Date of Award

Spring 2003

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Bill B. Elmore

Abstract

Microsystems, specifically microreactors, open the gate to new, improved analytical techniques while offering many advantages for a large number of applications in chemical engineering, pharmacy, medicine, and biotechnology. This study explored the feasibility of fabrication of microreactors using polydimethylsiloxane (PDMS) as a support for enzyme immobilization. Urease enzyme was used for catalyzing the conversion of urea to ammonia.

PDMS (polydimethylsiloxane) is a silicone-based elastomeric polymer. Traditional micromanufacturing technology was employed for reactor mold fabrication. The mold was fabricated based on photolithography techniques, and SU-8 photoresist was used to construct reactor structure templates. The resulting silicon-wafer based reactor molds were then used repeatedly to generate PDMS microreactors.

One advantage of using an immobilized enzyme system is that the bio-catalyst is retained within the reactor system and enables high concentrations to be maintained. Two enzyme immobilization methods were explored for use with PDMS microreactor systems. One used CMC (1-cyclohexyl-3-(2-morpholineoeethyl) carbodiimide metho-p-tolunensulfonate) as a crosslinker for covalently binding the enzyme to the PDMS microreactor surface. The other employed directly incorporating the enzyme into the uncured polymer. The latter method provided a higher urease activity and was used for most microreactor studies.

To allow an examination of reactor path length, two different reactor templates were applied for evaluation: straight- and wave-channel microreactors. The reactors were tested with different enzyme loadings, feed flowrates, channel lengths, and different operation environments. The wave-channel reactors exhibited considerably high urea conversions at relatively higher flowrates compared with the straight-channel reactors. Urea conversion was about 90% in wave-channel reactor with 0.001 ml/min flowrate and 0.01 g/g PDMS urease loading, whereas for straight-channel reactor, it is only about 10% urea conversion.

A mathematical model was developed for the microreactors tested. The predicted results were consistent with the experiment results for the straight-charnel reactors with short-channels. For the wave-channel reactors, the model showed large deviation from experimented results. The longer the channel length, the greater the deviation. Several assumptions were considered to account for the deviations: channel structure, ammonium ion inhibition, and reactive surface estimation.

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