Saori Sasaki

Swelling and mechanical properties of physically crosslinked poly(vinyl alcohol) hydrogels.

Physically crosslinked poly(vinyl alcohol) gels are versatile biomaterials due to their excellent biocompatibility. In the past decades, physically crosslinked poly(vinyl alcohol) and poly(vinyl alcohol)-based hydrogels have been extensively studied for biomedical applications. However, these materials have not yet been implemented due to their mechanical strength. Physically crosslinked poly(vinyl alcohol) gels consist of a swollen amorphous network of poly(vinyl alcohol) physically crosslinked by microcrystallites. Although the mechanical properties can be improved to some extent by controlling the distribution of microcrystallites on the nano- and micro-scales, enhancing the mechanical properties while maintaining high water content remains very difficult. It may be technologically impossible to significantly improve the mechanical properties while keeping the gel’s high water absorbance ability using conventional fabrication methods. Physical and chemical understandings of the swelling and mechanical properties of physically crosslinked poly(vinyl alcohol) gels are considered here; some promising strategies for their practical applications are presented. This review focuses more on the recent studies on swelling and mechanical properties of poly(vinyl alcohol) hydrogels, prepared using only poly(vinyl alcohol) and pure water with no other chemicals, as potential biomedical materials.

Effects of preparation temperature on swelling and mechanical properties of PVA cast gels

Poly(vinyl alcohol) cast gels (PVA cast gels) were prepared at different gelation temperatures, Tgel, and the swelling ratio and mechanical properties were characterized by the equilibrium weight measurement, a uniaxial loading test, a tearing force measurement, and a cyclic compression test at room temperature. The results showed that the swelling ratio decreased and both the breaking stress and the tearing energy increased with an increase in Tgel. The compression deformation energy of 50% reduction of its original size increased monotonically, while the hysteresis loss was not affected by Tgel. To understand these macroscopic properties, the network structures of the samples were examined by X-ray diffraction and Fourier transform infrared measurements. As a result, the size of the microcrystallites increased with an increase in Tgel and the average distance between the microcrystallites also increased. These results suggested that the structures of PVA networks, crosslinked by the microcrystallites, have essential roles in determining the mechanical properties of PVA cast gels. The macroscopic mechanical properties are discussed on the basis of the size, number, and distribution of microcrystallites as crosslinkers and the homogeneity of amorphous PVA networks, which were determined at gelation.

Elution of polymers from physically cross-linked poly(vinyl alcohol) gels

Elution of polymers from physically cross-linked poly(vinyl alcohol) (PVA) gel in its swollen state was investigated by measuring quantitatively the total carbon in the outer water using a Total Organic Carbon (TOC) analyzer. The dried samples, prepared by a cast-drying method, were immersed in pure water under different conditions. As a result, the elution was detected, and it gradually stopped in a limited amount of water. On the other hand, the polymer was successively released by exchanging the outer water repeatedly at long intervals (more than 21 h), which was suppressed by adding a drying treatment at each water exchange. Effects of the degree of polymerization of PVA on the elution ratio were also presented.

Change in molecular weight distribution by elution of polymers from PVA cast gel

Poly(vinyl alcohol) (PVA) cast gels with high degrees of polymerization and hydrolysis were prepared at room temperature. After sufficient exchange of the outer solvent by fresh pure water, the gel was re-dissolved in pure water at a high temperature. Using this solution, the cast gel was prepared again. This re-dissolution process was repeated again, and a third cast gel was prepared. During each swelling process after each cast-drying process, the amount of elution from the gel was measured and the molecular weight distributions of PVA in the gel as well as in the outer solvent were quantitatively evaluated. The molecular weight distribution of the eluted PVA was found to depend on that of the PVA in the gel; the average molecular weight gradually increased as the re-dissolution process was repeated, while the polydispersity indices of both the PVA from the gel and the eluted PVA decreased.

Effect of Gamma Ray Irradiation on Friction Property of Poly(vinyl alcohol) Cast-Drying on Freeze-Thawed Hybrid Gel

Poly(vinyl alcohol) (PVA) is a biocompatible polymer with low toxicity. It is possible to prepare physically cross-linked PVA gels having hydrogen bonds without using a cross-linking agent. The newly reported physically cross-linked PVA cast-drying (CD) on freeze-thawed (FT) hybrid gel has an excellent friction property, which is expected to be applied as a candidate material for artificial cartilage. Gamma ray sterilization for clinical applications usually causes additional chemical cross-linking and changes physical properties of gels. In this study, CD on FT hybrid gels were irradiated using gamma rays at a different dose rate and irradiance. The results showed the optimized irradiation conditions for gamma irradiated gels to retain excellent friction characteristics.