1 The height of chitinous ridges alone produces the entire structural colour palette Hemant Kumar Raut1 Qifeng Ruan1 Cédric Finet23 Vinodkumar Saranathan234 Joel K.W.

2025-04-27 0 0 767.52KB 12 页 10玖币

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The height of chitinous ridges alone produces the entire structural colour palette

Hemant Kumar Raut1†, Qifeng Ruan1†, Cédric Finet2,3, Vinodkumar Saranathan2,3,4, Joel K.W.

Yang1, Javier G. Fernandez1*

1 Engineering and Product Development, Singapore University of Technology and Design,

Singapore

2 Department of Biological Sciences, National University of Singapore, Singapore

3 Division of Science, Yale-NUS College, National University of Singapore, Singapore

4 Division of Sciences, School of Interwoven Arts and Sciences, Krea University, Sricity,

India

*Corresponding author: javier.fernandez@sutd.edu.sg (JGF)

†Equal contribution

H.K.R. now at École polytechnique fédérale de Lausanne, Switzerland

Q.R. now at Harbin Institute of Technology, China

Keywords: chitin, structural color, lepidoptera, ridges, Biomimetic

Abstract

The colourful wings of butterflies result from the interaction between light and the intricate

chitinous nanostructures on butterflies’ scales. This study demonstrates that just by

reproducing the chitinous ridges present in butterfly scales (i.e., without any other secondary

structure), the entire colour palette is achieved. This result was achieved using a new

methodology based on the controlled reproduction of parts of the biological structure of

complex chitinous systems using their native chemistry, enabling the isolation of different

features’ contributions. Here we isolate the contribution of the ridges and their variations as

producing and modulating colour hue. The results suggest that complicated butterfly scales

may be non-ideal solutions for producing colour when multifunctionality is not considered.

Main

Structural colour in arthropod cuticles, particularly on butterfly wings, is one of the most

ubiquitous and striking examples of ostentation in nature, and the nanopatterns producing

these colours have been extensively studied[1]. It is generally assumed that the primary

function of these colour-producing nanostructures in butterfly wing scales is to produce

colour; therefore, their extreme complexity is required for such a task. However, this rather

obvious assumption may be erroneous. Butterfly scales are a multipurpose convolution of

features resulting from constraints and needs that may or may not be related to the many

factors involved in the generation of colour[2]. Therefore, identifying the particular

contributions of each feature of the scales to determine their functionality is central to

understanding the physiological and evolutionary aspects of butterflies.

In bioengineering, the motivation to understand the biology of butterflies is derived from the

goal to artificially reproduce structural colour in chitinous objects[3, 4]. Knowing that

butterfly scales offer the solution to this, the aim of this study was to determine the basic

principles behind this solution, hidden within the multipurpose ‘noise’ of the colour-

producing structures.

Over a decade ago, we demonstrated that chitinous polymers retain their ability to crystallise

and form nanostructures after extraction from the cuticle. This ability can be explored to form

topographies of a few hundred nanometres[5, 6]. To explore the characteristics of chitinous

structural colour, we built upon those results, achieving the control and quality necessary to

make use of the optical properties of the material. This enabled the reproduction of the

structural chitinous ridges without the rest of the scales’ topography, effectively isolating their

contribution to the production of colour.

We used two-photon lithography to produce ridge structures[7] with a horizontal spacing of

3.4 µm to match the interridge distance across several genera[8, 9] (Fig. 1a). We then mapped

the achievable colour produced at the fixed interridge spacing by altering the vertical

dimensions from 150 nm to 2.4 μm. We kept the interridge distance constant because of the

consistency of the horizontal interridge distances across butterfly species and colours[10]. We

focused on the variation of the ridge height[11] inspired by recent evidence supporting the

idea that the alteration of the vertical dimension (i.e., the thickness of the lower lamina) is a

previously unconsidered evolutionary strategy to explore different colourations in

butterflies[12-15].

The structures produced in the synthetic photocurable resin were transferred to a silicone

elastomer through soft lithography and transformed into chitinous structures through a process

that involved casting chitosan (i.e., highly deacetylated chitin), which had been extracted from

shrimp shells and dispersed in a weak acetic acid solution (Fig. 1b). The controlled

evaporation of the solvent resulted in freestanding chitinous films containing a reproduction

of the original ridge-like nanotopography with a height scaled by a factor of 0.73 by the

vertical shrinking of chitosan during crystallisation[16] (Fig. 1c).

These experiments demonstrated that the otherwise transparent chitosan films

(Supplementary Fig. 1a) started to show colour when the ridges were under 400 nm in

height, and they covered the whole spectrum in the next 1.4 µm. Then, as the height continued

to increase, the colour sequence started repeating (Figs. 2a, 2b, Supplementary Fig. 1b).

Despite the strong and lineal correlation between the ridge height and the colour hue, when

these results were reproduced for similar ranges of interridge distances and ridge widths,

those parameters showed negligible influence on the resulting hue compared to the height

(Fig. 3a).

To obtain intuitive insight into the origin of the peaks and dips in the transmission spectra, we

simulated the structures and their effects on a plane wave normally incident on the chitinous

gratings (Fig. 3b, Supplementary Fig. 2). The system can be understood as a thin-film

interference reflector, such as those commonly occurring in butterfly scales [17, 18], in which

two waves propagate through chitinous polymer and air. This interpretation results in

simulated transmission spectra remarkably close to those measured in the physical samples.

When the phase difference between the two paths equals an even (odd) integer of π, the

constructive (destructive) interference of the two waves gives rise to large (small)

transmittance[7]. The phase difference between the two paths increases with the increment in

ridge height, resulting in the redshift in the peaks and dips, which we have reported as a

correlation between the height of the grating and the colour hue. The slight discrepancy

between the measured and simulated spectra could be ascribed to the deviation in the shape of

the ridges from the ideal cuboid and the variation in the refractive index from 1.56 at some

wavelengths[19]. It is worth noting that in the theoretical reproduction of the colour palette,

the supporting chitinous layer is not involved, which is in agreement with the independence of

the colour hue with the thickness of the supporting layer in the fabricated structures. This is

because the phase difference between the two waves is not altered when both waves pass

through the supporting layer of constant thickness before travelling through the chitinous

polymer/air-structured area. This could also be observed on the flat spectra obtained in the

unstructured and low structures (i.e., < 0.3 µm) surfaces (both theoretical and fabricated).

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1TheheightofchitinousridgesaloneproducestheentirestructuralcolourpaletteHemantKumarRaut1†,QifengRuan1†,CédricFinet2,3,VinodkumarSaranathan2,3,4,JoelK.W.Yang1,JavierG.Fernandez1*1EngineeringandProductDevelopment,SingaporeUniversityofTechnologyandDesign,Singapore2DepartmentofBiologicalSciences,Nationa...

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1 The height of chitinous ridges alone produces the entire structural colour palette Hemant Kumar Raut1 Qifeng Ruan1 Cédric Finet23 Vinodkumar Saranathan234 Joel K.W.

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