Porcine skin damage thresholds and histological damage characteristics from 1319-nm laser radiation

Abstract

There is an increasing use of near-infrared lasers in biomedical applications operating in the spectrum between 1300 and 1400 nm. To corroborate and expand the existing safety data for skin exposure to lasers in this wavelength region, the in-vivo ED50 damage thresholds were determined in Guizhou miniature pigs for 1319-nm laser radiation. Exposure durations of 0.4, 1.0, and 3.0 s and 1 / e2 beam diameters of 0.98 and 1.96 cm were employed. Damage lesion determinations were performed at 1- and 24 h post exposure. The Bliss probit analysis was employed to establish the ED50 damage thresholds. Histopathological studies of skin damage were performed at 48 h after irradiation to reveal the damage characteristics. The skin damage thresholds at 1 h post exposure, given in peak radiant exposure, were 35.5, 36.1, and 37.1 J / cm2 at exposure durations of 0.4, 1.0, and 3.0 s with the spot diameter of 0.98 cm, and 28.6 J / cm2 at exposure duration of 3.0 s with the spot diameter of 1.96 cm. At 24 h post exposure, the ED50s increased slightly. Histologically, the thermal damage characteristics at the near-threshold level included gathering of the nuclear chromatin and cell vacuolation in the epidermis and deposition of blood cells in the capillary vessels. However, at the apparently above-threshold level, the damage characteristics included obvious stretching of the nuclear chromatin in the epidermis, closing of the capillary lumen, structural change of collagen fibers, and coagulative necrosis of the hair follicle cells. The damage induced by this laser could go deep into the fatty tissue. The obtained results may contribute to the knowledge base for the damage mechanisms and expand the database for the refinement of laser safety standards in the wavelength range of 1300 to 1400 nm.

Keywords: 1319 nm; ED50 damage thresholds; infrared laser; photothermal effects; skin damage.

Fig. 1

Schematic drawing of the laser exposure setup for the determination of porcine skin damage thresholds at the wavelength of 1319 nm. The $1/e^2$ beam diameter arriving at the focusing lens was about 1.20 cm, and the selected $1/e^2$ beam diameters were measured to be $0.98\pm 0.01$ and $1.96\pm 0.02\mathrm{cm}$ at two target planes by repeated measurements prior to exposure. The distances between the lens and the target planes with the two spot sizes (0.98 and 1.96 cm) were about 108.2 and 161.3 cm, respectively.

Fig. 2

Typical graph showing the probit analysis of skin damages for Guizhou miniature pig. The incident beam diameter and exposure duration was 1.96 cm and 3.0 s, respectively. The red square points were experimentally determined damage probabilities at 1 h post exposure for corresponding power levels. By probit analysis, the ${\mathrm{ED}}_{50}$, the fiducial limits at the 95% confidence level, and the probit slope could be determined.

Fig. 3

Comparisons of the skin damage thresholds for lesions 1 h after exposure, between this work and the work of Oliver et al. Oliver et al. presented the damage thresholds for exposure durations of 0.25, 1.0, 2.5, and 10.0 s with the $1/e^2$ spot diameters of 0.61 and 0.97 cm, respectively. This work determined the damage thresholds for exposure durations of 0.4, 1.0, and 3.0 s with the $1/e^2$ spot diameter of 0.98 cm, and for exposure duration of 3.0 s with the $1/e^2$ spot diameter of 1.96 cm, respectively.

Fig. 4

Skin damage images induced by 1319 nm continuous-wave laser. The black circles indicated the lesions. (a) Immediately post exposure, (b) 1 h post exposure, (c) 24 h post exposure, (d) 7 days post exposure, (e) immediately post exposure, And (f) 24 h post exposure. Laser conditions from (a) to (d): beam diameter was 1.96 cm, exposure duration was 3.0 s, laser power was 16.0 W, and the radiant exposure was about 1.11 times of the ${\mathrm{ED}}_{50}$ damage threshold. Laser conditions of (e) and (f): beam diameter was 1.96 cm, exposure duration was 3.0 s, laser power was 24.0 W, and the radiant exposure was about 1.67 times of the ${\mathrm{ED}}_{50}$ damage threshold.

Fig. 5

Histological sections of skin tissue. (A) Normal skin tissue. (B) Damaged skin tissue at 48 h post exposure. Laser conditions: beam diameter 1.96 cm, exposure duration 3.0 s, and laser power 16.0 W. The radiant exposure was about 1.11 times of the ${\mathrm{ED}}_{50}$ damage threshold. (C) Skin tissue at the central region of a damage lesion at 48 h post exposure. Laser conditions: beam diameter 1.96 cm, exposure duration 3.0 s, and laser power 24.0 W. The radiant exposure was about 1.67 times of the ${\mathrm{ED}}_{50}$ damage threshold. (D) Skin tissue at the edge region of the same lesion as (C). Laser condition was the same as (C). The specimens seen in (C) and (D) were from the same tissue section but different locations. The circles from (a) to (e) represented following damage characteristics: (a) gathered nuclear chromatin in the epidermis, (b) blood cells deposited abundantly in the capillary vessels, (c) the cell nuclei in the upper dermis layer shrank with hyperchromatism, (d) obvious stretching of the nuclear chromatin in the epidermis, and (e) the structure of the collagen fibers changed obviously with homogenization characteristic.

Fig. 6

Histological sections of damaged skin fatty tissue at 48 h post exposure. Laser conditions: beam diameter 1.96 cm, exposure duration 3.0 s, and laser power 24.0 W.

Fig. 7

Comparisons of skin damage thresholds for 1070, 1319, and 2000 nm. The $1/e^2$ beam diameters for these wavelengths were all around 1.0 cm.