Optically Transparent and Color-Stable Elastomer with Structural Coloration under Elongation.

Optically transparent and colored elastomers with high toughness are expected to play an important role in the construction of advanced medical materials, wearable displays, and soft robots. In this study, we found that composite elastomers consisting of amorphous SiO2 particles homogeneously dispersed in high concentrations within a biocompatible acrylic polymer network exhibit optical transparency and bright structural colors. In the composite elastomers, the system in which the SiO2 particles form a colloidal amorphous array hardly changes its structural color hue despite deformation due to elongation. Furthermore, the composite elastomer of the SiO2 particles with the acrylic polymer network also results in high mechanical toughness. In summary, we have shown that the elastomer that exhibits fade-resistant structural coloration formed from safe materials can combine stable coloration and mechanical strength independent of their shape. This is expected to have new potential in future technologies to support our daily life.

Mechanically Tunable Hydrogels with Self-Healing and Shape Memory Capabilities from Thermo-Responsive Amino Acid-Derived Vinyl Polymers.

In this study, we report the fabrication and characterization of self-healing and shape-memorable hydrogels, the mechanical properties of which can be tuned via post-polymerization crosslinking. These hydrogels were constructed from a thermo-responsive poly(N-acryloyl glycinamide) (NAGAm) copolymer containing N-acryloyl serine methyl ester (NASMe) units (5 mol%) that were readily synthesized via conventional radical copolymerization. This transparent and free-standing hydrogel is produced via multiple hydrogen bonds between PNAGAm chains by simply dissolving the polymer in water at a high temperature (~90 °C) and then cooling it. This hydrogel exhibited moldability and self-healing properties. The post-polymerization crosslinking of the amino acid-derived vinyl copolymer network with glutaraldehyde, which acts as a crosslinker between the hydroxy groups of the NASMe units, tuned mechanical properties such as viscoelasticity and tensile strength. The optimal crosslinker concentration efficiently improved the viscoelasticity. Moreover, these hydrogels exhibited shape fixation (~60%)/memory (~100%) behavior owing to the reversible thermo-responsiveness (upper critical solution temperature-type) of the PNAGAm units. Our multifunctional hydrogel, with moldable, self-healing, mechanical tunability via post-polymerization crosslinking, and shape-memorable properties, has considerable potential for applications in engineering and biomedical materials.

Characterization of Hydration Water Bound to Choline Phosphate-Containing Polymers.

Zwitterionic methacrylate polymers with either choline phosphate (CP) (poly(MCP)) or phosphorylcholine (PC) (poly(MPC)) side groups were analyzed to characterize the bound hydration water molecules as nonfreezing water (NFW), intermediate water (IW), or free water (FW). This characterization was carried out by differential scanning calorimetry (DSC) of polymer/water systems, and the enthalpy changes of cold crystallization and melting were determined. The electron pair orientation of CP is opposite to that of PC, and the former binds the alkyl terminal groups at the phosphate esters. The numbers of NFW and IW molecules per monomer unit of poly(MCP) with an isopropyl terminal group were estimated to be 10.7 and 11.3 mol/mol, respectively, which were slightly greater than those of the poly(MCP) bearing an ethyl terminal group. More NFW and IW molecules hydrated the phosphobetaine polyzwitterions, poly(MCP) and poly(MPC), compared with carboxybetaine and sulfobetaine polymers. Moreover, the hydration states of polyelectrolytes were compared with the zwitterionic polymers. Finally, we discuss the relationship between the amount of hydration water and bio-inert properties.

Star-Shaped Peptide-Polymer Hybrids as Fast pH-Responsive Supramolecular Hydrogels.

Significant challenges have gone into the design of smart hydrogels, with numerous potential applications in the industrial, cosmetic, and biomedical fields. Herein, we report the synthesis of novel 4-arm self-assembling peptide-polyethylene glycol (PEG) hybrid star-shaped polymers and their comprehensive hydrogel properties. ß-sheet-forming oligopeptides with alternating hydrophobic Leu/ionizable Glu repeats and Cys residues were successfully conjugated to 4-arm PEG via a thiol-maleimide click reaction. The hybrid star-shaped polymers demonstrated good cytocompatibility and reversible ß-sheet (lightly acidic pH)-to-random coil (neutral and basic pH) transition in dilute aqueous solutions. At increasing polymer concentrations up to 0.5 wt %, the star-shaped polymers formed transparent hydrogels with shear-thinning and self-healing behaviors via ß-sheet self-assembly, as well as a conformation-dependent gel-sol transition. Interestingly, the star-shaped polymers responded rapidly to pH changes, causing gelation to occur rapidly within a few seconds from the change in pH. Hydrogel characteristics could be modulated by manipulating the length and net charge of the peptide blocks. Furthermore, these star-shaped polymers served as satisfactory network scaffolds that could respond to dynamic environmental changes in the pH-oscillation system, owing to their excellent gelation capability and pH sensitivity. As such, they are highly favorable for diverse applications, such as pH-responsive controlled release.

Infrared Spectra and Hydrogen-Bond Configurations of Water Molecules at the Interface of Water-Insoluble Polymers under Humidified Conditions.

Elucidating the state of interfacial water, especially the hydrogen-bond configurations, is considered to be key for a better understanding of the functions of polymers that are exhibited in the presence of water. Here, an analysis in this direction is conducted for two water-insoluble biocompatible polymers, poly(2-methoxyethyl acrylate) and cyclic(poly(2-methoxyethyl acrylate)), and a non-biocompatible polymer, poly(n-butyl acrylate), by measuring their IR spectra under humidified conditions and by carrying out theoretical calculations on model complex systems. It is found that the OH stretching bands of water are decomposed into four components, and while the higher-frequency components (with peaks at ∼3610 and ∼3540 cm-1) behave in parallel with the C═O and C-O-C stretching and CH deformation bands of the polymers, the lower-frequency components (with peaks at ∼3430 and ∼3260 cm-1) become pronounced to a greater extent with increasing humidity. From the theoretical calculations, it is shown that the OH stretching frequency that is distributed from ∼3650 to ∼3200 cm-1 is correlated to the hydrogen-bond configurations and is mainly controlled by the electric field that is sensed by the vibrating H atom. By combining these observed and calculated results, the configurations of water at the interface of the polymers are discussed.

Observing the repulsion layers on blood-compatible polymer-grafted interfaces by frequency modulation atomic force microscopy.

Blood-compatible materials that do not promote reactions in contact with human blood are required to support emerging medical technologies. The interfaces of poly(2-methoxyethyl acrylate) (PMEA), a blood-compatible polymer, and its analogues were investigated by frequency modulation atomic force microscopy (FM-AFM). The grafted polymers exhibited phase separation into polymer-rich and water-rich domains. Thin repulsive layers of hydrated polymer chains were observed in the water-rich domains of the blood-compatible polymers; on the other hand, such layers were not observed for the non-blood-compatible polymers. We report for the first time that FM-AFM enables characteristic repulsive layers composed of hydrated polymer chains in water-rich domains to be observed, which is a significant design factor for blood-compatible polymers.

Selective Accumulation to Tumor Cells with Coacervate Droplets Formed from a Water-Insoluble Acrylate Polymer.

Selective targeting of specific cells without the use of biological ligands has not been achieved. In the present study, we revealed that the coacervate droplets formed from poly(2-methoxyethyl acrylate) (PMEA) and its derivatives selectively accumulated to tumor cells. PMEA derivatives, which are insoluble acrylate polymers, induced coacervation in water to form polymer-dense droplets via hydrophobic interaction. Interestingly, the accumulation of coacervate droplets to tumor cells was involved in the bound water content of PMEA derivatives. Coacervate droplets with a high bound water content accumulated and internalized up to 36.6-fold higher in HeLa cervical tumor cells than in normal human fibroblasts (NHDF). Moreover, the interactions between coacervate droplets and plasma membrane components such as CD44 played a key role in this accumulation process. Therefore, coacervate droplets formed from PMEA derivatives have great clinical potential in tumor cell detection, development of alternative tumor-targeting ligands, and optimization of drug delivery carriers.

Hydration Mechanism in Blood-Compatible Polymers Undergoing Phase Separation.

Interactions involving intermediate water are crucial for the design of novel blood-compatible materials. Herein, we use a combination of atomic force microscopy, quartz crystal microbalance measurements, and soft X-ray emission spectroscopy to investigate the local hydrogen-bonded configuration of water on blood-compatible poly(2-methoxyethyl acrylate) and non-blood-compatible poly(n-butyl acrylate) grafted on a gold substrate. We find that the initially incorporated water induces polymer-dependent phase separation, facilitating further water uptake. For the blood-compatible polymer, tetrahedrally coordinated water coexists with water adsorbed on C═O groups in low-density regions of the grafted polymer surface, providing a scaffold for the formation of intermediate water. The amount of intermediate water is determined by the type of functional groups, local polymer configuration, and polymer morphology. Thus, blood compatibility is governed by the complex water/polymer interactions.

A ß-hairpin peptide with pH-controlled affinity for tumor cells.

Considering that the pH in the tumor microenvironment is dysregulated, we designed a ß-hairpin peptide (SSRFEWEFESSDPRGDPSSRFEWEFESS). The configuration of the peptide switched from a flexible linear to a rigid loop structure under weakly acidic conditions. The peptide internalized by tumor cells increased significantly under weakly acidic conditions.

Impact of simultaneous hydrolysis of OCP and PLGA on bone induction of a PLGA-OCP composite scaffold in a rat femoral defect.

Effect of the simultaneous hydrolysis of octacalcium phosphate (OCP) and poly (lactic-co-glycolic acid) (PLGA) was investigated on its osteoconductivity. PLGA soaked in phosphate buffered saline with 0%, 20%, and 40% OCP at 37°C for eight weeks indicated that when the OCP dose was increased, 1) the weight loss of PLGA increased, 2) the glass transition temperature of the PLGAs decreased, 3) the saturation degree in the saline moved to nearly saturated condition with respect to hydroxyapatite (HA) but was undersaturated with respect to OCP, and 4) OCP tended to convert to HA by X-ray diffraction and Fourier transform infrared spectroscopy. OCP/PLGA composites of 20% and 40% with more than 92% porosity were produced by combining OCP granules with 1,4-dioxane-solubilizing PLGA followed by lyophilization and then subjected to four- and eight-week in vivo implantation tests in 3 mm diameter rat femora defects. Microfocus X-ray computed tomography, histochemical and histomorphometric analyses showed that while bone formation was very limited with PLGA implantation, the extent of repair tended to increase with increasing OCP content in the PLGA, coupled with PLGA degradation, and bridge the defects with trabecular bone. Tartrate-resistant acid phosphatase-positive osteoclast-like cells were accumulated four weeks after implantation, while osteocalcin-positive osteoblastic cells appeared later at eight weeks, especially in 40% OCP/PLGA. These results suggest that OCP hydrolysis, with phosphate ion release, enhances PLGA hydrolysis, probably through the acid catalysis function of the protons supplied during the hydrolysis of OCP, thereby inducing PLGA biodegradation and new bone formation in the femoral defects. STATEMENT OF SIGNIFICANCE: Octacalcium phosphate (OCP) enhances osteoblasts and osteocytes differentiations during its hydrolysis accompanying inorganic ions exchange in this material. The present study found that the advancement of OCP hydrolysis under physiological conditions had an effect on poly (lactic-co-glycolic acid) (PLGA) degradation through its chemical environmental change around OCP, which was ascertained by the decreases in weight loss and glass transition temperature of PLGA with increasing the dose of OCP co-present. Rat femur-penetrated standardized severe defects were found to repair through bridging the cortical region defect margin. PLGA degradation could be enhanced through an acid catalyst function by protons derived from inorganic phosphate (Pi) ions through OCP hydrolysis under bone forming condition, resulting in showing a prominent bone regenerative capacity in OCP/PLGA composite materials.

Spontaneous Formation of Nanoparticles from Peptide-Vinyl Polymer Diblock Hybrids Prepared by RAFT Polymerization and Their Interactions with Cells.

Novel polymeric nanoparticles (NPs) with uniform sizes were prepared from peptide-vinyl polymer diblock hybrids by the self-organized precipitation method. Hybrid polymers of polystyrene (PSt) and tetrapeptide (cell-binding epitope RGDS, reverse SDGR, cationic KKKK, and anionic DDDD) were successfully synthesized by combining solid-phase peptide synthesis and reversible addition fragmentation chain transfer polymerization methods. Narrowly dispersed hybrid polymers (polydispersity index < 1.25, M n 14 000-17 000) were obtained. Altering the preparation conditions easily tuned the size and size distribution of the NPs. When the ζ-potentials for the NP suspensions were measured at pH 6.0, the obtained values corresponded to the net charge of each peptide segment. More importantly, the NPs could encapsulate fluorescent Nile red (NR) and magnetic iron oxide NP (MNP), which might be suitable for fluorescent imaging and magnet-induced patterning of cells, respectively. The interactions of NPs with cells (NIH/3T3 fibroblast) and the magnetic effects were examined for NR/MNP-loaded PSt-RGDS and -SDGR NPs. Both NPs were readily incorporated into cells, but only NR/MNP-loaded PSt-RGDS NP showed magnetic responsiveness in cell adhesion and cultures.

Photocleavable Peptide-Poly(2-hydroxyethyl methacrylate) Hybrid Graft Copolymer via Postpolymerization Modification by Click Chemistry To Modulate the Cell Affinities of 2D and 3D Materials.

Controlling the surface properties of engineered materials to enhance or reduce their cellular affinities remains a significant challenge in the field of biomaterials. We describe a universal technique for modulating the cytocompatibilities of two-dimensional (2D) and three-dimensional (3D) materials using a novel photocleavable peptide-grafted poly(2-hydroxyethyl methacrylate) (PHEMA) hybrid. The reversible addition-fragmentation chain transfer copolymerization of HEMA and propargyl acrylate was successfully controlled. The resultant alkyne-containing PHEMA was then used to modify the azide-terminated oligopeptides [Arg-Gly-Asp-Ser (RGDS)] with a photolabile 3-amino-3-(2-nitrophenyl)propanoic acid moiety via the copper-catalyzed alkyne-azide click chemistry. This strategy was readily used to decorate the surfaces of both hydrophilic and hydrophobic materials with RGDS peptides due to the high film-forming abilities of the PHEMA unit. The resultant thin film acted as an effective scaffold for improving cell adhesion and growth of NIH/3T3 fibroblasts and MC3T3-E1 osteoblast-like cells in vitro. In addition, UV irradiation of the surface led to the detachment of cells from the material surface accompanied by the photocleavage of RGDS grafts and enabled the 2D-patterning of cells and cell sheet engineering. The applicability of this system to 3D materials was investigated, and the cell adhesion was remarkably enhanced on a 3D-printed poly(lactic acid) object. This facile, biocompatible, and photoprocessable peptide-vinyl polymer hybrid system is valuable for its ability to advance the fields of tissue engineering, cell chips, and regenerative medicine.

A novel thermo-responsive multiblock architecture composed of a sequential peptide and an amino acid-derived vinyl polymer: toward protein-mimicking single-chain folding.

A novel multiblock architecture composed of an alternating ß-sheet forming oligopeptide and a thermo-responsive glycine-derived vinyl polymer was synthesized. The polymer exhibited lower critical solution temperature (LCST) behavior in water, unlike the behavior of a glycine-derived homopolymer, and formed nanoparticles through protein-mimicking folding driven by thermal cycle-induced ß-sheet formation.

Stepwise Thermo-Responsive Amino Acid-Derived Triblock Vinyl Polymers: ATRP Synthesis of Polymers, Aggregation, and Gelation Properties via Flower-Like Micelle Formation.

Novel thermo-responsive ABA-type triblock copolymers (poly(NAAMen-b-NAGMe240-b-NAAMen), n = 18-72) composed of naturally occurring amino acid-based vinyl polymer blocks such as poly(N-acryloyl-l-alanine methyl ester (poly(NAAMe)) as the A segment and poly(N-acryloyl-glycine methylester)(poly(NAGMe)) as the B segment have been synthesized by the atom transfer radical polymerization (ATRP). Their thermal behaviors were analyzed in dilute aqueous solutions by turbidimetry. The turbidity curves provided two-step LCST transitions, and a flower-like micelle formation was confirmed at the temperature region between the first and second LCST transitions by dynamic light scattering, AFM and TEM. At higher copolymer concentrations, hydrogels were obtained at temperatures above the first LCST due to network formation induced with the flower-like micelles as cross-linker. The hydrogels were found to be switched to a sol state when cooled below the first LCST. These hydrogels also exhibited self-healable and injectable capabilities, which were evaluated by rheological measurements.

Thermo-responsive polymer brushes on glass plate prepared from a new class of amino acid-derived vinyl monomers and their applications in cell-sheet engineering.

In this study, we present a novel thermo-responsive polymer platform that is based on the alanine methyl ester-containing homopolymer (PNAAMe) and the copolymer with glycine methyl ester-based vinyl monomer (P(NAAMe-co-NAGMe)) brushes prepared via surface-initiated atom transfer radical polymerization. Water contact angles for these brushes measured at different temperatures reveal that the polymer brushes collapse and dehydrate around 13°C and 25°C (TTs), respectively, upon elevating the temperature. At 37°C, seeded fibroblasts (NIH/3T3) adhere to and spread well onto these brush surfaces although the copolymer brush of P(NAAMe-co-NAGMe) depresses the number of adherent cells less than half of that for the homopolymer of PNAAMe after 24h of cell culture due to increment in hydrophilicity. To prepare the cell-sheet, the cells are seeded on both polymer brushes and cultured at 37°C in the presence of serum. After 4days, the cells proliferated confluently on these brush surfaces. Lowering the temperature to 4°C and 20°C below TT of each brush led to the cell-sheet detachment as a monolayer form from the polymer brushes accompanying with the switching of surface affinity.

Facile Synthesis of Multiblock Copolymers Containing Sequence-Controlled Peptides and Well-Defined Vinyl Polymers by Nitroxide-Mediated Polymerization.

Precisely incorporating a wide range of structural and functional multiblocks along a polymer backbone is a significant challenge in polymer chemistry and offers promising opportunities to design highly ordered materials, including controlled polymer folding. Herein, a facile and versatile strategy for preparing functional multiblock copolymers composed of sequential peptides and well-defined vinyl polymers with a narrow polydispersity is reported. Cyclic oligopeptides have been developed that contain an alkoxyamine bond in the framework. By using this type of cyclic initiator, peptide-containing multiblock copolymers are successfully synthesized by nitroxide-mediated polymerization of styrene. To demonstrate the versatility of this method, radical (co)polymerizations were carried out for different monomers (p-chlorostyrene, 4-vinylpyridine, and styrene/acrylonitrile) and by three different cyclic peptide initiators with specific amino acid sequences. The resultant multiblock copolymer is foldable through intramolecular interactions between peptide blocks. It is believed that this approach will significantly advance the field of controlled polymer synthesis for complex structures and single-chain folding.