What fire ants can teach us about making better self-healing materials

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Fire ants form rafts to survive floods. Credit: Robert Wagner

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Fire ants form rafts to survive floods. Credit: Robert Wagner

Fire ants form rafts to survive floods, but how do those connections work? And what can we learn from them? A professor at Binghamton University, State University of New York explores these questions to expand our knowledge of materials science.

When an area where fire ants live is hit by a flood, their survival response is to band together to form a floating ‘raft’ that floats and keeps the colony united. Think of it as a condensed, adaptive material in which the building blocks – individual ants – actually live.

Binghamton University assistant professor Rob Wagner led a study as part of the Vernerey Soft Matter Mechanics Lab at the University of Colorado Boulder, in which researchers examined the adaptive response of these living rafts. The goal is to understand how they change autonomously and change their mechanical properties, and then integrate the simplest and most useful discoveries into artificial materials.

“Living systems have always fascinated me, because they achieve things that our current engineering materials cannot achieve – can’t even achieve,” he said. “We manufacture bulk polymer systems, metals and ceramics, but they are passive. The components do not store energy and then convert it into mechanical work, as any living system does.”

Wagner sees this storage and conversion of energy as essential for simulating the smart and adaptive behavior of living systems.

Experiment to test how fire ant rafts responded to mechanical loading when stretched. Credit: Robert Wagner

In their most recent publication in the Proceedings of the National Academy of SciencesWagner and his co-authors from the University of Colorado examined how fire ant rafts responded to mechanical loading when stretched, and they compared the response of these rafts to dynamic, self-healing polymers.

“Many polymers are held together by dynamic bonds that break but can reform,” Wagner said. ‘If pulled slowly enough, these bonds have time to restructure the material so that – instead of breaking – it flows like the slime our children play with, or like soft-serve ice cream. However, if pulled very quickly, it breaks more like chalk. Because the rafts are held together by ants clinging to each other, their bonds can break and reform. So my colleagues and I thought they would do the same.”

But Wagner and his collaborators found that regardless of the speed at which they pulled the ant rafts, their mechanical response was virtually the same and they never flowed. Wagner speculates that the ants reflexively tighten and extend their grip when they feel force, because they want to stay together. They reject or turn off their dynamic behavior.

This phenomenon of bonds becoming stronger when force is applied is called catch bond behavior, and it likely improves cohesion for the colony, which makes sense for survival.

“If you pull on the typical bonds with any force, they are more likely to loosen and their lifespan decreases. You weaken the bond by pulling on it. That’s what you see in virtually any passive system,” Wagner said.

“But in living systems, because of their complexity, you can sometimes have bonds that last longer under a certain range of applied force. Some proteins do this mechanistically and automatically, but it’s not as if the proteins make a decision. ‘are simply arranged in such a way that when force is applied, these binding sites become visible and snap or ‘catch’.”

Wagner believes that mimicking these catch bonds in engineering systems could lead to artificial materials that exhibit autonomous, localized self-strengthening in areas of higher mechanical stress. This could extend the life of biomedical implants, adhesives, fiber composites, soft robotic components and many other systems.

Collective insect collections such as fire ant rafts are already inspiring researchers to develop materials with stimuli-responsive mechanical properties and behavior. A piece of paper inside Natural materials earlier this year – led by the Ware Responsive Biomaterials Lab at Texas A&M and with contributions from Wagner and his former thesis advisor, Professor Franck J. Vernerey – demonstrated how ribbons made of special gels or materials called liquid crystal elastomers can coil as a result of heating, and then intertwine with each other to form condensed, solid structures inspired by these ants

“A natural progression from this work is to answer the question of how we can make the interactions between these ribbons or other soft building blocks ‘capture’ under load, as the fire ants and some biomolecular interactions do,” Wagner said.

More information:
Robert J. Wagner et al., Capture bond kinetics plays an important role in fire ant raft cohesion under load, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2314772121

Magazine information:
Proceedings of the National Academy of Sciences

Natural materials

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