Cyclic deformation behavior of agarose hydrogels prepared at different gelation concentrations.

Effects of the gelation concentration (c0) on cyclic deformation behavior of agarose hydrogels have been examined. Agarose hydrogels prepared at c0 = 10 g L-1 (A10) show smaller Young's modulus than those prepared at c0 = 30 g L-1 (A30) even when the mechanical properties have been measured at the same agarose concentration 50 g L-1. Moreover, A10 has stronger tendency to exhibit the residual strain in the cyclic deformation curves compared to A30. These results can be explained based on the gelation mechanism specific to agarose, where helices of agarose aggregate via hydrogen bonding to be the crosslinks. It has been proposed from the optical rotation that the helix content just after gelation increases with c0, while the helix content for A10 was higher than that for A30 at 50 g L-1. Since the lower helix content at c0 for A10 results in less stable aggregates of agarose helices, it can be said that the cyclic deformation behavior as well as Young's modulus of agarose hydrogels primarily reflects the network structure determined by c0.

Effects of gelation concentration on cyclic deformation behavior of κ-carrageenan hydrogels.

The aim of this study is to elucidate the relationship between the network structure of κ-carrageenan hydrogels and their mechanical properties. First κ-carrageenan hydrogels have been prepared at different gelation concentrations, and then the mechanical behavior during four cyclic deformations has been examined at the same κ-carrageenan concentration. Young's modulus is higher for the gel prepared at 5 gL-1 (C05) compared to that for the gel prepared at 30 gL-1 (C30). C30 shows almost linear relation between the stress and the strain like an ideal rubber, while a residual strain appears in each cyclic deformation for C05. The extent of the residual strain depends on the maximum strain and the deformation speed, indicating that C05 deforms plastically to some extent. The residual strain for C05 decreases gradually even after a cyclic deformation and disappears in the case of a small strain as if there were a memory of the structure. The effects of the gelation concentration on the mechanical properties have been explained based on the network structure specific to κ-carrageenan hydrogels. The higher modulus for C05 has been attributed to the higher helix content and the plastic deformation of C05 to the loosely-aggregated crosslinks.

Addition of glycerol enhances the flexibility of gelatin hydrogel sheets; application for in utero tissue engineering.

Gelatin hydrogels are naturally derived scaffolds useful for tissue engineering because of their cytocompatibility and controllable degradability. However, they are brittle and inflexible when dry, which limits their use for in utero tissue engineering in large animal models. Therefore, in this study, we attempted to generate flexible gelatin sheets by adding various plasticizers with different molecular weights (MW). We systematically evaluated the flexibility, sustainability, and potential clinical utility of the resulting flexible gelatin sheets. Gelatin sheets with low-MW plasticizers, such as monosaccharides or sugar alcohols, showed a reduced tensile modulus in dynamic viscoelasticity, which reflected their actual flexibility. Wet gelatin sheets containing plasticizers showed higher tensile strength than the nonplasticizer control, although wet gelatin sheets under all conditions had a much lower tensile strength than dry gelatin sheets. In a functional study, gelatin sheets containing glycerol, which has the lowest MW among sugar alcohols, showed encouraging results, such as good fit to the curvature of the experimental animal, biocompatibility, and suitability for endoscopic approaches. The findings of this study should enable the expansion of future applications for flexible gelatin sheets.

Application of a Clapeyron-Type Equation to the Volume Phase Transition of Polymer Gels.

The applicability of the Clapeyron equation to the volume phase transition of cylindrical poly(N-isopropylacrylamide)-based gels under external force is reviewed. Firstly, the equilibrium conditions for the gels under tension are shown, and then we demonstrate that the Clapeyron equation can be applied to the volume phase transition of polymer gels to give the transition entropy or the transition enthalpy. The transition enthalpy at the volume phase transition obtained from the Clapeyron equation is compared with that from the calorimetry. A coefficient of performance, or work efficiency, for a gel actuator driven by the volume phase transition is also defined. How the work efficiency depends on applied force is shown based on a simple mechanical model. It is also shown that the force dependence of transition temperature is closely related to the efficiency curve. Experimental results are compared with the theoretical prediction.

Optical and mechanical properties of pullulan films studied by uniaxial stretching.

Optical properties of pullulan films were examined before and after uniaxial stretching to obtain the specific molecular characteristics of pullulan. It was shown that the specific optical rotation for pullulan film was not affected by uniaxial stretching, being the same as that for pullulan in aqueous solution. Combined with the mechanical measurement, the stress-optical coefficient for pullulan was estimated as a material constant on the basis of the modified stress-optical rule. The birefringence was positive and increased with time during the stress relaxation of uniaxially-stretched pullulan film. A set of the stress-optical coefficients CR=1.9×10-9Pa-1 and CG=-4.6×10-12Pa-1 were obtained, where the subscripts R and G stand for the rubber and the glass components respectively.

Rheological properties of concentrated solutions of gelatin in an ionic liquid 1-ethyl-3-methylimidazolium dimethyl phosphate.

Rheological properties of gelatin solutions were examined in concentrated regions. Gelatin species from porcine skin and from bovine bone were dissolved in an ionic liquid 1-ethyl-3-methylimidazolium dimethyl phosphate. The dynamic viscoelasticity data for the solutions exhibited rubbery plateaus, indicating the existence of entanglement coupling between gelatin chains in the solutions. From the analogy with rubber elasticity, assuming that the molecular weight between entanglements (Me) is the average mesh size of the entanglement network, Me for gelatin in the solutions were determined from the heights of the rubbery plateaus. Then the value of Me in the molten state (Me,melt), a material constant reflecting the chemical structure of polymer species, for gelatin was estimated to be 8.7×10(3). Compared to synthetic polyamides whose Me,melt were known, Me,melt for gelatin was significantly larger, which could be explained by the densely repeating amide bonds composing gelatin.

Effect of pH on the conformation of gellan chains in aqueous systems.

The optical rotation and fluorescence anisotropy for gellan aqueous systems were measured at pH 4, 7, and 10 to elucidate the effect of pH on the conformation of gellan chains. The optical rotation study suggests that pH affects the conformation of helical gellan chains and their aggregation behavior but the coil-helix transition temperature. By comparing the chain mobility estimated from the fluorescence anisotropy between different pH conditions, it has been revealed that the degree of expansion of random-coiled gellan chains varies with pH. These results indicate that the effect of pH is explained by the change in the anionic nature of gellan chains rather than in the shielding effect of hydrogen ions surrounding gellan chains as a cation species.

In situ measurement of circular dichroism of DNA adsorbing onto a solid surface.

The adsorption process of DNA dissolved in aqueous solutions onto the surface of polyethyleneimine (PEI) has been examined by the observation of in situ circular dichroism (CD), including time-resolved measurements, to elucidate the conformation of DNA at the liquid/solid interface. The adsorption process can be characterized by two stages that are characterized in terms of CD. In the first stage, time (t)<700 s, a slight change in the time-resolved CD spectra of DNA is observed, whereas the value of the induced CD of the dye intercalated in DNA is constant. This result can be explained by the interaction between DNA and PEI during the adsorption at the liquid/solid interface. The weakness of the interaction is attributed to the geometrical restriction of this interface. In the second stage, t>700 s, where no further adsorption occurs, a change in the induced CD as well as in the CD of DNA is observed. This change in the induced CD can be interpreted as a significant conformational change of DNA for stabilizing the ion complex with PEI.

Local chain mobility of gellan in aqueous systems studied by fluorescence depolarization.

The local chain mobility of a gellan, an electrolyte polysaccharide, in aqueous systems was examined with respect to the effect of the temperature, the concentration of gellan (c(G)), and the concentration of added salt (c(S)). The relaxation time of local motion was estimated for fluorescein isothiocyanate (FITC)-labeled-gellan by the fluorescence depolarization technique, and the chain mobility was discussed. The relaxation time increased with decreasing temperature, in particular when accompanying the coil-helix transition due to the great difference in chain mobility between the coil and the helical conformations. The effect of c(G) was observed for gellan solutions even below the critical concentration of chain entanglement (2 wt.-%) for well-expanded nonelectrolyte polymers with size similar to that of the gellan. This suggests that the actual excluded volume of gellan is larger than that of nonelectrolyte polymers due to the electrostatic repulsion between segments. The relaxation time for 0.2 wt.-% systems of gellan in coil conformation is independent of c(S), whereas a c(S) dependence of the relaxation time is clearly observed for 0.5 wt.-% systems. The degree of expansion of the gellan chain is independent of the shielding effect of cations on the electrostatic repulsion between gellan segments due to the stiffness of gellan chain. On the other hand, the c(G) as well as the c(S) dependence of the chain mobility is clearly observed for gellan in the helical conformation, examined over the concentration range, probably due to the partial aggregation of helices induced by the attractive interaction between gellan segments.

Molecular weight between entanglements for κ- and ι-carrageenans in an ionic liquid.

The molecular weight between entanglements (Me) for κ- and ι-carrageenans, sulfated galactans, was examined in concentrated solutions using an ionic liquid 1-butyl-3-methylimidazolium acetate as a solvent. The dynamic viscoelasticity data for the solutions measured at different temperatures were overlapped according to the time-temperature superposition principle, and the obtained master curves exhibited the flow and rubbery plateau zones, being typical of concentrated polymer solutions having entanglement coupling. The values of Me for κ- and ι-carrageenans in the solutions were determined from the plateau moduli. Then the values of Me in the molten state (Me,melt) estimated as a material constant to be 6.6×10(3) and 7.2×10(3), respectively. The close values of Me,melt for κ- and ι-carrageenans indicate that 4-sulfate group of ι-carrageenan are not so influential for the entanglement network. Compared with agarose, a non-sulfate galactan, carrageenans have larger values of average spacing between entanglements.

Rheological properties of concentrated solutions of galactomannans in an ionic liquid.

The rheological behavior of galactomannans in concentrated solutions was examined by using dynamic viscoelasticity measurements. Concentrated solutions of three galactomannans, guar gum, tara gum, and locust bean gum were prepared with an ionic liquid 1-butyl-3-methylimidazolium chloride as the solvent. Each galactomannan solution showed angular frequency dependence curves of the storage modulus and the loss modulus which were characteristic of a solution of entangled polymer chains. The molecular weight between entanglements (Me) was obtained from the plateau modulus and the concentration dependence of Me showed Me in the molten state (Me,melt) to be 4.6×10(3), 3.2×10(3), and 2.7×10(3) for guar gum, tara gum, and locust bean gum, respectively. It was found that the material constant Me,melt depends on the mannose/galactose ratio of the galactomannans. The number of monosaccharide units between entanglements in the molten state for the galactomannans varied within the range found for other polysaccharides such as cellulose and agarose in ionic liquids, suggesting that all the galactomannans take a random-coil conformation in ionic liquid solutions.

Gelation of gellan gum aqueous solutions studied by polarization modulation spectroscopy.

Circular birefringence (CB, or optical rotation) and linear birefringence (LB) were measured for gellan gum aqueous solutions with and without salt to examine the gelling system in the helical structure as well as in the orientation. It was found that gelling samples with salt show nonzero LB values, whereas LB is zero for the samples without salt even in the gel state. This difference can be explained by the thermal deformation of the system containing anisotropic aggregations of helices formed with the shielding effect of the added salt on the intramolecular and intermolecular electrostatic repulsions. Considering that the presence of LB in the system affects the estimation of CB, we developed an original procedure of the CB measurement to eliminate the contribution of LB. It was shown that our methods for eliminating the contribution of LB can improve the CB measurement for the gellan gum gel. The temperature dependence of [alpha] for the samples with salt in the gel state is quite different from that for the samples without salt, suggesting that the aggregates of helices in the samples containing a high concentration of salt form a supramolecular structure that contributes to CB.