Selective fusion of two membrane surrounded volumes is of great interest in nanochemistry and nanomedicine as it can pave the way for performing controlled nanoscale chemical reactions and for delivering a cargo (e.g., chemicals, genetic regulatory factors, etc.) to a desired living cell. The concept of cellular delivery is also known as targeted drug delivery and is quite a hot research topic internationally. Therefore, there have been efforts to develop various chemical molecules, proteins/peptides and physical approaches to trigger membrane fusion between synthetic giant unilamellar vesicles (GUVs) and/or live cells. However, most of the fusion methods reported so far do not provide sufficient control over which cells/GUVs are going to fuse. Moreover, some of these approaches are not sufficiently non-invasive to be applied to living cells without compromising their viability. In this work, we introduce a novel and extremely flexible physical method which can trigger membrane fusion in a highly selective manner not only between synthetic GUVs of different compositions, but also between live cells which remain viable after fusion.

Optical tweezers’ laser (1064 nm) is used to position the two desired cells and/or GUVs next to each other and in immediate contact. Then, the same laser is placed in the contact zone between the two adjacent membranes until one or more gold nanoparticles diffuse into the focus. Gold nanoparticles absorb part of near infrared light and dissipate the absorbed light as heat to their surroundings. The strong and localized generated heating is sufficient for local melting and/or expansion of lipids within the two adjoining membranes. Since exposure of the hydrophobic bilayer core to the aqueous medium is energetically unfavorable, immediate rearrangements and merging of the two membranes results in merging the two membranes thereby completes the fusion. Complete fusion is associated with lipid mixing and lumen mixing which are both imaged by a high resolution confocal microscope. The confocal imaging enables quantification of the associated lipid mixing and cargo mixing time scales.

Importantly, in cell-cell and cell-vesicle fusion experiments, the cells stay viable when fusing to a second cell or to a vesicle by the localized heating emitted from the trapped nanoheater