IR-spectroscopy of skin in vivo: Optimal skin sites and properties for non-invasive glucose measurement by photoacoustic and photothermal spectroscopy.

We have reported two methods to analyze glucose in the interstitial fluid of skin based on mid-infrared excitation with a tunable quantum cascade laser and photoacoustic or photothermal detection. These methods were evaluated for optimum skin locations to obtain reproducible glucose information. The lower part of the arm, the hypothenar, the tips of the index finger and the thumb were tested. The thumb appears to be the optimal skin location, followed by the index finger. Basic requirements for an optimum site are good capillary blood perfusion, low Stratum corneum thickness and the absence of fat layers. To obtain a correlation on such a site, spectra were recorded on volunteers continuously after blood glucose manipulation. However, continuous measurements on an in vivo sample such as the skin have to cope with physiological alterations such as the formation of sweat. We have used both detection schemes to investigate the acid mantle reformation after washing during time scales similar to continuous measurements for calibration spectra. We found that reconstitution of the acid mantle of skin may be seen in less than one hour. Precleaning of the measurement site may thus be useful for intermittent, but not for long term continuous measurements.