O 2 microsensors for minimally invasive tissue monitoring

Abstract

Tissue oxygenation is a key factor ensuring normal tissue functions and viability. Continuous real-time monitoring of the partial pressure of oxygen, p O 2 , in tissues gives insight into the dynamic fluctuations of O 2 supplies to tissues by blood circulation. Small oxygen sensors enable investigations of the spatial variation of p O 2 in tissues at different locations in relation to local microvessels. In this paper, p O 2 measurement 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 the p O 2 in 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 tissue p O 2 is deduced to be between 0 and 60 mmHg. In the study, p O 2 increases 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 in p O 2 following tissue compression and rise in p O 2 following 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 in p O 2 change with the time following tissue compression and pressure release reveals different dynamic mechanisms involved in the two transient phases. The elevated steady state p O 2 following reperfusion, which is approximately 20% higher than the pre-occlusion value, indicates localized reactive hyperaemia. Possible applications of O 2 microsensors in diseases, e.g. tumours, pressure ulcers, are also discussed.