We've long known the slime mold can determine the shortest path through a maze, or even model optimal railway systems. Now, a group of researchers has shown these amoeba-like single-cell organisms could be used to build general-purpose computers.

In a paper published last week on the academic research website arXiv, scientists from the University of the West of England confirm Physarum polycephalum slime molds can act as memristors, a new type of resistor, a key component of electrical circuits. The paper has not yet been submitted to any scientific journals.

Memristors, like resistors, regulate the flow of electricity through a circuit, but they can "remember" a particular charge even when it's turned off. This means they could be used to create more efficient computer memory. HP claims to have built a real-world memristor and has been working to commercialize the technology for the past few years, though some scientists have claimed HP's inventions are not actually memristors. Others, meanwhile, have looked to biological systems for examples of working memristors.

In 2008, researchers from the University of California, San Diego and the University of South Carolina published a paper observing that the *P. polycephalum'*s ability to adapt to stimulus is due to its ability to act as a living memristor. This was a theoretical paper focused on learning about the origins of intelligence, but Andrew Adamatzky, a professor of "unconventional computing" at the University of the West of England and a co-author of the recent study, already was looking at the computational potential of P. polycephalum.

For the past decade, Adamatzky researched the computational potential of slime molds. In 2009, he published a paper on how P. polycephalum can be used to model UK road systems, preceding the famous Tokyo railway paper. This new study was the logical next step in his research.

"Our paper involves experimentally measuring the electrical conductivity of living Physarum and discovering that it is a memristor," Ella Gale, another co-author of the paper, tells us. "We are measuring physical properties of the Physarum."

It turns out they do in fact work like memristors, given enough voltage. That creates the possibility of using slime molds to build new types of general-purpose computers. "If the Physarum is a memristor then, according to the current understanding, we can do implication logic which can form a Turing complete set of logic gates," says Gale.

The biggest obstacle to practical application at the moment is the lifespan of the P. polycephalum. But engineers might not need to actually incorporate living P. polycephalum to take advantage of this research. "Part of the idea of the ongoing research is to include metal nanoparticles and make computers that way, which may involve using living slime mold to lay down circuitry before killing it to leave behind wiring," she says. "This could be a cheaper and greener way to make circuits."

Specific applications will depend on how robust the computers are, Gales says. But environmental sensors are one possibility. "Slime mold is happy with damp conditions and sensitive to light so it could be used as a light sensor in a cave network or something."

There has in recent years been an explosion of interest in using alternative materials in computing. Some are inorganic, like the use of the nanomaterial graphene to create atom-thin transistors. But the use of living systems is one of the most captivating areas of research. The University of Leeds is trying to use bacteria to build hard drives, and there's even potential to use salmon testes in storage devices. But Gale says that this type of research helps us do more than just create new types of computers. It also helps us understand life itself.

"Computation might turn out to be the 'killer app' for understanding biological systems, the way that mathematics has turned out to be the killer app for understanding physical systems," she says.