Three‐Dimensional Printing of Multifunctional Nanocomposites: Manufacturing Techniques and Applications

The integration of nanotechnology into three‐dimensional printing (3DP) offers huge potential and opportunities for the manufacturing of 3D engineered materials exhibiting optimized properties and multifunctionality. The literature relating to different 3DP techniques used to fabricate 3D structures at the macro‐ and microscale made of nanocomposite materials is reviewed here. The current state‐of‐the‐art fabrication methods, their main characteristics (e.g., resolutions, advantages, limitations), the process parameters, and materials requirements are discussed. A comprehensive review is carried out on the use of metal‐ and carbon‐based nanomaterials incorporated into polymers or hydrogels for the manufacturing of 3D structures, mostly at the microscale, using different 3D‐printing techniques. Several methods, including but not limited to micro‐stereolithography, extrusion‐based direct‐write technologies, inkjet‐printing techniques, and popular powder‐bed technology, are discussed. Various examples of 3D nanocomposite macro‐ and microstructures manufactured using different 3D‐printing technologies for a wide range of domains such as microelectromechanical systems (MEMS), lab‐on‐a‐chip, microfluidics, engineered materials and composites, microelectronics, tissue engineering, and biosystems are reviewed. Parallel advances on materials and techniques are still required in order to employ the full potential of 3D printing of multifunctional nanocomposites.     

Stimulus Responsive 3D Assembly for Spatially Resolved Bifunctional Sensors

3D electronic/optoelectronic devices have shown great potentials for various applications due to their unique properties inherited not only from functional materials, but also from 3D architectures. Although a variety of fabrication methods including mechanically guided assembly have been reported, the resulting 3D devices show no stimuli‐responsive functions or are not free standing, thereby limiting their applications. Herein, the stimulus responsive assembly of complex 3D structures driven by temperature‐responsive hydrogels is demonstrated for applications in 3D multifunctional sensors. The assembly driving force, compressive buckling, arises from the volume shrinkage of the responsive hydrogel substrates when they are heated above the lower critical solution temperature. Driven by the compressive buckling force, the 2D‐formed membrane materials, which are pre‐defined and selectively bonded to the substrates, are then assembled to 3D structures. They include “tent,” “tower,” “two‐floor pavilion,” “dome,” “basket,” and “nested‐cages” with delicate geometries. Moreover, the demonstrated 3D bifunctional sensors based on laser induced graphene show capability of spatially resolved tactile sensing and temperature sensing. These multifunctional 3D sensors would open new applications in soft robotics, bioelectronics, micro‐electromechanical systems, and others.     

Fabrication and Deformation of 3D Multilayered Kirigami Microstructures

Mechanically guided 3D microassembly with controlled compressive buckling represents a promising emerging route to 3D mesostructures in a broad range of advanced materials, including single‐crystalline silicon (Si), of direct relevance to microelectronic devices. During practical applications, the assembled 3D mesostructures and microdevices usually undergo external mechanical loading such as out‐of‐plane compression, which can induce damage in or failure of the structures/devices. Here, the mechanical responses of a few mechanically assembled 3D kirigami mesostructures under flat‐punch compression are studied through combined experiment and finite element analyses. These 3D kirigami mesostructures consisting of a bilayer of Si and SU‐8 epoxy are formed through integration of patterned 2D precursors with a prestretched elastomeric substrate at predefined bonding sites to allow controlled buckling that transforms them into desired 3D configurations. In situ scanning electron microscopy measurement enables detailed studies of the mechanical behavior of these structures. Analysis of the load–displacement curves allows the measurement of the effective stiffness and elastic recovery of various 3D structures. The compression experiments indicate distinct regimes in the compressive force/displacement curves and reveals different geometry‐dependent deformation for the structures. Complementary computational modeling supports the experimental findings and further explains the geometry‐dependent deformation.     

Probing the pH Microenvironment of Mesenchymal Stromal Cell Cultures on Additive‐Manufactured Scaffolds

Despite numerous advances in the field of tissue engineering and regenerative medicine, monitoring the formation of tissue regeneration and its metabolic variations during culture is still a challenge and mostly limited to bulk volumetric assays. Here, a simple method of adding capsules‐based optical sensors in cell‐seeded 3D scaffolds is presented and the potential of these sensors to monitor the pH changes in space and time during cell growth is demonstrated. It is shown that the pH decreased over time in the 3D scaffolds, with a more prominent decrease at the edges of the scaffolds. Moreover, the pH change is higher in 3D scaffolds compared to monolayered 2D cell cultures. The results suggest that this system, composed by capsules‐based optical sensors and 3D scaffolds with predefined geometry and pore architecture network, can be a suitable platform for monitoring pH variations during 3D cell growth and tissue formation. This is particularly relevant for the investigation of 3D cellular microenvironment alterations occurring both during physiological processes, such as tissue regeneration, and pathological processes, such as cancer evolution.     

Enabling Robust Self‐Folding Origami by Pre‐Biasing Vertex Buckling Direction

Self‐folding is a powerful approach to fabricate materials with complex 3D forms and advanced properties using planar patterning steps, but suffers from intrinsic limitations in robustness due to the highly bifurcated nature of configuration space around the flat state. Here, a simple mechanism is introduced to achieve robust self‐folding of microscale origami by separating actuation into two discrete steps using different thermally responsive hydrogels. First, the vertices are pre‐biased to move in the desired direction from the flat state by selectively swelling one of the two hydrogels at high temperature. Subsequently, the creases are folded toward their target angles by activating swelling of the second hydrogel upon cooling to room temperature. Since each vertex can be individually programmed to move upward or downward, it is possible to robustly select the desired branch even in multi‐vertex structures with reasonably high complexity. This strategy provides key new principles for designing shaping‐morphing materials that avoid undesired distractor states, expanding their potential applications in areas such as soft robotics, sensors, mechanical metamaterials, and deployable devices.     

Research Advances in Microfiber Humidity Sensors

An overview of the numerous latest research in microfiber humidity sensors is carried out with a specific focus on measurement methods, humidity sensitive materials, probe structures, and sensing properties of different sensors. First, five mainstream measurement structures, including taper, fiber grating, coupler, resonator, and interferometer are reviewed. It is concluded that these measurement structures sense the physicochemical property variations of microfibers or sensitive films and exhibit the change of optical signal when exposed to environment. Second, the basic preparation methods, humidity‐sensing properties, and their advantages and disadvantages as humidity sensitive material are addressed. Then, the advantages and disadvantages of all the above sensing structures are also discussed and compared. Finally, the main existing problems and potential solutions of microfiber humidity sensors are pointed out.     

Microscale 3D Printing of Nanotwinned Copper

Nanotwinned (nt)‐metals exhibit superior mechanical and electrical properties compared to their coarse‐grained and nanograined counterparts. nt‐metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (additive manufacturing) of nt‐metals. Microscale 3D printing of nt‐Cu is demonstrated with high density of coherent twin boundaries using a new room temperature process based on localized PED (L‐PED). The 3D printed nt‐Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps. The L‐PED process enables direct 3D printing of layer‐by‐layer and complex 3D microscale nt‐Cu structures, which may find applications for fabrication of metamaterials, sensors, plasmonics, and micro/nanoelectromechanical systems.     

Layered PtSe2 for Sensing,Photonic, and (Opto‐)Electronic Applications

Since the first experimental discovery of graphene 16 years ago, many other 2D layered nanomaterials have been reported. However, the majority of 2D nanostructures suffer from relatively complicated fabrication processes that have bottlenecked their development and their uptake by industry for practical applications. Here, the recent progress in sensing, photonic, and (opto‐)electronic applications of PtSe₂, a 2D layered material that is likely to be used in industries benefiting from its high air‐stability and semiconductor‐technology‐compatible fabrication methods, is reviewed. The advantages and disadvantages of a range of synthesis methods for PtSe₂ are initially compared, followed by a discussion of its outstanding properties, and industrial and commercial advantages. Research focused on the broadband nonlinear photonic properties of PtSe₂, as well as reports of its use as a saturable absorber in ultrafast lasers, are then reviewed. Additionally, the advances that have been achieved in a range of PtSe₂‐based field‐effect transistors, photodetectors, and sensors are summarized. Finally, a conclusion on these results along with the outlook for the future is presented.     

Extrusion‐Based 3D Printing of Hierarchically Porous Advanced Battery Electrodes

A highly porous 2D nanomaterial, holey graphene oxide (hGO), is synthesized directly from holey graphene powder and employed to create an aqueous 3D printable ink without the use of additives or binders. Stable dispersions of hydrophilic hGO sheets in water (≈100 mg mL^?1) can be readily achieved. The shear‐thinning behavior of the aqueous hGO ink enables extrusion‐based printing of fine filaments into complex 3D architectures, such as stacked mesh structures, on arbitrary substrates. The freestanding 3D printed hGO meshes exhibit trimodal porosity: nanoscale (4–25 nm through‐holes on hGO sheets), microscale (tens of micrometer‐sized pores introduced by lyophilization), and macroscale (<500 µm square pores of the mesh design), which are advantageous for high‐performance energy storage devices that rely on interfacial reactions to promote full active‐site utilization. To elucidate the benefit of (nano)porosity and structurally conscious designs, the additive‐free architectures are demonstrated as the first 3D printed lithium–oxygen (Li–O₂) cathodes and characterized alongside 3D printed GO‐based materials without nanoporosity as well as nanoporous 2D vacuum filtrated films. The results indicate the synergistic effect between 2D nanomaterials, hierarchical porosity, and overall structural design, as well as the promise of a freeform generation of high‐energy‐density battery systems.     

Direct Fabrication of Freestanding and Patterned Nanoporous Junctions in a 3D Micro‐Nanofluidic Device for Ion‐Selective Transport

In the field of micro‐nanofluidics, a freestanding configuration of a nanoporous junction is highly demanded to increase the design flexibility of the microscale device and the interfacial area between the nanoporous junction and microchannels, thereby improving the functionality and performance. This work first reports direct fabrication and incorporation of a freestanding nanoporous junction in a microfluidic device by performing an electrolyte‐assisted electrospinning process to fabricate a freestanding nanofiber membrane and subsequently impregnating the nanofiber membrane with a nanoporous precursor material followed by a solidification process. This process also enables to readily control the geometry of the nanoporous junction depending on its application. By these advantages, vertically stacked 3D micro‐nanofluidic devices with complex configurations are easily achieved. To demonstrate the broad applicability of this process in various research fields, a reverse electrodialysis‐based energy harvester and an ion concentration polarization‐based preconcentrator are produced. The freestanding Nafion‐polyvinylidene fluoride nanofiber membrane (F‐NPNM) energy harvester generates a high power (59.87 nW) owing to the enlarged interfacial area. Besides, 3D multiplexed and multi‐stacked F‐NPNM preconcentrators accumulate multiple preconcentrated plugs that can increase the operating sample volume and the degree of freedom of handling. Hence, the proposed process is expected to contribute to numerous research fields related to micro‐nanofluidics in the future.     

3D-Printed Micro/Nano-Scaled Mechanical Metamaterials: Fundamentals,Technologies, Progress,Applications, and Challenges

Micro/nano-scaled mechanical metamaterials have attracted extensive attention in various fields attributed to their superior properties benefiting from their rationally designed micro/nano-structures. As one of the most advanced technologies in the 21st century, additive manufacturing (3D printing) opens an easier and faster path for fabricating micro/nano-scaled mechanical metamaterials with complex structures. Here, the size effect of metamaterials at micro/nano scales is introduced first. Then, the additive manufacturing technologies to fabricate mechanical metamaterials at micro/nano scales are introduced. The latest research progress on micro/nano-scaled mechanical metamaterials is also reviewed according to the type of materials. In addition, the structural and functional applications of micro/nano-scaled mechanical metamaterials are further summarized. Finally, the challenges, including advanced 3D printing technologies, novel material development, and innovative structural design, for micro/nano-scaled mechanical metamaterials are discussed, and future perspectives are provided. The review aims to provide insight into the research and development of 3D-printed micro/nano-scaled mechanical metamaterials.     

3D Mimicry of Native‐Tissue‐Fiber Architecture Guides Tendon‐Derived Cells and Adipose Stem Cells into Artificial Tendon Constructs

Tendon and ligament (T/L) function is intrinsically related with their unique hierarchically and anisotropically organized extracellular matrix. Their natural healing capacity is, however, limited. Here, continuous and aligned electrospun nanofiber threads (CANT) based on synthetic/natural polymer blends mechanically reinforced with cellulose nanocrystals are produced to replicate the nanoscale collagen fibrils grouped into microscale collagen fibers that compose the native T/L. CANT are then incrementally assembled into 3D hierarchical scaffolds, resulting in woven constructions, which simultaneously mimic T/L nano‐to‐macro architecture, nanotopography, and nonlinear biomechanical behavior. Biological performance is assessed using human‐tendon‐derived cells (hTDCs) and human adipose stem cells (hASCs). Scaffolds nanotopography and microstructure induce a high cytoskeleton elongation and anisotropic organization typical of tendon tissues. Moreover, the expression of tendon‐related markers (Collagen types I and III, Tenascin‐C, and Scleraxis) by both cell types, and the similarities observed on their expression patterns over time suggest that the developed scaffolds not only prevent the phenotypic drift of hTDCs, but also trigger tenogenic differentiation of hASCs. Overall, these results demonstrate a feasible approach for the scalable production of 3D hierarchical scaffolds that exhibit key structural and biomechanical properties, which can be advantageously explored in acellular and cellular T/L TE strategies.     

Printing Polymer Nanocomposites and Composites in Three Dimensions

Recent advances in materials science and three‐dimensional (3D) printing hold great promises to conceive new classes of multifunctional materials and components for functional devices and products. Various functionalities (e.g., mechanical, electrical, and thermal properties, magnetism) can be offered by the nano‐ and micro‐reinforcements to the non‐functional pure printing materials for the realization of advanced materials and innovative systems. In addition, the ability to print 3D structures in a layer‐by‐layer manner enables manufacturing of highly‐customized complex features and allows an efficient control over the properties of fabricated structures. Here, the authors present a brief overview mainly over the latest progresses in 3D printing of multifunctional polymer nanocomposites and microfiber‐reinforced composites including the benefits, limitations, and potential applications. Only those 3D printing techniques that are compatible with polymer nanocomposites and composites, that is, materials that have already been used as printing materials, are introduced. The very hot topic of 3D printing of thermoplastic composites featuring continuous microfibers is also briefly introduced.     

3D,Reconfigurable, Multimodal Electronic Whiskers via Directed Air Assembly

A batch‐assembly technique for forming 3D electronics on shape memory polymer substrates is demonstrated and is used to create dense, highly sensitive, multimodal arrays of electronic whiskers. Directed air flow at temperatures above the substrate's glass transition temperature transforms planar photolithographically defined resistive sensors from 2D precursors into shape‐tunable, deterministic 3D assemblies. Reversible 3D assembly and flattening is achieved by exploiting the shape memory properties of the substrate, enabling context‐driven shape reconfiguration to isolate/enhance specific sensing modes. In particular, measurement schemes and device configurations are introduced that allow for the sensing of temperature, stiffness, contact force, proximity, and surface texture and roughness. The assemblies offer highly spatiotemporally resolved, wide‐range measurements of surface topology (50 nm to 500 µm), material stiffness (200 kPa to 7.5 GPa), and temperature (0–100 °C), with response times of <250 µs. The development of a scalable process for 3D assembly of reconfigurable electronic sensors, as well as the large breadth and sensitivity of complex sensing modes demonstrated, has applications in the growing fields of 3D assembly, electronic skin, and human–machine interfaces.     

Bio‐Hybrid Cell‐Based Actuators for Microsystems

As we move towards the miniaturization of devices to perform tasks at the nano and microscale, it has become increasingly important to develop new methods for actuation, sensing, and control. Over the past decade, bio‐hybrid methods have been investigated as a promising new approach to overcome the challenges of scaling down robotic and other functional devices. These methods integrate biological cells with artificial components and therefore, can take advantage of the intrinsic actuation and sensing functionalities of biological cells. Here, the recent advancements in bio‐hybrid actuation are reviewed, and the challenges associated with the design, fabrication, and control of bio‐hybrid microsystems are discussed. As a case study, focus is put on the development of bacteria‐driven microswimmers, which has been investigated as a targeted drug delivery carrier. Finally, a future outlook for the development of these systems is provided. The continued integration of biological and artificial components is envisioned to enable the performance of tasks at a smaller and smaller scale in the future, leading to the parallel and distributed operation of functional systems at the microscale.     

Shaping Giant Membrane Vesicles in 3D‐Printed Protein Hydrogel Cages

Giant unilamellar phospholipid vesicles are attractive starting points for constructing minimal living cells from the bottom‐up. Their membranes are compatible with many physiologically functional modules and act as selective barriers, while retaining a high morphological flexibility. However, their spherical shape renders them rather inappropriate to study phenomena that are based on distinct cell shape and polarity, such as cell division. Here, a microscale device based on 3D printed protein hydrogel is introduced to induce pH‐stimulated reversible shape changes in trapped vesicles without compromising their free‐standing membranes. Deformations of spheres to at least twice their aspect ratio, but also toward unusual quadratic or triangular shapes can be accomplished. Mechanical force induced by the cages to phase‐separated membrane vesicles can lead to spontaneous shape deformations, from the recurrent formation of dumbbells with curved necks between domains to full budding of membrane domains as separate vesicles. Moreover, shape‐tunable vesicles are particularly desirable when reconstituting geometry‐sensitive protein networks, such as reaction‐diffusion systems. In particular, vesicle shape changes allow to switch between different modes of self‐organized protein oscillations within, and thus, to influence reaction networks directly by external mechanical cues.     

二维胶体晶体成形及应用研究进展

综述了二维胶体晶体的结构特点及成形驱动力,介绍了LB(Langmuir-Blodgett)转移法、毛细组装法、电场沉积组装法、特殊装置组装法等二维胶体晶体成形方法的原理、成形过程及优缺点,同时概述了二维胶体晶体在微加工及微图案化、纳米传感器、纳米磁存储等领域的应用情况。     

Inverse Opal Scaffolds with an Embossed Surface Pattern Prepared from Sticky Golf‐Ball‐Shaped Microparticle Assemblies

Inverse opal scaffolds presenting an embossed‐pattern surface are prepared from colloidal crystal assemblies of uniformly sized golf‐ball‐shaped microparticles. Post‐treatments, such as thermal annealing during the bridging of the microparticles for opal preparation, are avoided to prevent deterioration of surface patterns of the sacrificial template. This presents a new approach to increase the surface‐area‐to‐volume ratio (SAV) by the alteration of morphological features in sophisticated 3D structures that remain largely unexamined owing to difficulties in their preparation. Previous results observed in 2D surfaces that show effective performance improvement through an increase in contact area, especially in biomedical applications, also appear applicable to patterned inverse opal scaffolds based on comparable results obtained from cell cultures. As the field of application of opal and inverse opal structures is expanding due to their unique structural advantages, such as 3D interconnectivity and periodic structures, our strategy opens the door for the use of patterned surfaces on highly sophisticated 3D structures, improving their performance via an increase in SAV.     

Folding 2D Structures into 3D Configurations at the Micro/Nanoscale: Principles,Techniques, and Applications

Compared to their 2D counterparts, 3D micro/nanostructures show larger degrees of freedom and richer functionalities; thus, they have attracted increasing attention in the past decades. Moreover, extensive applications of 3D micro/nanostructures are demonstrated in the fields of mechanics, biomedicine, optics, etc., with great advantages. However, the mainstream micro/nanofabrication technologies are planar ones; therefore, they cannot be used directly for the construction of 3D micro/nanostructures, making 3D fabrication at the micro/nanoscale a great challenge. A promising strategy to overcome this is to combine the state‐of‐the‐art planar fabrication techniques with the folding method to produce 3D structures. In this strategy, 2D components can be easily produced by traditional planar techniques, and then, 3D structures are constructed by folding each 2D component to specific orientations. In this way, not only will the advantages of existing planar techniques, such as high precision, programmable patterning, and mass production, be preserved, but the fabrication capability will also be greatly expanded without complex and expensive equipment modification/development. The goal here is to highlight the recent progress of the folding method from the perspective of principles, techniques, and applications, as well as to discuss the existing challenges and future prospectives.     

3D Heteroatom‐Doped Carbon Nanomaterials as Multifunctional Metal‐Free Catalysts for Integrated Energy Devices

Sustainable and cost‐effective energy generation has become crucial for fulfilling present energy requirements. For this purpose, the development of cheap, scalable, efficient, and reliable catalysts is essential. Carbon‐based heteroatom‐doped, 3D, and mesoporous electrodes are very promising as catalysts for electrochemical energy conversion and storage. Various carbon allotropes doped with a variety of heteroatoms can be utilized for cost‐effective mass production of electrode materials. 3D porous carbon electrodes provide multiple advantages, such as large surface area, maximized exposure to active sites, 3D conductive pathways for efficient electron transport, and porous channels to facilitate electrolyte diffusion. However, it is challenging to synthesize and functionalize isotropic 3D carbon structures. Here, various synthesis processes of 3D porous carbon materials are summarized to understand how their physical and chemical properties together with heteroatom doping dictate the electrochemical catalytic performance. Prospects of attractive 3D carbon structural materials for energy conversion and efficient integrated energy systems are also discussed.