Effect of Body Geometry On Motility of Bacteria-Powered Microrrobots (BACTERIABOTS)

 

EFFECT OF BODY GEOMETRY ON MOTILITY OF BACTERIA-POWERED MICRORROBOTS (BACTERIABOTS)

Ali Sahari1, Bahareh Behkam1,2 1School of Biomedical Engineering and Sciences, Virginia Tech, Blacksburg VA 24061 2Mechanical Engineering Department, Virginia Tech, Blacksburg VA 24061

Abstract— Motility of micro-scale swimming robots falls in the realm of low Reynolds number, where viscous forces exerted on the robots are dominant over inertia. In this work, the importance of body geometry on the dynamics of bacteria propelled swimming microrobots (BacteriaBots) is investigated. We have experimentally demonstrated that oblong geometries enhance directionality of the micro-particles compared with spherical geometry.

  <>I.     INTRODUCTION

Mobile micro-robots have unique advantages such as the ability to access small spaces and the potential to be employed in large numbers as inexpensive agents of distributed systems for swarm robotic applications. Such micro-robots are envisioned to impact a diverse range of applications, including minimally invasive diagnosis and localized treatment of diseases, environmental monitoring, and homeland security [1]. Amongst the most significant obstacles to realization of mobile robots at micro-scale are the miniaturization of on-board actuators, power sources, and communication and control modules. Bio-hybrid approaches can be employed to address these challenges by integrating prokaryotic and eukaryotic cells within the robotic system [2-4]. BacteriaBot, a bio-hybrid micro-robot is constructed here by interfacing a microfabricated robot body with an ensemble of live engineered bacteria with the purpose of using the bacteria for actuation, sensing, communication and control. Motile behavior of bacteria is well characterized and falls under two characteristic modes of run and tumble. This random run and tumble behavior leads to the characteristic three dimensional random walk of bacteria and consequently to the stochastic motion of micro-objects propelled by bacteria.

Motility of spherical microstructures actuated by an ensemble of attached bacteria has been thoroughly characterized in previous literature [5-8] but a systematic study of the effect of microstructure geometry on propulsive behavior is currently missing. Mobile microrobots with optimal body geometries are envisioned to impact minimally invasive diagnosis, localized treatment of diseases and environmental monitoring. Limited particle diffusion and directional coefficient of drag are some of the attributes that are enhanced through such bio-hybrid systems. In this work, we have utilized a low-cost and high throughput technique to obtain non-spherical mico-particles and investigate the effect of particle shape on the motile behavior of the BacteriaBots.

  <>II.     Materials and methods <>A. BacteriaBot Body Fabrication A high throughput spherical particle casting and mechanical stretching under heat treatment, as previously described in [9], is used to produce non-spherical polystyrene (PS) particles which act as the engineered synthetic body of the robots. Briefly, 6 µm PS spheres (Polysciences) are casted in a 35 µm thick polyvinyl alcohol (PVA) film. The film is uniformly stretched in one dimension to generate voids around the micro-spheres. The casted PS micro-spheres are liquefied using a bath of hot mineral oil. For making football shaped particles, 0.5% glycerol is added to the PVA film as a plasticizer. After removal from bath and cooling, particles are released from the PVA film by soaking in 30% isopropyl alcohol (IPA): DI water solution at 80˚C, and are washed by centrifugation in a 30% IPA solution.

  <>B. Bacteria culture <>Escherichia coli (E. coli) strain MG1655 was cultured in Luria Broth (1% tryptone, 0.5% yeast extract, and 0.5% sodium chloride). The culture was grown to an optical density OD₆₀₀=0.5 at 37 ˚C.

  <>C. BacteriaBot Construction

The mixture of Poly-_L-lysine and micro-particles is incubated on a vortex mixer for one hour. Bacteria are centrifuged at 3000g for 5 min and then resuspended in motility medium (0.01M potassium phosphate, 0.067M sodium chloride, 10^-4M EDTA, 0.21M glucose, and 0.002% Tween-20). The poly-_L-lysine coated micro-particles are added to the bacteria allowing the bacteria to self-assemble on the particles.

D. Two-dimensional Single Particle Tracking The motion of the microbeads was captured at 20 frames per second using a Zeiss AxioObserver Z1 inverted microscope equipped with an AxioCam HSm camera. The images were analyzed using a two-dimensional (2D) particle tracking routine developed in MATLAB (MathWorks, Natick, MA). Briefly, using cell segmentation and image restoration, the artifacts existing in most of the captured images were removed. This was followed by noise removal and cell boundary recognition using a border following algorithm. Finally, the nearest-neighbor method was used to link segmented cells in successive frames and to generate the trajectories.

  <>III.     Results and Discussions

Two different particle geometries were fabricated using a casting and mechanical stretching method that was described earlier. Parameters used for the manufacturing and the scanning electron microscopy (SEM) images of the resulting geometries are given in Table 1 and Fig. 1, respectively. Representative images of the bacteriabots with different geometries are depicted in Fig. 2. The 2D image tracking routine was utilized to characterize the motion of the BacteriaBots. Of particular interest is how the body geometry will affect the directionality of the motion of BacteriaBot without the need for active steering. Directionality is defined as the ratio of the magnitude of the displacement vector to the total distance traveled. To prove that the bacteria attached to the mobile microbeads are the source of propulsion, a control experiment was performed. Minimal displacement of the control bead confirms that the bacteria attached to the mobile microbeads are the main source of propulsion. Therefore, any directed movement observed for this control bead would be neither due to diffusion nor due to the flow field generated by the free- swimming bacteria present in the background. The experimental results for directionality of the bacteria- propelled bodies with spherical, barrel, and football-shaped geometries are shown in Fig. 3. It can be seen that the barrel and football shaped bodies are propelled with a higher degree of directionality compared to the spherical robots. Two representative trajectories of the BacteriaBots are shown in Fig. 4. Number of attached bacteria for all geometries varies between 1-6; however, our experimental results do not seem to be significantly dependent upon the number of bacteria attached. This is consistent with our previous observations if the attached bacteria are uniformly distributed over the body, the overall force is expected to remain largely unchanged regardless of the number of attached bacteria [7].

 

It should be noted that more than the number of bacteria attached to the microbead, the location of attachment is expected to affect the overall behavior. If the areal attachment density becomes significantly nonuniform, we expect to see a change in the average net resultant force and consequently observe a change in overall speed and directionality [3]. <>IV.     Conclusion

Variation in motile behavior of BacteriaBots due to their body geometry can be very complex and can only be determined experimentally. These complexities are due to: (1) non-spherical geometries have varying coefficients of drag depending on their aspect ratio and the direction of motion, and (2) the varying radius of curvature on the surface of non-spherical geometries leads to preferential bacterial adhesion in certain locations. By utilizing a high throughput PS micro-particle manufacturing method, we characterized the motile behavior of BacteriaBots with spherical, barrel and football shaped bodies. It was shown that for BacteriaBots with uniform areal attachment density, body shape strongly affects the directionality of the motion.

We are currently examining the behavior of other geometrical shapes such as bullets. We are also investigating if the particles size will have an impact on the observed trend.

Acknowledgment

The authors would like to thank Professor Birgit Scharf in Biological Sciences Department at Virginia Tech for gifting the bacteria. This work was in part supported by the National Science Foundation (IIS-117519).

References

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[2] S. Martel, et al., "Controlled manipulation and actuation of micro-objects with magnetotactic bacteria," Applied Physics Letters, vol. 89, pp. 233904-233904-3, 2006.

[3] A. A. Julius, M. S. Sakar, E. Steager, U. K. Cheang, M. Kim, V. Kumar,and G. J. Pappas, “Harnessing bacterial power for micro scale manipulation and locomotion,” IEEE International Conference on Robotics and Automation (IEEE, Kobe, Japan, 2009).

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[5] B. Behkam and M. Sitti, "Bacterial flagella-based propulsion and on/off motion control of microscale objects," Applied Physics Letters, vol. 90, p. 023902, 2007.

[6] B. Behkam and M. Sitti, "Effect of quantity and configuration of attached bacteria on bacterial propulsion of microbeads," Applied Physics Letters, vol. 93, p. 223901, 2008.

[7] M. A. Traoré, et al., "Computational and experimental study of chemotaxis of an ensemble of bacteria attached to a microbead," Physical Review E, vol. 84, p. 061908, 2011.

[8] B. Behkam and M. Sitti, "Characterization of bacterial actuation of micro-objects," Proceedings of IEEE Conference on Robotics and Automation (ICRA) 2009, pp. 1022-1027.

[9] J. A. Champion, et al., "Making polymeric micro-and nanoparticles of complex shapes," Proceedings of the National Academy of Sciences, vol. 104, p. 11901, 2007.

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