Akira Harada

Substrate transport within biological tissues is diverse, with the most fundamental process being transport across cell membranes, which plays a crucial role in sustaining life. In this study, an artificial substrate transport system based on hydrogels by utilizing molecular recognition and stimuli‐responsive substrates is developed. α‐ and β‐Cyclodextrins are selected as host molecules, while adamantane serves as the guest molecule, enabling the adhesion of two hydrogels through self‐assembly. Under light stimulation, the light‐responsive dye, azobenzene derivative, is transported between the two hydrogels. This research provides new insights into the development of light‐controlled substance transport systems, which can be applied to biological substance delivery and the creation of smart materials.

In these decades, considerable attention has focused on supramolecular polymers due to their unique structures and properties. More recently, macroscopic supramolecular polymers have attracted increasing interest from not only biologists but also materials scientists inspired by the sophisticated structures and functions of living organisms. Since the functions of supramolecular polymers are strongly dependent on their shape, control of the shape is an important issue in controlling the functions of supramolecular polymers. However, the control of shape in macroscopic supramolecular assemblies has not yet been sufficiently investigated. Previously, we studied the macroscopic self-assembly behavior of super absorbent polymer (SAP) microparticles modified with β-cyclodextrin (βCD) and adamantane (Ad) residues (βCD(x)-SAP and Ad(y)-SAP microparticles, where x and y are the mol% contents of βCD and Ad residues, respectively). More elongated assemblies were formed at higher y, indicating that the shape of assemblies can be controlled by varying the interaction strength. The noteworthy is that 1-adamantanamine hydrochloride (AdNH₃Cl) assisted the formation of assemblies from βCD(x)-SAP and Ad(y)-SAP microparticles, indicating that AdNH₃Cl acts as a chemical stimulus for macroscopic assemblies of βCD(x)-SAP and Ad(y)-SAP microparticles. In this study, we have thus studied the assembling behavior of βCD(x)-SAP microparticles with Ad(y)-SAP microparticles and unmodified SAP microparticles assisted by AdNH₃Cl, as well as the shape of the resulting macroscopic assemblies. AdNH₃Cl assisted the formation of assemblies from βCD(16.2)-SAP and Ad(15.1)-SAP microparticles, in which AdNH₃Cl crosslinked the SAP microparticles through the formation of inclusion complexes of βCD residues with the Ad residue and the electrostatic interaction of ammonium and carboxylate residues. Assemblies of βCD(26.7)-SAP and unmodified SAP microparticles were formed at the concentrations of AdNH₃Cl ([AdNH₃Cl]₀) higher than a certain level (ca. 0.05 mM). The aspect ratio (a/b) of assemblies showed a maximum at [AdNH₃Cl]₀ ~ 0.10 mM, indicating that the chemical stimulus, i.e., addition of AdNH₃Cl, controls the shape of assemblies formed from βCD(26.7)-SAP and unmodified SAP microparticles. This study suggests that other stimuli, e.g., heat, pH, light, redox, and force, can be utilized to control the shape of macroscopic assemblies based on supramolecular interactions.

An efficient synthesis of cyclic polymers (CPs) is in high demand due to their unique properties. However, polymer cyclization generally occurs at low concentrations (0.1 g/L), and the synthesis of CPs at high concentrations remains a challenge. Herein an efficient cyclization of poly(ethylene glycol) (M-n = 2000 g/mol, 4000 g/mol) (PEG-2k, PEG-4k) in high concentration (80 g/L) is realized by the assistance of pseudopolyrotaxane (pPRx). Water-soluble pPRx with a U-like-shape inclusion motif is prepared by mixing the 2-hydroxypropyl-gamma-cyclodextrin (HP gamma CD) and PEG with (E)-3,4,5-trimethoxycinnamate (TCA-PEG-2k, TCA-PEG-4k). Subsequent irradiation of the pPRx solution (10-80 g/L) by UV light gives cyclic polymers through the intramolecular [2 + 2] photocycloaddition of the cinnamoyl moieties. The photoreaction of TCA-PEG-2k in the pPRx system gives cyclic monomers (C-1mer) as major products with a yield of 66% at 80 g/L. Additionally, the cyclization of TCA-PEG-4k also gives C-1mer as major products with a yield of 45% at a concentration of 80 g/L.

Practical applications like very thin stress-strain sensors require high strength, stretchability, and conductivity, simultaneously. One of the approaches is improving the toughness of the stress-strain sensing materials. Polymeric materials with movable cross-links in which the polymer chain penetrates the cavity of cyclodextrin (CD) demonstrate enhanced strength and stretchability, simultaneously. We designed two approaches that utilize elastomer nanocomposites with movable cross-links and carbon filler (ketjenblack, KB). One approach is mixing SC (a single movable cross-network material), a linear polymer (poly(ethyl acrylate), PEA), and KB to obtain their composite. The electrical resistance increases proportionally with tensile strain, leading to the application of this composite as a stress-strain sensor. The responses of this material are stable for over 100 loading and unloading cycles. The other approach is a composite made with KB and a movable cross-network elastomer for knitting dissimilar polymers (KP), where movable cross-links connect the CD-modified polystyrene (PSCD) and PEA. The obtained composite acts as a highly sensitive stress-strain sensor that exhibits an exponential increase in resistance with increasing tensile strain due to the polymer dethreading from the CD rings. The designed preparations of highly repeatable or highly responsive stress-strain sensors with good mechanical properties can help broaden their application in electrical devices.

We report unique conductive leaf-inspired (in particular, stomata-inspired) supramolecular gas sensors in which acetylated cyclodextrin derivatives rule the electric output. The gas sensors consist of polymers bearing acetylated cyclodextrin, adamantane, and carbon black. Host-guest complexes between acetylated cyclodextrin and adamantane corresponding to the closed stomata realize a flexible polymeric matrix. Effective recombination of the cross-links contributes to the robustness. As gas sensors, the supramolecular materials detect ammonia as well as various other gases at 1 ppm in 10 min. The free acetylated cyclodextrin corresponding to open stomata recognized the guest gases to alter the electric resistivity. Interestingly, the conductive device failed to detect ammonia gases at all without acetylated cyclodextrin. The molecular recognition was studied by molecular dynamics simulations. The gas molecules existed stably in the cavity of free acetylated cyclodextrin. These findings show the potential for developing wearable gas sensors.

The physical properties of cyclic polymers differ from those of linear polymers. To prepare cyclic polymers, the ends of a linear polymer must intramolecularly react. Cyclic polymers are typically synthesized at low concentration because intermolecular linkage is preferential to intramolecular linkage at high concentrations. Therefore, preparation of cyclic polymers at high concentrations is difficult. This study demonstrates a new approach to obtain cyclic polymers from highly concentrated polymer solutions using pseudo-polyrotaxanes (pPRxs). Cyclic polymers are prepared by penetrating a linear polymer into the cavity of a cyclic host to form pPRx and then the ends of the linear polymer are subsequently connected. Our approach allows the synthesis of cyclic polymers at concentration that do not allow conventional cyclic polymer synthesis. Specifically, although without pPRx, almost no cyclic polymers were obtained (<1%), with pPRx, the cyclic polymer was obtained as the main product (proportion of cyclic PEG 14%).

In this paper, the relationship between the structure that serves as metamaterial and the structure in which the band gap occurs is investigated. The results can be summarised as follows: 1)When structure is added to the main system, the conditions for metamaterialisation are part of the conditions for band gap generation. 2)The band gaps that occur when the main system itself undergoes periodic changes cannot be described as metamaterials. 3)The application of the band gap generation mechanism showed the possibility of creating propagation structures that reduce vibrations in the desired frequency range by coupling systems that do not have natural frequencies in the desired frequency range with coupling elements until the required magnitude is reached.

The proper combination of slow and fast relaxation components is important in the design of supramolecular hydrogels on toughness. Two kinetically distinct reversible cross-links via host–guest complexations realizes wide relaxation modes.

Steric bulkiness and water content plays an important role in mechanical properties of supramolecular hydrogels consisting of host-guest complexation as cross-links. With low and high water contents, the network mobility and the kinetics of the cross-links become dominant to the mechanical properties, respectively.