O2microsensors for minimally invasive tissue monitoring

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Abstract
Tissue oxygenation is a key factor ensuring normal tissue functions and viability. Continuous real-time monitoring of the partial pressure of oxygen,pO2, in tissues gives insight into the dynamic fluctuations of O2supplies to tissues by blood circulation. Small oxygen sensors enable investigations of the spatial variation ofpO2in tissues at different locations in relation to local microvessels. In this paper,pO2measurement using microelectrodes and biocompatible sensors is discussed and recent progress of their application in human skin is reviewed. Emphasis is given to working principles of a number of existing oxygen sensors and their potential application in vivo and in tissue engineering. Results on spatial and temporal variations of thepO2in human skin introduced by localized ischaemia-reperfusion are presented when the surface of the skin is covered by an oxygen-free paraffin oil layer and the range of the tissuepO2is deduced to be between 0 and 60 mmHg. In the study,pO2increases from 8.0±3.2 mmHg (n=6) at the surface of the skin to 35.2±8.0 mmHg (n=9) at a depth just above the subpapillary plexus. Temporal decay inpO2following tissue compression and rise inpO2following pressure release can be described using mono-exponential functions. The time constant for the exponential decay,t=8.44±1.53 s (n=7) is consistently greater than that for the exponential rises,τ′=4.75±0.82 s (n=6). The difference inpO2change with the time following tissue compression and pressure release reveals different dynamic mechanisms involved in the two transient phases. The elevated steady statepO2following reperfusion, which is approximately 20% higher than the pre-occlusion value, indicates localized reactive hyperaemia. Possible applications of O2microsensors in diseases, e.g. tumours, pressure ulcers, are also discussed.