Infoscience

Thesis

Two-dimensional microimpedance imaging for cell culture monitoring

This thesis presents the fabrication and design of an impedance imaging sensor, and its application to monitoring in-vitro skin culture. Three-dimensional tissue cultures are becoming increasingly important in biological and medical research. There is, however, a lack of low-cost techniques for monitoring cellular growth and migration in such systems. For single-layer cell cultures, microelectrodes have been successfully used to investigate the effect of various drugs on cell behaviour, but current systems yield no spatial information and are therefore not suitable for tissue culture. The objective of this work was thus to develop a technique for imaging the electrical properties of tissue cultures and other microscopic objects. Two different techniques were developed: a two-electrode (bipolar) system and a four-electrode (tetrapolar) system. The bipolar technique, "resistivity microprofiling", allowed measuring the resistivities of layered structures, whereas the four-electrode technique, "microimpedance tomography", was used to produce two-dimensional images of vertical cross-sections of a sample. Resistivity microprofiling is based on the principle that the measurement depth of bipolar measurements depends on the electrode size and separation; small electrodes are more sensitive to changes near the surface. To reconstruct the resistivity profile of the sample, a "serial partial resistance" algorithm, based on conformal mapping, was developed. Conformal mapping was also used extensively to optimise electrode design and model the frequency and impedance response of the sensor. An electrode width of 74% of the separation between the electrode midpoints maximized the bandwidth, and a relative electrode width of 55% maximized the measurement depth. The resistivity profiling was used to measure sedimentation rate of nanoparticles, and to determine the thickness and resistivity of growing skin cells. For calibration purposed so-called "tissue phantoms" composed of pHEMA were fabricated, and the relationship between resistivity and chemical composition was investigated. The imaging sensor was based on tetrapolar measurements between 16 planar microelectrodes (5 µm x 4 mm) integrated into a culture chamber. A commercial impedance analyser, combined with a front-end amplifier and a digital switch circuit, was used to measure the impedance. The internal resistivity of the sample was created by first making a large number of measurements using different electrode combinations, and then inverting the results. The commercial software RES2DINV, developed for geophysical prospecting, was used for the image inversion. A 50 µm insulating wire on top of the electrodes could be resolved, and its position determined. The acquisition time was approximately 30 seconds, allowing monitoring of events taking place on the order of minutes. Stem cells of type YF 29, a strain of human foreskin keratinocytes, were cultured on a feeder layer of mouse fibroblasts and allowed to form a pluristratified layer on the sensor surface. Cultured skin cells were locally removed from the electrode surface, and migration into the wounded area was followed over several days. Cell migration speeds of 300 nm/min following wounding were established. Triton-X was used to permeabilize the cell membranes and the resulting drop in tissue resistivity could be followed in real time. In the reconstructed images, the gradual decrease in resistivity as the substance penetrates into the tissue could be followed. The hardware is relatively inexpensive and the system is well adapted for monitoring a large number of cultures in parallel. The imaging sensor will most likely find uses in monitoring cell growth for tissue engineering applications as well as drug studies on tissue cultures. Furthermore, impedance imaging could be used to study wound healing around chronically implanted electrodes, a phenomenon not accessible using existing technologies.

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