Improved concentration and separation of particles in a 3D dielectrophoretic chip integrating focusing, aligning and trapping

This article presents a dielectrophoresis (DEP)-based microfluidic device with the three-dimensional (3D) microelectrode configuration for concentrating and separating particles in a continuous throughflow. The 3D electrode structure, where microelectrode array are patterned on both the top and bottom surfaces of the microchannel, is composed of three units: focusing, aligning and trapping. As particles flowing through the microfluidic channel, they are firstly focused and aligned by the funnel-shaped and parallel electrode array, respectively, before being captured at the trapping unit due to negative DEP force. For a mixture of two particle populations of different sizes or dielectric properties, with a careful selection of suspending medium and applied field, the population exhibits stronger negative DEP manipulated by the microelectrode array and, therefore, separated from the other population which is easily carried away toward the outlet due to hydrodynamic force. The functionality of the proposed microdevice was verified by concentrating different-sized polystyrene (PS) microparticles and yeast cells dynamically flowing in the microchannel. Moreover, separation based on size and dielectric properties was achieved by sorting PS microparticles, and isolating 5 μm PS particles from yeast cells, respectively. The performance of the proposed micro-concentrator and separator was also studied, including the threshold voltage at which particles begin to be trapped, variation of cell-trapping efficiency with respect to the applied voltage and flow rate, and the efficiency of separation experiments. The proposed microdevice has various advantages, including multi-functionality, improved manipulation efficiency and throughput, easy fabrication and operation, etc., which shows a great potential for biological, chemical and medical applications.     

New production method of convex microlens arrays for integrated fluorescence microfluidic detection systems

A new method for producing microlens array with large sag heights is proposed for integrated fluorescence microfluidic detection systems. Three steps in this production technique are included for concave microlens array formations to be integrated into microfluidic systems. First, using the photoresist SU-8 to produce hexagonal microchannel array is required. Second, UV curable glue is injected into the hexagonal microchannel array. Third, the surplus glue is rotated by a spinner at high velocity and exposed to a UV lamp to harden the glue. The micro concave lens molds are then finished and ready to produce convex microlens in poly methsiloxane (PDMS) material. This convex microlens in PDMS can be used for detecting fluorescence in microfluidic channels because a convex microlens plays the light convergence role for optical fiber detection.

     

New production method of convex microlens arrays for integrated fluorescence microfluidic detection systems

A new method for producing microlens array with large sag heights is proposed for integrated fluorescence microfluidic detection systems. Three steps in this production technique are included for concave microlens array formations to be integrated into microfluidic systems. First, using the photoresist SU-8 to produce hexagonal microchannel array is required. Second, UV curable glue is injected into the hexagonal microchannel array. Third, the surplus glue is rotated by a spinner at high velocity and exposed to a UV lamp to harden the glue. The micro concave lens molds are then finished and ready to produce convex microlens in poly methsiloxane (PDMS) material. This convex microlens in PDMS can be used for detecting fluorescence in microfluidic channels because a convex microlens plays the light convergence role for optical fiber detection.     

A novel technique for fabrication of micro- and nanofluidic device with embedded single carbon nanotube

This paper describes a novel technique for fabrication of micro- and nanofluidic device that consists of a carbon nanotube (CNT) and a polydimethylsiloxane (PDMS) microchannel. Single CNT was placed at desired locations using dielectrophoresis (DEP) and PDMS microchannel was constructed on the aligned CNT via photolithography and soft lithography techniques. This technique enables a CNT to be seamlessly embedded in a PDMS microchannel. Moreover, controlling the PDMS curing condition enables the construction of the device with or without a CNT (the device without CNT has a trace nanochannel in PDMS). Preliminary flow tests such as capillary effect and pressure-driven flow were performed with the fabricated devices. In the capillary effect tests, the flow stopped at the nanochannel in both devices. In the pressure-driven flow lower flow resistance was observed in the device with a CNT.     

Optimization and application of dry film photoresist for rapid fabrication of high-aspect-ratio microfluidic devices

Fabrication of high-aspect-ratio PDMS microfluidic devices with conventional SU-8 based soft photolithography is challenging, and often, the thickness of the master from which PDMS replicas are molded is non-uniform. Here, we present an optimized, low cost, fast prototyping microfabrication technique to make deep (up to 500 μm) and high-aspect-ratio (up to 10) microfluidic channels by producing masters by laminating a single or multiple layers of a thin dry film photoresist onto metal wafers. In particular, we explore the required exposure energy for different film thicknesses as well as the highest achievable channel depths and aspect ratios. The homogeneity of the depth of PDMS channels formed using these masters is quantified and found to be remarkably uniform over distances of 20 mm or more. The importance of the processing parameters, such as the exposure energy and development time on final feature size, wall angle, and channel aspect ratio, is investigated. In addition, we report some failure cases, the potential reasons, and strategies for making optimized devices. Potentially, deep microfluidic channels with a wide range of aspect ratios can be used to make long, homogenous separation devices that can be used in cell sorting, filtration, and flow cytometry. We believe the protocols we outline here will be of great utility to the microfluidics community.     

Radioactivity resistance evaluation of polymeric materials for application in radiopharmaceutical production at microscale

Microfluidic technologies are gaining increasing importance due to their capability of manipulating fluids at the microscale that should allow to synthesize many products with surprisingly high yields and short reaction times. In the lab-on-chip field researchers have developed microfluidic apparatuses to provide special equipments for producing positron emission tomography (PET) radiopharmaceuticals in a quicker, safer, and more reliable way compared to traditional vessel-based approaches. In this paper, we have selected a number of polymeric materials, such as polydimethylsiloxane (PDMS), SU-8, and Teflon-like coatings deposited on PDMS or hard substrates, to be used for the fabrication of micro apparatuses for radiosynthesis. Their radioactivity resistance was investigated employing different setups and the results analyzed by atomic force microscopy (AFM), optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). To evaluate undesired absorption effects in the investigated materials, the fluoride radioactive trapping inside microchannel was measured through autoradiography. We found out that polymeric materials such as SU-8 and Teflon coated on hard materials seem very appealing for fabricating microreactors for radiochemistry.     

Investigation and improvement of reversible microfluidic devices based on glass–PDMS–glass sandwich configuration

Reversibly assembled microfluidic devices are dismountable and reusable, which is useful for a number of applications such as micro- and nano-device fabrication, surface functionalization, complex cell patterning, and other biological analysis by means of spatial–temporal pattern. However, reversible microfluidic devices fabricated with current standard procedures can only be used for low-pressure applications. Assembling technology based on glass–PDMS–glass sandwich configuration provides an alternative sealing method for reversible microfluidic devices, which can drastically increase the sealing strength of reversibly adhered devices. The improvement mechanism of sealing properties of microfluidic devices based on the sandwich technique has not been fully characterized, hindering further improvement and broad use of this technique. Here, we characterize, for the first time, the effect of various parameters on the sealing strength of reversible PDMS/glass hybrid microfluidic devices, including contact area, PDMS thickness, assembling mode, and external force. To further improve the reversible sealing of glass–PDMS–glass microfluidic devices, we propose a new scheme which exploits mechanical clamping elements to reinforce the sealing strength of glass–PDMS–glass sandwich structures. Using our scheme, the glass–PDMS–glass microchips can survive a pressure up to 400 kPa, which is comparable to the irreversibly bonded PDMS microdevices. We believe that this bonding method may find use in lab-on-a-chip devices, particularly in active high-pressure-driven microfluidic devices.     

Optimized design and fabrication of a microfluidic platform to study single cells and multicellular aggregates in 3D

A microfluidic platform for cell motility analysis in a three-dimensional environment is presented. The microfluidic device is designed to study migration of both single cells and cell spheroids, in particular under spatially and temporally controlled chemical stimuli. A layout based on a central microchannel confined by micropillars and two lateral reservoirs was selected as the most effective. The microfluidics have an internal height of 350 μm to accommodate cell spheroids of a considerable size. The chip is fabricated using well-established micromachining techniques, by obtaining the polydimethylsiloxane replica from a Si/SU-8 master. The chip is then bonded on a 170-μm-thick microscope glass slide to allow high spatial resolution live microscopy. In order to allow the cost-effective and highly repeatable production of chips with high aspect ratio (5:1) micropillars, specific design and fabrication processes were optimized. This design permits spatial confinement of the gel where cells are grown, the creation of a stable gel–liquid interface and the formation of a diffusive gradient of a chemoattractant (>48 h). The chip accomplishes both the tasks of a microfluidic bioreactor system and a cell analysis platform avoiding critical handling of the sample. The experimental fluidic tests confirm the easy handling of the chip and in particular the effectiveness of the micropillars to separate the Matrigel? from the culture media. Experimental tests of (i) the stability of the gradient, (ii) the biocompatibility and (iii) the suitability for microscopy are presented.     

Fabrication of microfluidic channels based on melt-electrospinning direct writing

Microfluidics is a flourishing field, enabling a wide range of applications. However, the current fabrication methods for creating the microchannel structures of microfluidic devices, such as photolithography and 3D printing, mostly have the problems of time-consuming, high cost or low resolution. In this work, we developed a simple and flexible method to fabricate PDMS microfluidic channels, based on poly(ε-caprolactone) (PCL) master mold additive manufactured by a technique termed melt-electrospinning direct writing (MEDW). It relies on the following steps: (1) direct writing of micrometric PCL 2D or 3D pattern by MEDW. (2) Casting PDMS on the printed PCL pattern. (3) Peeling off of patterned PDMS from the embedded sacrificial PCL layer. (4) Bonding the PDMS with microchannel to another PDMS layer by hot pressing. The process parameters during MEDW such as collector speed, nozzle dimension and temperature were studied and optimized for the quality and dimension of the printed micropatterns. Multilayer fiber deposition was developed and applied to achieve microscale architectures with high aspect ratio. Thus, the microchannels fabricated by the proposed approach could possess tunable width and depth. Finally, T-shape and cross-channel devices were fabricated to create either laminar flow or microdroplets to illustrate the applicability and potential of this method for microfluidic device manufacture.     

A novel method to construct 3D electrodes at the sidewall of microfluidic channel

We report a simple, low-cost and novel method for constructing three-dimensional (3D) microelectrodes in microfluidic system by utilizing low melting point metal alloy. Three-dimensional electrodes have unique properties in application of cell lysis, electro-osmosis, electroporation and dielectrophoresis. The fabrication process involves conventional photolithography and sputtering techniques to fabricate planar electrodes, positioning bismuth (Bi) alloy microspheres at the sidewall of PDMS channel, plasma bonding and low temperature annealing to improve electrical connection between metal microspheres and planar electrodes. Compared to other fabrication methods for 3D electrodes, the presented one does not require rigorous experimental conditions, cumbersome processes and expensive equipments. Numerical analysis on electric field distribution with different electrode configurations was presented to verify the unique field distribution of arc-shaped electrodes. The application of 3D electrode configuration with high-conductive alloy microspheres was confirmed by particle manipulation based on dielectrophoresis. The proposed technique offers alternatives to construct 3D electrodes from 2D electrodes. More importantly, the simplicity of the fabrication process provides easy ways to fabricate electrodes fast with arc-shaped geometry at the sidewall of microchannel.     

Suspended SU-8 structures for monolithic microfluidic channels

SU-8 photoresist is commonly used in the field of microfabrication as structural material or for molding of microfluidic devices. One major limitation, however, is the difficulty to process partially freestanding SU-8 structures or monolithic closed cavities and channels on-chip. We propose here a simple method for the fabrication of suspended structures, in particular of monolithic SU-8 microchannels. The method is based on the processing of a SU-8 double-layer. Appropriate modification of the optical properties of the upper layer allows for selective crosslinking in the layer sandwich. This process is suitable for versatile layouts comprising open and hollow SU-8 structures on the same chip.     

PDMS microfluidics developed for polymer based photonic biosensors

In this work, advances in the fabrication technology and functional analysis of a polymer microfluidic system—as a significant part of a developed polymer photonic biosensor—are reported. Robust and cost-effective microfluidics in PDMS including sample preparation functions is designed and realized by using SU-8 moulding replica. Surface modification strategies using Triton X-100 and PDMS-PEO and their effect on device sealing and non-specific protein adsorption are investigated by contact angle measurement and in situ fluorescence microscopy.     

Rapid,simple and low cost fabrication of a microfluidic direct methanol fuel cell based on polydimethylsiloxane

In this paper a simple and rapid fabrication method for a microfluidic direct methanol fuel cell using polydimethylsiloxane (PDMS) as substrate is demonstrated. A gold layer on PDMS substrate as seed layer was obtained by chemical plating instead of conventional metal evaporation or sputtering. The morphology of the gold layer can be controlled by adjusting the ratio of curing agent to the PDMS monomer. The chemical properties of the gold films were examined. Then catalyst nanoparticles were grown on the films either by cyclic voltammetry or electrophoretic deposition. The microfluidic fuel cell was assembled by simple oxygen plasma bonding between two PDMS substrates. The cell operated at room temperature with a maximum power density around 6.28 mW cm^?2. Such a fuel cell is low-cost and easy to construct, and is convenient to be integrated with other devices because of the viscosity of the PDMS. This work will facilitate the development of miniature on-chip power sources for portable electronic devices.     

Low-stress fabrication of 3D polymer free standing structures using lamination of photosensitive films

Free-standing microstructures such as cantilevers, membranes or microchannels are building blocks of microfluidic systems and MEMS. As a complement to silicon, the large family of polymers offers many opportunities for micro and nanotechnologies. Their low temperature processing and the planarizing properties of many resists is a definitive advantage for system integration, paving the way to complete lab-on-chips. In this article, we investigate a fabrication process of polymeric free standing structures based on the lamination of SU-8, a thick epoxy photoresist. Our motivation is the hybrid integration of polymer microfluidic or MEMS components with silicon chips (e.g., integrated circuits or sensors). Compared to rigid substrates used in more conventional SU-8/SU-8 bonding process, the flexible photosensitive films used within this lamination technique allows a more homogeneous and reliable bonding at low pressure and temperature, and a 3D fabrication with an excellent level-to-level alignment. A parametric optimization of the lamination process is presented. The fabrication of a leakage-free 3D microfluidic network is demonstrated by stacking up to five layers. A polyethylene terephtalate layer has been employed to easily release the SU-8 devices. We show that this release layer also significantly decrease the curvature of the substrate by 32% and the related residual stress in a 100 μm SU-8 layer by at least 10%. Finally, we briefly describe the hybrid integration of a silicon sensor in a microfluidic network as a direct application of our lamination process to the fabrication of lab-on-chips.

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Sequential Field-Flow Cell Separation Method in a Dielectrophoretic Chip With 3-D Electrodes

This paper presents a sequential dielectrophoretic field-flow separation method for particle populations using a chip with a 3-D electrode structure. A unique characteristic of our chip is that the walls of the microfluidic channels also constitute the device's electrodes. This property confers the opportunity to use the electrodes' shape to generate not only the electric field gradient required for dielectrophoretic force but also a fluid velocity gradient. This interesting combination gives rise to a new solution for the dielectrophoretic separation of two particle populations. The proposed sequential field-flow separation method consists of four steps. First, the microchannel is filled with the mixture of the two populations of particle. Second, the particle populations are trapped in different locations of the microfluidic channels. The population, which exhibits positive dielectrophoresis (DEP), is trapped in the area where the distance between the electrodes is the minimum, while the other population that exhibits negative DEP is trapped in locations of maximum distance between electrodes. In the next step, increasing the flow in the microchannels will result in an increased hydrodynamic force that sweeps the cell population trapped by positive DEP out of the chip. In the last step, the electric field is removed, and the second population is swept out and collected at the outlet. For theoretical and experimental exemplification of the separation method, a population of viable and nonviable yeast cells was considered.     

Fabrication of three-dimensional hemispherical structures using photolithography

A photolithography technique using SU-8 and PDMS was developed to fabricate three-dimensional hemispherical structures. This technique utilized a mask-aligner and normal binary coded photomasks to generate hemispherical pits on SU-8, followed by PDMS molding to obtain an array of dome-shaped structures. Using this technique, a microfluidic device was fabricated with a patterning area that consisted of an array of 5 μm wells and dome-shaped structures with 10 μm diameter and 6 μm height. Encoded microbeads, 6 μm in size, were immobilized and patterned in the microfluidic device under flow conditions and a DNA hybridization experiment was performed to demonstrate the incorporation of encoded beads that would enable a high level of multiplexing in bioassays.     

Particle focusing in a contactless dielectrophoretic microfluidic chip with insulating structures

The focusing of biological and synthetic particles in microfluidic devices is a crucial step for the construction of many microstructured materials as well as for medical applications. The present study examines the feasibility of using contactless dielectrophoresis (cDEP) in an insulator-based dielectrophoretic (iDEP) microdevice to effectively focus particles. Particles 10?μm in diameter were introduced into the microchannel and pre-confined hydrodynamically by funnel-shaped insulating structures near the inlet. The particles were repelled toward the center of the microchannel by the negative DEP forces generated by the insulating structures. The microchip was fabricated based on the concept of cDEP. The electric field in the main microchannel was generated using electrodes inserted into two conductive micro-reservoirs, which were separated from the main microchannel by 20-μm-thick insulating barriers made of polydimethylsiloxane (PDMS). The impedance spectrum of the thin insulating PDMS barrier was measured to investigate its capacitive behavior. Experiments employing polystyrene particles were conducted to demonstrate the feasibility of the proposed microdevice. Results show that the particle focusing performance increased with increasing frequency of the applied AC voltage due to the reduced impedance of PDMS barriers at high frequencies. When the frequency was above 800?kHz, most particles were focused into a single file. The smallest width of focused particles distributed at the outlet was about 13.1?μm at a frequency of 1?MHz. Experimental results also show that the particle focusing performance improved with increasing applied electric field strength and decreasing inlet flow rate. The usage of the cDEP technique makes the proposed microchip mechanically robust and chemically inert.     

Continuous-flow ATP amplification system on a chip

In this study, we constructed a novel microfluidic device for continuous-flow ATP amplification, using the SU-8:PDMS method. Sepharose beads immobilized with adenylate kinase and pyruvate kinase was packed into a microfluidic chamber to form lamination layer. Dry film type SU-8 was suitable to form a very thick mold for beads column reactor and its dam structure. A good correlation between amplified luminescence and initial ATP concentration was observed in this system. The gradient of amplification when performing six cycles of continuous-flow ATP amplification was 1.72^N.     

Development of surface tension-driven microboats and microflotillas

Surface tension gradients may induce the motions of a floating solid fragment. This mechanism has been employed to drive miniaturized objects on a water surface. These objects could function, for example, as rotators. In the meanwhile, based on such a mechanism, we have recently developed microboats and microflotillas, and explored their movements on water surfaces. Isopropyl alcohol was adopted as the propellant. It had a surface tension much lower than water. Once the IPA exited the reservoir of a microboat or a microflotilla, the force induced by the difference between fore-and-aft surface tensions propelled the corresponding locomotive forwards. This article gives a simple review of the results that we have achieved to date, including exploration of SU-8 microboats, development of SU-8 microflotillas, and determination of driving mechanisms of these microboats and microflotillas. Two types of SU-8 microboats were developed, and their designs, fabrication and propulsion were explored. The first type of microboats did not have side rudders. They were capable of moving along their longitudinal directions. We considered the movements of this type of SU-8 boats in both horizontal and vertical planes. The second type of microboats had side rudders, which enabled these microboats to have radial motions. By varying the lengths of the side rudders, we were able to have a good control of the radial motions. To have more storage space, we further developed microflotillas, which each consisted of five SU-8 microboats that were linked with a Nylon rope. We explored design, fabrication and propulsion of these microflotillas, as well as their motions in both straight and circular channels. The developed boats and flotillas were capable of moving on water surfaces at speeds of order 0.1?m/s. Finally, we performed in situ observation of exchange processes of water and IPA inside the reservoir of a cm-scaled PDMS boat. Based on this observation, we interpreted driving mechanisms of the microboats and microflotillas. Existing approaches mainly use stationary devices for material supply and sensing detection. Compared with these devices, the developed boats and flotillas are mobile. This enables these miniaturized vehicles to actively deliver materials and sensors on demand for microfluidic applications.     

A new technology for microfluidic structures preparation based on a photoimageable ceramic

A new method for fabrication of hybrid ceramic-polymer structures with diversified geometry of microchannel was elaborated. This method is universal, non-complicated, and utilises commercially available materials and basic equipment for thick film technology and photolithography. A microchip for capillary electrophoresis was prepared as an example of microfluidic structure fabrication. The chip was prepared by using a photosensitive paste (dielectric FODEL 6050) which was screen printed onto a ceramic substrate, exposed through an appropriate mask, developed, fired and then glazed. In this way, we obtained the structure which can be bonded with poly(dimethylsiloxane) PDMS after oxygen plasma treatment. The application of transparent PDMS as a seal of the microchannel enabled the optical detection.