Sensing oxygen through skin using a red diode laser and fluorescence lifetimes

The most difficult impediments to transcutaneous optical sensing are the absorbance and scatter of light caused by skin and the lack of fluorescent sensing probes which can be excited at wavelengths over 600 nm. Furthermore, current optical sensing techniques rely on absorbance or fluorescence intensity measurements, both of which are sensitive to drifts in lamp intensity, changes in probe concentration and inner filter effects. We demonstrate oxygen sensing through a layer of skin by using red light which readily penetrates skin as diffusely scattered light. The oxygen sensitive osmium-ligand complex used in this study can be excited at 635-680 nm. In addition, we measure fluorescence lifetimes, which are inherently unaffected by factors that limit absorbance and fluorescence intensity measurements. By using phase fluorimetry and long lived fluorophores, we are able to demonstrate the potential for subdermal oxygen sensing with simple and inexpensive instrumentation. This work describes a paradigm for future non-invasive measurements of other analytes.