David A. Chang-Yen

A monolithic PDMS waveguide system fabricated using soft-lithography techniques

A monolithic waveguide system using poly(dimethyl siloxane) (PDMS) was designed, fabricated, and characterized. The waveguide demonstrated good confinement of light and relatively low attenuation at 0.40 dB/cm. The robustness and handling properties of the completed waveguides were excellent, and the process yield exceeded 96%. The waveguide did exhibit moderate temperature and humidity sensitivity but no temporal variation, and insertion loss remained stable over extended periods of time. Applications of this waveguide system in microscale sensing are immense, judging by the frequency of use of PDMS as the substrate for microfluidic and biomedical systems. The monolithic nature of the waveguides also reduces their cost and allows integration of optical pathways into existing PDMS-based microsystems.

Characterization of interconnects used in PDMS microfluidic systems

This paper reports the characterization of a microfluidic packaging technique involving the use of press-fit interconnects to microfluidic channels molded in PDMS. This packaging technique is implemented by, first, coring a small hole in the PDMS to access molded or buried microchannels using a modified 20 gauge needle; and second, inserting an unmodified needle into the hole to create a direct connection to the microchannel that requires no bonding or molding. The needles can then easily be removed and reinserted multiple times since the seal is created purely by the compression of the PDMS around the needle. The luer fitting on the needles can easily be connected to standard fluid fittings. The quality of the interconnects is correlated with observations of the PDMS after coring. Methods of coring examined include pushing straight through and twisting the coring tool by hand or by machine. These comparisons demonstrated that all methods can produce viable interconnects; however, machine coring was the most consistent. The interconnects were characterized mechanically primarily by measuring their leak resistance under pressure. Leak tests were performed on interconnects (1) fabricated using different methods, (2) experiencing rotation or bending and (3) fabricated at various linear densities. Static pressure testing revealed that interconnect pressure limits varied from 100 kPa to over 700 kPa depending on the fabrication method. Suggestions are presented on how the technique could be modified to reach much higher pressures. Interconnect flexibility testing demonstrated a minimum of 30° of bending and a maximum of 60° before failure depending on the direction rotated. Density testing showed that PDMS was strong enough to allow at least six interconnects on a 1 cm linear channel.

A Novel PDMS Microfluidic Spotter for Fabrication of Protein Chips and Microarrays

A novel polydimethyl siloxane (PDMS) microfluidic spotter system has been developed for the patterning of surface microarrays that require individually addressing each spot area and high probe density. Microfluidic channels are used to address each spot region, and large spot arrays can be addressed in parallel. Fluorescence intensity measurement of dye-spotted samples compared to control and pipetted drops demonstrated a minimum of three-fold increase in dye surface density compared to pin-spotted dyes. Surface plasmon resonance (SPR) measurement of protein-spotted samples as compared to pin-spotted samples demonstrated an 86-fold increase in protein surface concentration. The spotting system has been applied successfully to protein microarrays for SPR applications, in both a 12-spot linear and 48-spot two-dimensional (2-D) array format. This novel spotter system can be applied to the production of high-throughput arrays in the fields of genomics, proteomics, immunoassays, and fluorescence or luminescence assays

Large-area, high-aspect-ratio SU-8 molds for the fabrication of PDMS microfluidic devices

A relatively low-cost fabrication method using soft lithography and molding for large-area, high-aspect-ratio microfluidic devices, which have traditionally been difficult to fabricate, has been developed and is presented in this work. The fabrication process includes novel but simple modifications of conventional microfabrication steps and can be performed in any standard microfabrication facility. Specifically, the fabrication and testing of a microfluidic device for continuous flow deposition of bio-molecules in an array format are presented. The array layout requires high-aspect-ratio elastomeric channels that are 350 µm tall, extend more than 10 cm across the substrate and are separated by as little as 20 µm. The mold from which these channels were fabricated consisted of high-quality, 335 µm tall SU-8 structures with a high-negative aspect ratio of 17 on a 150 mm silicon wafer and was produced using spin coating and UV-lithography. Several unique processing steps are introduced into the lithographic patterning to eliminate many of the problems experienced when fabricating tall, high-aspect-ratio SU-8 structures. In particular, techniques are used to ensure uniform molds, both in height and quality, that are fully developed even in the deep negative-aspect-ratio areas, have no leftover films at the top of the structures caused by overexposure and no bowing or angled sidewalls from diffraction of the applied UV light. Successful microfluidic device creation was demonstrated using these molds by casting, curing and bonding a polydimethylsiloxane (PDMS) elastomer. A unique microfluidic device, requiring these stringent geometries, for continuous flow printing of a linear array of 16 protein and antibody spots has been demonstrated and validated by using surface plasmon resonance imaging of printed arrays.

Electrostatic self-assembly of a ruthenium-based oxygen sensitive dye using polyion–dye interpolyelectrolyte formation

Abstract The dye tris(2,2′-bipyridyl dichlororuthenium) hexahydrate has been successfully applied to glass and silica surfaces using the technique of layer by layer self-assembly. Solutions of both admixed polyion–dye and pure dye were used to attempt adsorption of the dye onto the substrates. Characterization of the constructed dye layers was performed using quartz crystal microbalance (QCM), UV-Vis absorbance spectrophotometry, fluorescence spectroscopy, and scanning electron microscopy (SEM) methods. Successive, controlled addition of multiple dye layers has been demonstrated, and exposure to different levels of dissolved oxygen shows that the films containing entrapped dye molecules retain sensing properties.

Integrated optical glucose sensor fabricated using PDMS waveguides on a PDMS substrate

This paper describes the design and fabrication of an integrated optical glucose sensing system using the combination of the oxygen sensitive dye tris(2,2’-bipyridyl) dichlororuthenium(II) hexahydrate and glucose oxidase. Layer-by-layer self-assembly is used to immobilize the dye/enzyme system onto the surface of the waveguides. Changes in the enzyme/dye system as it interacts with the surrounding environment are monitored using end-face interaction with light injected into waveguides. The waveguides are thermally-defined monolithic polydimethlysiloxane (PDMS) waveguide system, fabricated on a PDMS substrate. The method of waveguide fabrication is a radical departure from conventional microscale waveguide systems, and offers unique opportunities for integration of this sensor into existing microfluidic systems.

Integrated optical biochemical sensor fabricated using rapid-prototyping techniques

This paper details the design and fabrication of an integrated optical biochemical sensor using a select oxygen-sensitive fluorescent dye, tris(2,2’-bipyridyl) dichlororuthenium(II) hexahydrate, combined with polymeric waveguides that are fabricated on a glass substrate. The sensor uses evanescent interaction of light confined within the waveguide with the dye that is immobilized on the waveguide surface. Adhesion of the dye to the integrated waveguide surface is accomplished using a unique process of spin-coating/electrostatic layer-by-layer formation. Exposure of the dye molecules to the analyte and subsequent chemical interaction is achieved by directly coupling the fluid channel to the integrated waveguide. A unique fabrication aspect of this sensor is the inherent simplicity of the design, and the resulting rapidity of fabrication, while maintaining a high degree of functionality and flexibility.

Design, fabrication, and packaging of a practical multianalyte-capable optical biosensor

A unique evanescent-wave biosensor was designed and fabricated using simple and robust microfabrication technology. The sensor uses a microscale optical waveguide fabricated from NOA61 that is surface-altered with a custom chemical modification process, coupled with modified self-assembly of an oxygen-sensitive fluorescent dye and the enzyme glucose oxidase. To interface the analyte with the waveguide surface, a multilayer PDMS fluidic mold was designed to fit over the waveguide. Interfacing of both optics and fluidics was achieved using novel-generic methods. The entire device has been successfully fabricated and assembled, with analyte response testing completed using glucose solutions. The system also demonstrated inherent random noise insensitivity while making measurements.

A novel PDMS microfluidic spotter for fabrication of protein chips and microarrays

A novel PDMS microfluidic spotter system has been developed for the patterning of surface microarrays that require individually addressing each spot area and high probe density. Microfluidic channels are used to address each spot region and large spot arrays can be addressed in parallel. Fluorescence intensity measurement of dye-spotted samples compared to control and pipetted drops demonstrated a minimum of a three-fold increase in dye surface density. Surface plasmon resonance measurement of protein-spotted samples as compared to pin-spotted samples demonstrated an 86-fold increase in protein surface density. This novel spotter system can be applied to the production of high-throughput arrays in the fields of genomics, proteomics, immunoassays and fluorescence or luminescence assays.

A novel integrated optical dissolved oxygen sensor for cell culture and micro total analysis systems

Production of accurate microscale oxygen sensors has been conventionally attempted using amperometric methods, which prove to be difficult to miniaturize, mainly due to the requirement of a bulky reference electrode. This paper details the design and fabrication process for an optically based oxygen sensor using an oxygen-sensitive fluorescent dye combined with microfabricated Cytop/spl trade/ waveguides that are monolithically integrated into the sensing substrate. Application of the dye to the optical waveguides was accomplished using a unique electrostatic layer-by-layer deposition process that has allowed us to deposit ultrathin dye layers to specific areas of the substrate. Testing of the oxygen sensitivity of the immobilized dye was carried out.