So how exactly do you teach a piece of plastic to react to light?
The research behind this thing has also been presented in a paper in the journal Matter, a paper that fortunately is freely available (kudos!). According to the paper the piece of plastic in question is something called a liquid crystal polymer network. Polymers are long molecules consisting of smaller, identical pieces repeating in a long chain - cellulose for instance is a polymer consisting of long chains of identical glucose molecules. Plastics are generally polymers, like polyethene (chains of ethene molecules) or polystyrene (chains of styrene molecules - "poly" here just means "multiple" or "many").
Liquid crystals are also long molecules, some of them are in fact also polymers. The unique thing about them is that they behave partly like liquids, in that they flow between containers and change shape to fill the container, and partly like (what physicists mean when they talk about) crystals, in that the molecules arrange themselves in regular repeating patterns. Liquid crystals have a whole bunch of fascinating properties, in fact the last post on this blog was about their optical properties and what they could mean for lidars.
There are multiple earlier studies showing how you can connect a network of polymer molecules, which gives rigidity and structure to the final material, with liquid crystals (for instance there is this review article, although it's fairly technical). This can for example give you materials that bend in a specific way when heated, like the walking plastic does. The reason is that when the otherwise so well-ordered liquid crystal molecules are heated they move from their well-ordered positions to something more disorderly. If they would start out lying after one another in a long line, increased disorder would mean the line would start to bend and therefore cover a smaller distance. This part of the material would then contract. On the other hand, if the liquid crystal molecules are lying parallel to each other increased disorder may require them to move apart, so this part of the material will expand. In the walking piece of plastic one side has the liquid crystal molecules arranged in lines, while the other has them more or less parallel, so when the material is heated one side will contract and the other will expand. This leads to the material bending significantly when heated
We thus start out with a plastic materal that bends in a very specific way when it is heated. But how has it been made to react to light as well? The paper reveals that one side of the material was coated with a dye that absorbs light in a certain range of wavelengths. The absorbed light causes the dye to heat up and transfer heat to the surroundings, in this case the plastic. However, when it is just in a layer on the surface of the plastic it does not cause a temperature increase large enough to get the plastic to bend. Instead, the material was exposed to both light and heat at the same time. The heat lead both to the plastic bending and to the dye diffusing into the plastic (like the red pigment in tomato sauce spreading into the plastic of your leftover containers - if you use a plastic containers, that is). The more the dye diffused and spread into the plastic, the more efficiently it contributed to heating the plastic, further increasing the temperature. Once the dye was spread more or less evenly throughout the plastic, heating via the dye was efficient enought that once the material was cooled down and straightened out, shining a light on it caused enough of a temperature increase for it to bend again.
So does this mean that the piece of plastic has been taught something? The scientists themselves compare the process to conditioning and draw parallels to Pavlov's experiments on dogs. Pavlov's experiments showed that the dogs, who normally started to drool when given food, could also be made to drool at the sound of a bell, provided that they were first conditioned by hearing the bell while they were being fed. The scientists compare the heat that causes the plastic to bend to the food, the light to the bell and the simultaneous exposure to light and heat to the conditioning phase.
They also admit that there are problems with this comparison and that the plastic is (obviously) a much simpler system. An important difference is that Pavlov's dogs reacted also without any food being present, while the plastic isn't actually bending in the absence of heat - the heat is just provided in a different way. Another difference is that the piece of plastic cannot spontaneously "forget" its "conditioning" the way a living organism would - the dye will stay where it is. One could also have skipped the "conditioning" altogether and just mixed in the dye from the start, or exposed the material to only heat (rather than heat and light) to get the dye into the plastic, and achieved the same end result. All things considered, it is doubtful whether this should really be seen as a form of conditioning, and thereby learning. Maybe this research will give rise to a simple material model for some kinds of learning, but it is clearly very far from the kinds of learning seen in organisms.
