Takuya Katashima

State of Water, Molecular Structure, and Cytotoxicity of Silk Hydrogels

A novel technique was developed to regulate the bulk water content of silk hydrogels by adjusting the concentrations of silk proteins, which is helpful to investigate the effects of the state of water in polymeric hydrogel on its biological functions, such as cytotoxicity. Gelation of the silk hydrogel was induced with ethanol and its gelation behavior was analyzed by rheometry. The silk hydrogels prepared at various silk concentrations were characterized with respect to their water content, molecular and network structures, state of water, mechanical properties, and cytotoxicity to human mesenchymal stem cells. The network structure of silk hydrogel was heterogeneous with β-sheet and fibrillar structures. The influence of the state of water in the silk hydrogel on the cytotoxicity was recognized by means of differential scanning calorimetry and cell proliferation assay, which revealed that the bound water will support cell-adhesion proteins in the cellular matrix to interact with the surface of the silk hydrogels.

Strain energy density function of a near-ideal polymer network estimated by biaxial deformation of Tetra-PEG gel

We have performed various types of biaxial stretching for Tetra-PEG gels which correspond to nearly ideal polymer networks with small amounts of structural defects. We have found that (1) the Neo-Hookean (NH) model, which has been considered as a model for ideal polymer networks, evidently fails to describe the biaxial stress–strain data especially in the large strain regime; (2) the stress ratio σPS−y : σPS−x (where x and y are the stretching and constrained directions, respectively) in pure shear deformation is much larger than the expectation of the models with no explicit strain-coupling term, and the stress ratio increases with an increase in polymer concentration. These findings indicate that the two effects, i.e., finite extensibility and strain-coupling between different axes, should be explicitly introduced in the strain energy density function. We extend the Gent model, which considers the finite extensibility effect on the basis of the NH model, by adding an explicit strain-coupling term. This extended Gent model successfully describes the whole biaxial data for the Tetra-PEG gels with various polymer concentrations. This model employs the first and second invariants of deformation gradient tensor as the two variables, and it includes the three parameters, each of which represents the number of network strands, the strength of strain-coupling between different axes, and the maximum value of the first invariant.

Silk-Pectin Hydrogel with Superior Mechanical Properties, Biodegradability, and Biocompatibility

A new method is developed to prepare silk hydrogels and silk-pectin hydrogels via dialysis against methanol to obtain hydrogels with high concentrations of silk fibroin. The relationship between the mechanical and biological properties and the structure of the silk-pectin hydrogels is subsequently evaluated. The present results suggest that pectin associates with silk molecules when the silk concentration exceeds 15 wt%, suggesting that a silk concentration of over 15 wt% is critical to construct interacting silk-pectin networks. The silk-pectin hydrogel reported here is composed of a heterogeneous network, which is different from fiber-reinforced, interpenetrated networks and double-network hydrogels, as well as high-stiffness hydrogels (elastic modulus of 4.7 ± 0.9 MPa, elastic stress limit of 3.9 ± 0.1 MPa, and elastic strain limit of 48.4 ± 0.5%) with regard to biocompatibility and biodegradability.

Mechanical properties of tetra-PEG gels with supercoiled network structure

We investigate the effects of swelling and deswelling on the mechanical properties of tetra-polyethylene glycol gels with the precisely tuned polymerization degree of network strand (Nc) and polymer volume fraction at preparation (ϕ0) by varying the fraction of interest (ϕm). The ϕm-dependence of the elastic modulus exhibits a crossover at ϕc due to large contraction of the network strands (supercoiling) accompanying deswelling. The Obukhov model successfully describes the ϕm-dependence of the elastic modulus. We estimate the fractal dimension of network strands (Df) by analyzing the stress-elongation relationships at high stretching using Pincus blob concept. In the supercoiling region, Df increases with an increase in ϕm, which suggests that the gyration radius of network strands decreases with deswelling in affine manner. The extensibility increases with an increase in ϕm because the deswelling reduces the distance between the neighboring junctions. These findings will help to understand the structure and formation mechanism of supercoiling.

Ultimate elongation of polymer gels with controlled network structure

Tetra-PEG gel, which is a polymer gel with a well-controlled network structure, was utilized to examine the model predicting the ultimate elongation ratio of elastomeric materials, i.e., the Kuhn model. The effect of polymer fraction, degree of polymerization of network strands, and connectivity of the Tetra-PEG gel was discussed. The ultimate elongation ratio was estimated from the stress-elongation curves using the extended Gent model. The ultimate elongation ratio of the Tetra-PEG gels with different structural parameters did not obey the Kuhn model at all. Instead, we proposed a new semi-empirical model predicting the ultimate elongation ratio of Tetra-PEG gel.

Probing the cross-effect of strains in non-linear elasticity of nearly regular polymer networks by pure shear deformation

The pure shear deformation of the Tetra-polyethylene glycol gels reveals the presence of an explicit cross-effect of strains in the strain energy density function even for the polymer networks with nearly regular structure including no appreciable amount of structural defect such as trapped entanglement. This result is in contrast to the expectation of the classical Gaussian network model (Neo Hookean model), i.e., the vanishing of the cross effect in regular networks with no trapped entanglement. The results show that (1) the cross effect of strains is not dependent on the network-strand length; (2) the cross effect is not affected by the presence of non-network strands; (3) the cross effect is proportional to the network polymer concentration including both elastically effective and ineffective strands; (4) no cross effect is expected exclusively in zero limit of network concentration in real polymer networks. These features indicate that the real polymer networks with regular network structures have an explicit cross-effect of strains, which originates from some interaction between network strands (other than entanglement effect) such as nematic interaction, topological interaction, and excluded volume interaction.