The topical application of drugs, such as nicotinates, affects cutaneous blood flow. Such a biological response, which is dependent on the drug and the individual, can be measured noninvasively using laser Doppler flowmetry. We illustrate the kinetics of vasodilation caused by topically applied benzyl nicotinate using a new frequency-selective laser Doppler flowmeter. This flowmeter measures the blood flow in the superficial dermal plexus and the deeper lying larger capillaries simultaneously and indirectly by determining the flow velocity. Both sets of data are compared with the skin temperature and redness. Four biological parameters are measured consecutively on a skin area treated with gel containing benzyl nicotinate and on an untreated control area. A linear relationship between both blood flows is observed. However, no correlation is obtained between the microcirculation with either the cutaneous temperature or the redness. These results indicate the transport of the drug in the blood from the upper to the deeper capillaries. Cutaneous temperature and redness are unsuitable parameters to measure the kinetics of the blood flow after topical application of drugs.

Abstract:The laser Doppler (LD) technique has been applied to measurements of blood perfusion in a number of studies and is the most popular method for investigating microcirculation in patients. The skin is supplied with nutrients by the capillaries, the smallest blood vessels. The nutritive blood flow within the capillaries is very slow and should be distinguished from the blood flow of other vessels such as those in deeper layers of the skin. In this work, a novel laser Doppler flux meter was used. In contrast to the usual LD technique that only provides a mean flux value, partial flux values can be calculated which are correlated to different flow velocities by analyzing different Doppler frequencies. The influence of the changes in the microcirculation on the spectral LD signal was investigated on the inner part of the wrist to select frequency bands, which would allow the best discrimination of capillary blood flux. The LD measurements were performed during selective compression of the skin. Results were obtained from different volunteers and the Doppler signal intensity in a frequency range from 50 Hz to about 400 Hz was found to correlate best with the capillary blood flux of the skin.

A change in the water distribution of the human body, such as water retention in the skin, can be a symptom of a pathological condition including heart failure. Therefore, a sensor for the non-invasive quantification of the water content of the skin could be useful where continuous monitoring of patients is required to detect and avoid life threatening conditions. As water is a major absorber of tissue in the near-infrared wavelength range, the water content can be determined based on reflectance measurements. Measuring the diffuse reflectance at multiple distances from the point of illumination allows the determination of absorption in scattering media such as skin. The aim of this project was to develop a small and portable sensor based on light emitting diodes and photodiodes. Evaluation of the first functional sensor design has showed that the water content of tissue phantoms can be predicted with a prediction error of 1%. Further developments towards a sensor that can be applied in a future field study are ongoing.

With a cardanically mounted micromirror a confocal laser scanning microscope for in vivo imaging was built. A resolution of 0.6 µm laterally and 10 µm axially allows to image tissue and cells in good quality. Samples of skin and adhered cells are imaged either in reflection or in fluorescence with an excitation wavelength of 682 nm. Fluorescence of Indocyanine Green is detected in the wavelength range above 730 nm.

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