# Thermal hysteresis and its physical significance in lipid membranes?

We usually associate the phenomenon of hysteresis with memory in the system. For example, magnetic hysteresis is the basis of hard disks(if I recall it correctly). There are also electronic circuits(like Schmitt trigger) which exhibit hysteresis and thus memory.

Now coming to my question. I study about lipid membranes using all-atom classical molecular dynamics(MD) simulations. Recently, I read Sun & Böckmann's 2018 paper Membrane phase transition during heating and cooling: molecular insight into reversible melting (NB: PDF), which reports replication of phase transition in-silico in lipid membranes as a variable of temperature. But the interesting thing is that the phase changes at a temperature(T1) when you increase the temperature, but while reducing the temperature you see phase transition not at T1, but at T2. This is called Thermal Hysteresis.

• My question is what kind of memory this system has?
• Can we use this phenomenon for some practical use?
• And what biological significance does this possibly imply?

## 1 Answer

It seems the system has thermal memory, it is keeping track of whether there are spikes in temperature. It switches 'on' when the temperature is greater than $$~334\,$$K, just like magnetic memory on application of magnetic field.

I think that the system would be used by biological systems as a part of thermometer with memory. Biological systems are adaptive and may need to keep track of the temperature of the environment to activate and turn on certain genes which might be instrumental in a suitable response (like thermal shock). Obviously there are mechanisms within the cell that keep track of temperature, thermal memory just provides a better model for the environment by storing more information, which lets it not just track but also anticipate the environment. A high temperature spike to this system might prepare it for another subsequent spike, by the system not reverting to 'normal' even when the temperature has returned to 'normal' because that is how the system models the environment.

IMHO there might be practical uses for this kind of thing, there are magic metals (like nitinol) which remember their shape at certain temperatures. They can be crumpled, and then when they are heated back to a critical temperature, they deform to their original shape. If these lipids can do so too, then nanotechnology might find some use for them in storing and reconstructing nanostructures, I am just speculating of course.