A research group has reported a method to measure tissue oxygen content via O2 reduction at carbon paste electrodes CPEs using constant potential amperometry CPA.  This technology has been further refined to allow complete implantation (albeit, preclinical animal studies) and wireless telemetry of the data.  This technology has also been used to measure nitric oxide and glucose using similar electrodes.

For more information, see Characterisation of Carbon Paste Electrodes for Real-time Amperometric Monitoring of Brain Tissue Oxygen.