Temperature-sensitive releases from liposomes containing hydrophobically modified poly(N-isopropylacrylamide)

Abstract

Novel temperature-sensitive Liposomes containing hydrophobically modified poly(N-isopropylacrylamide) (HPNIPAM) and their release behaviors were investigated using calcein as a fluorescence probe. Above the lower critical solution temperature (LCST) of the polymer (e.g., 40‡C), the degree of calcein release in 280 sec from reverse-phase evaporation vesicles (REVs) of egg phosphatidylcholine (egg PC) was 43 %, while egg PC MLVs was 16 %. Such a large difference of release may be attributed to the lamellarity of Liposomes. The incorporation of dioleoyl-phosphatidylethanolamine (DOPE) into the PC bilayer enhanced the release by 10–13 % at 40‡C, probably due to the increased instability of mixture bilayers. Meanwhile, a temperature-sensitive device of DOPE Liposomes was prepared by using HPNIPAM as a stabilizer. The optimal ratio of HPNIPAM to lipid to stabilize the bilayer was 0.1. Above the LCST (e.g., 40‡C), the release percentage was about 80% of the entrapped calcein. DOPE Liposomes were the most temperature-sensitive among Liposomes tested. This is probably because DOPE Liposomes disintegrate into a non-liposomal phase, such as hexagonal (H£), by a thermal contraction of HPNIPAM.