Structural color hydrogels with lasting survivability are important for many applications, but they still lack anti-biodegradation capability. Thus, we herein present novel antibacterial structural color hydrogels by simply integrating silver nanoparticles (AgNPs) in situ into the hydrogel materials. Because the integrated AgNPs possessed wide and excellent antibacterial abilities, the structural color hydrogels could prevent bacterial adhesion, avoid hydrogel damage, and maintain their vivid structural colors during their application and storage. It was demonstrated that the AgNP-tagged poly(N-isopropylacrylamide) structural color hydrogels could retain their original thermal-responsive color transition even when the AgNP-free hydrogels were degraded by bacteria and that the AgNP-integrated self-healing structural color protein hydrogels could save their self-repairing property instead of being degraded by bacteria. These features indicated that the antibacterial structural color hydrogels could be amenable to a variety of practical biomedical applications.Keywords: AgNPs; antibacterial; colloidal crystal; hydrogel; structural color;

Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation. The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development. We believe this review will promote communications among biology, chemistry, physics, and materials science.

Enzymatic carriers have a demonstrated value for chemical reactions and industrial applications. Here, we present a novel kind of inverse opal hydrogel particles as the enzymatic carriers. The particles were negatively replicated from spherical colloidal crystal templates by using magnetic nanoparticles tagged acrylamide hydrogel. Thus, they were endowed with the features of monodispersity, small volume, complete penetrating structure, and controllable motion, which are all beneficial for improving the efficiency of biocatalysis. In addition, due to the ordered porous nanostructure, the inverse opal hydrogel particles were imparted with unique photonic band gaps (PBGs) and vivid structural colors for encoding varieties of immobilized enzymes and for constructing a multienzymes biocatalysis system. These features of the inverse opal hydrogel particles indicate that they are ideal enzymatic carriers for biocatalysis.Keywords: biocatalysis; hydrogel; inverse opal; particles; photonic crystal;

Microparticles have a demonstrated value for drug delivery systems. The attempts to develop this technology focus on the generation of featured microparticles for improving the function of the systems. Here, we present a new type ofmicroparticles with gelatinmethacrylate (GelMa) cores and poly(L-lactide-co-glycolide) (PLGA) shells for synergistic and sustained drug delivery applications. The microparticles were fabricated by using GelMa aqueous solution and PLGA oil solution as the raw materials of the microfluidic double emulsion templates, in which hydrophilic and hydrophobic actives, such as doxorubicin hydrochloride (DOX, hydrophilic) and camptothecine (CPT, hydrophobic), could be loaded respectively. As the inner cores were polymerized in the microfluidics when the double emulsions were formed, the hydrophilic actives could be trapped in the cores with high efficiency, and the rupture or fusion of the cores could be avoided during the solidification of the microparticle shells with other actives. The size and component of the microparticles can be easily and precisely adjusted by manipulating the flow solutions during the microfluidic emulsification. Because of the solid structure of the resultant microparticles, the encapsulated actives were released from the delivery systems only with the degradation of the biopolymer layers, and thus the burst release of the actives was avoided. These features of the microparticlesmake them ideal for drug delivery applications.微胶囊在药物递送系统中具有重要的应用价值. 目前关于该领域的研究主要集中于开发新型微胶囊来提高药物递送系统的效率. 本 文提出了一种可协同运输和缓慢释放药物的微胶囊, 其由明胶甲基丙烯酸接枝共聚物(GelMa)内核和聚乳酸羟基乙酸共聚物(PLGA)外壳 组成. 在微胶囊的制备过程中, 使用液滴微流控技术, 将溶有盐酸阿霉素(DOX)的GelMa水溶液和溶有喜树碱(CPT)的PLGA油溶液乳化成 均匀的双乳液模板, 通过紫外固化模板内核, 通过溶剂挥发固化模板壳层. 该过程避免了乳液的破损及包裹液的流出, 因此可显著提高药 物的包裹效率. 通过调节微流控的流速, 还可精确地调节微粒的尺寸和结构. 由于所制备的微胶囊内核和外壳都为固化状态, 其包裹的活 性药物只能随着载体材料的降解而缓慢释放出来, 这就避免了其他种类药物载体所面临的药物突释现象. 本研究所开发的微胶囊的这些 优良特性使其成为药物递送系统中的理想选择.

The fabrication of functional microcarriers capable of achieving in vivo-like three-dimensional cell culture is important for many tissue engineering applications. Here, inspired by the structure of Buddha beads, which are generally composed of moveable beads strung on a rope, we present novel cell microcarriers with controllable macropores and heterogeneous microstructures by using a capillary array microfluidic technology. Microfibers with a string of moveable and releasable microcarriers could be achieved by an immediate gelation reaction of sodium alginate spinning and subsequent polymerization of cell-dispersed gelatin methacrylate emulsification. The sizes of the microcarriers and their inner macropores could be well tailored by adjusting the flow rates of the microfluidic phases; this was of great importance in guaranteeing a sufficient supply of nutrients during cell culture. In addition, by infusing multiple cell-dispersed pregel solutions into the capillaries, the microcarriers with spatially heterogeneous cell encapsulations for mimicking physiological structures and functions could also be achieved.构建可用于细胞三维培养的多功能微载体在组织工程的应用中至关重要. 本文受佛珠手串中佛珠可以在绳子上自由滑动这一特殊结构的启发, 利用毛细管阵列微流控技术制备了一种具有可控大孔微结构的新型异质细胞微载体, 用于细胞三维培养.仿佛珠微载体的构建首先需要通过海藻酸钠与钙离子的快速凝胶化形成海藻酸钙纤维, 随即在纤维上包覆可聚合的细胞预聚溶液, 通过流体的剪切实现溶液乳化并将其同化聚合, 从而获得串有可以自由滑动的微载体的纤维串.纤维上释放的微载体中间的大孔结构的尺寸高度可控, 这一特点在微载体用于细胞三维培养中具有重要意义, 因为微载体中间的大孔结构能够有效保证载体内部细胞氧气、 营养物质的充分交换, 减少细胞坏死.此外, 通过将多种细胞预聚溶液引入微流控通道中, 还可以获得具有多组分异质结构的细胞微载体, 从而有望实现体内复杂的组织器官结构与功能的模拟.

Microcarriers have a demonstrated value for biomedical applications, in particular for drug delivery and three-dimensional cell culture. Attempts to develop this technique tend to focus on the fabrication of functional microparticles by using convenient methods with innovative but accessible materials. Inspired by the process of boiling eggs in everyday life, which causes the solidification of egg proteins, we present a new microfluidic “cooking” approach for the generation of egg-derived microcarriers for cell culture and drug delivery. As the egg emulsion droplets are formed with exquisite precision during the microfluidic emulsification, the resultant egg microcarriers present highly monodisperse and uniform morphologies at the size range of hundred microns to one millimeter. Benefiting from the excellent biocompatibility of the egg protein components, the obtained microcarriers showed good performances of cell adherence and growth. In addition, after a freezing treatment, the egg microcarriers were shown to have interconnected porous structures throughout their whole sphere, could absorb and load different kinds of drugs or other active molecules, and work as microcarrier-based delivery systems. These features point to the potential value of the microfluidic egg microcarriers in biomedicine.Egg microcarriers were generated by using a droplet microfluidics to emulsify the egg white solution and cooking them in situ. The excellent biocompatibility and rough surface of the egg microcarriers make them ideal for cell adherence and growth; while the interconnected porous structures of the freezing treated egg microcarriers impart them additional function for drug loading and delivery.Download high-res image (82KB)Download full-size image

Barcodes have a demonstrated value for multiplex high-throughput bioassays. The tendency of this technology is to pursue high sensitivity target screening. Herein, we presented a new type of inverse opal-structured poly(N-isopropylacrylamide) (pNIPAM) hydrogel photonic crystal (PhC) barcodes with the function of fluorescent signal self-amplification for the detection. During the bio-reaction process at body temperature, the pNIPAM hydrogel barcodes kept swelling, and their inverse opal structure with interconnected pores provided unblocked channels for the targets to diffuse into the voids of the barcodes and react. During the detection process, the barcodes were kept at a volume phase transition temperature (VPTT) to shrink their volume; this resulted in an obvious increase in the density of fluorescent molecules and signal amplification. It was demonstrated that the responsive barcodes could achieve the limits of detection (LOD) of α-fetoprotein (AFP) and carcinoembryonic antigen (CEA) at 0.623 ng mL−1 and 0.492 ng mL−1, respectively. In addition, the proposed barcodes showed good multiplex detection capacity with acceptable cross-reactivity, accuracy, and reproducibility, and the results were consistent with those of common clinical laboratory methods for the detection of clinical samples. These features of the inverse opal-structured responsive hydrogel barcodes indicate that they are ideal technology for high-sensitive multiplex bioassays.

Barcodes have a demonstrated value for multiplex high-throughput bioassays. The tendency of this technology is to pursue high sensitivity target screening. Herein, we presented a new type of inverse opal-structured poly(N-isopropylacrylamide) (pNIPAM) hydrogel photonic crystal (PhC) barcodes with the function of fluorescent signal self-amplification for the detection. During the bio-reaction process at body temperature, the pNIPAM hydrogel barcodes kept swelling, and their inverse opal structure with interconnected pores provided unblocked channels for the targets to diffuse into the voids of the barcodes and react. During the detection process, the barcodes were kept at a volume phase transition temperature (VPTT) to shrink their volume; this resulted in an obvious increase in the density of fluorescent molecules and signal amplification. It was demonstrated that the responsive barcodes could achieve the limits of detection (LOD) of α-fetoprotein (AFP) and carcinoembryonic antigen (CEA) at 0.623 ng mL−1 and 0.492 ng mL−1, respectively. In addition, the proposed barcodes showed good multiplex detection capacity with acceptable cross-reactivity, accuracy, and reproducibility, and the results were consistent with those of common clinical laboratory methods for the detection of clinical samples. These features of the inverse opal-structured responsive hydrogel barcodes indicate that they are ideal technology for high-sensitive multiplex bioassays.

Graphene oxide (GO) fibers with a spindle-knotted structure and photothermally responsive phase-transition behaviors were fabricated by combining spinning and emulsification procedures in microfluidics. The resultant GO fibers enabled fog-capture and NIR-triggered water-collection applications.

Barcodes-based suspension array have for demonstrated values in multiplex assay of tumor markers. Photonic barcodes which are encoded by their characteristic reflection peaks are the important supports for suspension array due to their stable code, low fluorescent background and high surface-volume ratio. Attempts to develop this technology tend to improve the function of the photonic barcodes. Here, we present a new type of hybrid hydrogel photonic barcodes for efficient multiplex assays. This photonic barcodes are hybrid inverse opal hydrogel composed of poly(ethylene glycol) diacrylate (PEG-DA) and agarose. The polymerized PEG-DA hydrogel could guarantee the stabilities of the inverse opal structure and its resultant code, while the agarose could offer active chemical groups for the probe immobilization and homogeneous water surrounding for the bioassay. In addition, the interconnected pores inverse opal structure could provide channels for biomolecules diffusing and reaction into the voids of barcodes. These features imparted the hybrid hydrogel photonic barcodes with limits of detection (LOD) of 0.78 ng/mL for carcinoembryonic antigen (CEA) and 0.21 ng/mL for α-fetoprotein (AFP), respectively. It was also demonstrated that the proposed barcodes showed acceptable accuracy and detection reproducibility, and the results were in acceptable agreement with those from common clinic method for the detections of practical clinical samples. Thus, our technique provides a new platform for simultaneous multiplex immunoassay.

Natural photonic crystals (PCs) such as those in butterfly wings have been vastly utilized in signal enhancement applications. In recent years, simulations are mostly related to the color generation phenomenon and reflection properties of butterfly wings, and few of them focus on the localized electric field. Herein, a general exploration of electromagnetic modeling of bioinspired PC-incorporated gold nanoparticles (AuNPs) for enhancing the localized electric field has been illustrated. With the aid of PC, the incorporated system can produce stronger electric fields than AuNP(s). The highest increment of |E|max is 1.96 times from AuNP alone for the AuNP-PC system and 1.86 times for the dimer-PC system.

AbstractAs an important characteristic of many creatures, structural colors play a crucial role in the survival of organisms. Inspired by these features, an intelligent structural color material with a heterogeneous striped pattern and stimuli-responsivity by fast self-assembly of colloidal nanoparticles in capillaries with a certain diameter range are presented here. The width, spacing, color, and even combination of the structural color stripe patterns can be precisely tailored by adjusting the self-assembly parameters. Attractively, with the integration of a near-infrared (NIR) light responsive graphene hydrogel into the structural color stripe pattern, the materials are endowed with light-controlled reversible bending behavior with self-reporting color indication. It is demonstrated that the striped structural color materials can be used as NIR-light-triggered dynamic barcode labels for the anti-counterfeiting of different products. These features of the bioinspired structural color stripe pattern materials indicate their potential values for mimicking structural color organisms, which will find important applications in constructing intelligent sensors, anti-counterfeiting devices, and so on.

Helical objects are among the most important and landmark structures in nature, and represent an emerging group of materials with unique spiral geometry; because of their enriched physical and chemical properties, they can have multiple functionalities. However, the fabrication of such complex helical materials at the micro- or nanoscale level remains a challenge. Here, a coaxial capillary microfluidic system, with the functions of consecutive spinning and spiraling, is presented for scalable generation of helical microfibers. The generation processes can be precisely tuned by adjusting the flow rates, and thus the length, diameter, and pitch of the helical microfibers are highly controllable. Varying the injection capillary design of the microfluidics enables the generation of helical microfibers with structures such as the novel Janus, triplex, core–shell, and even double-helix structures. The potential use of these helical microfibers is also explored for magnetically and thermodynamically triggered microsprings, as well as for a force indicator for contraction of cardiomyocytes. These indicate that such helical microfibers are highly versatile for different applications.

AbstractConsiderable efforts have been devoted to developing artificial micro/nanomotors that can convert energy into movement. A flow lithography integrated microfluidic spinning and spiraling system is developed for the continuous generation of bioinspired helical micromotors. Because the generation processes could be precisely tuned by adjusting the flow rates and the illuminating frequency, the length, diameter, and pitch of the helical micromotors were highly controllable. Benefiting from the fast online gelation and polymerization, the resultant helical micromotors could be imparted with Janus, triplex, and core–shell cross-sectional structures that have never been achieved by other methods. Owing to the spatially controlled encapsulation of functional nanoparticles in the microstructures, the helical micromotors can perform locomotion not only by magnetically actuated rotation or corkscrew motion but also through chemically powered catalytic reaction.

AbstractConsiderable efforts have been devoted to developing artificial micro/nanomotors that can convert energy into movement. A flow lithography integrated microfluidic spinning and spiraling system is developed for the continuous generation of bioinspired helical micromotors. Because the generation processes could be precisely tuned by adjusting the flow rates and the illuminating frequency, the length, diameter, and pitch of the helical micromotors were highly controllable. Benefiting from the fast online gelation and polymerization, the resultant helical micromotors could be imparted with Janus, triplex, and core–shell cross-sectional structures that have never been achieved by other methods. Owing to the spatially controlled encapsulation of functional nanoparticles in the microstructures, the helical micromotors can perform locomotion not only by magnetically actuated rotation or corkscrew motion but also through chemically powered catalytic reaction.

Diagnosing hematological disorders based on the separation and detection of cells in the patient's blood is a significant challenge. We have developed a novel barcode particle-based suspension array that can simultaneously capture and detect multiple types of blood cells. The barcode particles are polyacrylamide (PAAm) hydrogel inverse opal microcarriers with characteristic reflection peak codes that remain stable during cell capture on their surfaces. The hydrophilic PAAm hydrogel scaffolds of the barcode particles can entrap various plasma proteins to capture different cells in the blood, with little damage to captured cells.

There is a clinical need for tissue-engineered blood vessels that can be used to replace or bypass damaged arteries. The success of such grafts depends strongly on their ability to mimic native arteries; however, currently available artificial vessels are restricted by their complex processing, controversial integrity, or uncontrollable cell location and orientation. Here, we present new tubular scaffolds with specific surface microstructures for structural vessel mimicry. The tubular scaffolds are fabricated by rotationally expanding three-dimensional tubular inverse opals that are replicated from colloidal crystal templates in capillaries. Because of the ordered porous structure of the inverse opals, the expanded tubular scaffolds are imparted with circumferentially oriented elliptical pattern microstructures on their surfaces. It is demonstrated that these tailored tubular scaffolds can effectively make endothelial cells to form an integrated hollow tubular structure on their inner surface and induce smooth muscle cells to form a circumferential orientation on their outer surface. These features of our tubular scaffolds make them highly promising for the construction of biomimetic blood vessels.

Boronate affinity molecularly imprinted polymer inverse opal particles were developed for the multiplex label-free detection of glycoproteins with high sensitivity and specificity.

The development of effective drug screening platforms is an important task for biomedical engineering. Here, a novel methacrylated gelatin (GelMA) hydrogel-encapsulated core–shell photonic crystal (PhC) barcode particle was developed for three-dimensional cell aggregation culture and drug screening. The GelMA shells of the barcode particles enable creation of a three-dimensional extracellular matrix (ECM) microenvironment for cell adhesion and growth, while the PhC cores of the barcode particles provide stable diffraction peaks that can encode different cell spheroids during culture and distinguish their biological response during drug testing. The applicability of this cell spheroids-on-barcodes platform was investigated by testing the cytotoxic effect of tegafur (TF), a prodrug of 5-fluorouracil (5-FU), on barcode particle-loaded liver HepG2 and HCT-116 colonic tumor cell spheroids. The cytotoxicity of TF against the HCT-116 tumor cell spheroids was enhanced in systems using cocultures of HepG2 and NIH-3T3 cells, indicating the effectiveness of this multiple cell spheroids-on-barcodes platform for drug screening.

Hydrogel colloidal crystal composite materials have a demonstrated value in responsive photonic crystals (PhCs) via controllable stimuli. Although they have been successfully exploited to generate a gradient of color distribution, the soft hydrogels have limitations in terms of stability and storage caused by dependence on environment. Here, we present a practical strategy to fabricate free-standing PhC films with a stable gradient of structural colors using binary polymer networks. A colloidal crystal hydrogel film was prepared for this purpose, with continuously varying photonic band gaps corresponding to the gradient of the press. Then, a second polymer network was used to lock the inside non-close-packed PhC structures and color distribution of the hydrogel film. It was demonstrated that our strategy could bring about a solution to the angle-dependent structural colors of the PhC films by coating the surface with special microstructures.Keywords: angle independent; colloidal crystal; film; photonic crystal; structural color

The fabrication of heterogeneous microstructures, which exert precise control over the distribution of different cell types within biocompatible constructs, is important for many tissue engineering applications. Here, bioactive microfibers with tunable morphologies, structures, and components are generated and employed for creating different tissue constructs. Multibarrel capillary microfluidics with multiple laminar flows are used for continuously spinning these microfibers. With an immediate gelation reaction of the cell dispersed alginate solutions, the cell-laden alginate microfibers with the tunable morphologies and structures as the designed multiple laminar flows can be generated. The performances of the microfibers in cell culture are improved by incorporating bioactive polymers, such as extracellular matrix (ECM) or methacrylated gelatin (GelMA), into the alginate. It is demonstrated that a series of complex three-dimensional (3D) architectural cellular buildings, including biomimic vessels and scaffolds, can be created using these bioactive microfibers.Keywords: cell-encapsulation; hydrogel; microfiber; microfluidics; tissue engineering

A cavitation system was found in solid microcapsules with a membrane shell and a liquid core. By simply treating these microcapsules with hypertonic solutions, cavitation could be controllably triggered without special equipment or complex operations. A cavitation-formed vapor bubble was fully entrapped within the microcapsules, thus providing an advantageous method for fabricating encapsulated microbubbles with controllable dimensions and functional components.

A novel suspension array was developed that uses photonic crystal (PhC) microbubbles as barcode particles. The PhC microbubbles have an outer transparent polymeric shell, a middle PhC shell, and an inner bubble core, and they were achieved by extraction-derived self-assembly of colloidal nanoparticles in semipermeable solid microcapsules. The encoded elements of the microbubbles originated from their PhC structure with a coated shell, which not only improved the stability of the codes but also provided a flexible surface for bioassays. By using multicompartmental microcapsule templates, PhC microbubbles with substantial coding levels and controllable movement could also be achieved. In addition, as the size of the encapsulated bubbles could be tailored, the overall density of the PhC microbubbles could be adjusted to match the density of a detection solution and to remain in suspension. These remarkable properties make the PhC microbubbles excellent barcode particles.

Structural color pigments have attracted increasing interest in a wide variety of research fields. The color is usually angle dependent and iridescent. However, most applications of the pigments require constant color regardless of the viewing angle. Thus, the development of structural color pigments without iridescence is anticipated. Here, we present novel non-iridescent structural color pigments derived from liquid marble microreactors. Using hydrophobic microparticles to encapsulate colloidal crystal suspension drops in marble microreactors, the resultant pigments have hierarchical micro/nanostructures and ordered colloidal crystal arrays on their surfaces. These structural features impart the structural color pigments with the desired non-iridescence.

A novel colorimetric logic system based on the aptamer-cross-linked colloidal crystal hydrogel (CCH) was developed. With the input stimuli of Hg2+ and Ag+, the CCH displayed shrinking response and colour change corresponding to the logical “OR” and “AND” gate. The visualization of the logic output signals is realized.

Particle-based delivery systems have a demonstrated value for drug discovery and development. Here, we report a new type of particle-based delivery system that has controllable release and is self-monitoring. The particles were composed of poly(N-isopropylacrylamide) (pNIPAM) hydrogel with an inverse opal structure. The presence of macropores in the particles provides channels for active drug loading and release from the materials.

Inspired by the microstructure of the stem cell niche, which is generally composed of adjacent cell protection layers and an extracellular matrix (ECM), we present novel microfluidic porous microcarriers for cell culture that consist of external–internal connected scaffold structures and biopolymer matrix fillers. The biomimetic scaffold structure of the porous microcarriers not only avoids the imposition of shear forces on the encapsulated cells but also provides a confined microenvironment for cell self-assembly, whereas the biopolymers in the porous cores of the microcarriers can act as an ECM microenvironment to promote the formation of multicellular spheroid aggregates for biomedical applications.Keywords: cell culture; cell spheroid; microcarrier; microfluidics; niche

The generation of cell gradients is critical for understanding many biological systems and realizing the unique functionality of many implanted biomaterials. However, most previous work can only control the gradient of cell density and this has no effect on the gradient of cell orientation, which has an important role in regulating the functions of many connecting tissues. Here, we report on a simple stretched inverse opal substrate for establishing desired cell orientation gradients. It was demonstrated that tendon fibroblasts on the stretched inverse opal gradient showed a corresponding alignment along with the elongation gradient of the substrate. This “random-to-aligned” cell gradient reproduces the insertion part of many connecting tissues, and thus, will have important applications in tissue engineering.Keywords: biomaterial; cell gradient; cell orientation; colloidal crystal; inverse opal;

Colloidal photonic crystals (PhCs), periodically arranged monodisperse nanoparticles, have emerged as one of the most promising materials for light manipulation because of their photonic band gaps (PBGs), which affect photons in a manner similar to the effect of semiconductor energy band gaps on electrons. The PBGs arise due to the periodic modulation of the refractive index between the building nanoparticles and the surrounding medium in space with subwavelength period. This leads to light with certain wavelengths or frequencies located in the PBG being prohibited from propagating. Because of this special property, the fabrication and application of colloidal PhCs have attracted increasing interest from researchers. The most simple and economical method for fabrication of colloidal PhCs is the bottom-up approach of nanoparticle self-assembly. Common colloidal PhCs from this approach in nature are gem opals, which are made from the ordered assembly and deposition of spherical silica nanoparticles after years of siliceous sedimentation and compression. Besides naturally occurring opals, a variety of manmade colloidal PhCs with thin film or bulk morphology have also been developed. In principle, because of the effect of Bragg diffraction, these PhC materials show different structural colors when observed from different angles, resulting in brilliant colors and important applications. However, this angle dependence is disadvantageous for the construction of some optical materials and devices in which wide viewing angles are desired.Recently, a series of colloidal PhC materials with spherical macroscopic morphology have been created. Because of their spherical symmetry, the PBGs of spherical colloidal PhCs are independent of rotation under illumination of the surface at a fixed incident angle of the light, broadening the perspective of their applications. Based on droplet templates containing colloidal nanoparticles, these spherical colloidal PhCs can be generated by evaporation-induced nanoparticle crystallization or polymerization of ordered nanoparticle crystallization arrays. In particular, because microfluidics was used for the generation of the droplet templates, the development of spherical colloidal PhCs has progressed significantly. These new strategies not only ensure monodispersity, but also increase the structural and functional diversity of the PhC beads, paving the way for the development of advanced optoelectronic devices.In this Account, we present the research progress on spherical colloidal PhCs, including their design, preparation, and potential applications. We outline various types of spherical colloidal PhCs, such as close-packed, non-close-packed, inverse opal, biphasic or multiphasic Janus structured, and core–shell structured geometries. Based on their unique optical properties, applications of the spherical colloidal PhCs for displays, sensors, barcodes, and cell culture microcarriers are presented. Future developments of the spherical colloidal PhC materials are also envisioned.

Cell adhesion and alignment are two important considerations in tissue engineering applications as they can regulate the subsequent cell proliferation activity and differentiation program. Although many effects have been applied to regulate the adhesion or alignment of cells by using physical and chemical methods, it is still a challenge to regulate these cell behaviors simultaneously. Here, we present novel substrates with tunable nanoscale patterned structures for regulating the adhesion and alignment of cells. The substrates with different degrees of pattern orientation were achieved by customizing the amount of stretching applied to polymer inverse opal films. Cells cultured on these substrates showed an adjustable morphology and alignment. Moreover, soft hydrogels, which have poor plasticity and are difficult to cast into patterned structures, were applied to infiltrate the inverse opal structure. We demonstrated that the adhesion ratio of cells could be regulated by these hybrid substrates, as well as adjusting the cell morphology and alignment. These features of functional inverse opal substrates make them suitable for important applications in tissue engineering.

A facile microfluidic device was developed by inserting an annular capillary array into a collection channel for single-step emulsification of double emulsions. By inserting multiple inner-phase solutions into the capillary array, multicomponent double emulsions or microcapsules with inner droplets of different content could also be obtained from the device.

Diffraction gratings have a demonstrated value in optical applications, such as monochromators and spectrometers. Recent efforts have been directed at finding simple ways to manufacture diffraction gratings at low cost and under mild conditions. Here we present a practical strategy to fabricate a diffraction grating by simply treating an elastic photonic crystal film with a gradient of stress. The film was made of non-close-packed colloidal crystal arrays embedded in hydrogel polymer. Its photonic band gap (PBG) could be tuned precisely by using varying levels of pressure. Thus, when the elastic photonic crystal film was subjected to a stress gradient, a novel diffraction grating with continuously varying PBGs in the whole visible range could be achieved. The practical application of this type of diffraction grating was demonstrated in a miniaturized spectrometer system.Keywords: colloidal crystal; diffraction grating; photonic crystal; spectrometer

Janus particles with features of an anisotropic photonic band gap (PBG) structure and magnetic property have been achieved by phase separation and self-assembly of nanoparticles in microfluidic droplets. The resultant particles enable optical encoding and magnetically controllable motion, making them excellent functional encoded particles in biomedical applications.

A tunable fiber Bragg grating (FBG) was developed by using stress-responsive colloidal crystals. In addition, the FBGs with the function of dynamically filtering multiple wavelengths were also demonstrated by incorporating multiple colloidal crystal segments into the fibers.

An angle-independent photonic crystal (PhC) colorimetric sensor was developed by using a stimuli-response hydrogel to replicate the template arrays of isotropic photonic crystal beads (PCBs) for the detection of Hg2+.

Solid phase extraction (SPE) has emerged as the widely used technique for sample preparation in the analytical process. Recent research trends of SPE are toward developing novel adsorbents to enrich the analytes simply and effectively. In this study, we proposed the poly-3,4-ethylenedioxythiophene (PEDOT) nanoclusters as the SPE adsorbent. During the application, only a small amount of PEDOT nanoclusters was needed and placed in a pipet tip with glass wool on either side. Without complex preparation, the target analytes could be directly extracted from the sample onto the extraction material and eluted in this lab-in-a-pipet-tip system. The efficiency of this approach was demonstrated by detecting 20 kinds of sulfonamides (SAs) in honey with ultra-performance liquid chromatography/tandem mass spectrometry (UPLC–MS/MS). All the analytes were detected by multiple reaction monitoring (MRM) mode. The established method was extensively validated by determining the linearity (R2 ≥ 0.991), average recovery (88.4–105.0%) and precision (relative standard deviation ≤9.60%). Low detection limits (0.5–4 μg kg−1), wide linearity (10–250 μg kg−1) and short sample pretreatment time (20 min) were achieved under the optimized conditions. The absolute recoveries of the SAs at high level ranged from 71.1% to 91.4%. Due to its simplicity, selectivity and sensitivity, our new method has potential applications in quantitative analysis of the target compounds in complex samples.Highlights► We proposed the poly-3,4-ethylenedioxythiophene nanoclusters as new SPE adsorbent. ► The lab-in-a-pipet-tip system need only a tiny amount of PEDOT nanoclusters. ► Up to 20 kinds of sulfonamides was extracted in the lab-in-a-pipet-tip system. ► Our new method exhibited high simplicity, selectivity and sensitivity.

This Letter reports a simple method for the mass production of 3D colloidal photonic crystal beads (PCBs) by using a gravity-driven microfluidic device and online droplet drying method. Compared to traditional methods, the droplet templates of the PCBs are generated by using the ultrastable gravity as the driving force for the microfluidics, thus the PCBs are formed with minimal polydispersity. Moreover, drying of the droplet templates is integrated into the production process, and the nanoparticles in the droplets self-assemble online. Overall, this process results in PCBs with good morphology, low polydispersity, brilliant structural colors, and narrow stop bands. PCBs could be bulk generated by this process for many practical applications, such as multiplex-encoded assays and the construction of novel optical materials.

Inspired by the nipple arrays covering mosquitoes’ eyes and the heterogeneous textured bumps on beetles’ backs, we have developed a new kind of Janus particle with multiplexed features, such as different boss arrays and wettability compartmentalized on the same surface, and an anisotropic color and magnetic properties. The prepared Janus particles can be anchored at the air–water interface and act as a highly flexible barrier for preventing coalescence of water droplets. The incorporation of magnetic nanoparticles can give the Janus particles magnetic responsiveness for controlled transportation and coalescence of liquid marbles, while the structural colors in the Janus particles can be employed for barcoding of the encapsulated liquid marbles. We believe that these small Janus particles have great potential as components for constructing intelligent interfacial objects.

Barcode particles have a demonstrated value for multiplexed high-throughput bioassays. Attempts to develop this technology tend to focus on the generation of featured barcodes both with a large number of identifications to increase the throughput and with novel functions to improve the assays. Here, we report a new class of barcodes that are composed of multiple photonic crystal or magnetic-tagged ethoxylated trimethylolpropane triacrylate (ETPTA) cores and polyethylene glycol (PEG) hydrogel shells. These barcodes are prepared by polymerizing microfluidic multiple double emulsions. As the photonic crystal cores possess distinct reflection peaks, our barcodes allow for a substantial number of coding levels for multiplexing applications. The hydrogel shells surrounding the barcodes enable the creation of three-dimensional scaffolds for immobilizing probes. Moreover, the presence of magnetism in the barcodes confers their controllable movement under magnetic fields, which can be used to significantly increase the sensitivity of the bioassays and to simplify the processing. These features make photonic crystal barcodes ideal for biomedical applications.

We have developed a robust method for the visual detection of heavy metal ions (such as Hg2+ and Pb2+) by using aptamer-functionalized colloidal photonic crystal hydrogel (CPCH) films. The CPCHs were derived from a colloidal crystal array of monodisperse silica nanoparticles, which were polymerized within the polyacrylamide hydrogel. The heavy metal ion-responsive aptamers were then cross-linked in the hydrogel network. During detection, the specific binding of heavy metal ions and cross-linked single-stranded aptamers in the hydrogel network caused the hydrogel to shrink, which was detected as a corresponding blue shift in the Bragg diffraction peak position of the CPCHs. The shift value could be used to estimate, quantitatively, the amount of the target ion. It was demonstrated that our CPCH aptasensor could screen a wide concentration range of heavy metal ions with high selectivity and reversibility. In addition, these aptasensors could be rehydrated from dried gels for storage and aptamer protection. It is anticipated that our technology may also be used in the screening of a broad range of metal ions in food, drugs and the environment.

Boronate affinity molecularly imprinted polymer inverse opal particles were developed for the multiplex label-free detection of glycoproteins with high sensitivity and specificity.

An angle-independent photonic crystal (PhC) colorimetric sensor was developed by using a stimuli-response hydrogel to replicate the template arrays of isotropic photonic crystal beads (PCBs) for the detection of Hg2+.

Structural color pigments have attracted increasing interest in a wide variety of research fields. The color is usually angle dependent and iridescent. However, most applications of the pigments require constant color regardless of the viewing angle. Thus, the development of structural color pigments without iridescence is anticipated. Here, we present novel non-iridescent structural color pigments derived from liquid marble microreactors. Using hydrophobic microparticles to encapsulate colloidal crystal suspension drops in marble microreactors, the resultant pigments have hierarchical micro/nanostructures and ordered colloidal crystal arrays on their surfaces. These structural features impart the structural color pigments with the desired non-iridescence.

Graphene oxide (GO) fibers with a spindle-knotted structure and photothermally responsive phase-transition behaviors were fabricated by combining spinning and emulsification procedures in microfluidics. The resultant GO fibers enabled fog-capture and NIR-triggered water-collection applications.