Single flow-focusing device with downstream constriction narrowed by water sheath flow for bacterial-sized liposomes

Abstract In this study, we aimed to enable electron microscopy of the bacterial interior in bacteriophage infection research by developing bacterial-sized liposomes for metal labeling of intracellular structures. In flow-focusing microfluidic devices, the diameter of the core determines the liposome size; however, reducing the channel cross-sectional area to achieve smaller cores leads to a large pressure drop, making miniaturization difficult. To address this issue, we designed a narrow constriction downstream of the inner aqueous/carrying organic/outer aqueous (OA) focusing region, where the OA sheath flow effectively reduced the functional constriction size. This approach enabled the formation of bacterial-sized liposomes. Using a single flow-focusing device with a constriction width of 0.9 μm and a height of 0.8 μm, liposomes with a diameter of 2.2 μm were successfully generated. In future work, electrically fusing bacteria with liposomes encapsulating metal nanoparticles is expected to allow metal labeling of intracellular structures and enable scanning electron microscopy observation of the bacterial interior.