Optical Methods for Non-Invasive Determination of Skin Penetration: Current Trends, Advances, Possibilities, Prospects, and Translation into In Vivo Human Studies

Abstract

Information on the penetration depth, pathways, metabolization, storage of vehicles, active pharmaceutical ingredients (APIs), and functional cosmetic ingredients (FCIs) of topically applied formulations or contaminants (substances) in skin is of great importance for understanding their interaction with skin targets, treatment efficacy, and risk assessment—a challenging task in dermatology, cosmetology, and pharmacy. Non-invasive methods for the qualitative and quantitative visualization of substances in skin in vivo are favored and limited to optical imaging and spectroscopic methods such as fluorescence/reflectance confocal laser scanning microscopy (CLSM); two-photon tomography (2PT) combined with autofluorescence (2PT-AF), fluorescence lifetime imaging (2PT-FLIM), second-harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS), and reflectance confocal microscopy (2PT-RCM); three-photon tomography (3PT); confocal Raman micro-spectroscopy (CRM); surface-enhanced Raman scattering (SERS) micro-spectroscopy; stimulated Raman scattering (SRS) microscopy; and optical coherence tomography (OCT). This review summarizes the state of the art in the use of the CLSM, 2PT, 3PT, CRM, SERS, SRS, and OCT optical methods to study skin penetration in vivo non-invasively (302 references). The advantages, limitations, possibilities, and prospects of the reviewed optical methods are comprehensively discussed. The ex vivo studies discussed are potentially translatable into in vivo measurements. The requirements for the optical properties of substances to determine their penetration into skin by certain methods are highlighted.

Biodegradable calcium carbonate carriers for the topical delivery of clobetasol propionate

“…The efficiency of the carrier transportation was assessed using OCT, which was proven to be effective in determining the penetration and accumulation of particulate formulations in skin under native and non-destructive in vivo conditions. 89–91 This non-invasive imaging modality was also utilized further to monitor the process of the CP–CaCO 3 carriers’ degradation in skin. Previously, we have convincingly demonstrated that this method allows for a reliable assessment of hair follicle filling with the CaCO 3 carriers providing a strong correlation between the data obtained using OCT and the other investigation methods, including CLSM, 46,48,61 SEM, 46,61,92 and histological examination.…”

Review of optical methods for noninvasive imaging of skin fibroblasts—From in vitro to ex vivo and in vivo visualization

“…The advantage of TPE‐FM over OPE‐FM is the reducing fluorescence background and photobleaching that improves signal‐to‐noise ratio and provides high lateral and axial resolution, contrast and image quality, and lower risk of cell damage. Due to the red to near infrared excitation wavelengths, it is possible to reduce light attenuation in the skin and increase imaging depth [49]. TPE‐FM allows studying various characteristics of fibroblasts, such as size and metabolic status, assessing the influence of the environment, virus, and other effects.…”

“…CRM is useful for liquid samples/cultures analysis. The main disadvantage of CRM is that the light scattered by the cells is weak, and a long signal acquisition is required to obtain high‐quality images [154] that makes in vivo skin imaging impossible [49]. There are already prerequisites for developing CRM protocols for in vivo measurements, for instance, combining with skin optical clearing technique to enhance the image depth until papillary and reticular dermis [171].…”

“…In addition, TPE‐FM does not require a pinhole, which reduces the signal‐to‐noise ratio as in a confocal microscopy. Usually, femtosecond lasers with a wavelength in the first “therapeutic window” of skin transparency (≈700–1000 nm), are used in TPE‐FM [49].…”

“…Raman microspectroscopy (uses spontaneous Raman scattering) can determine the spatial distribution of individual molecular components in the skin, for example, melanin [63], α ‐helix and ß ‐sheet proteins [64–66], lipids and their structural organization [67–69], natural moisturizing factor molecules [68], DNA [70], carotenoids [71], water [72], and water mobility states [73]. Coherent anti‐Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) spectroscopy have better chemical selectivity, sensitivity, and fast image acquisition compared to conventional spontaneous Raman scattering spectroscopy [49, 74, 75].…”