FLOX—an oxygen-flux-measuring system using a phase-modulation method to evaluate the oxygen-dependent fluorescence lifetime

The direct relationship between the oxygen supply by skin blood flow and oxygen uptake (O 2 uptake) through the skin could be of importance for the diagnosis of circulatory disturbances and their consequences. A new measuring system has been developed to obtain simultaneously at three places the local O 2 uptake throug the skin. It uses the principle of the O 2-flux optode to measure the O 2 flux into the tissue. The luminescence-based-O 2 sensor has the well-known advantages (1) to be permeable for the analyte O 2, (2) to be flexible to cover larger areas of the surface and (3) not to cosume the analyte O 2. To avoiod the problems inherent to fluorescence intensity measurements the FLOX system (an oxygen-flux-measuring system) uses a phase-modulation measuring method to evaluate the oxygen-dependent fluorescence lifetime. The evaluation is based on the quadrature or incoherent envelope detection, which enables both phase angle (lifetime) and amplitude (intensity) to be received. The three sensor modules of the system consist of a light-emitting diode (LED) ( λ peak = 470 nm) as light source, optical filters, a bifurcated glass main frequency generation of the modulation and reference signals is perfommed by a direct digital synthesizer (DDS) with dual output. The system is PC-based and works at modulation frequencies in the range of 5–900 kHz. Depending on the used indicator dye and the measuring purpose, the frequency is adjusted for the optimum phase range. The first O 2-flux measurements on human skin with the multilayer O 2-flux sensor and reproducibility measurements with the FLOX system prove the ability of the method.