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Objective: ε‐Poly‐l‐lysine (PLL) is a cationic polymer consisting of 25–35 l‐lysine residues. Our previous study revealed that fluorescently labelled PLL can stain the stratum corneum (SC) via ionic interactions between PLL and SC constituents. In this study, to further clarify the mechanisms underlying the interaction between PLL and the SC, the staining properties of fluorescent PLL were compared with that of fluorescently labelled anionic dextran (aDex), which has approximately the same molecular weight as PLL. Methods: SC samples were collected by non‐invasive tape stripping and stained with fluorescent PLL and/or fluorescent aDex. Fluorescence images were acquired using a fluorescence microscope and then analysed. Results: The SC could be stained with either fluorescent PLL or aDex, both of which were inhibited by the addition of high concentrations of salt solutions. In particular, aDex staining was inhibited at a lower salt concentration than PLL staining. Moreover, PLL staining was inhibited under acidic conditions, while aDex staining was inhibited under neutral to alkaline conditions. Double staining of SC with both fluorescent polymers produced heterogeneous staining patterns: corneocytes stained with both polymers, corneocytes stained with PLL or aDex in a mutually exclusive manner, and unstained corneocytes. Staining of SC samples from the face was more extensive than staining of SC samples from the inside of the upper arm with both polymers. In addition, pretreatment of the SC with ethanol resulted in enhanced staining with both polymers. These results suggest that double staining of SC with both polymers can provide information on the damaged SC. Conclusion: Staining of SC with fluorescent PLL depends on its properties of a cationic and hydrophobic polymer with appropriate molecular size, which can distinguish the damaged SC. Double staining of SC with fluorescent PLL and aDex is a novel approach to obtain information for the analysis of skin conditions. [ABSTRACT FROM AUTHOR]

Background: ε‐Poly‐L‐lysine (PLL) is a cationic polymer consisting of 25 to 35 L‐lysine residues that adheres to the surface of skin as well as hair. However, the properties of PLL regarding its adhesion to the skin remain to be elucidated. In this study, we examined the staining of stratum corneum (SC) with fluorescence‐labeled PLL and explored its relationship with skin condition. Materials and methods: Alexa Fluor 488‐labeled PLL (AF‐PLL) was reacted with tape‐stripped stratum corneum (SC), and the staining properties were monitored by fluorescence microscopy. Clinical study was performed by measuring the water content of the cheek SC and transepidermal water loss (TEWL), and the tape‐stripped SC was subjected to staining with AF‐PLL. Results: AF‐PLL staining of the SC was inhibited at acidic pH or by the addition of high concentration of salt solution, suggesting the involvement of ionic interaction between PLL and the SC, at least in part. The AF‐PLL staining was inhibited by unlabeled PLL or various alkyl amines, but not by L‐lysine monomer. AF‐PLL staining was observed inside the corneocytes as well as surrounding cornified envelope. Clinical study revealed that AF‐PLL staining intensity of the SC was negatively correlated with its water content and positively correlated with its TEWL. Conclusion: PLL can efficiently adhere to SC and AF‐PLL staining of SC can be applied to evaluate skin conditions. [ABSTRACT FROM AUTHOR]