Study of the Freezing Behavior of Liposomes

The freezing behavior of liposomes such as internal freezing, contraction by dehydration and disruption of membrane after thawing was examined by microscopic observation. Liposome suspensions were prepared by gentle hydration method using phosphatidylcholine (egg) and distilled water. The observed liposomes ranged in diameter from 5 to 150 mu m. The sample of liposome suspensions was cooled from -1 degrees C to -50 degrees C at cooling rates of 1, 2 and 5 degrees C/min and heated to 0 degrees C at heating rate of 10 degrees C/min. As a result, two different patterns for freezing were observed: internal freezing and contraction without internal freezing. When the internal freezing was observed, the membrane was unexceptionally disrupted after thawing. When the internal freezing was not observed, two different cases were observed after thawing: disruptive and contractive conditions. These different freezing patterns were primarily dependent on the liposome size. In addition, the cooling rate became a key factor determining the freezing patterns in small liposomes.