Glassy dynamics as a predictive framework for lipid exchange across membranes

Biological processes that govern our lives are many, intertwined, and often difficult to understand. They involve countless interactions happening at once—molecules recognizing each other, signals being transmitted, and matter being transported with precise timing—making the underlying physical rules complex and hard to disentangle.

Physicists try to understand this complexity by building models that describe biological systems in terms of their basic components and interactions. Because these components are numerous and involve different physical aspects acting together, such models can capture biological behavior more or less accurately, often at the cost of becoming increasingly complicated and less predictive.

A useful way to approach this complexity is to look for simple organizing principles that capture what many processes have in common. One such principle is that life depends on boundaries. A concrete example is found in every living cell, which is wrapped in a fragile membrane made of lipids—fat-like molecules that separate the inside from the outside while still allowing exchange and communication.

These lipid membranes are far from static. They constantly renew themselves as lipid molecules slowly move from one membrane to another, a process that plays a role in cell communication, membrane remodelling, viral entry, and the functioning of lipid-based drug carriers. Yet, despite decades of research, predicting how fast such exchanges occur—and why different membranes behave so differently—has remained a major challenge.

In a paper published in Small, researchers from the EST laboratory at the Université libre de Bruxelles, led by Patricia Losada-Pérez and Simone Simon Napolitano, show that this apparent complexity can, in fact, be described in much simpler terms. Their work demonstrates that membrane dynamics can be summarized by a single, measurable thermodynamic quantity.

That quantity is how much a membrane expands when gently warmed. While this may sound like a purely physical detail, it turns out to encode both how difficult it is for the membrane to rearrange and how fast molecular exchange can occur. In other words, a soft equilibrium property provides direct insight into a dynamic biological process.

The reason lies in how membranes actually change. The study shows that lipid exchange is not a simple molecular hop from one membrane to another. Instead, it is a collective process: a lipid can leave only when many neighboring molecules rearrange together. Exchange occurs during rare moments when the membrane briefly loosens, allowing coordinated motion to take place.

This behavior closely resembles what physicists observe in glassy materials, such as everyday plastics, where molecules move through many small, cooperative displacements rather than smooth, continuous flow. Developed in collaboration with American theorists, the model places biological membranes within a broader physical framework that has already proven successful in other areas of soft-matter physics.

This thermodynamic framework is not limited to membranes. The same model has already been successfully used to describe how molecules stick onto surfaces, how they rearrange over time, and how they crystallize. In doing so, it has helped clarify the stability of pharmaceutical compounds and the long-term behavior of materials used in organic electronics—systems where predicting slow rearrangements is essential for performance and reliability. Applying this approach for the first time to molecules that are central to biological processes opens the way to extending these ideas to many other biological mechanisms, with the prospect of not only understanding them more deeply, but also learning how to control their behavior.