Developing active transport systems for micro cargo delivery is challenging because it requires overcoming the constraints imposed by low Reynolds numbers. We developed a bio-hybrid micro-swimmer, "Chlamylipo" consisting of the green alga Chlamydomonas reinhardtii, encapsulated within a giant liposome. Although internal encapsulation offers cargo protection, it necessitates a mechanism to transmit the propulsion force across a closed membrane. We demonstrated that Chlamylipo exhibited forward swimming and phototactic directional control. High-speed imaging of the membrane shape and fluid flow revealed that the driving force originated from periodic membrane deformations and was accompanied by characteristic fluid dynamics. Flow analysis showed rapid oscillations at tens of hertz corresponding to flagellar beating, superimposed on slower axial migration at approximately 4 Hz, associated with cell rotation. Corresponding flow signatures were also detected in the external fluid, indicating mechanical coupling of the lipid bilayer. Membrane domain tracking further revealed that the fluid motions inside and outside the membrane were coupled through viscous friction and membrane deformation, generating a characteristic four-vortex flow field consistent with a two-point force model. Collectively, these results suggest that membrane flow primarily reflects force transmission across the bilayer, whereas forward propulsion is primarily driven by periodic membrane deformation. This study elucidates the physical mechanism of force transmission in encapsulated swimmers, demonstrating that internal hydrodynamic power can effectively drive the motion of microscopic containers.
Shiomi et al. (Thu,) studied this question.