Study of chondrogenesis of umbilical cord mesenchymal stem cells in curdlan- poly(vinyl alcohol) composite hydrogels and its mechanical properties of freezing-thawing treatments.

The curdlan gel is a natural material produced by bacteria. It utilizes chemical cross-linking reactions to form a 3D porous composite hydrogel, increasing its porosity and water content, and improving its mechanical properties. It can be used in tissue repair and regenerative medicine. Curdlan-Poly(vinyl alcohol) (PVA) composite hydrogel can rapidly swell within 1 min due to its porous structure. Compression tests confirmed that it still maintains its original mechanical strength, even after five repeated freeze-thaw (FT) processes, making it suitable for long-term cryopreservation. The purpose of this study is to transplant umbilical cord mesenchymal stem cells (UC-MSCs) on Curdlan-PVA composite hydrogel and observe the chondrocytes on the material. The results of using 4',6-diamidino-2-phenylindole (DAPI), hematoxylin and eosin (H&E), calcein-acetoxymethyl ester (calcein AM), and Collagen type II-Fluorescein isothiocyanate (FITC) staining, confirmed that UC-MSCs can attach and differentiate into chondrocytes on 3D Curdlan-PVA composite hydrogel.

Editorial on the Special Issue "Advances in Composite Gels".

Polymer gels are soft materials composed of a large amount of solvent (water, organic solvent, and ionic liquid) and a polymer, and they are constructed using a three-dimensional network [...].

Structural and Mechanical Properties of Konjac Glucomannan Gels and Influence of Freezing-Thawing Treatments on Them.

Freezing has been widely used for long-term food preservation. However, freezing-thawing (FT) treatment usually influences the texture and structure of food gels such as konjac. For their texture control after FT treatment, it is important to clarify the structural change of food gels during the FT process. In this study, we investigated the aggregated structures of konjac glucomannan (GM) gels during the FT process using simultaneous synchrotron small-angle X-ray/wide-angle X-ray scattering (SAXS/WAXS) techniques. The FT treatment resulted in more crystallization of GM, and consequently, a large increase in compressive stress. In-situ SAXS/WAXS measurements revealed the following findings: on freezing, water molecules came out of the aggregated phase of GM and after the thawing, they came back into the aggregated phase, but the aggregated structure did not return to the one before the freezing; the gel network enhanced the inhomogeneity due to the growth of ice crystals during freezing. Furthermore, we examined the influence of additives such as polyvinyl (alcohol) (PVA) and antifreeze glycoprotein (AFGP) on the mechanical and structural properties of freeze-thawed GM gels. Although the addition of PVA and AFGP suppressed the crystallization of GM, it could not prevent the growth of ice crystals and the increase in the inhomogeneity of the gel network. As a result, the compressive stresses for freeze-thawed GM gels containing PVA or AFGP were significantly higher compared with those of GM gels without FT treatments, although they were lower than those of freeze-thawed GM gels. The findings of this study may be useful for not only the texture control of freeze-thawed foods but also the improvement of the mechanical performance of the biomaterials.

Mechanical Performance of Corn Starch/Poly(Vinyl Alcohol) Composite Hydrogels Reinforced by Inorganic Nanoparticles and Cellulose Nanofibers.

We investigated the mechanical properties of corn starch (CS)/poly(vinyl alcohol) (PVA)/borax hydrogels reinforced by clay platelets, silica (SiO2) nanospheres, or cellulose nanofibers (CNFs). The effects of these reinforcing agents on the tensile properties of the hydrogels were quite different; the fracture stress of SiO2/CS/PVA/borax composite hydrogels increased with SiO2 concentration, whereas that of clay/CS/PVA/borax composite hydrogels was high at a low clay concentration but low at high clay concentrations; for CNF/CS/PVA/borax composite hydrogels, although the elastic modulus was highly enhanced by adding CNF, the fracture stress was very low because of the stress relaxation during the elongation. This result came from differences in the dispersibility of each filler and the reinforcing ability. These composite hydrogels were constructed by multi-crosslinking, such as hydrogen bonding between CS and PVA, CS and PVA crystals, complexation between borate and PVA (partly CS), and the crosslinking between each filler and polymer. The self-healing ability of SiO2 and clay composite hydrogels was examined. As a result, the SiO2/CS/PVA/borax composite hydrogels possessed an excellent self-healing ability, whereas the clay/CS/PVA/borax composite hydrogels had a poor self-healing ability.

Robust and Highly Stretchable Chitosan Nanofiber/Alumina-Coated Silica/Carboxylated Poly (Vinyl Alcohol)/Borax Composite Hydrogels Constructed by Multiple Crosslinking.

We investigated the mechanical and structural properties of composite hydrogels composed of chitosan nanofiber (ChsNF), positively charged alumina-coated silica (ac-SiO2) nanoparticles, carboxylated poly (vinyl alcohol) (cPVA), and borax. ChsNF/cPVA/borax hydrogels without ac-SiO2 exhibited high Young's modulus but poor elongation, whereas cPVA/ac-SiO2/borax hydrogels without ChsNF had moderate Young's modulus but high elongation. ChsNF/ac-SiO2/cPVA/borax hydrogels using both ChsNF and ac-SiO2 as reinforcement agents exhibited high extensibility (930%) and high Young's modulus beyond 1 MPa at a high ac-SiO2 concentration. The network was formed by multiple crosslinking such as the complexation between borate and cPVA, the ionic complexation between ac-SiO2 and cPVA, and the hydrogen bond between ChsNF and cPVA. Structural analysis by synchrotron small-angle X-ray scattering revealed that the nanostructural inhomogeneity in ChsNF/ac-SiO2/cPVA/borax hydrogel was suppressed compared to those of the ChsNF/cPVA/borax and cPVA/ac-SiO2/borax hydrogels.

Mechanical Properties and Structures of Clay-Polyelectrolyte Blend Hydrogels.

Our recent studies have shown that the hydrogels prepared by blending clay, a dispersant of clay, and a polyelectrolyte (sodium polyacrylate (PAAS)) possess excellent mechanical properties. In order to clarify the mechanism of the toughness, we have so far investigated the effects of the composition, molecular mass of the polymer, and kinds of polymers on the mechanical properties. This study has focused upon the mechanical properties and structures of the clay/PAAS gels using three kinds of smectite clay minerals such as synthetic hectorite (laponite XLG), saponite (sumecton-SA), montmorillonite (kunipia-F), whose particle size becomes larger according to the sequence. Laponite/PAAS and sumecton/PAAS gels were quite tough for high compression, whereas kunipia-F/PAAS did not gelate. In comparison between sumecton/PAAS gel and laponite/PAAS gel, the mechanical property of the former gel was poorer than that of the latter gel due to the inhomogeneous distribution of clay platelets in the gel. Synchrotron small-angle X-ray scattering experiments revealed that their clay platelets laid down in the stretching direction under elongation. Furthermore, it was found that sumecton/PAAS gel under elongation was arranged with an interparticle distance of ~6.3 nm in the direction perpendicular to the stretching. Such local ordering under elongation may originate in local aggregation of sumecton platelets in the original state without elongation.

Mechanical, Swelling, and Structural Properties of Mechanically Tough Clay-Sodium Polyacrylate Blend Hydrogels.

We investigated the mechanical, swelling, and structural properties of mechanically tough clay/sodium polyacrylate (PAAS) hydrogels prepared by simple mixing. The gels had large swelling ratios, reflecting the characteristics of the constituent polymer. The swelling ratios initially increased with the increase of the swelling time, and then attained maximum values. Afterwards, they decreased with an increase of the swelling time and finally became constant. An increase in the clay concentration lead to a decrease in the swelling ratios, whereas an increase in the PAAS concentration lead to an increase in the swelling ratios. Tensile measurements indicated that the toughness for clay/PAAS (Mw = 3.50 × 106) gels was several hundred times larger than that of clay/PAAS (Mw = 5.07 × 105) gels, i.e., the use of ultra-high molecular weight PAAS is essential for fabricating mechanically tough clay/PAAS blend hydrogels. Synchrotron small-angle X-ray scattering (SAXS) results showed that the SAXS intensity measured at small scattering angles decreased with an increase in the clay concentration, indicating that the interparticle interactions were more repulsive at higher concentrations. The decrease of the scattering intensity at high clay concentrations was larger for the clay/PAAS (Mw = 5.07 × 105) gel system than for the clay/PAAS (Mw = 3.50 × 106) gel system.

A structural study of an organogel investigated by small-angle neutron scattering and synchrotron small-angle X-ray scattering.

We investigated structures of 12-hydroxystearic acid (12-HSA) gels in a wide temperature range from the gel to the sol states and the gelation process by means of small-angle neutron scattering (SANS) and time-resolved synchrotron small-angle X-ray scattering (SAXS), respectively. The SANS result shows that the size of the cross section of the crystalline fibers in the gel was not affected much by temperature change, although the density of the fibers decreased with the increase of temperature even in the gel state. The SANS profiles at various temperatures and compositions were well reduced by the product of the volume fraction of the fiber and the square of the scattering contrast. At temperatures above the melting point of the gels obtained from differential scanning calorimeter, the gel turned into a homogeneous solution. The time-resolved small-angle X-ray scattering experiments revealed the gelation process as mentioned below. At the early stage, the scattering behavior did not show any change (induction period). The induction period was longer at higher temperatures. In the second stage, an amorphous precursor appeared as an initial structure of crystalline nucleus. In the next stage, the fiber was developed via crystalline nucleation and growth. In this stage, the fibers grew with keeping the size of the cross section (the radius of gyration R ≅ 82 -84 Å) constant, and finally, the fiber growth was arrested because of gelation.

Reversible chain association/dissociation via a CO2 responsive crosslinking/decrosslinking system.

Reversible chain association/dissociation phenomenon via CO(2) responsive crosslinking/decrosslinking was detected in aqueous solutions of polyallylamine (PAA). The chain association/dissociation behavior was reversible and useful in the synthesis of porous crosslinked polystyrene, which suggested potential utility in the area of CO(2)-responsive separable adhesives, switches and sensors.

Viscoelastic effects on early stage of spinodal decomposition in dynamically asymmetric polymer blends.

Spinodal decomposition induced by a rapid pressure change was investigated for a dynamically asymmetric polymer blend [deuterated polybutadiene (DPB)/polyisoprene (PI)] with a composition of 50/50 wt/wt by using time-resolved small angle neutron scattering. The time change in the scattered intensity distribution with wave number (q) during the spinodal decomposition was found to be approximated by the Doi-Onuki theory [M. Doi and A. Onuki, J. Phys. II 2, 1631 (1992)]. The theoretical analysis yielded the q dependence of the Onsager kinetic coefficient which is characterized by the q(-2) dependence at qxive > 1 with the characteristic length xive being much larger than the radius of gyration of DPB or PI. The estimated xive agrees well with that obtained previously in the relaxation processes induced by pressure change within the one phase region for the same blend.

Viscoelastic effects in relaxation processes of concentration fluctuations in dynamically asymmetric polymer blends.

Relaxation processes of the concentration fluctuations induced by a rapid pressure change were investigated for a dynamically asymmetric polymer blend [deuterated polybutadiene (DPB)/polyisoprene (PI)] with a composition of 50-50 by weight by using time-resolved small-angle neutron scattering. The pressure change was carried out inside the single-phase of the blend with the cell designed for polymeric systems under high pressure and temperature. Time change in the scattered intensity distribution with wave number (q) during the relaxation processes was found to be approximated by Cahn-Hilliard-Cook linearized theory. The theoretical analysis yielded the q dependence of Onsager kinetic coefficient that is characterized by the q(-2) dependence at q(xi)(ve)>1 with the characteristic length xi(ve) (with xi(ve) being the viscoelastic length) being much larger than radius of gyration of DPB or PI. The estimated xi(ve) agrees well with that calculated using the Doi and Onuki theory that takes into account the viscoelastic effects arising from the dynamical asymmetry between the component polymers in the relaxation of concentration fluctuations.