The “Fed��?batch on a chip��?��?project, which was carried out in close cooperation with the Technical University of Delft, aims to miniaturize and parallelize micro bioreactors suitable for on-line screening of micro-organisms. This thesis describes an electrochemical sensor array which has been developed for this purpose. The sensor array is suitable for on-line monitoring pH, temperature, dissolved oxygen concentrations and viable biomass concentrations. pH is monitored using an Ion-Selective Field Effect Transistor (ISFET). Drift influence was effectively be reduced. The temperature sensor is a platinum thin-film thermistor with a nominal impedance of 1 kOhm. Dissolved oxygen concentration is monitored using an amperometric Ultra Micro Electrode Array (UMEA) which exhibits good selectivity and low flow dependence. Viable biomass concentration is monitored using impedance spectroscopy. Yeast suspension conductivity was found to increase linearly with the biomass concentration for frequencies above 1 MHz. The sensors were fabricated using cleanroom techniques and meet the floorplan of a 96-well microtiterplate. The sensor array was first calibrated with respect to conventional measurement methods in lab-scale fermentors with a working volume of 4 liters. The sensors were then applied in an array consisting of two microreactors with a working volume of 100 microliter. Therefore, the sensors were placed underneath bottomless wells taken from 96-well microtiterplates. A single aqueous Ag/AgCl reference electrode was placed in a separate reservoir connected to the microreactors using saltbridges consisting of Nafion hollow fiber membranes filled with medium. Mixing was achieved using a magnetic stirrer bar, fixated on an axle. Cultivations of Candida Utilis were performed and monitored. Immediately after the log phase, excessive cell death and sensor fouling appears to take place; this phenomenon requires further investigation. This thesis furthermore describes the development of microfluidics for pH control and fed-batch functionality. PDMS microvalves with a rounded to semicircular cross section were fabricated using a new fabrication process which takes advantage of isotropically etched microchannels in glass substrates. The valves were actuated using Braille displays, electromagnetically, and pneumatically. Pneumatic actuation leads to the most robust results, since the alignment between the actuator and the microchannel is fixed.
|Award date||29 Nov 2007|
|Place of Publication||Enschede|
|Publication status||Published - 29 Nov 2007|