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Maximizing the scattering of visible light within disordered nano-structured materials is essential for commercial applications such as brighteners, while also testing our fundamental understanding of light-matter interactions. The progress in the research field has been hindered by the lack of understanding how different structural features contribute to the scattering properties. Here we undertake a systematic investigation of light scattering in correlated disordered structures. We demonstrate that the scattering efficiency of disordered systems is mainly determined by topologically invariant features, such as the filling fraction and correlation length, and residual variations are largely accounted by the surface-averaged mean curvature of the systems. Optimal scattering efficiency can thus be obtained from a broad range of disordered structures, especially when structural anisotropy is included as a parameter. These results suggest that any disordered system can be optimised for whiteness and give comparable performance, which has far-reaching consequences for the industrial use of low-index materials for optical scattering.

The coral-algal symbiosis is the biological engine that drives one of the most spectacular structures on Earth: the coral reef. Here, living coral microhabitats are engineered using 3D bioprinting, as biomimetic model system of the coral-algal symbiosis. Various bioinks for the encapsulation of coral photosymbiotic microalgae (Breviolum psygmophilum) are developed and coral mass transfer phenomena are mimicked by 3D bioprinting coral tissue and skeleton microscale features. At the tissue–seawater interface, the biomimetic coral polyp and connective tissue structures successfully replicate the natural build-up of the O2 diffusive boundary layer. Inside the bioprinted construct, coral-like microscale gastric cavities are engineered using a multi-material bioprinting process. Underneath the tissue, the constructs mimic the porous architecture of the coral aragonite skeleton at the micrometer scale, which can be manipulated to assess the effects of skeletal architecture on stress-related hydrogen peroxide (H2O2) production. The bioprinted living coral microhabitats replicate the diffusion-related phenomena that underlie the functioning and breakdown of the coral-algal symbiosis and can be exploited for the additive manufacturing of synthetic designer corals.

This review paper provides a recent overview of current international research that is being conducted into the functional properties of cellulose as a nanomaterial. A particular emphasis is placed on fundamental and applied research that is being undertaken to generate applications, which are now becoming a real prospect given the developments in the field over the last 20 years. A short introduction covers the context of the work, and definitions of the different forms of cellulose nanomaterials (CNMs) that are most widely studied. We also address the terminology used for CNMs, suggesting a standard way to classify these materials. The reviews are separated out into theme areas, namely healthcare, water purification, biocomposites, and energy. Each section contains a short review of the field within the theme and summarizes recent work being undertaken by the groups represented. Topics that are covered include cellulose nanocrystals for directed growth of tissues, bacterial cellulose in healthcare, nanocellulose for drug delivery, nanocellulose for water purification, nanocellulose for thermoplastic composites, nanocellulose for structurally colored materials, transparent wood biocomposites, supercapacitors and batteries.

The self-assembly of block copolymers (BCPs) into photonic materials has drawn significant attention due to the flexibility and diversity of the building blocks that can be synthesized. This review provides a brief overview of the development of this research area and then discusses in detail recent advances in the use of BCPs as photonic pigments. In particular, the role of confinement upon the self-assembly process to form well-ordered lamellae is described and compared to correlated disordered structures derived from densely packed micelles. These systems respectively act as 1D (multilayered) photonic crystals and photonic glasses and are here critically compared in terms of their structural characteristics and optical performance. Finally, based on this understanding, the current challenges in this field are discussed and the potential for future developments is explored.

Corals have evolved as optimized photon augmentation systems, leading to space-efficient microalgal growth and outstanding photosynthetic quantum efficiencies. Light attenuation due to algal self-shading is a key limiting factor for the upscaling of microalgal cultivation. Coral-inspired light management systems could overcome this limitation and facilitate scalable bioenergy and bioproduct generation. Here, we develop 3D printed bionic corals capable of growing microalgae with high spatial cell densities of up to 109 cells mL-1. The hybrid photosynthetic biomaterials are produced with a 3D bioprinting platform which mimics morphological features of living coral tissue and the underlying skeleton with micron resolution, including their optical and mechanical properties. The programmable synthetic microenvironment thus allows for replicating both structural and functional traits of the coral-algal symbiosis. Our work defines a class of bionic materials that is capable of interacting with living organisms and can be exploited for applied coral reef research and photobioreactor design.

The formation of exciton-polaritons allows the transport of energy over hundreds of nanometres at velocities up to 10^6 m s^-1 in organic semiconductors films in the absence of external cavity structures.

The understanding of the interaction between light and complex, random structures is the key for designing and tailoring the optical appearance and performance of many materials that surround us, ranging from everyday consumer products, such as those for personal care, paints, and paper, to light diffusers used in the LED-lamps and solar cells. Here, it is demonstrated that the light transport in membranes of pure cellulose nanofibrils (CNFs) can be controlled to achieve bright whiteness in structures only a few micrometers thick. This is in contrast to other materials, such as paper, which require hundreds of micrometers to achieve a comparable appearance. The diffusion of light in the CNF membranes is shown to become anomalous by tuning the porosity and morphological features. Considering also their strong mechanical properties and biocompatibility, such white coatings are proposed as a new application for cellulose nanofibrils.

A common strategy to optimize whiteness in living organisms consists in using three-dimensional random networks with dense and polydisperse scattering elements constituted by relatively low-refractive index materials. Inspired by these natural architectures, we developed a fast and scalable method to produce highly scattering porous polymer films via phase separation. By varying the molecular weight of the polymer, we modified the morphology of the porous films and therefore tuned their scattering properties. The achieved transport mean free paths are in the micrometer range, improving the scattering strength of analogous low-refractive index systems, e.g. standard white paper, by an order of magnitude. The produced porous films show a broadband reflectivity of approximately 75 % whilst only 4 m thick. In addition, the films are flexible and can be readily index-matched with water (i.e. they become transparent when wet), allowing for various applications such as coatings with tunable transmittance and responsive paints.

The bright and intense blue-green coloration of the fruits of $\textit{Margaritaria nobilis}$ (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.

The marine world is incredibly rich in brilliant and intense colours. Photonic structures are found in many different species and provide extremely complex optical responses that cannot be achieved solely by pigments. In this study we examine the cuticular structure of the red alga Chondrus crispus (Irish Moss) using anatomical and optical approaches. We experimentally measure the optical response of the multilayer structure in the cuticle. Using finite-difference time-domain modelling, we demonstrate conclusively for the first time that the dimensions and organisation of lamellae are responsible for the blue structural colouration on the surface of the fronds. Comparison of material along the apical-basal axis of the frond demonstrates that structural colour is confined to the tips of the thalli and show definitively that a lack of structural colour elsewhere corresponds with a reduction in the number of lamellae and the regularity of their ordering. Moreover, by studying the optical response for different hydration conditions, we demonstrate that the cuticular structure is highly porous and that the presence of water plays a critical role in its ability to act as a structural light reflector.

The gyroid is a continuous and triply periodic cubic morphology which possesses a constant mean curvature surface across a range of volumetric ll fractions. Found in a variety of natural and synthetic systems which form through self-assembly, from buttery wing scales to block copolymers, the gyroid also exhibits an inherent chirality not observed in any other similar morphologies. These unique geometrical properties impart to gyroid structured materials a host of interesting optical properties. Depending on the length scale on which the constituent materials are organised, these properties arise from starkly di erent physical mechanisms (such as a complete photonic band gap for photonic crystals and a greatly depressed plasma frequency for optical metamaterials). This article reviews the theoretical predictions and experimental observations of the optical properties of two fundamental classes of gyroid structured materials: photonic crystals (wavelength scale) and metamaterials (subwavelength scale).

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We present a phase-sensitive near-field study of speckle fields from photonic crystals in the presence of disorder. We observe phase singularities (vortices) and analyze their statistical properties and screening effects. The experimental results show a clear polarization dependence, not only in their morphological parameters but also in their spatial distribution.

A complete control of the coupling between the modes of two photonic crystal microcavities is obtained by using a combination of several local tuning techniques. By a local and controlled modification of one cavity we are able to bring into resonance modes with a different polarization and spatial distribution, producing heteroatomic photonic molecules. Clear anticrossing is observed, denoting the mode hybridization which is in a very good agreement with finite difference time domain calculations.

Complex hierarchical architectures are ubiquitous in nature. By designing and controlling the interaction between elementary building blocks, nature is able to optimize a large variety of materials with multiple functionalities. Such control is, however, extremely challenging in man-made materials, due to the difficulties in controlling their interaction at different length scales simultaneously. Here, hierarchical cholesteric architectures are obtained by the self-assembly of cellulose nanocrystals within shrinking, micron-sized aqueous droplets. This confined, spherical geometry drastically affects the colloidal self-assembly process, resulting in concentric ordering within the droplet, as confirmed by simulation. This provides a quantitative tool to study the interactions of cellulose nanocrystals beyond what has been achieved in a planar geometry. Our developed methodology allows us to fabricate truly hierarchical solid-state architectures from the nanometer to the macroscopic scale using a renewable and sustainable biopolymer.

By a simple two-step procedure, large photonic strain sensors using a biocompatible cellulose derivative are fabricated. Transient color shifts of the sensors are explained by a theoretical model that consideres the deformation of cholesteric domains, which is in agreement with the experimental results. The extremely simple fabrication method is suitable for both miniaturization and large-sale manufacture, taking advantage of inexpensive and sustainable materials.

Aqueous colloidal suspensions, both man-made and natural, are part of our everyday life. The applicability of colloidal suspensions, however, is highly limited by the range of conditions over which they are stable. Here, we report a novel type of highly monodisperse ‘raspberry’ colloids, which are prepared in a single-step synthesis that relies on simultaneous dispersion and emulsion polymerisation. The resulting raspberry colloids behave almost like hard spheres. In aqueous solutions such prepared raspberries show unprecedented stability against aggregation over large variations of added salt concentrations without addition of surfactants or other stabilisers. We present simple DLVO-calculations performed on raspberries and smooth colloids showing that this stability results from our raspberries’ unique morphology preventing salt-induced colloidal aggregation, which extends our understanding of colloidal stability against salting. Our calculations are supported by salting experiments using a variety of salts and differently sized colloids with varying ‘roughness’. Further, the raspberies’ stability facilitates the formation of superspheres and thin films in which the raspberry colloids self-assemble into hexagonally close-packed photonic crystals with exquisite reproducibility. Introduction The ability to disperse charged colloids in water is generally determined by the interplay between attractive van der Waals and, YL thanks the CSC Cambridge scholarship for financial support. AC thanks D. Frenkel for discussions and acknowledges the ETN-COLLDENSE (H2020-MCSA-ITN-2014, Grant No. 642774). SV acknowledges the BBSRC David Phillips fellowship (BB/K014617/1), the European Research Council (ERC-2014-STG H2020 639088), and B. Frka-Petesic for discussions. GG thanks the Engineering and Physical Sciences Research Council (EPSRC, 1525292), VEJ the European Commission (Marie Curie Fellowship LODIS , 701455), JL the Marie Curie FP7 SASSYPOL ITN (607602), and MK EPSRC (EP/L027151/1) for financial support. EE, YL and SV thank the Winton Programme for the Physics of Sustainability.

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.

Structural colouration is widespread in the marine environment. Within the large variety of marine organisms, macroalgae represent a diverse group of more than 24,700 species. Some macroalgae have developed complex optical responses using different nanostructures and material compositions. In this review, we describe the mechanisms that are employed to produce structural colour in algae and provide a discussion on the functional relevance by analysing the geographical distribution and ecology in detail. In contrast to what is observed in the animal kingdom, we hypothesise that structural colour in algae predominantly functions for a non-communicative purpose, most likely protection from radiation damage, e.g. by harmful UV light. We suggest that the presence of structural colour in algae is likely influenced by local factors such as radiation intensity and turbidity of the water.

Funder: Chinese Scholarship Council : Cambridge Scholarship, Funder: Croucher Foundation : Cambridge International Scholarship, Hydroxypropyl cellulose (HPC) is a widely commercialized cellulose derivative. While it is typically used as a binder or stabilizer for foods and pharmaceuticals, it can also form a cholesteric liquid crystal in aqueous solution. Moreover, at high HPC concentrations this lyotropic and thermotropic mesophase is known to reflect structural color. However, it remains a challenge to retain this vibrant coloration into the solid state. Here, by combining the emulsification of a HPC mesophase with drying at elevated temperature, we produce solid microparticles that can reflect color across the visible spectrum, from blue to green and red. This method provides a facile and scalable pathway to fabricate structurally colored, edible pigments, which can displace existing synthetic additives used in a wide range of foods and cosmetics.

Over the past 150 years, our ability to produce and transform engineered materials has been responsible for our current high standards of living, especially in developed economies. However, we must carefully think of the effects our addiction to creating and using materials at this fast rate will have on the future generations. The way we currently make and use materials detrimentally affects the planet Earth, creating many severe environmental problems. It affects the next generations by putting in danger the future of the economy, energy, and climate. We are at the point where something must drastically change, and it must change now. We must create more sustainable materials alternatives using natural raw materials and inspiration from nature while making sure not to deplete important resources, i.e. in competition with the food chain supply. We must use less materials, eliminate the use of toxic materials and create a circular materials economy where reuse and recycle are priorities. We must develop sustainable methods for materials recycling and encourage design for disassembly. We must look across the whole materials life cycle from raw resources till end of life and apply thorough life cycle assessments (LCAs) based on reliable and relevant data to quantify sustainability. We need to seriously start thinking of where our future materials will come from and how could we track them, given that we are confronted with resource scarcity and geographical constrains. This is particularly important for the development of new and sustainable energy technologies, key to our transition to net zero. Currently ‘critical materials’ are central components of sustainable energy systems because they are the best performing. A few examples include the permanent magnets based on rare earth metals (Dy, Nd, Pr) used in wind turbines, Li and Co in Li-ion batteries, Pt and Ir in fuel cells and electrolysers, Si in solar cells just to mention a few. These materials are classified as ‘critical’ by the European Union and Department of Energy. Except in sustainable energy, materials are also key components in packaging, construction, and textile industry along with many other industrial sectors. This roadmap authored by prominent researchers working across disciplines in the very important field of sustainable materials is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the sustainable materials community. In compiling this roadmap, we hope to aid the development of the wider sustainable materials research community, providing a guide for academia, industry, government, and funding agencies in this critically important and rapidly developing research space which is key to future sustainability., The authors would like to thank The Faraday Institution ReLiB Project Grant Numbers FIRG005 and FIRG006, the UKRI Interdisciplinary Circular Economy Centre for Technology Metals (Met4Tech) Grant No. EP/V011855/1 and the EPSRC Critical Elements and Materials Network (CREAM) EP/R020140/1 for providing financial assistance for this research.