Aya Mizutani Akimoto

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.

Effect of reaction solvent on the preparation of thermo-responsive stationary phase through a surface initiated atom transfer radical polymerization.

Poly(N-isopropylacrylamide) (PIPAAm) brush grafted silica beads, a thermo-responsive chromatographic stationary phase, were prepared through a surface-initiated atom transfer radical polymerization (ATRP) using 2-propanol, N,N-dimethylformamide (DMF), and water as reaction solvents. The rate of grafting PIPAAm on silica bead surfaces was different and found to be dependent on the reactivity of reaction solvent. Temperature-dependent elution profiles of hydrophobic steroids from the prepared-beads-packed columns were found to be different, although the graft amounts of PIPAAm were similar on silica bead surfaces. Especially, prepared beads using 2-propanol exhibited a higher resolution than those using DMF. Calibration curves using glucose and pullulan suggested that beads prepared using DMF prohibited analytes to diffuse into the pores. On the contrary, beads prepared using 2-propanol allowed analytes to diffuse into the pores. The pore diameter of the prepared beads, measured by N(2) adsorption-desorption measurement, suggested that beads using 2-propanol has relatively larger pore diameter than those using DMF. Thus, the reaction solvent in surfaces-initiated ATRP affected the grafting configuration of PIPAAm on porous silica-bead surfaces, leading to the different separation efficiency of stationary phase for bioactive compounds.

Recent developments in self-oscillating polymeric systems as smart materials: from polymers to bulk hydrogels

As novel functional materials, we developed self-oscillating polymeric materials composed of synthetic polymers coupled with an oscillating chemical reaction, the so-called Belousov–Zhabotinsky (BZ) reaction. These materials are very different from traditional stimuli-responsive polymeric materials in terms of their autonomic, self-regulating, and rhythmically changing functions. In this review, we address recent advances in the field of self-oscillating smart materials from the microscopic scale (linear copolymers and block copolymers) to the macroscopic scale (cross-linked hydrogels). The unique features of self-oscillating materials are analogous to the diverse functions of living organisms. This overview has been prepared to promote the development of innovative self-oscillating materials for novel biomimetic material systems.

Recent Advances in Self-Oscillating Polymer Material Systems.

In 1996, we first reported self-oscillating polymer gels exhibiting autonomous swelling-deswelling oscillations driven by the Belousov-Zhabotinsky reaction. In contrast to conventional stimuli-responsive gels, the self-oscillating gel can autonomously and periodically change its volume in a closed solution without any external stimuli. Since the first report, the novel concept of self-oscillating gels has been expanded into various polymer and gel systems. Herein, we summarize recent advances in self-oscillating polymers and gels.

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.

Photocurable ABA triblock copolymer-based ion gels utilizing photodimerization of coumarin

Herein, we develop a photocurable ABA triblock copolymer-based ion gel, which can be converted from a thermally processable, physically crosslinked ion gel to a thermally and mechanically stable, chemically crosslinked ion gel via photoinduced dimerization. The A block consists of a random copolymer of N-isopropylacrylamide and a coumarin-containing acrylate monomer, while the B block consists of an ionic liquid-philic poly(ethylene oxide). Due to the upper critical solution temperature-type phase behavior of the A block, the ABA triblock copolymer undergoes gel-to-sol transitions in a hydrophobic ionic liquid as the temperature is increased. Furthermore, under ultraviolet (UV) light irradiation, the physical crosslinks formed by association of the A blocks in the gel at low temperatures become chemically crosslinked as a result of photodimerization of the coumarin moieties in the A block; this results in conversion from a thermo-reversible, physically crosslinked ion gel to a thermo-irreversible, chemically crosslinked ion gel. The rheological changes of the ion gel upon UV irradiation have been investigated in detail. In addition, photopatterning of the ion gel has been realized by exploiting the photocurable behavior of the ABA triblock copolymer in the ionic liquid.

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.

Macroscopic Adhesion of Thermoreversible ABC Triblock Copolymer-Based Hydrogels Via Boronic Acid-Sugar Complexation

Two complementary thermoreversible ABC triblock copolymers containing either phenylboronic acids with low pKa values or galactosyl groups in the hydrophilic B blocks are synthesized by sequential reversible addition-fragmentation chain transfer polymerization and subsequent modification of the functional groups. Both ABC triblock copolymers undergo reversible sol-to-gel transitions upon temperature change and form physically cross-linked hydrogels under physiological conditions. Furthermore, the spontaneous adhesion of these thermoreversible hydrogels via the formation of boronic esters between the phenylboronic acid and galactosyl groups under physiological conditions is realized for the first time.