A method for UV-bonding in the fabrication of glass electrophoretic microchips

Membrane integration into PDMS-free microfluidic platforms for organ-on-chip and analytical chemistry applications

Membranes play a crucial role in many microfluidic systems, enabling versatile applications in highly diverse research fields. However, the tight and robust integration of membranes into microfluidic systems requires complex fabrication processes. Most integration approaches, so far, rely on polydimethylsiloxane (PDMS) as base material for the microfluidic chips. Several limitations of PDMS have resulted in the transition of many microfluidic approaches to PDMS-free systems using alternative materials such as thermoplastics. To integrate membranes in those PDMS-free systems, novel alternative approaches are required. This review provides an introduction into microfluidic systems applying membrane technology for analytical systems and organ-on-chip as well as a comprehensive overview of methods for the integration of membranes into PDMS-free systems. The overview and examples will provide a valuable resource and starting point for any researcher that is aiming at implementing membranes in microfluidic systems without using PDMS.

Subwavelength-Polarized Quasi-Two-Dimensional Perovskite Single-Mode Nanolaser

When approaching the subwavelength or deep subwavelength scale, there is a fundamental trade-off between the ultimate shrinking size and the performance for miniaturized lasers. Herein, to overcome this trade-off, we investigated the excitonic gain nature of quasi-two-dimensional (quasi-2D) perovskites and revealed that both singlet excitons and polarons would make nearly the entire contribution within ∼50 ps to a high net gain of 558 cm^-1. Inspired by the gain characteristic, we successfully shrank the quasi-2D perovskites laser to the subwavelength scale using only a layer of ultraviolet glue and a glass substrate in the vertical dimension. In spite of the compact and simple cavity structure, single-mode lasing with a highly linear polarization degree of 81% and a quality factor of 1635 was achieved. The extremely short cavity, excellent lasing performance, and simple structure of the quasi-2D perovskite laser are expected to provide insights into next-generation integrated laser sources.

Femtosecond laser 3D micromachining: a powerful tool for the fabrication of microfluidic, optofluidic, and electrofluidic devices based on glass

Femtosecond lasers have unique characteristics of ultrashort pulse width and extremely high peak intensity; however, one of the most important features of femtosecond laser processing is that strong absorption can be induced only at the focus position inside transparent materials due to nonlinear multiphoton absorption. This exclusive feature makes it possible to directly fabricate three-dimensional (3D) microfluidic devices in glass microchips by two methods: 3D internal modification using direct femtosecond laser writing followed by chemical wet etching (femtosecond laser-assisted etching, FLAE) and direct ablation of glass in water (water-assisted femtosecond laser drilling, WAFLD). Direct femtosecond laser writing also enables the integration of micromechanical, microelectronic, and microoptical components into the 3D microfluidic devices without stacking or bonding substrates. This paper gives a comprehensive review on the state-of-the-art femtosecond laser 3D micromachining for the fabrication of microfluidic, optofluidic, and electrofluidic devices. A new strategy (hybrid femtosecond laser processing) is also presented, in which FLAE is combined with femtosecond laser two-photon polymerization to realize a new type of biochip termed the ship-in-a-bottle biochip.

Temperature-driven self-actuated microchamber sealing system for highly integrated microfluidic devices

We present here a novel microchamber sealing valve that is self-actuated by a pressure change during the temperature change in the thermal activation of reactions. Actuation of our valve requires only the use of the same heating device as employed for the reactions. A thermoplastic UV-curable polymer is used as a device material; the polymer allows realization of the temperature-driven valve actuation as well as the fabrication of multi-layered devices. The self-actuated valve achieves effective sealing of the microchamber for the polymerase chain reaction (PCR) even at 90 °C, which is essential for developing highly parallel PCR array devices without the need for complicated peripherals to control the valve operation.

Research Progress of Materials and Fabrication Technologies of Microfluidic Chip

We review the current typical materials of microfluidic chip and discuss the microfabrication technologies. A variety of materials exist for fabrication of microchip, including silicon, glass, quartz, polymers and paper. Early developments in microchip materials were focus on the silicon, glass and quartz by referring to the sophisticated microfabrication techniques from microelectronics field. Recently, the introductions of low-cost materials and easily fabricated techniques have offered more alternative ways for rapid prototyping of disposable devices.

Surface patterning of bonded microfluidic channels

Microfluidic channels in which multiple chemical and biological processes can be integrated into a single chip have provided a suitable platform for high throughput screening, chemical synthesis, detection, and alike. These microchips generally exhibit a homogeneous surface chemistry, which limits their functionality. Localized surface modification of microchannels can be challenging due to the nonplanar geometries involved. However, chip bonding remains the main hurdle, with many methods involving thermal or plasma treatment that, in most cases, neutralizes the desired chemical functionality. Postbonding modification of microchannels is subject to many limitations, some of which have been recently overcome. Novel techniques include solution-based modification using laminar or capillary flow, while conventional techniques such as photolithography remain popular. Nonetheless, new methods, including localized microplasma treatment, are emerging as effective postbonding alternatives. This Review focuses on postbonding methods for surface patterning of microchannels.

Low-temperature, simple and fast integration technique of microfluidic chips by using a UV-curable adhesive

In the fields of MicroElectroMechanical Systems (MEMS) and Lab On a Chip (LOC), a device is often fabricated using diverse substrates which are processed separately and finally assembled together using a bonding process to yield the final device. Here we describe and demonstrate a novel straightforward, rapid and low-temperature bonding technique for the assembly of complete microfluidic devices, at the chip level, by employing an intermediate layer of gluing material. This technique is applicable to a great variety of materials (e.g., glass, SU-8, parylene, UV-curable adhesive) as demonstrated here when using NOA 81 as gluing material. Bonding is firstly characterized in terms of homogeneity and thickness of the gluing layer. Following this, we verified the resistance of the adhesive layer to various organic solvents, acids, bases and conventional buffers. Finally, the assembled devices are successfully utilized for fluidic experiments.

Room-temperature intermediate layer bonding for microfluidic devices

In this work a novel room-temperature bonding technique based on chemically activated Fluorinated Ethylene Propylene (FEP) sheet as an intermediate between chemically activated substrates is presented. Surfaces of silicon and glass substrates are chemically modified with APTES bearing amine terminal groups, while FEP sheet surfaces are treated to form carboxyl groups and subsequently activated by means of EDC-NHS chemistry. The activation procedures of silicon, glass and FEP sheet are characterized by contact angle measurements and XPS. Robust bonds are created at room-temperature by simply pressing two amine-terminated substrates together with activated FEP sheet in between. Average tensile strengths of 5.9 MPa and 5.2 MPa are achieved for silicon-silicon and glass-glass bonds, respectively, and the average fluidic pressure that can be operated is 10.2 bar. Moreover, it is demonstrated that FEP-bonded microfluidic chips can handle mild organic solvents at elevated pressures without leakage problems. This versatile room-temperature intermediate layer bonding technique has a high potential for bonding, packaging, and assembly of various (bio-) chemical microfluidic systems and MEMS devices.

Room temperature UV adhesive bonding of CE devices

A simple low temperature adhesive 'stamp-and-stick' bonding procedure for lab-on-a-chip glass devices has been tested for capillary electrophoresis applications. This technique involves use of a mask aligner to transfer a UV-curable adhesive selectively onto the top CE substrate which is then aligned with and bonded to the bottom CE wafer. The entire bonding process can be carried out at room temperature in less than 30 minutes, involved only user-friendly laboratory operations, and provided a near 100% success rate. CE microchips made in this manner exhibited similar electroosmotic flow and separation characteristics as ones made via conventional high temperature thermal bonding. Equally important, the devices provided stable long-term performance over weeks of use, encompassing hundreds of individual CE runs without structural failure or any apparent change in operating characteristics. Finally, these devices exhibited excellent chip-to-chip reproducibility. Successful adaptation of the stamp-and-stick approach did require the development and testing of new but easily implemented structural features which were incorporated into the chip design and whose nature is described in detail.

CE chips fabricated by injection molding and polyethylene/thermoplastic elastomer film packaging methods

This study presents a new packaging method using a polyethylene/thermoplastic elastomer (PE/TPE) film to seal an injection-molded CE chip made of either poly(methyl methacrylate) (PMMA) or polycarbonate (PC) materials. The packaging is performed at atmospheric pressure and at room temperature, which is a fast, easy, and reliable bonding method to form a sealed CE chip for chemical analysis and biomedical applications. The fabrication of PMMA and PC microfluidic channels is accomplished by using an injection-molding process, which could be mass-produced for commercial applications. In addition to microfluidic CE channels, 3-D reservoirs for storing biosamples, and CE buffers are also formed during this injection-molding process. With this approach, a commercial CE chip can be of low cost and disposable. Finally, the functionality of the mass-produced CE chip is demonstrated through its successful separation of phiX174 DNA/HaeIII markers. Experimental data show that the S/N for the CE chips using the PE/TPE film has a value of 5.34, when utilizing DNA markers with a concentration of 2 ng/microL and a CE buffer of 2% hydroxypropyl-methylcellulose (HPMC) in Tris-borate-EDTA (TBE) with 1% YO-PRO-1 fluorescent dye. Thus, the detection limit of the developed chips is improved. Lastly, the developed CE chips are used for the separation and detection of PCR products. A mixture of an amplified antibiotic gene for Streptococcus pneumoniae and phiX174 DNA/HaeIII markers was successfully separated and detected by using the proposed CE chips. Experimental data show that these DNA samples were separated within 2 min. The study proposed a promising method for the development of mass-produced CE chips.

Rapid bonding of Pyrex glass microchips

A newly developed vacuum hot press system has been specially designed for the thermal bonding of glass substrates in the fabrication process of Pyrex glass microchemical chips. This system includes a vacuum chamber equipped with a high-pressure piston cylinder and carbon plate heaters. A temperature of up to 900 degrees C and a force of as much as 9800 N could be applied to the substrates in a vacuum atmosphere. The Pyrex substrates bonded with this system under different temperatures, pressures, and heating times were evaluated by tensile strength tests, by measurements of thickness, and by observations of the cross-sectional shapes of the microchannels. The optimal bonding conditions of the Pyrex glass substrates were 570 degrees C for 10 min under 4.7 N/mm(2) of applied pressure. Whereas more than 16 h is required for thermal bonding with a conventional furnace, the new system could complete the whole bonding processes within just 79 min, including heating and cooling periods. Such improvements should considerably enhance the production rate of Pyrex glass microchemical chips. Whereas flat and dust-free surfaces are required for conventional thermal bonding, especially without long and repeated heating periods, our hot press system could press a fine dust into glass substrates so that even the areas around the dust were bonded. Using this capability, we were able to successfully integrate Pt/Ti thin film electrodes into a Pyrex glass microchip.

A low temperature bonding of quartz microfluidic chip for serum lipoproteins analysis

A low-temperature bonding method for microfabrication of quartz microfluidic chips has been developed. The bonding process involved two steps: pre-bonding and post-annealing at low temperature. The bonding quality was evaluated by measuring the shear force at bonding interface and the electrical properties of the chips. Shear force of 5.66 MPa (566 N/cm(2)) was obtained in a bonded chip after a post-annealing at 200 degrees C for 6 h. We owe the strong bonding strength to the formation of Si-O-Si bonds at the bonding interface during the post-annealing stage. The bonding procedures were not sensitive to surrounding and could be performed in a routine laboratory without clean room conditions. The performance of the fabricated microfluidic chips was tested by capillary zone electrophoresis (CZE) of serum lipoproteins with laser-induced fluorescence (LIF). The low-density (LDL) and high-density (HDL) lipoproteins in the serum was separated completely by using tricine buffer with methylglucamine.

Design and fabrication of a multilayered polymer microfluidic chip with nanofluidic interconnects via adhesive contact printing

The design and fabrication of a multilayered polymer micro-nanofluidic chip is described that consists of poly(methylmethacrylate) (PMMA) layers that contain microfluidic channels separated in the vertical direction by polycarbonate (PC) membranes that incorporate an array of nanometre diameter cylindrical pores. The materials are optically transparent to allow inspection of the fluids within the channels in the near UV and visible spectrum. The design architecture enables nanofluidic interconnections to be placed in the vertical direction between microfluidic channels. Such an architecture allows microchannel separations within the chip, as well as allowing unique operations that utilize nanocapillary interconnects: the separation of analytes based on molecular size, channel isolation, enhanced mixing, and sample concentration. Device fabrication is made possible by a transfer process of labile membranes and the development of a contact printing method for a thermally curable epoxy based adhesive. This adhesive is shown to have bond strengths that prevent leakage and delamination and channel rupture tests exceed 6 atm (0.6 MPa) under applied pressure. Channels 100 microm in width and 20 microm in depth are contact printed without the adhesive entering the microchannel. The chip is characterized in terms of resistivity measurements along the microfluidic channels, electroosmotic flow (EOF) measurements at different pH values and laser-induced-fluorescence (LIF) detection of green-fluorescent protein (GFP) plugs injected across the nanocapillary membrane and into a microfluidic channel. The results indicate that the mixed polymer micro-nanofluidic multilayer chip has electrical characteristics needed for use in microanalytical systems.

Construction of microfluidic chips using polydimethylsiloxane for adhesive bonding

A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 degrees C for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 microm, a approximately 200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.

Modification of poly(methyl methacrylate) microchannels for highly efficient and reproducible electrophoretic separations of double-stranded DNA

This paper deals with dynamic coating of the microchannels fabricated on poly(methyl methacrylate) (PMMA) chips and DNA separation by microchip electrophoresis (MCE). After testing a number of polymers, including 2-hydroxyethyl cellulose, hydroxypropylmethyl cellulose, different sizes of poly(ethylene oxide) (PEO), and poly(vinyl pyrrolidone) (PVP), we found that coating of the PMMA microchannels with PEO(Mr = 6.0 x 10(5) g/mol) on the first layer is essential to minimize the interaction of DNA with PMMA surface. To achieve high efficiency, multilayer coating of PMMA chips with PEO, PVP, and PEO containing gold nanoparticles [PEO(GNP)] is important. A 2-(PEO-PVP)-PEO(GNP) PMMA chip, which was repeatedly coated with 1.0% PEO and 5.0% PVP twice, and then coated with 0.75% PEO(GNP) each for 30 min, provided a high efficiency (up to 1.7 x 10(6) plates/m) for the separation of DNA markers V (pBR 322/HaeIII digest) and VI (pBR 328/BgiI digest and pBR 328/HinfI digest) when using 0.75% PEO(GNP). With such a high efficiency, we demonstrated the separation of hsp65 gene fragments of Mycobacterium HaeIII digests by MCE within 90 s. The advantages of this approach to DNA analysis include ease of filling the microchannel with 0.75% PEO(GNP), rapidity, and reproducibility.

Fabrication and characterization of 20 nm planar nanofluidic channels by glass-glass and glass-silicon bonding

We have characterized glass-glass and glass-Si bonding processes for the fabrication of wide, shallow nanofluidic channels with depths down to the nanometer scale. Nanochannels on glass or Si substrate are formed by reactive ion etching or a wet etching process, and are sealed with another flat substrate either by glass-glass fusion bonding (550 degrees C) or an anodic bonding process. We demonstrate that glass-glass nanofluidic channels as shallow as 25 nm with low aspect ratio of 0.0005 (depth to width) can be achieved with the developed glass-glass bonding technique. We also find that silicon-glass nanofluidic channels, as shallow as 20 nm with aspect ratio of 0.004, can be reliably obtained with the anodic bonding technique. The thickness uniformity of sealed nanofluidic channels is confirmed by cross-sectional SEM analysis after bonding. It is shown that there is no significant change in the depth of the nanofluidic channels due to anodic bonding and glass-glass fusion bonding processes.

A new fabrication process for a microchip electrophoresis device integrated with a three-electrode electrochemical detector

We report here a novel and simple process for the fabrication of a poly(methyl methacrylate) (PMMA)-based microchip electrophoresis device, integrated with a screen-printed three-electrode electrochemical detector that does not require a replicate mold. In this approach, a photoresist layer constitutes both an adhesion layer and side walls of 50 mum wide and 50 mum tall microfluidic channels on a screen-printed three-electrode PMMA substrate. Openings were drilled for buffer reservoirs on an additional piece of PMMA, then the final device was bonded in a PMMA/photoresist/PMMA sandwich configuration. This process is inexpensive, less time-consuming, and simpler compared with traditional fabrication methods. The combination of this PMMA-based microchip fabrication together with screen-printed electrode technology holds great promise for the mass production of a single-use micrototal analytical system. Successful determination of uric acid and L-ascorbic acid with the presented system validates its utility. In combination with a suitable electrochemical detector, this device holds much promise for the determination of other analytes in various biological samples for medical and clinical diagnosis.

Microchip electrophoresis-based separation of DNA

Miniaturized instruments have developed very quickly in the last decade. This review is focused on the microchip electrophoresis-based separation of DNA. Fundamentals, including the chip format, substrates and fabrication technologies, fluid control, as well as various detection methods, are summarized. Array electrophoresis microchip and the on-chip integration of electrophoresis with other systems are introduced as well. In addition, the application of microchip electrophoresis in DNA sizing, genetic analysis and DNA sequencing are also presented in this paper.

Laser-induced fluorescence detection on multichannel electrophoretic microchips using microprocessor-embedded acousto-optic laser beam scanning

An improved method for fast scanning and fluorescence detection on multimicrochannel microchips is presented using acousto-optic-deflection-driven laser-beam scanning. A microprocessor embedded subsystem used in conjunction with LabView program as the human-machine interface for control of laser-beam scanning and data preprocessing allowed faster scanning and addressing speeds to be attained and improved attenuation calibration and the data sampling speed. This system allows for flexible, high-resolution fluorescence detection for multimicrochannel electrophoresis in a manner that can be applied to a number of high-throughput analysis applications. Incorporating an F-theta focusing lens into the optical set-up allowed for a laser spot as small as 10 microm to accurately be addressed to the center of microchannels. With this spot size, it will be possible to further increase the channel density in the scanning range without encountering crosstalk. Using a six-channel microchip (four separation channels, two alignment channels), the simultaneous separation and fluorescence detection of amino acids and DNA digest samples in four channels is illustrated. User-friendly interpretation of the separation data is facilitated not only by a peak alignment/normalization routine developed within the software, but also through improved signal-to-noise ratios obtained through exploitation of signal processing.