Block Copolymer Nanocomposites: Perspectives for Tailored Functional Materials

Heterogeneous materials in which the characteristic length scale of the filler material is in the nanometer range—i.e., nanocomposites—is currently one of the fastest growing areas of materials research. Polymer nanocomposites have expanded beyond the original scope of polymer–nanocrystal dispersions for refractive‐index tuning or clay‐filled homopolymers primarily pursued for mechanical reinforcement, to include a wide range of applications. This article highlights recent research efforts in the field of structure formation in block copolymer‐based nanocomposite materials, and points out opportunities for novel materials based on inclusion of different types of nanoparticles. The use of block copolymers instead of homopolymers as the matrix is shown to afford opportunities for controlling the spatial and orientational distribution of the nanoelements. This, in turn, allows much more sophisticated tailoring of the overall properties of the composite material.     

Versatile gradients of chemistry, bound ligands and nanoparticles on alumina nanopore arrays

Nanoporous alumina (PA) arrays produced by self-ordering growth, using electrochemical anodization, have been extensively explored for potential applications based upon the unique thermal, mechanical and structural properties, and high surface-to-volume ratio of these materials. However, the potential applications and functionality of these materials may be further extended by molecular-level engineering of the surface of the pore rims. In this paper we present a method for the generation of chemical gradients on the surface of PA arrays based upon plasma co-polymerization of two monomers. We further extend these chemical gradients, which are also gradients of surface charge, to those of bound ligands and number density gradients of nanoparticles. The latter represent a highly exotic new class of materials, comprising aligned PA, capped by gold nanoparticles around the rim of the pores. Gradients of chemistry, ligands and nanoparticles generated by our method retain the porous structure of the substrate, which is important in applications that take advantage of the inherent properties of these materials. This method can be readily extended to other porous materials.     

Remotely Light‐Powered Soft Fluidic Actuators Based on Plasmonic‐Driven Phase Transitions in Elastic Constraint

Materials capable of actuation through remote stimuli are crucial for untethering soft robotic systems from hardware for powering and control. Fluidic actuation is one of the most applied and versatile actuation strategies in soft robotics. Here, the first macroscale soft fluidic actuator is derived that operates remotely powered and controlled by light through a plasmonically induced phase transition in an elastomeric constraint. A multiphase assembly of a liquid layer of concentrated gold nanoparticles in a silicone or styrene–ethylene–butylene–styrene elastic pocket forms the actuator. Upon laser excitation, the nanoparticles convert light of specific wavelength into heat and initiate a liquid‐to‐gas phase transition. The related pressure increase inflates the elastomers in response to laser wavelength, intensity, direction, and on–off pulses. During laser‐off periods, heating halts and condensation of the gas phase renders the actuation reversible. The versatile multiphase materials actuate—like soft “steam engines”—a variety of soft robotic structures (soft valve, pnue‐net structure, crawling robot, pump) and are capable of operating in different environments (air, water, biological tissue) in a single configuration. Tailored toward the near‐infrared window of biological tissue, the structures actuate also through animal tissue for potential medical soft robotic applications.     

Orthogonal Transformations on Solid Substrates: Efficient Avenues to Surface Modification

The performance of solid substrates is not only governed by their molecular constitution, but is also critically influenced by their surface constitution at the solid/gas or solid/liquid interface. In here, we critically review the use of orthogonal chemical transformations (so‐called click chemistry) to achieve efficient surface modifications of materials ranging from gold and silica nanoparticles, polymeric films, and microspheres to fullerenes as well as carbon nanotubes. In addition, the functionalization of surfaces via click chemistry with biomolecules is explored. Although a large host of reactions fulfilling the click‐criteria exist, pericyclic reactions are most frequently employed for efficient surface modifications. The advent of the click chemistry concept has led—as evident from the current literature—to a paradigm shift in current approaches for materials modification: Away from unspecific and nonselective reactions to highly specific true surface engineering.     

Programmed Nanoparticle‐Loaded Nanoparticles for Deep‐Penetrating 3D Cancer Therapy

Tumors are 3D, composed of cellular agglomerations and blood vessels. Therapies involving nanoparticles utilize specific accumulations due to the leaky vascular structures. However, systemically injected nanoparticles are mostly uptaken by cells located on the surfaces of cancer tissues, lacking deep penetration into the core cancer regions. Herein, an unprecedented strategy, described as injecting “nanoparticle‐loaded nanoparticles” to address the long‐lasting problem is reported for effective surface‐to‐core drug delivery in entire 3D tumors. The “nanoparticle‐loaded nanoparticle” is a silica nanoparticle (≈150 nm) with well‐developed, interconnected channels (diameter of ≈30 nm), in which small gold nanoparticles (AuNPs) (≈15 nm) with programmable DNA are located. The nanoparticle (AuNPs)‐loaded nanoparticles (silica): (1) can accumulate in tumors through leaky vascular structures by protecting the inner therapeutic AuNPs during blood circulation, and then (2) allow diffusion of the AuNPs for penetration into the entire surface‐to‐core tumor tissues, and finally (3) release a drug triggered by cancer‐characteristic pH gradients. The hierarchical “nanoparticle‐loaded nanoparticle” can be a rational design for cancer therapies because the outer large nanoparticles are effective in blood circulation and in protection of the therapeutic nanoparticles inside, allowing the loaded small nanoparticles to penetrate deeply into 3D tumors with anticancer drugs.     

Dynamic internal gradients control and direct electric currents within nanostructured materials

Switchable nanomaterials--materials that can change their properties and/or function in response to external stimuli-have potential applications in electronics, sensing and catalysis. Previous efforts to develop such materials have predominately used molecular switches that can modulate their properties by means of conformational changes. Here, we show that electrical conductance through films of gold nanoparticles coated with a monolayer of charged ligands can be controlled by dynamic, long-range gradients of both mobile counterions surrounding the nanoparticles and conduction electrons on the nanoparticle cores. The internal gradients and the electric fields they create are easily reconfigurable, and can be set up in such a way that electric currents through the nanoparticles can be modulated, blocked or even deflected so that they only pass through select regions of the material. The nanoion/counterion hybrids combine the properties of electronic conductors with those of ionic gels/polymers, are easy to process by solution-casting and, by controlling the internal gradients, can be reconfigured into different electronic elements (current rectifiers, switches and diodes).     

The Generation of Nanoparticles in Miniemulsions

A versatile method for the generation of nanoparticulate metals, ceramics, and polymers based on synthesis in miniemulsions—highly stable small droplets in a continuous phase—is presented here. It is revealed that in addition to nanoparticles, encapsulated materials, polymer capsules (see Figure), and hollow particles can also be obtained by careful selection of the starting materials.     

The Role of Surface Chemistry on the Toxicity of Ag Nanoparticles

The role of surface chemistry on the toxicity of Ag nanoparticles is investigated using Saccharomyces cerevisiae yeast as a platform for evaluation. Combining the shape‐controlled synthesis of Ag nanoparticles with a comprehensive characterization of their physicochemical properties, an understanding is formed of the correlation between the physicochemical parameters of nanoparticles and the inhibition growth of yeast cells upon the introduction of nanoparticles into the cell culture system. Capping agents, surface facets, and sample stability—the three experimental parameters that are inherent from the wet‐chemical synthesis of Ag nanoparticles—have a strong impact on toxicity evaluation. Hence, it is important to characterize surface properties of Ag nanoparticles in the nature of biological media and to understand the role that surface chemistry may interplay to correlate the physicochemical properties of nanoparticles with their biological response upon exposure. This work demonstrates the great importance of surface chemistry in designing experiments for reliable toxicity evaluation and in mitigating the toxicity of Ag nanoparticles for their safe use in future commercialization.     

High‐Internal‐Phase Emulsion Tailoring Polymer Amphiphilicity towards an Efficient NIR‐Sensitive Bacteria Filter

Emulsions having a high internal‐phase volume fraction—termed as HIPEs for high internal phase emulsions—are in high demand as templates for functional macroporous materials. Designing molecular surfactants with appropriate amphiphilicity plays a critical role in the HIPE preparation. In this study, successful tailoring of the amphiphilicity of the originally hydrophobic block co‐polymer of polystyrene‐b‐polyvinylpyridine (PS‐b‐P4VP) is reported. In combination with trifluoroacetic acid, less than 5 wt% of the polymer–CF₃COOH system is feasible as a surfactant for HIPE preparation; this is lower than the amounts typically needed for commonly used commercial surfactants. Using the HIPEs as templates, well‐defined closed‐ and open‐cell macroporous triacrylate‐based monoliths are fabricated simply through the adjustment of the ratio of the water phase to oil phase. After coating the resulting macroporous material with polypyrrole nanoparticles, the system can be exploited as an NIR‐sensitive filter for bacteria; it not only excludes oversized bacteria, but it also kills the bacteria with the help of NIR‐induced heat.     

Ferritin Protein Regulates the Degradation of Iron Oxide Nanoparticles

Proteins implicated in iron homeostasis are assumed to be also involved in the cellular processing of iron oxide nanoparticles. In this work, the role of an endogenous iron storage protein—namely the ferritin—is examined in the remediation and biodegradation of magnetic iron oxide nanoparticles. Previous in vivo studies suggest the intracellular transfer of the iron ions released during the degradation of nanoparticles to endogenous protein cages within lysosomal compartments. Here, the capacity of ferritin cages to accommodate and store the degradation products of nanoparticles is investigated in vitro in the physiological acidic environment of the lysosomes. Moreover, it is questioned whether ferritin proteins can play an active role in the degradation of the nanoparticles. The magnetic, colloidal, and structural follow‐up of iron oxide nanoparticles and proteins in lysosome‐like medium confirms the efficient remediation of potentially harmful iron ions generated by nanoparticles within ferritins. The presence of ferritins, however, delays the degradation of particles due to a complex colloidal behavior of the mixture in acidic medium. This study exemplifies the important implications of intracellular proteins in processes of degradation and metabolization of iron oxide nanoparticles.     

Buoyancy‐Driven Gradients for Biomaterial Fabrication and Tissue Engineering

The controlled fabrication of gradient materials is becoming increasingly important as the next generation of tissue engineering seeks to produce inhomogeneous constructs with physiological complexity. Current strategies for fabricating gradient materials can require highly specialized materials or equipment and cannot be generally applied to the wide range of systems used for tissue engineering. Here, the fundamental physical principle of buoyancy is exploited as a generalized approach for generating materials bearing well‐defined compositional, mechanical, or biochemical gradients. Gradient formation is demonstrated across a range of different materials (e.g., polymers and hydrogels) and cargos (e.g., liposomes, nanoparticles, extracellular vesicles, macromolecules, and small molecules). As well as providing versatility, this buoyancy‐driven gradient approach also offers speed (<1 min) and simplicity (a single injection) using standard laboratory apparatus. Moreover, this technique is readily applied to a major target in complex tissue engineering: the osteochondral interface. A bone morphogenetic protein 2 gradient, presented across a gelatin methacryloyl hydrogel laden with human mesenchymal stem cells, is used to locally stimulate osteogenesis and mineralization in order to produce integrated osteochondral tissue constructs. The versatility and accessibility of this fabrication platform should ensure widespread applicability and provide opportunities to generate other gradient materials or interfacial tissues.     

Photocatalytic Micromotors Activated by UV to Visible Light for Environmental Remediation,Micropumps, Reversible Assembly,Transportation, and Biomimicry

Photocatalytic micromotors are light‐induced, chemically powered micromachines based on photocatalytic materials, activated by light illumination, and have redox reactions with environmental solutions to produce chemical gradients and bubbles that propel the micromachines through self‐diffusiophoresis, self‐electrophoresis, and bubble recoil. Due to the fact that excitation light relates largely to the bandgaps of selected materials, the development of photocatalytic micromotors has experienced an evolution from ultraviolet‐light‐activated to visible‐light‐activated and potentially biocompatible systems. Furthermore, due to the strong redox capacity and physical effects caused by the products or product gradients, photocatalytic micromotors have applications in environmental remediation, micropumps, reversible assembly, transportation, and biomimicry.     

Cover Picture: Metallodielectric Two‐Dimensional Photonic Structures Made by Electric‐Field Microstructuring of Nanocomposite Glasses (Adv. Mater. 24/2005)

Nanocomposite glasses containing metallic nanoparticles can be microstructured by electric‐field assisted dissolution of the embedded particles. As reported by Graener and co‐workers on p. 2983, any pattern of the electrode—down to the nanoscale—can be transferred onto the nanocomposite glass, giving 2D metallodielectric microstructures. The cover image shows as the background a regular array of squares with 2 μm periodicity produced using macroporous silicon as an electrode. The insets show the base material, the electrode, a representation of the dissolution process, and an enlarged view of the remaining silver nanoparticles.     

Reconfigurable nanoscale soft materials

We discuss recent research efforts towards understanding and implementing the physical rules needed to make materials—especially materials composed of nanoscale building blocks—that exhibit the defining characteristics of living systems: adaptive and evolving functional behavior. In particular, we highlight advancements in direct imaging and quantifying of kinetic pathways governing structural reconfiguration in model systems of colloidal nanoparticles as well as emerging opportunities brought by frontier efforts in synthesizing shape-shifting colloids and flexible electronics. Direct observation of kinetic “crossroads” in nanoparticle self-assembly and reconfiguration will offer insight into how these steps can be manipulated to design dynamic, potentially novel materials and devices. Moreover, these principles will not be limited to nanoparticles; when extended to building blocks like soft micelles and proteins, they have the potential to have a similar impact throughout the broader field of soft matter physics.     

Influence of Silicification on the Structural and Biological Properties of Buffer‐Mediated Collagen Hydrogels

A buffer‐mediated gelation route for collagen hydrogels that allows the formation of homogeneous composite and hybrid materials with various silica sources (i.e., colloidal silica and soluble silicates) at high concentration (up to 25 × 10^?3 M ) is described. Most significant improvement in rheological properties and proliferation of primary adult human dermal fibroblasts was obtained for the silicate‐based hybrid materials. A similar trend was observed in composite materials incorporating 14 nm SiO₂ nanoparticles, although to a much lesser extent, whereas larger colloids (80 and 390 nm) did not significantly impact mechanical stability and cell behavior. Modification of 80 nm particles surface with amine groups weakens the collagen‐mineral interface, resulting in the decrease of material stability and leading to particle aggregation during the course of cell proliferation experiments.     

Ultrastable Liquid–Liquid Interface as Viable Route for Controlled Deposition of Biodegradable Polymer Nanocapsules

Liquid–liquid interfaces are highly dynamic and characterized by an elevated interfacial tension as compared to solid–liquid interfaces. Therefore, they are gaining an increasing interest as viable templates for ordered assembly of molecules and nanoparticles. However, liquid–liquid interfaces are more difficult to handle compared to solid–liquid interfaces; their intrinsic instability may affect the assembly process, especially in the case of multiple deposition. Indeed, some attempts have been made in the deposition of polymer multilayers at liquid–liquid interfaces, but with limited control over size and stability. This study reports on the preparation of an ultrastable liquid–liquid interface based on an O/W secondary miniemulsion and its possible use as a template for the self‐assembly of polymeric multilayer nanocapsules. Such polymer nanocapsules are made of entirely biodegradable materials, with highly controlled size—well under 200 nm—and multi‐compartment and multifunctional features enriching their field of application in drug delivery, as well as in other bionanotechnology fields.     

Design of Multifunctional Fluorescent Hybrid Materials Based on SiO2 Materials and Core–Shell Fe3O4@SiO2 Nanoparticles for Metal Ion Sensing

Hybrid fluorescent materials constructed from organic chelating fluorescent probes and inorganic solid supports by covalent interactions are a special type of hybrid sensing platform that has gained much interest in the context of metal ion sensing applications owing to their excellent advantages, recyclability, and solubility/dispersibility in particular, as compared with single organic fluorescent molecules. In recent decades, SiO₂ materials and core–shell Fe₃O₄@SiO₂ nanoparticles have become important inorganic solid materials and have been used as inorganic solid supports to hybridize with organic fluorescent receptors, resulting in multifunctional fluorescent hybrid systems for potential applications in sensing and related research fields. Therefore, recent progress in various fluorescent‐group‐functionalized SiO₂ materials is reviewed, with a focus on mesoporous silica nanoparticles and core–shell Fe₃O₄@SiO₂ nanoparticles, as interesting fluorescent organic–inorganic hybrid materials for sensing applications toward essential and toxic metal ions. Selective examples of other types of silica/silicon materials, such as periodic mesoporous organosilicas, solid SiO₂ nanoparticles, fibrous silica spheres, silica nanowires, silica nanotubes, and silica hollow microspheres, are also mentioned. Finally, relevant perspectives of metal‐ion‐sensing‐oriented silica‐fluorescent probe hybrid materials are provided.     

Pure titanium particle loaded nanocomposites: study on the polymer/filler interface and hMSC biocompatibility

The integration of inorganic nanoparticles into polymer matrices allows for the modification of physical properties as well as the implementation of new features for unexplored application fields. Here, we propose the study of a new metal/polymer nanocomposite fabricated by dispersing pure Ti nanoparticles into a poly(methylmetacrilate) matrix via solvent casting process, to investigate its potential use as new biomaterial for biomedical applications. We demonstrated that Ti nanoparticles embedded in the poly(methylmetacrilate) matrix can act as reinforcing agent, not negatively influencing the biological response of human mesenchymal stem cell in terms of cytotoxicity and cell viability. As a function of relative amount and surface treatment, Ti nanoparticles may enhance mechanical strength of the composite—ranging from 31.1?±?2.5 to 43.7?±?0.7?MPa—also contributing to biological response in terms of adhesion and proliferation mechanisms. In particular, for 1?wt% Ti, treated Ti nanoparticles improve cell materials recognition, as confirmed by higher cell spreading-quantified in terms of cell area via image analysis—locally promoting stronger interactions at cell matrix interface. At this stage, these preliminary results suggest a promising use of pure Ti nanoparticles as filler in polymer composites for biomedical applications.     

Metal Nanoparticles with a Knack for Molecular Recognition

Cyclodextrin‐modified metal nanoparticles illustrate the role that protective organic shells can play in the modulation of the properties of nanoparticles, endowing them with molecular recognition abilities, as highlighted here. These nanocomposite materials are of fundamental interest for the organized self‐assembly of metal nanoparticles.     

Simultaneous or Sequential Orthogonal Gradient Formation in a 3D Cell Culture Microfluidic Platform

Biochemical gradients are ubiquitous in biology. At the tissue level, they dictate differentiation patterning or cell migration. Recapitulating in vitro the complexity of such concentration profiles with great spatial and dynamic control is crucial in order to understand the underlying mechanisms of biological phenomena. Here, a microfluidic design capable of generating diffusion‐driven, simultaneous or sequential, orthogonal linear concentration gradients in a 3D cell‐embedded scaffold is described. Formation and stability of the orthogonal gradients are demonstrated by computational and fluorescent dextran‐based characterizations. Then, system utility is explored in two biological systems. First, stem cells are subjected to orthogonal gradients of morphogens in order to mimic the localized differentiation of motor neurons in the neural tube. Similarly to in vivo, motor neurons preferentially differentiate in regions of high concentration of retinoic acid and smoothened agonist (acting as sonic hedgehog), in a concentration‐dependent fashion. Then, a rotating gradient is applied to HT1080 cancer cells and the change in migration direction is investigated as the cells adapt to a new chemical environment. The response time of ≈4 h is reported. These two examples demonstrate the versatility of this new design that can also prove useful in many applications including tissue engineering and drug screening.