mechanobiology and the design of microdevices and engineered tissues.

A novel metal-protected plasma treatment for the robust bonding of polydimethylsiloxane

We describe a method for the irreversible bonding of PDMS-based microfluidic components by exploiting the first reported "shelfable" plasma treatment of PDMS. Simultaneous plasma activation and protection of PDMS surfaces are achieved via RF magnetron sputtering of thin aluminium films in the presence of an argon plasma. In this process, Ar plasma exposure generates a hydrophilic, silanol-enriched polymer surface amenable to irreversible bonding to glass, PDMS or silicon substrates, while the aluminium film functions as a capping layer to preserve the surface functionality over several weeks of storage in ambient conditions. Prior to bonding, this protective aluminium layer is removed by immersion in an aqueous etchant, exposing the adhesive surface. Employing this technology, PDMS-glass and PDMS-PDMS microfluidic devices were fabricated and the adhesive strength was quantified by tensile and leakage testing. Bonding success rates in excess of 80% were demonstrated for both PDMS-glass and PDMS-PDMS assemblies sealed 24 h and 7 days following initial polymer surface activation. PDMS-glass microdevices performed optimally, displaying maximum adhesive strengths on the order of 5 MPa and burst flow rates of approximately 1 mL min(-1) (channel dimensions: l = 25 mm; w = 300 microm; h = 20 microm). These data demonstrate a significant improvement in performance over previously reported bonding technologies, resulting in the production of more robust, longer-lasting microfluidic systems that can withstand higher pressures and flow rates.

Integrated multilayer microfluidic device with a nanoporous membrane interconnect for online coupling of solid-phase extraction to microchip electrophoresis

An integrated microfluidic device was developed for online coupling of solid-phase extraction to microchip electrophoresis (chip SPE-CE). With a nanoporous membrane sandwiched between two PDMS substrates, SPE preconcentration and electrophoretic separation can be carried out in upper and lower fluidic layers, separately and sequentially. During the SPE process, the thin membrane can act as a fluid isolator to prevent intermixing between two fluidic channels. However, when a pulse voltage is applied, the membrane becomes a gateable interconnect so that a small plug of concentrated analytes can be online injected into the lower channel for subsequent separations. This multilayer design provides a universal solution to online SPE-CE hyphenation. Both electroosmotic flow and hydrodynamic pumps have been adopted for SPE operation. SPE was performed on a 2.5 mm long microcolumn, with two weirs on both sides to retain the C(18)-coated silica beads. Rhodamine 123 and FITC-labelled ephedrine were used to test the operational performance of the hyphenation system. High separation efficiency and thousand-fold signal enhancement were achieved.

Pumping fluids in microfluidic systems using the elastic deformation of poly(dimethylsiloxane)

This paper demonstrates a methodology for storing and pumping fluids that provide a useful capability for microfluidic devices. It uses microfluidic screw valves to isolate fluids in poly(dimethylsiloxane) (PDMS) microcompartments, in which the pressure of the liquid is stored in the elastic deformation of the walls and ceiling of the compartments. Fluids can be stored under pressure in these structures for months. When the valves are opened, the walls and ceiling push the fluid out of the compartments into microfluidic channels. The system has five useful characteristics: (i) it is made using soft lithographic techniques; (ii) it allows multiple reagents to be preloaded in devices and stored under pressure without any additional user intervention; (iii) it makes it possible to meter out fluids in devices, and to control rates of flow of fluids; (iv) it prevents the user from exposure to potentially toxic reagents; and (v) it is hand-operated and does not require additional equipment or resources.

Cell-based high content screening using an integrated microfluidic device

High content screening (HCS) has quickly established itself as a core technique in the early stage of drug discovery for secondary compound screening. It allows several independent cellular parameters to be measured in a single cell or populations of cells in a single assay. In this work, we describe high content screening for the multiparametric measurement of cellular responses in human liver carcinoma (HepG2) cells using an integrated microfluidic device. This device consists of multiple drug gradient generators and parallel cell culture chambers, in which the processes of liquid dilution and diffusion, micro-scale cell culture, cell stimulation and cell labeling can be integrated into a single device. The simple assay provides multiparametric measurements of plasma membrane permeability, nuclear size, mitochondrial transmembrane potential and intracellular redox states in anti-cancer drug-induced apoptosis of HepG2 cells. The established platform is able to rapidly extract the maximum of information from tumor cells in response to several drugs varying in concentration, with minimal sample and less time, which is very useful for basic biomedical research and cancer treatment.

Development of a microfluidic device for determination of cell osmotic behavior and membrane transport properties

An understanding of cell osmotic behavior and membrane transport properties is indispensable for cryobiology research and development of cell-type-specific, optimal cryopreservation conditions. A microfluidic perfusion system is developed here to measure the kinetic changes of cell volume under various extracellular conditions, in order to determine cell osmotic behavior and membrane transport properties. The system is fabricated using soft lithography and is comprised of microfluidic channels and a perfusion chamber for trapping cells. During experiments, rat basophilic leukemia (RBL-1 line) cells were injected into the inlet of the device, allowed to flow downstream, and were trapped within a perfusion chamber. The fluid continues to flow to the outlet due to suction produced by a Hamilton Syringe. Two sets of experiments have been performed: the cells were perfused by (1) hypertonic solutions with different concentrations of non-permeating solutes and (2) solutions containing a permeating cryoprotective agent (CPA), dimethylsulfoxide (Me(2)SO), plus non-permeating solute (sodium chloride (NaCl)), respectively. From experiment (1), cell osmotically inactive volume (V(b)) and the permeability coefficient of water (L(p)) for RBL cells are determined to be 41% [n=18, correlation coefficient (r(2)) of 0.903] of original/isotonic volume, and 0.32+/-0.05 microm/min/atm (n=8, r(2)>0.963), respectively, for room temperature (22 degrees C). From experiment (2), the permeability coefficient of water (L(p)) and of Me(2)SO (P(s)) for RBL cells are 0.38+/-0.09 microm/min/atm and (0.49+/-0.13) x 10(-3)cm/min (n=5, r(2)>0.86), respectively. We conclude that this device enables us to: (1) readily monitor the changes of extracellular conditions by perfusing single or a group of cells with prepared media; (2) confine cells (or a cell) within a monolayer chamber, which prevents imaging ambiguity, such as cells overlapping or moving out of the focus plane; (3) study individual cell osmotic response and determine cell membrane transport properties; and (4) reduce labor requirements for its disposability and ensure low manufacturing costs.

Compact microfluidic structures for generating spatial and temporal gradients

We present an improved microfluidic design for generating spatial and temporal gradients. The basic functional elements are bifurcated and trifurcated channels used to split flow between two and three channels, respectively. We use bifurcated channels on the exterior of the channel manifold and trifurcated channels in the interior with mixing tees to recombine flows. For N gradient-forming levels, the number of discrete steps in the gradient is 2(N) + 1, allowing a compact gradient-forming structure that is only 1.6 mm long and 0.5 mm wide. Control of the relative sample concentration at the inlets enables generation of gradients with varying slopes and offsets. The small total channel length allows faster switching (only 2.6 s) between gradients of different compositions than did previous designs, allowing complex temporal sequences and reducing total displacement volume and reagent use. The design permits opposing-gradient experiments and generation of complex nonlinear gradients. We fabricated and tested three channel designs with either three or four gradient-forming levels, 20- or 40-microm channel widths, 60- or 120-microm center-to-center channel spacings, and 9 or 17 output steps. These devices produced essentially identical high-quality linear gradients using both pressure-driven and electrokinetic flow.

Bulk modification of PDMS microchips by an amphiphilic copolymer

A simple and rapid bulk-modification method based on adding an amphiphilic copolymer during the fabrication process was employed to modify PDMS microchips. Poly(lactic acid)-poly(ethylene glycol) (PLA-PEG) was used as the additive substance. Compared to the native PDMS microchips, both the contact angle and the EOF of the bulk-modified PDMS microchips decreased. The effects of the additive loading and the pH on the EOF were investigated in detail. The bulk-modified PDMS microchips exhibited reproducible and stable EOF behavior. The application of the bulk-modified PDMS microchips was also studied and the results indicated that they could be successfully used to separate amino acids and to suppress protein adsorption.

Flow-focusing generation of monodisperse water droplets wrapped by ionic liquid on microfluidic chips: from plug to sphere

Generating droplets via microfluidic chips is a promising technology in microanalysis and microsynthesis. To realize room-temperature ionic liquid (IL)-water two-phase studies in microscale, a water-immiscible IL was employed as the continuous phase for the first time to wrap water droplets (either plugs or spheres) on flow-focusing microfluidic chips. The IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), could wet both hydrophilic and hydrophobic channel surfaces because of its dual role of hydrophilicity/hydrophobicity and extremely high viscosity, thus offering the possibility of wrapping water droplets in totally hydrophilic (THI), moderately hydrophilic (MHI), and hydrophobic (HO) channels. The droplet shape could be tuned from plug to sphere, with the volume from 6.3 nL to 65 pL, by adding an orifice in the focusing region, rendering the hydrophilic channel surface hydrophobic, and suppressing the Uw/UIL ratio below 1.0. Three different breakup processes were defined and clarified, in which the sub-steady breakup and steady breakup were essential for the formation of plugs and spheric droplets, respectively. The influences of channel hydrophilicity/hydrophobicity on droplet formation were carefully studied by evaluating the wetting abilities of water and IL on different surfaces. The superiority of IL over water in wetting hydrophobic surface led to the tendency of forming small, spheric aqueous droplets in the hydrophobic channel. This IL-favored droplet-based system represented a high efficiency in water/IL extraction, in which rhodamine 6G was extracted from aqueous droplets to [BMIM][PF6] in the hydrophobic orifice-included (HO-OI) channel in 0.51 s.

Guidance of actin filament elongation on filament-binding tracks

Biomolecular motors, which convert chemical energy into mechanical work in intracellular processes, have high potential in bionanotechnology in vitro as molecular shuttles or nanoscale actuators. In this context, guided elongation of actin filaments in vitro could be used to lay tracks for myosin motor-based shuttles or to direct nanoscale actuators based on actin filament end-tracking motors. To guide the direction of filament polymerization on surfaces, microcontact printing was used to create tracks of chemically modified myosin, which binds to, but cannot exert force on, filaments. These filament-binding tracks captured nascent filaments from solution and guided the direction of their subsequent elongation. The effect of track width and protein surface density on filament alignment and elongation rate was quantified. These results indicate that microcontact printing is a useful method for guiding actin filament polymerization in vitro for biomolecular motor-based applications.

Flow rate analysis of a surface tension driven passive micropump

A microfluidic passive pumping method relying on surface tension properties is investigated and a physical model is developed. When a small inlet drop is placed on the entrance of a microfluidic channel it creates more pressure than a large output drop at the channel exit, causing fluid flow. The behavior of the input drop occurs in two characteristic phases. An analytical solution is proposed and verified by experimental results. We find that during the first phase the flow rate is stable and that this phase can be prolonged by refilling the inlet drop to produce continuous flow in the microchannel.

Microfluidic logic gates and timers

We use surface tension-based passive pumping and fluidic resistance to create a number of microfluidic analogs to electronic circuit components. Three classes of components are demonstrated: (1) OR/AND, NOR/NAND, and XNOR digital microfluidic logic gates; (2) programmable, autonomous timers; and (3) slow, perfusive flow rheostats. The components can be implemented with standard pipettes and provide a means of non-electronic and autonomous preprogrammed control with potential utility in cell studies and high throughput screening applications.

Combining multiple optical trapping with microflow manipulation for the rapid bioanalytics on microparticles in a chip

An array of four independent laser traps is combined with a polydimethylsiloxane microfluidic chip to form a very compact system allowing parallel processing of biological objects. Strong three dimensional trapping allows holding objects such as functionalized beads in flows at speeds near 1 mm/s, enabling rapid processing. By pressure control of the inlet flows, the trapped objects can be put in contact with different solutions for analysis purpose. This setup, including a fluorescence excitation-detection scheme, offers the potential to perform complex biochemical manipulations on an ensemble of microparticles.

Monolithic Teflon membrane valves and pumps for harsh chemical and low-temperature use

Microfluidic diaphragm valves and pumps capable of surviving conditions required for unmanned spaceflight applications have been developed. The Pasteur payload of the European ExoMars Rover is expected to experience temperatures ranging between -100 degrees C and +50 degrees C during its transit to Mars and on the Martian surface. As such, the Urey instrument package, which contains at its core a lab-on-a-chip capillary electrophoresis analysis system first demonstrated by Mathies et al., requires valving and pumping systems that are robust under these conditions before and after exposure to liquid samples, which are to be analyzed for chemical signatures of past or present living processes. The microfluidic system developed to meet this requirement uses membranes consisting of Teflon and Teflon AF as a deformable material in the valve seat region between etched Borofloat glass wafers. Pneumatic pressure and vacuum, delivered via off-chip solenoid valves, are used to actuate individual on-chip valves. Valve sealing properties of Teflon diaphragm valves, as well as pumping properties from collections of valves, are characterized. Secondary processing for embossing the membrane against the valve seats after fabrication is performed to optimize single valve sealing characteristics. A variety of different material solutions are found to produce robust devices. The optimal valve system utilizes a membrane of mechanically cut Teflon sandwiched between two thin spun films of Teflon AF-1600 as a composite "laminated" diaphragm. Pump rates up to 1600 nL s(-1) are achieved with pumps of this kind. These high pumping rates are possible because of the very fast response of the membranes to applied pressure, enabling extremely fast pump cycling with relatively small liquid volumes, compared to analogous diaphragm pumps. The developed technologies are robust over extremes of temperature cycling and are applicable in a wide range of chemical environments.

Development of an on-chip injector for microchip-based flow analyses using laminar flow

A new on-chip injector for microchip-based flow analyses has been designed and characterized. The microchip design utilizes separate laminar flow streams of buffer and sample that are brought into parallel contact for a distance of 300 microm. The buffer flow stream is first routed through a conventional 6-port injection valve fitted with a 5 microm i.d. sample loop. When the 6-port valve is actuated from load to inject for a given time, the on-chip buffer flow stream is constricted and the sample flow stream is pressurized into the buffer flow channel. Once the valve returns to the load state the separate laminar flow streams resume. Fluorescence detection was used to characterize the injector and it was found that 50 injections of a 100 microM fluorescein sample led to an average peak height of 174.32 +/- 2.05 AFU (RSD 1.18%) and average peak skew of 1.37 +/- 0.06. The injector was also interfaced with amperometric detection. Injections of catechol solutions ranging in concentration from 500 nM to 100 microM resulted in a linear response (sensitivity = 2.49 pA microM(-1), r(2) = 0.998) and a limit of detection of 155 nM (S/N = 3). Compared to an off-chip injection scheme, plug dilution, band broadening, and peak asymmetry are much reduced. Finally, the injection and subsequent lysis of an erythrocyte sample was demonstrated, with an injected plug of erythrocytes being lysed 5.72 +/- 0.15 s after injection into a flow stream containing sodium dodecyl sulfate (n = 10). The new injection scheme does not require complex valving mechanisms or high pressures and enables reproducible injections from a continuous sample flow stream in a manner where changes in analyte concentration can be monitored with high temporal resolution.

Using pattern homogenization of binary grayscale masks to fabricate microfluidic structures with 3D topography

Because fluids at the microscale form three dimensional interfaces and are subject to three dimensional forces, the ability to create microstructures with modulated topography over large areas could greatly improve control over microfluidic phenomena (e.g., capillarity and mass transport) and enable exciting novel microfluidic applications. Here, we report a method for the fabrication of three-dimensional relief microstructures, based on the emergence of smooth features when a photopolymer is exposed to UV light through a transparency mask with binary motifs. We show that homogeneous features emerge under certain critical conditions that are also common to other, apparently unrelated, phenomena such as the emergence of macroscopic continuum properties of composite materials and the rates of ligand binding to cell membrane receptors. This fabrication method is simple and inexpensive, and yet it allows for the fabrication of microstructures over large areas (centimetres) with topographic modulation of features with characteristic dimensions smaller than 100 micrometres.

Development of an integrated direct-contacting optical-fiber microchip with light-emitting diode-induced fluorescence detection

In this paper, one poly(dimethylsiloxane) (PDMS) sandwich microchip integrated with one direct-contacting optical fiber was fabricated by using a thin-casting method. This novel integrated PDMS sandwich microchip included top glass plate, PDMS membrane replica with microfluidic networks and optical fiber, flat PDMS membrane and bottom glass plate. As the tip of excitation optical fiber completely contacted with the separation microchannel in this integrated microchip, it not only increased the excitation light intensity to achieve the high sensitivity, but also reduced the diameter of excitation beam to obtain high resolution. In addition, we found that this rigid PDMS sandwich microchip structure effectively prevented PDMS microchannel distortion from rigid optical fiber, and provided a substantial convenience for microchips manipulating. A blue light-emitting diode (LED) was applied as excitation source by using optical fiber to couple excitation light into its direct-contacting microchannel for fluorescence detection. The performances of this integrated PDMS sandwich microchip was demonstrated by separating the mixture of sodium fluorescein (SF) and fluorescein isothiocyanate isomer I (FITC), and showed a higher sensitive and resolution than those obtained from the conventional integrated optical-fiber PDMS microchip with a 100-microm distance between fiber tip and separation microchannel. Additionally, the reproducibility of this integrated microchip with LED-induced fluorescence detection was also examined by separation of a mixture of FITC-labeled amino acids.

Mass spectrometric imaging of peptide release from neuronal cells within microfluidic devices

Microfluidic devices are well suited for manipulating and measuring mass limited samples. Here we adapt a microfluidic device containing functionalized surfaces to chemically stimulate a small number of neurons (down to a single neuron), collect the release of neuropeptides, and characterize them using mass spectrometry. As only a small fraction of the peptides present in a neuron are released with physiologically relevant stimulations, the amount of material available for measurement is small, thereby requiring minimal sample loss and high-sensitivity detection. Although a number of detection schemes are used with microfluidic devices, mass spectrometric detection is used here because of its high information content, allowing the characterization of the released peptide complement. Rather than using an on-line approach, off-line analysis is used; after collection of the peptides onto a surface, mass spectrometric imaging interrogates that surface to determine the peptides released from the cell. The overall utility of this scheme is demonstrated using several device formats with measurement of neuropeptides released from Aplysia californica bag cell neurons.

Self-assembled epoxy-modified polymer coating on a poly(dimethylsiloxane) microchip for EOF inhibition and biopolymers separation

A straightforward approach to generate a stable and protein-resistant poly(dimethylsiloxane) (PDMS) surface using self-assembled hydrophilic polymers is demonstrated in this work. Epoxy-modified polymers were directly adsorbed from aqueous solution onto plasma oxidized PDMS based on H-bond interaction, and epoxies of polymer and silanols on oxidized PDMS surface were crosslinked by heating at 110 degrees C. The coating process could be completed within half hour. Poly(dimethylacrylamide-co-glycidyl methacrylate) (PDMA-co-GMA), poly(vinyl pyrrolidone)-g-glycidyl methacrylate (PVP-g-GMA) and poly(vinyl alcohol)-g-glycidyl methacrylate (PVA-g-GMA) (D. P. Wu, B. X. Zhao, Z. P. Dai, J. H. Qin and B. C. Lin, Lab Chip, 2006, 6, 942) were employed as examples here. Unlike PDMA, PVP, and PVA themselves, these epoxy-modified hydrophilic polymers could be directly used as static surface coatings on oxidized PDMS, and inhibited electroosmotic flow (EOF) within pH 3-11. It was also found that hard baking of PDMS at 150 degrees C for 24 hours before surface coating could greatly retard surface hydrophobicity recovery after oxygen plasma exposure, which strengthened epoxy-modified polymer coatings on oxidized PDMS surface, and resulted in EOF less than 0.2 x 10(-4) cm(2) V(-1) s(-1) (pH 9.0) within two weeks. On epoxy-modified polymer coated PDMS microchips, basic proteins, peptides and DNA fragments could be separated satisfactorily, in which more than 2 x 10(4) plates per 2 cm and less than 3% RSD (>8 runs) for migration time were obtained for lysozyme.

Microfluidic device for the discrimination of single-nucleotide polymorphisms in DNA oligomers using electrochemically actuated alkaline dehybridization

This work describes an integrated microfluidic (mu-fl) device that can be used to effect separations that discriminate single-nucleotide polymorphisms (SNP) based on kinetic differences in the lability of perfectly matched (PM) and mismatched (MM) DNA duplexes during alkaline dehybridization. For this purpose a 21-base single-stranded DNA (ssDNA) probe sequence was immobilized on agarose-coated magnetic beads, that in turn can be localized within the channels of a poly(dimethylsiloxane) microfluidic device using an embedded magnetic separator. The PM and MM ssDNA targets were hybridized with the probe to form a mixture of PM and MM DNA duplexes using standard protocols, and the hydroxide ions necessary for mediating the dehybridization were generated electrochemically in situ by performing the oxygen reduction reaction (ORR) using O2 that passively permeates the device at a Pt working electrode (Pt-WE) embedded within the microfluidic channel system. The alkaline DNA dehybridization process was followed using fluorescence microscopy. The results of this study show that the two duplexes exhibit different kinetics of dehybridization, rate profiles that can be manipulated as a function of both the amount of the hydroxide ions generated and the mass-transfer characteristics of their transport within the device. This system is shown to function as a durable platform for effecting hybridization/dehybridization cycles using a nonthermal, electrochemical actuation mechanism, one that may enable new designs for lab-on-a-chip devices used in DNA analysis.

Optofluidic trapping and transport on solid core waveguides within a microfluidic device

In this work we demonstrate an integrated microfluidic/photonic architecture for performing dynamic optofluidic trapping and transport of particles in the evanescent field of solid core waveguides. Our architecture consists of SU-8 polymer waveguides combined with soft lithography defined poly(dimethylsiloxane) (PDMS) microfluidic channels. The forces exerted by the evanescent field result in both the attraction of particles to the waveguide surface and propulsion in the direction of optical propagation both perpendicular and opposite to the direction of pressure-driven flow. Velocities as high as 28 mum/s were achieved for 3 mum diameter polystyrene spheres with an estimated 53.5 mW of guided optical power at the trapping location. The particle-size dependence of the optical forces in such devices is also characterized.

In-channel atom-transfer radical polymerization of thermoset polyester microfluidic devices for bioanalytical applications

A new technique for polymer microchannel surface modification, called in-channel atom-transfer radical polymerization, has been developed and applied in the surface derivatization of thermoset polyester (TPE) microdevices with poly(ethylene glycol) (PEG). X-ray photoelectron spectroscopy, electroosmotic flow (EOF), and contact angle measurements indicate that PEG has been grafted on the TPE surface. Moreover, PEG-modified microchannels have much lower and more pH-stable EOF, more hydrophilic surfaces and reduced nonspecific protein adsorption. Capillary electrophoresis separation of amino acid and peptide mixtures in these PEG-modified TPE microchips had good reproducibility. Phosducin-like protein and phosphorylated phosducin-like protein were also separated to measure the phosphorylation efficiency. Our results indicate that PEG-grafted TPE microchips have broad potential application in biomolecular analysis.

A microfabricated scaffold for retinal progenitor cell grafting

Diseases that cause photoreceptor cell degeneration afflict millions of people, yet no restorative treatment exists for these blinding disorders. Replacement of photoreceptors using retinal progenitor cells (RPCs) represents a promising therapy for the treatment of retinal degeneration. Previous studies have demonstrated the ability of polymer scaffolds to increase significantly both the survival and differentiation of RPCs. We report the microfabrication of a poly(glycerol-sebacate) scaffold with superior mechanical properties for the delivery of RPCs to the subretinal space. Using a replica molding technique, a porous poly(glycerol-sebacate) scaffold with a thickness of 45 microm was fabricated. Evaluation of the mechanical properties of this scaffold showed that the Young's modulus is about 5-fold lower and the maximum elongation at failure is about 10-fold higher than the previously reported RPC scaffolds. RPCs strongly adhered to the poly(glycerol-sebacate) scaffold, and endogenous fluorescence nearly doubled over a 2-day period before leveling off after 3 days. Immunohistochemistry revealed that cells grown on the scaffold for 7 days expressed a mixture of immature and mature markers, suggesting a tendency towards differentiation. We conclude that microfabricated poly(glycerol-sebacate) exhibits a number of novel properties for use as a scaffold for RPC delivery.

Dual functional, polymeric self-assembled monolayers as a facile platform for construction of patterns of biomolecules

We report a facile approach to the construction of patterns of biomolecules based on polymeric self-assembled monolayers (pSAMs) that possess dual functions: "bio-reactive (post-functionalizable)" and "bio-inert (anti-biofouling)" properties. To prepare pSAMs on Si/SiO2 wafers were synthesized new random copolymers by radical polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA), 3-(trimethoxysilyl)propyl methacrylate (TMSMA), and N-acryloxysuccinimide (NAS), and referred to as poly(TMSMA-r-PEGMA-r-NAS). Poly(TMSMA-r-PEGMA-r-NAS) was designed to play triple roles: "surface-reactive", "bio-reactive", and "bio-inert" ones. The pSAMs of poly(TMSMA-r-PEGMA-r-NAS) were formed on Si/SiO2 wafers with 1 h incubation of the substrates in the polymer solution, which showed approximately a 1 nm-thick film as measured by ellipsometry. After the formation of the pSAMs, the feasibility of the pSAMs as a dual functional surface (bio-inert and bio-reactive properties) was examined. The ability of the pSAMs to block nonspecific adsorption of proteins was evaluated against bovine serum albumin as a model protein. High-resolution N(1s) X-ray photoelectron spectroscopy (XPS) analysis on the protein adsorption revealed that significant reduction up to approximately 97% was observed compared to the unmodified Si/SiO2 wafer. In addition, micropatterns of streptavidin with high signal-to-noise ratios were achieved using microcontact printing (microCP) of NH2-bearing biotin onto the pSAMs of poly(TMSMA-r-PEGMA-r-NAS) on glass slides, which suggests that other biomolecules could also be efficiently immobilized onto the pSAMs with high specificity while minimizing nonspecific adsorption. On the other hand, the surface density of both bio-reactive and anti-biofouling functionality could be tailored by simply changing initial feed ratios of each monomer in the polymer synthesis: different molar ratios of the bio-reactive group (NAS: 33%, 20%, and 10%, respectively) were employed. When micropatterns of streptavidin were constructed, the pSAMs with 33% NAS moiety showed the highest immobilization of the protein. Taken together, the present dual functional, random copolymers may have warrant applications in the field of biosensors and biochips.

Microfluidic Perfusion Culture of Human Hepatocytes

Analysis using human cells has been widely used in place of animal experiments. To obtain culture environments closer to those with in vivo, perfusion culture using microfluidic devices is being studied instead of stationary culture such as in a culture dish. With conventional perfusion culture with microfluidic devices, pumping system is externally provided, causing a large dead volume of culture medium. As a result, applied drugs as well as metabolites and signal transmitters from cells are diluted. We minimized this dead volume by embedding micropumps within the device to realize a high concentration of metabolites and signal transmitters from cells by perfusion with small amounts of culture medium and its effects on the cells. Using Hep-G2, established from a human hepatoma, we successfully formed Hep-G2 spheroids which are not observed in conventional culture. Evaluating activity from the DNA amount and albumin produced, we found that Hep-G2 spheroids formed in our device showed higher activity than conventional 2-dimensional culture. We demonstrated that the functionally highly integrated on-chip perfusion cell culture microdevice provided cells with a culture environment close to that in vivo and promoted morphological change and expression of high activity in cells.

Polyurethane from biosource as a new material for fabrication of microfluidic devices by rapid prototyping

This paper presents the use of elastomeric polyurethane (PU), derived from castor oil (CO) biosource, as a new material for fabrication of microfluidic devices by rapid prototyping. Including the irreversible sealing step, PU microchips were fabricated in less than 1h by casting PU resin directly on the positive high-relief molds fabricated by standard photolithography and nickel electrodeposition. Physical characterization of microchannels was performed by scanning electron microscopy (SEM) and profilometry. Polymer surface was characterized using contact angle measurements and the results showed that the hydrophilicity of the PU surface increases after oxygen plasma treatment. The polymer surface demonstrated the capability of generating an electroosmotic flow (EOF) of 2.6 x 10(-4)cm(2)V(-1)s(-1) at pH 7 in the cathode direction, which was characterized by current monitoring method at different pH values. The compatibility of PU with a wide range of solvents and electrolytes was tested by determining its degree of swelling over a 24h period of contact. The performance of microfluidic systems fabricated using this new material was evaluated by fabricating miniaturized capillary electrophoresis systems. Epinephrine and l-DOPA, as model analytes, were separated in aqueous solutions and detected with end-channel amperometric detection.

Fully integrated microfluidic separations systems for biochemical analysis

Over the past decade a tremendous amount of research has been performed using microfluidic analytical devices to detect over 200 different chemical species. Most of this work has involved substantial integration of fluid manipulation components such as separation channels, valves, and filters. This level of integration has enabled complex sample processing on miniscule sample volumes. Such devices have also demonstrated high throughput, sensitivity, and separation performance. Although the miniaturization of fluidics has been highly valuable, these devices typically rely on conventional ancillary equipment such as power supplies, detection systems, and pumps for operation. This auxiliary equipment prevents the full realization of a "lab-on-a-chip" device with complete portability, autonomous operation, and low cost. Integration and/or miniaturization of ancillary components would dramatically increase the capability and impact of microfluidic separations systems. This review describes recent efforts to incorporate auxiliary equipment either as miniaturized plug-in modules or directly fabricated into the microfluidic device.

Addressing a vascular endothelium array with blood components using underlying microfluidic channels

Here, we show that an array of endothelial cells, addressable by an underlying microfluidic network of channels containing red blood cells, can be employed as an in vitro model of in vivo circulation to monitor cellular communication between different cell types in the drug discovery process.

Rapid fabrication of a microfluidic device with integrated optical waveguides for DNA fragment analysis

The fabrication and performance of a microfluidic device with integrated liquid-core optical waveguides for laser induced fluorescence DNA fragment analysis is presented. The device was fabricated through poly(dimethylsiloxane) (PDMS) soft lithography and waveguides are formed in dedicated channels through the addition of a liquid PDMS pre-polymer of higher refractive index. Once a master has been fabricated, microfluidic chips can be produced in less than 3 h without the requirement for a cleanroom, yet this method provides an optical system that has higher performance than a conventional confocal optical assembly. Optical coupling was achieved through the insertion of optical fibers into fiber-to-waveguide couplers at the edge of the chip and the liquid-fiber interface results in low reflection and scattering losses. Waveguide propagation losses are measured to be 1.8 dB cm(-1) (532 nm) and 1.0 dB cm(-1) (633 nm). The chip displays an average total coupling loss of 7.6 dB due to losses at the optical fiber interfaces. In the electrophoretic separation and detection of a BK virus PCR product, the waveguide system achieves an average signal-to-noise ratio of 570 +/- 30 whereas a commercial confocal benchtop electrophoresis system achieves an average SNR of 330 +/- 30. To our knowledge, this is the first time that a waveguide-based system has been demonstrated to have a SNR comparable to a commercially available confocal-based system for microchip capillary electrophoresis.

Synthesis of composite emulsions and complex foams with the use of microfluidic flow-focusing devices

A method is described for the formation of stable, composite aqueous emulsions of 1) combinations of distinct families of bubbles of nitrogen, 2) combinations of distinct families of droplets of an organic fluid (either perfluoro(methyl)decalin or hexadecane), and 3) combinations of bubbles and droplets. A system of two or three microfluidic flow-focusing units is coupled to a single outlet channel. The composite emulsions can be precisely tuned, both in their composition and in the number fraction of components--either bubbles or droplets--of different types. The use of microfluidic technology, with closely coupled flow-focusing units, guarantees that the emulsions are mixed locally at a controlled local stoichiometry. The emulsions self-assemble in a nonequilibrium process to form a wide variety of highly organized periodic lattices.

Microfabricated high-throughput electronic particle detector

We describe the design, fabrication, and use of a radio frequency reflectometer integrated with a microfluidic system, applied to the very high-throughput measurement of micron-scale particles, passing in a microfluidic channel through the sensor region. The device operates as a microfabricated Coulter counter [U.S. Patent No. 2656508 (1953)], similar to a design we have described previously, but here with significantly improved electrode geometry as well as including electronic tuning of the reflectometer; the two improvements yielding an improvement by more than a factor of 10 in the signal to noise and in the diametric discrimination of single particles. We demonstrate the high-throughput discrimination of polystyrene beads with diameters in the 4-10 microm range, achieving diametric resolutions comparable to the intrinsic spread of diameters in the bead distribution, at rates in excess of 15 x 10(6) beads/h.