Additive manufacturing with stimuli‐responsive materials—4D printing—is a rapidly growing field, with direct ink writing allowing deposition of a wide variety of materials. The synthesis of a humidity‐sensitive cholesteric liquid crystal oligomer ink is reported. With the responsive cholesteric ink, demonstrator devices exhibiting the ink's “four dimensionality” are printed in disparate fashions: as a structural color change or as a preprogrammed deformation mode. After printing, the photonic ink changes color in response to atmospheric humidity, demonstrated as a hydrochromic coating precisely deposited atop a 3D‐printed beetle. After activation in aqueous acid, the beetle exhibits vibrant color shifts across the visible spectrum. Alternatively, a scallop‐inspired actuator with a 3D‐programmed structural color is selectively treated with acid, to allow reversible “opening” and “closing” when exposed to humid and dry air, respectively. The ink enables additive manufacturing of both monolithic and multimaterial stimuli‐responsive, shape‐changing, structurally colored objects, toward broad application of cholesterics in future “smart,” 4D structurally colored devices.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adfm.202201766

Direct Ink Writing of 4D Structural Colors

The point-of-care (POC) method with affordability and portability for the sensitive detection of biological substances is an emerging topic in rapid disease screening and personalized medicine. In this work, we demonstrated a diverse responsive platform based on a dual-channel pressure sensor (DCPS). The DCPS had a multilayer flexible architecture consisting of a photonic hydrogel with chromatic transitions and a piezoresistive pressure sensor as the electrical data transmission unit, both of which had the property of pressure-induced mechanical stimulus feedback. By incorporating a platinum nanoparticles-labeled detection antibody (PtNPs-dAb) into the sandwich-type immunoreaction for the target carcinoembryonic antigen (CEA, as a model analyte), gas decomposition could be triggered by the addition of hydrogen peroxide (H2O2) to induce a significant increase under pressure in a closed chamber. Meanwhile, the DCPS enabled an accurate electrical signal output, and the photonic hydrogel converted spatiotemporal stimuli into eye-readable colorations with string brilliance. In this way, the target concentration could be quantificationally related to the electrical response and intuitively perceived through visible color alterations. Under optimal conditions, a sensitive determination of CEA was performed in a detectable range of 0.3-60 ng/mL with a limit of detection (LOD) of 0.13 ng/mL. In addition, the proposed protocol had satisfactory selectivity, accuracy, and reproducibility. Furthermore, an array-based immunoassay device was fabricated to conceptually validate its application potential in high-throughput biomedical detection and inspire a dual-signal POC diagnostic platform in a friendly way for resource-limited settings.

Point-of-Care Immunoassay Based on a Multipixel Dual-Channel Pressure Sensor Array with Visual Sensing Capability of Full-Color Switching and Reliable Electrical Signals.

Significance We propose a printable structural color ink composed of cholesteric cellulose liquid crystals together with gelatin and a thermal-responsive hydrogel. The ink is endowed with vivid structural colors and printability due to its constituents. Based on this, we print a series of graphics and three-dimensional (3D) objects with vivid color appearances. Moreover, the printed objects possess dual thermal responsiveness, which results in visible color change around body temperature. These performances, together with the biocompatibility of the constituents, indicate that the present ink represents a leap forward to the next-generation 3D printing and would unlock a wide range of real-life applications.

Cholesteric cellulose liquid crystal ink for three-dimensional structural coloration

Three-dimensional (3D) printing has emerged as a powerful tool for material, food, and life science research and development, where the technology's democratization necessitates the advancement of open-source platforms. Herein, we developed a hackable, multi-functional, and modular extrusion 3D printer for soft materials, nicknamed Printer.HM. Multi-printhead modules are established based on a robotic arm for heterogeneous construct creation, where ink printability can be tuned by accessories such as heating and UV modules. Software associated with Printer.HM were designed to accept geometry inputs including computer-aided design models, coordinates, equations, and pictures, to create prints of distinct characteristics. Printer.HM could further perform versatile operations, such as liquid dispensing, non-planar printing, and pick-and-place of meso-objects. By 'mix-and-match' software and hardware settings, Printer.HM demonstrated printing of pH-responsive soft actuators, plant-based functional hydrogels, and organ macro-anatomical models. Integrating affordability and open design, Printer.HM is envisaged to democratize 3D printing for soft, biological, and sustainable material architectures. 

/pdf/a-hackable-multi-functional-and-modular-extrusion-3d-printer-cdzqvzi0.pdf

A hackable, multi-functional, and modular extrusion 3D printer for soft materials

Responsive structural colors generated by the geometrical deformation of synthetic nanomaterials with a periodic skeleton corresponding to light wavelength through interference, diffraction, and scattering are recognized as extremely valuable information conversion methods. However, there lacks systematic generalization regarding the responsive structural colors generated from the response deformation of micro–nano structures. In this review, the recent progress on synthetic structural color materials (SCMs) and their size effects on color conversion are summarized and discussed. Different deformable micro–nano structures, such as microspheres, inverse opals, surface wrinkles, nano‐/micropillars, layered stacks, and others, have been summarized. Their common feature is the variation in structural color produced by the deformation of nanostructures, but the difference lies in the deformation mechanism and response to stimuli. It is aimed to figure out the relationship and mechanism between stimuli deformation and the variable structural color of the corresponding nanostructures, inspiring the creation of the next generation of intelligent SCMs. Finally, perspectives on the development of responsive SCMs derived from the geometrical deformation of synthetic nanomaterials are presented. Several potential stimuli‐deformable micro–nano structures are discussed. It is believed that stimuli deformation is a powerful tool for the fabrication of high‐performance responsive SCMs to advance photonic technology.

Responsive Structural Colors Derived from Geometrical Deformation of Synthetic Nanomaterials

Additive manufacturing is becoming increasingly important as a flexible technique for a wide range of products, with applications in the transportation, health, and food sectors. However, to develop additional functionality it is important to simultaneously control structuring across multiple length scales. In 3D printing, this can be achieved by employing inks with intrinsic hierarchical order. Liquid crystalline systems represent such a class of self‐organizing materials; however, to date they are only used to create filaments with nematic alignment along the extrusion direction. In this study, cholesteric hydroxypropyl cellulose (HPC) is combined with in situ photo‐crosslinking to produce filaments with an internal helicoidal nanoarchitecture, enabling the direct ink writing of solid, volumetric objects with structural color. The iridescent color can be tuned across the visible spectrum by exploiting either the lyotropic or thermotropic behavior of HPC during the crosslinking step, allowing objects with different colors to be printed from the same feedstock. Furthermore, by examining the microstructure after extrusion, the role of shear within the nozzle is revealed and a mechanism proposed based on rheological measurements simulating the nozzle extrusion. Finally, by using only a sustainable biopolymer and water, a pathway toward environmentally friendly 3D printing is revealed.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adfm.202108566

3D Printing of Liquid Crystalline Hydroxypropyl Cellulose—toward Tunable and Sustainable Volumetric Photonic Structures

The structural coloration of arthropods often arises from helicoidal structures made primarily of chitin. Although it is possible to achieve analogous helicoidal architectures by exploiting the self‐assembly of chitin nanocrystals (ChNCs), to date no evidence of structural coloration has been reported from such structures. Previous studies are identified to have been constrained by both the experimental inability to access sub‐micrometer helicoidal pitches and the intrinsically low birefringence of crystalline chitin. To expand the range of accessible pitches, here, ChNCs are isolated from two phylogenetically distinct sources of α‐chitin, namely fungi and shrimp, while to increase the birefringence, an in situ alkaline treatment is performed, increasing the intensity of the reflected color by nearly two orders of magnitude. By combining this treatment with precise control over ChNC suspension formulation, structurally colored chitin‐based films are demonstrated with reflection tunable from blue to near infrared.

/pdf/revealing-the-structural-coloration-of-self-assembled-chitin-1yx6q337.pdf

Revealing the Structural Coloration of Self‐Assembled Chitin Nanocrystal Films

Biodegradable photonic microspheres with structural colors are promising substitutes to polluting microbeads and toxic dyes. Here, amphiphilic polyester-block-poly(ethylene glycol) bottlebrush block copolymers (BBCPs) with polylactic acid or poly(ɛ-caprolactone) as the hydrophobic block are  synthesized and used to fabricate eco-friendly photonic pigments. Molecular parameters of BBCPs, including rigidity and symmetry, are precisely tailored by variation of side chain lengths, which enables effective manipulation of interfacial tension (γ). Organized spontaneous emulsion mechanism is enabled only when γ falls in a suitable range (10.6~14.3 mN/m), producing ordered water-in-oil-in-water multiple emulsions and ordered porous structures. Consequently, highly saturated and tunable structural colors are observed due to coherent light scattering from the porous structures. Such photonic materials are nontoxic as confirmed by careful safety tests using aquatic model organisms.

Precise Tailoring of Polyester Bottlebrush Amphiphiles toward Eco-Friendly Photonic Pigments via Interfacial Self-Assembly.

Abstract To unlock the widespread use of block copolymers as photonic pigments, there is an urgent need to consider their environmental impact (cf. microplastic pollution). Here we show how an inverse photonic glass architecture can enable the use of biocompatible bottlebrush block copolymers (BBCPs), which otherwise lack the refractive index contrast needed for a strong photonic response. A library of photonic pigments is produced from poly(norbornene‐graft‐polycaprolactone)‐block‐poly(norbornene‐graft‐polyethylene glycol), with the color tuned via either the BBCP molecular weight or the processing temperature upon microparticle fabrication. The structure–optic relationship between the 3D porous morphology of the microparticles and their complex optical response is revealed by both an analytical scattering model and 3D finite‐difference time domain (FDTD) simulations. Combined, this allows for strategies to enhance the color purity to be proposed and realized with our biocompatible BBCP system.

/pdf/deconvoluting-the-optical-response-of-biocompatible-photonic-1mhi2l90.pdf

Deconvoluting the Optical Response of Biocompatible Photonic Pigments

The brightest colours in nature often originate from the interaction of light with materials structured at the nanoscale. Different organisms produce such coloration with a wide variety of materials and architectures. In the case of bacterial colonies, structural colours stem for the periodic organization of the cells within the colony, and while considerable efforts have been spent on elucidating the mechanisms responsible for such coloration, the biochemical processes determining the development of this effect have not been explored. Here, we study the influence of nutrients on the organization of cells from the structurally coloured bacteria Flavobacterium strain IR1. By analysing the optical properties of the colonies grown with and without specific polysaccharides, we found that the highly ordered organization of the cells can be altered by the presence of fucoidans. Additionally, by comparing the organization of the wild-type strain with mutants grown in different nutrient conditions, we deduced that this regulation of cell ordering is linked to a specific region of the IR1 chromosome. This region encodes a mechanism for the uptake and metabolism of polysaccharides, including a polysaccharide utilization locus (PUL operon) that appears specific to fucoidan, providing new insight into the biochemical pathways regulating structural colour in bacteria.

Gea T. van de Kerkhof

Papers

3D Printing of Liquid Crystalline Hydroxypropyl Cellulose—toward Tunable and Sustainable Volumetric Photonic Structures

Revealing the Structural Coloration of Self‐Assembled Chitin Nanocrystal Films

Precise Tailoring of Polyester Bottlebrush Amphiphiles toward Eco-Friendly Photonic Pigments via Interfacial Self-Assembly.

Deconvoluting the Optical Response of Biocompatible Photonic Pigments

Polysaccharide metabolism regulates structural colour in bacterial colonies