Targeted delivery of colloids by swimming bacteria

Abstract

The possibility of exploiting motile microorganisms as tiny propellers represents a fascinating strategy for the transport of colloidal cargoes. However, delivery on target sites usually requires external control fields to steer propellers and trigger cargo release. The need for a constant feedback mechanism prevents the design of compact devices where biopropellers could perform their tasks autonomously. Here we show that properly designed three-dimensional microstructures can define accumulation areas where bacteria spontaneously and efficiently store colloidal beads. The process is stochastic in nature and results from the rectifying action of an asymmetric energy landscape over the fluctuating forces arising from collisions with swimming bacteria. As a result, the concentration of colloids over target areas can be strongly increased or depleted according to the topography of the underlying structures. Besides the significance to technological applications, our experiments pose some important questions regarding the structure of stationary probability distributions in non-equilibrium systems.

Propulsion and Chemotaxis in Bacteria‐Driven Microswimmers

“…Most of the studies on bacteria‐driven microswimmers adopt a similar design, which is a sphere‐shaped microbead driven by single or multiple flagellated bacteria attached to it in random locations and orientations 2, 6, 9, 11, 12, 13, 14, 17, 20, 21, 22, 23, 24, 25. This particular design is chosen for its easier fabrication, characterization, and analysis, and also isotropic physical properties, such as drag coefficient, in all orientations.…”

“…However, nature has its own solutions since billions of years ago: flagellated swimming bacteria can efficiently convert chemical energy into mechanical actuation with their nanoscale biomotors. Over the past decade, numerous studies have been conducted on harnessing the flagellated bacteria, such as E. coli and S. marcescens , as propellers for biohybrid microswimmers,1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 aiming to develop a new type of targeted drug delivery system for tumor therapy 14, 16, 18, 19. Recently, efforts have also been made to guide the motion of such bacteria‐driven microswimmers through taxis‐based14, 17, 20, 21, 22, 23, 24, 25 and magnetic steering16, 26 approaches.…”

Ratchet Effects in Active Matter Systems

“…In experiments with passive colloids immersed in a bath of swimming bacteria, Koumakis et al showed that introducing an asymmetric substrate that causes the bacteria to undergo rectification also produces transport of the passive colloids into or out of enclosed regions (70). Figure 12(a) shows the time evolution of the number of passive colloids in each chamber of a sample containing three layers of ratchets for a geometry that concentrates the colloids in the central chamber, while Fig.…”

External control strategies for self-propelled particles: Optimizing navigational efficiency in the presence of limited resources

“…As a consequence, the propulsion direction varies as a function of time. With experiments and simulations, it has been demonstrated that even in the presence of rotational diffusion, autonomous microswimmers can perform a directed motion when moving along planar or patterned walls, funnels, channels, or chemical gradients [9][10][11][12][13]. Alternatively, steering of active particles can also be achieved using feedback loops which control the particle propulsion depending on its position and orientation [14,15].…”