Polyphosphazenes containing lactic acid ester and methoxyethoxyethoxy side groups — Thermosensitive properties and, in vitro degradation, and biocompatibility

The thermosensitive and hydrolytic properties and biocompatibility of polyphosphazenes containing lactic acid ester and methoxyethoxyethoxy as co-substitutents were investigated. Depending on the type of lactic acid ester, these polyphosphazenes exhibited lower critical solution temperatures (LCST) (from 33 to 52 °C), which were almost concentration-independent in the range of 1.5 to 15 wt% of the polymers in aqueous solution. The salt effect on the thermosensitivity of the polymers was studied by measuring their LCST in aqueous solutions containing various salts. Bu4NBr and KI showed salting-in effects among the tested six salts, but NH4Br, NaBr, NH4Cl, and NaCl showed salting-out effects. Hydrolysis studies showed the rate of polymer hydrolysis decreased in the order of basic > acidic > neutral solution. The polyphosphazene with bulkier and more hydrophobic ester groups was more stable in hydrolysis. The results of a cytotoxicity study using an MTT assay method with HepG2 cell and K562/VCR cell showed that these polyphosphazenes and their degradation products were biocompatible. The thermoresponsiveness and biocompatibility of these biodegradable polyphosphazenes may favor the polymers as potential stimuli-responsive materials in biomedical applications.