Minimally invasive silicon probe for electrical impedance measurements in small animals

It is commonly accepted that electrical impedance provides relevant information about the physiological condition of living tissues. Currently, impedance measurements are performed with relatively large electrodes not suitable for studies in small animals due to their poor spatial resolution and to the damage that they cause to the tissue. A minimally invasive needle shaped probe for electrical impedance measurements of living tissues is presented in this paper. This micro-probe consists of four square platinum electrodes (300 microm x 300 microm) on a silicon substrate (9 mm x 0.6 mm x 0.5 mm) and has been fabricated by using standard Si microelectronic techniques. The electrodes are not equally spaced in order to optimise the signal strength and the spatial resolution. Characterisation data obtained indicate that these probes provide high spatial resolution (measurement radius <4 mm) with a useful wide frequency band going from 100 Hz to 100 kHz. A series of in vivo experiments in rat kidneys subjected to ischemia was performed to demonstrate the feasibility of the probes and the measurement system. The impedance modulus and phase were measured at 1 kHz since this frequency is sufficiently low to permit the study of the extracellular medium. The extracellular pH and K+ were also simultaneously measured by using commercial miniaturised Ion Selective Electrodes. The induced ischemia period (45 min) resulted in significant changes of all measured parameters (Delta/Z/ approximately 65%; DeltapH approximately 0.8; DeltaK+ approximately 30 mM).