Polymer-based responsive structural color materials are polymers with ordered microstructures that can change colors. They offer unique advantages and present a novel solution for many industries. Despite the presence of classic review papers on responsive photonic materials, a comprehensive review of polymer-based responsive structural color materials, including fundamental research and applications, is still lacking. This study provides a detailed overview of polymer-based responsive structural color materials, including polymer selection, structure fabrication, response mechanisms, and applications. The current challenges and future developments are also presented. It is hoped that this review will encourage interdisciplinary communications across physics, chemistry, biology, medicine, materials science, and engineering.

The nervous system is critical to the operation of various organs and systems, while novel methods with designable neural induction remain to exploit. Here, we present a conductive inverse opal film with anisotropic elliptical porous patterns for nerve orientation induction. The films are fabricated based on polystyrene (PS) inverse opal scaffolds with periodical elliptical porous structure and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) mixed polyacrylamide (PAAm) polymers fillers. It is demonstrated that the anisotropic elliptical surface topography allows the nerve cells to be induced into orientation connected with the stretching direction. Because of the anisotropic features of the film which can be stretched into different directions, nerve cells can be induced to grow in one or two directions, forming a neural network and promoting the connection of nerve cells. It is worth mentioning that the PEDOT:PSS-doped PAAm hydrogels endow the film with conductive properties, which makes the composite films be a suitable candidate for neurites growth and differentiation. All these features of the conductive and anisotropic inverse opal films imply their great prospects in biomedical applications.

Heart-on-a-chip plays an important role in revealing the biological mechanism and developing new drugs for cardiomyopathy. Tremendous efforts have been devoted to developing heart-on-a-chip systems featuring simplified fabrication, accurate imitation and microphysiological visuality. In this paper, the authors present a novel electroconductive and anisotropic structural color hydrogel by simply polymerizing non-close-packed colloidal arrays on super aligned carbon nanotube sheets (SACNTs) for visualized and accurate heart-on-a-chip construction. The generated anisotropic hydrogel consists of a colloidal array-locked hydrogel layer with brilliant structural color on one surface and a conductive methacrylated gelatin (GelMA)/SACNTs film on the other surface. It is demonstrated that the anisotropic morphology of the SACNTs could effectively induce the alignment of cardiomyocytes, and the conductivity of SACNTs could contribute to the synchronous beating of cardiomyocytes. Such consistent beating rhythm caused the deformation of the hydrogel substrates and dynamic shifts in structural color and reflection spectra of the whole hybrid hydrogels. More attractively, with the integration of such cardiomyocyte-driven living structural color hydrogels and microfluidics, a visualized heart-on-a-chip system with more consistent beating frequency has been established for dynamic cardiomyocyte sensing and drug screening. The results indicate that the electroconductive and anisotropic structural color hydrogels are potential for various biomedical applications.

Structural coloration is an intriguing phenomenon that exists widely in living creatures. A genuine bionic design of artificial structural colors requires accommodating of molecular interactions that contribute to dynamic optical responses over various external stimuli. Novel composite cellulose cholesteric liquid crystals are reported with additional molecular interactions and dynamic regulations as a bionic prototype for biological structural coloration. It is found that the incorporation of polymer monomers increases the amount of hydrogen bonds, leading to a broader range of color tunability. In addition, the crosslinking of the polymer is accompanied by a time-dependent structure alternation of the cellulose cholesteric liquid crystals, which offers a duration control of irradiation over the color. Based on these phenomena, a series of structural color materials is developed with dynamic photonic responsiveness and high-resolution patterns; their utility in wearable sensors, graphic displays, and information encryption is demonstrated. It is believed that this study presents a next-generation biomimetic photonic material design and applications for different areas.

Peripheral nerve injury (PNI), causing loss of sensory and motor function, is a complex and challenging disease in the clinic due to the restricted regeneration capacity. Nerve guidance conduits (NGCs) have become a promising substitute for peripheral nerve regeneration, but their efficacy is often limited. Here, inspired by the physiological structures of peripheral nerves, we present a conductive topological scaffold for nerve repair by modifying Morpho butterfly wing with reduced graphene oxide (rGO) nanosheets and methacrylated gelatin (GelMA) hydrogel encapsulated brain-derived neurotrophic factor (BDNF). Benefiting from the biocompatibility of GelMA hydrogel, the conductivity of rGO and parallel nanoridge structures of wing scales, PC12 cells, and neural stem cells grown on the modified wing have an increased neurite length with guided cellular orientation. In addition, the NGCs are successfully obtained by manually rolling up the scaffolds and exhibited great performance in repairing 10 mm sciatic nerve defects in rats, and we believe that the NGCs can be applied in reparing longer nerve defects in the future by further optimization. We also demonstrate the feasibility of electrically conductive NGCs based on the rGO/BDNF/GelMA-integrated Morpho butterfly wing as functional nerve regeneration conduits, which may have potential value for application in repairing peripheral nerve injuries.

In the drug therapy of tumor, efficient and stable drug screening platforms are required since the drug efficacy varies individually. Here, inspired by the microstructures of hepatic lobules, in which hepatocytes obtain nutrients from both capillary vessel and the central vein, we present a novel hierarchical hydrogel system with ordered micro-nano structure for liver cancer-on-a-chip construction and drug screening. The hierarchical hydrogel system was fabricated by using pregel to fill and replicate self-assembled colloidal crystal arrays and microcolumn array template. Due to the synergistic effect of its interconnected micro-nano structures, the resultant system could not only precisely control the size of cell spheroids but also realize adequate nutrient supply of cell spheroids. We have demonstrated that by integrating the hierarchical hydrogel system into a multichannel concentration gradients microfluidic chip, a functional liver cancer-on-a-chip could be constructed for high-throughput drug screening with good repeatability and high accuracy. These results indicated that the hierarchical hydrogel system and its derived liver cancer-on-a-chip are ideal platforms for drug screening and have great application potential in the field of personalized medicine.

Structural color materials have existed in nature for millions of years. Natural polymers constitute biological structural colors and provide an excellent resource for fabricating artificial structural color materials. Through either "top-down" or "bottom-up" methods, natural polymers could be processed into intricate micro/nano-structures and show unique photonic features; the resultant materials and devices find applications in a wide range of fields, including optical display, sensing, anti-counterfeiting, organ-on-a-chip, microcarriers, etc. In this review, the latest progress about structural color materials generated from natural polymers is provided. After introducing typical polymers involved in the biological structural coloration, artificial structural color materials derived from natural polymers, including their biomimetic design, fabrication, and application are summarized. The remaining challenges and future research prospects of this remarkable field are also discussed. It is believed that this review would accelerate the development of biomimetics, optics, and materials science.

Cellular mechanics are of great significance in biological functions and have stimulated the development of various techniques to detect and analyze it. Here, a novel heterogeneous structural color microfiber designed for dynamic cardiac mechanics sensing is presented. The microfiber is fabricated by the programmed injection microfluidic spinning method, which achieves the alternative injection of different solutions and generates axial heterogeneous components. This microfiber contains non‐close‐packed colloidal arrays for quantitative optical sensing and bioactive methacrylated gelatin (GelMA) for cardiac culture. When the cultivated cardiomyocytes recover autonomous beating cycles, the structural color section would be stretch and exhibits synchronous stretch cycles with dynamic color variation and wavelength shifts, which transforms microcosmic cell‐generated force into macroscopic optical signals. In addition, the single‐cell‐level mechanics detecting platform can be achieved by tuning the size and diameter of the heterogeneous structural color microfibers. These features contribute to the heterogeneous structural color microfiber to be an ideal platform for biomedical fields.

Department of Clinical Laboratory, Institute of Translational Medicine, The Affiliated Drum Tower Hospital of Nanjing University Medical School, 210008 Nanjing, China State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China Oujiang Laboratory (Zhejiang Lab for Regenerative Medicine, Vision and Brain Health), Wenzhou, Zhejiang 325001, China Chemistry and Biomedicine Innovation Center, Nanjing University, Nanjing 210023, China

Spiral ganglion neuron (SGN) degeneration can lead to severe hearing loss, and the directional regeneration of SGNs has shown great potential for improving the efficacy of auditory therapy. Here, a novel 3D conductive microstructure with surface topologies is presented by integrating superaligned carbon-nanotube sheets (SA-CNTs) onto Morpho Menelaus butterfly wings for SGN culture. The parallel groove-like topological structures of M. Menelaus wings induce the cultured cells to grow along the direction of its ridges. The excellent conductivity of SA-CNTs significantly improves the efficiency of cellular information conduction. When integrating the SA-CNTs with M. Menelaus wings, the SA-CNTs are aligned in parallel with the M. Menelaus ridges, which further strengthens the consistency of the surface topography in the composite substrate. The SA-CNTs integrated onto butterfly wings provide powerful physical signals and regulate the behavior of SGNs, including cell survival, adhesion, neurite outgrowth, and synapse formation. These features indicate the possibility of directed regeneration after auditory nerve injury.

Cellular mechanics plays an important role in physiological processes such as cell growth, differen-tiation, apoptosis and gene expression. Herein, we present a nano-imprinted structural color graphene film with anisotropic microgroove and two-dimensional (2D) photonic crystal structures for cardiomyocytes dynamic displaying. The anisotropic structural color graphene film was generated by a "sandwich" mold in one step with the assistance of two designed templates. Because of the microgroove structure and biocompatibility of the film, the cardiomyocytes could be induced into a highly ordered arrangement with recoverable autonomous beating. Meanwhile, the opposite surface of the film was imparted with defect-free 2D photonic crystal structures, giving it a vivid structural color and photonic band gap (PBG). We demonstrated that the flexible film would undergo synchronous volume or shape changes with the cardiomyocytes' elongation and contraction during the beating process, showing cyclic changes of the structural color, which in turn could be converted into force. In addition, by integrating this anisotropic structural color graphene film with microfluidic channels, a novel and accurate heart-on-a-chip platform was established for real-time cell monitoring and drug screening. These characteristics of the present anisotropic structural color graphene films make them extremely valuable in the field of biomedicine.

As one of the most complex structures in nature, butterflies have attracted wide interest over the past few decades. Inspired by these delicate structures and the marvelous derived properties, scientists have investigated and biomimetic fabricated several designs to replicate the structure and to apply the functional features. Here, we present up-to-date researches concerning butterfly-inspired functional materials in different fields. After introducing the basic properties and corresponding structures, the bio-mimic fabrication methods are clarified and concluded. We then concerned about the applications, combining the modified butterfly wing and the fabricated replicas. The challenges and prospects of the further development of the butterfly inspired functional materials are conclusively presented.

As simulators of organisms in Nature, soft robots have been developed over the past few decades. In particular, biohybrid robots constructed by integrating living cells with soft materials demonstrate the unique advantage of simulating the construction and functions of human tissues or organs, thus attracting extensive attention and research interest. Here, we present up-to-date studies concerning biohybrid robots with various biological actuators such as contractile cells and microorganisms. After presenting the basic components including biological components and synthetic materials, the controlling methods and locomotion modalities of biohybrid robots are clarified and summarized. We then focus on the applications, especially the biomedical applications, of the biohybrid robots including drug delivery, bioimaging, and tissue engineering. The challenges and prospects for the future development of biohybrid robots are also presented.

Bionic electronic skin (E-skin) that could convert external physical or mechanical stimuli into output signals has a wide range of applications including wearable devices, artificial prostheses, software robots, etc. Here, we present a chameleon-inspired multifunctional E-skin based on hydroxypropyl cellulose (HPC), Poly(Acrylamide-co-Acrylic acid) (PACA), and carbon nanotubes (CNTs) composited liquid-crystal hydrogel. We found that the HPC could still form cholesteric liquid-crystal photonic structures with the CNTs additive for enhancing their color saturation and PACA polymerization for locating their assembled periodic structures. As the composite hydrogel containing HPC elements and the PACA scaffold responds to different stimuli, such as temperature variations, mechanical pressure, and tension, it could correspondingly change its volume or internal nanostructure and report these as visible color switches. In addition, due to the additive of CNTs, the composite hydrogel could also output these stimuli as electrical resistance signals. Thus, the hydrogel E-skins had the ability of quantitatively feeding back external stimuli through electrical resistance as well as visually mapping the stimulating sites by color variation. This dual-signal sensing provides the ability of visible-user interaction as well as antiinterference, endowing the multifunctional E-skin with great application prospects.

Biological soft robots have attracted extensive attention and research because of their superiority in executing designed biomedical missions compared with conventional robots. Here, inspired by the crawling mechanism of snakes and caterpillars, a novel biological soft robot composed of asymmetric claws, a carbon nanotube (CNT)-induced myocardial tissue layer, and a structural color indicator is presented. The asymmetric claws can assist the whole soft robot to accomplish directional movement during the cardiomyocytes' contraction process. The oriented conduct of the CNT layer can regulate the cardiomyocytes' arrangement and improve their beating capability and the contraction performance. However, the structural color indicator provides a visualized monitoring approach to dynamically and immediately reflect the motion status of the biological soft robots. With these three functional layers, the cardiomyocyte-driven soft robot can greatly simulate the crawling behavior of a caterpillar. It is demonstrated that by integrating these soft robots in a microfluidic organ-on-a-chip system with multitrack construction, they can run along the tracks and exhibit different running speed based on the stimulus concentrations in the tracks. These features indicate the potential values of the cardiomyocyte-driven soft robots for providing an effective screening platform for clinical diseases.

Recently,organ-on-chips have become a fast-growing research field with the widespread development of microfluidic chips and synthetic materials in tissue engineering.Due to the existing cardiotoxicity of many cardiovascular drugs,heart-onchips which are promising to replace traditional animal models have been extensively researched and developed to mimic human organ functions in vitro.The heart-on-chips mainly focus on cardiac mechanics,which is regarded as the central indicator of in vitro heart models and drug testing.Traditional methods for the detection of myocardial mechanics have been demonstrated complex and inefficient in heart-on-chips.Therefore,photonic crystal materials with unique optical properties have attracted interests and have been introduced into the heart-on-chips,developing a visualized self-reporting system for cardiomyocytes activity monitoring.In this review,photonic crystal-based heart-on-chips for biosensing are introduced,as well as the fabrication methods and design criteria of them.The characterizations of the photonic crystal materials are classified into optical properties and structural properties,and their applications in cell culture and biosensing are further discussed.Then,several representative examples and developments of the integration of photonic crystal materials into microfluidic chips are described in detail.Finally,potentials and limitations are put forward to promote the development of the photonic crystal-based intelligent heart-on-chips.

Introduction Drug discovery through a large number of candidates with nearly unlimited possibilities is laborious, time-consuming, and incurs massive costs. It thus calls for fast and robust methods that allow for handling small amounts of reagents, automation, and high-throughput analysis. In this regard, droplet-based microfluidic platforms provide several unique advantages. Highly monodispersed droplets can be generated in a high frequency and serve as microcontainers with just a few femtoliter to nanoliter volume. Furthermore, each of them can be manipulated to initiate specific reactions in parallel. Consequently, droplet microfluidics is emerging as a useful tool for drug discovery and development.Areas covered The authors provide a brief overview of the latest developments of droplet-based microfluidic techniques for drug discovery and emphasize their applications at different stages, covering target selection, drug candidate identification, and preclinical research.Expert opinion Droplet-based microfluidics holds great potential in drug discovery due to its capability of reaction miniaturization and high-throughput analysis. However, its commercial applications are still at an early stage as the experiments are mostly implemented utilizing custom-built instruments in laboratory environments. Thus, joint efforts from scientists and engineers with multidisciplinary backgrounds are required to optimize the standardization and stability of droplet-based microfluidic platforms.

Structural color materials have been studied for decades because of their fascinating properties. Effects in this area are the trend to develop functional structural color materials with new components, structures, or morphologies for different applications. In this study, we found that the coassembled graphene oxide (GO) and colloid nanoparticles in droplets could form component phase separations, and thus, previously unknown anisotropic structural color particles (SCPs) with hemispherical colloidal crystal cluster and oblate GO component could be achieved. The anisotropic SCPs, as well as their inverse opal hydrogel derivatives, were endowed with brilliant structural colors and controllable capabilities of fixation, location, orientation, and even responsiveness due to their specific structure, morphology, and components. We have also demonstrated that the anisotropic hydrogel SCPs with these features were ideal candidates for dynamic cell monitoring and sensing. These properties indicate that the anisotropic SCPs and their derivatives have huge potential values in biomedical areas.

Heart‐on‐a‐chip based on microfluidic platform can simulate the structure and reveal the function of heart at the micrometer level, compensating the gap between organism and experiments in vitro. In this paper, a novel heart‐on‐a‐chip system integrated with reduced graphene oxide (rGO) hybrid anisotropic structural color film is designed for cardiac sensing and evaluation. This hybrid anisotropic film is based on the opposite adhesion properties of the polyethylene glycol diacrylate (PEGDA) and gelatin methacryloyl (GelMA). The PEGDA area with low adhesion rate has inverse opal structure and specific reflection peak, while microgroove‐patterned rGO‐doped GelMA area with high adhesion rate provides the cardiomyocytes with excellent growing environment and induced orientation property. Benefiting from the design, the cultured cardiomyocytes only adhere in specific area without affecting the surface microstructure of the structural color. When cardiomyocytes recover beating, its elongation and contraction will stretch the structure of PEGDA and result in a color shift, which realizes the transformation from micromechanics to macroscopic optics. In addition, the heart‐on‐a‐chip system based on the anisotropic structural color hydrogels and microfluidics provides an outstanding visible method for cardiac sensing, which is of great significance in cardiac pathophysiological studies and drug detection in vitro.

Stem-cell based in vitro differentiation for disease modeling offers great value to explore the molecular and functional underpinnings driving many types of cardiomyopathy and congenital heart diseases. Nevertheless, one major caveat in the application of in vitro differentiation of human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (hiPSC-CMs) involves the immature phenotype of the CMs. Most of the existing methods need complex apparatus and require laborious procedures in order to monitor the cardiac differentiation/maturation process and often result in cell death. Here we developed an intrinsic color sensing system utilizing a microgroove structural color methacrylated gelatin film, which allows us to monitor the cardiac differentiation process of hiPSC-derived cardiac progenitor cells in real time. Subsequently this system can be employed as an assay system to live monitor induced functional changes on hiPSC-CMs stemming from drug treatment, the effects of which are simply revealed through color diversity. Our research shows that early intervention of cardiac differentiation through simple physical cues can enhance cardiac differentiation and maturation to some extent. Our system also simplifies the previous complex experimental processes for evaluating the physiological effects of successful differentiation and drug treatment and lays a solid foundation for future transformational applications.