Temperature Depth Profiles Induced in Human Skin In Vivo Using Pulsed 975 nm Irradiation

The aim of this study was to determine the temperature depth profiles induced in human skin in vivo by using a pulsed 975 nm diode laser (with 5 ms pulse duration) and compare them with those induced by the more common 532 nm (KTP) and 1,064 nm (Nd:YAG) lasers. Quantitative assessment of the energy deposition characteristics in human skin at 975 nm should help design of safe and effective treatment protocols when using such lasers.

Temperature depth profiles induced in the human skin by the three lasers were determined using pulsed photothermal radiometry (PPTR). This technique involves time-resolved measurement of mid-infrared emission from the irradiated test site and reconstruction of the laser-induced temperature profiles using an earlier developed optimization algorithm. Measurements were performed on volar sides of the forearms in seven volunteers with healthy skin. At irradiation spot diameters of 3-4 mm, the radiant exposures were 0.24, 0.36, and 5.7 J/cm² for the 975, 532, and 1,064 nm lasers, respectively.

Upon normalization to the same radiant exposure of 1 J/cm ² , the assessed maximum temperature rise in the epidermis averaged 0.8 °C for the 975 nm laser, 7.4 °C for the 532 nm, and 0.6 °C for the 1,064 nm laser. The characteristic subsurface depth to which 50% of the absorbed laser energy was deposited was on average 0.31 mm at 975 nm irradiation, and slightly deeper at 1,064 nm, and 0.15 mm at 532 nm. The experimentally obtained relations were reproduced in a dedicated numerical simulation.

The assessed energy deposition characteristics show that the pulsed 975 nm diode laser is very suitable for controlled heating of the upper dermis as required, for example, for nonablative skin rejuvenation. The risks of nonselective overheating of the epidermis and subcutis are significantly reduced in comparison with irradiation at 532 and 1,064 nm, respectively. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.