Emulsions are essential formulations for cosmetics, and are the basis of product functions, properties, and usability. Many emulsification technologies have been developed and have contributed to the advancement of cosmetics. In this article, the product development research I have experienced, as well as the distinctive emulsification technologies developed by researchers associated with me, and how they have been applied to cosmetics will be introduced. Specific examples include: 1）Stabilization of low-viscosity emulsions, 2）Development of a high-oil-containing liquid crystal gel with spontaneous emulsification ability, and its subsequent development, 3）Stabilization of α-gel and formation of multilamellar emulsion in the development of highly moisturizing emulsions, 4）Achieving both long-lasting effect and usability, including high-water-containing W/O emulsions, powder-oriented W/O emulsion（Pickering emulsion）that must be shaken before use, and water-repellent O/W emulsion technologies, will be explained. In addition to technical aspects, various events experienced during the development process are also shown.

In the development of skincare products and topical medications, numerous penetration tests have been performed on both human and animal skin to confirm drug efficacy and safety. Typically, test skin is stored frozen to facilitate distribution and prevent degradation, and is subsequently used after thawing. While many studies have examined the effects of freezing and thawing on skin structure and drug permeability, results vary significantly due to differences in factors such as animal species, freezing temperatures, and durations. As a result, the effects of freezing and thawing on the skin currently remain controversial. In this study, we utilized fresh facia（l eyelid）skin obtained from the same individual immediately post-surgery to investigate the differences in drug permeability between frozen and unfrozen skin. Our findings indicated that the cumulative skin permeability of water-soluble caffeine and oil-soluble isopropylmethylphenol was lower in unfrozen skin compared to frozen skin. No significant difference was observed in esterase activity between frozen and unfrozen skin, suggesting that freezing and thawing may affect the physical barrier function of the skin. Additionally, we considered the concentration of residual drug in the stratum corneum, epidermis, and dermis post-permeation test, as well as the analysis of intercellular lipid structure using confocal Raman spectroscopy.

Shampoos typically consist of cationic polymers, anionic surfactants, and amphoteric surfactants as their primary components. During use, when these formulations are diluted with water, they often undergo liquid-liquid phase separation, resulting in the formation of a dilute phase and a concentrated phase. This concentrated phase, known as coacervate, is rich in polymers and surfactants and plays a significant role in determining the sensory characteristics and overall user experience of shampoos. The quantity and quality of the coacervate are widely acknowledged to influence the foaming, spreading, and conditioning properties of shampoo. Traditionally, shampoos have relied on sulfate-based anionic surfactants. However, there has been a growing shift towards non-sulfate formulations due to consumer demand for milder and potentially less irritating alternatives. Despite this trend, studies examining coacervation phenomena in non-sulfate shampoos remain limited. In this study, we analyzed four commercially available non-sulfate shampoos to investigate the relationship between coacervate properties̶such as formation region, quantity, particle size, ζ-potential, and viscoelasticity̶and the perceived user experience. Our findings reveal that the characteristics of the coacervate from non-sulfate shampoos significantly influence the sensory attributes of formulas, including foaming, sliminess, quickness of rinsing, and smoothness at rising and drying. This study highlights the importance of understanding coacervate behavior in the formulation of non-sulfate shampoos to optimize user satisfaction. Further research into the interplay between formulation ingredients and coacervate properties could provide valuable insights for the development of next-generation shampoos.

Handwashing is critically important in preventing infection as it prevents pathogens on your hands from entering your body through your mouth or eyes. In this study, we examined the persistence of the antibacterial effect after handwashing with a hand soap formulation containing fatty acid salts and poly diallyldimethylammonium chloride（PDADMAC）on human skin. To achieve the persistence of this effect, we utilized the coacervation technology, which involves complex precipitation due to the interaction between anionic surfactants（fatty acid salts）and cationic polymers（PDADMAC）, caused by dilution during the washing process. The results of the persistence of the antibacterial effect on human skin against E. coli and S. aureus showed significantly higher efficacy even 4 h after handwashing compared to handwashing without hand soap, suggesting that the use of this formulation in handwashing may provide the persistence of antibacterial effect on the hands. In addition, when the results of the persistence of the antibacterial effect and the residual components on human skin were compared with those on model skin, similar trends were observed in both cases. This indicates that the evaluation using model skin can effectively predict the results on human skin. The findings of this study contribute to reducing the risk of contact infection after handwashing.