Injectable and biodegradable supramolecular hydrogels by inclusion complexation between poly(organophosphazenes) and α-cyclodextrin

Biodegradable poly(organophosphazenes) containing side chains of various oligo(ethylene glycol) methyl ethers (mPEGs) and glycine ethyl ester units were synthesized and characterized. Novel supramolecular-structured hydrogel systems based on the inclusion complex between the mPEG grafted polyphosphazenes and α-cyclodextrin were prepared in aqueous media. The gelation time depended on the length of the mPEG side chains, the molar ratio between mPEG repeat units and α-cyclodextrin, and the concentration of the polymeric gel precursors. The rheological measurements of the supramolecular hydrogels indicate a fast gelation process and flowable character under a large strain. The hydrogel systems demonstrate unique structure-related reversible gel-sol transition properties at a certain temperature due to the reversible supramolecular assembly. The formation of a channel-type inclusion complex induced gelation mechanism was studied by DSC, TGA, ¹³C CP/MAS NMR, and X-ray diffraction techniques. The strong potential of the system for injectable drug delivery applications was explored with the use of bovine serum albumin as a model protein for in vitro release studies. All the supramolecular hydrogels studied showed disintegration by dethreading of the α-cyclodextrin. Polymers with longer poly(ethylene glycol) side chains had better stability and slower protein release profiles. The molecular weights of the polymers were monitored by GPC to show the biodegradability of the hydrogel system.