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LSSS 2017-2018


Life Sciences Seminar Series


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Silvia Vignolini

University of Cambridge, Cambridge, UK

Structural colours in plants: mechanisms and functions

Selected Publications

Evolutionary-Optimized Photonic Network Structure in White Beetle Wing Scales.Wilts BD, Sheng X, Holler M, Diaz A, Guizar-Sicairos M, Raabe J, Hoppe R, Liu SH, Langford R, Onelli OD, Chen D, Torquato S, Steiner U, Schroer CG, Vignolini S, Sepe A
Adv Mater 2017 Jun 22


Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The "color" white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light. Opaque white materials therefore must be much thicker than periodic structures. It is known that flying insects create "white" in extremely thin layers. This raises the question, whether evolution has optimized the wing scale morphology for white reflection at a minimum material use. This hypothesis is difficult to prove, since this requires the detailed knowledge of the scattering morphology combined with a suitable theoretical model. Here, a cryoptychographic X-ray tomography method is employed to obtain a full 3D structural dataset of the network morphology within a white beetle wing scale. By digitally manipulating this 3D representation, this study demonstrates that this morphology indeed provides the highest white retroreflection at the minimum use of material, and hence weight for the organism. Changing any of the network parameters (within the parameter space accessible by biological materials) either increases the weight, increases the thickness, or reduces reflectivity, providing clear evidence for the evolutionary optimization of this morphology.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets.Frka-Petesic B, Guidetti G, Kamita G, Vignolini S
Adv Mater 2017 Aug; 29(32)


The self-assembly of cellulose nanocrystals is a powerful method for the fabrication of biosourced photonic films with a chiral optical response. While various techniques have been exploited to tune the optical properties of such systems, the presence of external fields has yet to be reported to significantly modify their optical properties. In this work, by using small commercial magnets (≈ 0.5-1.2 T) the orientation of the cholesteric domains is enabled to tune in suspension as they assemble into films. A detailed analysis of these films shows an unprecedented control of their angular response. This simple and yet powerful technique unlocks new possibilities in designing the visual appearance of such iridescent films, ranging from metallic to pixelated or matt textures, paving the way for the development of truly sustainable photonic pigments in coatings, cosmetics, and security labeling.

Development of structural colour in leaf beetles.Onelli OD, Kamp TV, Skepper JN, Powell J, Rolo TDS, Baumbach T, Vignolini S
Sci Rep 2017 May 2; 7(1):1373


Structural colours in living organisms have been observed and analysed in a large number of species, however the study of how the micro- and nano-scopic natural structures responsible of such colourations develop has been largely ignored. Understanding the interplay between chemical composition, structural morphology on multiple length scales, and mechanical constraints requires a range of investigation tools able to capture the different aspects of natural hierarchical architectures. Here, we report a developmental study of the most widespread strategy for structural colouration in nature: the cuticular multilayer. In particular, we focus on the exoskeletal growth of the dock leaf beetle Gastrophysa viridula, capturing all aspects of its formation: the macroscopic growth is tracked via synchrotron microtomography, while the submicron features are revealed by electron microscopy and light spectroscopy combined with numerical modelling. In particular, we observe that the two main factors driving the formation of the colour-producing multilayers are the polymerization of melanin during the ecdysis and the change in the layer spacing during the sclerotisation of the cuticle. Our understanding of the exoskeleton formation provides a unique insight into the different processes involved during metamorphosis.

Disordered Cellulose-Based Nanostructures for Enhanced Light Scattering.Caixeiro S, Peruzzo M, Onelli OD, Vignolini S, Sapienza R
ACS Appl Mater Interfaces 2017 Mar 8; 9(9):7885-7890


Cellulose is the most abundant biopolymer on Earth. Cellulose fibers, such as the one extracted form cotton or woodpulp, have been used by humankind for hundreds of years to make textiles and paper. Here we show how, by engineering light-matter interaction, we can optimize light scattering using exclusively cellulose nanocrystals. The produced material is sustainable, biocompatible, and when compared to ordinary microfiber-based paper, it shows enhanced scattering strength (×4), yielding a transport mean free path as low as 3.5 μm in the visible light range. The experimental results are in a good agreement with the theoretical predictions obtained with a diffusive model for light propagation.

Colour formation on the wings of the butterfly Hypolimnas salmacis by scale stacking.Siddique RH, Vignolini S, Bartels C, Wacker I, Hölscher H
Sci Rep 2016 Nov 2; 6:36204


The butterfly genus Hypolimnas features iridescent blue colouration in some areas of its dorsal wings. Here, we analyse the mechanisms responsible for such colouration on the dorsal wings of Hypolimnas salmacis and experimentally demonstrate that the lower thin lamina in the white cover scales causes the blue iridescence. This outcome contradicts other studies reporting that the radiant blue in Hypolimnas butterflies is caused by complex ridge-lamellar architectures in the upper lamina of the cover scales. Our comprehensive optical study supported by numerical calculation however shows that scale stacking primarily induces the observed colour appearance of Hypolimnas salmacis.

Shape Memory Cellulose-Based Photonic Reflectors.Espinha A, Guidetti G, Serrano MC, Frka-Petesic B, Dumanli AG, Hamad WY, Blanco Á, López C, Vignolini S
ACS Appl Mater Interfaces 2016 Nov 23; 8(46):31935-31940


Biopolymer-based composites enable to combine different functionalities using renewable materials and cost-effective routes. Here we fabricate novel thermoresponsive photonic films combining cellulose nanocrystals (CNCs) with a polydiolcitrate elastomer exhibiting shape memory properties, known as hydroxyl-dominant poly(dodecanediol-co-citrate) (PDDC-HD). Iridescent films of CNCs are first made by evaporation-induced self-assembly, then embedded in the PDDC-HD prepolymer, and finally cured to obtain a cross-linked composite with shape memory properties. The fabricated samples are characterized by polarized optical microscopy, scanning electron microscopy, and thermomechanical cycling. The obtained hybrid material combines both intense structural coloration and shape memory effect. The association of stiff cellulose nanocrystals and soft polydiolcitrate elastomer enhances the overall mechanical properties (increased modulus and reduced brittleness). This hybrid nanocomposite takes advantage of two promising materials and expands their possibilities to cover a wide range of potential applications as multiresponsive devices and sensors. As they perform from room to body temperatures, they could be also good candidates for biomedical applications.

Flexible Photonic Cellulose Nanocrystal Films.Guidetti G, Atifi S, Vignolini S, Hamad WY
Adv Mater 2016 Dec; 28(45):10042-10047


The fabrication of self-assembled cellulose nanocrystal (CNC) films of tunable photonic and mechanical properties using a facile, green approach is demonstrated. The combination of tunable flexibility and iridescence can dramatically expand CNC coating and film barrier capabilities for paints and coating applications, sustainable consumer packaging products, as well as effective templates for photonic and optoelectronic materials and structures.