Beetles living in the dark teach us how to make sustainable colors

"Extreme scarcity conditions have enabled natural materials to evolve into some of the most extraordinary materials on Earth, such as incredibly strong spider silk and impact-resistant seashells," said Javier Fernandez, Associate Professor of Singapore University of Technology and Design (SUTD).

Throughout history, scientists have consistently turned to nature for inspiration to solve problems and develop new technologies, from da Vinci's flying machines modelled after birds to efficient swimsuits that mimic shark skin. A decade ago, Assoc Prof Fernandez proposed to use nature as not only a source of inspiration for materials science, but a blueprint of how natural molecules must be organised to recreate the extraordinary properties of natural materials. "Matching natural molecules with their native organisation enables their use without modification, resulting in materials that remain fully integrated into the natural ecological cycles," he added.

Assoc Prof Fernandez's research in bioinspired engineering focuses on chitin. As Earth's second most abundant organic molecule, chitin is renewable and part of every ecological cycle. It is also the material nature uses to produce some of its most exceptional structures, such as an insect's light and stiff wings, a seashell's tough exterior, and a butterfly's remarkable colours. Therefore, controlling it has broad implications in engineering due to its versatility and sustainability.

In an earlier study, Assoc Prof Fernandez and his team found that isolated chitin can aggregate and create strong materials while maintaining its optical function. Their latest study, 'Large-scale artificial production of coleoptera cuticle iridescence and its use in conformal biodegradable coatings' built on these results by learning from beetles how to efficiently use chitin to produce colour at large scales. They, however, did not learn it from colourful beetles.

While beetles living on plants use complex structures to produce vibrant and iridescent colours for many tasks, from communicating information to confusing predators, some dark-coloured species living in concealed/dark environments produce faint colour reflections without apparent use for them. It is this mechanism that interests Assoc Prof Fernandez, as it involves simple structures that can be easily implemented in manufacturing processes.

Beetles living in dark environments have their exoskeleton covered by chitin folds, which help them move easily through mud and damp areas. Interestingly, when those folds combine with the melanin-rich background responsible for their dark colour, their cuticles become iridescent, reflecting different colours when exposed to light. The researchers found that the periodicity of the folds is not naturally optimised to produce colour. However, the team could optimise it artificially and was able to produce, with this simplified mechanism, iridescent chitinous colours comparable to those generated by the complex structures of the brightly coloured beetles living on leaves.

This simple construction allowed the team, in just one year, to upscale the production of colour from microscopic samples used as proof of concept, to A4-sized films, the largest example of structural colour produced with its native molecule to date. These results are not just significant in theory, but also technologically relevant. "Since chitin is FDA-approved for medical and cosmetics use, it provides a health and environment-friendly alternative to synthetic materials used in those applications," explained Assoc Prof Fernandez. Adding to their past results on using chitin to produce consumables locally, the team expects to incorporate colour structurally in general manufacturing, removing the need to include artificial dyes.

Moving forward, Assoc Prof Fernandez views bioinspired manufacturing as a mutually beneficial synergy between biology and technology, enabling the technological use of new materials informed by biological designs and helping researchers create controlled models to understand biological systems better.

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