Bionanotubule Formation from Surface-Attached Liposomes Using Electric Fields

Spontaneous formation of long-range (millimeters) membrane-bound nanotubules from surface-immobilized liposomes is possible by application of modest electric fields (2 -20 V/cm), providing a novel fabrication strategy for these hollow cylindrical structures. Stable tubes generally aligned with the applied electric field were created from liposomes prepared with phosphatidylcholine (PC), phosphatidic acid (PA), phosphoethanolamine (PE), and cholesterol. The minimum voltage which causes nanotubular formation (the onset voltage) and the average number of tubules per liposome of varying composition was examined with fluorescent microscopy using labeled phospholipids. Generally, the onset voltages ranged between 4 and 15 V/cm and depended on the mother vesicle composition. The results of this study suggest that increasing the charged lipid content can decrease the onset voltage. Conversely, a cholesterol content of more than 30% (by mass) was found to hinder extension of lipid tubules. Basic calculations that assume lipid migration and domain formation on the mother liposome as a nucleating site for tubule extension are assessed and suggest this is a reasonable model to describe the mechanism of tubular growth from immobilized liposomes.