Electromechanical properties of pressure-actuated poly(dimethylsiloxane) microfluidic push-down valves

Optimization of Oligomer Stamping Technique for Normally Closed Elastomeric Valves on Glass Substrate

Microscale elastomeric valves are an integral part of many lab-on-chip applications. Normally closed valves require lower actuation pressures to form tight seals, making them ideal for portable devices. However, fabrication of normally closed valves is typically more difficult because the valve structure must be selectively bonded to its substrate. In this work, an oligomer stamping technique for selective bonding of normally closed valves is optimized for bonding of PDMS devices on glass substrates. Contact angle and blister bursting testing measurements are used to quantitatively characterize the oligomer stamping process for the first time, and recommendations are made for plasma treatment conditions, microstamping technique, and valve construction. Glass-PDMS devices are ideal for lab-on-chip systems that integrate electrodes on the rigid glass substrate. Here, integrated electrodes are used to assess valve performance, demonstrating electrical isolation in excess of 8 MΩ over the biologically relevant frequency range in the closed state. Further, electrical measurement is used to demonstrate that the valve design can operate under a pulsed actuation scheme, sealing to withstand fluid pressures in excess of 200 mbar.

Surface Modification of Glass/PDMS Microfluidic Valve Assemblies Enhances Valve Electrical Resistance

Microfluidic instrumentation offers unique advantages in biotechnology applications including reduced sample and reagent consumption, rapid mixing and reaction times, and a high degree of process automation. As dimensions decrease, the ratio of surface area to volume within a fluidic architecture increases, which gives rise to some of the unique advantages inherent to microfluidics. Thus, manipulation of surface characteristics presents a promising approach to tailor the performance of microfluidic systems. Microfluidic valves are essential components in a number of small volume applications and for automated microfluidic platforms, but rigorous evaluation of the sealing quality of these valves is often overlooked. In this work, the glass valve seat of hybrid glass/PDMS microfluidic valves was surface modified with hydrophobic silanes, octyldimethylchlorosilane (ODCS) or (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane (PFDCS), to investigate the effect of surface energy on electrical resistance of valves. Valves with ODCS- or PFDCS-modified valve seats both exhibited >70-fold increases in electrical resistance (>500 GΩ) when compared to the same valve design with unmodified glass valve seats (7 ± 3 GΩ), indicative of higher sealing capacity. The opening times for valves with ODCS- or PFDCS-modified valve seats was ca. 5× shorter compared to unmodified valve seats, whereas the closing time was up to 8× longer for modified valve seats, although the total closing time was ≤1.5 s, compatible with numerous microfluidic valving applications. Surface modified valve assemblies offered sufficient electrical resistance to isolate sub-pA current signals resulting from electrophysiology measurement of α-hemolysin conductance in a suspended lipid bilayer. This approach is well-suited for the design of novel microfluidic architectures that integrate fluidic manipulations with electrophysiological or electrochemical measurements.

Normally closed plunger-membrane microvalve self-actuated electrically using a shape memory alloy wire

Various microfluidic architectures designed for in vivo and point-of-care diagnostic applications require larger channels, autonomous actuation, and portability. In this paper, we present a normally closed microvalve design capable of fully autonomous actuation for wide diameter microchannels (tens to hundreds of μm). We fabricated the multilayer plunger-membrane valve architecture using the silicone elastomer, poly-dimethylsiloxane (PDMS) and optimized it to reduce the force required to open the valve. A 50-μm Nitinol (NiTi) shape memory alloy wire is incorporated into the device and can operate the valve when actuated with 100-mA current delivered from a 3-V supply. We characterized the valve for its actuation kinetics using an electrochemical assay and tested its reliability at 1.5-s cycle duration for 1 million cycles during which we observed no operational degradation.

Microfluidic multiplexing of solid-state nanopores

Although solid-state nanopores enable electronic analysis of many clinically and biologically relevant molecular structures, there are few existing device architectures that enable high-throughput measurement of solid-state nanopores. Herein, we report a method for microfluidic integration of multiple solid-state nanopores at a high density of one nanopore per (35 µm²). By configuring microfluidic devices with microfluidic valves, the nanopores can be rinsed from a single fluid input while retaining compatibility for multichannel electrical measurements. The microfluidic valves serve the dual purpose of fluidic switching and electric switching, enabling serial multiplexing of the eight nanopores with a single pair of electrodes. Furthermore, the device architecture exhibits low noise and is compatible with electroporation-based in situ nanopore fabrication, providing a scalable platform for automated electronic measurement of a large number of integrated solid-state nanopores.

Elastomeric microvalves as tunable nanochannels for concentration polarization

Elastomeric microvalves in poly(dimethylsiloxane) (PDMS) devices are today's paradigm for massively parallel microfluidic operations. Here, we report that such valves can act as nanochannels upon closure. When tuning nanospace heights between ~55 nm and ~7 nm, the nanofluidic phenomenon of concentration polarization could be induced. A wide range of concentration polarization regimes (anodic and cathodic analyte focusing and stacking) was achieved simply by valve pressure actuation. Electro-osmotic flow generated a counterpressure which also could be used to actuate between concentration polarization regimes. 1000-fold preconcentration of fluorescein was achieved in just 100 s in the anodic focusing regime. After valve opening, a concentrated sample plug could be transported through the valve, though at the cost of some defocusing. Reversible nanochannels open new avenues for integrating electrokinetic operations and assays in large scale integrated microfluidics.

Microfluidic cell volume sensor with tunable sensitivity

We report the fabrication and validation of a microfluidic cell volume sensor integrated on a multi-layered polydimethylsiloxane (PDMS) microchip with a tunable detection volume for dynamic control of sensitivity, enabling the detection of individual Escherichia coli and microparticles.

Microfluidic very large scale integration (mVLSI) with integrated micromechanical valves

Microfluidic chips with a high density of control elements are required to improve device performance parameters, such as throughput, sensitivity and dynamic range. In order to realize robust and accessible high-density microfluidic chips, we have fabricated a monolithic PDMS valve architecture with three layers, replacing the commonly used two-layer design. The design is realized through multi-layer soft lithography techniques, making it low cost and easy to fabricate. By carefully determining the process conditions of PDMS, we have demonstrated that 8 × 8 and 6 × 6 μm(2) valve sizes can be operated at around 180 and 280 kPa differential pressure, respectively. We have shown that these valves can be fabricated at densities approaching 1 million valves per cm(2), substantially exceeding the current state of the art of microfluidic large-scale integration (mLSI) (thousands of valves per cm(2)). Because the density increase is greater than two orders of magnitude, we describe this technology as microfluidic very large scale integration (mVLSI), analogous to its electronic counterpart. We have captured and tracked fluorescent beads, and changed the electrical resistance of a fluidic channel by using these miniaturized valves in two different experiments, demonstrating that the valves are leakproof. We have also demonstrated that these valves can be addressed through multiplexing.

Rapid detection of avian influenza H5N1 virus using impedance measurement of immuno-reaction coupled with RBC amplification

Avian influenza virus (AIV) subtype H5N1 was first discovered in the 1990 s and since then its emergence has become a likely source of a global pandemic and economic loss. Currently accepted gold standard methods of influenza detection, viral culture and rRT-PCR, are time consuming, expensive and require special training and laboratory facilities. A rapid, sensitive, and specific screening method is needed for in-field or bedside testing of AI virus to effectively implement quarantines and medications. Therefore, the objective of this study was to improve the specificity and sensitivity of an impedance biosensor that has been developed for the screening of AIV H5. Three major components of the developed biosensor are immunomagnetic nanoparticles for the separation of AI virus, a microfluidic chip for sample control and an interdigitated microelectrode for impedance measurement. In this study polyclonal antibody against N1 subtype was immobilized on the surface of the microelectrode to specifically bind AIV H5N1 to generate more specific impedance signal and chicken red blood cells (RBC) were used as biolabels to attach to AIV H5N1 captured on the microelectrode to amplify impedance signal. RBC amplification was shown to increase the impedance signal change by more than 100% compared to the protocol without RBC biolabels, and was necessary for forming a linear calibration curve for the biosensor. The use of a second antibody against N1 offered much greater specificity and reliability than the previous biosensor protocol. The biosensor was able to detect AIV H5N1 at concentrations down to 10(3) EID(50)ml(-1) in less than 2h.

A versatile valve-enabled microfluidic cell co-culture platform and demonstration of its applications to neurobiology and cancer biology

A versatile microfluidic platform allowing co-culture of multiple cell populations in close proximity with separate control of their microenvironments would be extremely valuable for many biological applications. Here, we report a simple and compact microfluidic platform that has these desirable features and allows for real-time, live-cell imaging of cell-cell interactions. Using a pneumatically/hydraulically controlled poly(dimethylsiloxane) (PDMS) valve barrier, distinct cell types can be cultured in side-by-side microfluidic chambers with their optimum culture media and treated separately without affecting the other cell population. The platform is capable of both two-dimensional and three-dimensional cell co-culture and through variations of the valve barrier design, the platform allows for cell-cell interactions through either direct cell contact or soluble factors alone. The platform has been used to perform dynamic imaging of synapse formation in hippocampal neurons by separate transfection of two groups of neurons with fluorescent pre- and post-synaptic protein markers. In addition, cross-migration of 4T1 tumor cells and endothelial cells has been studied under normoxic and hypoxic conditions, which revealed different migration patterns, suggesting the importance of the microenvironments in cell-cell interactions and biological activities.

High-pressure on-chip mechanical valves for thermoplastic microfluidic devices

A facile method enabling the integration of elastomeric valves into rigid thermoplastic microfluidic chips is described. The valves employ discrete plugs of elastomeric polydimethylsiloxane (PDMS) integrated into the thermoplastic substrate and actuated using a threaded stainless steel needle. The fabrication process takes advantage of poly(ethylene glycol) (PEG) as a sacrificial molding material to isolate the PDMS regions from the thermoplastic flow channels, while yielding smooth contact surfaces with the PDMS valve seats. The valves introduce minimal dead volumes, and provide a simple mechanical means to achieve reproducible proportional valving within thermoplastic microfluidic systems. Burst pressure tests reveal that the valves can withstand pressures above 12 MPa over repeated open/close cycles without leakage, and above 24 MPa during a single use, making the technology well suited for applications such as high performance liquid chromatography. Proportional valve operation is demonstrated using a multi-valve chemical gradient generator fabricated in cyclic olefin polymer.