Published papers

Vesicles, closed bilayer structures composed of amphiphiles, have attracted considerable attention as functional materials. Structural stability and stimulus responsiveness are required for next-generation functional vesicles. However, there is a dilemma between these properties because the desired membrane structure varies in terms of structural stability and stimulus sensitivity. Herein, we propose a new approach for the development of giant vesicles (GVs) through the molecular design and synthesis of amphiphiles with or without amide linkages, forming hydrogen bonding. From the 1H NMR analysis and fluorescence spectra of environment-responsive probes, intermolecular hydrogen bonding between the amide linkages in the membrane contributed to the enhanced structural stability of the GVs. Moreover, by adding amphiphiles containing a photoresponsive azobenzene moiety to GVs composed of amphiphiles with or without amide linkages, a distinct mechanism of photoresponsive deformation was observed: the former exhibited large and irreversible deformation, while the latter showed a modest and reversible manner due to the photoisomerisation of azobenzene under ultraviolet and subsequent visible light illumination. This difference was also attributed to the membrane structure affected by intermolecular hydrogen bonding. Based on these results, the finding provides a molecular methodology for developing highly functional vesicles.

The study of self-propelled motion in soft matter systems has garnered significant interest owing to its potential applications in microfluidics, soft robotics, and autonomous system design. Understanding the molecular mechanisms underlying motility is crucial for advancing these applications. This study investigates the self-propelled motion of lauronitrile oil droplets in aqueous surfactant solutions, focusing on the impact of different surfactant molecular structures on droplet dynamics. This study compares surfactants with ester and amide linkages, highlighting their critical role in modulating interfacial tension and driving Marangoni convection, a key factor behind droplet movement. Surfactants with ester linkages exhibit a high affinity for lauronitrile and rapidly adsorb at the oil–water interface, generating strong Marangoni flows and driving fast droplet motion. In contrast, amide-containing surfactants exhibit slower adsorption and weaker interactions with lauronitrile, leading to reduced or absent motion. These findings provide new insights into the molecular mechanisms underlying the self-propelled droplet behavior in non-equilibrium systems and contribute to a deeper understanding of self-organizing phenomena.

Nonequilibrium fluid patterns, such as Marangoni contraction, coffee rings, and tears of wine, are generated in binary solutions spread on a substrate during their evaporation. In this study, we observed another type of nonequilibrium behavior exhibited by binary solutions as they infiltrate porous materials and undergo evaporation. A binary solution comprising hexane and ethanol was brought into contact with the chromatography paper to facilitate infiltration into the paper’s pores. Because the experimental setup was in an open environment, infiltration and evaporation occurred simultaneously. The wicking front exhibited an initial rapid advancement, followed by subsequent receding and readvancing. Additionally, the bifurcation of the wicking front after the onset of its readvancement was confirmed by monitoring the temporal evolution of the spatial luminance distribution and temperature distribution on the surface of the chromatography paper. Chromatographic development of a hydrophilic dye was conducted in this experimental setup in an open environment. Additionally, it was confirmed that the receding and readvancing of the wicking front represented the transposition of the bifurcated wicking fronts.

The movement of supramolecular self-assemblies has been induced by the application of external stimuli that cause a shift to non-equilibrium conditions. However, few supramolecular chemical systems exist in which multiple different functions are associated with movement. Herein, we observed multiple phase transition of micrometer-sized lauronitrile oil droplets in a solution of cationic surfactants with aniline skeletons in the presence of metal ions. These dynamics include tactic motion and subsequent formation of aggregates with membrane structures, in a linear-type channel. These dynamic behaviors result from variations in the interfacial tension at the droplet surface and the chemical compositions owing to the interactions between system components. The current findings may provide a methodology for designing molecular systems in which multi-step phase transitions of supramolecular self-assemblies are coupled to movement in non-equilibrium conditions.

Multicellular organisms demonstrate a hierarchical organization where multiple cells collectively form tissues, thereby enabling higher-order cooperative functionalities beyond the capabilities of individual cells. Drawing inspiration from this biological organization, assemblies of multiple protocells are developed to create novel functional materials with emergent higher-order cooperative functionalities. This paper presents new artificial tissues derived from multiple vesicles, which serve as protocellular models. These tissues are formed and manipulated through non-covalent interactions triggered by a salt bridge. Exhibiting pH-sensitive reversible formation and destruction under neutral conditions, these artificial vesicle tissues demonstrate three distinct higher-order cooperative functionalities: transportation of large cargoes, photo-induced contractions, and enhanced survivability against external threats. The rapid assembly and disassembly of these artificial tissues in response to pH variations enable controlled mechanical task performance. Additionally, the self-healing property of these artificial tissues indicates robustness against external mechanical damage. The research suggests that these vesicles can detect specific pH environments and spontaneously assemble into artificial tissues with advanced functionalities. This leads to the possibility of developing intelligent materials with high environmental specificity, particularly for applications in soft robotics.

Factors influencing on in vitro evaluation of UV protecting ability of sunscreens were analyzed. It was found that any factors making the sunscreen layer spatially inhomogeneous, such as directional viscous fingering during the sunscreen application, dewetting of applied sunscreen layer, and the surface roughness of the standard PMMA plate, alter the UV transmittance. New application procedure and new type of flat hydrophilic plate were developed for inhibiting the generation of spatial inhomogeneity in applied sunscreen layer. The method created by the combination of these newly developed technologies succeeded in providing reliable and reproducible in vitro evaluation of UV protecting ability.

Motion is an essential feature of living systems. Microorganisms autonomously change their nature in response to slight changes in the surrounding environment induced by external stimuli and exhibit unique motion modes. Various self-propelled objects have been constructed to understand these behaviors. Towards achievement of such purpose, the precise settings of experimental conditions including fabrication of objects with a shape anisotropy have been made attempts in the field of active matter and supramolecular chemistry. This review describes the recent progress in inducing the self-propelled motion of artificial objects. If life-like dynamic behavior such as self-propelled motion can be designed and experimentally induced from molecular properties, it will be easier to control functions expressed as outputs. This will lead to not only a better understanding of the complex functions in living systems, but also the fabrication of exotic materials with life-like properties.

Factors affecting the in vitro Sun Protection Factor (SPF) and Ultraviolet-A Protection Factor (UVA-PF) of sunscreens were analyzed for verifying the validity and reliability of the ISO24443 evaluation method. UV absorbance measurements by different spectrophotometers did not lead to the large difference in in vitro SPF/UVA-PF, although the UV absorbance determined by each spectrophotometer exhibited relatively large difference when it was larger than 2. On the other hand, relatively large difference was found in in vitro SPF/UVA-PF by utilizing European Cosmetic and Perfumery Association (Colipa) 1994 or UV-solar simulated radiation (UV-SSR) for the spectral irradiance. Appropriateness of employing the coefficient of adjustment for the determination of in vitro UVA-PF was also found to be reexamined.

For the development of new functional materials for various applications, such as drug or gene delivery and environmental remediation, the relationship between function and morphology has been considered an important aspect for controlling affinity to the targets. However, there are only a few reports on this relationship because the molecular strategy for the precise control of vesicle shape has been restricted. Herein, we report the photocontrol of vesicle shape using azobenzene-containing amphiphilic switches. The addition of the photoswitches to vesicles composed of phospholipids yielded non-spherical vesicles. They exhibited reversible deformation between a non-spherical and spherical shape in response to ultraviolet and visible light illumination, respectively. From the results of the 1H NMR analyses, fluorescence spectra of the environmentally responsive probes, and Π-A isotherm measurements, the reversible photoresponsivity can be attributed to the changes in the vesicular membrane structures caused by migration of the photoswitches between the aqueous phase and membrane through their photoisomerization.

Multicellular organisms have hierarchical structures where multiple cells collectively form tissues with complex 3D architectures and exhibit higher-order functions. Inspired by this, to date, multiple protocell models have been assembled to form tissue-like structures termed prototissues. Despite recent advances in this research area, the programmed assembly of protocells into prototissue fibers with emergent functions still represents a significant challenge. The possibility of assembling prototissue fibers will open up a way to a novel type of prototissue subunit capable of hierarchical assembly into unprecedented soft functional materials with tunable architectures, modular and distributed functionalities. Herein, the first method to fabricate freestanding vesicle-based prototissue fibers with controlled lengths and diameters is devised. Importantly, it is also shown that the fibers can be composed of different specialized modules that, for example, can endow the fiber with magnetotaxis capabilities, or that can work synergistically to take an input diffusible chemical signals and transduce it into a readable fluorescent output through a hosted enzyme cascade reaction. Overall, this research addresses an important challenge of prototissue engineering and will find important applications in 3D bio-printing, tissue engineering, and soft robotics as next-generation bioinspired materials.

Infiltration of binary solution of hexane and ethanol into chromatography paper associated with their evaporation was found to generate unexpected initial rapid advancement of wicking front followed by its receding and readvancing in our previous research. In the present study, paper chromatography development of hydrophobic dye, Sudan III, and hydrophilic dye, Acid Blue 9, was carried out using binary solutions of hexane and ethanol in open environment, allowing the developing solvent been evaporated. Sudan III was developed with initial rapid advancing wicking front, while Acid Blue 9 was scarcely developed. On the other hand, Acid Blue 9 was developed with the readvancing second wicking front, while the spot of Sudan III scarcely migrated. Thus, the unexpected illusional phenomenon, overtaking the spot of Sudan III by the spot of Acid Blue 9, was observed. The readvancement of the second wicking front was found to be enhanced as increasing the relative humidity in the environment. Surface temperature of the chromatography paper was measured during the chromatographic development in open environment to show that it became lower than the dew point when the experiments were carried out in relatively high humidity. Solubility of Sudan III in a binary solution of ethanol and water remarkably decreased as increasing the content of water. It was thus suggested that the water vapor condensation to induce water mixing into the mobile phase to decrease the solubility of Sudan III to inhibit its chromatographic development to realize the illusional spot overtaking of dyes of their chromatographic development associated with solvent evaporation.

© 2019 by Japan Oil Chemists’ Society. Cosmetic standard poly methyl methacrylate (PMMA) UV evaluation plates, i.e., roughened PMMA plates, are used in the EU and the US for the in vitro evaluation of UV protection abilities of sunscreen, such as in vitro UVA-PF and critical wavelength for the Broad-spectrum approval. In our previous studies, inhomogeneity in the thickness of a pseudo-sunscreen layer applied on a flat quartz plate was observed to alter UV transmission. Thus, the surface roughness of the standard plates should have a significant influence on the in vitro evaluation. In the present study, we have analyzed the surface structures of three cosmetic standard PMMA UV evaluation plates: Helioplates HD6 (Sa = 6 µm) and ISO plates (Ra = 2 and 5 µm). A decamethylcyclopentasiloxane and 2-propanol-mixed solvent solution of acrylsilicone resin was prepared, and the solution was added dropwise onto the plates. After the evaporation of the solvents, the plates were cut and the cross section was analyzed using SEM-EDS. The distribution of silicon atoms at the cross-section suggested that the maximum depth of penetrating of acrylsilicone resin was larger than Ra for all the standard plates, and the surface structure was significantly different for each standard plate. In addition, cracks into which the acrylsilicone resin deeply penetrated were observed on the surface of some plates. Clear-solution-type pseudo-sunscreen samples in which UV absorbers and acrylsilicone resin were dissolved were deposited on the standard roughened PMMA plates. It was observed that the addition of acrylsilicone resin drastically changed the net UV transmission. The degree to which the hollows of the roughened surface were filled with the pseudo-sunscreen samples determined the variation of UV transmittance.

Control of powder alignment is essential for maximizing the functionality of color cosmetics and sunscreens. Various surface treatments were applied to nanosized titanium dioxide to modify their surface characteristics. Such modifications can be used to control the behavior of dispersions in cosmetics, enabling them to align uniformly. The powders were mixed with solvents and applied to a cellulose triacetate film. The features of powder alignment on the film were evaluated using several approaches. When the type of surface treatment changed by varying the weight ratio, there was no significant correlation between its alignment and treatment. However, when we focused on the pseudo-HLB each treated pigment, their alignments were correlated. It was confirmed that the powders subjected to the appropriate surface treatment combinations from the pseudo-HLB standpoint made it possible to align uniformly and create a smooth coating film. As a result, it has a high UV-shielding ability. The surface-treated powders in this study were found to change the UV shielding ability and surface roughness of the layer formed when they were formed by spreading the sample powder dispersion and drying of the film. It was suggested that the pseudo-HLB, which is calculated based on the chemical structure after the surface treatment process, is useful for choosing the optimum surface treatment to create a uniformly aligned pigment layer.

Self-propelled droplets are of considerable interest as an appropriate model for understanding the self-propulsion of objects in the fields of nonequilibrium physics and nonlinear science. Several research groups have reported the monodirectional motion of droplets, that is, chemotaxis, using stimuli-responsive materials. However, the precise control of chemotaxis remains challenging from the perspective of synthetic chemistry because chemotactic motion is primarily induced by the consumption of reactive oil or surfactants. Herein, we report a chemical system containing pH-responsive fumaric acid derivatives, in which the oil droplet exhibited positive chemotaxis over a wide pH range–from basic to acidic conditions. From the measurements of the interfacial tension between the oil and aqueous phases, it was deduced that the positive chemotaxis was due to heterogeneity in the interfacial tension of the droplet surface, which was accompanied by the production of surface-active compounds in the pH gradient in a linear-type channel.

A new method was developed for the in vitro sun protection factor (SPF) evaluation of sunscreen samples. A new type of substrate, a hydroxyalkyl cellulose-coated plate, was also prepared specifically for hydrophilic samples. This new substrate was required because hydrophilic samples would be unlikely to wet the surface of the standard cosmetic PMMA UV evaluation plate. A super-hydrophilic quartz plate was prepared by corona-discharge treatment before an aqueous solution of hydroxyalkyl cellulose was spread on it. A flat and uniform hydroxyalkyl cellulose film was subsequently formed through the evaporation of water. Special care was taken to inhibit the generation of spatial non-uniformity. Six hydrophilic sunscreen samples with in vivo SPF values of 56, 55, 52, 25, 15, and 4, were then applied to the prepared hydroxyalkyl cellulose-coated plate, as well as a super-hydrophilic quartz plate and a flat hydrophobic PMMA plate. The thicknesses of the applied layers were determined using a wheel-shaped wet film thickness gauge immediately after the application, and UV transmission was measured using an SPF analyzer. The value of in vitro SPF was calculated from the UV absorbance and the thickness of the layer. For two out of the six samples, PMMA plate could not be available, as the samples were unable to wet the PMMA surface. Relatively small differences were observed between the in vitro SPF values when the super-hydrophilic and hydroxyalkyl cellulose-coated plates were used. Samples exhibiting higher in vivo SPF were also associated with higher in vitro SPF values, although a linear relationship was not observed. In contrast to the super-hydrophilic plate whose half-life of the super-hydrophilicity is only approximately five days, the hydrophilicity of the hydroxyalkyl cellulose-coated plate scarcely varied during six months of storage. Finally, a simplified evaluation method was also proposed. The validity of the method was verified through a ring test where three researchers employed this method in different laboratories at three independent organizations.

Objective: Titanium dioxide powders are contained in a large class of colour cosmetics and sunscreen formulas. When they are used, the formation of a uniform functional powder layer on the skin is an important factor to show their functionality, such as aesthetic and UV protection. Attempts were made to extract the factors that affect the UV shielding ability of the deposited powder layer. Methods: Seven kinds of surface treatments were conducted on nano-sized titanium dioxide powder to modify the surface characteristics. Dispersion samples were prepared by mixing these powders with liquids, such as mixed solutions of cyclopentasiloxane, isododecane, coconuts alkane and dimethicone using a disperser and a bead mill. The dispersions were applied using an applicator on cellulose triacetate film, polycarbonate film and polymethyl pentene film. Laser microscope observation and micro-gloss glossmeter analysis were carried out to assess the flatness of the deposited powder layers, and the UV shielding ability was evaluated using SPF analyser. Results: Factors whose influences on the structure and UV shielding ability of the deposited powder layer being analysed were pseudo-HLB of the powders, liquids for preparing the dispersions and material of the substrates. Higher UV shielding ability was attained when powders having pseudo-HLB at around 6 were employed independent from the kinds of liquids and substrates. Flatness of the deposited layer was found to enhance the UV shielding ability of the UV-B region, while that of the UV-A region was scarcely influenced by the flatness. Employing lower surface tension liquids for preparing the dispersions and materials exhibiting lower polar components of surface free energy as substrates tended to enhance the UV shielding ability. Conclusion: Surface treatments conducted on the powders in this study were found to change UV shielding ability, especially UV-B shielding ability, and the relation between pseudo-HLB and UV shielding ability was scarcely influenced by the kinds of liquids. Both surface tension of liquids and the polar component of surface free energy of substrates affected the UV shielding ability. It was suggested that pseudo-HLB calculated based on chemical structure becomes useful information to choose optimum surface treatment to make uniform powder alignment independent from the surrounding environment.

There have been several attempts to construct supramolecular chemical systems that mimic the phase transitions in living systems. However, most of these phase transitions are one-to-one and induced by one stimulus or chemical; there have been few reports on the pathway-dependent phase transition of supramolecular self-assemblies in multi-step. To induce multistep phase transitions, molecular crystals were prepared that contained a cationic amphiphile bearing azobenzene and disulfide groups. A reducing agent caused the crystals to become vesicles, and adjacent, non-touching vesicles fused under UV and subsequent visible light. Adding a reducing agent to the worm-like aggregates that were generated after UV irradiation of the original crystals resulted in the growth of sheet-like aggregates. 1H NMR and fluorescence anisotropy measurements showed that a series of phase transitions was induced by changes in the phase structures from molecular conversions of the reactive amphiphiles. The multiple pathway-dependent phase transitions of supramolecular self-assemblies can provide a methodology for developing new stimuli-responsive materials that exhibit the desirable properties under specific circumstances from a systems chemistry viewpoint.

Collective motions are the spontaneous emergence of the ordered movement in living systems. Various constructive approaches have been attempted toward understanding the mechanism. However, few studies have been on the motion induced by biomimetic artificial systems based on molecular conversions. In this study, we report an artificial system of oil droplets in surfactant solutions, wherein collective motion and pattern formation occur concurrently, using the conversion of photoresponsive surfactants. Assembly/dispersion of multiple oil droplets and appearance/disappearance of circular patterns around them occur under UV/visible light illumination, respectively. Moreover, whether the assembled droplets move laterally or not depends on the number of droplets, which is considered analogous to biological organisms in response to population density. These phenomena occur because the photoisomerization of surfactants induces the gradient of interfacial tension, generating specific flow fields around droplets. Our results could be useful for understanding the mechanism of life motion in terms of physicochemical aspects.

Amphiphiles form various molecular self-assemblies in the aqueous phase, such as vesicles and coacervates, depending on molecular properties. Recently, phase transitions between vesicles and coacervates have drawn much attention because the switching between “closed” and “open” compartments could achieve the controllability of molecular sequestrations. We expect that the photoinduced drastic change in the molecular structure of catanionic amphiphiles comprising azobenzene-containing cationic amphiphiles (Azo) and anionic sodium dodecyl sulfate causes the transition. Self-assemblies prepared by a thin-film hydration method were observed using microscopy under UV and subsequent visible-light illumination. Additionally, investigation of selectivity for molecular sequestration and assay of enzymatic reactions in the self-assemblies were conducted. Photoinduced reversible phase transitions between vesicles and coacervates occurred owing to the photoisomerization of Azo. The formed coacervates had selectivity for molecular sequestration. Additionally, the transition achieved fusion of different vesicles via coacervates. Although coacervates are generally known to facilitate enzymatic reactions, the reactions were inhibited in the presence of the formed self-assemblies due to the deactivation of enzymes. Our results represent rapid and non-invasive systems of reversible vesicle–coacervate transitions. Furthermore, the findings provide a new aspect of coacervates in which catanionic low-molecular-weight amphiphiles could act as suppressors of chemical reactions.

Mannosylerythritol lipids (MELs), which are one of the representative sugar-based biosurfactants (BSs) produced by microorganisms, have attracted much attention in various fields in the sustainable development goals (SDGs) era. However, they are inseparable mixtures with respect to the chain length of the fatty acids. In this study, self-assembling properties and structure-activity relationship (SAR) studies of recovery effects on damaged skin cells using chemically synthesized MELs were investigated. It was revealed, for the first time, that synthetic and homogeneous MELs exhibited significant self-assembling properties to form droplets or giant vesicles. In addition, a small difference in the length of the fatty acid chains of the MELs significantly affected their recovery effects on the damaged skin cells. MELs with medium or longer length alkyl chains exhibited much higher recovery effects than that of C18-ceramide NP.