Hydroxypropyl-cellulose (HPC), a derivative of naturally abundant cellulose, can self-assemble into helical nanostructures that lead to striking colouration from Bragg reflections. The helical periodicity is very sensitive to pressure, rendering HPC a responsive photonic material. Recent advances in elucidating these HPC mechano-chromic properties have so-far delivered few real-world applications, which require both up-scaling fabrication and digital translation of their colour changes. Here we present roll-to-roll manufactured metre-scale HPC laminates using continuous coating and encapsulation. We quantify the pressure response of the encapsulated HPC using optical analyses of the pressure-induced hue change as perceived by the human eye and digital imaging. Finally, we show the ability to capture real-time pressure distributions and temporal evolution of a human foot-print on our HPC laminates. This is the first demonstration of a large area and cost-effective method for fabricating HPC stimuli-responsive photonic films, which can generate pressure maps that can be read out with standard cameras.

Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self‐assembling into a lyotropic chiral nematic phase in aqueous solution. This liquid crystalline phase reflects right‐handed circular polarized light of a specific color as a function of the HPC weight fraction. Here, it is demonstrated that, by introducing a crosslinking agent, it is possible to drastically alter the visual appearance of the HPC mesophase in terms of the reflected color, the scattering distribution, and the polarization response, resulting in an exceptional matte appearance in solid‐state films. By exploiting the interplay between order and disorder, a robust and simple methodology toward the preparation of polarization and angular independent color is developed, which constitutes an important step toward the development of real‐world photonic colorants.

Hydroxypropyl cellulose (HPC) is an edible, cost‐effective and widely used derivative of cellulose. Under lyotropic conditions in water, HPC forms a photonic, liquid crystalline mesophase with an exceptional mechanochromic response. However, due to insufficient physical cross‐linking photonic HPC can flow freely as a viscous liquid, preventing the exploitation of this mechanochromic material in the absence of any external encapsulation or structural confinement. Here this challenge is addressed by mixing HPC and gelatin in water to form a self‐supporting, viscoelastic, and edible supramolecular photonic hydrogel. It is demonstrated that the structural coloration, mechanochromism and non‐Newtonian shear‐thinning behavior of the lyotropic HPC solutions can all be retained into the gel state. Moreover, the rigidity of the HPC‐gel provides a 69% shorter mechanochromic relaxation time back to its initial color when compared to the liquid HPC–water only system, broadening the dynamic color range of HPC by approximately 2.5× in response to a compressive pressure. Finally, the ability to formulate the HPC‐gels in a scalable fashion from only water and “food‐grade” constituents unlocks a wide range of potential applications, from response‑tunable mechanochromic materials and colorant‐free food decoration, to short‐term sensors in, for example, biodegradable “smart labels” for food packaging.

Cellulose nanocrystals (CNCs) can spontaneously assemble into chiral nematic films capable of reflecting circularly polarized light in the visible range. As many other photonic materials obtained by bottom-up approaches, CNC films often display defects that greatly impact their visual appearance. Here, we study the optical response of defects in photonic CNC films, coupling optical microscopy with hyperspectral imaging, and we compare it to optical simulations of discontinuous cholesteric structures of increasing complexity. Cross-sectional SEM observations of the film structure guided the choice of simulation parameters and showed excellent agreement with experimental optical patterns. More importantly, it strongly suggests that the last fraction of CNCs to self-assemble, upon solvent evaporation, does not undergo the typical nucleation and growth pathway, but a spinodal decomposition, an alternative self-assembly pathway so far overlooked in cast films and that can have far-reaching consequences on choices of CNC sources and assembly conditions.

Above a critical concentration, CNCs behave as lyotropic liquid crystal with a helicoidal-layered structure forming a chiral nematic architecture leading to vivid iridescent films with photonic properties. [2] It was suggested from earlier research that the chirality of cellulose nanocrystals could arise from the structure of tactoids, which are considered the intermediate state connecting the isotropic phase and the macroscopic liquid crystalline phase. [3] More recently, however, it has been shown that the CNC chiral nematic pitch was found to rely on the presence of crystallite bundles, commonly observed in CNC suspensions but hitherto unassociated with their chiral self-assembly. [4] These new findings indicate that CNC crystallite bundles are the missing link in the hierarchical transfer of chirality from the molecular to colloidal level. [4] Such chiral nematic structures have attracted considerable attention not only in the fabrication of functional colored coatings and pigments [5] and sensors [6] but also as templating agents for functional materials [7,8] and as a host matrix for a wide range of nonchiral molecular guests [9] or nanoparticles. [10] To date, all the reported chiral nematic CNC films have been produced via solution casting or evaporation-induced selfassembly. [11,12] Under solvent evaporation, this structure can be retained in a solid dry form due to the onset of kinetic arrested state. [13] Such evaporation induced self-assembly (EISA) process is reported to take place in three stages. [14] The first stage consists of the beginning of evaporation where CNCs remain isotropically suspended while their concentration increases and form into large-scale assemblies. The second phase is the gel state which takes place after a partial water evaporation. During this stage, domains with stacked structures can be identified related to the anisotropic state of CNCs. [14][15] In the third stage, the film becomes colored due to compression of the mesophase [13] and the reflection of the left circularly polarized light. The wavelength of the reflected light decreases as evaporation slowly proceeds (hours to weeks). The slow evaporation process is necessary for the large tactoids to form, thereby leading to uniform color in the films. [16] Critically, developing a fast technique to produce industrially scalable CNC chiral nematic structures is an important challenge in this field. Moreover, CNC Nano-enabled, bio-based, functional materials are key for the transition to a sustainable society as they can be used, owing to both their performance and nontoxicity, to gradually replace existing nonrenewable engineering materials. Cellulose nanocrystals (CNCs), produced by acid hydrolysis of cellulosic biomass, have been shown to possess distinct self-assembly, optical, and electromechanical properties, and are anticipated to play an important role in the fabrication of photonic, optoelectronic, and functional hybrid materials. To facilitate CNCs' technological viability, a method suitable for industrial ex...

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Although HPC allows access to different colors by simply altering its concentration in water (approx. 60-70 wt%), this tunability comes at the cost of color stability. [14] In particular, the reflected wavelength will blueshift into the ultraviolet as the mesophase dries, ultimately resulting in transparent films. [15] As such, the photonic applications of HPC have typically focused upon sealing an aqueous mesophase within nonpermeable media, [16][17][18] for use as, e.g., a mechanochromic sensor. [3,17,19,20] However, to enable its use as a colorant it is preferable to employ HPC in the solid state, where the pitch is fixed to reflect a specific color. To date, several methods

Metrics & MoreArticle RecommendationsCONSPECTUS: Polysaccharides are a class of biopolymers that are widely exploited in living organisms for a diversity of applications, ranging from structural reinforcement to energy storage. Among the numerous types of polysaccharides found in the natural world, cellulose is the most abundant and widespread, as it is found in virtually all plants. Cellulose is typically organized into nanoscale crystalline fibrils within the cell wall to give structural integrity to plant tissue. However, in several species, such fibrils are organized into helicoidal nanostructures with a periodicity comparable to visible light (i.e., in the range 250−450 nm), resulting in structural coloration. As such, when taking bioinspiration as a design principle, it is clear that helicoidal cellulose architectures are a promising approach to developing sustainable photonic materials. Different forms of cellulose-derived materials have been shown to produce structural color by exploiting self-assembly processes. For example, crystalline nanoparticles of cellulose can be extracted from natural sources, such as cotton or wood, by strong acid hydrolysis. Such "cellulose nanocrystals" (CNCs) have been shown to form colloidal suspensions in water that can spontaneously self-organize into a cholesteric liquid crystal phase, mimicking the natural helicoidal architecture. Upon drying, this nanoscale ordering can be retained into the solid state, enabling the specific reflection of visible light. Using this approach, colors from across the entire visible spectrum can be produced, alongside striking visual effects such as iridescence or a metallic shine. Similarly, polymeric cellulose derivatives can also organize into a cholesteric liquid crystal. In particular, edible hydroxypropyl cellulose (HPC) is known to produce colorful mesophases at high concentrations in water (ca. 60−70 wt %). This solution state behavior allows for interesting visual effects such as mechanochromism (enabling its use in low-cost colorimetric pressure or strain sensors), while trapping the structure into the solid state enables the production of structurally colored films, particles and 3D printed objects.In this article, we summarize the state-of-the-art for CNC and HPC-based photonic materials, encompassing the underlying selfassembly processes, strategies to design their photonic response, and current approaches to translate this burgeoning green technology toward commercial application in a wide range of sectors, from packaging to cosmetics and food. This overview is supported by a summary of the analytical techniques required to characterize these photonic materials and approaches to model their optical response. Finally, we present several unresolved scientific questions and outstanding technical challenges that the wider community should seek to address to develop these sustainable photonic materials.