Ultrathin thermoresponsive self-folding 3D graphene

Engineered Living Structures with Shape-Morphing Capability Enabled by 4D Printing with Functional Bacteria

Engineered living structures with the incorporation of functional bacteria have been explored extensively in recent years and have shown promising potential applications in biosensing, environmental remediation, and biomedicine. However, it is still rare and challenging to achieve multifunctional capabilities such as material production, shape transformation, and sensing in a single-engineered living structure. In this study, we demonstrate bifunctional living structures by synergistically integrating cellulose-generating bacteria with pH-responsive hydrogels, and the entire structures can be precisely fabricated by three-dimensional (3D) printing. Such 3D-printed bifunctional living structures produce cellulose nanofibers in ambient conditions and have reversible and controlled shape-morphing properties (usually referred to as four-dimensional printing). Those functionalities make them biomimetic versions of silkworms in the sense that both can generate nanofibers and have body motion. We systematically investigate the processing-structure-property relationship of the bifunctional living structures. The on-demand separation of 3D cellulose structures from the hydrogel template and the living nature of the bacteria after processing and shape transformation are also demonstrated.

Determinative scrolling and folding of membranes through shrinking channels

Flat membranes ubiquitously transform into mysterious complex shapes in nature and artificial worlds. Behind the complexity, clear determinative deformation modes have been continuously found to serve as basic application rules but remain unfulfilled. Here, we decipher two elemental deformation modes of thin membranes, spontaneous scrolling and folding as passing through shrinking channels. We validate that these two modes rule the deformation of membranes of a wide thickness range from micrometer to atomic scale. Their occurrence and the determinative fold number quantitatively correlate with the Föppl-von Kármán number and shrinkage ratio. The unveiled determinative deformation modes can guide fabricating foldable designer microrobots and delicate structures of two-dimensional sheets and provide another mechanical principle beyond genetic determinism in biological morphogens.

Active Materials for Functional Origami

In recent decades, origami has been explored to aid in the design of engineering structures. These structures span multiple scales and have been demonstrated to be used toward various areas such as aerospace, metamaterial, biomedical, robotics, and architectural applications. Conventionally, origami or deployable structures have been actuated by hands, motors, or pneumatic actuators, which can result in heavy or bulky structures. On the other hand, active materials, which reconfigure in response to external stimulus, eliminate the need for external mechanical loads and bulky actuation systems. Thus, in recent years, active materials incorporated with deployable structures have shown promise for remote actuation of light weight, programmable origami. In this review, active materials such as shape memory polymers (SMPs) and alloys (SMAs), hydrogels, liquid crystal elastomers (LCEs), magnetic soft materials (MSMs), and covalent adaptable network (CAN) polymers, their actuation mechanisms, as well as how they have been utilized for active origami and where these structures are applicable is discussed. Additionally, the state-of-the-art fabrication methods to construct active origami are highlighted. The existing structural modeling strategies for origami, the constitutive models used to describe active materials, and the largest challenges and future directions for active origami research are summarized.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Scrolling reduced graphene oxides to induce room temperature magnetism via spatial coupling of defects

Due to its intriguing features and numerous applications, graphene has garnered a lot of interest in recent years. However, it is still very difficult to create graphene-based room-temperature magnets without transition metals or rare earth elements since pristine graphene is inherently diamagnetic due to the delocalized π bonding network. Herein, room-temperature ferromagnetism with a saturation magnetization of 0.93 emu g-1 (300 K) is achieved in defect-rich-reduced graphene oxide (DR-rGO) nanoscrolls by creating a spatial coupling of defects. The experiments and DFT calculations verify that spatial coupling of defects could enhance Rudermann-Kittel-Kasuya-Yosida interactions to induce magnetism in graphene. It displays high-efficiency electromagnetic wave absorption performance with a minimal reflection loss of -62.1 dB and an effective absorption bandwidth of 7.8 GHz (3.0 mm) thanks to greatly improved magnetism. This breakthrough serves as a building block for the creation of room-temperature magnetic carbon materials and expands their applications in many pertinent domains.

Wrap-like transfer printing for three-dimensional curvy electronics

Three-dimensional (3D) curvy electronics has wide-ranging application in biomedical health care, soft machine, and high-density curved imager. Limited by material properties, complex procedures, and coverage ability of existing fabrication techniques, the development of high-performance 3D curvy electronics remains challenging. Here, we propose an automated wrap-like transfer printing prototype for fabricating 3D curvy electronics. Assisted by a gentle and uniform pressure field, the prefabricated planar circuits on the petal-like stamp are integrated onto the target surface intactly with full coverage. The driving pressure for the wrapping is provided by the strain recovery of a prestrained elastic film triggered by the air pressure control. The wrapping configuration and strain distribution of the stamp are simulated by finite element analysis, and the pattern and thickness of the stamps are optimized. Demonstration of this strategy including spherical meander antenna, spherical light-emitting diode array, and spherical solar cell array illustrates its feasibility in the development of complex 3D curvy electronics.

Thermoresponsive Conductivity of Graphene-Based Fibers

Smart materials are versatile material systems which exhibit a measurable response to external stimuli. Recently, smart material systems have been developed which incorporate graphene in order to share on its various advantageous properties, such as mechanical strength, electrical conductivity, and thermal conductivity as well as to achieve unique stimuli-dependent responses. Here, a graphene fiber-based smart material that exhibits reversible electrical conductivity switching at a relatively low temperature (60 °C), is reported. Using molecular dynamics (MD) simulation and density functional theory-based non-equilibrium Green's function (DFT-NEGF) approach, it is revealed that this thermo-response behavior is due to the change in configuration of amphiphilic triblock dispersant molecules occurring in the graphene fiber during heating or cooling. These conformational changes alter the total number of graphene-graphene contacts within the composite material system, and thus the electrical conductivity as well. Additionally, this graphene fiber fabrication approach uses a scalable, facile, water-based method, that makes it easy to modify material composition ratios. In all, this work represents an important step forward to enable complete functional tuning of graphene-based smart materials at the nanoscale while increasing commercialization viability.

Perfect optical absorption in a single array of folded graphene ribbons

Due to its one atom thickness, optical absorption (OA) in graphene is a fundamental and challenging issue. Practically, the patterned graphene-dielectric-metal structure is commonly used to achieve perfect OA (POA). In this work, we propose a novel scenario to solve this issue, in which POA is obtained by using free-standing folded graphene ribbons (FGRs). We show several local resonances, e.g. a dipole state (Mode-I) and a bound state in continuum (BIC, Mode-II), will cause very efficient OA. At normal incidence, by choosing appropriate folding angle θ, 50% absorptance by the two states is easily achieved; at oblique incidence, the two states will result in roughly 98% absorptance as incidence angle φ≈40^∘. It is also interesting to see that the system has asymmetric OA spectra, e.g. POA of the former (latter) state existing in reverse (forward) incidence, respectively. Besides the angles θ and φ, POA here can also be actively tuned by electrostatic gating. As increasing Fermi level, POA of Mode-I will undergo a gradual blueshift, while that of Mode-II will experience a rapid blueshift and then be divided into three branches, due to Fano coupling to two guided modes. In reality, the achieved POA is well maintained even the dielectric substrates are used to support FGRs. Our work offers a remarkable scenario to achieve POA, and thus enhance light-matter interaction in graphene, which can build an alternative platform to study novel optical effects in general two-dimensional (2D) materials. The folding, mechanical operation in out-of-plane direction, may emerge as a new degree of freedom for optoelectronic device applications based on 2D materials.

Transfer printing technologies for soft electronics

Soft electronics have received increasing attention in recent years, owing to their wide range of applications in dynamic nonplanar surface integration electronics that include skin electronics, implantable devices, and soft robotics. Transfer printing is a widely used assembly technology for micro- and nano-fabrication, which enables the integration of functional devices with flexible or elastomeric substrates for the manufacturing of soft electronics. Through advanced materials and process design, numerous impressive studies related to transfer printing strategies and applications have been proposed. Herein, a discussion of transfer printing technologies toward soft electronics in terms of mechanisms and example demonstrations is provided. Moreover, the perspectives on the potential challenges and future directions of this field are briefly discussed.

Self-Assembly of a Graphene Oxide Liquid Crystal for Water Treatment

Adsorbents, especially those with high removal efficiency, long life, and multi-purpose capabilities, are the most crucial components in an adsorption system. By taking advantage of the liquid-like mobility and crystal-like ordering of liquid crystal materials, a liquid crystal induction method is developed and applied to construct three-dimensional graphene-based adsorbents featuring excellent shape adaptability, a distinctive pore structure, and abundant surface functional groups. When the monoliths are used for water restoration, the large amount of residual oxygen-containing groups is more susceptible to electrophilic attack, thus contributing to cation adsorption (up to 705.4 mg g^-1 for methylene blue), while the connected microvoids between the aligned graphene oxide sheets facilitate mass transfer, e.g., the high adsorption capacity for organic pollutants (196.2 g g^-1 for ethylene glycol) and the high evaporation rate for water (4.01 kg m^-2 h^-1). This work gives a practical method for producing high-performance graphene-based functional materials for those applications that are sensitive to surface and mass transfer properties.

4D Multiscale Origami Soft Robots: A Review

Time-dependent shape-transferable soft robots are important for various intelligent applications in flexible electronics and bionics. Four-dimensional (4D) shape changes can offer versatile functional advantages during operations to soft robots that respond to external environmental stimuli, including heat, pH, light, electric, or pneumatic triggers. This review investigates the current advances in multiscale soft robots that can display 4D shape transformations. This review first focuses on material selection to demonstrate 4D origami-driven shape transformations. Second, this review investigates versatile fabrication strategies to form the 4D mechanical structures of soft robots. Third, this review surveys the folding, rolling, bending, and wrinkling mechanisms of soft robots during operation. Fourth, this review highlights the diverse applications of 4D origami-driven soft robots in actuators, sensors, and bionics. Finally, perspectives on future directions and challenges in the development of intelligent soft robots in real operational environments are discussed.

Humidity-Controlled Dynamic Engineering of Buckling Dimensionality in MoS2 Thin Films

Dynamic engineering of buckling deformation is of vital importance as it provides multiphase modulation of thin film devices. In particular, dynamic switch of buckles between one-dimensional (1D) and two-dimensional (2D) configurations in a single film system on rigid substrates is intriguing but very challenging. The current approach to changing buckling configuration is mainly achieved by varying the built-in stress at the film-substrate interface, but it is difficult to realize dynamic engineering on rigid substrates. Herein, we report a dynamic engineering of buckling deformation in MoS₂ thin films by humidity-tuned interfacial adhesion. With the change of humidity, the MoS₂ thin films deform from 1D telephone-cord buckles to 2D web-like buckles due to the hydrophilic nature of both MoS₂ and substrate. Such 1D-to-2D evolution of buckles is attributed to the weakened interfacial adhesion of mixed deformation modes induced by humidity, which is verified by finite-element modeling. These buckled films further find potential applications as patterned templates for liquid condensation and sensing units for tactile sensors. Our work not only demonstrates the humidity-controlled dimensionality engineering of buckles in MoS₂ thin films but also sheds light on the functional applications of buckled films based on their profile features.

Assembly of complex 3D structures and electronics on curved surfaces

Electronic devices with engineered three-dimensional (3D) architectures are indispensable for frictional-force sensing, wide-field optical imaging, and flow velocity measurement. Recent advances in mechanically guided assembly established deterministic routes to 3D structures in high-performance materials, through controlled rolling/folding/buckling deformations. The resulting 3D structures are, however, mostly formed on planar substrates and cannot be transferred directly onto another curved substrate. Here, we introduce an ordered assembly strategy to allow transformation of 2D thin films into sophisticated 3D structures on diverse curved surfaces. The strategy leverages predefined mechanical loadings that deform curved elastomer substrates into flat/cylindrical configurations, followed by an additional uniaxial/biaxial prestretch to drive buckling-guided assembly. Release of predefined loadings results in an ordered assembly that can be accurately captured by mechanics modeling, as illustrated by dozens of complex 3D structures assembled on curved substrates. Demonstrated applications include tunable dipole antennas, flow sensors inside a tube, and integrated electronic systems capable of conformal integration with the heart.

A new dimension for magnetosensitive e-skins: active matrix integrated micro-origami sensor arrays

Magnetic sensors are widely used in our daily life for assessing the position and orientation of objects. Recently, the magnetic sensing modality has been introduced to electronic skins (e-skins), enabling remote perception of moving objects. However, the integration density of magnetic sensors is limited and the vector properties of the magnetic field cannot be fully explored since the sensors can only perceive field components in one or two dimensions. Here, we report an approach to fabricate high-density integrated active matrix magnetic sensor with three-dimensional (3D) magnetic vector field sensing capability. The 3D magnetic sensor is composed of an array of self-assembled micro-origami cubic architectures with biased anisotropic magnetoresistance (AMR) sensors manufactured in a wafer-scale process. Integrating the 3D magnetic sensors into an e-skin with embedded magnetic hairs enables real-time multidirectional tactile perception. We demonstrate a versatile approach for the fabrication of active matrix integrated 3D sensor arrays using micro-origami and pave the way for new electronic devices relying on the autonomous rearrangement of functional elements in space.

Plasmonic 3D Self-Folding Architectures via Vacuum Microforming

3D self-folding microarchitectures have been studied enormously since the past decade, because of the potential of utilizing the third dimension to reach a new level of device integration. However, incorporating various functionalities is a great challenge, due to the limited folding force and choice of materials. In particular, self-folding microarchitectures with advanced optical properties have yet to be demonstrated. Here, a unique folding technique is developed, namely vacuum microforming, successfully demonstrating the self-folding of microcubes that can be completed within 30 ms, a few orders of magnitudes faster as compared to various established strategies reported so far. Simultaneously, a metal-insulator-metal (MIM) plasmonic nanostructure is fabricated, invoking strong gap plasmon to obtain a wide and robust angle-independent optical behavior and high environmental sensitivity that is close to the theoretical limit. It is successfully proven that such superb plasmonic properties are well preserved in 3D architectures throughout the folding process. The nanofabrication method together with the self-folding strategy not only provide the fastest folding process so far, compatible for high-volume fabrication, but also create new opportunities in integrating various functionalities, more specifically, optical properties for untethered optical sensing and identification.

An embedded interface regulates the underwater actuation of solvent-responsive soft grippers

In this work, we report the role of an embedded interface between two polymer thin films in determining the overall folding and actuation characteristics of a bilayer system applied for gripping submerged objects. Along with the material properties and geometry of the individual films involved, the strength of the embedded interface governs the folding behaviour of the bilayer when exposed to a solvent. The concentration gradient developed across the film thickness when exposed to the solvent results in the deformation of the film. The evolution of concentration through the film thickness as a function of time is closely related to the interface strength. It affects various aspects of the deformation, such as the direction of folding, curvature attained, and actuation rate. In this work, we have varied the strength of the interface between solvent responsive chitosan and hydrophobic Poly(methyl-methacrylate) (PMMA) by treating the substrate (chitosan) with varying concentrations of silane before coating. Experimentally, the folding characteristics of the solvent responsive bilayer films have been investigated for four different interfacial strengths. A coupled diffusion-deformation model for the film and a cohesive zone model for the interface is developed to provide insights into the underlying mechanism behind the observations made. Finally, the application of the bilayer as a gripper for submerged objects for two different types of interfaces is demonstrated. Interestingly, in this approach, the medium where the object is immersed acts as a trigger for folding the grippers.

Shape-Changing Particles: From Materials Design and Mechanisms to Implementation

Demands for next-generation soft and responsive materials have sparked recent interest in the development of shape-changing particles and particle assemblies. Over the last two decades, a variety of mechanisms that drive shape change have been explored and integrated into particulate systems. Through a combination of top-down fabrication and bottom-up synthesis techniques, shape-morphing capabilities extend from the microscale to the nanoscale. Consequently, shape-morphing particles are rapidly emerging in a variety of contexts, including photonics, microfluidics, microrobotics, and biomedicine. Herein, the key mechanisms and materials that facilitate shape changes of microscale and nanoscale particles are discussed. Recent progress in the applications made possible by these particles is summarized, and perspectives on their promise and key open challenges in the field are discussed.

Recyclable silicone elastic light-triggered actuator with a reconfigurable Janus structure and self-healable performance

A recyclable silicone elastic light-triggered actuator with reconfigurable Janus structure and self-healable performance is reported, which was fabricated via heterogeneous crosslinking induced by a gradient intensity of UV light due to CNTs accretion.

Thermalized buckling of isotropically compressed thin sheets

The buckling of thin elastic sheets is a classic mechanical instability that occurs over a wide range of scales. In the extreme limit of atomically thin membranes like graphene, thermal fluctuations can dramatically modify such mechanical instabilities. We investigate here the delicate interplay of boundary conditions, nonlinear mechanics, and thermal fluctuations in controlling buckling of confined thin sheets under isotropic compression. We identify two inequivalent mechanical ensembles based on the boundaries at constant strain (isometric) or at constant stress (isotensional) conditions. Remarkably, in the isometric ensemble, boundary conditions induce a novel long-ranged nonlinear interaction between the local tilt of the surface at distant points. This interaction combined with a spontaneously generated thermal tension leads to a renormalization group description of two distinct universality classes for thermalized buckling, realizing a mechanical variant of Fisher-renormalized critical exponents. We formulate a complete scaling theory of buckling as an unusual phase transition with a size-dependent critical point, and we discuss experimental ramifications for the mechanical manipulation of ultrathin nanomaterials.

A method for designing tunable chiral mechanical carbon networks for energy storage

A method is proposed for designing tunable chiral nano-networks using partly hydrogenated graphene ribbons and carbon nanotubes (CNTs). In the network, the hydrogenated graphene ribbons (HGRs) act as basic components, which connect each other via CNT joints. Each component contains two HGR segments and an internal graphene joint (G-J2) or CNT joint (CNT-J2). Since the two HGR segments are hydrogenated at opposite surfaces, they may wind in chiral about the internal joint to form a scroll (G-J2-scroll or CNT-J2-scroll) or about the two end joints to form CNT-J4-scrolls. In general, a G-J2-scroll is formed more easily than both a CNT-J4-scroll and a CNT-J2-scroll. Because of scrolling, the surface energy is reduced. This reduction is converted to and stored as deformation potential energy. By means of molecular-dynamics simulations, we studied the final configurations of two types of networks from the same components, the maximum shrinkage, and their capacity of energy storage for potential application of energy storage or as large-deformable components in a nano-device. The results indicate that the network reaches a stable state when the shrinkage reaches 70% of the two in-plane dimensions.

Tailoring the multistability of origami-inspired, buckled magnetic structures via compression and creasing

Origami-inspired multistable structures are gaining increasing interest because of their potential applications in fields ranging from deployable structures to reconfigurable microelectronics. The multistability of such structures is critical for their applications but is challenging to manipulate due to the highly nonlinear deformations and complex configurations of the structures. Here, a comprehensive experimental and computational study is reported to tailor the multistable states of origami-inspired, buckled ferromagnetic structures and their reconfiguration paths. Using ribbon structures as an example, a design phase diagram is constructed as a function of the crease number and compressive strain. As the crease number increases from 0 to 7, the number of distinct stable states first increases and then decreases. The multistability is also shown to be actively tuned by varying the strain from 0% to 40%. Furthermore, analyzing energy barriers for reconfiguration among the stable states reveals dynamic changes in reconfiguration paths with increasing strains. Guided by studies above, diverse examples are designed and demonstrated, from programmable structure arrays to a soft robot. These studies lay out the foundation for the rational design of functional, multistable structures.

Environmentally Adaptive Shape-Morphing Microrobots for Localized Cancer Cell Treatment

Microrobots have attracted considerable attention due to their extensive applications in microobject manipulation and targeted drug delivery. To realize more complex micro-/nanocargo manipulation (e.g., encapsulation and release) in biological applications, it is highly desirable to endow microrobots with a shape-morphing adaptation to dynamic environments. Here, environmentally adaptive shape-morphing microrobots (SMMRs) have been developed by programmatically encoding different expansion rates in a pH-responsive hydrogel. Due to a combination with magnetic propulsion, a shape-morphing microcrab (SMMC) is able to perform targeted microparticle delivery, including gripping, transporting, and releasing by "opening-closing" of a claw. As a proof-of-concept demonstration, a shape-morphing microfish (SMMF) is designed to encapsulate a drug (doxorubicin (DOX)) by closing its mouth in phosphate-buffered saline (PBS, pH ∼ 7.4) and release the drug by opening its mouth in a slightly acidic solution (pH < 7). Furthermore, localized HeLa cell treatment in an artificial vascular network is realized by "opening-closing" of the SMMF mouth. With the continuous optimization of size, motion control, and imaging technology, these magnetic SMMRs will provide ideal platforms for complex microcargo operations and on-demand drug release.

Self-Rolling of Monolayer Graphene for Ultrasensitive Molecular Sensing

The controllable manipulation of graphene to create three-dimensional (3D) structures is an intriguing approach for favorably tuning its properties and creating new types of 3D devices. However, due to extremely low bending stiffnesses, it is rather challenging to construct monolayer graphene into stable 3D structures. Here, we demonstrate the stable formation of monolayer graphene microtubes with accompanying pre-patterned strain layers. The diameter of graphene microtubes can be effectively tuned by changing the thickness of the strain layers. Benefiting from a high surface-to-volume ratio of the tubular geometry, the 3D geometry leads to a prominent Raman enhancement, which was further applied to molecular sensing. The R6G molecules on graphene microtubes can be detected even for a concentration as low as 10^-11 M. We believe that this method can be a generalized way to realize the 3D tubular structure of other 2D materials.

A novel temperature sensor based on three-dimensional buried-gate graphene field effect transistor

Temperature sensor is one of the primarily developed and most proverbially utilized sensors. Owing to the limitations of their characteristics (stability, thermal conductivity, and thermal contact area), traditional temperature sensors may exhibit drawbacks of high production cost and large volume. In this paper, a three-dimensional (3D) buried-gate graphene field effect transistors (GFETs) are proposed as a novel sensor for temperature detection, which possess a 3D microtube structure by self-rolled-up technology. Compared to conventional two-dimensional (2D) devices, the 3D devices would have tinier area and higher integration. Two main reasons that would affect the resistance of the graphene are the graphene electro-phonon coupling and the thermal expansion effect. In addition, by applying the COMSOL Multiphysics software, it has been demonstrated that the microtube would deform to a certain extent when the temperature increases. And the strain on the 3D devices is proved to be greater than that of the 2D devices. Experimental results show that 3D GFETs could realize temperature detection between 30 °C and 150 °C, and its resistance increases with temperature rising. Furthermore, the maximum achieved temperature coefficient of resistance (TCR) is 0.41% °C^-1and the hysteresis error is only 3.85%. By virtue of the 3D microtube, not only more superior temperature detection could be achieved, but also more devices are integrated in unit area. The 3D temperature sensor possesses superior sensitivity, repeatability and stability, which contributes a new approach to develop the high-performance temperature sensor.

Hierarchical Assembly of Two-Dimensional Polymers into Colloidosomes and Microcapsules

Hierarchical assembly of two-dimensional (2D) polymers to 3D microstructures provides new means of creating functional materials with exotic properties for extensive applications. Herein, we report an approach of assembling 2D covalent organic framework (COF) colloidosomes or microcapsules from small molecules. We polymerized monomers to produce narrowly distributed COF particles with average particle sizes greater than 490 nm, which were further used as stabilizers to prepare various water-in-oil Pickering emulsions with droplet sizes of 10-120 μm on average. The emulsion droplets were subsequently applied as templates for interfacial polymerization of the same monomers. The COF microcapsules with varied diameters and shell thicknesses of 0.2-3.1 μm were thus obtained, which possessed good stability, high crystallinity, and surface areas no less than 540 m²/g. The approach also permits facile loading of water-soluble substances such as salts, dyes, or proteins. The loaded molecules demonstrated different permeability against the shell, in which 98% of the encapsulated salts could be released in 1 h while only 18% of dye molecules and almost none of the fluorescent proteins diffused out from the microcapsules. Such an assembling approach may greatly extend the applications of 2D polymers and their microcapsules.

Ultrathin Polydopamine-Graphene Oxide Hybrid Coatings on Polymer Filters with Improved Filtration Performance and Functionalities

Thin polymer fiber mats, in particular those made of nonwoven polypropylene (PP) fibers, are extensively used for medical and industrial filtration. The recent pandemic has increased the demand for the fabrication of protective masks. The nonwoven PP filter has limitations in filtration efficiency and lacks advanced functionalities. Here, we propose a simple, effective, and low-cost method to functionalize PP filters and endow antimicrobial and photothermal properties. Our approach is based on the deposition of an ultrathin hybrid coating composed of graphene oxide (GO) and polydopamine on the surface of PP filters by spray-coating. The complementary properties and synergic effects of GO and polydopamine in the ultrathin coating improved the filtration efficiency of the PP filter by 20% with little change in pressure drop. Single component coatings did not result in similar improvements in performance. The ultrathin coating also makes the surface of the filter more hydrophilic with negative charges. The photothermal property of GO enables a rapid temperature increase of the surface-coated filter upon light irradiation for easy sterilization. Furthermore, cationic polymer brushes can be grafted to the ultrathin hybrid coating, which adds the highly desired antimicrobial property to the PP filters for their more effective protection against microorganisms.

A microfluidic field-effect transistor biosensor with rolled-up indium nitride microtubes

Field-effect-transistor (FET) biosensors capable of rapidly detecting disease-relevant biomarkers have long been considered as a promising tool for point-of-care (POC) diagnosis. Rolled-up nanotechnology, as a batch fabrication strategy for generating three-dimensional (3D) microtubes, has been demonstrated to possess unique advantages for constructing FET biosensors. In this paper, we report a new approach combining the two fascinating technologies, the FET biosensor and the rolled-up microtube, to develop a microfluidic diagnostic biosensor. We integrated an excellent biosensing III-nitride material-indium nitride (InN)-into a rolled-up microtube and used it as the FET channel. The InN possesses strong, intrinsic, and stable electron accumulation (~10¹³ cm^-2) on its surface, thereby providing a high device sensitivity. Multiple rolled-up InN microtube FET biosensors fabricated on the same substrate were integrated with a microfluidic channel for convenient fluids handling, and shared the same external electrode (inserted into the microchannel outlet) for gating voltage modulation. Using human immunodeficiency virus (HIV) antibody as a model disease marker, we characterized the analytical performance of the developed biosensor and achieved a limit of detection (LOD) of 2.5 pM for serum samples spiked with HIV gp41 antibodies. The rolled-up InN microtube FET biosensor represents a new type of III-nitride-based FET biosensor and holds significant potential for practical POC diagnosis.

Hybridized Radial and Edge Coupled 3D Plasmon Modes in Self-Assembled Graphene Nanocylinders

Current graphene-based plasmonic devices are restricted to 2D patterns defined on planar substrates; thus, they suffer from spatially limited 2D plasmon fields. Here, 3D graphene forming freestanding nanocylinders realized by a plasma-triggered self-assembly process are introduced. The graphene-based nanocylinders induce hybridized edge (in-plane) and radial (out-of-plane) coupled 3D plasmon modes stemming from their curvature, resulting in a four orders of magnitude stronger field at the openings of the cylinders than in rectangular 2D graphene ribbons. For the characterization of the 3D plasmon modes, synchrotron nanospectroscopy measurements are performed, which provides the evidence of preservation of the hybridized 3D graphene plasmons in the high precision curved nanocylinders. The distinct 3D modes introduced in this paper, provide an insight into geometry-dependent 3D coupled plasmon modes and their ability to achieve non-surface-limited (volumetric) field enhancements.

Spontaneous Folding Growth of Graphene on h-BN

Graphene has been the subject of much research, with structural engineering frequently used to harness its various properties. In particular, the concepts of graphene origami and kirigami have inspired the design of quasi-three-dimensional graphene structures, which possess intriguing mechanical, electronic, and optical properties. However, accurate controlling the folding process remains a big challenge. Here, we report the discovery of spontaneous folding growth of graphene on the h-BN substrate via adopting a simple chemical vapor deposition method. Folded edges are formed when two stacked graphene layers share a joint edge at a growth temperature up to 1300 °C. Using first-principles density functional theory calculations, the bilayer graphene with folded edges is demonstrated to be more stable than that with open edges. Utilizing this novel growth mode, hexagram bilayer graphene containing entirely sealed edges is eventually realized. Our findings provide a route for designing graphene devices with a new folding dimension.

Designing Mechanical Metamaterials with Kirigami-Inspired, Hierarchical Constructions for Giant Positive and Negative Thermal Expansion

Advanced mechanical metamaterials with unusual thermal expansion properties represent an area of growing interest, due to their promising potential for use in a broad range of areas. In spite of previous work on metamaterials with large or ultralow coefficient of thermal expansion (CTE), achieving a broad range of CTE values with access to large thermally induced dimensional changes in structures with high filling ratios remains a key challenge. Here, design concepts and fabrication strategies for a kirigami-inspired class of 2D hierarchical metamaterials that can effectively convert the thermal mismatch between two closely packed constituent materials into giant levels of biaxial/uniaxial thermal expansion/shrinkage are presented. At large filling ratios (>50%), these systems offer not only unprecedented negative and positive biaxial CTE (i.e., -5950 and 10 710 ppm K^-1 ), but also large biaxial thermal expansion properties (e.g., > 21% for 20 K temperature increase). Theoretical modeling of thermal deformations provides a clear understanding of the microstructure-property relationships and serves as a basis for design choices for desired CTE values. An Ashby plot of the CTE versus density serves as a quantitative comparison of the hierarchical metamaterials presented here to previously reported systems, indicating the capability for substantially enlarging the accessible range of CTE.

Advances in Stimuli-Responsive Soft Robots with Integrated Hybrid Materials

Hybrid stimuli-responsive soft robots have been extensively developed by incorporating multi-functional materials, such as carbon-based nanoparticles, nanowires, low-dimensional materials, and liquid crystals. In addition to the general functions of conventional soft robots, hybrid stimuli-responsive soft robots have displayed significantly advanced multi-mechanical, electrical, or/and optical properties accompanied with smart shape transformation in response to external stimuli, such as heat, light, and even biomaterials. This review surveys the current enhanced scientific methods to synthesize the integration of multi-functional materials within stimuli-responsive soft robots. Furthermore, this review focuses on the applications of hybrid stimuli-responsive soft robots in the forms of actuators and sensors that display multi-responsive and highly sensitive properties. Finally, it highlights the current challenges of stimuli-responsive soft robots and suggests perspectives on future directions for achieving intelligent hybrid stimuli-responsive soft robots applicable in real environments.

A novel three-dimensional Ag nanoparticles/reduced graphene oxide microtubular field effect transistor sensor for NO2 detections

A novel three-dimensional (3D) microtubular NO2 field effect transistor (FET) sensor has been fabricated from 2D reduced graphene oxide (rGO) nanosheets decorated with Ag nanoparticles, by applying the self-roll-up technique. The electrical properties of 2D and 3D Ag NP/rGO FET sensors have been investigated and compared. Finally, the performance of the 3D sensors has been demonstrated, where the preliminary results show that our 3D Ag NP/rGO FET NO2 sensor exhibits a relatively fast response (response time of 116 s) to 20 parts per million NO2 with a response of 4.92% at room temperature at zero bias voltage and 2 V source-drain bias voltage. Moreover, characteristics of our 3D Ag NP/rGO FET sensors, e.g. response, response and recovery times, have been demonstrated to be tuned by adjusting the applied source-drain and gate biases. Compared to the 2D geometry, our 3D geometry occupies less device area, but with the same sensing area. This study provides a new way to optimize sensing device performance, and promotes its development for miniaturized and integrated gas-sensing applications for indoor health and safety detection, outdoor environmental monitoring, industrial pollution monitoring and beyond.

Tannic acid modified graphene/CNT three-dimensional conductive network for preparing high-performance transparent flexible heaters

In this paper, the eco-friendly plant polyphenol, tannic acid (TA) was demonstrated as a non-covalent modifier for carbon nanotubes (CNTs), as well as a stripping medium to achieve exfoliated graphite to graphene by microfluidization. High-performance transparent flexible heater (TFH) with an embedded structure had been successfully fabricated by integrating conductive nanocomposites (TA-functionalized grapheme/TA-functionalized CNT/PEDOT:PSS; TG/TCNT/PEDOT) into waterborne polyurethane (WPU) film. Such a film exhibited favorable optical transmittance and sheet resistance (T = ca. 80% at 550 nm, R_s = 62.5 Ω/sq.), low root mean square (rms) roughness (approximately 0.37 nm), excellent adhesion and mechanical stability (the sheet resistance remained almost constant after 1000 bending cycle test for the bending radius of 10 mm), which are ideal as transparent heaters with high thermal efficiency. For TG/TCNT/PEDOT-WPU TFHs, the temperature increased rapidly and reached a steady state within 20 s with the maximum temperature reached to 116 °C, when the applied voltage was 20 V. Moreover, no variation in temperature was observed after the repeated heating-cooling tests and long-time stability test, indicating that TG/TCNT/PEDOT-WPU TCFs can be used as high performance TFHs. These TFH's are expected to be suitable for vehicle defrosting, smart windows, portable heating, smart wearable devices, etc.

In situ monitoring of electrical and optoelectronic properties of suspended graphene ribbons during laser-induced morphological changes

Exploring ways to tune and improve the performance of graphene is of paramount importance in creating functional graphene-based electronic and optoelectronic devices. Recent advancements have shown that altering the morphology of graphene can have a pronounced effect on its properties. Here, we present a practical and facile method to manipulate the morphology of a suspended graphene ribbon using a laser to locally induce heating while monitoring its electrical and optoelectronic properties in situ. Electrical measurements reveal that the conductance of suspended graphene transistors can be tuned by modifying its morphology. Additionally, scanning photocurrent measurements show that laser-induced folded graphene ribbons display significantly enhanced localized photocurrent responses in comparison with their flat counterparts. Moreover, the localization of the laser-induced heating allows for a series of folds to be induced along the entire graphene ribbon, creating targeted photocurrent enhancement. Through further investigations, it is revealed that the photo-thermoelectric effect is the primary mechanism for the increased photocurrent response of the device. Our experimental results explore the mechanisms and consequences of the folding process as well as provide a strategy to manipulate morphology and physical properties of graphene for future engineering of electronics and optoelectronics.

Recent advances in graphene monolayers growth and their biological applications: A review

Development of two-dimensional high-quality graphene monolayers has recently received great concern owing to their enormous applications in diverging fields including electronics, photonics, composite materials, paints and coatings, energy harvesting and storage, sensors and metrology, and biotechnology. As a result, various groups have successfully developed graphene layers on different substrates by using the chemical vapor deposition method and explored their physical properties. In this direction, we have focused on the state-of-the-art developments in the growth of graphene layers, and their functional applications in biotechnology. The review starts with the introduction, which contains outlines about the graphene and their basic characteristics. A brief history and inherent applications of graphene layers followed by recent developments in growth and properties are described. Then, the application of graphene layers in biodevices is reviewed. Finally, the review is summarized with perspectives and future challenges along with the scope for future technological applications.

Asymmetric elastoplasticity of stacked graphene assembly actualizes programmable untethered soft robotics

There is ever-increasing interest yet grand challenge in developing programmable untethered soft robotics. Here we address this challenge by applying the asymmetric elastoplasticity of stacked graphene assembly (SGA) under tension and compression. We transfer the SGA onto a polyethylene (PE) film, the resulting SGA/PE bilayer exhibits swift morphing behavior in response to the variation of the surrounding temperature. With the applications of patterned SGA and/or localized tempering pretreatment, the initial configurations of such thermal-induced morphing systems can also be programmed as needed, resulting in diverse actuation systems with sophisticated three-dimensional structures. More importantly, unlike the normal bilayer actuators, our SGA/PE bilayer, after a constrained tempering process, will spontaneously curl into a roll, which can achieve rolling locomotion under infrared lighting, yielding an untethered light-driven motor. The asymmetric elastoplasticity of SGA endows the SGA-based bi-materials with great application promise in developing untethered soft robotics with high configurational programmability.

An intelligent film actuator with multi-level deformation behaviour

Smart materials with simply reversible deformation or reconfigurability have shown great potential in artificial muscles, robots, controlled displays, etc. However, the combination of reversible and reconfigured functions in responsive materials, which can endow them with mature and complex actuation performance similar to that of living things, is still a great challenge. In this regard, we report an intelligent graphene oxide (GO)/polyvinylidene fluoride (PVDF) film with reconfigured structures resulting from inner plastic deformation after treatment at elevated temperature (40-80 °C), which simultaneously expresses basically and secondarily reversible deformation ability of its original and reconfigured states in response to external stimuli (e.g. IR light and moisture), respectively. As a result, this film achieves unique multi-level actuation behaviour by combining reversible and reconfigured functions. Based on this, an "Artificial Pupil" with two switchable light penetration modes under the different reconfigured states was designed and developed. Furthermore, many special and reconfigured 3D structures (e.g. cubic boxes, pyramids) have been well explored, which is promising for applications in future materials engineering and biomimetic devices.

Electron Irradiation Driven Nanohands for Sequential Origami

Sequence plays an important role in self-assembly of 3D complex structures, particularly for those with overlap, intersection, and asymmetry. However, it remains challenging to program the sequence of self-assembly, resulting in geometric and topological constrains. In this work, a nanoscale, programmable, self-assembly technique is reported, which uses electron irradiation as "hands" to manipulate the motion of nanostructures with the desired order. By assigning each single assembly step in a particular order, localized motion can be selectively triggered with perfect timing, making a component accurately integrate into the complex 3D structure without disturbing other parts of the assembly process. The features of localized motion, real-time monitoring, and surface patterning open the possibility for the further innovation of nanomachines, nanoscale test platforms, and advanced optical devices.

Bidirectional Self-Folding with Atomic Layer Deposition Nanofilms for Microscale Origami

Origami design principles are scale invariant and enable direct miniaturization of origami structures provided the sheets used for folding have equal thickness to length ratios. Recently, seminal steps have been taken to fabricate microscale origami using unidirectionally actuated sheets with nanoscale thickness. Here, we extend the full power of origami-inspired fabrication to nanoscale sheets by engineering bidirectional folding with 4 nm thick atomic layer deposition (ALD) SiN_x-SiO₂ bilayer films. Strain differentials within these bilayers result in bending, producing microscopic radii of curvature. We lithographically pattern these bilayers and localize the bending using rigid panels to fabricate a variety of complex micro-origami devices. Upon release, these devices self-fold according to prescribed patterns. Our approach combines planar semiconductor microfabrication methods with computerized origami design, making it easy to fabricate and deploy such microstructures en masse. These devices represent an important step forward in the fabrication and assembly of deployable micromechanical systems that can interact with and manipulate micro- and nanoscale environments.

Shape Morphable Hydrogel/Elastomer Bilayer for Implanted Retinal Electronics

Direct fabrication of a three-dimensional (3D) structure using soft materials has been challenging. The hybrid bilayer is a promising approach to address this challenge because of its programable shape-transformation ability when responding to various stimuli. The goals of this study are to experimentally and theoretically establish a rational design principle of a hydrogel/elastomer bilayer system and further optimize the programed 3D structures that can serve as substrates for multi-electrode arrays. The hydrogel/elastomer bilayer consists of a hygroscopic polyacrylamide (PAAm) layer cofacially laminated with a water-insensitive polydimethylsiloxane (PDMS) layer. The asymmetric volume change in the PAAm hydrogel can bend the bilayer into a curvature. We manipulate the initial monomer concentrations of the pre-gel solutions of PAAm to experimentally and theoretically investigate the effect of intrinsic mechanical properties of the hydrogel on the resulting curvature. By using the obtained results as a design guideline, we demonstrated stimuli-responsive transformation of a PAAm/PDMS flower-shaped bilayer from a flat bilayer film to a curved 3D structure that can serve as a substrate for a wide-field retinal electrode array.

Visualization of materials using the confocal laser scanning microscopy technique

The development of materials science always benefits from advanced characterizations. Currently, imaging techniques are of great technological importance in both fundamental and applied research on materials. In comparison to conventional visualization methods, confocal laser scanning microscopy (CLSM) is non-invasive, with macroscale and high-contrast scanning, a simple and fast sample preparation procedure as well as easy operation. In addition, CLSM allows rapid acquisition of longitudinal and cross-sectional images at any position in a material. Therefore, the CLSM-based visualization technique could provide direct and model-independent insight into material characterizations. This review summarizes the recent applications of CLSM in materials science. The current CLSM approaches for the visualization of surface structures, internal structures, spatial structures and reaction processes are discussed in detail. Finally, we provide our thoughts and predictions on the future development of CLSM in materials science. The purpose of this review is to guide researchers to build a suitable CLSM approach for material characterizations, and to open viable opportunities and inspirations for the development of new strategies aiming at the preparation of advanced materials. We hope that this review will be useful for a wide range of research communities of materials science, chemistry, and engineering.

Moulding three-dimensional curved structures by selective heating

It is of interest to fabricate curved surfaces in three dimensions from homogeneous material in the form of flat sheets. The aim is not just to obtain a surface which has a desired intrinsic Riemannian metric, but to get the desired embedding inR 3 up to translations and rotations. In this paper, we demonstrate three generic methods of moulding a flat sheet of thermo-responsive plastic by selective contraction induced by targeted heating. These methods do not involve any cutting and gluing, which is a property they share with origami. The first method is inspired by tailoring, which is the usual method for making garments out of plain pieces of cloth. Unlike usual tailoring, this method produces the desired embedding inR 3 . The second method just aims to bring about the desired new Riemannian metric via an appropriate pattern of local contractions, without directly controlling the embedding. The third method is based on triangulation, and seeks to induce the desired local distances. This results in getting the desired embedding inR 3 . The second and the third methods, and also the first method for the special case of surfaces of revolution, are algorithmic in nature. We explain these methods and show examples.