Mikrofluidiksystem zur lokalisierten elektrischen Impedanzspektroskopie
- A microfluidic system for localized electrical impedance spectroscopy
El Hasni, Akram; Schnakenberg, Uwe (Thesis advisor); Knoch, Joachim (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019
By using microsystem technologies, microfluidic systems can be developed which are suitable for the characterization of biological cell cultures or single cells. In this work, a microfluidic chip has been developed and characterized which allows electrical impedance spectroscopy of fixed microparticles or single cells (mouse oocytes). The microchannel has four traps to hydrodynamically immobilize cells or microparticles. The traps are equipped with a new arrangement of four integrated microelectrodes to carry out electrical impedance measurements. To optimize the microchannel design with respect to the hydrodynamic properties, a numerical flow simulation has been performed. The production of the microfluidic chip was carried out using microsystem fabrication processes. A complex multilayer process based on negative-tone photoresist SU-8 has been developed. To reduce the capacitive effect of the electrodes in the low frequency range, the electrodes were electrochemically coated with a conductive biocompatible polymer layer. The characterization of this surface modification was conducted using extensive electrochemical measurements and interpretation with the aid of suitable models. The clear increase of the measurement range was mainly attributed to the shift of the lower cutoff frequency. The verification of function was carried out using impedance measurements of polystyrene microparticles with a diameter of 40, 50 and 60 µm, respectively. The dependence of the impedance spectra on the particle size for four measurement configurations was demonstrated. Measured maximum impedance change and FEM simulations showed a good agreement. In a second experiment, measurements of conductive polymer microparticles with an insulating coating has been conducted. These microparticles simulated the composition of a biological cell with its conductive core and the insulating thin layer. Using a comparison with measurements of homogeneous microparticles of the same size, the sensitivity of the measurement configurations has been determined. Furthermore, applying the Maxwell-Wagner approximation, the electrical conductivity of the core was calculated. Using measurements of mouse oocytes, the applicability of the microfluidic chip for the characterization of single cells has been successfully demonstrated. The effect of the zona pellucida, a glycoprotein layer surrounding the egg cell, on the impedance spectrum was investigated in frequency range of 10 Hz - 10 MHz using two cell types: intact egg cells and egg cells without zona pellucida. The result showed a clear difference of the impedance change for the two cell types. The relative impedance increase for the oocytes without zona pellucida showed larger values in the range of 30 - 50 % than the intact egg cells, which showed values in the range of 10 - 15 %. These results agreed with findings of electrorotation measurements. The developed measurement system provides the basis for a future measurement system for non-destructive and objective quality assessment of in-vitro fertilization of egg cells with the hardening of the zona pellucida playing an important role. The future measurement system will be used to increase the fertilization rate in the reproductive technology.