Dynamics of Microfabricated Enzyme Electrodes

This work sets out to meet some of the demands created by advancements in glucose oxidase enzyme electrode fabrication techniques. The application of microfabrication techniques to enzyme electrodes has enabled not only greater control over enzyme geometry but also the possibility of monolithic low-power fully wireless implantable biosensors with sensor-on-CMOS construction. Such efforts must be guided by a strong grasp of the theory of diffusion-limited electrochemistry of the products of enzymatically catalyzed reactions. Low power requirements demand a full understanding of sensor turn-on transients and the reduced device size impacts diffusion phenomena and increases the importance of considerations such as oxygen recycling from the reaction at the working electrode. With analytical solutions to the nonlinear differential equations involved not forthcoming, there is a need for sophisticated simulation tools that build upon other efforts in the field and deliver novel capabilities. Such simulation tools must also be complemented by robust, convenient, reproducible, and ideally automated empirical measurement tools to enable the design-test-redesign iteration process to converge quickly to the desired outcomes. This work presents the development of finite element simulations of enzyme electrodes incorporating full two-substrate enzyme kinetics, a dynamic simulation of the sensor environment, and a full treatment of oxygen recycling at the working electrode. While the simulations presented in this work are carried out with axisymmetric RZ meshes, they are ready for use with full 3D meshes. It additionally presents the development of an automated wafer-scale measurement system enabling the testing of up to twenty sensors in parallel, still on the wafer on which they were fabricated. We present this with the hope that the ability to attack the problem from both sides --- better in silico simulation and faster and more controlled in vitro iteration --- should assist in the development of new sensing technologies. We also present selected results studied through the use of these tools, in particular the determination of the impact of enzyme geometry on sensor response. These results show the promise of thin-film deposition via spin-coating and vapor deposition crosslinking to enable the kind of fast response-time high-sensitivity electrodes that are needed for achieving monolithic wireless implantable biosensors.