Research Collection Journal Article Alternative moth-eye nanostructures: antireflective properties and composition of dimpled corneal nanocoatings in silk-moth ancestors Author(s): Kryuchkov, Mikhail; Lehmann, Jannis; Schaab, Jakob; Cherepanov, Vsevolod; Blagodatski, Artem; Fiebig, Manfred; Katanaev, Vladimir L. Publication Date: 2017 Permanent Link: https://doi.org/10.3929/ethz-b-000191061 Originally published in: Journal of Nanobiotechnology 15, http://doi.org/10.1186/s12951-017-0297-y Rights / License: Creative Commons Attribution 4.0 International This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Kryuchkov et al. J Nanobiotechnol (2017) 15:61 DOI 10.1186/s12951-017-0297-y Journal of Nanobiotechnology SHORT COMMUNICATION Open Access Alternative moth‑eye nanostructures: antirefective properties and composition of dimpled corneal nanocoatings in silk‑moth ancestors Mikhail Kryuchkov1, Jannis Lehmann2, Jakob Schaab2, Vsevolod Cherepanov3, Artem Blagodatski1,3, Manfred Fiebig2 and Vladimir L. Katanaev1,3* Abstract Moth-eye nanostructures are a well-known example of biological antirefective surfaces formed by pseudoregular arrays of nipples and are often used as a template for biomimetic materials. Here, we provide morphological charac- terization of corneal nanostructures of moths from the Bombycidae family, including strains of domesticated Bombyx mori silk-moth, its wild ancestor Bombyx mandarina, and a more distantly related Apatelodes torrefacta. We fnd high diversifcation of the nanostructures and strong antirefective properties they provide. Curiously, the nano-dimple pat- tern of B. mandarina is found to reduce refectance as efciently as the nanopillars of A. torrefacta. Access to genome sequence of Bombyx further permitted us to pinpoint corneal proteins, likely contributing to formation of the antire- fective nanocoatings. These fndings open the door to bioengineering of nanostructures with novel properties, as well as invite industry to expand traditional moth-eye nanocoatings with the alternative ones described here. Keywords: Moth-eye structures, Antirefective nanocoatings, Biomimetic materials, Silkmoth Introduction the packing order of the nanostructures contribute to the Moth-eye nanostructures—pseudo-regular arrays of anti-refectivity [11–14]. nanopillars frst described on corneal surfaces of the noc- Model insect organisms, permitting genetic manipu- turnal moth Spodoptera eridania [1]—served as a para- lations and/or providing complete information on their digm for bio-inspired antirefective coating with broad genome sequences, have been instrumental in advancing technological applications, from solar cells to art paint- the research on insect molecular, developmental, and cell ings [2–5]. Gradually matching the refractive index of air biology [15]. Following our research on the corneal nano- to that of the lens material, these nanostructures mini- coatings in Drosophila melanogaster [16, 17], we are now mize light refectance and maximize perception. While turning to another famous model insect—the silkmoth physics of the anti-refectance by nanopillar arrays is Bombyx mori. well-understood, permitting reliable simulations [6, 7], More than 1000 diferent B. mori silkworm strains exist nature also designed other types of corneal nanocoatings worldwide, having diferent phenotypic and genomic fea- [8], of which some were shown to play the antirefective tures [18, 19]. Te silkworm was domesticated in East function too [9, 10]. Numerous studies on artifcial nano- Asia from wild B. mandarina moths some 5000 years coatings showed that the shape, dimensions, as well as ago, losing several features essential in the wild habitat on the expense of maximizing silk production [20]. Bom- byx moths’ genomes have been fully sequenced [18, 19], *Correspondence: [email protected] 1 Department of Pharmacology and Toxicology, University of Lausanne, increasing their importance as genetic model organisms. Rue du Bugnon 27, 1011 Lausanne, Switzerland While many aspects of the Bombyx biology have been Full list of author information is available at the end of the article analyzed, these insects have not been previously studied © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Kryuchkov et al. J Nanobiotechnol (2017) 15:61 Page 2 of 7 in terms of their corneal morphology and properties. We corneal surfaces of B. mandarina, to the samples from hypothesized that distinct corneal nanocoatings may be two diferent B. mori strains obtained from Japan (Jp) and found in the wild vs. domesticated silkmoths. Vietnam (Vn, see “Materials and methods”; Fig. 1a–c). Unusually for Lepidopterans, diferent species of which Results and discussion so far have displayed diferent varieties of nanopillars, To explore diferences between corneal nanocoatings in in some species fused into mazes or parallel strands [8], wild and domestic silkmoths, an atomic-force micros- corneal surfaces of B. mandarina reveal a clear nano- copy (AFM) analysis has been performed, comparing the dimpled pattern (Fig. 1d), previously described in insects Fig. 1 Corneal nanocoatings in Bombyx moths. a–c Photographs of B. mandarina (a), B. mori from Vietnam (b) and B. mori from Japan (c). d–f Representative AFM scans (5 5 µm) of corneal surfaces of the Bombyx species presented in a–c. The height dimension of the surface (in nm) is × indicated by the color scale next to (f) with the mean set to zero. g, h Calculation of the height of protrusions (from the lowest point up to the next highest point (g) and their broadness (h) of B. mandarina (in red), B. mori [Vn] (in orange), and B. mori [Jp] (in green); n 50 = Kryuchkov et al. J Nanobiotechnol (2017) 15:61 Page 3 of 7 of other orders, such as earwigs or Carabidae beetles [8]. Curiously, corneae of the two B. mori strains reveal dif- ferent degrees of corruption of this pattern, with the Vn strain depicting a dimple-to-maze transition previously seen in other insects (e.g. bug from Pyrrhocoridae fam- ily [8]), and the Jp strain—a complete degeneration into sporadic irregularities (Fig. 1e–h). Fourier analysis [16, 21, 22] confrms this visual inspection, indicating that the B. mandarina and B. mori [Vn] corneal structures are close to quasi-random, while the B. mori [Jp]—to completely random structures (Additional fle 1: Figure S1). Fully ran- dom structures are known to show less anti-refectance than the quasi-random structures [11, 12], such as those we see in B. mandarina and B. mori [Vn] (Additional fle 1: Figure S1). As B. mori [Jp] corneae further possess randomization in the broadness and depth of the nano- structures (Fig. 1), we may expect even stronger loss of Fig. 2 Antirefective function of corneal nanocoatings from Bom- the anti-refectivity. Using fnite-diference time domain- bycidae moths. a–d 3D AFM representation (3 3 µm) of corneal × based approximations of the Maxwell function, simula- nanocoatings of B. mori [Jp] (a), B. mori [Vn] (b), B. mandarina (c) and tions of the refectance pattern of structured surfaces were A. torrefacta (d). e Ratio of the experimentally measured refection found to match the results received in the process of the spectra to the average refectance of B. mori [Jp] measured for B. mori [Jp] (green), B. mori [Vn] (orange), B. mandarina (red) and A. torrefacta optical experiments [6, 7, 23, 24]. However, such simula- (gray). Data present as mean SD, n 3 (for the experimental data ± = tions in general do not predict antirefective properties of for B. mori [Vn] n 2) nanostructures, whose dimensions are below 30–50 nm = [7, 25], such as those we see in the Bombyx species. Given these uncertainties, we decided to directly measure refectivity from corneal surfaces of the Bom- Turing reaction–difusion model was proposed to govern byx moths. Tis analysis reveals strong reduction of the the formation and diversity of the insect corneal nano- refected light in the broad visible spectrum from the structures [8], which predicts corneal protein(s) to serve dimpled corneal surfaces of B. mandarina and the dim- as the key slow-difusing component of this reaction and ple-to-maze surfaces of B. mori [Vn], as compared to the the building blocks of the resulting structures. To prove irregularly rough surface of B. mori [Jp] (Fig. 2). Remark- directly that corneal proteins are important for formation ably, the antirefective properties provided by the Bombyx of the antirefective nanocoatings, we treated corneae nano-dimpled coating even exceeded those provided by of B. mandarina with a detergent (see “Materials and the nano-pillar arrays of another Bombycidae moth, Apa- methods”), and analyzed the resulting samples for their telodes torrefacta (Fig. 2d, e). While the nano-dimpled B. morphology and anti-refectance. Remarkably, we fnd mandarina arrays (as the nano-pillar A. torrefacta arrays) that removal of proteins purges away the nano-dimpled appear to provide uniform broadband anti-refectivity, coatings (Additional fle 2: Figure S2), correlating with a the dimple-to-maze B. mori [Vn] structures are efcient strong loss of the anti-refective potential of the corneal at low wavelengths and start to decrease anti-refectivity surface (Additional fle 2: Figure S2b). Tese features, at >650 nm (Fig. 2e). With the theoretical assessment of together with the detailed inspection of the remnant these fndings currently missing, we are left to speculate nanostructures’ height and broadness (Additional fle 2: that either the lack of uniformity in the overall morphol- Figure S2c, d), indicate that protein extraction brings ogy of the dimple-to-maze nanostructures (Figs.