Slime Mold Resistors (part 1)

Slime Mold Resistors (part 1)

Slime Mold Resistors (part 1)

I found Andrew Adamatzky’s research very helpful in getting to know slime mold at the intersection of biology and electronics. I’ve been trying to replicate ways to coax slime mold growth to make conductive wires as well. In an chapter contributed to Advances in Physarum Machines (2016) a book he also edited, Adamatzky reported that:

In 25 experiments we measured resistance of protoplasmic tubes on agar blobs. In calculations we assumed length of a tube is 1cm, and diameter is 0.03cm. We found that minimum resistance recorded is 80Ωcm, maximum resistance is 2560Ωcm, median 625Ωcm, and average 825Ωcm. Resistivity of Physarum substantially varies from one experiment to another: standard deviation calculated is 776, which is just slightly below average of Physarum wire resistivity. Average resistivity of Physarum protoplasmic tubes is of the same rank as resistivity of a cardiac muscle of a dog, and skeletal muscles of a dog and a human [18](p. 234)

That’s a pretty high resistivity! It’s also fascinating that the resistance of slime mold tubes approximates that of our own “skeletal muscles”, as well as that of dogs.

As I started tracking the way slime mold moves along and feeds itself, I’ve been surprised at how it seems to be resisting my own attempts at making it grow into a wire. When I told them about my failed experiments last Tuesday, friends at DIYbio Toronto designed and laser-cut a microfluidic chamber on acrylic. We thought pouring agar mixed with oat powder would help the Physarum hang out in the chamber longer than with non-nutrient medium. And to get it to hang out in the etched pattern even longer, I also put an inoculated slime mold oat at one end of the chamber, and another moist oat at the other end, thinking once the slime mold would have populated that as well, we’d get a happy camper.

Picture taken right after inoculation, Wednesday May 3, 2:00am.

So I stayed up late last Tuesday making both non-nutrient and nutrient —oat powder-laced— agar gels and experimented with different setups, the one above among them.

When I checked the slime mold macrofluidic chamber the following day, I got elated: the slime mold had grown one of its filaments and made a connection!

Picture taken the following day (May 3, around 5:00pm)

It was a small connection, but I thought with a day or two more, it would get much bigger.

Two days later, as I checked the chamber, the mold surprised me again. This time, it seemed it had retracted the small tube it had grown to reach the new oat flake. Instead of moving into that new one, it made itself more comfortable in its older home-oat-flake:

Picture taken on Friday May 5, 1:44am

Seeing that the old flake was all slimy and was perhaps going to start developing unfriendly bacterial cultures, I replaced that flake with another one more than twelve hours later:

Picture taken on Friday May 5, 4:23pm

On Saturday: still nothing growing in that direction. I thought: “okay, maybe it doesn’t like my new oat flake for some reason”. That new oat flake came from an ACME brand bag of oats: were they processed in some way that hindered the bacterial culture growth the slime mold feeds on? Really not sure. Especially considering that original flake the mold seemed happy to colonize came from the same bag. I switched brands and tried with a new one regardless, thinking maybe the slime mold would be tempted to find its way back to the newer oat flake and make a connection.

But again, I was wrong. Or maybe I was right: perhaps the slime mold did make its way to the new flakes, quickly ate what it found, and decided to go on foraging for better bacterial pasture from its headquarter flake again. Another —completely different—possibility is that any new flake placed in a zone where the slime mold went (and left its smelly waste products) will quickly get slimy as well. The slime mold might not think it fair game to get back to where it once was, especially if its waste products are somewhat toxic to it. After all, the two pictures above and the one below were taken two days apart: this can be long in slime mold land. A better way to visualize this will involve a non-invasive way to take time-lapse pictures of the mold.

Then the final blow came: the slime mold was obviously trying to jump ship when I took this picture of it yesterday:

The slime mold might have fed on —and seemed fed up with— both oat flakes. In retrospect, it looks like such a small setup can feel constraining for the slime mold. In the hopes of getting the slime to stay and bloom in growth in the etched pattern, I instead found that experimental setups with microfluidic chambers better involve short time spans. The attempt at microfluidic domestication resulted in appreciating how born to be wild that critter is!

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