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 t o bilayer patches on hydrophilic and hydrophobic solid supports were visual ized in situ by Tapping Mode atomic force microscopy in liquid. In a smooth manner, the attached liposomes spread and flattened from the outer edges t oward the center until the two membrane bilayers were stacked on top of eac h other. The top bilayer then either rolls or slides over the bottom bilaye r, and the adjacent edges join to form a larger membrane patch. This is cle arly 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, alt hough the attached liposomes seemed to be distorted and the resultant membr ane edges were uneven. This work has provided visual and detailed informati on on liposome coalescence (fusion) onto solid supports and demonstrated ho w the atomic force microscope can be used to study the process.