How Polymers and People Remember

There has been a recent innovation out of a General Motors research lab that has captured my attention, and it has nothing to do with fast new cars or wasted opportunities on fuel efficient ones  (http://www.technologyreview.com/energy/24718/?a=f). Actually it is a way in which they are using a unique polymer that sound to many people like science fiction (think shape changers). The basic idea is that this particular polymer has the ability to become up to 4 distinct shapes while maintaining the ability to return to its original shape. These shapes are activated by temperature. So, if exposed to one temperature, they become one shape, and when exposed to another, they become another. The report headlines are saying these are polymers with “memory”. Actually to me, who works with polymers like elastomers, the idea of our materials having memory is nothing new. In fact we rely on that memory for creating parts made from them, and everyone needs this in using them. This is everything from heart valves to car tires. We say memory in our lectures when talking about elasticity. That is, something that is elastic appears to have a memory of sorts. Imagine a scale for instance. It is basically a metallic spring inside a box. When you stand on the scale a spring compresses, taking on a different shape, and giving a weight reading on a dial or screen. When you step off of the scale the spring returns to its original form, and the display returns to 0. So we could in a way say that the spring in the scale has a memory of what it was like to not have a person standing on it. When we are talking about elastomers, the material “memory” is a bit more nuanced. An elastomer doesn’t fully recover its original form, like the spring, because it is not just elastic; it is visco elastic. It behaves not just like a spring, but also like a liquid. This is all a graduate course, and one that I love teaching, but it is a long story. An elastomer does almost recover, but not entirely. We still say that if it is close enough (imagine a rubber band) it does have memory. It is just that the memory is not as perfect as the memory of the spring. It is however capable of doing a lot more interesting things, including taking on new shapes, and new properties. The reason this all works is due to the molecular nature of polymers. Polymers are made of many long chain molecules that work together. When they are at rest they are generally entangled around each other into a rat’s nest of strings. Many polymer scientists use the picture of a bowl of spaghetti when visualizing this. When they are pulled, or sheared, they tend to straighten out, becoming less tangled. When released they go back to the tangled up state. Hence the tangled up state is statistically and to our eyes similar to what it was before being pulled apart. We can say then that these long molecules remembered where to go. Truthfully though they didn’t go exactly back to the positions they were in before. They went just close enough. This is why the memory is not perfect.

This metaphor to memory, or suggestion that it is like animal memory (including human memory), is not really such a stretch. I have written before about re- consolidated memories in people http://putmanonart.blogspot.com/2009/05/re-consolidated-theatre.html. The idea is that we never truly remember any event from our past, but instead an approximation of that memory based on the last time we remembered it. If we now imagine the brain, we see a tangled web of axons and dendrites, and memories which are occurring through neural synapses. This also looks a bit like a bowl of spaghetti. These are electrically charged chemically induced connections between neurons (brain cells). When we remember an event the firing of these synapses never arrive at the receptors in the exact way that they did the previous time. So in essence we are like the elastomer. We remember where we have been, but only to a statistical accuracy that is fairly good.

There is another similar thing with elastomeric polymers and the human brain. The memory tends to get worse with age. Even the best elastomers experience fatigue and degradation throughout time. A tire for instance can work for 100,000Km but will eventually wear out. This may very well be like our brains as well. 

This blog may very well be the last bit of evidence that people will need when saying I am stuck in a lab too much. I am comparing the complexity of humanity to a piece of rubber. They may be right, but the idea is not to diminish the brain. In fact it is that spaghetti bowl in our heads that allowed us to figure out how to use materials to do these complicated things. It is just that polymer technology and neuroscience are generally not linked in the way way other biomimicy technologies are. This blog is actually again an admiration of nature, which did something spectacular before we even knew it was happening.