Study of SU-8 to make a Ni master-mold: Adhesion, sidewall profile, and removal

For disposable microfluidic devices, easy and inexpensive fabrication is essential. Consequently, replication of microfluidic devices, using injection molding or hot embossing, from a master-mold is widely used. However, the conventional master-mold fabrication technique is unsatisfactory in terms of time and costs. In this regard, direct Ni growth (electroplating) from a back plate is promising when the photoresist is well-defined. Here, we demonstrate the use of SU-8 as a photoresist to define the Ni-growth region. We accomplish this application by focusing on the adhesion, the sidewall profile, and the removal of SU-8: the adhesion is enhanced by controlling the exposure dose, the soft-baking time, and by choosing the adhesion-promoting layer; the sidewall profile is regulated by selecting the intensity of each exposed wavelength, showing an aspect ratio of up to 20.9; and, easy removal is achieved by choosing a proper photoresist-stripper. Using the master-mold fabricated by this method, we test the mechanical stability of the features according to the aspect ratio and length; in the hot embossing process, the features are stable in the aspect ratio of up to 5.8 at a length of 200 microm. In addition, the plastic devices fabricated from this method are applied to the passive stop valves, showing a capillary pressure (-0.2 to -7.2 kPa).     

Patterning of a random copolymer of poly[lactide-co-glycotide-co-(epsilon-caprolactone)] by UV embossing for tissue engineering

The random copolymer, poly[lactide-co-glycotide-co-(epsilon-caprolactone)] (PLGACL) diacrylate was prepared by ring-opening polymerization of L-lactide, glycolide, and epsilon-caprolactone initiated with tetra(ethylene glycol). The diacrylated polymers were extensively characterized. With a UV embossing method, these copolymers were successfully fabricated into microchannels separated by microwalls with a high aspect (height/width) ratio. The PLGACL network films showed good cytocompatibility. Varieties of microstructures were fabricated, such as 10 x 40 x 60, 10 x 80 x 60, 25 x 40 x 60, or 25 x 80 x 60 microm(3) structures (microwall width x microchannel width x microwall height). The results demonstrated that smooth muscle cells (SMCs) can grow not only on the microchannel surfaces but also on the surfaces of the microwall and sidewall. The SMCs aligned along the 25 microm wide microwall with an elongated morphology and proliferated very slowly in comparison to those on the smooth surface with a longer cell-culture term. Few cells could attach and spread on the surface of the 40 microm wide microchannel, while the cells flourished on the 80 microm, or more than 80 microm, wide microchannel with a spindle morphology. The biophysical mechanism mediated by the micropattern geometry is discussed. Overall, the present micropattern, consisting of biodegradable and cytocompatible PLGACL, provides a promising scaffold for tissue engineering.     

Three-dimensional closed microfluidic channel fabrication by stepper projection single step lithography: the diabolo effect

Microfluidic devices are currently being used in many types of biochemical microsystems for liquid phase analysis in the frame of medical applications. This paper presents a new technique for the realization of microfluidic channels using SU-8, a commonly used epoxy-based negative photo-resist. These microchannels were fabricated by a single stepper UV-photolithography process. By changing the process parameters, e.g. the optical focus depth and the UV exposure dose, well-defined, covered microchannels with various dimensions and aspect ratios were realized and proven to be effective for the fluid transport by capillarity. This technique can easily be used for the fabrication of microfluidic devices in the microanalysis and lab-on-chip applications realm.     

Design and fabrication of integrated solid-phase extraction-zone electrophoresis microchip

Integrated solid-phase extraction-zone electrophoresis (SPE-ZE) device has been designed and fabricated on microchip. The structures were fabricated by using multiple layers of SU-8 polymer with a novel technique that enables easy alignment and high yield of the chips. SU-8 adhesive bonding has two major advantages: it enables bonding of high aspect ratio pillars and it results in fully SU-8 microchannels with uniform electrokinetic flow properties. The SPE-ZE device has a fluidic reservoir with 15:1 high aspect ratio pillars for bead filters that act as a SPE part in the chip structure. The separation unit is a 25 mm long electrophoresis channel starting from the outlet of SPE reservoir. Argon laser-induced fluorescence (LIF) detector was used to monitor simultaneously the SPE reservoir and the detection site at the end of the electrophoresis channel. Flow characteristics and electric field distributions were simulated with Femlab software. Fluorescein was used as the analyte for detecting the operational performance of the chip. Adsorption, bead rinsing, elution and detection were tested to verify functioning of the chip design.     

Characterization of SU-8 for electrokinetic microfluidic applications

The characterization of SU-8 microchannels for electrokinetic microfluidic applications is reported. The electroosmotic (EO) mobility in SU-8 microchannels was determined with respect to pH and ionic strength by the current monitoring method. Extensive electroosmotic flow (EOF), equal to that for glass microchannels, was observed at pH > or =4. The highest EO mobility was detected at pH > or =7 and was of the order of 5.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer. At pH < or =3 the electroosmotic flow was shown to reverse towards the anode and to reach a magnitude of 1.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer (pH 2). Also the zeta-potential on the SU-8 surface was determined, employing lithographically defined SU-8 microparticles for which a similar pH dependence was observed. SU-8 microchannels were shown to perform repeateably from day to day and no aging effects were observed in long-term use.     

高聚物微流控芯片镍阳模的简易加工技术

提出了一种简便快速制作高聚物微流控芯片镍阳模的新方法。采用抛光镍片作为电铸基底,涂覆SU-8光胶层后,光刻得到SU-8微结构。以镍基片作为阳极,用16～30A/dm2的电流密度电解刻蚀5min,清除SU-8微结构间隙底部镍片表面的氧化物,并刻蚀得到10～20μm深的凹坑,有效地提高了随后电沉积镍结构和基底镍片间结合力。利用SU-8微结构作为电铸模板,以镍基片作为阴极,电铸5h后制得了微结构倾角为83°深宽比较大的镍阳模。实现了在普通化学实验室中长寿命镍阳模的制作。用热压法制得500多片聚甲基丙烯酸甲酯(PMMA)聚合物芯片,并成功用于DNA片段的分离。     

Dielectrophoresis switching with vertical sidewall electrodes for microfluidic flow cytometry

A novel dielectrophoresis switching with vertical electrodes in the sidewall of microchannels for multiplexed switching of objects has been designed, fabricated and tested. With appropriate electrode design, lateral DEP force can be generated so that one can dynamically position particulates along the width of the channel. A set of interdigitated electrodes in the sidewall of the microchannels is used for the generation of non-uniform electrical fields to generate negative DEP forces that repel beads/cells from the sidewalls. A countering DEP force is generated from another set of electrodes patterned on the opposing sidewall. These lateral negative DEP forces can be adjusted by the voltage and frequency applied. By manipulating the coupled DEP forces, the particles flowing through the microchannel can be positioned at different equilibrium points along the width direction and continue to flow into different outlet channels. Experimental results for switching biological cells and polystyrene microbeads to multiple outlets (up to 5) have been achieved. This novel particle switching technique can be integrated with other particle detection components to enable microfluidic flow cytometry systems.     

Monolithic integration of well-ordered nanoporous structures in the microfluidic channels for bioseparation

Gel electrophoresis and capillary gel electrophoresis are widely used for the separation of biomolecules. With increasing demand in the miniaturized devices such as lab-on-a-chip, it is necessary to integrate such a separation component into a chip format. Here, we describe a simple approach to fabricate robust three-dimensional periodic porous nanostructures inside the microchannels for the separation of DNA molecules. In our approach, the colloidal crystals were first grown inside the microchannel using evaporation assisted self-assembly process. Then the void spaces among the colloidal crystals were filled with epoxy-based negative tone photoresist (SU-8). UV radiation was used to cure the photoresist at the desired area inside the microchannel. After subsequent development and nanoparticle removal, the well-ordered nanoporous structures inside the microchannel were obtained. Our results indicated that it was possible to construct periodic porous nanostructures inside the microchannels with cavity size around 300 nm and interconnecting pores around 30 nm. The mobility of large DNA molecules with different sizes was measured as a function of the applied electric field in the nanoporous materials. It was also demonstrated that 1 kilo-base pair (kbp) DNA ladders could be separated in such an integrated system within 10 min under moderate electric field.     

Supercooled micro flows and application for asymmetric synthesis

Supercooled micro flow in microchannels was demonstrated. In order to clarify fundamental properties of the supercooling state in microchannels, freezing temperature was measured in the microchannels having widths ranging from 70 microm to 300 microm. The freezing temperature decreased with decreasing width of the microchannel when the microchannel wall was chemically modified with octadecylsilane group. The lowest freezing temperature was observed as -28 degrees C for water in the 70 microm wide microchannel. By contrast, the freezing temperature was -15 degrees C and did not depend on width when a microchannel with a bare glass surface was used. Next, the freezing point was measured with flow rates ranging from 0.1 to 2.0 microl min(-1) and no dependence on flow rate was observed. Then, the supercooled micro flow was applied to an asymmetric reaction in micro two-phase flow of aqueous and CH2Cl2 phases. As expected from thermodynamic prediction, enantiomeric selectivity increased in the supercooled state of water.     

Interfacing SH-SY5Y human neuroblastoma cells with SU-8 microstructures

Microwell structures were fabricated using SU-8 photoresist for engineering a quasi-three-dimensional (quasi-3D) microenvironment for cultured neuronal cells. SH-SY5Y human neuroblastoma cells were successfully integrated into microwells of a nominal diameter of 100 microm, with or without 10-microm wide microchannels connecting neighboring microwells, in an aspect ratio (ratio of structure depth over width) of approximately 1. With the help of polyethylene glycol stamping and laminin coating, a neuronal-like network was achieved by integrating populations of SH-SY5Y cells with a microwell network pattern. Resting membrane potential establishment was evaluated with confocal microscopy and the potentiometric fluorescent dye tetramethylrhodamine methyl ester. It was found that the intra/extracellular fluorescent intensity ratio (R) was 2.4+/-1.4 [n (number of cells measured)=112] for SH-SY5Y cells on flat SU-8 substrates on day 5 into differentiation, which was not significantly different from the ratio on day 13 into differentiation, 2.0+/-1.8 (n=104) (P>0.05). For cells in the microwell network structures, R was 4.8+/-4.7 (n=51) and 3.9+/-3.2 (n=62) on days 5 and 13 into differentiation, respectively (P>0.5). Cells within the network structures had higher R ratios than on flat substrates, for either day 5 or 13 into differentiation (P<0.01). These results demonstrated that the well network structures, or topographically patterned substrates, were more suitable formats for promoting SH-SY5Y cell resting membrane potential establishment than flat substrates, suggesting the potential to control cellular function through substrate topography engineering.     

Application of magnetohydrodynamic actuation to continuous flow chemistry

Continuous flow microreactors with an annular microchannel for cyclical chemical reactions were fabricated by either bulk micromachining in silicon or by rapid prototyping using EPON SU-8. Fluid propulsion in these unusual microchannels was achieved using AC magnetohydrodynamic (MHD) actuation. This integrated micropumping mechanism obviates the use of moving parts by acting locally on the electrolyte, exploiting its inherent conductive nature. Both silicon and SU-8 microreactors were capable of MHD actuation, attaining fluid velocities of the order of 300 microm s(-1) when using a 500 mM KCl electrolyte. The polymerase chain reaction (PCR), a thermocycling process, was chosen as an illustrative example of a cyclical chemistry. Accordingly, temperature zones were provided to enable a thermal cycle during each revolution. With this approach, fluid velocity determines cycle duration. Here, we report device fabrication and performance, a model to accurately describe fluid circulation by MHD actuation, and compatibility issues relating to this approach to chemistry.     

Polyimide and SU-8 microfluidic devices manufactured by heat-depolymerizable sacrificial material technique

The following paper describes a sacrificial layer method for the manufacturing of microfluidic devices in polyimide and SU-8. The technique uses heat-depolymerizable polycarbonates embedded in polyimide or SU-8 for the generation of microchannels and sealed cavities. The volatile decomposition products originating from thermolysis of the sacrificial material escape out of the embedding material by diffusion through the cover layer. The fabrication process was studied experimentally and theoretically with a focus on the decomposition of the sacrificial materials and their diffusion through the polyimide or SU-8 cover layer. It is demonstrated that the sacrificial material removal process is independent of the actual channel geometry and advances linearly with time unlike conventional sacrificial layer techniques. The fabrication method provides a versatile and fast technique for the manufacturing of microfluidic devices for applications in the field of microTAS and Lab-on-a-Chip.     

SDS-CGE of proteins in microchannels made of SU-8 films

This work describes the SDS-CGE of proteins carried out in microchannels made of the negative photoresist EPON SU-8. Embedded electrophoretic microchannels have been fabricated with a multilayer technology based on bonding and releasing steps of stacked SU-8 films. This technology allows the monolithic integration of the electrodes in the device. A high wafer fabrication yield and mass production compatibility guarantees low costs and high reliability. A poly(methyl methacrylate) (PMMA) packaging allows an easy setup and replacement of the device for electrophoresis experiments. In addition, the wire-bonding step is avoided. The electrophoretic mobilities of four proteins have been measured in microchannels filled with polyacrylamide. Different pore sizes have been tested obtaining their Ferguson plots. Finally, a separation of two proteins (20 and 36 kDa) has been carried out confirming that this novel device is suitable for protein separation. A resolution of 2.75 is obtained. This is the first time that this SU-8 microfluidic technology has been validated for SDS-CGE of proteins. This technology offers better separation performance than glass channels, at lower costs and with an easy packaging procedure.     

Oscillating laminar electrokinetic flow in infinitely extended rectangular microchannels

This paper has addressed analytically the problem of laminar flow in microchannels with rectangular cross-section subjected to a time-dependent sinusoidal pressure gradient and a sinusoidal electric field. The analytical solution has been determined based on the Debye-Hückel approximation of a low surface potential at the channel wall. We have demonstrated that Onsager's principle of reciprocity is valid for this problem. Parametric studies of streaming potential have shown the dependence of the electroviscous effect not only on the Debye length, but also on the oscillation frequency and the microchannel width. Parametric studies of electroosmosis demonstrate that the flow rate decreases due to an increase in frequency. The obtained solutions for both the streaming potential and electroosmotic flows become those for flow between two parallel plates in the limit of a large aspect ratio.     

Development of novel microscale system as immobilized enzyme bioreactor

This study involves a novel method for immobilized enzyme catalysis. The focus of the work was to design and construct a microscale bioreactor using microfabrication techniques traditionally employed within the semiconductor industry. Enzymes have been immobilized on the microreactor walls by incorporating them directly into the wall material. Fabricated microchannels have cross-sectional dimensions on the order of hundreds of micrometers, constructed using polydimethylsiloxane cast on silicon/SU-8 molds. The resulting ratio of high surface area to volume creates an efficient, continuous-flow reaction system. Transverse features also containing enzymes were molded directly into the channels.     

A stable intermediate wetting state after a water drop contacts the bottom of a microchannel or is placed on a single corner

It is considered that, after a water drop contacts the base of a roughness groove, water should immediately fill this roughness groove. Subsequently, Cassie-Baxter wetting state is transited to that of Wenzel. Accordingly, one of the criteria used to judge the transition from Cassie-Baxter to Wenzel states is whether a water drop has contact with the base of a roughness groove. In this work, through theoretical and experimental investigations, we show that this transition criterion does not always hold true in the case of microchannels. We first theoretically prove that, when an angle criterion is satisfied, there may exist an intermediate wetting state inside a microchannel after a water drop contacts the bottom of the microchannel. In this wetting state, water does not completely fill the microchannel, and air pockets still exist in its bottom corners. Also, the wetting state is stable in the sense that its energy state is lower than that of the Wenzel model. According to the angle criterion, such intermediate states may exist, for example, in microchannels with vertical sidewalls, when contact angles on the inner surfaces of these microchannels are larger than 135°. In addition to microchannels, the aforementioned intermediate state may also exist on a single corner (which is formed by a horizontal plate and an inclined plate), when the angle criterion is met. After theoretical modeling, we then conduct four types of tests on single corners and microchannels to validate the angle criterion. In these tests, once the angle criterion is met, stable intermediate states are observed on the corresponding samples. In addition, it is found from the two types of tests conducted on microchannels that, once Laplace pressure inside a water drop is gradually reduced, such an intermediate wetting state may be transited back to the original Cassie-Baxter state. On the other hand, the Wenzel state may not have such a reversal transition unless an additional force is applied to overcome energy barrier between Wenzel and Cassie-Baxter states.     

SU-8 as a structural material for labs-on-chips and microelectromechanical systems

Since its introduction in the nineties, the negative resist SU-8 has been increasingly used in micro- and nanotechnologies. SU-8 has made the fabrication of high-aspect ratio structures accessible to labs with no high-end facilities such as X-ray lithography systems or deep reactive ion etching systems. These low-cost techniques have been applied not only in the fabrication of metallic parts or molds, but also in numerous other micromachining processes. Its ease of use has made SU-8 to be used in many applications, even when high-aspect ratios are not required. Beyond these pattern transfer applications, SU-8 has been used directly as a structural material for microelectromechanical systems and microfluidics due to its properties such as its excellent chemical resistance or the low Young modulus. In contrast to conventional resists, which are used temporally, SU-8 has been used as a permanent building material to fabricate microcomponents such as cantilevers, membranes, and microchannels. SU-8-based techniques have led to new low-temperature processes suitable for the fabrication of a wide range of objects, from the single component to the complete lab-on-chip. First, this article aims to review the different techniques and provides guidelines to the use of SU-8 as a structural material. Second, practical examples from our respective labs are presented.     

High-throughput microfluidics: improved sample treatment and washing over standard wells

Fluid flow in microchannels is used to treat or wash samples and can be incorporated into high-throughput applications such as drug screening, which currently use standard microtiter wells for performing assays. This paper provides theoretical and experimental data comparing microchannels and standard wells on the metrics of sample washing and experimental error in treatment concentrations. It is shown numerically and experimentally that microchannel concentration can be approximated with an inverse linear relationship to input volume. The experimentally supported mathematical approximation and error propagation methods are used to compare the accuracy and precision of treatments in microchannels vs. standard wells. Mathematical results suggest microchannels can provide 10 or more times the treatment precision of standard wells for volume ratios typical of high-throughput screening. Passive-pumping and diffusion are utilized to improve microchannel accuracy and precision even further in a treat-wait-treat method. The advantages of microchannels outlined here can have large-scale effects on cost and accuracy in screening applications.     

Hydrodynamic control of the interface between two liquids flowing through a horizontal or vertical microchannel

We derive a Fourier expansion for estimating laminar hydrodynamic spreading when two immiscible liquids flow steadily, side by side, at low Reynolds number, in a horizontal microchannel of rectangular cross-section. When the aspect ratio of the height of the channel to its width is small this expansion can be truncated after a couple of terms and solved to yield the steady-state position of the boundary in terms of the constant volumetric flow rates, the aspect ratio and the viscosities of the two liquids. Our formula shows that we can control the location of the liquid boundary at the outlet of the microchannel by adjusting the relative volumetric flow rates of the two inlet streams. If the microchannel is oriented vertically and the volumetric flow rates are very low, the position of the boundary is also influenced by gravitational forces that depend on the relative densities of the two liquids. We have tested solutions to our analytical expansions by running computational fluid dynamics simulations and have found excellent agreement. When the aspect ratio is 0.01 the leading term in the Fourier expansion for the spreading has an accuracy of about 1%. This accuracy deteriorates to about 12% when the aspect ratio is 0.1. An understanding of these spreading mechanisms is fundamental to the design of T-sensors and related microfluidic devices.     

Fabrication of SU-8 based microchip electrophoresis with integrated electrochemical detection for neurotransmitters

A new SU-8 based microchip capillary electrophoresis (MCE) device has been developed for the first time with integrated electrochemical detection. Embedded electrophoretic microchannels have been fabricated with a multilayer technology based on bonding and releasing steps of stacked SU-8 films. This technology has allowed the monolithic integration in the device of the electrochemical detection system based on platinum electrodes. The fabrication of the chips presented in this work is totally compatible with reel-to-reel techniques, which guarantee a low cost and high reliability production. The influence of relevant experimental variables, such as the separation voltage and detection potential, has been studied on the SU-8 microchip with an attractive analytical performance. Thus, the effective electrical isolation of the end-channel amperometric detector has been also demonstrated. The good performance of the SU-8 device has been proven for separation and detection of the neurotransmitters, dopamine (DA) and epinephrine (EP). High efficiency (30,000-80,000 N/m), excellent precision, good detection limit (450 nM) and resolution (0.90-1.30) has been achieved on the SU-8 microchip. These SU-8 devices have shown a better performance than commercial Topas (thermoplastic olefin polymer of amorphous structure) microchips. The low cost and versatile SU-8 microchip with integrated platinum film electrochemical detector holds great promise for high-volume production of disposable microfluidic analytical devices.