Researchers at the University of Twente and Utrecht University have packed rigid, rod-shaped particles into soft lipid containers the size of a living cell, and watched the container and its contents reshape each other. The vesicles’s form determines how the rods line up; the tightly packed rods, in turn, bend the container into new shapes. This provides a minimal model for how physical coupling between a soft boundary and internal filaments can help cellular structures organise from within.
Living cells are crowded with filaments. These thread-like scaffolds hold a cell in shape, push it forward when it moves and pull it apart when it divides, all inside a soft membrane that bends and flows around them. The filaments shape the membrane, and the membrane in turn constrains the filaments.
Physicists understand one half of that exchange, but mostly for rigid containers. Pack enough rod-shaped particles into a fixed box and they switch from a disordered jumble to neat alignment, much like matches settling when you shake the box. What happens when the container can give way had barely been tested. A flexible wall can deform to make room for its contents, so the familiar rules no longer hold.
Shape steers order, order bends shape
The team encapsulated rigid silica rods, each about 4.5 micrometres long, inside giant lipid vesicles between 6 and 14 micrometres across. Each vesicle held 10 to 100 rods. Two-colour confocal microscopy let them reconstruct both the vesicle wall and every rod inside it in three dimensions, and computer simulations predicted the same behaviour.
As the rods were packed in more tightly, they moved through three stages. First, a random jumble, then alignment in a single direction, and finally that same alignment arranged into neat layers. An elongated vesicle made the rods line up sooner, at lower densities than a round one would. Geometry alone could tip the contents into order.
The reverse held as well. At high packing, the layered rods pushed back on their container, bending round vesicles into flat, plate-like shapes that take extra energy to hold. Vesicles filled with simple spheres never did this, so the new shapes came from the rods themselves. “The rods do not simply arrange themselves passively. The vesicle’s shape guides how they line up, and the rods in turn reshape the vesicle,” says Hanumantha Rao Vutukuri.
All of this is reversible. By gently changing a vesicle’s volume or the area of its membrane, the researchers moved the rods between jumbled, aligned and layered states, and the vesicle between round, elongated and plate-like, then back again.
Stand-in for a living cell
The vesicle is a stand-in for a living cell, stripped of all its chemistry. That it still organises itself this way shows the coupling needs no biological machinery to appear. Plain physics is enough. The same back-and-forth between filaments and a soft membrane is at work when cells crawl, divide and swallow material from outside, so the model gives biologists a clean way to study those processes one force at a time.
The findings also point the other way, towards making materials. Layered rod structures are normally grown by letting a droplet dry out, which limits the shapes you can reach. A flexible container offers a gentler route, and it opens up forms that a drying droplet cannot make, including the flat, plate-like arrangements seen here.
About the research
The study, “Dynamic bidirectional coupling of membrane morphology and rod organization in flexible vesicles”, has been published in PNAS. It was led by Dr Hanumantha Rao Vutukuri of the Active Soft Matter and Bio-inspired Materials Lab (), part of the University of Twente’s MESA+ Institute and the Faculty of S&T.
PhD researcher Stijn van der Ham (University of Twente) and André F. V. Matias (Utrecht University) are joint first authors, with Marjolein Dijkstra (Utrecht University) as co-corresponding author. The work was supported in part by the Dutch Research Council (NWO) and a European Research Council (ERC) Consolidator Grant.
The publication also marks a personal milestone for Vutukuri’s group: Stijn van der Ham, Vutukuri’s first PhD student, has just graduated after defending his PhD, with this work becoming his fourth high-impact publication, following papers in Nature Communications, ACS Nano, and Nano Letters. Rao says: “I am very proud of my group; after our recent Science paper, this PNAS publication marks our second high-impact study in just two months, making this a very exciting and rewarding period for the team.”
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