Three-dimensional micro/nanoscale architectures: fabrication and applications

Stimuli-Mediated Macrophage Switching, Unraveling the Dynamics at the Nanoplatforms-Macrophage Interface

Macrophages play an essential role in immunotherapy and tissue regeneration owing to their remarkable plasticity and diverse functions. Recent bioengineering developments have focused on using external physical stimuli such as electric and magnetic fields, temperature, and compressive stress, among others, on micro/nanostructures to induce macrophage polarization, thereby increasing their therapeutic potential. However, it is difficult to find a concise review of the interaction between physical stimuli, advanced micro/nanostructures, and macrophage polarization. This review examines the present research on physical stimuli-induced macrophage polarization on micro/nanoplatforms, emphasizing the synergistic role of fabricated structure and stimulation for advanced immunotherapy and tissue regeneration. A concise overview of the research advancements investigating the impact of physical stimuli, including electric fields, magnetic fields, compressive forces, fluid shear stress, photothermal stimuli, and multiple stimulations on the polarization of macrophages within complex engineered structures, is provided. The prospective implications of these strategies in regenerative medicine and immunotherapeutic approaches are highlighted. This review will aid in creating stimuli-responsive platforms for immunomodulation and tissue regeneration.

Harnessing the power of biological macromolecules in hydrogels for controlled drug release in the central nervous system: A review

Hydrogels have immense potential in revolutionizing central nervous system (CNS) drug delivery, improving outcomes for neurological disorders. They serve as promising tools for controlled drug delivery to the CNS. Available hydrogel types include natural macromolecules (e.g., chitosan, hyaluronic acid, alginate), as well as hybrid hydrogels combining natural and synthetic polymers. Each type offers distinct advantages in terms of biocompatibility, mechanical properties, and drug release kinetics. Design and engineering considerations encompass hydrogel composition, crosslinking density, porosity, and strategies for targeted drug delivery. The review emphasizes factors affecting drug release profiles, such as hydrogel properties and formulation parameters. CNS drug delivery applications of hydrogels span a wide range of therapeutics, including small molecules, proteins and peptides, and nucleic acids. However, challenges like limited biodegradability, clearance, and effective CNS delivery persist. Incorporating 3D bioprinting technology with hydrogel-based CNS drug delivery holds the promise of highly personalized and precisely controlled therapeutic interventions for neurological disorders. The review explores emerging technologies like 3D bioprinting and nanotechnology as opportunities for enhanced precision and effectiveness in hydrogel-based CNS drug delivery. Continued research, collaboration, and technological advancements are vital for translating hydrogel-based therapies into clinical practice, benefiting patients with CNS disorders. This comprehensive review article delves into hydrogels for CNS drug delivery, addressing their types, design principles, applications, challenges, and opportunities for clinical translation.

Wavelength-Dependent Shaping of Azopolymer Micropillars for Three-Dimensional Structure Control

Surfaces endowed with three-dimensional (3D) mesostructures, showing features in the nanometer to micrometer range, are critical for applications in several fields of science and technology. Finding a fabrication method that is simultaneously inexpensive, simple, fast, versatile, highly scalable, and capable of producing complex 3D shapes is still a challenge. Herein, we characterize the photoreconfiguration of a micropillar array of an azobenzene-containing polymer at different light wavelengths and demonstrate the tailoring of the surface geometry and its related functionality only using light. By changing the irradiated light wavelength and its polarization, we demonstrate the fabrication of various complex isotropic and anisotropic 3D mesostructures from a single original pristine geometry. Quantitative morphological analyses revealed an interplay between the decay rate of absorbed light intensity, micropillar volume preservation, and the cohesive forces between the azopolymer chains as the origin of distinctive wavelength-dependent 3D structural remorphing. Finally, we show the potentialities of this method in surface engineering by photoreshaping a single original micropillar surface into two sets of different mesostructured surfaces exhibiting tunable hydrophobicity in a wide water contact angle range. Our study opens up a new paradigm for fabricating functional 3D mesostructures in a simple, low-cost, fast, and scalable manner.

Micromolding of Thermoplastic Polymers for Direct Fabrication of Discrete, Multilayered Microparticles

Soft lithography provides a convenient and effective method for the fabrication of microdevices with uniform size and shape. However, formation of an embossed, connective film as opposed to discrete features has been an enduring shortcoming associated with soft lithography. Removing this residual layer requires additional postprocessing steps that are often incompatible with organic materials. This limits adaptation and widespread realization of soft lithography for broader applications particularly in drug discovery and drug delivery fields. A novel and versatile approach is demonstrated that enables fabrication of discrete, multilayered, fillable, and harvestable microparticles directly from any thermoplastic polymer, even at very high molecular weights. The approach, isolated microparticle replication via surface-segregating polymer blend mold, utilizes a random copolymer additive, designed with a highly fluorinated segment that, when blended with the mold's matrix, spontaneously orients to the surface conferring an extremely low surface energy and nonwetting properties to the template. The extremely nonwetting properties of the mold are further utilized to load soluble biologics directly into the built-in microwells in a rapid and efficient manner using an innovative screen-printing approach. It is believed that this approach holds promise for fabrication of large-array, 3D, complex microstructures, and is a significant step toward clinical translation of microfabrication technologies.

Process improvement of high aspect ratio nano-gratings based on synchrotron x-ray

To achieve better structural accuracy and aspect ratio, nano-gratings with a vertical angle close to 90° and a depth-to-width ratio of about 8 were prepared by synchrotron radiation. The optimal exposure dose and development time were determined to be 0.006 (A·h) and 6 min, respectively, by observing the surface loss and roughness of the gratings with slit widths of 150 nm and 250 nm under different conditions. To obtain the desired rectangular grating structure, the experimental conditions were optimized with the help of controlled variables experimental method. With the mask-to-photoresist pitch and the development and drying temperatures of 20μm and 23 °C, the optimized depth-to-width ratio of the nano-gratings with a slit width of 250 nm can reach 8.28. The cone angle can reach 88.4°. The aspect ratio of the nano-gratings with a slit width of 150 nm is 7.18, and its cone angle is 87.1°.

Suppression of self-stratification in colloidal mixtures with high Péclet numbers

The non-equilibrium assembly of bimodal colloids during evaporative processes is an attractive means to achieve gradient or stratified layers in thick films. Here, we show that the stratification of small colloids on top of large is prevented when the viscosity of the continuous aqueous phase is too high. We propose a model where a too narrow width of the gradient in concentration of small colloids suppresses the stratification.

New Era of Electroceuticals: Clinically Driven Smart Implantable Electronic Devices Moving towards Precision Therapy

In the name of electroceuticals, bioelectronic devices have transformed and become essential for dealing with all physiological responses. This significant advancement is attributable to its interdisciplinary nature from engineering and sciences and also the progress in micro and nanotechnologies. Undoubtedly, in the future, bioelectronics would lead in such a way that diagnosing and treating patients' diseases is more efficient. In this context, we have reviewed the current advancement of implantable medical electronics (electroceuticals) with their immense potential advantages. Specifically, the article discusses pacemakers, neural stimulation, artificial retinae, and vagus nerve stimulation, their micro/nanoscale features, and material aspects as value addition. Over the past years, most researchers have only focused on the electroceuticals metamorphically transforming from a concept to a device stage to positively impact the therapeutic outcomes. Herein, the article discusses the smart implants' development challenges and opportunities, electromagnetic field effects, and their potential consequences, which will be useful for developing a reliable and qualified smart electroceutical implant for targeted clinical use. Finally, this review article highlights the importance of wirelessly supplying the necessary power and wirelessly triggering functional electronic circuits with ultra-low power consumption and multi-functional advantages such as monitoring and treating the disease in real-time.

Chemically modified nucleic acids and DNA intercalators as tools for nanoparticle assembly

The self-assembly of inorganic nanoparticles to larger structures is of great research interest as it allows the fabrication of novel materials with collective properties correlated to the nanoparticles' individual characteristics. Recently developed methods for controlling nanoparticle organisation have enabled the fabrication of a range of new materials. Amongst these, the assembly of nanoparticles using DNA has attracted significant attention due to the highly selective recognition between complementary DNA strands, DNA nanostructure versatility, and ease of DNA chemical modification. In this review we discuss the application of various chemical DNA modifications and molecular intercalators as tools for the manipulation of DNA-nanoparticle structures. In detail, we discuss how DNA modifications and small molecule intercalators have been employed in the chemical and photochemical DNA ligation in nanostructures; DNA rotaxanes and catenanes associated with reconfigurable nanoparticle assemblies; and DNA backbone modifications including locked nucleic acids, peptide nucleic acids and borane nucleic acids, which affect the stability of nanostructures in complex environments. We conclude by highlighting the importance of maximising the synergy between the communities of DNA chemistry and nanoparticle self-assembly with the aim to enrich the library of tools available for the manipulation of nanostructures.

3D-Printed Complex Microstructures with a Self-Sacrificial Structure Enabled by Grayscale Polymerization and Ultrasonic Treatment

Complex three-dimensional (3D) microstructures are attracting more and more attention in many applications such as microelectromechanical systems, biomedical engineering, new materials, new energy, environmental protection, and wearable electronics. However, fabricating complex 3D microstructures by 3D printing techniques, especially those with long suspended structures, needs to introduce additional supporting structures, which are difficult to be removed. Here, we propose a simple method in which the supporting structures can be easily removed by optimizing their size and the grayscale value working with ultrasonic treatment in ethanol solution. The 3D microstructures and the supporting structures made of the same insoluble materials are fabricated simultaneously by using a projection microstereolithography system with a dynamic mask. The results demonstrate that the supporting structures play a key role in the fabrication of the long suspended structures while they can be easily removed. The removal time decreases with the increase in the height of the supporting microstructures, and the breaking force and shearing force of the supporting structures increase with the increase in their grayscale and the diameter. In addition, theory and the multiphysics simulation validate that the stress concentration at the top and the bottom of the supporting structures due to the cavitation from ultrasonic vibration dominates the removal of the supporting structures. Finally, a tree-like structure is precisely fabricated by using our method. The present study provides a new way for the removal of the supporting structures for 3D printed suspended microstructures.

Additive Manufacturing of Conducting Polymers: Recent Advances, Challenges, and Opportunities

Conducting polymers (CPs) have been attracting great attention in the development of (bio)electronic devices. Most of the current devices are rigid two-dimensional systems and possess uncontrollable geometries and architectures that lead to poor mechanical properties presenting ion/electronic diffusion limitations. The goal of the article is to provide an overview about the additive manufacturing (AM) of conducting polymers, which is of paramount importance for the design of future wearable three-dimensional (3D) (bio)electronic devices. Among different 3D printing AM techniques, inkjet, extrusion, electrohydrodynamic, and light-based printing have been mainly used. This review article collects examples of 3D printing of conducting polymers such as poly(3,4-ethylene-dioxythiophene), polypyrrole, and polyaniline. It also shows examples of AM of these polymers combined with other polymers and/or conducting fillers such as carbon nanotubes, graphene, and silver nanowires. Afterward, the foremost applications of CPs processed by 3D printing techniques in the biomedical and energy fields, that is, wearable electronics, sensors, soft robotics for human motion, or health monitoring devices, among others, will be discussed.

3D NiCo-Layered double Hydroxide@Ni nanotube networks as integrated free-standing electrodes for nonenzymatic glucose sensing

Nickel cobalt layered double hydroxide (NiCo-LDH)-based materials have recently emerged as catalysts for important electrochemical applications. However, they frequently suffer from low electrical conductivity and agglomeration, which in turn impairs their performance. Herein, we present a catalyst design based on integrated, self-supported nickel nanotube networks (Ni-NTNWs) loaded with NiCo-LDH nanosheets, which represents a binder-free, hierarchically nanostructured electrode architecture combining continuous conduction paths and openly accessible macropores of low tortuosity with an ultrahigh density of active sites. Similar to macroscale metallic foams, the NTNWs serve as three-dimensionally interconnected, robust frameworks for the deposition of active material, but are structured in the submicron range. Our synthesis is solely based on scalable approaches, namely templating with commercial track-etched membranes, electroless plating, and electrodeposition. Morphological and compositional characterization proved the successful decoration of the inner and outer nanotube surfaces with a conformal NiCo-LDH layer. Ni-NTNW electrodes and hydroxide-decorated variants showed excellent performance in glucose sensing. The highest activity was achieved for the catalyst augmented with NiCo-LDH nanosheets, which surpassed the modification with pure Ni(OH)₂. Despite its low thickness of 20 µm, the optimized catalyst layer provided an outstanding sensitivity of 4.6 mA mM^-1 cm^-2, a low detection limit of 0.2 µM, a fast response time of 5.3 s, high selectivity and stability, and two linear ranges covering four orders of magnitude, up to 2.5 mM analyte. As such, derivatized interconnected metal nano-networks represent a promising design paradigm for highly miniaturized yet effective catalyst electrodes and electrochemical sensors.

Periodic planar Fabry-Perot nanocavities with tunable interference colors based on filling density effects

Structural colors of high performance and economically feasible fabrication are desired in various applications. Herein, we demonstrate that reflective full-color filters based on the interference effect can be realized in periodic Fabry-Perot (F-P) nanocavity arrays of the same thickness. Enabled by simply adjusting the nanocavity size and array period, the resonant wavelengths can be successively tuned in the whole visible light range, which is mainly attributed to the varied effective refractive index introduced by the different filling density of the F-P nanocavity. Compared to the plasmonic colors utilizing the similar nanostructures, the proposed interference colors offer unique advantages of higher color contrast, wider gamut, and lower fabrication requirements. Besides, these color filters do not involve modulating the vertical dimensions of the F-P nanocavities, which is conducive to the monolithic integration of multicolor optical cavities and their large-area applications in consumable products combined with replica patterning techniques, such as nanoimprinting and soft lithography.

Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials

The supercapacitor has been widely seen as one of the most promising emerging energy storage devices, by which electricity is converted from chemical energy and stored. Two-dimensional (2D) metal oxides/hydroxides (TMOs/TMHs) are revolutionizing the design of high-performance supercapacitors because of their high theoretical specific capacitance, abundance of electrochemically active sites, and feasibility for assembly in hierarchical structures by integrating with graphitic carbon, conductive polymers, and so on. The hierarchical structures achieved can not only overcome the limitations of using a single material but also bring new breakthroughs in performance. In this article, the research progress on 2D TMOs/TMHs and their use in hierarchical structures as supercapacitor materials are reviewed, including the evolution of supercapacitor materials, the configurations of hierarchical structures, the electrical properties regulated, and the existence of advantages and drawbacks. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

A Programmable Nanofabrication Method for Complex 3D Meta-Atom Array Based on Focused-Ion-Beam Stress-Induced Deformation Effect

Due to their unique electromagnetic properties, meta-atom arrays have always been a hotspot to realize all kinds of particular functions, and the research on meta-atom structure has extended from two-dimensions (2D) to three-dimensions (3D) in recent years. With the continuous pursuit of complex 3D meta-atom arrays, the increasing demand for more efficient and more precise nanofabrication methods has encountered challenges. To explore better fabrication methods, we presented a programmable nanofabrication method for a complex 3D meta-atom array based on focused-ion-beam stress-induced deformation (FIB-SID) effect and designed a distinctive nanostructure array composed of periodic 3D meta-atoms to demonstrate the presented method. After successful fabrication of the designed 3D meta-atom arrays, measurements were conducted to investigate the electric/magnetic field properties and infrared spectral characteristics using scanning cathodoluminescence (CL) microscopic imaging and Fourier transform infrared (FTIR) spectroscopy, which revealed a certain excitation mode induced by polarized incident IR light near 8 μm. Besides the programmability for complex 3D meta-atoms and wide applicability of materials, a more significant advantage of the method is that a large-scale array composed of complex 3D meta-atoms can be processed in a quasi-parallel way, which improves the processing efficiency and the consistency of unit cells dramatically.

Metal Oxide and Hydroxide-Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need-Tailored Devices

Energy storage devices that efficiently use energy, in particular renewable energy, are being actively pursued. Aqueous redox supercapacitors, which operate in high ionic conductivity and environmentally friendly aqueous electrolytes, storing and releasing high amounts of charge with rapid response rate and long cycling life, are emerging as a solution for energy storage applications. At the core of these devices, electrode materials and their assembling into rational configurations are the main factors governing the charge storage properties of supercapacitors. Redox-active metal compounds, particularly oxides and hydroxides that store charge via reversible valence change redox reactions with electrolyte ions, are prospective candidates to optimize the electrochemical performance of supercapacitors. To address this target, collaborative investigations, addressing different streams, from fundamental charge storage mechanisms and electrode materials engineering to need-tailored device assemblies, are the key. Over the last few years, significant achievements in metal oxide and hydroxide-based aqueous supercapacitors have been reported. This work discusses the most recent achievements and trends in this field and brings into the spotlight the authors' viewpoints.

Super-resolution interference lithography enabled by non-equilibrium kinetics of photochromic monolayers

Highly parallelized optical super-resolution lithography techniques are key for realizing bulk volume nanopatterning in materials. The majority of demonstrated STED-inspired lithography schemes are serial writing techniques. Here we use a recently developed model spirothiopyran monolayer photoresist to study the non-equilibrium kinetics of STED-inspired lithography systems to achieve large area interference lithography with super-resolved feature dimensions. The linewidth is predicted to increase with exposure time and the contrast is predicted to go through a maximum, resulting in a narrow window of optimum exposure. Experimental results are found to match with high quantitative accuracy. The low photoinhibition saturation threshold of the spirothiopyran renders it especially conducive for parallelized large area nanopatterning. Lines with 56 and 92 nm FWHM were obtained using serial and parallel patterning, respectively. Functionalization of surfaces with heterobifunctional PEGs enables diverse patterning of any desired chemical functionality on these monolayers. These results provide important insight prior to realizing a highly parallelized volume nanofabrication technique.

The non-equilibrium kinetics of spirothiopyran monolayers are studied to enable large area interference lithography with feature dimensions that circumvent the diffraction barrier.

3D-Printed Hydrogel Composites for Predictive Temporal (4D) Cellular Organizations and Patterned Biogenic Mineralization

Materials chemistries for hydrogel scaffolds that are capable of programming temporal (4D) attributes of cellular decision-making in supported 3D microcultures are described. The scaffolds are fabricated using direct-ink writing (DIW)-a 3D-printing technique using extrusion to pattern scaffolds at biologically relevant diameters (≤ 100 µm). Herein, DIW is exploited to variously incorporate a rheological nanoclay, Laponite XLG (LAP), into 2-hydroxyethyl methacrylate (HEMA)-based hydrogels-printing the LAP-HEMA (LH) composites as functional modifiers within otherwise unmodified 2D and 3D HEMA microstructures. The nanoclay-modified domains, when tested as thin films, require no activating (e.g., protein) treatments to promote robust growth compliances that direct the spatial attachment of fibroblast (3T3) and preosteoblast (E1) cells, fostering for the latter a capacity to direct long-term osteodifferentiation. Cell-to-gel interfacial morphologies and cellular motility are analyzed with spatial light interference microscopy (SLIM). Through combination of HEMA and LH gels, high-resolution DIW of a nanocomposite ink (UniH) that translates organizationally dynamic attributes seen with 2D gels into dentition-mimetic 3D scaffolds is demonstrated. These analyses confirm that the underlying materials chemistry and geometry of hydrogel nanocomposites are capable of directing cellular attachment and temporal development within 3D microcultures-a useful material system for the 4D patterning of hydrogel scaffolds.

A critical and quantitative review of the stratification of particles during the drying of colloidal films

For a wide range of applications, films are deposited from colloidal particles suspended in a volatile liquid. There is burgeoning interest in stratifying colloidal particles into separate layers within the final dry film to impart properties at the surface different to the interior. Here, we outline the mechanisms by which colloidal mixtures can stratify during the drying process. The problem is considered here as a three-way competition between evaporation of the continuous liquid, sedimentation of particles, and their Brownian diffusion. In particle mixtures, the sedimentation of larger or denser particles offers one means of stratification. When the rate of evaporation is fast relative to diffusion, binary mixtures of large and small particles can stratify with small particles on the top, according to physical models and computer simulations. We compare experimental results found in the scientific literature to the predictions of several recent models in a quantitative way. Although there is not perfect agreement between them, some general trends emerge in the experiments, simulations and models. The stratification of small particles on the top of a film is favoured when the colloidal suspension is dilute but when both the concentration of the small particles and the solvent evaporation rate are sufficiently high. A higher particle size ratio also favours stratification by size. This review points to ways that microstructures can be designed and controlled in colloidal materials to achieve desired properties.

Fabrication of fillable microparticles and other complex 3D microstructures

Three-dimensional (3D) microstructures created by microfabrication and additive manufacturing have demonstrated value across a number of fields, ranging from biomedicine to microelectronics. However, the techniques used to create these devices each have their own characteristic set of advantages and limitations with regards to resolution, material compatibility, and geometrical constraints that determine the types ofmicrostructures that can be formed.We describe a microfabrication method, termed StampEd Assembly of polymer Layers (SEAL), and create injectable pulsatile drug-delivery microparticles, pH sensors, and 3D microfluidic devices that we could not produce using traditional 3D printing. SEAL allows us to generate microstructures with complex geometry at high resolution, produce fully enclosed internal cavities containing a solid or liquid, and use potentially any thermoplastic material without processing additives.

Applications and perspectives of nanomaterials in novel vaccine development

Vaccines show great potential for both prophylactic and therapeutic use in infections, cancer, and other diseases. With the rapid development of bio-technologies and materials sciences, nanomaterials are playing essential roles in novel vaccine formulations and can boost antigen effectiveness by operating as delivery systems to enhance antigen processing and/or as immune-potentiating adjuvants to induce or potentiate immune responses. The effect of nanoparticles in vaccinology showed enhanced antigen stability and immunogenicity as well as targeted delivery and slow release. However, obstacles remain due to the lack of fundamental knowledge on the detailed molecular working mechanism and in vivo bio-effects of nanoparticles. This review provides a broad overview of the current improvements in nanoparticles in vaccinology. Modern nanoparticle vaccines are classified by the nanoparticles' action based on either delivery system or immune potentiator approaches. The mechanisms of interaction of nanoparticles with the antigens and the immune system are discussed. Nanoparticle vaccines approved for use are also listed. A fundamental understanding of the in vivo bio-distribution and the fate of nanoparticles will accelerate the rational design of new nanoparticles comprising vaccines in the future.

Organosilicon modification to enhance the stability of black phosphorus nanosheets under ambient conditions

An organosilicon agent, TMSCl, was used for the surface coordination of bare BP nanosheets to generate the more stable TMSCl@BP.

Self-assembly of a nano hydrogel colloidal array for the sensing of humidity

Traditional artificial opals are assembled from silica or polystylene colloidals which have poor swellability and a lower response to stimuli. A novel three-dimensional photonic crystal array sensor which has a high stability and desired structural colour was fabricated from the self assembly of nano hydrogel colloids. The nano hydrogel colloids were prepared by co-polymerisation of N-isopropylacrylamide, functional monomer acrylic acid and N-tert-butylacrylamide. The relative humidity from 20% to 100% could be detected rapidly via the reflection spectrum of the nano hydrogel colloidal array with a maximum amount of red shift of 24 nm. The response kinetics for humidity of the nano hydrogel colloidal array were investigated, and correspondingly, a rational response mechanism of the compactness of the close-packed structure caused by the swelling of the nano hydrogel colloidal array was discussed. The nano hydrogel colloidal array sensor presented good reversibility and can be reused for at least five rounds.

A simple and low-cost humidity sensor based on self-assembled three dimensional nanohydrogel colloidal array was prepared for humidity sensing.

Platelet-Facilitated Photothermal Therapy of Head and Neck Squamous Cell Carcinoma

Here, we present a platelet-facilitated photothermal tumor therapy (PLT-PTT) strategy, in which PLTs act as carriers for targeted delivery of photothermal agents to tumor tissues and enhance the PTT effect. Gold nanorods (AuNRs) were first loaded into PLTs by electroporation and the resulting AuNR-loaded PLTs (PLT-AuNRs) inherited long blood circulation and cancer targeting characteristics from PLTs and good photothermal property from AuNRs. Using a gene-knockout mouse model, we demonstrate that the administration of PLT-AuNRs and localizing laser irradiation could effectively inhibit the growth of head and neck squamous cell carcinoma (HNSCC). In addition, we found that the PTT treatment augmented PLT-AuNRs targeting to the tumor sites and in turn, improved the PTT effects in a feedback manner, demonstrating the unique self-reinforcing characteristic of PLT-PTT in cancer therapy.

Biomimetic self-cleaning surfaces: synthesis, mechanism and applications

With millions of years of natural evolution, organisms have achieved sophisticated structures, patterns or textures with complex, spontaneous multifunctionality. Among all the fascinating characteristics observed in biosystems, self-cleaning ability is regarded as one of the most interesting topics in biomimicry because of its potential applications in various fields such as aerospace, energy conversion and biomedical and environmental protection. Recently, in-depth studies have been carried out on various compelling biostructures including lotus leaves, shark skins, butterfly wings and gecko feet. To understand and mimic their self-cleaning mechanisms in artificial structures, in this article, recent progress in self-cleaning techniques is discussed and summarized. Based on the underlying self-cleaning mechanisms, the methods are classified into two categories: self-cleaning with water and without water. The review gives a succinct account of the detailed mechanisms and biomimetic processes applied to create artificial self-cleaning materials and surfaces, and provides some examples of cutting-edge applications such as anti-reflection, water repellence, self-healing, anti-fogging and micro-manipulators. The prospectives and directions of future development are also briefly proposed.

Stratification in binary colloidal polymer films: experiment and simulations

When films are deposited from mixtures of colloidal particles of two different sizes, a diverse range of functional structures can result. One structure of particular interest is a stratified film in which the top surface layer has a composition different than in the interior. Here, we explore the conditions under which a stratified layer of small particles develops spontaneously in a colloidal film that is cast from a binary mixture of small and large polymer particles that are suspended in water. A recent model, which considers the cross-interaction between the large and small particles (Zhou et al., Phys. Rev. Lett., 2017, 118, 108002), predicts that stratification will develop from dilute binary mixtures when the particle size ratio (α), initial volume fraction of small particles (ϕ_S), and Péclet number are high. In experiments and Langevin dynamics simulations, we systematically vary α and ϕ_S in both dilute and concentrated suspensions. We find that stratified films develop when ϕ_S is increased, which is in agreement with the model. In dilute suspensions, there is reasonable agreement between the experiments and the Zhou et al.

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In concentrated suspensions, stratification occurs in experiments only for the higher size ratio α = 7. Simulations using a high Péclet number, additionally find stratification with α = 2, when ϕ_S is high enough. Our results provide a quantitative understanding of the conditions under which stratified colloidal films assemble. Our research has relevance for the design of coatings with targeted optical and mechanical properties at their surface.

Additive Manufacturing: Unlocking the Evolution of Energy Materials

The global energy infrastructure is undergoing a drastic transformation towards renewable energy, posing huge challenges on the energy materials research, development and manufacturing. Additive manufacturing has shown its promise to change the way how future energy system can be designed and delivered. It offers capability in manufacturing complex 3D structures, with near-complete design freedom and high sustainability due to minimal use of materials and toxic chemicals. Recent literatures have reported that additive manufacturing could unlock the evolution of energy materials and chemistries with unprecedented performance in the way that could never be achieved by conventional manufacturing techniques. This comprehensive review will fill the gap in communicating on recent breakthroughs in additive manufacturing for energy material and device applications. It will underpin the discoveries on what 3D functional energy structures can be created without design constraints, which bespoke energy materials could be additively manufactured with customised solutions, and how the additively manufactured devices could be integrated into energy systems. This review will also highlight emerging and important applications in energy additive manufacturing, including fuel cells, batteries, hydrogen, solar cell as well as carbon capture and storage.

Deterministic Integration of Biological and Soft Materials onto 3D Microscale Cellular Frameworks

Complex 3D organizations of materials represent ubiquitous structural motifs found in the most sophisticated forms of matter, the most notable of which are in life-sustaining hierarchical structures found in biology, but where simpler examples also exist as dense multilayered constructs in high-performance electronics. Each class of system evinces specific enabling forms of assembly to establish their functional organization at length scales not dissimilar to tissue-level constructs. This study describes materials and means of assembly that extend and join these disparate systems-schemes for the functional integration of soft and biological materials with synthetic 3D microscale, open frameworks that can leverage the most advanced forms of multilayer electronic technologies, including device-grade semiconductors such as monocrystalline silicon. Cellular migration behaviors, temporal dependencies of their growth, and contact guidance cues provided by the nonplanarity of these frameworks illustrate design criteria useful for their functional integration with living matter (e.g., NIH 3T3 fibroblast and primary rat dorsal root ganglion cell cultures).

Silicon nanocrystal-based photonic crystal slabs with broadband and efficient directional light emission

Light extraction from a thin planar layer can be increased by introducing a two-dimensional periodic pattern on its surface. This structure, the so-called photonic crystal (PhC) slab, then not only enhances the extraction efficiency of light but can direct the extracted emission into desired angles. Careful design of the structures is important in order to have a spectral overlap of the emission with extraction (leaky) modes. We show that by fabricating PhC slabs with optimized dimensions from silicon nanocrystals (SiNCs) active layers, the extraction efficiency of vertical light emission from SiNCs at a particular wavelength can be enhanced ∼ 11 times compared to that of uncorrugated SiNCs-rich layer. More importantly, increased light emission can be obtained in a broad spectral range and, simultaneously, the extracted light can stay confined within relatively narrow angle around the normal to the sample plane. We demonstrate experimentally and theoretically that the physical origin of the enhancement is such that light originating from SiNCs first couples to leaky modes of the PhCs and is then efficiently extracted into the surrounding.

Nanomaterial-based bone regeneration

Bone diseases/injuries have been driving an urgent quest for bone substitutes for bone regeneration. Nanoscaled materials with bone-mimicking characteristics may create suitable microenvironments to guide effective bone regeneration. In this review, the natural hierarchical architecture of bone and its regeneration mechanisms are elucidated. Recent progress in the development of nanomaterials which can promote bone regeneration through bone-healing mimicry (e.g., compositional, nanocrystal formation, structural, and growth factor-related mimicking) is summarized. The nanoeffects of nanomaterials on the regulation of bone-related biological functions are highlighted. How to prepare nanomaterials with combinative bone-biomimicry features according to the bone healing process is prospected in order to achieve rapid bone regeneration in situ.

Printable ink lenses, diffusers, and 2D gratings

Advances in holography have led to applications including data storage, displays, security labels, and colorimetric sensors. However, existing top-down approaches for the fabrication of holographic devices are complex, expensive, and expertise dependent, limiting their use in practical applications. Here, ink-based holographic devices have been created for a wide range of applications in diffraction optics. A single pulse of a 3.5 ns Nd:YAG laser allowed selective ablation of ink to nanofabricate planar optical devices. The practicality of this method is demonstrated by fabricating ink-based diffraction gratings, 2D holographic patterns, optical diffusers, and Fresnel zone plate (FZP) lenses by using the ink. The fabrication processes were rationally designed using predictive computational modeling and the devices were fabricated within a few minutes demonstrating amenability for large scale printable optics through industrial manufacturing. It is anticipated that ink will be a promising diffraction optical material for the rapid printing of low-cost planar nanophotonic devices.

pH-Switchable Stratification of Colloidal Coatings: Surfaces "On Demand"

Stratified coatings are used to provide properties at a surface, such as hardness or refractive index, which are different from underlying layers. Although time-savings are offered by self-assembly approaches, there have been no methods yet reported to offer stratification on demand. Here, we demonstrate a strategy to create self-assembled stratified coatings, which can be switched to homogeneous structures when required. We use blends of large and small colloidal polymer particle dispersions in water that self-assemble during drying because of an osmotic pressure gradient that leads to a downward velocity of larger particles. Our confocal fluorescent microscopy images reveal a distinct surface layer created by the small particles. When the pH of the initial dispersion is raised, the hydrophilic shells of the small particles swell substantially, and the stratification is switched off. Brownian dynamics simulations explain the suppression of stratification when the small particles are swollen as a result of reduced particle mobility, a drop in the pressure gradient, and less time available before particle jamming. Our strategy paves the way for applications in antireflection films and protective coatings in which the required surface composition can be achieved on demand, simply by adjusting the pH prior to deposition.

Three-Dimensional Polymeric Mechanical Metamaterials Fabricated by Multibeam Interference Lithography with the Assistance of Plasma Etching

The pentamode structure is a type of mechanical metamaterial that displays dramatically different bulk and shear modulus responses. In this study, a face-centered cubic (FCC) polymeric microstructure was fabricated by using SU8 negative-type photoresists and multibeam interference exposure. Isotropic plasma etching is used to control the solid-volume fraction; for the first time, we obtained a structure with the minimum solid-volume fraction as low as 15% that still exhibited high structural integrity. Using this method, we reduced the width of atom-to-atom connections by up to 40 nm. We characterize the effect of the connection area on the anisotropy of the mechanical properties using simulations. Nanoindentation measurements were also conducted to evaluate the energy dissipation by varying the connection area. The Young's/shear modulus ratio is 5 times higher for the etched microstructure than that of the bulk SU8 materials. The use of interference lithography may enable the properties of microscale materials to be engineered for various applications, such as MEMS.

Electrospun nanofibers for cancer diagnosis and therapy

The advent of nanotechnology has provided unprecedented opportunities for nanomedicine. Electrospun nanofibers have some astounding features such as high loading capacity, extremely large surface area and porosity, high encapsulation efficiency, ease of modification, combination of diverse therapies, low cost and great benefits. These remarkable structure-dependent properties have far reaching application potential in cancer diagnosis and therapy such as ultra-sensitive sensing systems for point-of-care cancer detection, targeted cancer cell capture, and functional and smart anticancer drug delivery systems. This review summarizes the principal mechanism of electrospun nanofibers and a variety of modified electrospun nanofibers, illustrates their application in biosensors for cancer detection, and enumerates their application in implantable drug delivery for cancer therapy.

Self-folding nanostructures with imprint patterned surfaces (SNIPS)

A significant need in nanotechnology is the development of methods to mass-produce three-dimensional (3D) nanostructures and their ordered assemblies with patterns of functional materials such as metals, ceramics, device grade semiconductors, and polymers. While top-down lithography approaches can enable heterogeneous integration, tunability, and significant material versatility, these methods enable inherently two-dimensional (2D) patterning. Bottom-up approaches enable mass-production of 3D nanostructures and their assemblies but with limited precision, and tunability in surface patterning. Here, we demonstrate a methodology to create Self-folding Nanostructures with Imprint Patterned Surfaces (SNIPS). By a variety of examples, we illustrate that SNIPS, either individually or in ordered arrays, are mass-producible and have significant tunability, material heterogeneity, and patterning precision.

Synthesis of a hierarchical MoSe2/C hybrid with enhanced electrochemical performance for supercapacitors

A facile one-step hydrothermal strategy was successfully developed to fabricate a 3D hierarchical MoSe₂/C hybrid with triethylene glycol as a structure-directing agent and carbon source.

Solid-state fluorescent composite phosphor based on cellulose grafted with carbon dots for temperature sensing

Blue phosphors consisting of cellulose and carbon dots (CDs) have been prepared successfully for the first time, which show excellent fluorescent temperature sensing properties.

Layered nano-gratings by electron beam writing to form 3-level diffractive optical elements for 3D phase-offset holographic lithography

A multi-level nanophotonic structure is a major goal in providing advanced optical functionalities as found in photonic crystals and metamaterials. A three-level nano-grating phase mask has been fabricated in an electron-beam resist (ma-N) to meet the requirement of holographic generation of a diamond-like 3D nanostructure in photoresist by a single exposure step. A 2D mask with 600 nm periodicity is presented for generating first order diffracted beams with a preferred π/2 phase shift on the X- and Y-axes and with sufficient 1(st) order diffraction efficiency of 3.5% at 800 nm wavelength for creating a 3D periodic nanostructure in SU-8 photoresist. The resulting 3D structure is anticipated to provide an 8% complete photonic band gap (PBG) upon silicon inversion. A thin SiO2 layer was used to isolate the grating layers and multiple spin-coating steps served to planarize the final resist layer. A reversible soft coating (aquaSAVE) was introduced to enable SEM inspection and verification of each insulating grating layer. This e-beam lithographic method is extensible to assembling multiple layers of a nanophotonic structure.

High sensitivity of a carbon nanowall-based sensor for detection of organic vapours

The high sensitivity in response, selectivity and reversibility was achieved on a carbon nanowall-based sensor for the vapor detection of volatile organic compounds, which were tested by an electrical resistance method during adsorption and desorption cycles.