Imagine a microscale machine that can make electricity one moment – charging your phone, lighting your house – and produce a fuel the next – powering your car, creating raw materials for a range of products. This device you’re imagining, according to physicist Moh El-Naggar, isn’t a sleek electronic device, packed with transistors and silicon wafers. It’s almost certainly a microorganism.
Such biomachines aren’t available yet, but a new wave of materials scientists, biophysicists, and synthetic biologists is working hard to make them possible. “With traditional electronics,” explains El-Naggar, “it’s hard to make a multifunctional system. We’re really good at building machines with a preferred function, but to make it switch from doing A to B, that’s hard.” Biology, on the other hand, is significantly more tunable, with a dizzying array of metabolic branch points that can be pushed toward distinct products. When prompted with appropriate regulator molecules, genes can be turned up or down, moving metabolites down different pathways like trains at a junction.
The flow of electrons – the currency of biological transformations – is central to this vision, and understanding how these charges move through biological structures is a subject of active study. El-Naggar is a professor of physics at the University of Southern California, and his “BioNano” group’s investigations have generated significant buzz over the last few years. In December, he was bestowed a Presidential Early Career Award for Scientists and Engineers, an exclusive honor that earns recipients an audience with the President.
“I want to do for electron transport in biology what physicists have done for electron transport in computers, cell phones, or electronic chips,” says El-Naggar. And as a trained physicist who got involved with relatively messy biological systems 7 years ago, he’s well positioned to do so. By applying a rigorously quantitative mentality and custom-built conductivity sensors to cultures of metal-breathing microbes, he has gained an unprecedented view into the strange world of extracellular electron transfer.
Off-loading electrons to metals like iron or manganese outside the cell allows certain organisms to unplug the pipeline of energetic internal reactions and sustain growth in a highly unusual way. But what wasn’t known until recently was how such microbes – Geobacter and Shewanella most famous among them – were able to pull off the feat. Three different mechanisms seem capable of mediating the electron transfer: direct cell-mineral contact, “shuttles” that ferry electrons from cell to mineral, and – El-Naggar’s specialty – the evocatively named “nanowires” that act as conductive cables. Along with key collaborators Yuri Gorby and Ken Nealson, El-Naggar has progressively demystified the characteristics of these appendages and opened up a strange new world of microbial systems.
First they showed otherworldly images of cells tangled in networks of nanowires. Then they constructed microscale sensors to prove they were conductive. Now they’re working to describe their composition and better model the charge transport physics involved.
This progression of discoveries underlies what El-Naggar characterizes as a rapidly evolving field, where theoretical frameworks lag distantly behind transformative observations. “The tools for investigating these biological systems aren’t the limiting factor,” he explains. “Understanding the transport physics itself is the challenge.”
The scale of observed biological electron transport has grown exponentially over the last decade. Electron tunneling, the nanometer-scale movement that drives many enzymatic reactions, is understood relatively well from a theoretical perspective. But nanowires conduct charge over distances thousands of times longer, and centimeter-scale filaments increase the range by roughly ten million times.
In order to build the “toolbox of modular bioelectronic components” that El-Naggar envisions, many basic questions need to be answered. “The fundamentals aren’t well known, and once we’ve got those, we can start to envision multiple applications,” from biological batteries to climate change to pathogen investigations.
And while he shies away from the attention that accompanies such recognition, El-Naggar appreciates the Presidential Early Career Award as a validation of his work. “When we started doing physics based on measurements of electron transport in microbiology, it wasn’t something a lot of people were doing,” he recalls. “But what this award means, is that now, people have noticed.”
