Strat-M® positioning for skin permeation studies: A comparative study including EpiSkin® RHE, and human skin

In the development and optimization of dermatological products, In Vitro Permeation Testing (IVPT) is pivotal for controlled study of skin penetration. To enhance standardization and replicate human skin properties reconstructed human skin and synthetic membranes are explored as alternatives. Strat-M® is a membrane designed to mimic the multi-layered structure of human skin for IVPT. For instance, in Strat-M®, the steady-state fluxes (JSS) of resorcinol in formulations free of permeation enhancers were found to be 41 ± 5 µg/cm2·h for the aqueous solution, 42 ± 6 µg/cm2·h for the hydrogel, and 40 ± 6 µg/cm2·h for the oil-in-water emulsion. These results were closer to excised human skin (5 ± 3, 9 ± 2, 13 ± 6 µg/cm2·h) and surpassed the performance of EpiSkin® RHE (138 ± 5, 142 ± 6, and 162 ± 11 µg/cm2·h). While mass spectrometry and Raman microscopy demonstrated the qualitative molecular similarity of EpiSkin® RHE to human skin, it was the porous and hydrophobic polymer nature of Strat-M® that more faithfully reproduced the skin's diffusion-limiting barrier. Further validation through similarity factor analysis (∼80-85%) underscored Strat-M®'s significance as a reliable substitute for human skin, offering a promising approach to enhance realism and reproducibility in dermatological product development.

Efficiency of emulsifier-free emulsions in delivering caffeine and α-tocopherol to human skin

OBJECTIVE

Increasing consumer demand for natural and environmentally friendly products is driving the cosmetic industry to seek greener and safer processes. High-frequency ultrasound technology (HFUT) stabilizes emulsions without adding emulsifying surfactants (ES). In this work, the formulation characteristics of an HFUT-treated emulsion and a Reference emulsion were compared for both caffeine and α -tocopherol.

METHODS

A comparison was made between ES-free emulsions and the Reference emulsions based on droplet size, viscosity, pH, and rheology behavior for both active cosmetic ingredients. The permeation of caffeine and the skin retention of a-tocopherol were studied in vitro using Franz diffusion cells on human skin biopsies, considered the gold standard for permeation assays.

RESULTS

The formulations developed were stable and showed suitable droplet size distribution. In the case of ES-free emulsions, the average droplet size was inferior to 1.5 μm regardless of the polarity of the active. All formulations presented a shear-thinning pseudoplastic behavior, an attribute usually desired for cosmetic products. The skin permeation studies showed that in the case of caffeine (model hydrophilic molecule), the ES-free emulsion presented a delivery capacity similar to that of the Reference emulsion. However, for α-tocopherol (highly lipophilic model molecule), differences were observed in the distribution of the active in the stratum corneum with an advantage for the Reference emulsion, probably due to the impact of surfactants on the SC lipids.

CONCLUSION

This work demonstrates that HFUT is a reliable tool able to prepare stable ES-free emulsions loaded with hydrophilic or lipophilic active ingredients. Skin permeation studies confirm that the emulsions produced by HFUT promote the delivery of the actives to the human skin. In the case of α-tocopherol, the delivery efficiency was lower than with the Reference emulsion, especially in the SC layers, due to the absence of surfactants. Nevertheless, the ES-free emulsion still represents a good compromise between efficacy and the need for green cosmetics in the market.

Highlighting the efficiency of ultrasound-based emulsifier-free emulsions to penetrate reconstructed human skin

OBJECTIVE

The cosmetic industry endeavours to strengthen the greener and safer claims of processes to respond to the high demand from customers for natural and environmentally friendly products. High Frequency Ultrasonication Technology (HFUT) is a physical process enabling the stabilisation of emulsions without requiring additional ingredients such as Emulsifying Surfactants (ES) to be introduced into the formulations. In this study, key formulation characteristics of an emulsion synthesised by HFUT and a reference emulsion (RE) were compared, as well as the permeation kinetics of caffeine, used as a model active cosmetic ingredient, from both types of emulsions.

METHODS

The pH, particle size and viscosity of emulsions prepared by the HFUT and the RE were determined and compared. The permeation of caffeine from the HFUT emulsion and the RE applied to the surface of reconstructed human epidermis (RHE) models was compared.

RESULTS

The ES-free formulations prepared by HFUT displayed a nearly 2-fold lower average particle size and over 3-fold greater viscosity, compared to the RE. Despite these differences, the absence of ES in the HFUT emulsion did not significantly alter the permeation kinetics of caffeine through RHE. The caffeine steady-state flux, lag time and permeability coefficients differed by 20 to 30% only.

CONCLUSION

This study demonstrates the potential of the HFUT to yield topical cosmetic products with lower requirements ingredients-wise, without losing efficacy, supporting the possible implementation of the technology in the cosmetic industry.

Confocal Raman Spectroscopic Imaging for Evaluation of Distribution of Nano-Formulated Hydrophobic Active Cosmetic Ingredients in Hydrophilic Films

Film-forming systems are highly relevant to the topical administration of active ingredients (AI) to the body. Enhanced contact with the skin can increase the efficacy of delivery and penetration during prolonged exposure. However, after the evaporation of volatile solvents to form a thin film, the distribution of the ingredient should remain homogenous in order to ensure the effectiveness of the formula. This is especially critical for the use of hydrophobic molecules that have poor solubility in hydrophilic films. In order to address this concern, hydroxyphenethyl esters (PHE) of Punica granatum seed oil were prepared as a nanosuspension stabilised by poloxamers (NanoPHE). NanoPHE was then added to a formulation containing polyvinyl alcohol (PVA) as a film forming agent, Glycerol as a plasticiser and an antimicrobial agent, SepicideTM HB. Despite their reliability, reference methods such as high-performance liquid chromatography are increasingly challenged due to the need for consumables and solvents, which is contrary to current concerns about green industry in the cosmetics field. Moreover, such methods fail to provide spatially resolved chemical information. In order to investigate the distribution of ingredients in the dried film, Confocal Raman imaging (CRI) coupled to Non-negatively Constrained Least Squares (NCLS) analysis was used. The reconstructed heat maps from a range of films containing systematically varying PHE concentrations highlighted the changes in spectral contribution from each of the ingredients. First, using NCLS scores it was demonstrated that the distributions of PVA, Glycerol, SepicideTM HB and PHE were homogenous, with respective relative standard deviations (RSD) of 3.33%, 2.48%, 2.72% and 6.27%. Second, the respective relationships between ingredient concentrations in the films and their Raman responses, and the spectral abundance were established. Finally, a model for absolute quantification for PHE was be constructed using the percentage of spectral abundance. The prepared %w/w concentrations regressed against predicted %w/w concentrations, displaying high correlation (R2 = 0.995), while the Root Mean Squared Error (0.0869% w/w PHE) confirmed the precision of the analysis. The mean percent relative error of 3.75% indicates the accuracy to which the concentration in dried films could be determined, further supporting the suitability of CRI for analysis of composite solid film matrix. Ultimately, it was demonstrated that nanoformulation of hydrophobic PHE provides homogenous distribution in PVA based film-forming systems independent of the concentration of NanoPHE used in the formula.

Freezing Weakens the Barrier Function of Reconstructed Human Epidermis as Evidenced by Raman Spectroscopy and Percutaneous Permeation

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.