Disaggregation and fibrillation during sol-gel transition of alginate hydrogels.

The rheological and morphological characteristics of Ca-crosslinked alginate hydrogels with two different M/G ratios, α-L-guluronate (G)-rich and ß-D-mannuronate (M)-rich, each with one alginic acid concentration, were investigated. It was found that the stiffness and elasticity of alginate hydrogels are derived from the thickness and density of the fibril network structures. In aqueous alginate solution, ball-like aggregates of alginates are present. Time-resolved small-angle X-ray scattering and time-domain nuclear magnetic resonance measurements suggest that the disaggregation of alginate aggregates and loose fibrillation occur in the early stage of the sol-gel transition. After these induction stage, direct gelation is finally caused by the formation of the egg-box junction. G-rich alginate hydrogel has a higher stiffness and a thicker and denser fibril network structure than M-rich alginate hydrogel. The former also exhibits faster and more significant changes in physical properties during the sol-gel transition.

Identification of the growth cone as a probe and driver of neuronal migration in the injured brain.

Axonal growth cones mediate axonal guidance and growth regulation. We show that migrating neurons in mice possess a growth cone at the tip of their leading process, similar to that of axons, in terms of the cytoskeletal dynamics and functional responsivity through protein tyrosine phosphatase receptor type sigma (PTPσ). Migrating-neuron growth cones respond to chondroitin sulfate (CS) through PTPσ and collapse, which leads to inhibition of neuronal migration. In the presence of CS, the growth cones can revert to their extended morphology when their leading filopodia interact with heparan sulfate (HS), thus re-enabling neuronal migration. Implantation of an HS-containing biomaterial in the CS-rich injured cortex promotes the extension of the growth cone and improve the migration and regeneration of neurons, thereby enabling functional recovery. Thus, the growth cone of migrating neurons is responsive to extracellular environments and acts as a primary regulator of neuronal migration.

Low-Temperature Behaviors, Cold Crystallization, and Glass Transition in Poly(vinylpyrrolidone) Aqueous Solution.

Intermediate water is a kind of water around the biocompatible polymer, such as poly(vinylpyrrolidone) (PVP), that exhibits the phenomenon of cold crystallization. We investigate the low-temperature behavior of PVP aqueous solution using small- and wide-angle X-ray scattering and total neutron scattering measurements. The ice formation speed of the intermediate water is extremely reduced by confinement in the PVP moiety during the cooling process. However, around the glass transition temperature, the water-rich phase expands and orders the hydrogen-bond network, behaving as ice nuclei. During the heating process, cubic ice is formed first and then fills the water-rich region. After saturation of the cubic ice formation, the ice transforms from the cubic to the hexagonal ice form.

Packing structure of chains and rings in an expanded liquid Se80Te20 mixture near the semiconductor to metal transition.

X-ray scattering measurements of an expanded liquid Se(80)Te(20) mixture in the temperature range between 300 and 1000 °C and at pressures of 10, 200, and 1600 bar have been carried out. The reverse Monte Carlo simulation and Voronoi-Delaunay void analyses have been applied to clarify the relationship between the voids and chain geometries in the intermediate scale near the semiconductor to metal (SC-M) transition at 800 °C under 200 bar accompanied by a shrinkage of molar volume. The structure of the liquid mixture can be envisaged in terms of a packing of covalently bonded chains and interstitial voids. The thermal expansion leads to a decrease in the number of chains around a chain and is compensated for by empty spaces (voids). The packing density of helical chains decreases and so voids supported by chains increase with rising temperature. At high temperature the shortening of chains and frequent transfer of lone pair electrons on Se (or Te) atoms cause modification of the helical chain to ring and zigzag chain conformations. The stacking of zigzag chains, joined to layers above 800 °C, leads to the formation of metallic domains separated by voids. The shrinkage of molar volume near the SC-M transition arises through the progressive filling of the voids around chains with (Se, Te)(5, 6, 7, 8) rings.

Quasielastic neutron scattering investigation of motion of water molecules in n-propyl alcohol-water mixture.

The dynamics of water molecules in the n-propyl alcohol-water mixtures is investigated by using quasielastic neutron scattering measurements. The dynamic structure factor S(Q,E) obtained from incoherent scattering of hydrogen atoms of water is fitted with jump diffusion and relaxing cage models. The diffusion constant obtained from the relaxing cage model, which gives better fitting with S(Q,E), shows better agreement to the experimental value than that of jump diffusion model. The dependence of translational relaxation time tau(T)(Q) and stretched exponent beta(T)(Q) on the fraction of hydrophobic hydrating water molecules in the solution is discussed.