<i>In vivo</i> efficacy investigation of long-wavelength infrared femtosecond lasers for skin rejuvenation

Ablative lasers such as erbium-doped laser and carbon dioxide laser are currently primary tools for skin rejuvenation and treating dermatological disorders. However, during treatment, as the thermal effect exerts on both target and normal tissues simultaneously, significant effectiveness is often accompanied by a high risk of adverse reactions. To attain an appropriate thermal diffusion and thus favorable therapeutic outcome and fewer side effects, collagen-resonant femtosecond (fs) lasers hold promise as innovative tools for laser cosmetic treatments. In this study, we report, for the first time to the best of our knowledge, an <i>in vivo</i> experiment of fs laser resurfacing with collagen-resonant wavelengths of 6.1 and 7.5 μm, via an optical parametric amplifier. Our results demonstrate that long-wavelength infrared (LWIR) lasers effectively enhance the components of the dermal matrix without causing dermal ablation. The structure of collagen fiber is significantly improved with a substantial amount of new collagen formation. The increased expression of various collagen types in immunofluorescence image further demonstrates the efficacy of the LWIR fs laser in skin rejuvenation. In addition, improvement in the epidermis is more pronounced at a wavelength of 6.1 μm, with a more suitable depth of action. We anticipate that LWIR fs laser could become widely applicable in clinical settings for skin regeneration and rejuvenation.