For the first time, a research team led by Prof. Dr. Alex Plajer at the University of Bayreuth has recreated a fibre system with hierarchical order and controlled bundling in a fully synthetic aqueous system. The starting point is a zinc-containing planar molecule linked to a temperature-responsive polymer. In dilute aqueous solution, these building blocks spontaneously assemble into nanofibres. Remarkably, water molecules themselves become incorporated as structural components of the nanofibres instead of merely acting as a solvent.
When the temperature is raised above 32°C, the initially independent nanofibres bundle together into micrometre-sized structures. The same bundling process can also be triggered by changes in salt concentration or solvent composition. Reversing the respective stimulus, for example by lowering the temperature or adjusting the salt concentration or solvent composition, causes the bundles to dissociate again. The process is therefore fully reversible.
Crucially, this higher-order organisation protects the otherwise fragile building blocks, just as in the cytoskeleton. While individual nanofibres rapidly disassemble under dilution, acidic conditions, or chemical attack, bundled fibres remain stable. “We were even able to demonstrate selective protection of the fibres. In a mixture containing bundled and unprotected fibres, only the bundle-forming fibres survive, while the others disintegrate. This behaviour resembles the selective compartmentalisation observed in cells, where distinct regions or organelles are formed within the cellular environment,” explains Merlin Stühler, first author of the study and a doctoral researcher in Alex Plajer’s group.
This principle became particularly evident in an unusual experiment. Upon irradiation with UV light, the zinc complex heats itself photothermally and maintains the fibres in their protected, bundled state for as long as energy is supplied by the light source. Once irradiation ceases, the bundles dissociate and the building blocks disintegrate under acidic conditions. “In this way, we have created an analogy to mechanisms used by acid-resistant bacteria, which actively expend energy to maintain their internal balance under hostile conditions,” says Merlin Stühler. “Our system literally survives only while energy is being supplied. It therefore exists in a state far from thermodynamic equilibrium, a defining feature of biological systems.”
The hierarchical bundling not only enhances fibre stability but can also be observed with the naked eye. Upon heating, the fibres form a hydrogel at concentrations around 200 times lower than those required in comparable polymer systems. Cooling reverses the process, returning the gel to a liquid state. Gelation can be triggered by light irradiation, while the gelation temperature can simultaneously be controlled through salt concentration or solvent composition. Even in the hydrogel state, the building blocks remain protected against acid and chemical attack.
“Beyond the fundamental insight that biological organisational principles can be transferred to synthetic systems, we see clear application potential for our material. Its reversible, repeatedly stimulus-responsive gelation at comparatively low material concentrations makes the system attractive for 3D printing, where it could serve as a recyclable, ‘switchable’ printing resin,” concludes Prof. Plajer. The work also provides a novel design strategy for sensing and optoelectronics, owing to the light-responsive photophysical properties of the zinc-containing building block, as well as for the development of more robust and intelligent materials in nanotechnology and biotechnology.
