Design of PDMS microlenses bonded to a lab-CD chip for ELISA applications

A novel device comprising polydimethyl-siloxane (PDMS) microlenses bonded to a microfluidic compact disk (CD) is proposed for enzyme-linked immunosorbent assay (ELISA) applications. The PDMS microlenses were fabricated using a simple soft replica molding method and were bonded to the microfluidic CD using oxygen plasma treatment. A commercial software tool (ZEMAX) has been used to analyze the focal length of the microlens. A laser-induced fluorescence bio-detection system, consisting of the integrated microfluidic CD/PDMS microlenses and an optical detection module, was constructed and used to examine the enzymatic reaction of 3-(4-hydroxy) phenly propionic acid. The experimental results show that the PDMS microlens focusing effect yields a significant improvement in the intensity of the detected fluorescence signals. As a result, the proposed device represents an ideal solution for ELISAs and other high-sensitivity bio-detection applications.     

Bioinspired Microfibers with Embedded Perfusable Helical Channels

Materials with microchannels have attracted increasing attention due to their promising perfusability and biomimetic geometry. However, the fabrication of microfibers with more geometrically complex channels in the micro‐ or nanoscale remains a big challenge. Here, a novel method for generating scalable microfibers with consecutive embedded helical channels is presented using an easily made coaxial microfluidic device. The characteristics of the helical channel can be accurately controlled by simply adjusting the flow rate ratio of the fluids. The mechanism of the helix formation process is theorized with newly proposed heterogenerated rope‐coil effect, which enhances the tunability of helical patterns and promotes the comprehension of this abnormal phenomenon. Based on this effect, microfibers with embedded Janus channels and even double helical channels are generated in situ by changing the design of the device. The uniqueness and potential applications of these tubular microfibers are also demonstrated by biomimetic supercoiling structures as well as the perfusable and permeable spiral vessel.     

Hydrogel-based microreactors as a functional component of microfluidic systems

A simple two-step method for fabricating poly(ethylene glycol) (PEG) hydrogel-based microreactors and microsensors within microfluidic channels is described. The intrachannel micropatches contain either a dye, which can report the pH of a solution within a fluidic channel, or enzymes that are able to selectively catalyze specific reactions. Analytes present within the microfluidic channel are able to diffuse into the micropatches, encounter the enzymes, and undergo conversion to products, and then the products interact with the coencapsulated dye to signal the presence of the original substrate. The micropatches are prepared by photopolymerizing the PEG precursor within the channel of a microfluidic system consisting of a poly(dimethylsiloxane) mold and a glass plate. Exposure takes place through a slit mask oriented perpendicular to the channel, so the size of the resulting micropatch is defined by the channel dimensions and the width of the slit mask. Following polymerization, the mold is removed, leaving behind the micropatch(es) atop the glass substrate. The final microfluidic device is assembled by irreversibly binding the hydrogel-patterned glass slide to a second PDMS mold that contains a larger channel. Multiple micropatches containing the same or different enzymes can be fabricated within a single channel. The viability of this approach is demonstrated by sensing glucose using micropatches copolymerized with glucose oxidase, horseradish peroxidase, and a pH-sensitive dye.     

Design and analysis of single-cell capture microfluidic device

The aim of our study was to develop microfluidic devices using microchannel technology with the capability of capturing single cells. We analyzed and compared the cell-capturing efficiencies of series-loop microchannel and parallel-loop microchannel devices that were produced using polydimethylsiloxane (PDMS). Each set of microchannels was composed of a main flow channel and several branch channels with capturing zones. The microfluidic devices were designed to use the differences in flow rates between the main flow channel and the branch channels as a means of capturing single cells based on size and sequestering them within the microstructure of multiple capture zones. The data indicated that the flow medium encountered significant resistance in the series-loop microchannel device, which resulted in an inability to hold the captured cells within any of the capture zones. Flow resistance was, however, greatly reduced in the parallel-loop microchannel device compared to the series-loop device, and single cells were captured in all the capturing zones of the device. Our data suggest that the parallel-loop microchannel technology has significant potential for development toward high-throughput platforms capable of capturing single cells for physiological analyses at the single-cell level.     

Addressable Microfluidic Polymer Chip for DNA‐Directed Immobilization of Oligonucleotide‐Tagged Compounds

A microfluidic polymer chip for the self‐assembly of DNA conjugates through DNA‐directed immobilization is developed. The chip is fabricated from two parts, one of which contains a microfluidic channel produced from poly(dimethylsiloxane) (PDMS) by replica‐casting technique using a mold prepared by photolithographic techniques. The microfluidic part is sealed by covalent bonding with a chemically activated glass slide containing a DNA oligonucleotide microarray. The dimension of the PDMS–glass microfluidic chip is equivalent to standard microscope slides (76 × 26 mm²). The DNA microarray surface inside the microfluidic channels is configured through conventional spotting, and the resulting DNA patches can be conveniently addressed with compounds containing complementary DNA tags. To demonstrate the utility of the addressable surface within the microfluidic channel, DNA‐directed immobilization (DDI) of DNA‐modified gold nanoparticles (AuNPs) and DNA‐conjugates of the enzymes glucose oxidase (GOx) and horseradish peroxidase (HRP) are carried out. DDI of AuNPs is used to demonstrate site selectivity and reversibility of the surface‐modification process. In the case of the DNA–enzyme conjugates, the patterned assembly of the two enzymes allows the establishment and investigation of the coupled reaction of GOx and HRP, with particular emphasis on surface coverage and lateral flow rates. The results demonstrate that this addressable chip is well suited for the generation of fluidically coupled multi‐enzyme microreactors.     

Capillary-assembled microchip for universal integration of various chemical functions onto a single microfluidic device

A novel concept for assembling various chemical functions onto a single microfluidic device is proposed. The concept, called a capillary-assembled microchip, involves embedding chemically functionalized capillaries into a lattice microchannel network fabricated on poly(dimethylsiloxane) (PDMS). The network has the same channel dimensions as the outer dimensions of the capillaries. In this paper, we focus on square capillaries to be embedded into a PDMS microchannel network having a square cross section. The combination of hard glass square capillary and soft square PDMS channel allows successful fabrication of a microfluidic device without any solution leakage, and which can use diffusion-based two-solution mixing. Two different types of chemically modified capillaries, an ion-sensing capillary and a pH-sensing capillary, are prepared by coating a hydrophobic plasticized poly(vinyl chloride) membrane and a hydrophilic poly(ethyleneglycol) membrane containing functional molecules onto the inner surface of capillaries. Then, they are cut into appropriate lengths and arranged on a single microchip to prepare a dual-analyte sensing system. The concept proposed here offers advantages inherent to using a planar microfluidic device and of chemical functionality of immobilized molecules. Therefore, we expect to fabricate various types of chemically functionalized microfluidic devices soon.     

Advection Flows‐Enhanced Magnetic Separation for High‐Throughput Bacteria Separation from Undiluted Whole Blood

A major challenge to scale up a microfluidic magnetic separator for extracorporeal blood cleansing applications is to overcome low magnetic drag velocity caused by viscous blood components interfering with magnetophoresis. Therefore, there is an unmet need to develop an effective method to position magnetic particles to the area of augmented magnetic flux density gradients while retaining clinically applicable throughput. Here, a magnetophoretic cell separation device, integrated with slanted ridge‐arrays in a microfluidic channel, is reported. The slanted ridges patterned in the microfluidic channels generate spiral flows along the microfluidic channel. The cells bound with magnetic particles follow trajectories of the spiral streamlines and are repeatedly transferred in a transverse direction toward the area adjacent to a ferromagnetic nickel structure, where they are exposed to a highly augmented magnetic force of 7.68 µN that is much greater than the force (0.35 pN) at the side of the channel furthest from the nickel structure. With this approach, 91.68% ± 2.18% of Escherichia coli (E. coli) bound with magnetic nanoparticles are successfully separated from undiluted whole blood at a flow rate of 0.6 mL h^?1 in a single microfluidic channel, whereas only 23.98% ± 6.59% of E. coli are depleted in the conventional microfluidic device.     

Cover Picture: Fabrication of Elastomeric Wires by Selective Electroless Metallization of Poly(dimethylsiloxane) (Adv. Mater. 1/2008)

Tricia Carmichael and co‐workers employ a simple, low‐cost method for the fabrication of patterned metal films on elastomeric poly(dimethylsiloxane) (PDMS) substrates, as described on p. 59. The metal/PDMS composites are electrically conductive and mechanically flexible, making them suitable for use in the fabrication of lightweight, flexible devices such as wearable electronics, biocompatible sensors, and artificial nerves, skins, and muscles. Copper wires on PDMS remain conductive when subjected to linear strains of up to 52 %. The utility of these wires is demonstrated by using them as laminated top contacts in an organic light‐emitting device.     

Electrokinetic protein preconcentration using a simple glass/poly(dimethylsiloxane) microfluidic chip

We discovered that a protein concentration device can be constructed using a simple one-layer fabrication process. Microfluidic half-channels are molded using standard procedures in PDMS; the PDMS layer is reversibly bonded to a glass base such as a microscope slide. The microfluidic channels are chevron-shaped, in mirror image orientation, with their apexes designed to pass within approximately 20 microm of each other, forming a thin-walled section between the channels. When an electric field is applied across this thin-walled section, negatively charged proteins are observed to concentrate on the anode side of it. About 10(3)-10(6)-fold protein concentration was achieved in 30 min. Subsequent separation of two different concentrated proteins is easily achieved by switching the direction of the electric field in the direction parallel to the thin-walled section. We hypothesize that a nanoscale channel forms between the PDMS and the glass due to the weak, reversible bonding method. This hypothesis is supported by the observation that, when the PDMS and glass are irreversibly bonded, this phenomenon is not observed until a very high E-field was applied and dielectric breakdown of the PDMS is observed. We therefore suspect that the ion exclusion-enrichment effect caused by electrical double layer overlapping induces cationic selectivity of this nanochannel. This simple on-chip protein preconcentration and separation device could be a useful component in practically any PDMS-on-glass microfluidic device used for protein assays.     

Torque-actuated valves for microfluidics

This paper describes torque-actuated valves for controlling the flow of fluids in microfluidic channels. The valves consist of small machine screws (> or =500 microm) embedded in a layer of polyurethane cast above microfluidic channels fabricated in poly(dimethylsiloxane) (PDMS). The polyurethane is cured photochemically with the screws in place; on curing, it bonds to the surrounding layer of PDMS and forms a stiff layer that retains an impression of the threads of the screws. The valves were separated from the ceiling of microfluidic channels by a layer of PDMS and were integrated into channels using a simple procedure compatible with soft lithography and rapid prototyping. Turning the screws actuated the valves by collapsing the PDMS layer between the valve and channel, controlling the flow of fluids in the underlying channels. These valves have the useful characteristic that they do not require power to retain their setting (on/off). They also allow settings between "on" and "off" and can be integrated into portable, disposable microfluidic devices for carrying out sandwich immunoassays.     

p型氧化镍薄膜晶体管的微流控法制备研究

可以图形化和沉积同时进行的镀膜技术可有效简化器件制备流程, 从而降低成本。本工作研究了一种新型的图形化沉积镀膜技术-微流控法: 将宽度及间隔均为80 μm、沟槽深度为2 μm左右的PDMS模板与衬底贴合构筑微流通道, 毛细力作用下前驱液可在微流通道内流动, 并在衬底表面形成图形化的液膜, 最后经热处理完成图行化的薄膜沉积。此外, 分析了硝酸镍/2-甲氧基乙醇前驱体的热分解过程和不同温度退火下前驱体粉末的相结构演化规律。最终利用微流控法图形化沉积技术制备了图形化的氧化镍沟道, 并构筑了薄膜晶体管器件。优化后的薄膜晶体管表现出典型的p型特征, 场效应迁移率可达0.8 cm²·V^-1·s^-1。     

Recirculation of nanoliter volumes within microfluidic channels

A microfluidic device is described, capable of recirculating nanoliter volumes in restricted microchannel segments. The device consists of a PDMS microfluidic structure, reversibly sealed to a glass substrate with integrated platinum electrodes. The integrated electrodes generate electroosmotic flow locally, which results in a cycling flow in the channel segment between the two electrodes in case one channel exit is closed (dead-end channel). This cycling flow is a consequence of the counterbalancing hydrodynamic pressure against the electroosmotically generated flow. Acid-base indicators were employed to study the formation of H(+) and OH(-) at both the in-channel electrodes. The formation of acid can locally change the zeta-potential of the channel wall, which will affect the flow profile. Using this method, small analyte volumes can be mixed for prolonged times within well-defined channel segments and/or exposed to in-channel sensor surfaces.     

Writing and Functionalisation of Suspended DNA Nanowires on Superhydrophobic Pillar Arrays

Nanowire arrays and networks with precisely controlled patterns are very interesting for innovative device concepts in mesoscopic physics. In particular, DNA templates have proven to be versatile for the fabrication of complex structures that obtained functionality via combinations with other materials, for example by functionalisation with molecules or nanoparticles, or by coating with metals. Here, the controlled motion of the a three‐phase contact line (TCL) of DNA‐loaded drops on superhydrophobic substrates is used to fabricate suspended nanowire arrays. In particular, the deposition of DNA wires is imaged in situ, and different patterns are obtained on hexagonal pillar arrays by controlling the TCL velocity and direction. Robust conductive wires and networks are achieved by coating the wires with a thin layer of gold, and as proof of concept conductivity measurements are performed on single suspended wires. The plastic material of the superhydrophobic pillars ensures electrical isolation from the substrate. The more general versatility of these suspended nanowire networks as functional templates is outlined by fabricating hybrid organic–metal–semiconductor nanowires by growing ZnO nanocrystals onto the metal‐coated nanowires.     

Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)

This paper describes a procedure that makes it possible to design and fabricate (including sealing) microfluidic systems in an elastomeric material [Formula: see text] poly(dimethylsiloxane) (PDMS) [Formula: see text] in less than 24 h. A network of microfluidic channels (with width >20 μm) is designed in a CAD program. This design is converted into a transparency by a high-resolution printer; this transparency is used as a mask in photolithography to create a master in positive relief photoresist. PDMS cast against the master yields a polymeric replica containing a network of channels. The surface of this replica, and that of a flat slab of PDMS, are oxidized in an oxygen plasma. These oxidized surfaces seal tightly and irreversibly when brought into conformal contact. Oxidized PDMS also seals irreversibly to other materials used in microfluidic systems, such as glass, silicon, silicon oxide, and oxidized polystyrene; a number of substrates for devices are, therefore, practical options. Oxidation of the PDMS has the additional advantage that it yields channels whose walls are negatively charged when in contact with neutral and basic aqueous solutions; these channels support electroosmotic pumping and can be filled easily with liquids with high surface energies (especially water). The performance of microfluidic systems prepared using this rapid prototyping technique has been evaluated by fabricating a miniaturized capillary electrophoresis system. Amino acids, charge ladders of positively and negatively charged proteins, and DNA fragments were separated in aqueous solutions with this system with resolution comparable to that obtained using fused silica capillaries.     

Design of a Microchannel‐Nanochannel‐Microchannel Array Based Nanoelectroporation System for Precise Gene Transfection

A micro/nano‐fabrication process of a nanochannel electroporation (NEP) array and its application for precise delivery of plasmid for non‐viral gene transfection is described. A dip‐combing device is optimized to produce DNA nanowires across a microridge array patterned on the polydimethylsiloxane (PDMS) surface with a yield up to 95%. Molecular imprinting based on a low viscosity resin, 1,4‐butanediol diacrylate (1,4‐BDDA), adopted to convert the microridge‐nanowire‐microridge array into a microchannel‐nanochannel‐microchannel (MNM) array. Secondary machining by femtosecond laser ablation is applied to shorten one side of microchannels from 3000 to 50 μm to facilitate cell loading and unloading. The biochip is then sealed in a packaging case with reservoirs and microfluidic channels to enable cell and plasmid loading, and to protect the biochip from leakage and contamination. The package case can be opened for cell unloading after NEP to allow for the follow‐up cell culture and analysis. These NEP cases can be placed in a spinning disc and up to ten discs can be piled together for spinning. The resulting centrifugal force can simultaneously manipulate hundreds or thousands of cells into microchannels of NEP arrays within 3 minutes. To demonstrate its application, a 13 kbp OSKM plasmid of induced pluripotent stem cell (iPSC) is injected into mouse embryonic fibroblasts cells (MEFCs). Fluorescence detection of transfected cells within the NEP biochips shows that the delivered dosage is high and much more uniform compared with similar gene transfection carried out by the conventional bulk electroporation (BEP) method.     

Transport, location, and quantal release monitoring of single cells on a microfluidic device

A novel microfluidic device has been developed for on-chip transport, location, and quantal release monitoring of single cells. The microfluidic device consists of a plate of PDMS containing channels for introducing cells and stimulants and a glass substrate into which a cell micro-chamber was etched. The two tightly reversibly sealed plates can be separated for respective cleaning, which significantly extends the lifetime of the microchip that is frequently clogged in cell analysis experiments. Using hydraulic pressure, single cells were transported and located on the microfluidic chip. After location of a single PC12 cell on the microfluidic chip, the cell was stimulated by nicotine that was also introduced through the micro-channels, and the quantum release of dopamine from the cell was amperometricly detected with our designed carbon fiber microelectrode. The results have demonstrated the convenience and efficiency of using the microfluidic chip for monitoring of quantal release from single cells and have offered a facile method for the analysis of single cells on microfluidic devices.     

Self-sealed vertical polymeric nanoporous-junctions for high-throughput nanofluidic applications

We developed a reliable but simple integration method of polymeric nanostructure in a poly(dimethylsiloxane) (PDMS)-based microfluidic channel, for nanofluidic applications. The Nafion polymer junction was creased by infiltrating polymer solution between the gaps created by mechanical cutting, without any photolithography or etching processes. The PDMS can seal itself with the heterogeneous polymeric nanoporous material between the PDMS/PDMS gap due to its flexibility without any (covalent) bonding between PDMS and the polymer materials. Thus, one can easily integrate the nanoporous-junction into a PDMS microchip in a leak-free manner with excellent repeatability. We demonstrated nanofluidic preconcentration of proteins (beta-phycoerythrin) using the device. Because the polymeric junction spans across the entire microchannel height, the preconcentration was achieved with high-pressure field or even in large channels, with the dimensions of 1000 microm width x 100 microm depth.     

Magnetometry of Individual Polycrystalline Ferromagnetic Nanowires

Ferromagnetic nanowires are finding use as untethered sensors and actuators for probing micro‐ and nanoscale biophysical phenomena, such as for localized sensing and application of forces and torques on biological samples, for tissue heating through magnetic hyperthermia, and for microrheology. Quantifying the magnetic properties of individual isolated nanowires is crucial for such applications. Dynamic cantilever magnetometry is used to measure the magnetic properties of individual sub‐500 nm diameter polycrystalline nanowires of Ni and Ni₈₀Co₂₀ fabricated by template‐assisted electrochemical deposition. The values are compared with bulk, ensemble measurements when the nanowires are still embedded within their growth matrix. It is found that single‐particle and ensemble measurements of nanowires yield significantly different results that reflect inter‐nanowire interactions and chemical modifications of the sample during the release process from the growth matrix. The results highlight the importance of performing single‐particle characterization for objects that will be used as individual magnetic nanoactuators or nanosensors in biomedical applications.     

Measurement of Local Si‐Nanowire Growth Kinetics Using In situ Transmission Electron Microscopy of Heated Cantilevers

A technique to study nanowire growth processes on locally heated microcantilevers in situ in a transmission electron microscope has been developed. The in situ observations allow the characterization of the nucleation process of silicon wires, as well as the measurement of growth rates of individual nanowires and the ability to observe the formation of nanowire bridges between separate cantilevers to form a complete nanowire device. How well the nanowires can be nucleated controllably on typical cantilever sidewalls is examined, and the measurements of nanowire growth rates are used to calibrate the cantilever‐heater parameters used in finite‐element models of cantilever heating profiles, useful for optimization of the design of devices requiring local growth.