Electrical impedance of stainless steel needle electrodes

We present experimental findings regarding variability and stability of the electrical impedance properties of medical grade stainless steel needle electrodes in vitro. Monopolar impedance spectra (1 Hz to 1 MHz) were measured and scanning electron microscope images were obtained for five needle types with active electrode area from 0.28 to 0.7 mm(2). A saline tank (0.9% NaCl) was used as tissue model. Measurements were done before and after electrolytic treatment with anodic and cathodic DC currents of 1 muA. With active electrode areas below 1 mm(2), high influence from electrode polarization impedance (EPI) was expected at low frequencies (LF). For higher frequencies (HF) the EPI decreases and the impedance of the surrounding tissue is more pronounced. The hypothesis tested was that the EPI at LF would depend upon contact area, alloy composition, surface structure, and treatment of the active electrode, and at HF upon the electrode area geometry, and the specific resistivity of saline. Our results show large differences in electrical properties between needle types. After electrolytic treatment the EPI decreased. After 5-48 h of saline exposure the EPI increased, both for treated and untreated needles. Cathodic treatment gave lower impedance and drift than anodic or no treatment.