Immobilization of Liposomes into Porous Anodized Aluminum Oxide and Intact and On-demand Release by Ultrasonic Irradiation

Synthetic anionic liposomes represent useful models of extracellular vesicles for understanding their biological functions. Their integration into nanomaterials holds a great potential for various applications including drug delivery systems, separation techniques and containers. In this study, we report that porous anodized aluminum oxide (AAO) membranes serve as useful platforms for the integration of synthetic anionic liposomes. The examination using fluorophore (NBD)-labelled liposomes (average diameter, 122 nm) revealed that liposomes were successfully immobilized into AAO pores with no leakage for one week. We demonstrated that the immobilized liposomes could be released from AAO pores rapidly and on-demand by only one-minute ultrasonic irradiation while they did not leak spontaneously during storage in the buffer solution. Significantly, the released liposomes were found to be intact by scanning electron-assisted dielectric microscopy (SE-ADM) and spectroscopic techniques, with their size and morphology remaining unchanged compared to the liposomes before immobilization into AAO. Systematic analysis of a series of liposomes suggested that the immobilization of anionic liposomes proceeded though an ion-exchange mechanism. Intactness of released liposomes would be ascribed to the relatively weak attraction with the pore surface due to the electrostatic repulsion. The utilization of AAO membranes as beneficial platforms for integrating liposomes is anticipated to expedite the development of functional composite materials incorporating liposomes for drug delivery systems.