image: Brilliant structural color in the exoskeleton of the flower beetle, Torynorrhina flammea. Photo courtesy of Ling Li.
Beetles are creatures with built-in body armor. They are tiny tanks covered with hard shells, also known as exoskeletons, protecting their soft, skeleton-less bodies inside. In addition to providing armored protection, the beetle's exoskeleton offers functions like sensory feedback and hydration control. Notably, the exoskeletons of many beetles are also brilliantly colored and patterned, which enhances visual communication with other beetles and organisms.
Ling Li, lead investigator and assistant professor in mechanical engineering, has joined colleagues from six other universities to investigate the interplay between mechanical and optical performance in beetle exoskeletons. They discovered that the structures providing mechanical support are also key players in optical framework. Their findings were published in the Proceedings of the National Academy of Sciences.
The team has come together to answer questions of how the exoskeletal material achieves remarkable mechanical and optical functions at the same time, and which function dominates the structural design at nano- and micrometer scales.
Their focus was narrowed to a specific species: the flower beetle. This small scarab beetle lives in the rainforests of southeast Asia and is noted for displaying brilliant colors, ranging from deep blue to green, to orange and to red. These colorful shells are composed of two main layers that combine for protection, communication, and hydration.
How a beetle's colored armor works
Li and his team launched their research from knowledge of a beetle's shell composition: their outer exocuticle layer contains a unique microstructure with only 1/30 of a millimeter thickness. Its composition is a stack of horizontal nanoscale layers inserted with vertical microscale pillars, providing the exoskeleton with optical coloration and mechanical strength at the same time.
Unlike pigment-based colorations, the optical appearance of the flower beetle results from the exoskeleton's microstructure. The nanolayered region consists of two alternating material compositions, which selectively reflect light of certain colors. This phenomenon is called structural color or photonic color.
Structural color is a common strategy to produce coloration in nature, as seen in butterfly wings, bird feathers, and even some plants and mollusk shells. In 2015, Li and his colleagues discovered that a type of limpet found in Europe develops its iridescent blue color in its shells through a similar multilayered microstructure out of the mineral calcite, the same material found in chalk.
In addition to providing coloration, the exoskeletal shell of beetles needs to be strong and damage tolerant, Li explained. The flower beetle achieves this through reinforcement of its shell's vertical micropillars. When the microstructure is pierced, the shell's micropillars hold a seal around the site of the puncture. This prohibits the beetle's wing from tearing, cracking, or delaminating. The micropillars are also able to spring backward, thus reducing the size of the damage site intruded by the incoming object after unloading.
Micropillars with multiple jobs
Knowing that the mechanical and optical functions were linked, the team sought to discover which of the two were primary.
By collaborating with Mathias Kolle from MIT, the team developed an optical modeling program to simulate the optical response of the beetle's microstructure. They found that the presence of micropillars, while reducing some degree of optical reflection, is able to redistribute the reflected light to a greater angular range. This contributes to the beetle's ability to "send out" the optical signals to its potential receivers.
At the same time, mechanically, the presence of micropillars increases the stiffness, strength, and mechanical robustness of the structure by preventing the formation of shear bands, improving the damage resistance of the outer layer, and localizing damage to the exoskeleton.
After gaining an understanding of the basic mechanisms for optical coloration and mechanical reinforcement, Li and his team studied how the arrangement and size of an exoskeleton's micropillars impact both factors.
They found that a balance had to be struck: If there were many micropillars, the mechanical strength would be improved, Li explained. However, this would degrade the structural color, because the area percentage of the horizontal multilayer would be reduced.
The final objective was to determine which property, optical or mechanical, is more optimized when evolution "designs" the microstructure. To answer this question, the team examined the microstructure of flower beetles from the same species group, but with different colors.
Optical function won the day. They found that the size and distribution of the micropillars in beetles of differing colors were indeed optimized for achieving the most efficient light redistribution. The improvement of mechanical properties, particularly the stiffness, appeared not to be optimized, since the microstructure was not entirely covered with the stiffer and stronger micropillars. This result indicated that optical performance took priority over mechanical performance during the evolution of this peculiar multilayer, micropillar structure.
"This work presents a remarkable example of how nature achieves multifunctionality with unique microstructural designs," said Li. "We believe the material strategies revealed in this work can be used in designing photonic coating materials with robust mechanical performance. Our interdisciplinary approach based on materials, optics, mechanics, and biology also offers an important avenue to understanding the evolution at a materials level."