Shape-Memory Balloon Structures by Pneumatic Multi-material 4D Printing

4D printing is an attractive approach for manufacturing structures that can adopt new shapes or functionalities after printing. However, 4D printing methods and materials that can be used to achieve structures with complex shapes and excellent mechanical properties simultaneously are still lacking. Here, a novel 4D printing is developed where multi-material digital light process 3D printing of shape memory polymers (SMPs) fabricates a structure that is later transformed into a complex 3D shape with robust mechanical properties by pneumatic manipulation. In this method, the shape change is controlled by the spatial distributions of SMPs, which is designed by finite element analysis. Experimental investigations are carried out to print various structured balloons with predefined intricate shapes, including a structure in dog-like shape and a surface with the human face contour. These structures are also endowed with robust mechanical stiffness and lightweight features, which allow this new 4D printing approach for potential applications in biomedical devices, reconfigurable structures, and metamaterials.     

Complex‐Shaped Cellulose Composites Made by Wet Densification of 3D Printed Scaffolds

Cellulose is an attractive material resource for the fabrication of sustainable functional products, but its processing into structures with complex architecture and high cellulose content remains challenging. Such limitation has prevented cellulose‐based synthetic materials from reaching the level of structural control and mechanical properties observed in their biological counterparts, such as wood and plant tissues. To address this issue, a simple approach is reported to manufacture complex‐shaped cellulose‐based composites, in which the shaping capabilities of 3D printing technologies are combined with a wet densification process that increases the concentration of cellulose in the final printed material. Densification is achieved by exchanging the liquid of the wet printed material with a poor solvent mixture that induces attractive interactions between cellulose particles. The effect of the solvent mixture on the final cellulose concentration is rationalized using solubility parameters that quantify the attractive interparticle interactions. Using X‐ray diffraction analysis and mechanical tests, 3D printed composites obtained through this process are shown to exhibit highly aligned microstructures and mechanical properties significantly higher than those obtained by earlier additively manufactured cellulose‐based materials. These features enable the fabrication of cellulose‐rich synthetic structures that more closely resemble the exquisite designs found in biological materials grown by plants in nature.     

4D Printing of Hydrogels: A Review

3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.     

3D Printing of a Biocompatible Double Network Elastomer with Digital Control of Mechanical Properties

The majority of 3D‐printed biodegradable biomaterials are brittle, limiting their application to compliant tissues. Poly(glycerol sebacate) acrylate (PGSA) is a synthetic biocompatible elastomer and compatible with light‐based 3D printing. In this article, digital‐light‐processing (DLP)‐based 3D printing is employed to create a complex PGSA network structure. Nature‐inspired double network (DN) structures consisting of interconnected segments with different mechanical properties are printed from the same material in a single shot. Such capability has not been demonstrated by any other fabrication techniques so far. The biocompatibility of PGSA is confirmed via cell‐viability analysis. Furthermore, a finite‐element analysis (FEA) model is used to predict the failure of the DN structure under uniaxial tension. FEA confirms that the DN structure absorbs 100% more energy before rupture by using the soft segments as sacrificial elements while the hard segments retain structural integrity. Using the FEA‐informed design, a new DN structure is printed and tensile test results agree with the simulation. This article demonstrates how geometrically‐optimized material design can be easily and rapidly constructed by DLP‐based 3D printing, where well‐defined patterns of different stiffnesses can be simultaneously formed using the same elastic biomaterial, and overall mechanical properties can be specifically optimized for different biomedical applications.     

Customizable and Reconfigurable Surface Properties of Printed Micro-objects by 3D Direct Laser Writing via Nitroxide Mediated Photopolymerization

Photoactivated Reversible Deactivation Radical Polymerization (RDRP) technologies have emerged very recently in the field of 3D printing systems especially at the macroscale in vat-photopolymerization-based processes such as digital light processing (DLP). Contrary to conventional free radical photopolymerization, photoRDRP in 3D printing leads to 3D objects with living character and thus confers them the unique ability to be post-modified after fabrication. While 3D direct laser writing (3D DLW) by two photon polymerization has become a standard for fabrication of complex 3D micro-objects, the use of RDRP and its associated benefits has so far been under-investigated at that scale. Herein, a photoresist suitable for 3D DLW based on nitroxide mediated photopolymerization (NMP2) is developed. The photopolymerization efficiency for fabrication of micro-structures and their subsequent post-modification are investigated regarding the laser power and the wavelength of excitation. Moreover, highly tunable, precise, and successive surface patterning of 2D and 3D multi-material microstructures are demonstrated thanks to the spatial and temporal control offered by the photo-induced post-modification. This work highlights new directions to be explored in order to accelerate the adoption of RDRP in 3D printing based on photopolymerization.     

Patterned Carbon Nitride–Based Hybrid Aerogel Membranes via 3D Printing for Broadband Solar Wastewater Remediation

Controlled scalable assembly of 2D building blocks into macroscopic 3D architectures is highly significant. However, the assembly of g‐C₃N₄ into tailored, 3D architectures is not yet reported. Here, a 3D printing methodology to enable the programmable construction of carbon nitride–based hybrid aerogel membranes with patterned macroscopic architectures is proposed. g‐C₃N₄ nanosheets (CNNS) are used as the building block, and sodium alginate (SA) increases the viscosity of the ink to obtain the desired rheological properties. Three printing routes, including printing directly in air and in the supporting reservoirs composed of CaCl₂/glycerol solution or Pluronic F127, are demonstrated for printing versatility. The printed Au nanobipyramid–CNNS–SA hybrid aerogels exhibit broadband visible‐light absorption and superior solar wastewater remediation performance with excellent cyclic stability and easy manipulation features. Remarkably, the activity of the 3D‐printed aerogel is about 2.5 times of that of the contrast sample, attributing to the enhanced liquid velocity and solution diffusion efficiency because of the 3D‐printed structure, which is demonstrated by experimental and theoretical simulations. This approach can be extended to the macroscopic assembly of other 2D materials for myriad applications.     

Analysis of electrically conductive silver ink on stretchable substrates under tensile load

Electrical conductors were printed by the screen printing method on stretchable PVC substrates and on fabrics. Polymer thick film silver ink was used as the conductive medium. The electrical performance and the structure of the ink film were investigated in unloaded conditions and under strain. In addition, the ink film morphology was examined. The goal of this study was to provide information for developing a strain sensor for large strain levels using the materials under investigation. An additional aim was to assist the integration of electronics into other structures. The results showed that strain sensitive structures can be made using the materials selected for this study and these materials provide an opportunity to develop strain sensors. The structures also tolerated large strain levels and thus they can be integrated into other materials which are exposed to strain.     

A Subtractive Photoresist Platform for Micro‐ and Macroscopic 3D Printed Structures

The selective removal of structural elements plays a decisive role in 3D printing applications enabling complex geometries. To date, the fabrication of complex structures on the microscale is severely limited by multistep processes. Herein, a subtractive photoresist platform technology that is transferable from microscopic 3D printing via direct laser writing to macroscopic structures via stereolithography is reported. All resist components are readily accessible and exchangeable, offering fast adaptation of the resist's property profile. The micro‐ and macroprinted structures can be removed in a facile fashion, without affecting objects based on standard photoresists. The cleavage is analyzed by time‐lapse optical microscopy as well as via in‐depth spectroscopic assessment. The mechanical properties of the printed materials are investigated by nanoindentation. Critically, the power of the subtractive resist platform is demonstrated by constructing complex 3D objects with flying features on the microscale.     

A Versatile Self‐Organization Printing Method for Simplified Tandem Organic Photovoltaics

Despite recent dramatic enhancements in power conversion efficiencies (PCEs) resulting in values over 10%, the manufacturing of tandem organic solar cells (OSCs) via current printing technologies is subject to tremendous challenges. Existing complicated tandem structures consisting of six or more component layers have been a major obstacle that significantly increases the complexity of printing processes and substantially sacrifices the PCE for printed devices. Here, an innovative printing method is reported that simplifies the fabrication process of the tandem OSCs. By developing a new printing technique using a nanocomposites containing interfacial and photoactive materials, a simultaneously printed bilayer of consisting of interfacial and photoactive layers, achieved through vertical self‐organization, is successfully demonstrated, resulting in tandem OSCs with only four printed layers. Moreover, by rigorously controlling the molecular weight of the interfacial materials, the self‐assembly characteristics are improved and an efficient tandem OSC is yielded with a PCE of 9.1% achieved in printed layers.     

Self-Healing,Photothermal-Responsive,and Shape Memory Polyurethanes for Enhanced Mechanical Properties of 3D/4D Printed Objects

Additive manufacturing is a promising technology that can directly fabricate structures with complex internal geometries, which is barely achieved by traditional manufacturing. However, the mechanical properties of fused deposition modeling (FDM)-printed objects are inferior to those of conventionally manufactured products. To improve the mechanical properties of the printed products, a series of novel thermoplastic polyurethanes with self-healing properties, intrinsic photothermal effects, and excellent printability are designed and synthesized by introducing dynamic oxime–carbamate bonds and hydrogen bonds into the polymer chains. On-demand introduction of near-infrared (NIR) irradiation, direct heating, and sunlight irradiation enhances interfacial bonding strength and thus improve the mechanical properties of the printed product. Additionally, mechanical anisotropy of the printed products can be sophistically manipulated by regulating the self-healing conditions. Support-free printing and healing of damaged printed products are also achieved owing to the self-healing properties of the material. Moreover, the as-prepared materials exhibit shape-memory properties NIR irradiation or direct heating effectively triggers shape-memory recovery and demonstrates their potential in 4D printing by printing a man-like robot. This study not only provides a facile strategy for obtaining high-performance printed products but also broadens the potential applications of FDM technology in intelligent devices.     

Mechanical Properties Tailoring of 3D Printed Photoresponsive Nanocellulose Composites

3D printing technologies allow control over the alignment of building blocks in synthetic materials, but compositional changes often require complex multimaterial printing steps. Here, 3D printable materials showing locally tunable mechanical properties are produced in a single printing step of Direct Ink Writing. These new inks consist of a polymer matrix bearing biocompatible photoreactive cinnamate derivatives and up to 30 wt% of anisotropic cellulose nanocrystals. The printed materials are mechanically versatile and can undergo further crosslinking upon illumination. When illuminating the material and controlling the irradiation doses, the Young's moduli can be adjusted between 15 and 75 MPa. Moreover, spatially controlled illumination allows patterning stiff geometries, resulting in 3D printed structures with segments of different mechanical properties tailoring the mechanical behavior under compression. The high design freedom implemented by 3D printing and photopatternability opens the venue to rapid manufacturing of devices for applications such as prosthetics or soft robotics where the 3D shapes and mechanical properties must be tailored for personalized load cases.     

Concurrent Formation of Metallic Glass During Laser Forward Transfer 3D Printing

In recent years, bulk metallic glasses (BMGs) have drawn much research attention and are shown to be of industrial interest due to their superior mechanical properties and resistance to corrosion. In spite of the interest in harnessing MG for microelectromechanical systems devices, there are limitations in manufacturing such micrometer‐scale structures. A novel approach for the fabrication of 3D MG structures using laser‐induced forward transfer (LIFT) is demonstrated. Inherent tremendous cooling rates associated with the metal LIFT process (≈10¹⁰ k s^?1) make the formation of a variety of BMGs accessible, including also various binary compositions. In this work, it is demonstrated that LIFT printing of ZrPd‐based metallic glass microstructures can also be performed under ambient conditions. X‐ray diffraction analysis of the printed structures reveals > 95% of amorphous metal phase. Taking advantage of the properties of BMG, high quality printing of high aspect ratio BMG pillars, and microbridges are demonstrated. It is also shown how a composite, amorphous‐crystalline metal structure with a required configuration can be fabricated using multimaterial LIFT printing. The inherent high resolution of the method combined with the noncontact and multimaterial printing capacity makes LIFT a valuable additive manufacturing technique to produce metallic glass‐based devices.     

Advances in 4D Printing: Materials and Applications

4D printing has attracted tremendous interest since its first conceptualization in 2013. 4D printing derived from the fast growth and interdisciplinary research of smart materials, 3D printer, and design. Compared with the static objects created by 3D printing, 4D printing allows a 3D printed structure to change its configuration or function with time in response to external stimuli such as temperature, light, water, etc., which makes 3D printing alive. Herein, the material systems used in 4D printing are reviewed, with emphasis on mechanisms and potential applications. After a brief overview of the definition, history, and basic elements of 4D printing, the state‐of‐the‐art advances in 4D printing for shape‐shifting materials are reviewed in detail. Both single material and multiple materials using different mechanisms for shape changing are summarized. In addition, 4D printing of multifunctional materials, such as 4D bioprinting, is briefly introduced. Finally, the trend of 4D printing and the perspectives for this exciting new field are highlighted.     

Aerosol jet printed top grids for organic optoelectronic devices

Aerosol jet deposited metallic grids are very promising as transparent electrodes for large area organic solar cells and organic light emitting diodes. However, the homogeneity and the printing speed remain a challenge. We report homogeneous and rapidly printed metallic lines based on a complex-based metal–organic silver ink using a processing temperature of 140 °C. We show that inhomogeneities, which are present in printed structures at increased printing speeds and mainly caused by drying effects, can be improved by adding high boiling point solvents. We demonstrate solution processed highly conductive and transparent hybrid electrodes on inverted organic solar cells comprising digitally printed top silver grids.     

3D Printed Actuators: Reversibility,Relaxation, and Ratcheting

Additive manufacturing strives to combine any combination of materials into 3D functional structures and devices, ultimately opening up the possibility of 3D printed machines. It remains difficult to actuate such devices, thus limiting the scope of 3D printed machines to passive devices or necessitating the incorporation of external actuators that are manufactured differently. Here, 3D printed hybrid thermoplast/conducter bilayers are explored, which can be actuated by differential heating caused by externally controllable currents flowing through their conducting faces. The functionality of such actuators is uncovered and it is shown that they allow to 3D print, in one pass, simple flexible robotic structures that propel forward under step‐wise applied voltages. Moreover, exploiting the thermoplasticity of the nonconducting plastic parts at elevated temperatures, it is shown that how strong driving leads to irreversible deformations—a form of 4D printing—which also enlarges the range of linear response of the actuators. Finally, it is shown that how to leverage such thermoplastic relaxations to accumulate plastic deformations and obtain very large deformations by alternatively driving both layers of a bilayer; this is called ratcheting. The strategy is scalable and widely applicable, and opens up a new approach to reversible actuation and irreversible 4D printing of arbitrary structures and machines.     

Ceramic synthetic substrates using solid freeform fabrication

Three-dimensional (3-D) printing techniques were used to create lost molds for advanced inhomogeneous materials made of ceramics. The utility of this process is the ability to manipulate low-loss materials, such as alumina illustrated here, into precise, fine features as periodic macroscopic inclusions in a larger host material or air. Three periodic structures were formed using solid freeform (SFF) fabrication techniques; a two-dimensional (2-D) all-dielectric electromagnetic bandgap (EBG) substrate, a 2-D metallo-dielectric substrate filter, and a 3-D woodpile EBG structure. The design of these structures is discussed and the fabricational steps are presented in detail. This process has been utilized to develop a simple two-pole filter embedded in a substrate at 10 GHz, a single resonator at 30 GHz, and a 3-D bandgap structure from 88 to 115 GHz. The wide range of features and structures that can be accurately fabricated are highlighted.     

Novel in-line method for patterned deposition of conductive structures

We report on a novel method for patterned deposition of conductive polymers, which is derived from and fully compatible with fast in-line printing methods. The method is aimed at improving resolution, homogeneity and edge sharpness of printed conductive structures for applications in the field of printed electronics. The surface energy of a foil substrate, which has been increased by means of corona treatment, was reduced in selected areas through contact with materials that have suitable surface properties and can be patterned at micrometer scale (i.e. printing plates). A conductive fluid was sprayed onto the chemically heterogeneous surface. The fluid accumulated at areas with higher surface energy. After the drying process, the conductive polymer structures were used as source/drain contacts for the preparation of OFETs.     

3D Bioprinting using UNIversal Orthogonal Network (UNION) Bioinks

Three-dimensional (3D) bioprinting is a promising technology to produce tissue-like structures, but a lack of diversity in bioinks is a major limitation. Ideally each cell type would be printed in its own customizable bioink. To fulfill this need for a universally applicable bioink strategy, a versatile bioorthogonal bioink crosslinking mechanism that is cell compatible and works with a range of polymers is developed. This family of materials is termed UNIversal, Orthogonal Network (UNION) bioinks. As demonstration of UNION bioink versatility, gelatin, hyaluronic acid (HA), recombinant elastin-like protein (ELP), and polyethylene glycol (PEG) are each used as backbone polymers to create inks with storage moduli spanning from 200 to 10 000 Pa. Because UNION bioinks are crosslinked by a common chemistry, multiple materials can be printed together to form a unified, cohesive structure. This approach is compatible with any support bath that enables diffusion of UNION crosslinkers. Both matrix-adherent human corneal mesenchymal stromal cells and non-matrix-adherent human induced pluripotent stem cell-derived neural progenitor spheroids are printed with UNION bioinks. The cells retained high viability and expressed characteristic phenotypic markers after printing. Thus, UNION bioinks are a versatile strategy to expand the toolkit of customizable materials available for 3D bioprinting.     

基于 3D 打印的THz 波导成型技术研究进展

本文首先介绍了太赫兹波导和3D 打印技术的发展现状。3D 打印作为一项新兴的技术,以数字模型文件为基础, 运用粉末状金属或塑料等可粘合材料通过逐层打印的方法构造实体,打破了传统THz 波导技术的局限性。本文介绍的3D 打印THz 波导利用聚合树脂作为打印材料,打印完成的THz 波导在其传输通路上镀500nm 的金,金的厚度足以支持THz 传播。利用这种方法可以打印出直波导、三维弯曲面、三维Y 劈和U 型波导等多种结构。3D 打印THz 波导除传输损耗 略高外,其传输模式及其特性与传统的金属波导基本一致,这种额外的传输损耗归咎于商业3D 打印机的精度。     

Automated fabrication of hybrid printed electronic circuits

Printed electronics offer great potential for new applications such as Internet of Things devices and wearables. However, to date, only a limited number of electronic functions and integration densities can be realised by printing processes. Hence, hybrid printed electronic circuits are actually created by mounting silicon electronic components. Since both printed materials and processes are continuously evolving, an accompanying structured development methodology is required. This paper highlights a digital workflow from design to automated fabrication using the example of a demonstrator circuit. A multi-layer vector ink-jet printing process to print electronic devices onto foil substrates with three functional inks is presented. This printing process is improved using a newly set-up printing system: Integrating a piezo print head into the path planning of the printing system and its control as a virtual stepper axis enable highly precise vector printing. This leads to printed resistors with low tolerances. Adaptations of surface mount technology for assembling silicon electronic components onto printed foil substrates are discussed. Finally, image processing methods to cope with deformations of the flexible foil substrates in the fabrication process are introduced.