SU-8 Microchannels Research Articles

Direct Fabrication of SU-8 Microchannel across an Embedded Chip for Potentiometric Bilayer Lipid Membrane Sensor

Direct Fabrication of SU-8 Microchannel across an Embedded Chip for Potentiometric Bilayer Lipid Membrane Sensor

Dielectrophoretic trapping and impedance detection of Escherichia coli, Vibrio cholera, and Enterococci bacteria.

In this work, a dielectrophoretic impedance measurement (DEPIM) lab-on-chip device for bacteria trapping and detection of Escherichia coli, Vibrio cholerae, and Enterococcus is presented. Through the integration of SU-8 negative photoresist as a microchannel and the precise alignment of the SU-8 microchannel with the on-chip gold interdigitated microelectrodes, bacteria trapping efficiencies of up to 97.4%, 97.7%, and 37.7% were achieved for E. coli, V. cholerae, and Enterococcus, respectively. The DEPIM device enables a high detection sensitivity, which requires only a total number of 69 ± 33 E. coli cells, 9 ± 2 Vibrio cholera cells, and 36 ± 13 Enterococcus cells to observe a discernible change in system impedance for detection. Nonetheless, the corrected limit of detection for Enterococcus is 95 ± 34 after taking into consideration the lower trapping efficiency. In addition, a theoretical model is developed to allow for the direct estimation of the number of bacteria through a linear relationship with the change in the reciprocal of the overall system absolute impedance.

Electrodeposition of thin chitosan membrane in freestanding SU-8 microfluidic channel for molecular addressing by capillary effect

The electrodeposition of thin chitosan membrane in freestanding microfluidic channel with capillary effect for biomolecule assembly of biosensor is investigated. A dry SU-8 sheet is used to fabricate freestanding fluidic microchannel by microelectromechanical systems (MEMS) technology. The hydrophobic surface of SU-8 microchannel transformed to hydrophilic surface by plasma treatment. The thin chitosan membrane is deposited on hydrophilic SU-8 microchannel by capillary effect, and miniaturization of sensor device is obtained. Membrane thickness of chitosan in freestanding channel can be controlled by concentration of chitosan solution and deposition time. The results show that thin chitosan membrane can be deposited by capillary effect in freestanding channel, and the deposited chitosan thin film shows great potential for immobilizing enzyme on the surface for compact biosensor device.

Heterogeneous Integration of CMOS Sensors and Fluidic Networks Using Wafer-Level Molding.

Direct sensing in liquids using CMOS-integrated optical and electrical sensors is attractive for lab-on-chip applications, where close physical proximity between sample and sensor can obviate optical lenses, enhance electrical sensitivity, and decrease noise due to parasitics. However, controlled delivery of fluid samples to the chip surface presents an ongoing challenge for lab-on-CMOS development, where traditional wire-bond packaging prevents integration of planar microfluidics. In this paper, we present a method for scalable heterogeneous integration of microfluidic channels and silicon-integrated circuit substrates using a commercial fan-out wafer-level packaging approach. The planar surface supports multiple approaches for fluidic integration; we present both a stacked laser-cut fluidic assembly and the fabrication of monolithic SU-8 microchannels over the IC surface. As a proof-of-principle, both electrical and fluidic routing are provided to a custom 0.18-m CMOS optical sensor IC, and optical transmission and fluorescence measurement experiments are demonstrated.

Promotion and guidance of neural network formation on SU-8 photoresist microchannels adjusted with multilayer films.

Induction of neural stem/progenitor cells (NSPCs) and establishment of neural network are important issues on neural engineering. In this work, a platform was designed to control and evaluate the differentiation of NSPCs, neurite direction, and to promote the neurite outgrowth. Polyelectrolyte multilayer (PEM) films provide surface properties by and have been used to regulate NSPCs differentiation in our previous study. Herein, a culture platform composed of SU-8 microchannel and PEM films was designed to achieve the goal of promoting NSPCs differentiation and to evaluate the effect of PEM films on the guidance of neural network formation. In this culture platform, NSPCs were induced into functional neurons, and neural network formation was accomplished on ITO glass-PEM films successfully.

Interfacing microchip isoelectric focusing with on-chip electrospray ionization mass spectrometry

In this work, we demonstrate the interfacing of microchip capillary isoelectric focusing (cIEF) with online mass spectrometric (MS) detection via a fully integrated, on-chip sheath flow electrospray ionization (ESI) emitter. Thanks to the pH-dependent surface charge of the SU-8 polymer cIEF can be successfully run in native SU-8 microchannels without need for surface pretreatment prior to analysis. On the other hand, the inherent electroosmotic flow (EOF) taking place in SU-8 microchannels at high pH can be exploited to electrokinetic mobilization of the focused pH gradient toward the MS and no external pumps are required. In addition to direct coupling of a cIEF separation channel to an ESI emitter, we developed a two-dimensional separation chip for two-step, multiplex cIEF-transient-isotachophoretic (tITP) separation. In this case, cIEF is performed in the first dimension (effective L=20mm) and tITP in the second dimension (L=35mm) followed by ESI/MS. As a result, the migration order is affected by both the pI values (cIEF) and the intrinsic electrophoretic mobilities (tITP) of the sample components. The selectivity of the separation system was shown to be different from pure cIEF or pure ITP, which allows at best for baseline separation of two compounds with nearly identical pI values. The repeatabilities of the migration times of the two-step cIEF-tITP separation were 3.1–6.8% RSD (n=3). Thanks to the short separation channel, relatively short focusing times of 60–270s (depending on the applied focusing potential) were sufficient for establishment of the pH gradient and cIEF separation of the sample components, yielding total analysis times (including loading, focusing, and mobilization) well below 10min.

Fabrication of Self-Packaged Seamless Nanoporous SU-8 Microchannels

Fabrication of a self-packaged partial semicircular nanoporous microchannel is demonstrated using electrospinning, self-limiting ultraviolet lithography, and thermal-reflow packaging processes with SU-8. The fabrication process offers unique advantages of microresolution channel patterning, alignment free seamless integration of nanofibers in a microchannel by self-packaged reflow, and easy formation of 3-D nanoporous channels. A fabricated nanoporous microchannel with a channel length of 7 mm, a channel width of 73.85 $\mu $ m, and a channel height of $34.65~\mu $ m shows slow fluidic flow through the channel, functioning as an effective nanochannel with an equivalent channel height of 800 nm and a radius of 857 nm, and therefore, its reduction ratios of the channel height and radius are $\sim 97.7$ % and 97.8%, respectively.

SAW-based fluid atomization using mass-producible chip devices.

Surface acoustic wave (SAW)-based fluid atomizers are ideally suited to generate micrometer-sized droplets without any moving parts or nozzles. Versatile application fields can be found for instance in biomedical, aerosol or thin film technology, including medical inhalators or particle deposition for advanced surface treatment. Such atomizers also show great potential for on-chip integration and can lead to economic production of hand-held and even disposable devices, with either a single functionality or integrated in more complex superior systems. However, this potential was limited in the past by fluid supply mechanisms inadequate for mass production, accuracy and reliability. In this work, we briefly discuss existing fluid supply methods and demonstrate a straightforward new approach suited for reliable and cost-effective mass-scale manufacturing of SAW atomizer chips. Our approach is based on a fluid supply at the boundary of the acoustic beam via SU-8 microchannels produced by a novel one-layer/double-exposure photolithography method. Using this technique, we demonstrate precise and stable fluid atomization with almost ideal aerosol plume geometry from a dynamically stabilized thin fluid film. Additionally, we demonstrate the possibility of in situ altering the droplet size distribution by controlling the amount of fluid available in the active region of the chip.

Fabrication of three-dimensional SU-8 microchannels by proton beam writing for microfluidics applications: Fluid flow characterisation

The proton beam writing (PBW) technique was used to fabricate microfluidic structures in SU-8 resist. A network of the buried channels was fabricated as part of a project to develop functional microfluidic device for neuronal studies and self-powered microfluidics. Protons with energies between 2.5MeV and 0.75MeV were used to fabricate the buried channels with a minimum feature size of around 1μm and depths of 40–55μm. Roughness of channels sidewalls was around 2.5nm rms. Exposure regime and examples of functional networks fabricated using PBW are described. COMSOL Multiphysics® software was used to model the flow characteristics of fluid in the SU-8 microchannels structured by PBW. The results obtained using PBW are compared with the structures fabricated by UV-lithography.

Low Temperature Irreversible Poly(DiMethyl) Siloxane Packaging of Silanized SU8 Microchannels: Characterization and Lab-on-Chip Application

In this paper, we describe and characterize a novel method, based on silanization, for strong bonding of SU8 microchannels to poly(dimethyl)siloxane (PDMS) flexible covers. First, the SU8 surface treatment process (silanization) is characterized through atomic force microscopy and contact angle measurements. The aging study proves grafting stability during more than two days. Silanized SU8 patterns and PDMS cover are finally bonded to seal the microchannel network. Such assembled microdevices can be used without leakage at flow rates above 2.4 mL/min, corresponding to 1.2 MPa if the PDMS deformation is neglected. The bonding tensile pressure exceeds 1.5 MPa, proving the packaging strength. Furthermore, SU8-PDMS composite devices display stable bonding after several weeks of storage. This rapid low cost and low temperature bonding technique is finally successfully employed to fabricate a fully packaged biochip for electric and fluidic handling of biological cells.

Fabrication and characterization of a microfluidic module for chemical gradient generation utilizing passive pumping.

We introduce a micro-biochemical administration module (μBAM) for generating chemical gradients for use in axonal guidance studies. The device is designed to be simple to use, require minimal packaging, and be operated using only a pipette. A passive pumping mechanism is utilized to pump liquid through a SU-8 microchannel and then the micropore on the Parylene cap of the microchannel. The achievable flow rate delivery through the micropore was characterized and manipulated by varying the drop volumes used to passively drive fluid flow into the device. Biochemicals controllably delivered using this module can be combined with neuronal cell cultures to form chemical gradients for axonal guidance studies.

Fabrication and performance evaluation of microfluidic organic light emitting diode

In this study, we fabricated a microfluidic organic light emitting diode (OLED) and evaluated its performance. The microchip consisted of a 3×3 matrix array of OLED pixels in SU-8 microchannels sandwiched by indium tin oxide (ITO) anode and cathode pairs. Liquid organic semiconductors introduced into the microchannels were employed as the light emitters. Electroluminescence was successfully observed at the emitting area of the microchannels. A current density of 0.298mA/cm2 was measured at 70V. In our prototype microfluidic OLED, the patterning of the liquid emitters was confirmed in the microchannels on a single chip. This result shows that the proposed structure can be applicable for liquid-based display.

Development of polysilicon devices for microfluidic thermal instrumentation

In microfluidics applications such as biology and chemistry, for which lab-on-chip and microreactors have been developed, the temperature control through a microchannel is of first interest. In this paper, we present a simple and generic technology for thermal sensing applications based on the fabrication of polysilicon diodes that can be implemented either on silicon, fused silica or silicon carbide substrates. Thermal sensitivities were characterized and as a first experimental approach a dedicated set-up was developed for the temperature measurement of a water flow through a SU-8 microchannel.

Integration of nanosensors into a sealed microchannel in a hybrid lab-on-a-chip device

Commonly used assembly methods for microfluidic devices rely heavily on direct, manual bonding of different components with limited visual aids. The manual operation is likely to cause damage to existing structures, especially to nanoscale sensors on the substrate. Here we report a novel approach to integrate nanosensors into a lab-on-a-chip device with total elimination of operational errors from the manual bonding process. The microfluidic components are composed of an ultraviolet (UV) light-defined, cross-linked SU-8 microchannel and a polydimethylsiloxane (PDMS) top cover. The hybrid microfluidic structure provides a fully sealed microchannel and precisely positioned features. Well-organized single-walled carbon nanotube (SWNT) thin films are deposited and aligned across the electrodes on a silicon substrate with dielectrophoresis. The assembly of SWNTs is carried out in a sealed microchannel which eliminates the potential damage of the nanosensors during the bonding process. The SWNT devices are configured as two-terminal resistor-type sensors with the metal electrodes as the probing pads and the dielectrophoretically captured SWNTs as the sensing elements. To demonstrate the feasibility of this integration approach, an assembled SWNT device is measured as an integrated flow sensor to monitor the flow rate in the microchannel. This lab-on-a-chip device is designed as a platform that can be expanded for more applications. For example, with minimal modifications, the device can be used in chemical sensing, biosensing, and medical research.

SU-8 as Hydrophobic and Dielectric Thin Film in Electrowetting-on-Dielectric Based Microfluidics Device

Electrowetting-on-dielectric (EWOD) based droplet actuation in microfluidic chip is designed and fabricated. EWOD is used as on-chip micro-pumping scheme for moving fluid digitally in Lab-on-a-chip devices. For enabling this scheme, stacked deposition of thin dielectric and hydrophobic layer in that order between microchannel and electrodes is done. The present paper investigates the potential use of SU-8 as hydrophobic layer in conjunction of acting as dielectric in the device. The objective for the investigation is to lower the cost and a thin simplification in fabrication process of EWOD-based devices. We have done design and optimization of dimensions of electrode array including gap between arrays for EWOD micropump. Design and optimization are carried out in CoventorWare. The designing is followed by fabrication of device and analysis for droplet motion. The fabrication of the device includes array of electrodes over the silicon surface and embedding them in hydrophobic SU-8 layer. Water droplet movement in the order of microliter of spherical shape is demonstrated. It has been shown that an SU-8 microchannel in the current design allows microfluidic flow at tens of voltages comparable with costlier and more complicated to fabricate designs reported in the literature.

Dynamic coating of SU‐8 microfluidic chips with phospholipid disks

In this work, PEG-stabilized phosphatidylcholine lipid aggregates (disks), mimicking mammalian cell membranes, were introduced as a new biofouling resistant coating for SU-8 polymer microchannels. A rapid and simple method was developed for immobilization of PEGylated phosphatidylcholine disks in microchannels. Microfluidic chips made from SU-8, PDMS, or glass were dynamically coated with the PEGylated disks followed by characterization of their surface chemistry before and after coating. On the basis of the observed changes in EOF and nonspecific protein adsorption, the affinity of the PEGylated disks was shown to be particularly strong toward SU-8. The PEG-lipid coating enabled permanent change in EOF in SU-8 microchannels with an initial value of 4.5 x 10(-8) m(2) V(-1) s(-1), decreasing to 2.1 x 10(-8) m(2) V(-1) s(-1) (immediately after modification), and, eventually, to 1.5 x 10(-8) m(2) V(-1) s(-1) (7 days after modification) for 9 mM sodium borate (pH 10.5) as BGE. As determined by the Wilhelmy plate measurements and microchip-CE analysis of BSA, the PEG-lipid coating also enabled efficient biofouling shield against protein adsorption, similar to that of low amounts of SDS (3.5 mM) or Tween-20 (80 microM) as buffer additives. These results suggest that dynamically attached PEG-lipid aggregates provide stable, biomimicking surface modification that efficiently reduces biofouling on SU-8.

MEMS-based Coulter counter for cell counting and sizing using multiple electrodes

A novel MEMS-based Coulter counter is designed, fabricated and tested in this paper. The Coulter counter is used for detection and monitoring the impedance changes of cells as a function of time after they are mixed with different experimental extracellular environments. The device consists of a multilayer of the SU-8 microchannel which is divided into a passive mixing region, a focusing region using negative dielectrophoretic forces and a measuring region by multiple electroplated vertical electrode pairs. The devices were tested with both fluidic and electrical functionality using dyed fluids, microbeads with different dimensions suspended in saline water and fibroblast cells in anisosmotic phosphate buffered saline. The results are presented and discussed. The device ultimately aims at testing time-sensitive cell characteristics after exposure to different extracellular media with enhanced sensitivity.

Suspended SU-8 structures for monolithic microfluidic channels

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

Note: Microelectromechanical systems Coulter counter for cell monitoring and counting

This note describes the design, fabrication, and testing of a novel microelectromechanical systems Coulter counter. The Coulter counter will be used to detect and monitor impedance changes of cells as a function of time in response to different experimental extracellular environments. The device consists of SU-8 (negative photoresist) microchannels, vertical electroplated electrodes, polydimethylsiloxane cover, and is divided into a passive mixing region, a focusing region using negative dielectrophoretic forces, and a measuring region defined by multiple electroplated electrode pairs. The devices were tested using both microbeads in saline water and fibroblast cells in phosphate buffered saline solution. The results show that the proposed microsystem is capable of monitoring impedance of cells at different positions along the Coulter microchannel.

Characterisation of the fabrication process of freestanding SU-8 microstructures integrated in printing circuit board in microelectromechanical systems

The characterisation of the fabrication process to develop freestanding SU-8 structures integrated in printing circuit board in microelectromechanical systems (PCBMEMS) technology is presented. SU-8 microcantilevers, microbridges, microchannels and micromembranes have been fabricated following the described procedure. Adherence between FR4 substrate and SU-8 has been examined using the destructive blister method, evaluating the surface energy. Residual thermal stress has also been analysed for this integration and compared when using other substrates. Moreover, an estimation of the copper wet etching with cupric chloride has been performed in order to understand how this isotropic etching affects the geometry of the copper structures. Finally, stiction has been observed and examined, estimating the adhesion energy responsible for this effect.