Controllable erosion time and profile in poly(ethylene glycol) hydrogels by supramolecular structure of hydrolyzable polyrotaxane

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 alpha -cyclodextrins (alpha -CDs) and a PEG chain capped with bulky end groups via ester linkages was used as a cross-linker in the PEG hydrogels, where alpha -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/alpha -CD ratio. The PEG/alpha -CD ratio, indicating the number of PEG chains linked with one alpha -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/alpha -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 alpha -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.