A Method for Growing Bio-memristors from Slime Mold

The overall goal of this procedure is to standardize the implementation of Physarum polycephalum based memristors, to begin facilitating their integration into technology. Using these methods we can begin investigating the practical applications of Physarum polycephalum based components, outside the laboratory and in the real world. The main advantage of this technique is that it renders the components into a form that is both accessible to non-experts and feasible to integrate into an analytical schematic.

We first have the idea for this method when we developed a musical processing system that harnessed Physarum polycephalum based memristors for real-time performance. By developing a system that was based on these components for outside of the laboratory, issues of practicality had to be addressed. To begin, first load the high-impact polystyrene into the 3D printer.

Then, prepare the 3D printer for use. Set the print bed temperature to 85 degrees Celsius, and the extruder to 230 degrees Celsius. Once at the target temperatures, loosen the idler arm, insert the filament, and push down, until the polystyrene extrudes from the hot end.

Then, re-tighten the filament idler arm and remove the extruded material. Next, in the software control, load the STL model file for the 3D receptacle. And if there are high or low print qualities to choose from, make the print high quality, while also ensuring that the correct material profile is selected.

After collecting the parts, use a thin wire brush to gently clear the electrode socket of any imperfections that may interfere in assembling the device. Now, on the printer, replace the HIPS filament with a cleaning filament. And run an ample volume of material through the print head to ensure a thorough cleaning.

Next, load the printer with an electrically conductive polylactic acid filament that has a low volume resistivity. Then, set the print bed temperature to 60 degrees Celsius, and the extruder to 230 degrees Celsius. When the temperatures are reached, extrude several centimeters of the filament to help ensure that all particulates from previous sessions are removed.

Now, load the STL file for the electrode, and in the print settings, specify the required settings for the layer height, shelf thickness, bottom and top thicknesses, and the fill density. Once printed, let the electrodes cool to room temperature on the print bed before handling them. This ensures the part does not become warped and misshaped.

To assemble the parts, first gently slide the electrode into each of the two chambers. It should go in without much force. Next, using a sharp scalpel, carefully cut a 10mm piece of PVC tubing, making each cut straight and clean.

Then, gently ease each end of the 10mm PVC tubing over the rim of the two electrodes. Once connected, clip the two chambers into the base. In preparation, make two percent non-nutrient agar.

Once the agar has cooled to about 55 degrees Celsius, use a 2mL pipette to transfer agar into the receptacle's chambers. Hover the nib of the pipette about 5mm above the surface and slowly fill the wells up to the bottom of the connecting tube hole, taking care to ensure that no air bubbles are introduced. Immediately after filling the wells, place a lid on each of the chambers and set the receptacle aside until the agar has set and reached room temperature.

Then, place an oat flake on the agar in each chamber. Next, remove one 2mL blob of pseudopods from a 12-hour starved culture of plasmodium. To promote speedy growth, try to take the protoplasm from the most active anterior of the organism.

These receptacles are quite robust and should be ready for use within 12 hours. Receptacles are ready once the plasmodium has forged a connection between the two electrodes. This can be easily distinguished by examining the organism through the clear PVC tube.

Five samples were prepared as described, and monitored via time-lapse imagery to review their growth time. Within 10 hours of inoculation, all five samples connected the two electrodes. One did so in under two hours.

The IV profile of our memristor is its most defining feature. Instantaneous current measurements were made at each point of a 160-step voltage sign wave. Each voltage step had a static dwell time of two seconds.

The experiments were conducted at room temperature in an unlit room. There was a strong relationship in single sample curves measured at different time steps and voltage ranges and in sample to sample curves. Hysteresis loops had relatively consistent lobe sizes as well as pinch locations.

This is reminiscent of the ideal memristor footprint where pinch points are always singular and almost consistently at zero voltage and current. After the initial IV measurements were completed, tests were made on each sample once per day, until they failed to present data in a memristor-like fashion. Of the five samples, all functioned for at least seven days.

Over time, the protoplasmic tubes became thicker and there was a decrease in overall resistance over a day or so, before an increase, with some samples measuring in the 10 to the negative 4th range for amperage during 10 volt runs, despite running in the 10 to the negative 5th range during their earlier tests. After watching this video, you should be able to experiment with biological memristors without having access to well resourced laboratories and expensive equipment. This technique has paved the way for researchers in the field of unconventional computing to explore the use of biological memristors in practical applications.

Once mastered, the receptacles can be set up in under an hour, and growth will be completed within 12 hours. While attempting this procedure, it's important to remember that growth speed is highly dependent on the state of the Physarum polycephalum culture. Following this procedure, further experiments can be performed in order to answer additional questions like, What is the biological mechanism behind Physarum polycephalum memristance?