Bacterial ratchet motors

Abstract

Self-propelling bacteria are a nanotechnology dream. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns. Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices. Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively. Here we show that asymmetric environments can produce a spontaneous and unidirectional rotation of nanofabricated objects immersed in an active bacterial bath. The propulsion mechanism is provided by the self-assembly of motile Escherichia coli cells along the rotor boundaries. Our results highlight the technological implications of active matter's ability to overcome the restrictions imposed by the second law of thermodynamics on equilibrium passive fluids.

Fig. 1.

Gear shapes. Optical microscopy images of microfabricated gears suspended in water. A SEM image of type I gear is also shown in B.

Fig. 2.

Rectifying bacterial motions with asymmetric boundaries. (A) A pictorial representation of the mechanism through which the asymmetric shape of the inclined teeth induces spontaneous alignment and locking of self-propelled bacteria in the concave corners. Bacteria are drawn with a white head pointing in the direction of self-propulsion. Black arrows represent the forces exerted by bacteria on the walls. Equal and opposite reaction forces reorient the cell body along the wall. (B) Two E. coli cells interacting with gear boundaries. The cell pointed out by the white arrow aligns parallel to the wall and slides towards the corner where it gets stuck, contributing a torque. The other one, pointed out by the black arrow, aligns along the wall and slides back into the bulk solution. See also Movie S1. (C) Four bacterial cells spontaneously align and cooperatively push in the same direction against the wall.

Fig. 3.

Bacterial driven micromotor. A nanofabricated asymmetric gear (48-μm external diameter, 10-μm thickness) rotates clockwise at 1 rpm when immersed in an active bath of motile E. coli cells, visible in the background. The gear is sedimented at a liquid–air interface to reduce friction. The yellow circle points to a black spot on the gear that can be used for visual angle tracking.

Fig. 4.

Analysis of rotational motion at a liquid–air interface. (A) Cumulative rotational angle of microgears in a bacterial bath. Green lines refer to type I asymmetric gears. The two gears have opposite orientations, resulting in opposite spinning directions. The blue line refers to a type III asymmetric gear with smaller teeth. No net rotation is observed for type II symmetric gears as shown by the red line. (B) Time fluctuations of rotational frequency for type I gear. Fluctuations around the nonzero mean value are because of randomness in bacterial number and local arrangement along the boundaries. The same gear in a bacteria-free buffer fluctuates much less as shown by the gray curve. (C) A symmetric type II gear in a bacterial bath fluctuates with an angular velocity of zero mean and a standard deviation comparable to the asymmetric case.

Fig. 5.

Effect of bacterial concentration on rotational motion. (A) The green solid line is the cumulative rotational angle of a type I gear in a highly concentrated bacterial suspension (7 × 10¹⁰ bacteria/mL). As a comparison, we report as a dashed green line data for the same gear at a lower concentration (∼10¹⁰ bacteria/mL). By increasing the concentration, the average angular speed increases by about a factor of 2 but fluctuates much more. Solid and dashed red lines refer to the large type IV gears respectively in the higher and lower concentrations. (B) Time correlations of rotational speed fluctuations. Green disks refer to type I gears whereas red triangles to gears of type IV. Solid and dashed lines are exponential fits for, respectively, the higher and lower concentrations.

Fig. 6.

Effect of boundary condition on rotational motion. (A) The blue (red) line is the cumulative rotational angle of a type I asymmetric gear spinning over a liquid–glass (liquid–oil) interface. As a comparison, we report as a green line data for the same gear on a liquid–air interface. (B) Time fluctuations of rotational frequency. Color coding is the same as in A.