Published papers

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.

© 2020 The Authors Ultrafine bubble (UFB) is a bubble with a diameter of less than 1 μm. Little attention has been paid to the defoaming and removal of UFBs. This study proposes a method to destabilize UFBs by using indirect ultrasonic irradiation. Besides, the destabilization mechanism of UFB was investigated. The ultrasonic frequency was 1.6 MHz and the dissipated power was 30 W. UFB dispersions were prepared using two different types of bubble generators: pressurized dissolution method and swirling liquid flow method. The effects of ultrasonic irradiation on the stability of UFBs were evaluated by particle tracking analysis (PTA) and electrophoretic zeta potential measurement. Results showed that the indirect ultrasonic irradiation for 30 min reduced the number concentration of UFBs by 90% regardless of the generation method. This destabilization was attributed to a decrease in the magnitude of zeta potential of UFBs due to the changes in pH and electrical conductivity. These changes in the electrochemical properties were caused by the formation of nitric acid. To study the destabilization mechanism, the pH of the UFB dispersions were modified by titration; the chemical and mechanical effects of ultrasound were separately examined. It was found that not only the chemical effect caused by the formation of nitric acid but also the mechanical effect contributed to the destabilization of UFB. Feasibility studies were also performed for UFBs in an aqueous surfactant solution and UFBs in a solid particle dispersion. The proposed method selectively destabilized UFBs in the solutions.

© 2020 Wiley-VCH GmbH For a variety of applications of microbubbles, it is necessary to clarify the dissolution dynamics and the lifetime of a microbubble. Although various models have been proposed, predicting the dissolution rate of a microbubble in a static fluid with a surfactant is still difficult. In this study, dissolution behavior was predicted with taking into account the mass transfer inhibition effect due to a surfactant at the bubble surface. The shrinking behavior of an oxygen microbubble was observed with several surfactants and the experimental results were in good agreement with model predictions.

© 2020 Wiley-VCH GmbH This study investigated submicron-sized bubbles, more precisely ultrafine bubbles (UFBs). The number concentration, size, and scattered light intensity of UFBs were measured by particle tracking analysis (PTA). The results showed that storage temperature had a significant effect, whereas the storage containers had little effect on the long-term stability of UFBs. It was found that the second-order aggregation rate equation could predict the stability of UFBs; however, there were some differences from the theory at high storage temperatures. The features of the PTA were also discussed in detail.

The microcapillary was combined with the liquid-liquid slug flow to build the separation field. The partition behavior of seventeen kinds of substrates was investigated by using six kinds of water/organic biphasic systems under hydrothermal conditions (25 - 190° C at 10 MPa). The scale of hydrophobicity (HF) of water/organic biphasic system was estimated. The HF value for the biphasic system used here was comparable to the conventional aqueous two-phase systems and depended on the (de)solvation of substrates by water and organic solvent. Besides, the vortex field in the slug contributed to the enhanced mass transfer of substrates. Those results are available for the selection of water/organic biphasic system as the reactive separation of target materials.

Because of their useful characteristics, microbubbles attracted much interest in industry and medicine. In this study, we developed a novel method of generating microbubbles through the chemical absorption of ammonia (NH3). By using an ultrahigh-speed camera, consecutive images of bubble formation were captured, and the microbubble generation mechanism was investigated. The effects of the gas flow rate and ammonia concentration in the liquid phase on the size of the microbubbles were examined. The hydrogen (H2) gas flow rate was found not to demonstrate a significant effect on the bubble diameter. The diameter of the microbubbles increased as the superficial velocity of the gas mixture (NH3 – H2) decreased.

The sufficient provision of oxygen is mandatory for enzymatic oxidations in aqueoussolution, however, in process optimization this still is a bottleneck that cannot beovercome with the established methods of macrobubble aeration. Providing highermass transfer performance through microbubble aerators, inefficient aeration canbe overcome or improved. Investigating the mass transport performance in a modelprotein solution, the microbubble aeration results in higher kLa values related to theapplied airstream in comparison with macrobubble aeration. Comparing the aera-tors at identical kLa of 160 and 60 1/h, the microbubble aeration is resulting in 25and 44 times enhanced gas utility compared with aeration with macrobubbles. Toprove the feasibility of microbubbles in biocatalysis, the productivity of a glucoseoxidase catalyzed biotransformation is compared with macrobubble aeration as wellas the gassaving potential. In contrast to the expectation that the same pro-ductivities are achieved at identically applied kLa, microbubble aeration increasedthe gluconic acid productivity by 32% and resulted in 41.6 times higher oxygenutilization. The observed advantages of microbubble aeration are based on the largevolumespecific interfacial area combined with a prolonged residence time, whichresults in a high mass transfer performance, less enzyme deactivation by foamformation, and reduced gas consumption. This makes microbubble aerators favor-able for application in biocatalysis.

During the optimization of processes in biocatalysis, the limitations of mass transfer and enzyme stability have to be overcome. To achieve these goals, aeration with microbubbles is advantageous by providing favorable properties, which include high gas utility, low-pressure drop, and reduced foam formation. In this study, the influences of microbubble, classic sub-millibubble and macrobubble aeration on enzyme stability and gas utility are compared. Using five enzymes, the half-life was investigated by comparing the microbubble aeration with a 2 μm porous sintered frit to the macrobubble aeration with a 5 mm open pipe, both adjusted to provide a constant kLa of 160 h−1. Furthermore, the half-life of a glucose oxidase was investigated by variation of the kLa in the range of 40 to 350 h−1 and the mode of the aeration system comparing the 2 μm and 10 μm sintered frit as well as the 5 mm open pipe. In both experimental set-ups, it was demonstrated that the aeration with microbubbles using the 2 μm sintered frit was beneficial for reaching high levels of enzyme stability. From this, we discovered that the interfacial area renewal rate plays an important role in the process of enzyme deactivation, which changes depending on the aeration mode.

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.

Although the mechanism of ultrafine bubble generation has not yet been elucidated, we believe that the state of gas-saturated water before the generation of ultrafine bubbles is the most important factor in the mechanism of ultrafine bubble generation. Therefore, we proposed the hypothesis that the spatial distribution of dissolved gas molecules among the water molecules before the ultrafine bubble formation is stared has a significant effect on the formation of ultrafine bubble suspending water. Based on this hypothesis, we succeeded in generating ultrafine bubbles of sufficient number concentration in the condensate by rapidly condensing a well mixture of non-condensable gas and vapor by cooling. The number concentration of the generated ultrafine bubbles increased almost in proportion to the condensation rate of the vapor. Therefore, it was found that promoting the formation of bubble nuclei from dissolved gas molecules existing between water molecules in the condensate and suppressing bubble growth led to an increase in the number concentration of ultrafine bubbles. The principle of ultrafine bubble generation in this invention not only simplifies the generation of ultrafine bubble water but also enables the control of the number concentration of ultrafine bubbles in water.