In vivo near-infrared spectroscopy of rat skin tissue with varying blood glucose levels

We have performed in vivo measurements of near-infrared rat skin absorption in the 4000-5000 cm-1 spectral range (2.0-2.5 μm wavelength) during a glucose clamp experiment. The goal of this work is to identify the presence of glucose-specific spectral information in order to evaluate the requirements for a noninvasive transcutaneous glucose instrument. Skin spectra are collected using an FTIR spectrometer coupled with a fiber-optic interface. In the experiment, an animal is allowed to stabilize at a euglycemic level for three hours while blood glucose values are monitored using samples taken from an arterial catheter. The blood glucose level is then increased above 30 mM by venous infusion of glucose and held for two hours, after which it is allowed to return to normal. Spectra are recorded continuously during the procedure and are analyzed to identify changes due to the glucose variations. Because the change in absorbance due to an increase in glucose concentration is small compared to changes due to other variations (e.g., the thickness of the skin sample), a simple subtraction of absorbance spectra from the hyperglycemic and euglycemic phases is not instructive. Instead, a set of principal components is determined from the euglycemic period where the glucose concentration is constant. We then examine the change in absorbance during the hyperglycemic period that is orthogonal to these principal components. We find that there are significant similarities between these orthogonal variations and the net analyte signal of glucose, which suggests that glucose spectral information is present.