Supramolecular-structured hydrogels were prepared on basis of the inclusion complexation between poly(ethylene glycol) grafted dextrans and α-cyclodextrins (α-CDs) in aqueous media. The inclusion ...complexes from the PEG grafted dextrans showed a unique gel−sol phase transition which cannot be obtained from usual polymer inclusion complexes that form crystalline precipitates. The gel−sol transition was based on the supramolecular assembly and dissociation, and the transition was reversible with hysteresis. The transition temperature was controllable by variation in the polymer concentration and the PEG content in the graft copolymers as well as the stoichiometric ratio between the guest and host molecules. The properties of the hydrogel were characterized by DSC, X-ray diffraction, and 13C CP/MAS NMR. The X-ray diffraction data indicated that the gel contains a channel-type crystalline structure, demonstrated by a strong reflection at 2θ = 20° (d = 4.44 Å). It was confirmed from the DSC and 13C CP/MAS NMR measurements that all the PEG grafts participate in the complexation. A phase-separated structure consisting of hydrophobic and channel-type crystalline PEG inclusion complex domains and hydrated dextran matrices was suggested as the internal structure, which comprises the supramolecular-structured hydrogel.
A series of poly(ethylene glycol) (PEG) hydrogels cross-linked by a hydrolyzable polyrotaxane was prepared and the hydrolytic erosion behavior was characterized. The hydrolyzable polyrotaxane ...consisting of many α-cyclodextrins (α-CDs) and a PEG chain capped with bulky end groups via ester linkages was used as a cross-linker in the PEG hydrogels, where α-CDs in the polyrotaxane were linked with another PEG chains to form hydrophilic PEG networks. From the result of the erosion study, the time to reach complete gel erosion was found to be prolonged by decreasing the polyrotaxane content and increasing the PEG/α-CD ratio. The PEG/α-CD ratio, indicating the number of PEG chains linked with one α-CD molecule, is considered to make the environment of the polyrotaxane more aqueous and lead to the hydrolysis of ester linkages in the polyrotaxane. However, the higher PEG/α-CD ratio prolonged the time of the hydrogel erosion. These results indicate the enhanced stability of ester hydrolysis in the hydrogels with highly water swollen state. Such an anomalous phenomenon may be due to the structural characteristic of the polyrotaxane: ester linkages may be included within the cavity of α-CDs, resulting in their enhanced stability. The erosion profile of the hydrogels was changeable by the M n of PEG−bisamine, independent of the polyrotaxane content. The hydrogels cross-linked by the polyrotaxane can be new candidates as long-term stable but actually hydrolyzable hydrogels for polymeric scaffolding in tissue engineering.
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