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 rum long and 200 nm thick. The electrode widths ranged from 20 to 500/mum. 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 mum 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.