Bacteria: A new power generator

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2010

Researchers develop a procedure that uses bacteria as a power source for running mechanical micromachines. 

KEY CONCEPTS
• A procedure has been developed to enable swimming bacteria to turn gears with teeth of varying number, shape and arrangement.
• Gear movement does not occur until the bacteria reach a certain concentration because some type of friction is present that can only be overcome with a certain number of bacteria.
• Adjustment of the concentration of air or oxygen that is placed in contact with the bacteria can regulate gear movement.

In development of new sources of power, researchers are looking to emulate how nature uses energy in an efficient manner. One case in point is the process of photosynthesis in which plants capture sunlight and transform it through a series of pathways into chemical energy that can be used as a food source or fuel.

Many of us in the lubricant field are familiar with dealing with the negative characteristics of microbes such as bacteria. For example, bacteria are known to proliferate in metalworking fluid systems because this medium is an excellent nutrient. The result is that the metalworking fluid is decomposed, leading to inferior product performance and the potential for health and safety concerns.

But if harnessed in the proper manner, bacteria could be used as a source of power for running mechanical micromachines. One reason for this potential is that bacteria swimming in a specific medium have been found to reduce the viscosity of the fluid.

Dr. Igor Aronson, physicist and principal investigator at Argonne National Laboratory in Argonne, Ill., says, “We suspended bacteria in a free-standing liquid film that was prepared by stretching two supporting crossed pairs of fibers into a square window approximately 200 micrometers thick with dimensions that were 7 millimeters-by-7 millimeters.” 

“Through the analysis of the decay time of a macroscopic vortex created in the film by a moving magnetically actuated probe and the measurement of torque generated on a rotating magnetic particle immersed in the film, we measured the viscosity of the liquid film by itself and in the presence of swimming bacteria.” 

The result is that the use of swimming bacteria reduces the viscosity of the liquid film by a factor of seven (1). This work indicates that if placed in the proper environment, the mechanical energy generated by swimming bacteria can be harnessed over a long timeframe as a power source without fluid viscosity becoming a problem.

An experiment demonstrating the potential for using bacteria in this fashion has not been conducted until now.

MOVING GEARS
Researchers at Argonne National Laboratory and Northwestern University developed a procedure to enable swimming bacteria to turn gears with teeth of varying number, shape and arrangement. Aronson says, “Laws of thermodynamics prohibit extraction of useful work from the Brownian motion of particles in equilibrium, which means that it is impossible to create directed motion of the gears in water.” 

“When placed in close proximity, molecules that collide randomly with gears will not turn them in a certain direction. Bacteria that are swimming near gears are self-propelled particles that can convert chemical energy of the nutrient into mechanical energy that turns gears.”

The researchers placed gears that are 380 micrometers in diameter and 50 micrometers thick into a liquid film suspension with bacteria and their nutrient, bacterial broth. Each gear was made from SU-8 photoresist using conventional photolithography.

The gears were twice as dense as the film, which meant that they sank to the lower fluid/air interface. Bacillus subtilis (also known as hay bacillus or grass bacillus), a common gram-positive bacteria, was used in this study. Aronson says, “This type of bacteria swim very well in this medium and can move an incredible 20 times their body length per second. They also do not present any harm to people.”

The bacteria move gears with asymmetric teeth by sliding along until they are trapped in the v junction. Due to the asymmetry, they are trapped in one direction and will move the gear in that direction. Gears with symmetric teeth were used as a control, and no movement was seen with bacteria because of their orientation. Aronson says, “In the case of symmetric gear, bacteria pushed from both sides, which led to zero momentum. The symmetric gears did not rotate but, rather, just fluctuated in the film.”

A variety of different gears being turned by bacteria is shown in Figure 2. Diagram “I” and “J” show that arrangements can be prepared to have two gears engage each other by rotating in opposite directions.


Figure 2. Bacteria placed in a medium with nutrient and oxygen can rotate gears. The frequency of the rotation is a function of the number of bacteria present and concentration of oxygen. (Courtesy of Argonne National Laboratory)

The rate of gear rotation is a function of the concentration of bacteria present in the film. Aronson explains, “Gear movement does not occur until the bacteria concentration reaches 1x1010 per cubic centimeter. We believe that some sort of friction is present in the surface film that can only be overcome by a critical force that requires a certain number of bacteria.”

The optimal concentration for bacteria is between 1 and 4x1010 per cubic centimeter. Above this range, the gears will stop rotating. Aronson says, “At a very high concentration, bacteria literally cannot swim because there are too many of them crowded into a finite space. This condition also prompts bacteria to secrete certain chemicals, which are the precursor for the generation of a biofilm.”

Gear movement can be regulated by adjusting the concentration of air or oxygen that is placed in contact with the bacteria. Aronson says, “We found that gear rotation stops when the oxygen in the atmosphere is completely replaced with nitrogen, leading to an anaerobic environment. Without access to oxygen, bacteria cannot undergo respiration and will stop moving.”

This experiment also served to prove that the motion of the bacteria is the sole reason why gears are rotating. Other factors controlling the rate of rotation are the size, number and shape of the gear teeth.

In the initial experiments, gear rotation occurred for six to eight minutes and was restricted by the level of nutrient. Eventually, a high enough concentration of metabolic products is generated from the bacteria to increase the viscosity of the film and reduce gear rotation. Aronson says, “Sustaining the motion for a longer period of time is feasible if fresh fluid containing nutrient can be constantly pumped into the system while metabolic byproducts are removed.”

The ability of an ordinary species of bacteria to turn gears means there is potential for injecting energy generated by microorganisms into a system to power micromachines in the future. Further information can be found in a recent article (2) or by contacting Aronson at aronson@anl.gov. 

REFERENCES
1. Sokolov, A. and Aronson, I. (2009), “Reduction of Viscosity in Suspension of Swimming Bacteria,” Physical Review Letters, 103 (14), 148101.
2. Sokolov, A., Apodaca, M., Grzybowski, B. and Aronson, I. (2010), “Swimming Bacteria Power Microscopic Gears,” Proceedings National Academy of Sciences, 107 (3), pp. 969-974.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.