10,000 feet down
Extreme bacteria provide an unexpected boost to manufacturing
Ten thousand feet below Vicksburg, Virginia, bacteria lurking in the hot, dark recesses of ancient rocks spend their lives transforming metal into magnetic nanoparticles. Some folks think that’s a little creepy. Tommy Phelps thinks it could be the start of a new industry.
Phelps, an ORNL biologist, has spent decades studying extremophiles, organisms that have adapted to life under difficult conditions. The Vicksburg bacteria, recovered from gas drilling core samples, are called Thermoanaerobacter ethanolicus and live under millions of tons of rock in total darkness and in temperatures as high as70°C (158°F).
These extreme bacteria produce magnetic excretions on the outside of their cell membranes through a process known as nanofermentation, which converts salts, metals (iron in this case), and other nutrients into metal-containing nanoparticles. Scientists think these bacteria, which also excrete ethanol and acetic acid, produce the nanoparticles as part of a chemical balancing act that prevents levels of the other two waste products from becoming toxic.
Metal-containing nanoparticles are used in a variety of manufacturing processes, from battery and solar cell production to electronics. However, while these particles are useful and versatile, they can also be fairly expensive to produce. Enter T. ethanolicus.
Phelps and a cast of collaborators that numbers in the dozens are trying to make “industrially relevant” nanoparticles using biological processes that work not only in the laboratory but also on an industrial scale. “Some of our production models suggest we might be able to generate this material at less than one percent of the cost of existing commercial processes,” he says. “That’s a result of the scalability issues and high energy demands associated with current industrial processes.”
T. ethanolicus requires only food and warmth. In fact, as it is grown in larger and larger volumes, less energy is required because, as the bacteria become more numerous, they begin heating their environment themselves.
Phelps and his colleagues didn’t initially see anything too remarkable about T. ethanolicus—it’s not all that unusual for extremeophiles to produce nanoparticles. However, when they determined that the particles were magnetite, a magnetic form of iron oxide, they took a closer look. Magnetism is a quality that is in high demand for a range of existing and hypothetical nanoparticle applications. The particles produced by T. ethanolicus were unusually numerous. “It’s not unusual to wait a couple weeks to see a significant amount of magnetite form on an organism,” Phelps says. “We were seeing copious amounts form within a couple of days.”
Eventually, other researchers began to suggest ways to persuade T. ethanolicus to produce additional materials. By adding metals besides iron to the bacteria’s diet of metal-containing salts, Phelps and his colleagues found that they could incorporate them into the nanoparticles as well. They were surprised that, when they added zinc to the mix, the resulting magnetite was 30 to 50 percent more magnetic than magnetite produced using iron alone. “From there,” Phelps says, “we set out on a couple tangents. One of our big successes was producing cadmium sulfide nanoparticles—the kind used to make thin films for use in solar cells. Next year we will begin replacing this process with one that produces zinc sulfide, which is more environmentally friendly.”
Once researchers recognized T. ethanolicus’ distinctive qualities, they began thinking seriously about the implications of the organism’s qualities for industry. “We realized we could make kilogram-size and larger batches of these particles,” Phelps explains. “That’s something that isn’t seen much in nanotech research. A lot of nanoparticle processes look like they have potential in the laboratory, but when people consider producing them in larger quantities, they see they have scalability issues. We think nanofermentation gets around many of those problems.”
Scaling up production
The nanoparticles Phelps and his group cultivate are easily harvested. Because they are made on the outside of microorganisms, they spontaneously break loose and settle to the bottom of the fermentation vessel. Phelps notes that as production of the bacteria was scaled up from test tubes to 30-liter vessels, researchers noticed an increase in particle production per liter of culture. “Unlike many other processes,” he explains, “when this simple biological process is scaled up, production increases because there are fewer potential sources of problems. For example, if you put a drop of something into a test tube, you might kill everything in the container; however, if you put the same drop into a 30-liter vessel, you might kill everything within a centimeter, but that’s all. So, in some respects, it’s easier to control this kind of process on a larger scale. We think that’s why we have seen better results.”
The next step will be to move production to ORNL’s Manufacturing Demonstration Facility (MDF), a facility supported by the Department of Energy’s Energy Efficiency and Renewable Energy Advanced Manufacturing Office. The MDF will house an 800-liter pilot plant designed to produce kilogram scale quantities of materials for industrial, academic and laboratory partners. “There are lots of people who have ideas about potential uses for nanoparticles but don’t have away to produce enough product to test on an industrial scale,” he says. “We will be able to provide that capability.”
Phelps foresees that as researchers refine their ability to produce nanoparticles from naturally occurring bacteria, the next step will be to look beyond the natural cells.“We are still learning what the bacteria can do,” he says. “It has a lot of potential as well as limitations. At what point do we start changing the organisms? I think that will happen soon. If some of our potential research opportunities come to fruition, we’ll be looking at genetically modifying some of the machinery within the cells to enhance their capabilities and scalability.”
Some of these opportunities include developing nanoparticles for medical purposes. Phelps and his colleagues have had conversations with medical investigators about, among other things, producing nanoparticles for use in enhancing various types of medical scans by improving medical tracer materials and targeting the delivery of medicines to specific parts of the body.
Tailored to processes
In the near term, Phelps says the biggest challenge is ensuring that the products that come out of ORNL’s nanofermentation effort can be incorporated easily into recognized industrial processes. “As we design the industrial-scale system at the MDF, we’re asking industry what nanoparticle characteristics are most important to them,” he says. “ Is it size? Is it shape? Is it chemical reactivity? It’s critically important that we reach out to industry and other researchers to see what they want, as well as talk to people who have developed similar processes, so we can learn from them in a collaborative fashion rather than reinventing the wheel.”
As the laboratory’s nanofermentation research grows, it is focused on three goals: manufacturing quality nanoparticles, expanding the range of particles that can be made using T. ethanolicus or a similar organism, and scaling production up to the point that ORNL can provide kilogram-size quantities of these materials for its industrial and laboratory partners. Phelps emphasizes that this effort continues to rely heavily on materials scientists, chemists, engineers and others researchers from across the laboratory. “No single person could have taken this idea and run with it,” Phelps says. “It has taken a team. This is the kind of research national laboratories are here for.” —Jim Pearce