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
