Thermal processes in human skin in radiation by led

Abstract

Basic researches has shown the prospectivity and high efficiency of the visible spectrum of radiation on biological objects. Since the heat from the radiation source is one of the exposure factors, in this article we’ll review the thermal processes in human skin exposed by irradiation, and analyze the effect of blood flow velocity in the circulatory system on the heating of the LED irradiated skin area and subcutaneous layers. LED is reviewed as the radiation source.

Mathematical model. The publications about the impact of electromagnetic radiation include descriptions of the different models of human skin structure (seven-layer and three-layer).

In this study the three-layer model, consisting of three parts was used. The upper part – the epidermis  is a multilayered epithelium, with the horny outer layer. The thickness of the epidermis is from from 0.02 mm to 2.4 mm in various parts of the body. The bottom epidermis layer is lying on the basal membrane. It contains the melanocytes, the cells protecting skin from the harmful sunlight effects. The inner skin – dermis – has a thickness of from 0.5 mm to 5 mm. Blood vessels are located there.

Heat problem was solved by heating of a semi-infinite body by LEDs radiation, penetrating into the body to a certain depth. It is recognized, that the blood vessels are inside the body. Dissipation of heat, caused by absorption of radiation, is determined by the blood flow velocity. Despite the simplicity of this model, it helped to obtain data, allowing definition of some recommendations related to the regimen of LEDs radiation of tissue.

Depth of penetration into biological tissue is a significant parameter. It increases with the wavelength increase. Thus, the depth of radiation penetration from ultraviolet to the orange part of the optical spectrum is gradually increased from 1-20 microns to 2.5 mm, with a sharp increase of the penetration depth in the red part (up to 20-30 mm). Short-wave infrared radiation with wavelengths of 0.76 to 1.5 microns is relatively absorbed by biological tissues, so they penetrate deeply (to 3-7 cm).

In the experiments, the red LEDs were used. The thickness of the epidermis is taken to be 0.6 mm, the coefficient of absorption of red light in it is equal to 3.5 mm-1. The energy fraction absorbed in this layer is small, so the presence of the layer can be ignored and considered as a homogeneous body tissue.

Calculations. The following data on thermophysical skin parameters were used: k = 0,5 W/(m grad), с = 3500 J/(kg grad), ρ = 1100 kg/m3, ρ = 1060 kg/m3. Blood stream density in vessels of dermis f is equal to 15 ml/(min·100 g). In SI units f = 2,5×10-6 m3/(kg·sec). Radiation source – LED with aperture of 5 mm diameter. Radiating power – 10 mW, maximum radiation – on 630 nm wavelength. Radiation intensity I is then equal to 500 W/m2. Red LED radiation penetration depth δ = 10 mm.

Conclusions. When human skin is exposed to irradiation by LED, its heating it strongly depends on the blood flow rate in the irradiated tissues. Even at low power (10 mW) heating can reach 50 °C, if the circula- tion is difficult (for example, in certain pressing on the skin). In such heat skin reddening or even burn may be observed.