Characterization of temperature induced changes in liposomes coated with poly(N-isopropylacrylamide-co-methacrylic acid)

Abstract

Positively charged liposomes were coated with the negatively charged and temperature sensitive poly(N-isopropylacrylamide-co-methacrylic acid) by electrostatic deposition. Too low or too high polymer concentrations lead to unstable suspensions. However, intermediate polymer concentrations (0.05-0.2 wt.%) result in relatively stable suspensions of polymer-coated liposomes. At elevated temperatures the thickness of the polymer layer around the coated liposomes increased sharply at 40 °C, due to the formation of polymer multilayers. At higher temperatures, a contraction of the adsorbed polymer layer was observed. The uncoated liposomes exhibited an interesting transition in size and intensity of the scattered light when heated, attributed to the transition from the gel to liquid crystalline phase. Rheo-SALS (small angle light scattering under shear conditions) measurements demonstrated that the polymer coating was stable under shear at physiological temperature. It also revealed an anomalous high scattered intensity of the uncoated liposomes compared to the coated liposomes. This discrepancy was diminished at higher temperatures, and can probably be attributed to the change from a non-spherical, polyhedron-like conformation of the uncoated liposomes in the gel phase to a spherical shape above the phase transition.

Keywords: Coating; Electrostatic deposition; Gel to liquid crystalline phase transition; Liposome; Non-spherical; PNIPAAM (poly(N-isopropylacrylamide)); Rheo-SALS; Thermoresponsive.