Showing papers in "Nature Reviews Materials in 2018"

Non-fullerene acceptors for organic solar cells

Abstract: Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction organic solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochemical stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused aromatic cores with strong electron-accepting end groups. The key structure–property relationships, donor–acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field. Non-fullerene acceptors have been widely used in organic solar cells over the past 3 years. This Review focuses on the two most promising classes of non-fullerene acceptors — rylene diimide-based materials and fused-ring electron acceptors — and discusses structure–property relationships, donor– acceptor matching criteria and device physics, as well as future research directions for the field.

A cost and resource analysis of sodium-ion batteries

Abstract: Sodium-ion batteries are an appealing alternative to lithium-ion batteries because they use raw materials that are less expensive, more abundant and less toxic. The background leading to such promises is carefully assessed in terms of cell and battery production, as well as raw material supply risks, for sodium-ion and modern lithium-ion batteries.

All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes

Abstract: Thermally activated delayed fluorescence (TADF) emitters, which produce light by harvesting both singlet and triplet excitons without noble metals, are emerging as next-generation organic electroluminescent materials. In the past few years, there have been rapid advances in molecular design criteria, our understanding of the photophysics underlying TADF and the applications of TADF materials as emitters in organic light-emitting diodes (OLEDs). This topic is set to remain at the forefront of research in optoelectronic organic materials for the foreseeable future. In this Review, we focus on state-of-the-art materials design and understanding of the photophysical processes, which are being leveraged to optimize the performance of OLED devices. Notably, we also appraise dendritic and polymeric TADF emitters — macromolecular materials that offer the potential advantages of low cost, solution processable and large-area OLED fabrication. Thermally activated delayed fluorescence (TADF) emitters are promising electroluminescent materials for next-generation organic light-emitting diodes (OLEDs). In this Review, the molecular design, photophysical characteristics and OLEDs composed of small-molecule, dendritic and polymeric TADF emitters are discussed.

Nanomaterials derived from metal–organic frameworks

Abstract: The thermal transformation of metal–organic frameworks (MOFs) generates a variety of nanostructured materials, including carbon-based materials, metal oxides, metal chalcogenides, metal phosphides and metal carbides. These derivatives of MOFs have characteristics such as high surface areas, permanent porosities and controllable functionalities that enable their good performance in sensing, gas storage, catalysis and energy-related applications. Although progress has been made to tune the morphologies of MOF-derived structures at the nanometre scale, it remains crucial to further our knowledge of the relationship between morphology and performance. In this Review, we summarize the synthetic strategies and optimized methods that enable control over the size, morphology, composition and structure of the derived nanomaterials. In addition, we compare the performance of materials prepared by the MOF-templated strategy and other synthetic methods. Our aim is to reveal the relationship between the morphology and the physico-chemical properties of MOF-derived nanostructures to optimize their performance for applications such as sensing, catalysis, and energy storage and conversion. Nanomaterials derived from metal–organic frameworks (MOFs) show good performance in sensing, gas storage, catalysis and energy-related applications. In this Review, the influence of the morphology of MOF-derived nanostructures on their performance is elucidated, and the opportunities in this field are discussed.

Majorana zero modes in superconductor–semiconductor heterostructures

Abstract: Realizing topological superconductivity and Majorana zero modes in the laboratory is a major goal in condensed-matter physics. In this Review, we survey the current status of this rapidly developing field, focusing on proposals for the realization of topological superconductivity in semiconductor–superconductor heterostructures. We examine materials science progress in growing InAs and InSb semiconductor nanowires and characterizing these systems. We then discuss the observation of robust signatures of Majorana zero modes in recent experiments, paying particular attention to zero-bias tunnelling conduction measurements and Coulomb blockade experiments. We also outline several next-generation experiments probing exotic properties of Majorana zero modes, including fusion rules and non-Abelian exchange statistics. Finally, we discuss prospects for implementing Majorana-based topological quantum computation.

Biomedical applications of soft robotics

Abstract: Soft robotics enables the design of soft machines and devices at different scales. The compliance and mechanical properties of soft robots make them especially interesting for medical applications. Depending on the level of interaction with humans, different levels of biocompatibility and biomimicry are required for soft materials used in robots. In this Review, we investigate soft robots for biomedical applications, including soft tools for surgery, diagnosis and drug delivery, wearable and assistive devices, prostheses, artificial organs and tissue-mimicking active simulators for training and biomechanical studies. We highlight challenges regarding durability and reliability, and examine traditional and novel soft and active materials as well as different actuation strategies. Finally, we discuss future approaches and applications in the field. Soft robots have broad applications in medicine. In this Review, biomedical applications, including surgery, drug delivery, prostheses, wearable devices and artificial organs, are discussed in the context of materials, actuation strategies and challenges.

Scalable fabrication of perovskite solar cells

Abstract: Perovskite materials use earth-abundant elements, have low formation energies for deposition and are compatible with roll-to-roll and other high-volume manufacturing techniques. These features make perovskite solar cells (PSCs) suitable for terawatt-scale energy production with low production costs and low capital expenditure. Demonstrations of performance comparable to that of other thin-film photovoltaics (PVs) and improvements in laboratory-scale cell stability have recently made scale up of this PV technology an intense area of research focus. Here, we review recent progress and challenges in scaling up PSCs and related efforts to enable the terawatt-scale manufacturing and deployment of this PV technology. We discuss common device and module architectures, scalable deposition methods and progress in the scalable deposition of perovskite and charge-transport layers. We also provide an overview of device and module stability, module-level characterization techniques and techno-economic analyses of perovskite PV modules. Perovskite solar cells (PSCs) have emerged as a revolutionary class of photovoltaic technology. Here, we review recent progress and challenges in scaling up PSCs towards commercialization. We discuss several areas, including device architectures, deposition methods, scalable deposition of perovskite and charge transport layers, device stability, module-level characterization and techno-economic analyses.

Advances in organ-on-a-chip engineering

Abstract: Predicting the effects of drugs before human clinical trials is at the heart of drug screening and discovery processes. The cost of drug discovery is steadily increasing owing to the limited predictability of 2D cell culture and animal models. The convergence of microfabrication and tissue engineering gave rise to organ-on-a-chip technologies, which offer an alternative to conventional preclinical models for drug screening. Organ-on-a-chip devices can replicate key aspects of human physiology crucial for the understanding of drug effects, improving preclinical safety and efficacy testing. In this Review, we discuss how organ-on-a-chip technologies can recreate functions of organs, focusing on tissue barrier properties, parenchymal tissue function and multi-organ interactions, which are three key aspects of human physiology. Specific organ-on-a-chip systems are examined in terms of cell sources, functional hallmarks and available disease models. Finally, we highlight the challenges that need to be overcome for the clinical translation of organ-on-a-chip devices regarding materials, cellular fidelity, multiplexing, sensing, scalability and validation. Organ-on-a-chip devices can recreate key aspects of human physiology in vitro, offering an alternative to animal models for preclinical drug testing. This Review examines how tissue barrier properties, parenchymal tissue function and multi-organ interactions can be recreated in organ-on-a-chip systems and applied for drug screening.

Material platforms for spin-based photonic quantum technologies

Abstract: A central goal in quantum optics and quantum information science is the development of quantum networks to generate entanglement between distributed quantum memories. Experimental progress relies on the quality and efficiency of the light–matter quantum interface connecting the quantum states of photons to internal states of quantum emitters. Quantum emitters in solids, which have properties resembling those of atoms and ions, offer an opportunity for realizing light–matter quantum interfaces in scalable and compact hardware. These quantum emitters require a material platform that enables stable spin and optical properties, as well as a robust manufacturing of quantum photonic circuits. Because no emitter system is yet perfect and different applications may require different properties, several light–matter quantum interfaces are being developed in various platforms. This Review highlights the progress in three leading material platforms: diamond, silicon carbide and atomically thin semiconductors. Atom-like quantum emitters in solids have emerged as promising building blocks for quantum information processing. In this Review, recent advances in three leading material platforms—diamond, silicon carbide and atomically thin semiconductors—are summarized, with a focus on applications in quantum networks

3D printing of soft robotic systems

Abstract: Soft robots are capable of mimicking the complex motion of animals. Soft robotic systems are defined by their compliance, which allows for continuous and often responsive localized deformation. These features make soft robots especially interesting for integration with human tissues, for example, the implementation of biomedical devices, and for robotic performance in harsh or uncertain environments, for example, exploration in confined spaces or locomotion on uneven terrain. Advances in soft materials and additive manufacturing technologies have enabled the design of soft robots with sophisticated capabilities, such as jumping, complex 3D movements, gripping and releasing. In this Review, we examine the essential soft material properties for different elements of soft robots, highlighting the most relevant polymer systems. Advantages and limitations of different additive manufacturing processes, including 3D printing, fused deposition modelling, direct ink writing, selective laser sintering, inkjet printing and stereolithography, are discussed, and the different techniques are investigated for their application in soft robotic fabrication. Finally, we explore integrated robotic systems and give an outlook for the future of the field and remaining challenges. 3D printing can be used to directly fabricate soft robots. This Review discusses advances in 3D printing technologies and soft materials for the fabrication of soft robotic systems with sophisticated capabilities, such as 3D movement and responsiveness to the environment.

Extracellular matrix-based materials for regenerative medicine

Abstract: In tissue engineering and regenerative medicine, a biomaterial provides mechanical support and biochemical signals to encourage cell attachment and modulate cell behaviour. Nature’s template for a biomaterial is the extracellular matrix (ECM). The ECM contains intrinsic biochemical and mechanical cues that regulate cell phenotype and function in development, in homeostasis and in response to injury. The use of ECM-based materials in biomedical research has advanced from coating cell culture plates with purified ECM components to the design of ECM-mimicking biomaterials and the engineering of decellularized tissues aimed at recapitulating the dynamics, composition and structure of the ECM. In this Review, we highlight important matrix properties and functions in the context of tissue engineering and regenerative medicine, consider techniques such as proteomics for the investigation of matrix structure and composition and discuss different engineering strategies for the design of matrix-mimicking biomaterials. Tissue, whole organ and cell culture decellularization approaches are examined for their potential to preserve the tissue-specific biochemical composition and ultrastructure of the ECM and for the development of biomaterials that promote the formation of functional tissues in clinical applications. Finally, we investigate challenges and opportunities of ECM biomaterials for the design of organotypic models to study disease progression, for the ex vivo creation of engineered tissue and for the clinical translation of functional tissue reconstruction strategies in vivo. The extracellular matrix is nature’s template for an ideal biomaterial to guide tissue homeostasis and repair. In this Review, matrix-mimicking biomaterials and decellularized matrices are discussed for their potential to reconstruct and repair tissues in vitro and in vivo.

Accelerating the discovery of materials for clean energy in the era of smart automation

Abstract: The discovery and development of novel materials in the field of energy are essential to accelerate the transition to a low-carbon economy. Bringing recent technological innovations in automation, robotics and computer science together with current approaches in chemistry, materials synthesis and characterization will act as a catalyst for revolutionizing traditional research and development in both industry and academia. This Perspective provides a vision for an integrated artificial intelligence approach towards autonomous materials discovery, which, in our opinion, will emerge within the next 5 to 10 years. The approach we discuss requires the integration of the following tools, which have already seen substantial development to date: high-throughput virtual screening, automated synthesis planning, automated laboratories and machine learning algorithms. In addition to reducing the time to deployment of new materials by an order of magnitude, this integrated approach is expected to lower the cost associated with the initial discovery. Thus, the price of the final products (for example, solar panels, batteries and electric vehicles) will also decrease. This in turn will enable industries and governments to meet more ambitious targets in terms of reducing greenhouse gas emissions at a faster pace. The discovery and development of advanced materials are imperative for the clean energy sector. We envision that a closed-loop approach, which combines high-throughput computation, artificial intelligence and advanced robotics, will sizeably reduce the time to deployment and the costs associated with materials development.

Biofabrication strategies for 3D in vitro models and regenerative medicine

Abstract: Organs are complex systems composed of different cells, proteins and signalling molecules that are arranged in a highly ordered structure to orchestrate a myriad of functions in our body. Biofabrication strategies can be applied to engineer 3D tissue models in vitro by mimicking the structure and function of native tissue through the precise deposition and assembly of materials and cells. This approach allows the spatiotemporal control over cell–cell and cell–extracellular matrix communication and thus the recreation of tissue-like structures. In this Review, we examine biofabrication strategies for the construction of functional tissue replacements and organ models, focusing on the development of biomaterials, such as supramolecular and photosensitive materials, that can be processed using biofabrication techniques. We highlight bioprinted and bioassembled tissue models and survey biofabrication techniques for their potential to recreate complex tissue properties, such as shape, vasculature and specific functionalities. Finally, we discuss challenges, such as scalability and the foreign body response, and opportunities in the field and provide an outlook to the future of biofabrication in regenerative medicine. Biofabrication can be applied to replicate tissues and organs for regenerative medicine and for the creation of 3D in vitro tissue models. In this Review, the recent advances in biomaterials and biofabrication technologies are discussed, and challenges and opportunities are highlighted.

Nanofibrils in nature and materials engineering

Abstract: Nanofibrillar materials, such as cellulose, chitin and silk, are highly ordered architectures, formed through the self-assembly of repetitive building blocks into higher-order structures, which are stabilized by non-covalent interactions. This hierarchical building principle endows many biological materials with remarkable mechanical strength, anisotropy, flexibility and optical properties, such as structural colour. These features make nanofibrillar biopolymers interesting candidates for the development of strong, sustainable and biocompatible materials for environmental, energy, optical and biomedical applications. However, recreating their architecture is challenging from an engineering perspective. Rational design approaches, applying a combination of theoretical and experimental protocols, have enabled the design of biopolymer-based materials through mimicking nature's multiscale assembly approach. In this Review, we summarize hierarchical design strategies of cellulose, silk and chitin, focusing on nanoconfinement, fibrillar orientation and alignment in 2D and 3D structures. These multiscale architectures are discussed in the context of mechanical and optical properties, and different fabrication strategies for the manufacturing of biopolymer nanofibril-based materials are investigated. We highlight the contribution of rational material design strategies to the development of mechanically anisotropic and responsive materials and examine the future of the material-by-design paradigm. Nanofibrils are abundant and critical structural components in nature that can be exploited for novel and sustainable materials. In this Review, hierarchical design strategies for cellulose, silk and chitin nanofibrils in nature and in materials engineering are discussed.

Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond

Abstract: The magnetic fields generated by spins and currents provide a unique window into the physics of correlated-electron materials and devices. First proposed only a decade ago, magnetometry based on the electron spin of nitrogen-vacancy (NV) defects in diamond is emerging as a platform that is excellently suited for probing condensed matter systems; it can be operated from cryogenic temperatures to above room temperature, has a dynamic range spanning from direct current to gigahertz and allows sensor–sample distances as small as a few nanometres. As such, NV magnetometry provides access to static and dynamic magnetic and electronic phenomena with nanoscale spatial resolution. Pioneering work has focused on proof-of-principle demonstrations of its nanoscale imaging resolution and magnetic field sensitivity. Now, experiments are starting to probe the correlated-electron physics of magnets and superconductors and to explore the current distributions in low-dimensional materials. In this Review, we discuss the application of NV magnetometry to the exploration of condensed matter physics, focusing on its use to study static and dynamic magnetic textures and static and dynamic current distributions. The spin of the nitrogen-vacancy (NV) defect in diamond acts as a sensitive, atomic-sized magnetic field sensor that provides nanoscale access to the properties of condensed matter systems. This Review introduces NV magnetometry and discusses its application to the exploration of static and dynamic magnetism and electric current distributions.

Design, fabrication and control of origami robots

Abstract: Origami robots are created using folding processes, which provide a simple approach to fabricating a wide range of robot morphologies. Inspired by biological systems, engineers have started to explore origami folding in combination with smart material actuators to enable intrinsic actuation as a means to decouple design from fabrication complexity. The built-in crease structure of origami bodies has the potential to yield compliance and exhibit many soft body properties. Conventional fabrication of robots is generally a bottom-up assembly process with multiple low-level steps for creating subsystems that include manual operations and often multiple iterations. By contrast, natural systems achieve elegant designs and complex functionalities using top-down parallel transformation approaches such as folding. Folding in nature creates a wide spectrum of complex morpho-functional structures such as proteins and intestines and enables the development of structures such as flowers, leaves and insect wings. Inspired by nature, engineers have started to explore folding powered by embedded smart material actuators to create origami robots. The design and fabrication of origami robots exploits top-down, parallel transformation approaches to achieve elegant designs and complex functionalities. In this Review, we first introduce the concept of origami robotics and then highlight advances in design principles, fabrication methods, actuation, smart materials and control algorithms. Applications of origami robots for a variety of devices are investigated, and future directions of the field are discussed, examining both challenges and opportunities. Inspired by biological systems, engineers are exploring origami folding with smart material actuation to enable intrinsically actuated designs with complex functionalities and easy fabrication. This Review highlights recent advances in the design, fabrication and control of these origami robots.

Transport and interactions of nanoparticles in the kidneys

Abstract: Kidneys are a major organ for blood filtration and waste elimination and thus play a key role in the transport and clearance of nanoparticles in vivo. The interactions of nanoparticles with different kidney compartments can be precisely regulated by modulating their size, shape and surface chemistry. The quantitative understanding of nanoparticle–kidney interactions at the molecular level is important for improving disease targeting, precisely controlling nanoparticle transport and clearance, and minimizing the potential health hazards of nanomedicines. In this Review, we summarize the glomerular filtration of macromolecules and nanoparticles in the kidney and survey kidney imaging techniques for the study of nanoparticle–kidney interactions ex vivo and in vivo. We investigate the different transport mechanisms of nanoparticles in the kidneys and discuss size, charge and shape dependencies in renal clearance. Nanoparticles are then investigated for the preclinical and clinical detection and treatment of diseases such as kidney dysfunction and cancer. Finally, challenges and opportunities for renal-clearable nanoparticles are highlighted. Interactions of nanoparticles with the kidneys affect their transport, clearance, targeting, therapeutic efficacy and biosafety in the body. This Review discusses nano–bio interactions of nanoparticles in the kidneys and highlights their potential for the detection and treatment of disease.

Tissue–electronics interfaces: from implantable devices to engineered tissues

Abstract: Advances in electronic devices have opened opportunities for extracting a variety of data from the human body, and for the treatment of diseases. In this Review, tissue properties affecting device integration are described and electronic systems interfacing with organs and engineered tissues are highlighted.

Epitaxial growth of hybrid nanostructures

Abstract: Hybrid nanostructures are a class of materials that are typically composed of two or more different components, in which each component has at least one dimension on the nanoscale. The rational design and controlled synthesis of hybrid nanostructures are of great importance in enabling the fine tuning of their properties and functions. Epitaxial growth is a promising approach to the controlled synthesis of hybrid nanostructures with desired structures, crystal phases, exposed facets and/or interfaces. This Review provides a critical summary of the state of the art in the field of epitaxial growth of hybrid nanostructures. We discuss the historical development, architectures and compositions, epitaxy methods, characterization techniques and advantages of epitaxial hybrid nanostructures. Finally, we provide insight into future research directions in this area, which include the epitaxial growth of hybrid nanostructures from a wider range of materials, the study of the underlying mechanism and determining the role of epitaxial growth in influencing the properties and application performance of hybrid nanostructures. Epitaxial hybrid nanostructures can show different functionalities and superior performance in applications from those of the individual components. This Review discusses the methods of preparation and techniques for characterization of epitaxial hybrid nanostructures with various architectures, and examines the role of epitaxial growth in influencing the properties and application performance of hybrid nanostructures.

Photoresponsive biomaterials for targeted drug delivery and 4D cell culture

Abstract: Biological signalling is regulated through a complex and tightly choreographed interplay between cells and their extracellular matrix. The spatiotemporal control of these interactions is essential for tissue function, and disruptions to this dialogue often result in aberrant cell fate and disease. When disturbances are well understood, correct biological function can be restored through the precise introduction of therapeutics. Moreover, model systems with modifiable physiochemical properties are needed to probe the effects of therapeutic molecules and to investigate cell–matrix interactions. Photoresponsive biomaterials benefit from spatiotemporal tunability, which allows for site-specific therapeutic delivery in vivo and 4D modulation of synthetic cell culture platforms to mimic the dynamic heterogeneity of the human body in vitro. In this Review, we discuss how light can be exploited to modify different biomaterials in the context of photomediated drug delivery and phototunable cell culture platforms. We survey various photochemistries for their applicability in vitro and in vivo and for the biochemical and biophysical modification of materials. Finally, we highlight emerging tools and provide an outlook for the field of photoresponsive biomaterials. Light can initiate chemistries with high spatial and temporal control. In this Review, photoresponsive biomaterials developed for controlled drug delivery and complex tissue engineering are investigated with a focus on photochemistries that provide dynamic precision.

Point defect engineering in thin-film solar cells

Abstract: Control of defect processes in photovoltaic materials is essential for realizing high-efficiency solar cells and related optoelectronic devices. Native defects and extrinsic dopants tune the Fermi level and enable semiconducting p–n junctions; however, fundamental limits to doping exist in many compounds. Optical transitions from defect states can enhance photocurrent generation through sub-bandgap absorption; however, these defect states are also often responsible for carrier trapping and non-radiative recombination events that limit the voltage in operating solar cells. Many classes of materials, including metal oxides, chalcogenides and halides, are being examined for next-generation solar energy applications, and each technology faces distinct challenges that could benefit from point defect engineering. Here, we review the evolution in the understanding of point defect behaviour from Si-based photovoltaics to thin-film CdTe and Cu(In,Ga)Se2 technologies, through to the latest generation of halide perovskite (CH3NH3PbI3) and kesterite (Cu2ZnSnS4) devices. We focus on the chemical bonding that underpins the defect chemistry and the atomistic processes associated with the photophysics of charge-carrier generation, trapping and recombination in solar cells. Finally, we outline general principles to enable defect control in complex semiconducting materials. Point defects have a key role in determining the performance of photovoltaic materials. In this Review, we assess defect processes in a range of photovoltaic materials and outline how point defect engineering could be used to improve the efficiency of solar cells.

Liquid, glass and amorphous solid states of coordination polymers and metal–organic frameworks

Abstract: The field of coordination polymers and metal–organic frameworks has to date focused on the crystalline state. More than 60,000 crystalline metal–organic framework structures, formed from highly ordered arrays of metal nodes connected by organic ligands in at least one dimension, have been identified. However, interest in non-crystalline systems is growing, with amorphous solids, glasses and liquids identified as possessing similar metal–ligand bonding motifs to their crystalline cousins. In this Review, we provide an overview of the structural design, properties and potential applications of non-crystalline coordination polymers and metal–organic frameworks. In particular, we highlight recent reports of glasses that result from the melt quenching of the liquid states of these topical classes of materials. Finally, we provide a perspective on the future of the non-crystalline domain of coordination polymers and metal–organic frameworks. There is increasing interest in the liquid, glass and amorphous solid states of coordination polymers and metal–organic frameworks. In this Review, we discuss the background and terminology of this emerging field, categorize example structures and provide an outlook for the future direction of the field.

Heusler, Weyl and Berry

Abstract: Heusler compounds, initially discovered by Fritz Heusler more than a century ago, have grown into a family of more than 1,000 compounds, synthesized from combinations of more than 40 elements. Recently, by incorporating heavy elements that can give rise to strong spin–orbit coupling, non-trivial topological phases of matter, such as topological insulators, have been discovered in Heusler materials. Moreover, interplay between the symmetry, spin–orbit coupling and magnetic structure allows for the realization of a wide variety of topological phases through Berry curvature design. The topological properties of Heusler compounds can be manipulated by various external perturbations, resulting in exotic properties, such as the chiral anomaly and large anomalous, spin and topological Hall effects. In addition, the non-zero Berry curvature that arises as a result of non-collinear order gives rise to a non-zero anomalous Hall effect. Besides this k-space Berry curvature, Heusler compounds with non-collinear magnetic structures also possess real-space topological states in the form of magnetic antiskyrmions, which have not yet been observed in other materials. In this Review, we discuss Heusler compounds from a topological perspective and the connection between the topology and the symmetry properties, spin gapless semiconductors, magnetic compensated ferrimagnets, non-collinear order in ferromagnetic and antiferromagnetic Heusler compounds, the anomalous Hall effect and, finally, magnetic antiskyrmions. Together with the new topological viewpoint and the high tunability, novel physical properties and phenomena await discovery in Heusler compounds.

Miniature soft robots — road to the clinic

Abstract: Soft small robots offer the opportunity to non-invasively access human tissue to perform medical operations and deliver drugs; however, challenges in materials design, biocompatibility and function control remain to be overcome for soft robots to reach the clinic.

Uncovering the structure-function relationship in spider silk

Abstract: All spiders produce protein-based biopolymer fibres that we call silk. The most studied of these silks is spider dragline silk, which is very tough and relatively abundant compared with other types of spider silks. Considerable research has been devoted to understanding the relationship between the molecular structure and mechanical properties of spider dragline silks. In this Review, we overview experimental and computational studies that have provided a wealth of detail at the molecular level on the highly conserved repetitive core and terminal regions of spider dragline silk. We also discuss the role of the nanocrystalline β-sheets and amorphous regions in determining the properties of spider silk fibres, endowing them with strength and elasticity. Additionally, we outline imaging techniques and modelling studies that elucidate the importance of the hierarchical structure of silk fibres at the molecular level. These insights into structure–function relationships can guide the reverse engineering of spider silk to enable the production of superior synthetic fibres. Experimental and computational studies reveal numerous aspects of the molecular and hierarchical structure of spider silk and of its molecular dynamics. In this Review, we discuss the structure–function relationships of spider silk that can be elucidated from these studies and how this knowledge may enable the reverse engineering of spider silk.

Graphene-based integrated photonics for next-generation datacom and telecom

Abstract: Graphene is an ideal material for optoelectronic applications. Its photonic properties give several advantages and complementarities over Si photonics. For example, graphene enables both electro-absorption and electro-refraction modulation with an electro-optical index change exceeding 10−3. It can be used for optical add–drop multiplexing with voltage control, eliminating the current dissipation used for the thermal detuning of microresonators, and for thermoelectric-based ultrafast optical detectors that generate a voltage without transimpedance amplifiers. Here, we present our vision for graphene-based integrated photonics. We review graphene-based transceivers and compare them with existing technologies. Strategies for improving power consumption, manufacturability and wafer-scale integration are addressed. We outline a roadmap of the technological requirements to meet the demands of the datacom and telecom markets. We show that graphene-based integrated photonics could enable ultrahigh spatial bandwidth density, low power consumption for board connectivity and connectivity between data centres, access networks and metropolitan, core, regional and long-haul optical communications.

Plasmonic polymer nanocomposites

Abstract: The optical properties of metal nanoparticles, particularly their localized surface plasmon effects, are well established. These plasmonic nanoparticles can respond to their surroundings or even influence the optical processes (for example, absorption, fluorescence and Raman scattering) of molecules located at their surface. As a result, plasmonic nanoparticles have been developed for multiple purposes, ranging from the detection of chemicals and biological molecules to light-harvesting enhancement in solar cells. By dispersing the nanoparticles in polymers and creating a hybrid material, the robustness, responsiveness and flexibility of the system are enhanced while preserving the intrinsic properties of the nanoparticles. In this Review, we discuss the fabrication and applications of plasmonic polymer nanocomposites, focusing on applications in optical data storage, sensing and imaging and photothermal gels for in vivo therapy. Within the nanocomposites, the nanoporosity of the matrix, the overall mechanical stability and the dispersion of the nanoparticles are important parameters for achieving the best performance. In the future, translation of these materials into commercial products rests on the ability to scale up the production of plasmonic polymer nanocomposites with tailored optical features.

Tissue repair and regeneration with endogenous stem cells

Abstract: In contrast to non-mammalian vertebrates, mammals and humans have limited innate capacity for the self-regeneration of tissues and organs owing to differences in genetics, development, immune systems and tissue complexity. Endogenous stem cells are tissue-specific adult stem cells with the capacity to self-renew and differentiate into specific cell types. Therefore, endogenous stem cells are being explored for the regeneration of tissues in situ and in vivo. Stem cells reside in specific niches in the body, and stem cell activation depends on progressive changes in the niche. Niches are specific and instructive microenvironments that can be recreated using biomaterial-based scaffolds. Such scaffolds can be fabricated into a variety of shapes and formulations, and they can be functionalized with biochemical and biophysical cues to guide stem cell fate and migration. In this Review, we discuss important differences in the self-regeneration abilities of non-mammalian vertebrates and mammals, including humans, and investigate adult stem cell populations and their niches involved in tissue repair and regeneration. We highlight natural and synthetic biomaterials and their potential for improving applications of endogenous stem cells and examine the role of interspecies chimaeras in regenerative medicine.

Inorganic semiconductor biointerfaces

Abstract: Biological systems respond to and communicate through biophysical cues, such as electrical, thermal, mechanical and topographical signals. However, precise tools for introducing localized physical stimuli and/or for sensing biological responses to biophysical signals with high spatiotemporal resolution are limited. Inorganic semiconductors display many relevant electrical and optical properties, and they can be fabricated into a broad spectrum of electronic and photonic devices. Inorganic semiconductor devices enable the formation of functional interfaces with biological material, ranging from proteins to whole organs. In this Review, we discuss fundamental semiconductor physics and operation principles, with a focus on their behaviour in physiological conditions, and highlight the advantages of inorganic semiconductors for the establishment of biointerfaces. We examine semiconductor device design and synthesis and discuss typical signal transduction mechanisms at bioelectronic and biophotonic interfaces for electronic and optoelectronic sensing, optoelectronic and photothermal stimulation and photoluminescent in vivo imaging of cells and tissues. Finally, we evaluate cytotoxicity and highlight possible new material components and biological targets of inorganic semiconductor devices. Inorganic semiconductor devices enable the formation of functional interfaces with cells and tissues to detect or provide physical stimuli. In this Review, inorganic semiconductor materials are discussed for electronic and optoelectronic sensing, optoelectronic and photothermal stimulation and photoluminescent in vivo imaging.

Biomaterials for cell transplantation

Abstract: Cell transplantation holds immense potential for reversing diseases that are currently incurable and for regenerating tissues. However, poor cell survival, cell aggregation and lack of cell integration into the host tissue constitute major challenges for the clinical translation of cell transplantation approaches. Biomaterials can influence cell behaviour in vitro and in vivo. The mechanical and biochemical properties of biomaterials can be tailored to affect cell survival, differentiation and migration. Therefore, the integration of advanced material design with stem cell biology may hold the key to improving the efficacy of cell transplantation. In this Review, we discuss biomaterial design strategies for their potential to influence the fate of transplanted cells and to manipulate the host microenvironment. We examine how biomaterial properties can be modulated to improve transplanted cell survival, differentiation and cell engraftment and how the host tissue can be manipulated for cell transplantation by inducing plasticity and vascularization. Finally, we emphasize the importance of the host immune cells for tissue repair and cell transplantation and discuss strategies to tune the immune response through modulating the mechanical properties, architecture, chemistry and functionalization of biomaterials. Cells can be transplanted into the body to both repair injured or diseased tissue and restore tissue function. This Review discusses how biomaterial design strategies can be used to improve cell survival, influence the fate of transplanted cells, and favourably manipulate the host microenvironment and the immune system.