Butterfly Wings Hold the Key to Revolutionary New Materials: MIT Researchers Uncover the Secretsof Scale Formation
Cambridge, MA – Researchers at MIT have discovered arevolutionary new way to understand the formation of butterfly wing scales, potentially leading to the development of innovative materials with unique optical, thermal, and mechanical properties. Their findings, published in the journal Cell Reports Physical Science, reveal how the intricate ridges on butterfly scales form through a process called buckling.
Butterfly wings are covered inhundreds of thousands of tiny scales, resembling miniature tiles on a paper-thin roof. These scales, though microscopic, possess remarkable complexity. Their corrugated ridges contribute to water absorption, heat dissipation, and light reflection, giving butterfly wings their dazzling iridescence.
MIT researchers have now captured the initial moments of a butterfly’s metamorphosis, when these scales begin to develop their distinctive ridge patterns. Using advanced imaging techniques, they observed the microscopic features on the wings of butterflies as they transformedwithin their chrysalises.
The team captured images of individual scales emerging from the wing membrane, revealing how their initially smooth surfaces begin to wrinkle, forming tiny parallel undulations. These corrugated structures eventually grow into the intricate ridges that define the function of the mature scales.
Buckling is an instability that, asengineers, we typically try to avoid, explained Mathias Kolle, Associate Professor of Mechanical Engineering at MIT. But in this case, the organism is using buckling to initiate the growth of these intricate, functional structures.
The researchers discovered that the transition of the scales from a smooth to a corrugated surface is likely the result ofbuckling – a general mechanism that describes how smooth surfaces wrinkle when they grow within a confined space.
Nature’s Engineering Inspiration
The MIT team is working to visualize more stages of butterfly wing growth, hoping to gain insights that could guide the design of advanced functional materials in the future.
Given the multifunctionality of butterfly scales, we want to understand and mimic these processes to sustainably design and manufacture novel functional materials, Kolle added. These materials will exhibit tailored optical, thermal, chemical, and mechanical properties for applications in textiles, building surfaces, vehicles – really anything that requires surfaces that perform based on their micro- andnanoscale structural features.
The research team, led by Jan Totz, a former postdoc at MIT, and Anthony McDougal, a current postdoc, meticulously dissected the thin, paper-like chrysalis of the painted lady butterfly (Vanessa cardui), exposing the growing wing membrane. They placed a smallglass slide over the exposed area and used a specialized microscope developed by Peter So, Professor of Mechanical Engineering and Biomedical Engineering at MIT, to capture continuous images of the scales emerging from the wing membrane.
This method allowed them to observe the scales growing in precise, overlapping patterns, similar to tiles on a roof. The imagesprovided the most continuous visualization of live butterfly wing scale growth at the microscopic level.
The Mechanism of Ridge Development
In their new study, the team focused on a specific time window in scale development, capturing the initial formation of the intricate ridges on individual scales in a live butterfly. Scientists knew that these ridges,running parallel along the length of each scale, are responsible for many of the wing scale’s functions.
With limited knowledge about how these ridges form, the MIT team aimed to record the continuous formation of the ridges in a developing butterfly and decipher the mechanism behind this biological phenomenon.
We looked at butterfly wing developmentover 10 days and took thousands of measurements of the surface changes of a single butterfly scale, said McDougal. We could see that the early scale surface was very smooth. As the butterfly grew, the scale surface started to undulate a little bit, and then, at around 41% ofdevelopment, we saw this very regular pattern of the fully undulated, primordial scale. The whole process took about 5 hours and set the structural foundation for the patterned ridges that would follow.
Investigating the Cause of Buckling
What caused these initial ridges to appear in such a precise arrangement? The researcherssuspected that buckling might be at play. Buckling is a mechanical process where a material bends inward when subjected to compressive forces. For example, an empty soda can buckles when pressed down from the top. Buckling can also occur when a material grows under constraint or when it is fixed.
The scientists observed that as thebutterfly scale’s cell membrane grew, it was effectively fixed in places by bundles of actin – long filaments that run beneath the growing cell membrane and act as a supporting scaffold as the scale takes shape. The researchers theorized that the constraint imposed by the actin bundles on the growing membrane was similar to the ropes on a hotair balloon. They proposed that as the butterfly wing scale grew, it would buckle between the underlying actin filaments, forming the initial parallel ridges of the scale in a buckling manner.
A Theoretical Model of Scale Formation
To test this idea, the MIT team investigated a theoretical model that describes the general mechanics of buckling. They incorporated image data into the model, such as measurements of the scale membrane’s height at different early stages of development, as well as the varying spacing of the actin bundles across the growing membrane. They then ran the model forward in time to see if its fundamental mechanics of buckling would produce the same ridge patterns observedin the actual butterflies.
Through this modeling, the researchers found that we can go from a flat surface to a more undulated surface. From a mechanical standpoint, this suggests that buckling of the membrane is most likely the cause of these strikingly ordered ridges.
Conclusions and Implications for Materials Science
We wantto learn from nature, not just the function of these materials but also how they are formed, said McDougal. Say, for example, you want to make a wrinkled surface, which is useful for a variety of applications, this provides two very easily tunable ‘knobs’ to customize how you wrinkle thosesurfaces. You can change the spacing of the things that are fixing the material, or you can change how much material grows between those fixed points, and we found that the butterfly uses both of those strategies.
The research team’s findings have significant implications for the development of new materials with tailored properties. By understanding themechanisms behind the formation of butterfly wing scales, researchers can potentially mimic these processes to create materials with enhanced optical, thermal, and mechanical properties. This could lead to innovations in various fields, including textiles, construction, and transportation.
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