From liposomes to supported, planar bilayer structures on hydrophilic and hydrophobic surfaces: an atomic force microscopy study.

The sequence of events involved in the transition from attached liposomes to bilayer patches on hydrophilic and hydrophobic solid supports were visualized in situ by Tapping Mode atomic force microscopy in liquid. In a smooth manner, the attached liposomes spread and flattened from the outer edges toward the center until the two membrane bilayers were stacked on top of each other. The top bilayer then either rolls or slides over the bottom bilayer, and the adjacent edges join to form a larger membrane patch. This is clearly visible from the apparent height of 6.0-7.5 nm of the single bilayer, measured in situ. The addition of calcium appeared to increase the rate of the processes preventing the visualization of the intermediate stages. The same intermediate steps appeared to be present on hydrophobic surfaces, although the attached liposomes seemed to be distorted and the resultant membrane edges were uneven. This work has provided visual and detailed information on liposome coalescence (fusion) onto solid supports and demonstrated how the atomic force microscope can be used to study the process.