Assessment of ultraviolet radiation impact on human skin tissue using double-exposure digital holographic interferometry.

Significance: We are all exposed to ultraviolet (UV) radiation coming from the Sun, electronic devices, and artificial sources used in medicine, industry, cosmetics, and other fields, and as it can penetrate the skin, it poses a health risk. In this research, the effects of UV radiation on human skin exposed to different energy doses are evaluated using digital holographic interferometry (DHI), which is proposed as a useful tool to assess the changes caused by skin surface displacement and stiffness values. These two indicators, and their representation in pseudo-three-dimensional (3D) images, will be used as biomarkers, and their quantification will help to better understand the effects of UV rays on human skin.

Aim: This research is centered on studying human skin tissue samples (HSTs) with double-exposure DHI; this non-invasive optical technique is able to detect alterations in its mechanical response as it changes caused by UV radiation falling on the skin surface, and such response is compared with the one of non-irradiated samples allowing us to correlate the changes in displacement and stiffness resulting from exposure to different doses of UV radiation.

Approach: Acoustic waves are sent to the HST to induce vibrations and displacements on their surface; the resulting vibration patterns are monitored through an out-of-plane sensitive DHI setup. The full-field-of-view quantification of the displacements in the $z$ -direction (normal to the surface) is quickly determined by processing the digital holograms, and with the amplitude of the displacements, skin stiffness is calculated. Both the surface displacements and their corresponding stiffness values correctly reveal the effects caused by the different UV radiation doses falling on the HST surface, a matter discussed in detail.

Results: The resonant frequencies and the 3D shape of the vibration showing the displacement and stiffness of human skin with and without radiation were found, and graphs were constructed using those data. A negative correlation is observed between the amount of UV energy applied and the changes in displacements, whereas a positive correlation is observed between stiffness and UV dose. The plot serves as a calibration plot and thus can be used to predict, from the optical data, the displacement and stiffness as a function of the UV dose. In addition, some critical changes in skin stiffness may indicate aging or dehydration in the skin, and this may be useful to achieve better skin care. These data indicate that UV light induces skin stiffening. The amplitude variation in displacement/strain and stiffness allows differentiation between skin tissues without and with UV radiation.

Conclusions: The optical non-invasive DHI technique offers a whole field of view assessment of the UV effects on the HST without touching the skin. With the aid of DHI, it is possible to retrieve the pseudo-3D phase map of the skin, and from it, valuable data on the displacement of the skin surface, and thus, its stiffness can be obtained; in addition, many potential benefits can be derived from its use, such as protection against skin diseases.