KJN

Nature Nanotechnology, Published online: 04 November 2019; doi:10.1038/s41565-019-0576-x

New nature‐inspired composites are developed for dental applications. These composites are similar to human teeth in terms of hardness, stiffness, and strength, and are extremely damage tolerant. They can also effectively alleviate the abrasion to opposing teeth and display outstanding machinability. These properties are unprecedented in current dental materials and thereby make the composites promising as new‐generation tooth replacements.

Abstract

Making replacements for the human body similar to natural tissue offers significant advantages but remains a key challenge. This is pertinent for synthetic dental materials, which rarely reproduce the actual properties of human teeth and generally demonstrate relatively poor damage tolerance. Here new bioinspired ceramic–polymer composites with nacre‐mimetic lamellar and brick‐and‐mortar architectures are reported, which resemble, respectively, human dentin and enamel in hardness, stiffness, and strength and exhibit exceptional fracture toughness. These composites are additionally distinguished by outstanding machinability, energy‐dissipating capability under cyclic loading, and diminished abrasion to antagonist teeth. The underlying design principles and toughening mechanisms of these materials are elucidated in terms of their distinct architectures. It is demonstrated that these composites are promising candidates for dental applications, such as new‐generation tooth replacements. Finally, it is believed that this notion of bioinspired design of new materials with unprecedented biologically comparable properties can be extended to a wide range of material systems for improved mechanical performance.

Monolithic silicone lenses containing integral optical filters are prepared by casting a liquid silicone pre‐polymer solution onto a porous silicon photonic crystal, whose surface nanostructure is engineered to form a nearly hemispherical lens. The self‐adherent lenses can convert a smartphone camera into a high‐fidelity fluorescence microscope without complicated fixturing.

Abstract

In this work, both light‐shaping and image magnification features are integrated into a single lens element using a moldless procedure that takes advantage of the physical and optical properties of mesoporous silicon (PSi) photonic crystal nanostructures. Casting of a liquid poly(dimethylsiloxane) pre‐polymer solution onto a PSi film generates a droplet with a contact angle that is readily controlled by the silicon nanostructure, and adhesion of the cured polymer to the PSi photonic crystal allows preparation of lightweight (10 mg) freestanding lenses (4.7 mm focal length) with an embedded optical component (e.g., optical rugate filter, resonant cavity, and distributed Bragg reflector). The fabrication process shows excellent reliability (yield 95%) and low cost and the lens is expected to have implications in a wide range of applications. As a proof‐of‐concept, using a single monolithic lens/filter element it is demonstrated: fluorescence imaging of isolated human cancer cells with rejection of the blue excitation light, through a lens that is self‐adhered to a commercial smartphone; shaping of the emission spectrum of a white light emitting diode to tune the color from red through blue; and selection of a narrow wavelength band (bandwidth 5 nm) from a fluorescent molecular probe.

An autonomous and degradable active microneedle delivery platform obviating external stimuli is presented. Active microneedles employ entrapped magnesium microparticles as built‐in engines capable of generating vigorous convective fluid flows for deeper and faster intradermal payload delivery. The “built‐in” active delivery strategy holds considerable promise for transdermal biomedical applications offering an attractive efficient delivery route compared to traditional passive microneedle patches.

Abstract

The use of microneedles has facilitated the painless localized delivery of drugs across the skin. However, their efficacy has been limited by slow diffusion of molecules and often requires external triggers. Herein, an autonomous and degradable, active microneedle delivery platform is introduced, employing magnesium microparticles loaded within the microneedle patch, as the built‐in engine for deeper and faster intradermal payload delivery. The magnesium particles react with the interstitial fluid, leading to an explosive‐like rapid production of H₂ bubbles, providing the necessary force to breach dermal barriers and enhance payload delivery. The release kinetics of active microneedles is evaluated in vitro by measuring the amount of IgG antibody (as a model drug) that passed through phantom tissue and a pigskin barrier. In vivo experiments using a B16F10 mouse melanoma model demonstrate that the active delivery of anti‐CTLA‐4 (a checkpoint inhibitor drug) results in greatly enhanced immune response and significantly longer survival. Moreover, spatially resolved zones of active and passive microneedles allow a combinatorial rapid burst response along with slow, sustained release, respectively. Such versatile and effective autonomous dynamic microneedle delivery technology offers considerable promise for a wide range of therapeutic applications, toward a greatly enhanced outcome, convenience, and cost.

Elemental 2D materials are some of the more exciting nanomaterials of interest due to their unique and tailorable properties. Recent progress in these exciting materials relative to their potential impact in applications including electronics, optoelectronics, and energy systems is presented.

Abstract

As elemental main group materials (i.e., silicon and germanium) have dominated the field of modern electronics, their monolayer 2D analogues have shown great promise for next‐generation electronic materials as well as potential game‐changing properties for optoelectronics, energy, and beyond. These atomically thin materials composed of single atomic variants of group III through group VI elements on the periodic table have already demonstrated exciting properties such as near‐room‐temperature topological insulation in bismuthene, extremely high electron mobilities in phosphorene and silicone, and substantial Li‐ion storage capability in borophene. Isolation of these materials within the postgraphene era began with silicene in 2010 and quickly progressed to the experimental identification or theoretical prediction of 15 of the 18 main group elements existing as solids at standard pressure and temperatures. This review first focuses on the significance of defects/functionalization, discussion of different allotropes, and overarching structure–property relationships of 2D main group elemental materials. Then, a complete review of emerging applications in electronics, sensing, spintronics, plasmonics, photodetectors, ultrafast lasers, batteries, supercapacitors, and thermoelectrics is presented by application type, including detailed descriptions of how the material properties may be tailored toward each specific application.

A novel solar evaporation system is developed by hanging hydrophilic photothermal fabric in air and immersing two fabric edges in seawater. Under the irradiation of sunlight, the hanging fabric can continuously and efficiently produce a watersteam for fresh water and concentrated brine for the chlor‐alkali industry without solid‐salt accumulation, offering a promising method for large‐scale desalination.

Abstract

Solar‐enabled evaporation for seawater desalination is an attractive, renewable, and environment‐friendly technique, and tremendous progress has been achieved by developing various photothermal membranes. However, traditional photothermal membranes directly float on water, resulting in some limitations such as unavoidable heat‐loss to bulk water and severe salt accumulation. To solve these problems, a hydrophilic, polymer nanorod‐coated photothermal fabric is designed and fabricated, and then an indirect‐contact evaporation system by hanging the fabric is demonstrated. The two ends of the fabric are designed to be in contact with seawater to guide water flow through capillary suction. Both arc‐shaped top/bottom surfaces of the hanging fabrics are exposed to air, which can prevent heat dissipation to bulk seawater and facilitate the double‐surface evaporation upon sunlight irradiation. Our design leads to an efficient evaporation rate of 1.94 kg m⁻² h⁻¹ and high solar efficiency of 89.9% upon irradiation with sunlight (1.0 kW m⁻²). Importantly, the highly concentrated brine can drip from the bottom of the arc‐shaped fabric, without the appearance of solid‐salt accumulation. This indirect‐contact evaporation system establishes a new path to continuously and economically produce watersteam from seawater for fresh‐water and concentrated brine for the chlor‐alkali industry.

The intercalation of lithium and sodium in black phosphorus with orientation‐dependent channels and distinct anisotropic pathways is discovered using in situ transmission electron microscopy combined with density functional theory calculations. The atomic structure evolution along zigzag and armchair directions and the relevant changes in chemical states are elucidated, which offers a fundamental understanding of intercalation mechanisms.

Abstract

Black phosphorus (BP) with unique 2D structure enables the intercalation of foreign elements or molecules, which makes BP directly relevant to high‐capacity rechargeable batteries and also opens a promising strategy for tunable electronic transport and superconductivity. However, the underlying intercalation mechanism is not fully understood. Here, a comparative investigation on the electrochemically driven intercalation of lithium and sodium using in situ transmission electron microscopy is presented. Despite the same preferable intercalation channels along [100] (zigzag) direction, distinct anisotropic intercalation behaviors are observed, i.e., Li ions activate lateral intercalation along [010] (armchair) direction to form an overall uniform propagation, whereas Na diffusion is limited in the zigzag channels to cause the columnar intercalation. First‐principles calculations indicate that the diffusion of both Li and Na ions along the zigzag direction is energetically favorable, while Li/Na diffusion long the armchair direction encounters an increased energy barrier, but that of Na is significantly larger and insurmountable, which accounts for the orientation‐dependent intercalation channels. The evolution of chemical states during phase transformations (from Li _x P/Na _x P to Li₃P/Na₃P) is identified by analytical electron diffraction and energy‐loss spectroscopy. The findings elucidate atomistic Li/Na intercalation mechanisms in BP and show potential implications for other similar 2D materials.

Nature Nanotechnology, Published online: 03 October 2019; doi:10.1038/s41565-019-0557-0

A high‐throughput inkjet printing approach is developed, and used to fabricate 25 mixed perovskite films from the sequential inkjet printing of four pure precursors in a fast and reproducible manner. The obtained film properties database enables to efficiently screen perovskite constituents for photovoltaic applications, highlighting the benefit of this approach.

Abstract

The exploration and optimization of numerous mixed perovskite compositions are causing a strong demand for high‐throughput synthesis. Nevertheless high‐throughput fabrication of perovskite films with representative film properties, which can efficiently screen the perovskite compositions for photovoltaic applications, has rarely been explored. A high‐throughput inkjet printing approach that can automatically fabricate perovskite films with various compositions with high reproducibility and high speed is developed. The automatic sequential printing of four precursors forms 25 mixed films in a fast and reproducible manner. The obtained bandgaps, photoluminescence (PL) peak positions, and PL lifetimes allow for the efficient screening of perovskite compositions for photovoltaic applications. To exemplify this concept, among 25 tested films, two compositions CH₃NH₃PbBr_0.75I_2.25 (MA) and (HC(NH₂)₂)_0.75(CH₃NH₃)_0.25PbBr_0.75I_2.25 (FA_0.75MA_0.25) with a long (237 ns) and short (49.0 ns) PL lifetime, respectively, are screened out for device investigations. As expected, the MA‐based device exhibits a much higher efficiency (19.0%) than that (15.3%) of the FA_0.75MA_0.25 counterpart. This efficiency improvement is mainly ascribed to a smaller dark saturate current density, a lower level of energetic disorder, more efficient charge transfer and decreased charge recombination losses, which are consistent with the much longer PL lifetime in the database.

A new approach to clean the surface of graphene is reported by using a force‐engineered “lint roller”, which is enabled by selectively removing intrinsic surface contaminants on graphene. The as‐obtained superclean graphene can be transferred to dielectric substrates with significantly reduced polymer residues and exhibits superior electronic and optical properties such as ultrahigh carrier mobility and low contact resistance.

Abstract

Contamination is a major concern in surface and interface technologies. Given that graphene is a 2D monolayer material with an extremely large surface area, surface contamination may seriously degrade its intrinsic properties and strongly hinder its applicability in surface and interfacial regions. However, large‐scale and facile treatment methods for producing clean graphene films that preserve its excellent properties have not yet been achieved. Herein, an efficient postgrowth treatment method for selectively removing surface contamination to achieve a large‐area superclean graphene surface is reported. The as‐obtained superclean graphene, with surface cleanness exceeding 99%, can be transferred to dielectric substrates with significantly reduced polymer residues, yielding ultrahigh carrier mobility of 500 000 cm² V⁻¹ s⁻¹ and low contact resistance of 118 Ω µm. The successful removal of contamination is enabled by the strong adhesive force of the activated‐carbon‐based lint roller on graphene contaminants.

A versatile water‐assisted printing technique is demonstrated to fabricate single‐ and multilayer graphene ink based devices onto 3D objects of different shapes, curvatures, and textures.

Abstract

Printing has drawn a lot of attention as a means of low per‐unit cost and high throughput patterning of graphene inks for scaled‐up thin‐form factor device manufacturing. However, traditional printing processes require a flat surface and are incapable of achieving patterning onto 3D objects. Here, a conformal printing method is presented to achieve functional graphene‐based patterns onto arbitrarily shaped surfaces. Using experimental design, a water‐insoluble graphene ink with optimum conductivity is formulated. Then single‐ and multilayered electrically functional structures are printed onto a sacrificial layer using conventional screen printing. The print is then floated on water, allowing the dissolution of the sacrificial layer, while retaining the functional patterns. The single‐ and multilayer patterns can then be directly transferred onto arbitrarily shaped 3D objects without requiring any postdeposition processing. Using this technique, conformal printing of single‐ and multilayer functional devices that include joule heaters, resistive deformation sensors, and proximity sensors on hard, flexible, and soft substrates, such as glass, latex, thermoplastics, textiles, and even candies and marshmallows, is demonstrated. This simple strategy promises to add new device and sensing functionalities to previously inert 3D surfaces.

Liquid metal droplets are tailored to remain liquid at temperatures as low as −85 °C. Their polymer composites are soft and stretchable even at extreme cold conditions and show exceptional thermal and electrical performance. This unique combination of properties is enabling for emerging technologies, including self‐powered electronics, deep‐sea underwater robots, and space applications.

Abstract

Elastomers embedded with droplets of liquid metal (LM) alloy represent an emerging class of soft multifunctional composites that have potentially transformative impact in wearable electronics, biocompatible machines, and soft robotics. However, for these applications it is crucial for LM alloys to remain liquid during the entire service temperature range in order to maintain high mechanical compliance throughout the duration of operation. Here, LM‐based functional composites that do not freeze and remain soft and stretchable at extremely low temperatures are introduced. It is shown that the confinement of LM droplets to micro‐/nanometer length scales significantly suppresses their freezing temperature (down to −84.1 from −5.9 °C) and melting point (down to −25.6 from +17.8 °C) independent of the choice of matrix material and processing conditions. Such a supercooling effect allows the LM inclusions to preserve their fluidic nature at low temperatures and stretch with the surrounding polymer matrix without introducing significant mechanical resistance. These results indicate that LM composites with highly stabilized droplets can operate over a wide temperature range and open up new possibilities for these emerging materials, which are demonstrated with self‐powered wearable thermoelectric devices for bio‐sensing and personal health monitoring at low temperatures.

Membrane separation is widely used for purifying water, but membrane fouling causes serious membrane flux attenuation, which is an obstacle for its application. The pore size of the polypyrrole‐dodecylbenzene sulfonate membrane can be intelligently controlled by simply applying a redox potential, so that it would effectively alleviate membrane fouling and achieve selective separation.

Abstract

Antifouling and selectivity are major challenges for membrane separation technology. Herein, a polypyrrole‐dodecylbenzene sulfonate (PPy‐DBS) membrane with tunable pores is fabricated to alleviate pore blocking and achieve selective separation. The insertion/extraction of ions during the electrical redox process causes the change of PPy‐DBS volume, so that the membrane pores can be tuned in situ by applying an external redox potential. The pore size of a fouled membrane is enlarged under an oxidation voltage, then the membrane is backwashed to eliminate foulants in the pores, after which membrane pore size is recovered under a reduction voltage. Thus, membrane fouling can be effectively alleviated by adjusting the membrane pore size combined with cleaning. The specific flux of PPy‐DBS membrane increased by 21.91% after applying the voltages and backwashing, and it exhibits great recycling performance. Moreover, the distribution of humic acid macromolecules in the permeate significantly decreased, proving the enhanced sieving effect of smaller membrane pores under negative voltage. This study provides an intelligent strategy for fouling prevention and selective separation in water treatment.

Drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). It is evident that as roughness increases, the threshold force to initiate drop movement decreases and that the surface friction properties depend on the topography characteristics (i.e., height and spacing).

Abstract

Superhydrophobic surfaces are extensively investigated in the literature, yet the phenomenon of drop motion on such surfaces and the corresponding friction properties of surfaces with different topography are not sufficiently analyzed. Here, drop motion on hydrophobic and superhydrophobic surfaces with different size topography is investigated for drops of largely varying viscosity (i.e., water and glycerol). The threshold force required to initiate drop movement is probed, the drop motion (velocity and acceleration) is analyzed, and the friction force on each surface is calculated. It is evident that as roughness increases, the threshold force to initiate 20 µL drop motion decreases; the lowest value for water is 17.9 ± 4.0 µN. For glycerol, the lowest threshold force value is 22.3 ± 5.9 µN. The results also indicate that this threshold force required for the initiation of the drop motion seems to be higher than that when the drop starts moving. Finally, this force (being proportional to the contact line) is expected to be about half smaller for 5 µL droplets. Water drops obtain higher velocities and accelerations by an order of magnitude compared to glycerol drops, which is attributed to the combinational effect of the higher hysteresis and the larger contact line of glycerol drops.

Metal‐phenolic network vesicles enclosing biocatalytic metal‐organic frameworks show functional and structural similarities to native cells, which can perform stimulus‐responsive, localized, and programmed biocascade reactions, mimicking multiple cellular events including metabolism, communication, and programmed degradation. Moreover, they exhibit ultrastability under diverse physical and chemical conditions, providing a novel material approach for artificial cell systems design.

Abstract

Artificial cells or cell mimics have drawn significant attention in cell biology and material science in the last decade and its development will provide a powerful toolbox for studying the origin of life and pave the way for novel biomedical applications. Artificial cells and their subcompartments are typically constructed from a semipermeable membrane composed of liposomes, polymersomes, hydrogels, or simply aqueous droplets enclosing bioactive molecules to perform cellular‐mimicking activities such as compartmentalization, communication, metabolism, or reproduction. Despite the rapid progress, concerns regarding their physical stability (e.g., thermal or mechanical) and tunability in membrane permeability have significantly hindered artificial cells systems in real‐life applications. In addition, developing a facile and versatile system that can mimic multiple cellular tasks is advantageous. Here, an ultrastable, multifunctional and stimulus‐responsive artificial cell system is reported. Constructed from metal‐phenolic network membranes enclosing enzyme‐containing metal‐organic frameworks as organelles, the bionic cell system can mimic multiple cellular tasks including molecular transport regulation, cell metabolism, communication and programmed degradation, and significantly extends its stability range across various chemical and physical conditions. It is believed that the development of such responsive cell mimics will have significant potentials for studying cellular reactions and have future applications in biosensing and drug delivery.

A novel solar evaporation system is developed by hanging hydrophilic photothermal fabric in air and immersing two fabric edges in seawater. Under the irradiation of sunlight, the hanging fabric can continuously and efficiently produce a watersteam for fresh water and concentrated brine for the chlor‐alkali industry without solid‐salt accumulation, offering a promising method for large‐scale desalination.

Abstract

Solar‐enabled evaporation for seawater desalination is an attractive, renewable, and environment‐friendly technique, and tremendous progress has been achieved by developing various photothermal membranes. However, traditional photothermal membranes directly float on water, resulting in some limitations such as unavoidable heat‐loss to bulk water and severe salt accumulation. To solve these problems, a hydrophilic, polymer nanorod‐coated photothermal fabric is designed and fabricated, and then an indirect‐contact evaporation system by hanging the fabric is demonstrated. The two ends of the fabric are designed to be in contact with seawater to guide water flow through capillary suction. Both arc‐shaped top/bottom surfaces of the hanging fabrics are exposed to air, which can prevent heat dissipation to bulk seawater and facilitate the double‐surface evaporation upon sunlight irradiation. Our design leads to an efficient evaporation rate of 1.94 kg m⁻² h⁻¹ and high solar efficiency of 89.9% upon irradiation with sunlight (1.0 kW m⁻²). Importantly, the highly concentrated brine can drip from the bottom of the arc‐shaped fabric, without the appearance of solid‐salt accumulation. This indirect‐contact evaporation system establishes a new path to continuously and economically produce watersteam from seawater for fresh‐water and concentrated brine for the chlor‐alkali industry.

In article number https://doi.org/10.1002/adfm.2019032461903246, Fumito Araoka, Suk‐Won Choi, and co‐workers demonstrate circularly polarized luminescence (CPL) from a nanosegregated mesophase consisting only of achiral molecules. CPL is induced through self‐assembled chiral aggregates (in the chiral super nanospace) formed only by achiral molecules. The results introduce the possibility of developing a novel technique for constructing practical CPL active materials.

Inspired by transport‐mediated healing in bones, this work demonstrates low‐energy rapid healing of cellular metal at room temperature. Electrochemical reactions transport metal ions from a metal source to fractured locations, and an insulating coating restricts the healing to fractured sites. This technique enables full recovery of tensile strength after fracture, strengthening to prevent future fracture, and healing of electrical conductivity.

Abstract

Healing metallic materials involves high temperatures and large energy inputs. This work demonstrates rapid, effective, low‐energy, and room‐temperature healing of metallic materials by using electrochemistry and polymer‐coated cellular nickel to mimic the transport‐mediated healing of bone. The polymer coating enables selective healing only at the fracture site, electrochemical reactions transport metal ions from a metal source to fractured areas, and the cellular structure of the metal allows facile ion transport to healing sites and effective recovery of strength and toughness when the cellular metal is subjected to three types of damage (scission fracture, tensile failure, and plastic deformation). Using this strategy, samples fractured in tension and by scission recover 100% of their tensile strength in as little as 10 and 4 h of healing. The healing process is stochastic, thus a statistical method is used to quantify and predict the likelihood of achieving target healing strengths based on energy input. This electrochemistry‐based approach enables the first demonstration of room‐temperature healing of structural metallic materials and requires several orders of magnitude less energy than many previously reported metal healing techniques.

Recently, transistor‐based artificial synapses have received much attention due to their good stability, relatively controllable test parameters, and clear operating mechanisms. In addition, they can perform concurrent learning, in which synaptic weight can be performed without interrupting the signal transmission process. This review summarizes recent advances in transistor‐based artificial synapses.

Abstract

Simulating biological synapses with electronic devices is a re‐emerging field of research. It is widely recognized as the first step in hardware building brain‐like computers and artificial intelligent systems. Thus far, different types of electronic devices have been proposed to mimic synaptic functions. Among them, transistor‐based artificial synapses have the advantages of good stability, relatively controllable testing parameters, clear operation mechanism, and can be constructed from a variety of materials. In addition, they can perform concurrent learning, in which synaptic weight update can be performed without interrupting the signal transmission process. Synergistic control of one device can also be implemented in a transistor‐based artificial synapse, which opens up the possibility of developing robust neuron networks with significantly fewer neural elements. These unique features of transistor‐based artificial synapses make them more suitable for emulating synaptic functions than other types of devices. However, the development of transistor‐based artificial synapses is still in its very early stages. Herein, this article presents a review of recent advances in transistor‐based artificial synapses in order to give a guideline for future implementation of synaptic functions with transistors. The main challenges and research directions of transistor‐based artificial synapses are also presented.