Abstract-An evanescent-wave biosensor was designed and fabricated using simple and robust microfabrication technology. The significance of the sensor is the design method, in which a multiphysical approach is used to draw from much broader field than is customary for these sensor types. The sensor uses a microscale SU-8 optical waveguide that is surface-altered with a custom chemical modification process, coupled with modified self-assembly of a fluorescent dye and enzyme. To interface the analyte with the waveguide surface, a multilayer PDMS fluidic mold is designed to fit over the waveguide. Interfacing of both optics and fluidics are achieved using specifically-designed couplings. The entire device has been successfully fabricated and assembled, with preliminary analyte response testing completed. Keywords – Multiphysical, biosensor, design. I. INTRODUCTION HE design and development of cutting-edge microanalysis systems has consistently centered on elemental design, particularly towards characterization of single components. Generally, these studies are limited to one or two engineering fields, such as enzyme interaction with a simple electrical system in the case of an amperometric oxygen biosensor. Use of more complex physical systems can lead to much more capable systems, provided the designer is willing to deal with the possible interactions. For example, an optical chemical sensing system can be combined with an enzymatic system to produce a very flexible biosensor. However, the convolution of design interactions can easily produce unintended difficulties in fabrication, packaging and operation of an integrated system. For a microanalysis system design to be practical, regardless of the application, all elements of the fabrication, testing and final use must be considered at the design outset. The purpose of this study is to examine the interactions of a biochemical sensing system designed using this philosophy to better understand the design implications from conceptual stages to final implementation. II. METHODOLOGY The optical waveguide is composed of a 130µm x 130µm SU-8 strip waveguide fabricated on a glass substrate, with integrated packaging to 125µm silica fibers. The choice of waveguide materials was important in the initial design stages, as the surface properties of the waveguide affected the dye/enzyme immobilization process, and ultimately required the development of an entirely new SU-8 surface modification process. The waveguide was modified using sulfuric acid to render the surface negatively charged at neutral pH and capable of supporting electrostatic layer-by- layer self-assembled films. To accelerate the self-assembly process, spin assembly of the dye film is used [1-2]. The oxygen-sensitive dye tris(2,2’-bipyridyl dichlororuthenium) hexahydrate was used as a transducer molecule for the enzyme glucose oxidase [3-5]. An interpolyelectrolyte complex of the dye with the negatively-charged polyion poly(sodium styrenesulfonate) (PSS) was created and alternately layered on the surface of the modified waveguide with positively-charged polyion poly(diallyl dimethylammonium) chloride (PDDA). Glucose oxidase was then layered over the dye film using conventional layer-by-layer self-assembly with the polyion poly(ethyl enimine) (PEI). A PDMS channel fabricated using a multilayer SU-8 mold was mated to the waveguide structure. The channel was designed to restrict diffusion of the analyte to the enzyme from solution to a single direction, facilitating the use of single dimension, transient, finite-difference equations to model the system. The significance of this technique was that the fluidic system was modified based on the analyte/oxygen diffusion model, rather than attempting to create a three-dimensional model of a simpler fluidic channel. Optical packaging of the waveguide was also simplified by using a monolithic Design and Fabrication of a Multianalyte-Capable Optical Biosensor Using a Multiphysics Approach D. A. Chang-Yen 1 , B. K. Gale 1 1 Department of Mechanical Engineering, University of Utah, Salt Lake City, UT T Fig. 1. Diagram of assembled SU-8 waveguide sensor. The unusual cross-sectional area of the fluidic channel was created to limit the analyte exposure to the waveguide surfaces, allowing only 1- dimensional analyte diffusion to the immobilized dye/enzyme. Fig. 2. Photograph of assembled sensor. The excitation and emission optical fibers can be seen entering and exiting the device. The PDMS above the fluid channel (also shown in Figure 1) was thinned to allow rapid diffusion of oxygen to the fluid and waveguide surface.