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Optical Methods for Non-Invasive Determination of Skin Penetration: Current Trends, Advances, Possibilities, Prospects, and Translation into In Vivo Human Studies. Pharmaceutics 2023; 15:2272. [PMID: 37765241 PMCID: PMC10538180 DOI: 10.3390/pharmaceutics15092272] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2023] [Revised: 08/19/2023] [Accepted: 08/24/2023] [Indexed: 09/29/2023] Open
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.
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Human glabrous skin contains crystallized urea dendriform structures in the stratum corneum which affect the hydration levels. Exp Dermatol 2023. [PMID: 37015835 DOI: 10.1111/exd.14802] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2023] [Revised: 03/15/2023] [Accepted: 03/24/2023] [Indexed: 04/06/2023]
Abstract
Glabrous skin is hair-free skin with a high density of sweat glands, which is found on the palms, and soles of mammalians, covered with a thick stratum corneum. Dry hands are often an occupational problem which deserves attention from dermatologists. Urea is found in the skin as a component of the natural moisturizing factor and of sweat. We report the discovery of dendrimer structures of crystalized urea in the stratum corneum of palmar glabrous skin using laser scanning microscopy. The chemical and structural nature of the urea crystallites was investigated in vivo by non-invasive techniques. The relation of crystallization to skin hydration was explored. We analysed the index finger, small finger and tenar palmar area of 18 study participants using non-invasive optical methods, such as laser scanning microscopy, Raman microspectroscopy and two-photon tomography. Skin hydration was measured using corneometry. Crystalline urea structures were found in the stratum corneum of about two-thirds of the participants. Participants with a higher density of crystallized urea structures exhibited a lower skin hydration. The chemical nature and the crystalline structure of the urea were confirmed by Raman microspectroscopy and by second harmonic generated signals in two-photon tomography. The presence of urea dendrimer crystals in the glabrous skin seems to reduce the water binding capacity leading to dry hands. These findings highlight a new direction in understanding the mechanisms leading to dry hands and open opportunities for the development of better moisturizers and hand disinfection products and for diagnostic of dry skin.
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Perspective of Future SERS Clinical Application Based on Current Status of Raman Spectroscopy Clinical Trials. Front Chem 2021; 9:665841. [PMID: 34354978 PMCID: PMC8329355 DOI: 10.3389/fchem.2021.665841] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2021] [Accepted: 06/18/2021] [Indexed: 12/18/2022] Open
Abstract
Raman spectroscopy has emerged as a promising tool in biomedical analysis and clinical diagnosis. The development of surface-enhanced Raman scattering spectroscopy (SERS) improved the detection limit with ultrahigh sensitivity and simplicity. More and more Raman spectroscopy clinical trials (R-PCT) have been conducted recently. However, there is a lack of an up-to-date review summarizing the current status of Raman clinical trials performed until now. Hence, the clinical trials for Raman were retrieved from the International Clinical Trials Registration Platform. We summarized the clinical characteristics of 55 registered Raman spectroscopy clinical trials (R-RSCTs) and 44 published Raman spectroscopy clinical trials (P-RSCTs). This review could assist researchers and clinicians to understand the current status of Raman spectroscopy clinical research and perhaps could benefit the reasonable and accurate design of future SERS studies.
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Understanding humectant behaviour through their water-holding properties. Int J Cosmet Sci 2021; 43:601-609. [PMID: 34228831 DOI: 10.1111/ics.12723] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2021] [Accepted: 07/05/2021] [Indexed: 11/27/2022]
Abstract
OBJECTIVE Humectants perform essential roles in the formulation of topical moisturizing products in terms of delivery of active ingredients, consumer experience and biophysical behaviour. How they retain and release water is key to understanding their behaviour. METHODS Dynamic vapour sorption (DVS) was used to monitor the dehydration kinetics of three humectants widely used in topical formulations-glycerine, dexpanthenol and urea. Model aqueous solutions with concentrations of 20% w/w were tested and compared against pure deionized water. RESULTS The three humectants varied in their ability to retain water during the dehydration process. Dexpanthenol was able to retain water most efficiently during the latter stages of dehydration. Urea demonstrated evidence of crystallization during the final stage of water loss, which was not shown by glycerine or dexpanthenol. CONCLUSIONS Humectants perform vital roles in the formulation of consumer acceptable topical products including the delivery of actives to the skin. Their ability to influence water movement in the skin is also essential for the maintenance of stratum corneum flexibility. DVS assessment of aqueous solutions has demonstrated how the behaviour of three commonly used humectants differs. Knowledge of the mechanisms by which these humectants operate enables the formulator to develop topical products optimized for the roles for which they are intended.
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Changes in Skin Barrier Function after Repeated Exposition to Phospholipid-Based Surfactants and Sodium Dodecyl Sulfate In Vivo and Corneocyte Surface Analysis by Atomic Force Microscopy. Pharmaceutics 2021; 13:pharmaceutics13040436. [PMID: 33804924 PMCID: PMC8063842 DOI: 10.3390/pharmaceutics13040436] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2021] [Revised: 03/16/2021] [Accepted: 03/20/2021] [Indexed: 11/17/2022] Open
Abstract
(1) Background: The aim of the study was to evaluate the effect of pure lecithins in comparison to a conventional surfactant on skin in vivo. (2) Methods: Physiological skin parameters were evaluated at the beginning and the end of the study (day 1 and day 4) (n = 8, healthy forearm skin) with an Aquaflux®, skin-pH-Meter, Corneometer® and an Epsilon® sensor. Confocal Raman spectroscopy was employed to monitor natural moisturizing factor, urea and water content of the participants' skin. Tape strips of treated skin sites were taken and the collected corneocytes were subjected to atomic force microscopy. Circular nano objects were counted, and dermal texture indices were determined. (3) Results: Transepidermal water loss was increased, and skin hydration was decreased after treatment with SDS and LPC80. Natural moisturizing factor and urea concentrations within the outermost 10 µm of the stratum corneum were lower than after treatment with S75 or water. Dermal texture indices of skin treated with SDS were higher than skin treated with water (control). (4) Conclusions: Results suggest very good (S75) or good (LPC80) skin-tolerability of lecithin-based surfactants in comparison to SDS and encourage further investigation.
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Raman spectroscopic analysis of skin penetration and moisturizing effects of Bionics vernix caseosa cream compared with Vaseline. Technol Health Care 2021; 29:327-334. [PMID: 33682769 PMCID: PMC8150656 DOI: 10.3233/thc-218030] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
BACKGROUND: The stratum corneum (SC) is the outermost layer of human skin and deemed as barrier against chemical exposure and water loss. Moisturizers have beneficial effects in treating dry skin, especially the SC. Confocal Raman spectroscopy (CRS) was used to evaluate the efficacy of moisturizers on skin hydration and penetration, with such agents posing inherent characteristics of being noninvasive, nondestructive, timesaving, and cost effective. Bionics vernix caseosa (BVC) cream mimics the composition of vernix caseosa (VC), which could protect the newborn skin. METHODS: This research applied CRS to evaluate the penetration depth and water content variation during the intervention with two moisturizers, BVC cream and Vaseline. Volunteers received the 2 h application of BVC cream and Vaseline on the forearms. The evaluations on 0 h, 2 h, 4 h and 6 h were performed clinical assessment. Experimental data was processed by least square method and analysis of variance (ANOVA). RESULTS: The penetration depth of Vaseline was deeper than that of Bionics vernix caseosa cream. Specifically, BVC cream penetrated 18 μm into human skin, while Vaseline penetrated at least 20 μm. Compared with Vaseline, only BVC cream increased skin hydration, with a moisturizing effect lasting for 4 h. At 6 h, the Vaseline moisturizing effect decreased significantly.
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Abstract
Many dermatological studies have had limited success in revealing skin function because conventional histological methods are known to affect skin components. Recent progress in non-invasive optical imaging has enabled non-invasive visualization of the structure of each skin layer. However, it remains difficult to identify individual skin components. Alternatively, it is possible to obtain molecular vibrational signatures using spontaneous Raman scattering microscopy. Spontaneous Raman scattering microscopy requires long acquisition times and is rarely applied to skin imaging, especially because skin components, such as water and transepidermal agents, undergo relatively rapid changes. Consequently, non-linear Raman microscopies, such as coherent anti-Stokes Raman scattering and stimulated Raman scattering, have gradually been applied to acquire molecular imaging of skin tissue. In this review, the applications of Raman microscopies used to evaluate skin and research trends are presented. The applications of spontaneous Raman microscopy to in vivo human skin evaluation are first demonstrated with typical applications. Finally, the latest application of coherent Raman scattering microscopy to visualize 3D intracellular morphologies in the human epidermis during differentiation is described.
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Clinical evidences of urea at medium concentration. Int J Clin Pract 2020; 74 Suppl 187:e13815. [PMID: 33249708 DOI: 10.1111/ijcp.13815] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/29/2020] [Accepted: 11/02/2020] [Indexed: 02/06/2023] Open
Abstract
Urea-based topical compounds at medium concentrations (15%-30%) represent useful dermatological agents for their humectant and keratolytic effects by enhancing stratum corneum hydration and by loosening epidermal keratin, respectively. The aim of this paper is to review the clinical evidences of the use of 15%-30% urea as single topical agent. Although limited evidence supports the use of these concentrations of urea in skin disorders characterised by xerosis and hyperkeratosis, in clinical practice they are largely used especially in xerosis of limited skin areas, in which the side effects are tolerable, or hyperkeratosis involving large or more sensitive (eg, face, genital region, etc) areas, in which higher concentration may be irritant. In addition, urea at medium concentrations is used in combination with other substances including topical antifungals as penetration enhancer.
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Freezing Weakens the Barrier Function of Reconstructed Human Epidermis as Evidenced by Raman Spectroscopy and Percutaneous Permeation. Pharmaceutics 2020; 12:E1041. [PMID: 33143093 PMCID: PMC7694161 DOI: 10.3390/pharmaceutics12111041] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2020] [Revised: 10/20/2020] [Accepted: 10/28/2020] [Indexed: 12/23/2022] Open
Abstract
The development and characterization of reconstructed human epidermis (RHE) is an active area of R&D. RHE can replace animal tissues in pharmaceutical, toxicological and cosmetic sciences, yielding scientific and ethical advantages. RHEs remain costly, however, due to consumables and time required for their culture and a short shelf-life. Storing, i.e., freezing RHE could help reduce costs but to date, little is known on the effects of freezing on the barrier function of RHE. We studied such effects using commercial EpiSkin™ RHE stored at -20, -80 and -150 °C for 1 and 10 weeks. We acquired intrinsic Raman spectra in the stratum corneum (SC) of the RHEs as well as spectra obtained following topical application of resorcinol in an aqueous solution. In parallel, we quantified the effects of freezing on the permeation kinetics of resorcinol from time-dependent permeation experiments. Principal component analyses discriminated the intrinsic SC spectra and the spectra of resorcinol-containing RHEs, in each case on the basis of the freezing conditions. Permeation of resorcinol through the frozen RHE increased 3- to 6-fold compared to fresh RHE, with the strongest effect obtained from freezing at -20 °C for 10 weeks. Due to the extensive optimization and standardization of EpiSkin™ RHE, the effects observed in our work may be expected to be more pronounced with other RHEs.
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The application of label-free imaging technologies in transdermal research for deeper mechanism revealing. Asian J Pharm Sci 2020; 16:265-279. [PMID: 34276818 PMCID: PMC8261078 DOI: 10.1016/j.ajps.2020.07.004] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2020] [Revised: 05/23/2020] [Accepted: 07/06/2020] [Indexed: 12/13/2022] Open
Abstract
The penetration behavior of topical substances in the skin not only relates to the transdermal delivery efficiency but also involves the safety and therapeutic effect of topical products, such as sunscreen and hair growth products. Researchers have tried to illustrate the transdermal process with diversified theories and technologies. Directly observing the distribution of topical substances on skin by characteristic imaging is the most convincing approach. Unfortunately, fluorescence labeling imaging, which is commonly used in biochemical research, is limited for transdermal research for most topical substances with a molecular mass less than 500 Da. Label-free imaging technologies possess the advantages of not requiring any macromolecular dyes, no tissue destruction and an extensive substance detection capability, which has enabled rapid development of such technologies in recent years and their introduction to biological tissue analysis, such as skin samples. Through the specific identification of topical substances and endogenous tissue components, label-free imaging technologies can provide abundant tissue distribution information, enrich theoretical and practical guidance for transdermal drug delivery systems. In this review, we expound the mechanisms and applications of the most popular label-free imaging technologies in transdermal research at present, compare their advantages and disadvantages, and forecast development prospects.
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Topical application of highly concentrated water-in-oil emulsions: Physiological skin parameters and skin penetration in vivo - A pilot study. Int J Pharm 2019; 571:118694. [PMID: 31525444 DOI: 10.1016/j.ijpharm.2019.118694] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2019] [Revised: 09/05/2019] [Accepted: 09/11/2019] [Indexed: 01/29/2023]
Abstract
Important aspects in the development of new dermal drug delivery systems are the formulations' physicochemical properties and stability. Moreover, their influence on skin physiology and their penetration performance in vivo are of crucial interest. We have recently developed novel concentrated water-in-oil emulsions based on a non-ionic silicone surfactant; the present study deals with the effect of these formulations on physiological skin parameters of healthy volunteers after repeated application. Variations in skin condition and barrier integrity were investigated using classical biophysical and spectroscopic techniques. After four weeks of continuous treatment, no signs of skin irritation could be observed. Both tested emulsions had a positive effect on skin properties despite their relatively high water content and low lipid content. In vivo tape stripping studies revealed penetrated amounts of the incorporated model drug fluorescein sodium of almost 50% of the applied dose, with a superior performance of emulsions with isopropyl myristate when compared to liquid paraffin. In summary, our study confirmed the suitability of the developed W/O emulsions for pharmaceutic and cosmetic applications.
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Novel confocal Raman microscopy method to investigate hydration mechanisms in human skin. Skin Res Technol 2019; 25:653-661. [PMID: 30932226 DOI: 10.1111/srt.12698] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2018] [Accepted: 02/14/2019] [Indexed: 11/29/2022]
Abstract
BACKGROUND Skin hydration is essential for maintaining stratum corneum (SC) flexibility and facilitating maturation events. Moisturizers contain multiple ingredients to maintain and improve skin hydration although a complete understanding of hydration mechanisms is lacking. The ability to differentiate the source of the hydration (water from the environment or deeper skin regions) upon application of product will aid in designing more efficacious formulations. MATERIALS AND METHODS Novel confocal Raman microscopy (CRM) experiments allow us to investigate mechanisms and levels of hydration in the SC. Using deuterium oxide (D2 O) as a probe permits the differentiation of endogenous water (H2 O) from exogenous D2 O. Following topical application of D2 O, we first compare in vivo skin depth profiles with those obtained using ex vivo skin. Additional ex vivo experiments are conducted to quantify the kinetics of D2 O diffusion in the epidermis by introducing D2 O under the dermis. RESULTS Relative D2 O depth profiles from in vivo and ex vivo measurements compare well considering procedural and instrumental differences. Additional in vivo experiments where D2 O was applied following topical glycerin application increased the longevity of D2 O in the SC. Reproducible rates of D2 O diffusion as a function of depth have been established for experiments where D2 O is introduced under ex vivo skin. CONCLUSION Unique information regarding hydration mechanisms are obtained from CRM experiments using D2 O as a probe. The source and relative rates of hydration can be delineated using ex vivo skin with D2 O underneath. One can envision comparing these depth-dependent rates in the presence and absence of topically applied hydrating agents to obtain mechanistic information.
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Abstract
Skin hydration is a complex process that influences the physical and mechanical properties of skin. Various technologies have emerged over the years to assess this parameter, with the current standard being electrical probe-based instruments. Nevertheless, their inability to provide detailed information has prompted the use of sophisticated spectroscopic and imaging methodologies, which are capable of in-depth skin analysis that includes structural and composition details. Modern imaging and spectroscopic techniques have transformed skin research in the dermatological and cosmetics disciplines, and are now commonly employed in conjunction with traditional methods for comprehensive assessment of both healthy and pathological skin. This article reviews current techniques employed in measuring skin hydration, and gives an account on their principle of operation and applications in skin-related research.
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NLC versus nanoemulsions: Effect on physiological skin parameters during regular in vivo application and impact on drug penetration. Int J Pharm 2018; 549:343-351. [DOI: 10.1016/j.ijpharm.2018.08.007] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2018] [Revised: 07/30/2018] [Accepted: 08/08/2018] [Indexed: 12/22/2022]
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Confocal Raman spectroscopy: In vivo measurement of physiological skin parameters - A pilot study. J Dermatol Sci 2017; 88:280-288. [PMID: 28826690 DOI: 10.1016/j.jdermsci.2017.08.002] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2017] [Revised: 07/31/2017] [Accepted: 08/02/2017] [Indexed: 11/20/2022]
Abstract
BACKGROUND In vivo application of confocal Raman spectroscopy (CRS) allows non-invasive depth measurement of the skin. Thereby obtained knowledge of the skin composition is essential to reliably assess the actual skin state. Besides other components, the skin cholesterol concentration is of interest; however, little is known about its connection to the cholesterol concentration quantified in venous blood. OBJECTIVE In this study, the skin composition of the volar forearm was characterised in vivo using CRS. In particular, the potential of CRS as a non-invasive method to determine cholesterol levels was validated. METHODS Raman spectra of the volar forearm of 15 participants were recorded twice within two weeks. Depth concentration profiles for major skin components were generated. Stratum corneum (SC) thickness was calculated from water concentration profiles. In order to examine the usability of dermal CRS for cholesterol level determination, results were compared to fasting total cholesterol values in venous blood as determined by an enzymatic method. RESULTS Depth concentration profiles for the skin components of interest showed a comparable curve progression for the participants. It was possible to link changes in concentration to physiological processes. Moreover, age-related differences could be found. Several novel mathematical approaches for the comparison of the skin cholesterol content and the blood cholesterol concentration have been developed. However, no correlation passed the Bonferroni multiple testing correction. CONCLUSION CRS serves as useful tool for the in vivo monitoring of skin components and hydration. Concentration depth profiles provide information about the current skin condition. No distinct correlation between the skin and blood cholesterol concentration was found within the scope of the present study. Concerning this matter, the heterogeneous distribution of cholesterol in the skin may be a factor influencing these results.
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In vivo Raman Microspectroscopy: Intra- and Intersubject Variability of Stratum Corneum Spectral Markers. Skin Pharmacol Physiol 2016; 29:102-9. [DOI: 10.1159/000445079] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2015] [Accepted: 02/28/2016] [Indexed: 11/19/2022]
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Penetration of silver nanoparticles into porcine skin ex vivo using fluorescence lifetime imaging microscopy, Raman microscopy, and surface-enhanced Raman scattering microscopy. JOURNAL OF BIOMEDICAL OPTICS 2015; 20:051006. [PMID: 25394476 DOI: 10.1117/1.jbo.20.5.051006] [Citation(s) in RCA: 51] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/26/2014] [Accepted: 10/21/2014] [Indexed: 06/04/2023]
Abstract
In order to investigate the penetration depth of silver nanoparticles (Ag NPs) inside the skin, porcine ears treated with Ag NPs are measured by two-photon tomography with a fluorescence lifetime imaging microscopy (TPT-FLIM) technique, confocal Raman microscopy (CRM), and surface-enhanced Raman scattering (SERS) microscopy. Ag NPs are coated with poly-N-vinylpyrrolidone and dispersed in pure water solutions. After the application of Ag NPs, porcine ears are stored in the incubator for 24 h at a temperature of 37°C. The TPT-FLIM measurement results show a dramatic decrease of the Ag NPs' signal intensity from the skin surface to a depth of 4 μm. Below 4 μm, the Ag NPs' signal continues to decline, having completely disappeared at 12 to 14 μm depth. CRM shows that the penetration depth of Ag NPs is 11.1 ± 2.1 μm. The penetration depth measured with a highly sensitive SERS microscopy reaches 15.6 ± 8.3 μm. Several results obtained with SERS show that the penetration depth of Ag NPs can exceed the stratum corneum (SC) thickness, which can be explained by both penetration of trace amounts of Ag NPs through the SC barrier and by the measurements inside the hair follicle, which cannot be excluded in the experiment.
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