Mirko Hofmann

Micro-bioreactors for fed-batch fermentations with integrated online monitoring and microfluidic devices

In this study an array of micro-bioreactors based on the format of 48-well microtiter plates (MTP) is presented. The process parameters pH-value and biomass are monitored online by a combination of different sensors, the biolector measurement technology and conductance measurements. A microfluidic device dispenses two fluids individually into each well for controlling the pH-value of fermentations. The micro-bioreactor consists of four wells and two reservoirs. In each well a polyimide foil with platinum electrodes for conductance measurements is integrated. The microfluidic device is fabricated using softlithographic techniques and utilizes pneumatically actuated microvalves. The device is able to dispense volumes below 5nl. Finally, fermentations of Escherichia coli are carried out in the micro-bioreactor system. During the fermentation, the pH-value is measured optically and the biomass development is monitored by the scattered light signal. Meanwhile, the pH-value is controlled by dispensing sodium hydroxide and phosphoric acid. This micro-bioreactor demonstrates the possibility of online monitored and pH-controlled fermentations in micro-scale. The pH-value in the uncontrolled culture varies within the range of 6.46-8.83 whereas the pH-value in the controlled cultures can be kept within 6.85-7.07. This results in an increase in biomass in the pH-controlled culture compared to the nearly completely inhibited pH-uncontrolled culture.

Long-term stability of PDMS-based microfluidic systems used for biocatalytic reactions

This paper reports on the changes of PDMS interface characteristics due to the long-term influence of aqueous alkaline solutions, which are frequently used fluids in biocatalytic reactions. Soft lithographic techniques were used to produce polymeric microfluidic systems containing a fluidic layer with multiple cavities for biocatalytic reactions and a pneumatic control layer. The surface energy, the surface roughness and the absorption of liquids on PDMS are analysed as they are important factors affecting the microfluidic current. The results obtained can be used to provide design guidelines for adapting PDMS-based microfluidic devices in long-term bio catalysis reactions.

Galvanically decoupled impedance spectroscopy for biological high-throughput-screening in microtiter plates

A flew method for measuring the impedance spectra of biological cell cultures in microtiter plates (MTP) is proposed. The application offers the opportunity of measuring the complex impedance of aqueous solutions without galvanic contact. A thin polyimide film is used as dielectric layer between two metal electrodes and the solutions. The impedance of these cells or aqueous solutions is determined by measuring the impedance of the whole application and subtracting the constant capacitance of the dielectric layer. This calculation is based on a newly developed equivalent electrical circuit. To show the applicability of the sensor, yeast cells (Hansenula polymorpha wt) are monitored at frequencies between 500 kHz and 15 MHz. The results demonstrate the ability of the sensor to measure even the relaxation step of the capacitance of living cells. Because of possible side effects, the influence of aqueous solutions on the polyimide film in general is investigated as well