Vacuum-assisted thermal bonding of plastic capillary electrophoresis microchip imprinted with stainless steel template

A Preliminary Experimental Study of Polydimethylsiloxane (PDMS)-To-PDMS Bonding Using Oxygen Plasma Treatment Incorporating Isopropyl Alcohol

Polydimethylsiloxane (PDMS) is a widely used material for soft lithography and microfabrication. PDMS exhibits some promising properties suitable for building microfluidic devices; however, bonding PDMS to PDMS and PDMS to other materials for multilayer structures in microfluidic devices is still challenging due to the hydrophobic nature of the surface of PDMS. This paper presents a simple yet effective method to increase the bonding strength for PDMS-to-PDMS using isopropyl alcohol (IPA). The experiment was carried out to evaluate the bonding strength for both the natural-cured and the heat-cured PDMS layer. The results show the effectiveness of our approach in terms of the improved irreversible bonding strength, up to 3.060 MPa, for the natural-cured PDMS and 1.373 MPa for the heat-cured PDMS, while the best bonding strength with the existing method in literature is 1.9 MPa. The work is preliminary because the underlying mechanism is only speculative and open for future research.

Recent Advances in Thermoplastic Microfluidic Bonding

Microfluidics is a multidisciplinary technology with applications in various fields, such as biomedical, energy, chemicals and environment. Thermoplastic is one of the most prominent materials for polymer microfluidics. Properties such as good mechanical rigidity, organic solvent resistivity, acid/base resistivity, and low water absorbance make thermoplastics suitable for various microfluidic applications. However, bonding of thermoplastics has always been challenging because of a wide range of bonding methods and requirements. This review paper summarizes the current bonding processes being practiced for the fabrication of thermoplastic microfluidic devices, and provides a comparison between the different bonding strategies to assist researchers in finding appropriate bonding methods for microfluidic device assembly.

Heterogeneous Bonding of PMMA and Double-Sided Polished Silicon Wafers through H2O Plasma Treatment for Microfluidic Devices

In this work we report on a rapid, easy-to-operate, lossless, room temperature heterogeneous H2O plasma treatment process for the bonding of poly(methyl methacrylate) (PMMA) and double-sided polished (DSP) silicon substrates by for utilization in sandwich structured microfluidic devices. The heterogeneous bonding of the sandwich structure produced by the H2O plasma is analyzed, and the effect of heterogeneous bonding of free radicals and high charge electrons (e−) in the formed plasma which causes a passivation phenomenon during the bonding process investigated. The PMMA and silicon surface treatments were performed at a constant radio frequency (RF) power and H2O flow rate. Changing plasma treatment time and powers for both processes were investigated during the experiments. The gas flow rate was controlled to cause ionization of plasma and the dissociation of water vapor from hydrogen (H) atoms and hydroxyl (OH) bonds, as confirmed by optical emission spectroscopy (OES). The OES results show the relative intensity peaks emitted by the OH radicals, H and oxygen (O). The free energy is proportional to the plasma treatment power and gas flow rate with H bonds forming between the adsorbed H2O and OH groups. The gas density generated saturated bonds at the interface, and the discharge energy that strengthened the OH-e− bonds. This method provides an ideal heterogeneous bonding technique which can be used to manufacture new types of microfluidic devices.

Fabrication of Polymerase Chain Reaction Plastic Lab-on-a-Chip Device for Rapid Molecular Diagnoses

Purpose:

We aim to fabricate a thermoplastic poly(methylmethacrylate) (PMMA) Lab-on-a-Chip device to perform continuous- flow polymerase chain reactions (PCRs) for rapid molecular detection of foodborne pathogen bacteria.

Methods:

A miniaturized plastic device was fabricated by utilizing PMMA substrates mediated by poly(dimethylsiloxane) interfacial coating, enabling bonding under mild conditions, and thus avoiding the deformation or collapse of microchannels. Surface characterizations were carried out and bond strength was measured. The feasibility of the Lab-on-a-Chip device for performing on-chip PCR utilizing a lab-made, portable dual heater was evaluated. The results were compared with those obtained using a commercially available thermal cycler.

Results:

A PMMA Lab-on-a-Chip device was designed and fabricated for conducting PCR using foodborne pathogens as sample targets. A robust bond was established between the PMMA substrates, which is essential for performing miniaturized PCR on plastic. The feasibility of on-chip PCR was evaluated using Escherichia coli O157:H7 and Cronobacter condimenti, two worldwide foodborne pathogens, and the target amplicons were successfully amplified within 25 minutes.

Conclusions:

In this study, we present a novel design of a low-cost and high-throughput thermoplastic PMMA Lab-on-a-Chip device for conducting microscale PCR, and we enable rapid molecular diagnoses of two important foodborne pathogens in minute resolution using this device. In this regard, the introduced highly portable system design has the potential to enable PCR investigations of many diseases quickly and accurately.

Recent Progress in Lab-on-a-Chip Technology and Its Potential Application to Clinical Diagnoses

We present the construction of the lab-on-a-chip (LOC) system, a state-of-the-art technology that uses polymer materials (i.e., poly[dimethylsiloxane]) for the miniaturization of conventional laboratory apparatuses, and show the potential use of these microfluidic devices in clinical applications. In particular, we introduce the independent unit components of the LOC system and demonstrate how each component can be functionally integrated into one monolithic system for the realization of a LOC system. In specific, we demonstrate microscale polymerase chain reaction with the use of a single heater, a microscale sample injection device with a disposable plastic syringe and a strategy for device assembly under environmentally mild conditions assisted by surface modification techniques. In this way, we endeavor to construct a totally integrated, disposable microfluidic system operated by a single mode, the pressure, which can be applied on-site with enhanced device portability and disposability and with simple and rapid operation for medical and clinical diagnoses, potentially extending its application to urodynamic studies in molecular level.

Microchip technology in mass spectrometry

Microfabrication of analytical devices is currently of growing interest and many microfabricated instruments have also entered the field of mass spectrometry (MS). Various (atmospheric pressure) ion sources as well as mass analyzers have been developed exploiting microfabrication techniques. The most common approach thus far has been the miniaturization of the electrospray ion source and its integration with various separation and sampling units. Other ionization techniques, mainly atmospheric pressure chemical ionization and photoionization, have also been subject to miniaturization, though they have not attracted as much attention. Likewise, all common types of mass analyzers have been realized by microfabrication and, in most cases, successfully applied to MS analysis in conjunction with on-chip ionization. This review summarizes the latest achievements in the field of microfabricated ion sources and mass analyzers. Representative applications are reviewed focusing on the development of fully microfabricated systems where ion sources or analyzers are integrated with microfluidic separation devices or microfabricated pums and detectors, respectively. Also the main microfabrication methods, with their possibilities and constraints, are briefly discussed together with the most commonly used materials.

Plasticizer-assisted bonding of poly(methyl methacrylate) microfluidic chips at low temperature

As an important phthalate plasticizer, dibutyl phthalate (DBP) was employed to decrease the bonding temperature of poly(methyl methacrylate) (PMMA) microfluidic chips in this work based on the fact that it can lower the glass transition temperature of PMMA. The channel plates of the PMMA microchips were fabricated by the UV-initiated polymerization of prepolymerized methyl methacrylate between a silicon template and a PMMA plate. Prior to bonding, DBP solution in isopropanol was coated on PMMA covers. When isopropanol in the coating was allowed to evaporate in air, DBP was left on the PMMA covers. Subsequently, the DBP-coated covers were bonded to the PMMA channel plates at 90 degrees C for 10min under pressure. The channels in the complete microchips had been examined by optical microscope and scanning electron microscope. The results indicated that high quality bonding was achieved below the glass transition temperature of PMMA ( approximately 105 degrees C). The performance of the PMMA microfluidic chips sealed by plasticizer-assisted bonding has been demonstrated by separating and detecting ionic species by capillary electrophoresis in connection with contactless conductivity detection.

Ultraviolet-based bonding for perfluoropolyether low aspect-ratio microchannels and hybrid devices

Producing solvent-resistant microfluidic devices is a challenge for analytical chemistry and biochemistry. We demonstrate a simple and low-cost fabrication approach for the realization of solvent-resistant microchannels based on perfluoropolyether elastomers, exhibiting very low aspect ratios (0.01). The strength of the microchannels sealing is evaluated through the maximum internal pressure (1.52 MPa) prior to device failure, due to delamination at the bonded interface. This approach allows the elastic properties of silicone elastomers, suitable for high quality external connections, to be combined with the non-swelling character of perfluoropolyethers.

Mechanical and chemical analysis of plasma and ultraviolet-ozone surface treatments for thermal bonding of polymeric microfluidic devices

Here we have demonstrated that radio frequency plasma and ultraviolet-ozone (UVO) surface modifications are effective treatments for enabling the thermal bonding of polymeric microfluidic chips at temperatures below the T(g) (glass transition temperature) of the polymer. The effects of UVO and plasma treatments on the surface properties of a cyclic polyolefin and polystyrene were examined with X-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM) surface roughness measurements and surface adhesion measurements with AFM force-distance data. Three-point bending tests using a dynamic mechanical analyzer (DMA) were used to characterize the bond strength of thermally sealed polymer parts and the cross-sections of the bonded microchannels were evaluated with scanning electron microscopy (SEM). The experimental results demonstrated that plasma and UVO surface treatments cause changes in the chemical and physical characteristics of the polymer surfaces, resulting in a decrease in T(g) at the surface, and thus allowing the microfluidic chips to be effectively bonded at temperatures lower than the T(g) of the bulk polymer without losing the intended channel geometry.

Characterization and performance of injection molded poly(methylmethacrylate) microchips for capillary electrophoresis

Injection molded poly(methylmethacrylate) (IM-PMMA), chips were evaluated as potential candidates for capillary electrophoresis disposable chip applications. Mass production and usage of plastic microchips depends on chip-to-chip reproducibility and on analysis accuracy. Several important properties of IM-PMMA chips were considered: fabrication quality evaluated by environmental scanning electron microscope imaging, surface quality measurements, selected thermal/electrical properties as indicated by measurement of the current versus applied voltage (I-V) characteristic and the influence of channel surface treatments. Electroosmotic flow was also evaluated for untreated and O2 reactive ion etching (RIE) treated surface microchips. The performance characteristics of single lane plastic microchip capillary electrophoresis (MCE) separations were evaluated using a mixture of two dyes-fluorescein (FL) and fluorescein isothiocyanate (FITC). To overcome non-wettability of the native IM-PMMA surface, a modifier, polyethylene oxide was added to the buffer as a dynamic coating. Chip performance reproducibility was studied for chips with and without surface modification via the process of RIE with O2 and by varying the hole position for the reservoir in the cover plate or on the pattern side of the chip. Additionally, the importance of reconditioning steps to achieve optimal performance reproducibility was also examined. It was found that more reproducible quantitative results were obtained when normalized values of migration time, peak area and peak height of FL and FITC were used instead of actual measured parameters.

Polymer based microchip for combined capillary electrophoresis and electrochemical detection

We have developed Microsystems with capillary electrophoresis and an electrochemical detector (CE-ECD). The microfabricated CE-ECD systems can be used as a disposable device and the characteristics have been optimized for application in electrochemical detection. The system has been realized on a Polystyrene (PS) polymer chip with gold electrodes. The injection and separation channels were produced by relatively simple and inexpensive methods. An electro-osmotic flow system (EOF) and a three-electrode electrochemical detector were fabricated on the same substrate in a single process. Cyclic voltammetry has been used to test the electrodes behaviour with catechol and dopamine in buffers of different pH. In this article, we give an overview on the methodological aspects of coupling ED (electrochemical Detection) with Capillary Electrophoretic systems.

Fabrication of microfluidic devices using dry film photoresist for microchip capillary electrophoresis

An inexpensive, disposable microfluidic device was fabricated from a dry film photoresist using a combination of photolithographic and hot roll lamination techniques. A microfluidic flow pattern was prefabricated in a dry film photoresist tape using traditional photolithographic methods. This tape became bonded to a poly(methyl methacrylate) (PMMA) sheet with prepouched holes when passed through a hot roll laminator. A copper working electrode and platinum decoupler was readily incorporated within this microchip. The integrated microchip device was then fixed in a laboratory-built Plexiglas holder prior to its use in microchip capillary electrophoresis. The performance of this device with amperometric detection for the separation of dopamine and catechol was examined. The separation was complete within 50 s at an applied potential of 200 V/cm. The relative standard deviations (RSD) of analyte migration times were less than 0.71%, and the theoretical plate numbers for dopamine and catechol were 3.2 x 10(4) and 4.1 x 10(4), respectively, based on a 65 mm separation channel.

Fabrication and characterization of poly(methylmethacrylate) microfluidic devices bonded using surface modifications and solvents

The fabrication of polymer microchips allows inexpensive, durable, high-throughput and disposable devices to be made. Poly(methylmethacrylate) (PMMA) microchips have been fabricated by hot embossing microstructures into the substrate followed by bonding a cover plate. Different surface modifications have been examined to enhance substrate and cover plate adhesion, including: air plasma treatment, and both acid catalyzed hydrolysis and aminolysis of the acrylate to yield carboxyl and amine-terminated PMMA surfaces. Unmodified PMMA surfaces were also studied. The substrate and cover plate adhesion strengths were found to increase with the hydrophilicity of the PMMA surface and reached a peak at 600 kN m(-2) for plasma treated PMMA. A solvent assisted system has also been designed to soften less than 50 nm of the surface of PMMA during bonding, while still maintaining microchannel integrity. The extent to which both surface modifications and solvent treatment affected the adhesion of the substrate to the cover plate was examined using nanoindentation methods. The solvent bonding system greatly increased the adhesion strengths for both unmodified and modified PMMA, with a maximum adhesion force of 5500 kN m(-2) achieved for unmodified PMMA substrates. The bond strength decreased with increasing surface hydrophilicity after solvent bonding, a trend that was opposite to what was observed for non-solvent thermal bonding.

Low temperature bonding of poly(methylmethacrylate) electrophoresis microchips by in situ polymerisation

A novel method for bonding poly(methyl methacrylate) (PMMA) electrophoresis microchips at the temperature below the glass transition temperature of PMMA based on in situ polymerization has been demonstrated. Methyl methacrylate (MMA) containing initiators was allowed to prepolymerize in an 85 degrees C water bath for 8 min and 15 min to produce a bonding solution and a dense molding solution, respectively. The channel plate of the PMMA microchip was fabricated by the UV-initiated polymerization of the molding solution between a nickel template and a PMMA plate at room temperature. Prior to bonding, the blank cover was coated with a thin layer of the bonding solution and was bonded to the channel plate at 95 degrees C for 20 min under the pressure of binder clips. The attractive performance of the PMMA chips bonded by the new approach has been demonstrated by separating and detecting dopamine, catechol, three cations, and three organic acids in connection with end-column amperometric detection and contactless conductivity detection.

A polymeric master replication technology for mass fabrication of poly(dimethylsiloxane) microfluidic devices

A protocol of producing multiple polymeric masters from an original glass master mold has been developed, which enables the production of multiple poly(dimethylsiloxane) (PDMS)-based microfluidic devices in a low-cost and efficient manner. Standard wet-etching techniques were used to fabricate an original glass master with negative features, from which more than 50 polymethylmethacrylate (PMMA) positive replica masters were rapidly created using the thermal printing technique. The time to replicate each PMMA master was as short as 20 min. The PMMA replica masters have excellent structural features and could be used to cast PDMS devices for many times. An integration geometry designed for laser-induced fluorescence (LIF) detection, which contains normal deep microfluidic channels and a much deeper optical fiber channel, was successfully transferred into PDMS devices. The positive relief on seven PMMA replica masters is replicated with regard to the negative original glass master, with a depth average variation of 0.89% for 26-microm deep microfluidic channels and 1.16% for the 90 mum deep fiber channel. The imprinted positive relief in PMMA from master-to-master is reproducible with relative standard deviations (RSDs) of 1.06% for the maximum width and 0.46% for depth in terms of the separation channel. The PDMS devices fabricated from the PMMA replica masters were characterized and applied to the separation of a fluorescein isothiocyanate (FITC)-labeled epinephrine sample.

Recent developments in electrochemical detection for microchip capillary electrophoresis

Significant progress in the development of miniaturized microfluidic systems has occurred since their inception over a decade ago. This is primarily due to the numerous advantages of microchip analysis, including the ability to analyze minute samples, speed of analysis, reduced cost and waste, and portability. This review focuses on recent developments in integrating electrochemical (EC) detection with microchip capillary electrophoresis (CE). These detection modes include amperometry, conductimetry, and potentiometry. EC detection is ideal for use with microchip CE systems because it can be easily miniaturized with no diminution in analytical performance. Advances in microchip format, electrode material and design, decoupling of the detector from the separation field, and integration of sample preparation, separation, and detection on-chip are discussed. Microchip CEEC applications for enzyme/immunoassays, clinical and environmental assays, as well as the detection of neurotransmitters are also described.