Mass transfer and gas-phase calibration of implanted oxygen sensors.

A protocol is described for validation of implanted oxygen sensors, in which sensors are calibrated in the gas phase where concentration boundary layers are absent. Calibration prior to sensor implantation and confirmation after sensor explantation allows separation of tissue mass transfer effects from sensor variance and drift. A model is given here that describes the oxygen-dependent signal current in terms of oxygen mass transfer to the sensor, permeability of the sensor membrane, and electrode area. The parameter used in the model to describe mass transfer to implanted sensors is consistent with experimental observations and allows comparisons with nonimplanted sensors. This method provides a bridge between the complementary approaches of empirical calibration and model-based calculation for determining oxygen concentration from the sensor response.