Pontus Linderholm

Cell Culture Imaging Using Microimpedance Tomography

We present a novel, inexpensive, and fast microimpedance tomography system for two-dimensional imaging of cell and tissue cultures. The system is based on four-electrode measurements using 16 planar microelectrodes (5 mum x 4 mm) integrated into a culture chamber. An Agilent 4294A impedance analyzer combined with a front-end amplifier is used for the impedance measurements. Two-dimensional images are obtained using a reconstruction algorithm. This system is capable of accurately resolving the shape and position of a human hair, yielding vertical cross sections of the object. Human epithelial stem cells (YF 29) are also grown directly on the device surface. Tissue growth can be followed over several days. A rapid resistivity decrease caused by permeabilized cell membranes is also monitored, suggesting that this technique can be used in electroporation studies.

Resistivity probing of multi-layered tissue phantoms using microelectrodes

We present the use of an array of rectangular microelectrodes to discriminate between different resistivities in a thin, layered sample. Each electrode was 8 mm long and 200 nm thick. The electrode widths ranged from 20 to 500 microm. The electrodes were designed such that all pairs of consecutive electrodes had the same relative geometry, and therefore identical cell constants. A hydrogel-based tissue phantom, made by photopolymerization of 2-hydroxyethyl methacrylate (HEMA), was developed. By changing the hydrogel composition and the ionic strength of the storage medium, the resistivity of the hydrogels could be tuned between 100 omegam and 100 komegam. Using bipolar measurements, the tissue phantoms were characterized in the frequency range from 100 Hz to 30 MHz. The relative resistivity distribution of a three-layered structure composed of 120 microm sheets could be calculated and was shown to agree to within 7% of the bulk measurements. Potential clinical applications for this technique include probing of epithelial tissue and skin cancer screening.

Long-term in vivo impedance changes of subretinal microelectrodes implanted in dystrophic P23H rats.

Retinal prostheses are being developed to restore vision in blind patients with photoreceptor degeneration. Electrodes arrays were subretinally implanted in transgenic P23H rats with their photoreceptors degenerated. Electrical stability of the implants was evaluated by long-term monitoring of their impedance changes. Electrode impedances were found to increase by two log units over a three weeks period whereas no impedance increase was noted when the implants were located in the vitreous. In case of hemorrhage or major fibrous reactions, the impedance continued to increase steadily. After explantation, it recovered its initial value indicating no deterioration of the implant. Although the glial cell layer at the surface of the subretinal space was slightly larger, no major glial reaction was seen in direct contact to the implant. These results indicate that no functional testing should be considered before at least three weeks post implantation.