Hiroharu Ajiro

Layer‐by‐Layer Assembly Through Weak Interactions and Their Biomedical Applications

The surface design and control of substrates with nanometer- or micrometer-sized polymer films are of considerable interest for both fundamental and applied studies in the biomedical field because of the required surface properties. The layer-by-layer (LbL) technique was discovered in 1991 by Decher and co-workers for the fabrication of polymer multilayers constructed mainly through electrostatic interaction. The scope and applicability of this LbL assembly has been extended by introducing molecularly regular conformations of polymers or proteins by employing, for the first time, weak interactions such as van der Waals interactions and biological recognition. Since these weak interactions are the sum of the attractive or repulsive forces between parts of the same molecule, they allow macromolecules to be easily arranged into the most stable conformation in a LbL film. By applying this characteristic feature, the template polymerization of stereoregular polymers, stereoregular control of surface biological properties, drastic morphological control of biodegradable nano materials, and the development of three-dimensional cellular multilayers as a tissue model were successfully achieved. It is expected that LbL assembly using weak interactions will promote further interest into fundamental and applied studies on the design of surface chemistry in the biomedical field.

Tunable drug-loading capability of chitosan hydrogels with varied network architectures

Advanced bioactive systems with defined macroscopic properties and spatio-temporal sequestration of extracellular biomacromolecules are highly desirable for next generation therapeutics. Here, chitosan (CT) hydrogels were prepared with neutral or negatively charged cross-linkers in order to promote selective electrostatic complexation with charged drugs. CT was functionalized with varied dicarboxylic acids, such as tartaric acid, poly(ethylene glycol) bis(carboxymethyl) ether, 1,4-phenylenediacetic acid and 5-sulfoisophthalic acid monosodium salt (PhS), whereby PhS was hypothesized to act as a simple mimetic of heparin. Attenuated total reflectance Fourier transform infrared spectroscopy showed the presence of CO amide I, N-H amide II and CO ester bands, providing evidence of covalent network formation. The cross-linker content was reversely quantified by proton nuclear magnetic resonance on partially degraded network oligomers, so that 18 mol.% PhS was exemplarily determined. Swellability (SR: 299 ± 65-1054 ± 121 wt.%), compressibility (E: 2.1 ± 0.9-9.2 ± 2.3 kPa), material morphology and drug-loading capability were successfully adjusted based on the selected network architecture. Here, hydrogel incubation with model drugs of varied electrostatic charge, i.e. allura red (AR, doubly negatively charged), methyl orange (MO, negatively charged) or methylene blue (MB, positively charged), resulted in direct hydrogel-dye electrostatic complexation. Importantly, the cationic compound, MB, showed different incorporation behaviours, depending on the electrostatic character of the selected cross-linker. In light of this tunable drug-loading capability, these CT hydrogels would be highly attractive as drug reservoirs towards e.g. the fabrication of tissue models in vitro.

A stereocomplex of poly(lactide)s with chain end modification: simultaneous resistances to melting and thermal decomposition

The simultaneous improvement of the melting temperature (T(m) = 224 °C) and the decomposition temperature (T(10) = 359 °C) of poly(lactide)s was achieved by the stereocomplex formation of poly(l-lactide) and poly(d-lactide) with bio-based aromatic groups at both initiating and terminating chain ends.

Anionic polymerization of o‐substituted styrene derivatives: Control of reactivity and stereochemistry by aminomethyl group

Ortho-substituted styrenes, such as 2-(N,N-dimethylaminomethyl)styrene (1), 2-(1-pyrrolidinylmethyl)styrene (2), and 2-[(S)-2-(1-pyrrolidinylmethyl)-1-pyrrolidinylmethyl]styrene (3), were synthesized, and the effects of the ortho-substituents on the polymerizability and stereoregularity of the obtained polymers using the anionic method were examined. The bulkiness and coordination of the ortho-substituted amino groups to the counter cation significantly affected the polymerizability and stereochemistry of the obtained polymers. The anionic and radical polymerizations of 2 with a less hindered ortho-substituent afforded polymers in good yields, whereas those of 1 and 3 resulted in lower yields. The anionic polymerization of 3 bearing an optically active diamine derivative at the ortho-position with n-butyllithium in toluene at 0 °C gave a polymer with a high stereoregularity and stable regular conformation based on the stereoregular backbone structure.

Solvent effects on isotactic poly(methyl methacrylate) crystallization and syndiotactic poly(methacrylic acid) incorporation in porous thin films prepared by stepwise stereocomplex assembly.

The solvent effects on the crystallization of porous isotactic (it) poly(methyl methacrylate) (PMMA) thin films as well as the incorporation behavior of syndiotactic (st) poly(methacrylic acid) (PMAA) into the porous films were investigated. The porous it-PMMA thin films were prepared by the extraction of st-PMAA from a stepwise layer-by-layer (LbL) assembly composed of it-PMMA and st-PMAA. The X-ray diffraction pattern of the it-PMMA thin films after immersion in acetonitrile/water (4/6, v/v) showed two characteristic peaks of a crystalline it-PMMA double-stranded helix (2theta = 9 degrees and 14 degrees , d = 0.96 and 0.62 nm). This suggested that acetonitrile promoted the crystallization of the thin films, because water is a nonsolvent for PMMA. The surface structural change of the it-PMMA films was also analyzed by atomic force microscopy. The it-PMMA conformation was maintained after crystallization as observed by infrared spectroscopy. The incorporation percentages of st-PMAA into the porous it-PMMA thin films under various solvent conditions were estimated using a quartz crystal microbalance. The incorporation of st-PMAA decreased as the it-PMMA films crystallized or with growing thickness of the porous it-PMMA films. This suggested that the polymer-polymer interactions and the entanglement of the it-PMMA chains played an important role. The incorporation was promoted as the acetonitrile content in the st-PMAA solution increased, indicating that it was necessary for st-PMAA incorporation to solvate it-PMMA.

Polylactide Block Copolymers using Trimethylene Carbonate with Methoxyethoxy Side Groups for Dual Modification of Hydrophilicity and Biodegradability

Novel block copolymers using the monomers 5-(2-methoxyethyoxymethyl)-5-methyl-[1,3]-dioxa-2-one (TMCM-MOE1OM) as a hydrophilic segment and lactides as a hydrophobic segment were designed in order to prepare controllable degradation polymers by dynamic polymer rearrangement based on the hydrophilicity. When the copolymer film contacted water, the hydrophobic polylactide (PLA) segments tend to be buried under the TMCM-MOE1OM segments due to the hydrophilicity of the methoxyethoxy groups. The copolymers were hardly degraded by both proteinase K and lipase, while both of their homopolymers, poly(trimethylene carbonate) and PLA, were degraded, which suggests that the rearrangement of the TMCM-MOE1OM segments at the outermost surface significantly improved the degradation ratio.

Cell Adhesion and Proliferation on Poly(N-vinylacetamide) Hydrogels and Double Network Approaches for Changing Cellular Affinities

Poly(N-vinylacetamide) hydrogels (PNVA gels) were synthesized to investigate their basic characteristics for biomedical applications such as water contact angles, protein uptake, and mouse fibroblasts (L-929) cell adhesion. Because PNVA gels show hydrophilic features, double network (DN) hydrogels were prepared by the secondary polymerization of N-vinylacetamide (NVA) or acrylamide (AAm) in PNVA gels (NVA/NVA DN gels and NVA/AAm DN gels, respectively), in order to vary PNVA gel features for biocompatibility. Contact angles for both DN gels decreased to around 20 degrees, whereas both PNVA and PAAm gels were over 30 degrees. On the other hand, more protein tended to adsorb to DN gels than single network hydrogels. Compared to PNVA gel, cell adhesion and proliferation on NVA/NVA DN gel were improved with less swelling ratio and much protein uptake, while no significant difference was observed on NVA/AAm DN gel, probably due to more hydrophilic character, supported by lowest water contact angle. These complicated structure change in DN gels would provide a new methodology for tuning the biocompatibility of hydrogels and for controlling surface hydrophilic characteristics and network structures.

Stereocomplex Film Using Triblock Copolymers of Polylactide and Poly(ethylene glycol) Retain Paxlitaxel on Substrates by an Aqueous Inkjet System

The stereocomplex formation of poly(L,L-lactide) (PLLA) and poly(D,D-lactide) (PDLA) using an inkjet system was expanded to the amphiphilic copolymers, using poly(ethylene glycol) (PEG) as a hydrophilic polymer. The diblock copolymers, which are composed of PEG and PLLA (MPEG-co-PLLA) and PEG and PDLA (MPEG-co-PDLA), were employed for thin-film preparation using an aqueous inkjet system. The solvent and temperature conditions were optimized for the stereocomplex formation between MPEG-co-PLLA and MPEG-co- PDLA. As a result, the stereocomplex was adequately formed in acetonitrile/water (1:1, v/v) at 40 °C. The aqueous conditions improved the stereocomplex film preparation, which have suffered from clogging when using the organic solvents in previous work. The triblock copolymers, PLLA-co-PEG-co-PLLA and PDLA-co-PEG-co-PDLA, were employed for square patterning with the inkjet system, which produced thin films. The amphiphilic polymer film was able to retain hydrophobic compounds inside. The present result contributed to the rapid film preparation by inkjet, retaining drugs with difficult solubility in water, such as paclitaxel within the films.

Oil gels with a chemically cross-linked copolymer of a trimethylene carbonate derivative and L-lactide: preparation and stereocomplex formation within gels

Oil gels made of low-toxic components were prepared using chemically cross-linked copolymer, which was composed of a poly(trimethylene carbonate) derivative and poly(L-lactide). The poly(L-lactide) moiety in the gels could form stereocomplexes with poly(D-lactide).

Hydrogen-Bonded Multilayer Films Based on Poly(N-vinylamide) Derivatives and Tannic Acid

Layer-by-layer (LbL) assembly based on hydrogen-bonding interactions is generating great interest for biomedical applications because it is composed of neutral polymers, while LbL assembly based on electrostatic interaction requires polycations which may induce toxicity issues. As a neutral polymer, poly(N-vinylamide), which has low toxicity compared to poly(acrylamide), has the potential to fabricate LbL thin films via hydrogen-bonding interactions. Herein we report interpolymer complexes of poly(N-vinylamide)s and natural polyphenol tannic acid to form the multilayered thin film. Poly(N-vinylformamide) and poly(N-vinylacetamide), which are water-soluble and insoluble in acetonitrile, could not form complexes with TA in water. On the other hand, N-alkylated poly(N-vinylamide) such as poly(N-ethyl-N-vinylformamide) and poly(N-methyl-N-vinylacetamide) was soluble in acetonitrile and allowed the LbL assembly to proceed with TA. Furthermore, the QCM frequency shift with films composed of poly(N-ethyl-N-vinylformamide) and TA were stable in water, while those of poly(N-methyl-N-vinylacetamide) and TA were instable in water, possibly because formamide has lower steric hindrance compared to acetamide to allow stronger hydrogen-bonding interactions to take place. Thus, LbL assembly reactions with alkylated poly(N-vinylamide)s and TA were investigated and revealed that poly(N-ethyl-N-formamide) and TA, which are water-soluble, effectively interacted with one another to generate water-stable hydrogen-bonded multilayered films.