Pili Power

A close-up of the microbial nanowires that could one day replace synthetic conductors. Any resemblance to a penis is purely coincidental. Image courtesy of UMass Amherst, Anna Klimes, and Ernie Carbone. Elbert van Putten is a scientist and PhD candidate at the University of Twente in Enschede, Netherlands. The focus of his research is imaging with scattered light, which is a fancy way of saying makin’ pictures. Mark Tuominen is a professor of physics at the University of Massachusetts, Amherst. Recently, he worked with a team of physicists and microbiologists who discovered that the bacterium Geobacter sulfurreducens has the ability to conduct electricity and one day could replace boring old metal wires and batteries. Geobacter sulfurrenducens are bacteria that live in colonies found under the ground, where there’s no oxygen. Instead of respiring by breathing, they have to get rid of electrons, so they pass them along to iron or magnesium atoms in the sediment where they live that will accept electrons. Back in 2005 we found they had these little nanofilaments—they look like hair growing off the sides of the bacteria’s bodies—that conduct electricity. That’s kind of interesting, but what we just discovered in our latest experiments is that within these colonies, the nanofilaments are all intertwined into a metal-like network that can conduct over very long distances. The bacteria is about one micrometer in size, that’s a millionth of a meter. (For reference, a red blood cell is about six micrometers.) But these filaments—biologists call them pili—can grow to be 20 micrometers or longer. The thing is, that’s not the maximum limit of their reach, because when the bacteria live in a colony the pili of one are touching the pili of another, which touch yet another pili and so on. The properties of these nanowire networks are very much like those of synthetic organic conductors that have been created in labs and are now used in some products, like organic light-emitting diodes. Apparently nature knows how to make organic conductors out of proteins, and we find that very surprising. That’s why it’s kind of a big discovery. There will be applications for these nanowires, some in the near term and some that are more speculative and long term. In the near term, knowing what we know about this bacterial electric network will help us do things like design better microbial fuel cells, which are an organic source of electrical energy. In the longer term, we’re looking at this material and how nature uses it. For instance, it’s a very entangled network—it looks like spaghetti—and it has a lot of surface area capable of storing a lot of energy in a small amount of space. We’re starting to explore how to do that, but it’s still a ways off. Right now, we’ve just made the basic discovery. We haven’t tinkered around with the material that much so we don’t know whether it can conduct much better than it does right now or whether it’s pretty much as good as it gets. We don’t completely understand the system yet. We’ve measured a lot of the properties of the nanofilaments, and we see that they behave like organic conductors, but we don’t know the detail, the atomic and molecular structure of the pili right now. So we’re going to be doing a lot of biology and physics to figure out what the structure is and maybe how to replicate it. Will organic materials one day completely replace inorganic materials in electronics? I would say probably not. A lot of effort and technology has gone into making high-performance silicon electronics, and it’s hard to beat silicon when it comes to computers or cell phones. But in other cases where we are mostly driven by cost, like solar cells, I think we’ll eventually see lots of organic materials out there. Silicon solar cells are too expensive, and that’s why we don’t have them on every roof in the US. With this bacteria, nature has figured out how to make conductors out of common natural materials, and it’s apparently very efficient because there’s not a lot of energy involved in how it uses these raw materials to produce itself and electricity. It’s definitely going to be an efficient way of making materials. The question is how we can take advantage of it.