Tsukuru Masuda

Self-Oscillating Polymer Brushes†

An autonomous functional surface has been designed by using self-oscillating polymers that convert the chemical energy of the Belousov-Zhabotinsky reaction into conformational changes of the polymer chains (see picture: red: hydrophobic/collapsed, green: hydrophilic/extended). Self-oscillating polymer brushes were grafted onto the inner surface of a glass capillary, and autonomous propagation of a chemical wave was observed.

Control of swelling–deswelling behavior of a self-oscillating gel by designing the chemical structure

We have developed a novel “self-oscillating” gel that exhibits an autonomous mechanical oscillation without any external stimuli. Here the ternary self-oscillating polymer composed of N-isopropylacrylamide and N-(3-aminopropyl)methacrylamide and Ru(bpy)3 was newly synthesized by atom transfer radical polymerization (ATRP) and the gel was prepared. For the self-oscillating polymers and gels with various compositions, their phase transition and self-oscillating behaviors were investigated considering the potential for biomedical application. It was demonstrated that the swelling–deswelling behavior of the self-oscillating gels can be controlled by changing the composition ratio of the free amino group present in the polymer and the conjugated Ru(bpy)3 moieties. Therefore the hydrophilic/hydrophobic balance is controllable and adjustable by this composition ratio.

Artificial cilia as autonomous nanoactuators: Design of a gradient self-oscillating polymer brush with controlled unidirectional motion

A gradient self-oscillating polymer brush has been designed to achieve controlled unidirectional motion. A gradient self-oscillating polymer brush surface with ordered, autonomous, and unidirectional ciliary motion has been designed. The self-oscillating polymer is a random copolymer composed of N-isopropylacrylamide and ruthenium tris(2,2′-bipyridine) [Ru(bpy)3], which acts as a catalyst for an oscillating chemical reaction, the Belousov-Zhabotinsky reaction. The target polymer brush surface was designed to have a thickness gradient by using sacrificial-anode atom transfer radical polymerization. The gradient structure of the polymer brush was confirmed by x-ray photoelectron spectroscopy, atomic force microscopy, and ultraviolet-visible spectroscopy. These analyses revealed that the thickness of the polymer brush was in the range of several tens of nanometers, and the amount of Ru(bpy)3 increased as the thickness increased. The gradient polymer brush induced a unidirectional propagation of the chemical wave from the region with small Ru(bpy)3 amounts to the region with large Ru(bpy)3 amounts. This spatiotemporal control of the ciliary motion would be useful in potential applications to functional surface such as autonomous mass transport systems.

Fabrication of Micropatterned Self-Oscillating Polymer Brush for Direction Control of Chemical Waves

The propagation control of chemical waves via a pentagonal patterned structure in a self-oscillating polymer brush composed of N-isopropylacrylamide and a metal catalyst for the Belousov-Zhabotinsky (BZ) reaction is reported. The patterned self-oscillating polymer brush is prepared by combining surface-initiated atom transfer radical polymerization and maskless photolithography. Surface modification is confirmed by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, 3D measuring laser microscopy, and fluorescence microscopy. The polymer brush patterns are fabricated with gaps between the pentagonal regions, and investigations on the effect of the gap distance on the BZ reaction reveal that at the appropriate distance, chemical waves propagate across the array from the plane to the corner between the patterns. Unidirectional control is achieved not only in the 1D array, but also in a 2D curved array. This patterned self-oscillating polymer brush is a novel and advantageous approach for creating an autonomous dynamic soft interface.

Design of a Tunable Self‐Oscillating Polymer with Ureido and Ru(bpy)3 Moieties

An upper critical solution temperature (UCST)-type self-oscillating polymer was designed that exhibited rhythmic soluble-insoluble changes induced by the Belousov-Zhabotinsky (BZ) reaction. The target polymers were prepared by conjugating Ru(bpy)3 , a catalyst for the BZ reaction, to ureido-containing poly(allylamine-co-allylurea) (PAU) copolymers. The Ru(bpy)3 -conjugated PAUs exhibited a UCST-type phase-transition behavior, and the solubility of the polymer changed in response to the alternation in the valency of Ru(bpy)3 . The ureido content influences the temperature range of self-oscillation, and the oscillation occurred at higher temperatures than conventional LCST-type self-oscillating polymers. Furthermore, the self-oscillating behavior of the Ru-PAU could be regulated by addition of urea, which is a unique tuning strategy. We envision that novel self-oscillating polymers with widely tunable soluble-insoluble behaviors can be rationally designed based these UCST-type polymers.

Thermoresponsive surface-grafted gels: Controlling the “bulk” volume change properties by “surface” localized polymer grafting with various densities

We prepared poly(N-isopropylacrylamide-r-N-3-(aminopropyl)methacrylamide) (poly(NIPAAm-r-NAPMAm)) gels with poly NIPAAm (PNIPAAm) grafted only in the surface region (so-called thermoresponsive surface-grafted gels) with various graft densities and investigated the effect of the graft density on the bulk volume change properties, shrinking and swelling, in response to temperature changes. Initiators for atom-transfer radical polymerization (ATRP) and structurally analogous compounds were introduced at certain ratios onto the surface regions of the gels, and a subsequent activator regeneration by electron transfer ATRP of NIPAAm was conducted in aqueous media. The graft densities and molecular weights of the grafted polymers were evaluated from the increment in the dry mass of the gels and the amount of introduced ATRP initiators, which was measured by elemental analyses. Three-dimensional measuring laser microscopy revealed that the prepared gels had graft-density-dependent fine wrinkle structures on their surfaces. The surface-grafted gels induced the formation of skin layers during the shrinking process in response to a temperature increase, and their permeability strongly depended on the graft density. The graft density also controlled the kinetics of the swelling behavior in response to a temperature decrease. These physical properties were discussed on the basis of Youngs modulus of the surface determined by an atomic force microscopy force curve measurement and the homogeneity of the surface polymer network observed by cryo-scanning electron microscopy. This makes it possible to arbitrarily control the characteristics of gels as open or semiclosed systems, which was uniquely determined by the designs of the surface gel networks.

Protic Ionic Liquids for the Belousov–Zhabotinsky Reaction: Aspects of the BZ Reaction in Protic Ionic Liquids and Its Use for the Autonomous Coil–Globule Oscillation of a Linear Polymer

Herein, we describe the physicochemical aspects of the Belousov-Zhabotinsky (BZ) reaction in hydrated protic ionic liquids (PILs) combined with different cations with a hydrogen sulfate ([HSO4-]) anion. PILs were prepared from the neutralization reactions of sulfuric acid with 13 different aliphatic amines. The amine structure was selected to investigate the effect of the number of active protons, alkyl chain length (hydrophilicity), and cationic linearity on the mechanism of the BZ reaction. The pKa values of PILs were significantly higher (pKa = 1.0-2.5) than those of inorganic acids (H2SO4 = -3.0; HNO3 = -1.4), the conventional proton source in a BZ reaction. A periodic redox oscillation was observed in Ru(bpy)3 when appropriate amounts of BZ reaction substrates (NaBrO3 oxidant; malonic acid reductant) were added to the hydrated PILs. A long-lasting BZ oscillation was realized when hydrophilic cations (ammonium, ethylammonium, and dimethylethylammonium) were employed. Interestingly, a large ΔA5000 (oscillation amplitude of absorbance for the scale of the oscillation stability observed after 5000 s from the initiation of the BZ reaction) was achieved for PILs possessing less than four carbon atoms in their cationic structure. The apparent BZ oscillation activation energy (Ea) in the hydrated PILs was estimated to be ∼40 kJ mol-1, which is 30 kJ mol-1 less than that observed in a conventional system. The catalytic reaction in the BZ reaction subprocess suppresses the total activation energy of the reaction. To realize long-lasting self-oscillating polymeric materials acting under milder condition, we demonstrated an autonomous coil-globule polymer chain transition (BZ-driven) that directly converts chemical energy to mechanical motion in hydrated PILs without freely diffusing Ru(bpy)3 metal catalyst. Ethylammonium hydrogen sulfate ([ea-H+][HSO4-]) is selected as the suitable proton source for the BZ reaction. A well-defined self-oscillating polymer that incorporated Ru(bpy)3 metal catalyst into the polymer backbone accompanied by good compatibility in hydrated [ea-H+][HSO4-] is successfully prepared by ATRP, followed by postmodification of the metal catalyst. The rhythmic solubility changes of the polymer under milder conditions, realized by combination with PILs, will expand the potential applications of PILs to novel functional materials.

A Thermoresponsive Cationic Comb-Type Copolymer Enhances Membrane Disruption Activity of an Amphiphilic Peptide

Membrane active peptides (MAPs) have potential applications in drug delivery systems and as antimicrobials. We previously showed that a cationic comb-type copolymer, poly(allylamine)- graft-dextran (PAA- g-Dex), forms a soluble inter-polyelectrolyte complex with an anionic MAP, the E5 peptide, resulting in significant enhancement of the membrane disruption activity of E5. In this study, we designed a novel comb-type cationic copolymer composed of a PAA main chain and thermoresponsive poly( N-isopropylacrylamide) graft chains (PAA- g-PNIPAAm). We hypothesized that the thermoresponsive hydrophilic/hydrophobic transition of the grafted polymer would regulate the membrane disruption activity of E5 peptide. Both the binding affinity of the complex and the membrane disruption activity of E5/PAA- g-PNIPAAm were found to be enhanced above the phase transition temperature of the grafted chain. Our analysis suggests that the hydrophilic/hydrophobic environment around the cationic polymer chain plays important roles in the enhancement of the activity of the anionic peptide.

Allosteric Control of Peroxidase-Mimicking DNAzyme Activity with Cationic Copolymers

Control of protein conformation and function, induced by the binding of an effector, plays significant roles in modulating biochemical reaction. Although the DNAzymes catalytic activity is similar to protein-based enzymes, reports of allosterically controlled DNAzymes are still limited except for aptamer-DNAzymes hybrrids. Here, we report allosteric control of peroxidase-mimicking DNAzyme activity using cationic copolymers. The DNAzyme requires a structured G-quadruplex core and hemin for activity, and the DNAzyme with a parallel G-quadruplex core has higher DNAzyme activity than DNAzymes based on other types of structure. We previously reported that a cationic copolymer composed of a cationic backbone and hydrophilic dextran side chains selectively stabilizes parallel G-quadruplex structures. In this study, we investigated effects of the cationic copolymer on peroxidase-mimicking DNAzyme activity. The cationic copolymer enhanced the DNAzyme activity by more than 30-fold by stabilizing the parallel G-quadruplex structure. Furthermore, reversible allosteric control of DNAzyme activity was achieved by adding cationic and anionic polymers.

Dynamic electrical behaviour of a thermoresponsive polymer in well-defined poly(N-isopropylacrylamide)-grafted semiconductor devices

Herein, a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAAm) was grafted on a Ta2O5 gate surface by surface-initiated atom transfer radical polymerization. We found that the phase transition behaviour from swelling state to deswelling state in response to temperature change was electrically detected in real time by using the PNIPAAm-grafted gate FET.