Thermosensitive phase behavior and drug release of in situ gelable poly(N-isopropylacrylamide-co-acrylamide) microgels

In situ gelable poly(N-isopropylacrylamide-co-acrylamide) microgels were prepared by precipitation polymerization in the presence of various amounts of N,N′-methlenebisacrylamide as a crosslinker. The diameters of microgels were in the range of 200–300 nm with narrow distributions as determined by photo correlation spectroscopy. The equilibrium swelling ratio and thermosensitive properties of the microgels increased with decreasing crosslinker content. The volume phase transition of microgels dispersions at high concentrations were investigated by phase diagrams. The microgels dispersions experienced four phases when the temperature was increased: semitranslucent swollen gel, clear flowable suspension, cloud flowable suspension, and white shrunken gel. The related phase transition temperatures were influenced by crosslinker content and the concentration of the microgel dispersions. Herein, the gelation temperature was changed by more than 20 °C, shrinking temperatures were slightly changed by about 3 °C, and cloud point temperatures showed almost no change. The three phase transition temperatures of microgels dispersed in phosphate-buffered saline solutions were lower than that in water. As drug carriers, the release rates of bleomycin from bleomycin-loaded microgel dispersions exhibited diffusion control at human body temperature.