It is easy to underestimate the brains of insects and yet even the most unremarkable of insects have a repertoire of powerful and more importantly flexible behaviors that are, as of yet, impossible to implement computationally. The humble fruit fly (Drosophila) is arguably the most important model organism in science besides Homo sapiens. Work done on the fruit fly has been the subject of six Nobel Prizes.( Spoiler: My intentions are much less grand).
While working on how to make a grid to give electric shocks to flies(see last log), I came up with the idea of etching a pattern of interdigitated arrays on a piece of copper tape. I had originally intended to cut out thin strips and create the same pattern but I thought of giving this a try. I wasn't very hopeful as I had never etched PCBs before. I ordered a copper tape online and while waiting for it to arrive I made some ferric chloride by dissolving iron nails in dil HCl and then oxidizing it with H2O2.
The copper tape was really good and easy to work with. I pasted it on a transparent plastic film. I printed a schematic of the array on glossy paper using a laser jet printer. The toner transfer was performed using acetone as I could not heat it. It was not very effective but okay-ish so for the second attempt I went a little adventurous and also ironed it at the lowest temp setting. The combination of acetone and iron gave really nice results. If someone is experienced then I think it is possible to get really thin tracks.
The possibilities for making DIY wearables using this technique are immense.
Copper tape(single sided)
Interdigitated array printable image.(made using MS Paint)
6cm x 5cm piece of copper tape on transparent plastic sheet.
After the toner transfer. The small imperfections, a result of either my incompetence or the irregularities of the tape surface or a combination of both, were managed by using a permanent marker.
The etching using FeCl3.
Due to the sticky adhesive on the tape some of the toner was a pain to remove. But the nice thing is that the glue is super strong and resistant to alcohol and acetone.
I had originally underestimated this part of the design. I had thought that a few wires spaced close enough that a fly lands on two at any given time will be easy to fabricate. With this in mind, I (with some help from fellow primates) meticulously placed thin copper wires on a plastic sheet. The edges were glued with silicone(as it had to be non-toxic and resistant to alcohol which would be used to wash the apparatus quite frequently). This sheet was then folded into a cylinder and placed into a disposable syringe as shown in the picture. This is where it all fell apart. The wires shorted, a few came off while others deformed. A major problem seems to be the fact that thin copper wires offer too little surface area for adhesion.
Right now I have two options in mind:
1) Use conductive copper tape(cheap but i have no idea how I am going to cut it into thin strips)
2) Use copper nanoparticle based conductive ink suspended in a two part epoxy. ( cheap and sexy but drawing thin lines with that will be challenge. The issue of durability on a plastic substrate is also an area of concern)
I would really appreciate any and all suggestions that does not involve ordering a custom flexible PCB from China.
In order to condition the flies to a stimulus, a variety of positive or negative treatments can be used. The general finding is that negative reinforcement(like electric shock or quinine which is awfully distasteful) is more effective than positive reinforcement (like sugar water) both in terms of memory retention and number of training runs required. Repeated exposure to quinine is known to reduce the propensity of flies to explore the maze which may be due to a suppression of appetite so I am not going to use that. This also saves me the headache and cost of sourcing quinine hydrochloride which is now rarely used as a medicine. Low current AC shock is less damaging to the flies. The shock is traditionally administered through a grid made out of conductive strips separated by a small distance such that a fly is likely to step on two adjacent strips simultaneously and become part of the circuit. I have plans to build this out of thin copper wires but haven't started on that yet.
The circuit for generating the AC current is simple. It essentially consists of a 555 timer wired in astable mode to generate a square wave of frequency around 50 Hz. This square wave is fed into a step up transformer which will be connected to the grid. The circuit diagram is shown below.
The output of the 555 is fed into the base of a npn transistor BD139. This is not an ideal choice as I found out it heats up quite significantly(though not enough to cause catastrophic failure). I will change this with a MOSFET in the future. The step-up transformer is actually a 220v - 12 v step-down transformer connected in reverse. For low current applications this works well although the losses are quite high as the "primary" coil has a high resistance. I manage to get around 50V at the output.
For ease of use, I conceptualized a unit detachable arms to allow cleaning(because with insects olfactory cues are a major confound) and customization. To this end, I designed the thing in the shape of a trapezoid with glass sides and top and bottom for visibility and adapters for each of the three arms. The adapters were made by drilling holes into plastic caps. The adapters were glued onto the front and back walls of the chamber. This required these two walls to have holes. Now from the pictures you could tell that my glass-working skills are not that great so I did not attempt drilling holes onto glass. Since visibility from this aspect was not a prerequisite, I made these part out of epoxy, a technique that I had learned while working on a previous project. The sides of the chamber were put together using RTV silicone glue since this was the most effective adhesive for glass. The plastic caps(adapters) were glued using cyanoacrylate.
The glass pieces.
The front and back made out of epoxy. The holes were cut out while the epoxy was curing using the plastic caps which were to be attached in them.
There have been many(though not that many) behavioral experiments on Drosophila. The simplest of these rely on a Pavlovian conditioning involving two stimuli associated with either a positive or negative(or neutral) outcome. This is implemented by a T or Y-maze where each arm of the maze has a particular stimuli and its associated outcome. For an animal to be conditioned in this manner it must possess two crucial cognitive abilities-- associative memory and long term memory. These experiments have been used to study fly models of human diseases such as Alzheimer's. I intend to build a Y-maze to initially replicate some of these classic experiments and then design so new ones and hopefully learn something new. In particular, I am interested in investigating whether flies can count. You read that right. I am talking about a numerical sense in flies. This is not a crazy thing to ask especially when you consider how crucial an ability it is to be able to count and do arithmetic. It is an undeniable selective advantage and may have evolved quite early in evolution. Still not convinced? Check this
2019 paper out by Howard et al . They report that honey bees possess a rudimentary ability to not just count but also do simple arithmetic. Honey bees are small and their brains are even smaller but they perform complicated navigational maneuvers and are social. Fruit flies are in comparison much simpler(and have smaller brains). Can they perform these same feats?