Centre d’Élaboration de Matériaux et d’Etudes Structurales


Accueil > Recherche > NeO : Nano-Optique et Nanomatériaux pour l’Optique > Cristaux liquides pour l’optique

Biomimicry of insect carapaces

Twisted cholesteric liquid crystal patterns are found in the iridescent tessellated cuticles of many insects and a few fruits. Their accurate replication is extremely difficult since discontinuous patterns and colours must coexist in a single layer within continuous structures. We approach the problem of the high-fidelity capture of the structural complexity observed in nature by focusing on patterned, iridescent carapaces of insects with a nin-uniform twisted organization of chitin fibres. We use cholesteric liquid-crystalline materials to mimic biological cholesteric liquid crystals, which choice is yet underutilized in the toolbox of researchers and engineers working in the domain of biomimicry. We attempt to reproduce the textural, structural and color properties of biological structures at several length scales. We made optimal use of resources and stages during the fabrication procedure, in the spirit of eco-design.

Contact : Michel Mitov, mitov[AT]cemes.fr

We focus on the emblematic case of the scarab beetle Chrysina gloriosa. The cuticle of C. gloriosa exhibits green and silver bands with selective and broad light reflections, respectively. Several requirements have to be met : the biomimetic material must be a monolayer ; exhibit a disruptive character of green and silver colors ; display the continuity of the twisted structures in all directions with, in the upper part of the film, an alternate mix of variable and fixed orientations of the helical axis ; include a pitch gradient related to reflection colors ranging from green to near IR ; and exhibit pitch variations in similar ranges.

By means of a single sequence based on self-organization, precise control of a single-piece sample structure composed of two different coloured patterns with the same unique pitch gradient was enabled. A multicriterion comparison reveals a very high level of biomimicry.

Chrysina gloriosa in its juniper habitat, with an ideal cross view of a pair of green and silver stripes.

 In addition to this set of conceptual advances, we present a concrete use of the material for advanced optical tags in cryptography. The functionalities of synthetic and biological materials are relevant to optical communication and camouflage.

The present design involves a high versatility of chiral patterns unreached by the current manufacturing techniques such as metallic layer vacuum deposition, template embossing and various forms of lithography which are limited and often prohibitively expensive.

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Biomimetic sample with a piece of carapace.

 Reference : A. Scarangella, V. Soldan and M. Mitov, Biomimetic design of Iridescent Insect Cuticles with Tailored, Self-Organized Cholesteric Patterns, Nature Communications, 11, 4108 (2020).

https://www.nature.com/articles/s41467-020-17884-0

Funding : ANR (Agence Nationale de la Recherche, France), COLEOPTIX (Grant : ANR-17-CE30-0025).

 

From biomimicry to bioinspiration : time-temperature indicators

 

Previously, we have researched the transfer of strategy and biological properties to artificial films : colors, spectral signature, textures, structures up to function.Mimicry is the quest.By a bio-inspiring approach, we start from the natural structure to create a physical property and function not necessary related to the insect.Cuticles of Chrysina genus suggests us the design of reflectors which in turn inspire the manufacture of tags recording the thermal history of a product.


 Intrigued by the observation of Bragg gratings with a depth-dependent periodicity in the carapace of Chrysina beetles, we have determined the experimental conditions leading to their transcription into synthetic materials. We correlate the optical properties of such reflectors with their internal morphology as observed by transmission electron microscopy. With the use of a single parameter, the reflection color is made time-tunable. Different spectral bands, from golden yellow to near-infrared are available and irreversibility of the final color is reached. On the basis of the design concept and these properties, these hybrid chiral–achiral materials inspire the fabrication of smart reflective labels. When encapsulated in the package of a product to be kept in cold conditions, such a vaccine, the label records the history of the product conservation. Two kinds of information based on color changes are recorded : qualitative information reporting that the product was kept outside of the specified storage temperature and quantitative information giving an indication of the time elapsed since then.

 

Structural analysis of samples for different annealing times. Transmission electron microscopy transverse views accompanied by the half pitch (distance between two bright stripes) as a function of the normalized depth (local depth over the total thickness of the sample). Scale bar = 2 μm.

 

Référence : Cécilia Boyon, Vanessa Soldan, and Michel Mitov, Bioinspired, Cholesteric Liquid-Crystal Reflectors with Time-Controlled Coexisting Chiral and Achiral Structures, ACS Applied Materials & Interfaces, 13, 30118 (2021).

https://pubs.acs.org/doi/full/10.1021/acsami.1c08218

Funding : ANR, projet COLEOPTIX (ANR-17-CE30-0025).