Breaking the limitation of traditional acid dissolving methods for chitosan by creating an alkali/urea hydrogen-bonded chitosan complex, a new solvent (4.5 wt % LiOH/7 wt % KOH/8 wt % urea aqueous solution) was used to successfully dissolve chitosan via the freezing−thawing process, for the first time. Subsequently, high strength hydrogels with unique nanofibrous architecture were constructed from the chitosan alkaline solution. The results from 13 C NMR, laser light scattering, atomic force microscopy, transmission electron microscopy, and scanning electron microscopy confirmed that chitosan easily aggregated in the solution and could self-assemble in parallel to form perfect regenerated nanofibers induced by heating. At elevated temperature and concentration, the regenerated chitosan nanofibers could entangle and cross-link with each other through hydrogen bonds to form hydrogels. The novel chitosan hydrogels exhibited homogeneous architecture and high strength as a result of the strong networks woven with the compact nanofibers. The compression fracture stress of the chitosan hydrogels was nearly 100 times that of the chitosan hydrogels prepared by the traditional acid dissolving method, revealing that the nanofibrous network microstructures contributed greatly to the reinforcement of the hydrogels. Furthermore, the chitosan hydrogels exhibited excellent biocompatibility and safety as well as a smart controlled drug release behavior triggered by acid. Therefore, we opened up a completely new avenue to construct high strength chitosan hydrogels for applications in biomedicine. ■ INTRODUCTIONHydrogels are composed of three-dimensional polymer networks that contain abundant water in the porous structures, and their soft and rubbery consistency and low interfacial tension with water or biological fluids are common to human tissues. 1−3 Recently, the potential of polysaccharide-based hydrogels as biomaterials has been widely recognized due to their excellent biocompatibility, biological activity, safety, and biodegradability. 4−6 Chitosan, the unique alkaline polysaccharide derived from the deacetylation of chitin, is readily soluble in dilute acidic solutions, and chitosan hydrogels can be regenerated by using alkaline coagulating bath. Thanks to their intrinsic biocompatibility, nontoxicity, biodegradability, strong affinity, antimicrobial activity, and low immunogenicity, chitosan hydrogels are considered to have potential applications in a wide variety of fields such as water treatment, food industry, catalysis, agriculture, and biomedicine. 7−12 However, the poor mechanical properties, the "Achilles' heel" of chitosan hydrogels, are serious impediments for their practical applications. 13,14 To date, several methods such as chemical cross-linking, nanofillers reinforcement (e.g., nanoclay, silica nanoparticles, carbon nanotube, and graphene), and blending with other polymers have been used to enhance their mechanical strength. However, these techniques resulted in only a moderate enhancement and sometimes even...

The design, synthesis, and self-assembly of a series of precisely defined, nonspherical, polyhedral oligomeric silsesquioxane (POSS)-based molecular Janus particles are reported. The synthesis aims to fulfill the "click" philosophy by using thiol-ene chemistry to efficiently install versatile functionalities on one of the POSS cages. In such a way, both the geometrical and chemical symmetries were broken to create the Janus feature. These particles self-organize into hierarchically ordered supramolecular structures in the bulk. For example, the Janus particle with isobutyl groups on one POSS and carboxylic groups on the other self-assembles into a bilayered structure with head-to-head, tail-to-tail arrangements of each particle, which further organize into a three-dimensional orthorhombic lattice. While the ordered structure in the layers was lost upon heating via a first-order transition, the bilayered structure persisted throughout. This study provides a model system of well-defined molecular Janus particles for the general understanding of their self-assembly and hierarchical structure formation in the condensed state.

An approach to multicomponent coordination-driven self-assembly of the first terpyridine-based, shape-persistent, giant two-dimensional D(6h) supramacromolecular spoked wheel is reported. Mixing core T6, rim T3, and Zn(II) or Cd(II) ions in a stoichiometric ratio (1:6:12) permitted the selective generation of a highly symmetric spoked wheel in 94% isolated yield via geometric and thermodynamic control. The products were characterized by a combination of traveling-wave ion mobility mass spectrometry and NMR techniques together with TEM imaging, which agreed with computational simulations.

High power‐conversion efficiencies (PCEs) from polymer solar cells with a low‐temperature‐annealed sol–gel‐derived molybdenum oxide (S‐MoOx) thin film as a buffer layer demonstrate that the S‐MoOx (i.e., solution‐processed) thin film can serve as the hole extraction layer in polymer solar cells in the quest for high performance.

High strength chitin/poly(vinyl alcohol) (PVA) composite hydrogels (RCP) were constructed by adding PVA into chitin dissolved in a NaOH/urea aqueous solution, and then by cross-linking with epichlorohydrin (ECH) and freezing-thawing process. The RCP hydrogels were characterized by field emission scanning electron microscopy, FTIR, differential scanning calorimetry, solid-state (13)C NMR, wide-angle X-ray diffraction, and compressive test. The results revealed that the repeated freezing/thawing cycles induced the bicrosslinked networks consisted of chitin and PVA crystals in the composite gels. Interestingly, a jellyfish gel-like structure occurred in the RCP75 gel with 25 wt % PVA content in which the amorphous and crystalline PVA were immobilized tightly in the chitin matrix through hydrogen bonding interaction. The freezing/thawing cycles played an important role in the formation of the layered porous PVA networks and the tight combining of PVA with the pore wall of chitin. The mechanical properties of RCP75 were much higher than the other RCP gels, and the compressive strength was 20× higher than that of pure chitin gels, as a result of broadly dispersing stress caused by the orderly multilayered networks. Furthermore, the cell culture tests indicated that the chitin/PVA composite hydrogels exhibited excellent biocompatibility and safety, showing potential applications in the field of tissue engineering.