Assessment of a 3050/3200 nm fiber laser system for ablative fractional laser treatments in dermatology

Abstract

Background and objectives: Mid-infrared (IR) ablative fractional laser treatments are highly efficacious for improving the appearance of a variety of dermatological conditions such as photo-aged skin. However, articulated arms are necessary to transmit the mid-IR light to the skin, which restricts practicality and clinical use. Here, we have assessed and characterized a novel fiber laser-pumped difference frequency generation (DFG) system that generates ablative fractional lesions and compared it to clinically and commercially available thulium fiber, Erbium:YAG (Er:YAG), and CO2 lasers.

Materials and methods: An investigational 20 W, 3050/3200 nm fiber laser pumped DFG system with a focused spot size of 91 µm was used to generate microscopic ablation arrays in ex vivo human skin. Several pulse energies (10-70 mJ) and pulse durations (2-14 ms) were applied and lesion dimensions were assessed histologically using nitro-blue tetrazolium chloride stain. Ablation depths and coagulative thermal damage zones were analyzed across three additional laser systems.

Results: The investigational DFG system-generated deep (>2 mm depth) and narrow (<100 µm diameter) ablative lesions surrounded by thermal coagulative zones of at least 20 µm thickness compared to 13, 40, and 320 µm by the Er:YAG, CO2 , and Thulium laser, respectively.

Conclusion: The DFG system is a small footprint device that offers a flexible fiber delivery system for ablative fractional laser treatments, thereby overcoming the requirement of an articulated arm in current commercially available ablative lasers. The depth and width of the ablated microcolumns and the extent of surrounding coagulation can be controlled; this concept can be used to design new treatment procedures for specific indications. Clinical improvements and safety are not the subject of this study and need to be explored with in vivo clinical studies.

Keywords: difference frequency generation; fiber delivery; fractional ablation; fractional laser.

Conflict of interest statement

The authors declare no conflicts of interest.

© 2022 The Authors. Lasers in Surgery and Medicine published by Wiley Periodicals LLC.

Figures

Figure 1

Photograph of the prototype ablative fractional fiber laser pumped DFG system. The small footprint base unit has output wavelengths of 1030 and 1560 nm, which are fiber delivered to a scanner handpiece. The scanner handpiece contains DFG crystals that convert the laser light wavelength to 3050 and 3200 nm (ratio of 2:1). DFG, difference frequency generation.

Figure 2

Two‐stage conversion setup scheme. Each stage consists of a periodically poled lithium niobate crystal that generates light with an output frequency that is the difference between the two input frequencies according to the DFG process. Image adapted from Gulyashko et al. DFG, difference frequency generation; Er, erbium; Yb, ytterbium.

Figure 3

Blow‐off, steady‐state, and Hibst models to fit the ablation depth versus radiant exposure curves of the Er:YAG (2940 nm), fiber laser‐pumped DFG (3050/3200 nm), CO2 (10600 nm), and Thulium (1940 nm) laser in ex vivo human tissue. The Hibst model (with the extra adjustable parameter γ) results in a better fit for every wavelength compared to the blow‐off and steady‐state model. DFG, difference frequency generation; Er, erbium.

Figure 4

Horizontal histology sections of ex vivo human skin treated with (A) Thulium, (B) Er:YAG, (C) fiber laser pumped DFG, and (D) CO2 laser stained with NBTC. For corresponding vertical histology sections below (E–H) laser parameters were chosen to generate 1‐mm‐deep ablated fractional lesions. The horizontal sections show clear differences in the size of the coagulation zone. The Thulium laser parameters are 108 µm spot size, 854 mJ pulse energy, 7 ms pulse width, and 1940 nm wavelength. The Er:YAG laser parameters are 250 µm spot size, 50 mJ pulse energy, 0.25 ms pulse width, and 2940 nm wavelength. The fiber laser‐pumped DFG parameters are 91 µm spot size, 20 mJ pulse energy, 4 ms pulse width, and 3050/3200 nm wavelength. The CO2 laser parameters are 120 µm spot size, 40 mJ pulse energy, 1 ms pulse width, and 10600 nm wavelength. DFG, difference frequency generation; Er, erbium; NBTC, nitro‐blue tetrazolium chloride.

Figure 5

The graph shows the effect of the pulse train duration on the ablation depth and coagulation zone thickness in ex vivo human tissue using the fiber laser‐pumped DFG. The energy per pulse stays constant at 10 mJ (radiant exposure of 154 J/cm2) for the pulse train duration of 2, 4, 6, 8, 10, 12, and 14 ms. DFG, difference frequency generation.