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.     

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.     

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.     

Hybrid microfluidic cartridge formed by irreversible bonding of SU-8 and PDMS for multi-layer flow applications

SU-8 and polydimethylsiloxane (PDMS) are both transparent materials with properties very convenient for rapid prototyping of microfluidic systems. However, previous efforts of combining these two materials failed due to poor adhesion between them. Herein, we introduce a promising low-temperature technique (< 100 °C) to irreversibly bond two or more structured layers of SU-8 and PDMS to create hybrid stacks. This offers new possibilities in design and fabrication of enclosed three-dimensional microstructures and microchannels with simple soft-lithography techniques. The potential of this method is demonstrated by the fabrication of a new version of our microfluidic sensor cartridge that was reported recently¹.     

Rapid prototyping of poly(methyl methacrylate) microfluidic systems using solvent imprinting and bonding

We have developed a method for rapid prototyping of hard polymer microfluidic systems using solvent imprinting and bonding. We investigated the applicability of patterned SU-8 photoresist on glass as an easily fabricated template for solvent imprinting. Poly(methyl methacrylate) (PMMA) exposed to acetonitrile for 2 min then had an SU-8 template pressed into the surface for 10 min, which provided appropriately imprinted channels and a suitable surface for bonding. After a PMMA cover plate had also been exposed to acetonitrile for 2 min, the imprinted and top PMMA pieces could be bonded together at room temperature with appropriate pressure. The total fabrication time was less than 15 min. Under the optimized fabrication conditions, nearly 30 PMMA chips could be replicated using a single patterned SU-8 master with high chip-to-chip reproducibility. Relative standard deviations were 2.3% and 5.4% for the widths and depths of the replicated channels, respectively. Fluorescently labeled amino acid and peptide mixtures were baseline separated using these PMMA microchips in <15s. Theoretical plate numbers in excess of 5000 were obtained for a approximately 3 cm separation distance, and the migration time relative standard deviation for an amino acid peak was 1.5% for intra-day and 2.2% for inter-day analysis. This new solvent imprinting and bonding approach significantly simplifies the process for fabricating microfluidic structures in hard polymers such as PMMA.     

Fabrication of enclosed SU-8 tips for electrospray ionization-mass spectrometry

We describe a novel electrospray tip design for MS which is fabricated completely out of SU-8 photoepoxy. A three-layer SU-8 fabrication process provides fully enclosed channels and tips. The tip shape and alignment of all SU-8 layers is done lithographically and is therefore very accurate. Fabrication process enables easy integration of additional fluidic functions on the same chip. Separation channels can be made with exactly the same process. Fluidic inlets are made in SU-8 during the fabrication process and no drilling or other postprocessing is needed. Channels have been fabricated and tested in the size range of 10 microm x 10 microm-50 microm x 200 microm. Mass spectrometric performance of the tips has been demonstrated with both pressure-driven flow and EOF. SU-8 microtips have been shown to produce stable electrospray with EOF in a timescale of tens of minutes. With pressure driven flow stable spray is maintained for hours. Taylor cone was shown to be small in volume and well defined even with the largest channel cross section. The spray was also shown to be well directed with our tip design.     

Thermoplastic elastomers for microfluidics: towards a high-throughput fabrication method of multilayered microfluidic devices

Multilayer soft lithography of polydimethylsiloxane (PDMS) is a well-known method for the fabrication of complex fluidic functions. With advantages and drawbacks, this technique allows fabrication of valves, pumps and micro-mixers. However, the process is inadequate for industrial applications. Here, we report a rapid prototyping technique for the fabrication of multilayer microfluidic devices, using a different and promising class of polymers. Using styrenic thermoplastic elastomers (TPE), we demonstrate a rapid technique for the fabrication and assembly of pneumatically driven valves in a multilayer microfluidic device made completely from thermoplastics. This material solution is transparent, biocompatible and as flexible as PDMS, and has high throughput thermoforming processing characteristics. We established a proof of principle for valving and mixing with three different grades of TPE using an SU-8 master mold. Specific viscoelastic properties of each grade allow us to report enhanced bonding capabilities from room temperature bonding to free pressure thermally assisted bonding. In terms of microfabrication, beyond classically embossing means, we demonstrate a high-throughput thermoforming method, where TPE molding experiments have been carried out without applied pressure and vacuum assistance within an overall cycle time of 180 s. The quality of the obtained thermoplastic systems show robust behavior and an opening/closing frequency of 5 Hz.     

Nanoparticles fabricated by selective reaction of Fe100-xPtx alloy films during imidization of polyamic acid

Nanoparticles with different morphology and composition were fabricated inside a polyimide (PI) matrix based on selectively oxidizing a layer of Fe(100-x)Pt(x) alloy metal film sandwiched between two PI precursor layers. Gamma-Fe2O3, Pt, and Fe3Pt nanoparticles were formed in a monolayer between two PI layers, depending on the alloy film composition and curing conditions. These particles were well-crystallized and sized between 4 and 10 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film preferentially reacted with the organic matrix whereas Pt remained metallic throughout the curing process, which enabled fabrication of particles different morphology and composition. This process can be easily extended to other alloy films, which provides an opportunity to fabricate nanoparticles relatively easily with desired composition and morphology embedded in an inert organic matrix.     

Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds

Scalable and cost effective patterning of polymer structures and their surface textures is essential to engineer material properties such as liquid wetting and dry adhesion, and to design artificial biological interfaces. Further, fabrication of high-aspect-ratio microstructures often requires controlled deep-etching methods or high-intensity exposure. We demonstrate that carbon nanotube (CNT) composites can be used as master molds for fabrication of high-aspect-ratio polymer microstructures having anisotropic nanoscale textures. The master molds are made by growth of vertically aligned CNT patterns, capillary densification of the CNTs using organic solvents, and capillary-driven infiltration of the CNT structures with SU-8. The composite master structures are then replicated in SU-8 using standard PDMS transfer molding methods. By this process, we fabricated a library of replicas including vertical micro-pillars, honeycomb lattices with sub-micron wall thickness and aspect ratios exceeding 50:1, and microwells with sloped sidewalls. This process enables batch manufacturing of polymer features that capture complex nanoscale shapes and textures, while requiring only optical lithography and conventional thermal processing.     

Controlling structure from the bottom-up: structural and optical properties of layer-by-layer assembled palladium coordination-based multilayers

Layer-by-layer assembly of two palladium coordination-based multilayers on silicon and glass substrates is presented. The new assemblies consist of rigid-rod chromophores connected by terminal pyridine moieties to palladium centers. Both colloidal palladium and PdCl2(PhCN)2 were used in order to determine the effect of the metal complex precursor on multilayer structure and optical properties. The multilayers were formed by an iterative wet-chemical deposition process at room temperature in air on a siloxane-based template layer. Twelve consecutive deposition steps have been demonstrated resulting in structurally regular assemblies with an equal amount of chromophore and palladium added in each molecular bilayer. The optical intensity characteristics of the metal-organic films are clearly a function of the palladium precursor employed. The colloid-based system has a UV-vis absorption maximum an order of magnitude stronger than that of the PdCl2-based multilayer. The absorption maximum of the PdCl2-based film exhibits a significant red shift of 23 nm with the addition of 12 layers. Remarkably, the structure and physiochemical properties of the submicron scale PdCl2-based structures are determined by the configuration of the approximately 15 angstroms thick template layer. The refractive index of the PdCl2-based film was determined by spectroscopic ellipsometry. Well-defined three-dimensional structures, with a dimension of 5 microm, were obtained using photopatterned template monolayers. The properties and microstructure of the films were studied by UV-vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray reflectivity (XRR), scanning electron microscopy (SEM), and aqueous contact angle measurements (CA).     

Multi-step surface functionalization of polyimide based evanescent wave photonic biosensors and application for DNA hybridization by Mach-Zehnder interferometer

The process of surface functionalization involving silanization, biotinylation and streptavidin bonding as platform for biospecific ligand immobilization was optimized for thin film polyimide spin-coated silicon wafers, of which the polyimide film serves as a wave guiding layer in evanescent wave photonic biosensors. This type of optical sensors make great demands on the materials involved as well as on the layer properties, such as the optical quality, the layer thickness and the surface roughness. In this work we realized the binding of a 3-mercaptopropyl trimethoxysilane on an oxygen plasma activated polyimide surface followed by subsequent derivatization of the reactive thiol groups with maleimide-PEG₂-biotin and immobilization of streptavidin. The progress of the functionalization was monitored by using different fluorescence labels for optimization of the chemical derivatization steps. Further, X-ray photoelectron spectroscopy and atomic force microscopy were utilized for the characterization of the modified surface. These established analytical methods allowed to derive information like chemical composition of the surface, surface coverage with immobilized streptavidin, as well as parameters of the surface roughness. The proposed functionalization protocol furnished a surface density of 144 fmol mm⁻² streptavidin with good reproducibility (13.9% RSD, n = 10) and without inflicted damage to the surface. This surface modification was applied to polyimide based Mach-Zehnder interferometer sensors to realize a real-time measurement of streptavidin binding validating the functionality of the MZI biosensor. Subsequently, this streptavidin surface was employed to immobilize biotinylated single-stranded DNA and utilized for monitoring of selective DNA hybridization. These proved the usability of polyimide based evanescent photonic devices for biosensing application.     

Cleavage of branched polyethylene by chemical filling

Branched polyethylene, melt-crystallized in commercial fabrication processes, cleaves into two layers when exposed to a prolonged “chemical filling” treatment (i.e., formation of filler in situ by interdiffusion of two reactive permeants). Cleavage has been observed in film and blown bottles. With film, progressive changes in experimental conditions from one sample to another, shifted the cleavage plane from near the surface to deeper lying planes. Separation of a thin surface layer requires more filler deposit than does separation into layers of more equal thickness. These observations suggest that a well defined layer structure may exist in branched polyethylene and that cohesive bonding is stronger between layers near the surface of the film than it is between deeper lying layers. Linear polyethylene showed slight layer separation after prolonged chemical filling, but clean cut separation of large areas was not achieved. This behavior may indicate that the cohesive bonding between layers is much stronger in linear polyethylene than in branched polyethylene.     

Microstencils for the patterning of nontraditional materials

A microfabrication technique that uses a photolithographically patterned film as a microstencil has been developed. This microstencil has a bilayer structure comprised of parylene and SU-8 films with thicknesses from 4 to 100 microm. The parylene layer enables the microstencil to be mechanically peeled from hydrophilic substrates. Since no chemicals are required to release the microstencil, this technique can be used to pattern chemically and biologically sensitive materials. The amount of material deposited can be automatically controlled by the height of the SU-8 structures or externally controlled by spin coating or other thin film deposition techniques. This patterning method is very versatile and has been used to pattern features as small as 25 by 25 microm on silicon, glass, and polymer substrates. As an initial demonstration, we have patterned wax, cells, proteins, sol, and CYTOP.     

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.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Anisotropic reactive mesogens transfer onto polyimides-mixture layer via imprinting method for continuous pretilt angle control

We present a new approach for the alignment process to orient liquid crystal (LC) molecules. The process consists of fabrication of a stamp composed of reactive mesogens (RMs), transfer of RM from the stamp onto polyimide (PI) layers, and LC device fabrication using the resulting RM/PI layers. The RM stamp was made anisotropic by a rubbing treatment, and this anisotropy was transferred to the PI layers via contact imprinting, leading to an achievement of uniform LC alignment. In addition, the pretilt angle of the LC molecules could be controlled by varying the content of homogeneous PI in the PI layers. We describe the effects of high pretilt angle upon the electro-optical (EO) characteristics of twisted-nematic (TN) cells based on the RM transfer film/blended PI layers.     

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.     

Direct immobilization of cholesteryl-TEG-modified oligonucleotides onto hydrophobic SU-8 surfaces

We introduce a rapid, simple one-step procedure for the high-yield immobilization of cholesteryl-tetraethyleneglycol-modified oligonucleotides (chol-DNA) at hydrophobic sites made of SU-8 photoresist. Topographic structures of SU-8 were microfabricated on microscope glass coverslips sputtered with a Ti/Au layer. Upon application, chol-DNA adsorbed to the SU-8 structures from solution, leaving the surrounding gold surface free of chol-DNA. chol-DNA immobilization is complete within 15 min and yields a surface coverage in the range of 20-95 pmol/cm(2), which corresponds to a film density of 10(12)-10(13) molecules/cm(2). chol-DNA immobilization is stable and can be sustained despite rinsing, drying, dry storage for several hours, and rehydration of chips. Furthermore, complementary DNA in solution hybridizes efficiently to immobilized chol-DNA.     

电沉积技术制作高聚物微流控芯片模具

利用电沉积技术制作微流控芯片金属模具,方法是:使用新型超厚光刻胶SU8胶作近紫外光刻,并在光刻后的图案上电沉积金属Ni,之后去胶,最终获得金属模具.该法减小了电沉积工作量.采用反向电流预处理基底、并适当增加电铸液的添加剂以及脱模后真空退火,即可明显提高电沉积微结构与基底的结合力.用此金属模具成功热压了PMMA,制成了微流控芯片.     

Photochemical modification and patterning of SU-8 using anthraquinone photolinkers

Bioactive protein patterns and microarrays achieved by selective localization of biomolecules find various applications in biosensors, bio-microelectromechanical systems (bio-MEMS), and in basic protein studies. In this paper we describe simple photochemical methods to fabricate two-dimensional patterns on a Novolac A derivative polymer (SU-8) and, subsequently, their functionalization with biomolecules. Anthraquinone (AQ) derivatives are used to chemically modify and pattern SU-8 surfaces. Features as small as 20 mum are obtained when using uncollimated light. The X-Y spatial resolution of micropatterned AQ molecules is improved to 1.5 mum when a collimated light source is used. This micropatterning process will be important for the functionalization of MEMS-based biosensors. The method saves several processing steps and can be integrated in cleanroom fabrication thus avoiding contamination of the sensor surfaces.