A Tailorable Family of Elastomeric‐to‐Rigid, 3D Printable,Interbonding Polymer Networks

Soft materials with widely tailorable mechanical properties throughout the material's volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense sectors. Herein, a platform of 3D printable soft polymer networks with unprecedented tunability of stiffness of nearly three orders of magnitude (MPa to GPa) and an inherent capability to interbond is reported. The materials are based on dynamic covalent polymer networks with variable density of crosslinkers attached to prepolymer backbones via a temperature‐reversible Diels–Alder (DA) reaction. Inherent flexibility of the prepolymer chains and controllable crosslinking density enable 3D printed networks with glass transition temperatures ranging from just a few degrees to several tens of degrees Celsius. Materials with an elastomeric network demonstrate a fast and spontaneous self‐healing behavior at room temperature both in air and under water—a behavior difficult to achieve with other crosslinked materials. Reversible dissociation of DA networks at temperatures exceeding ≈120 °C allows for reprintability, while control of the stereochemistry of DA attachments enables reprogrammable shape memory behavior. The introduced platform addresses current major challenges including control of polymer interbonding, enhanced mechanical performance of printed parts, and reprocessability of 3D‐printed crosslinked materials in the absence of solvent.     

3D‐Printable Antimicrobial Composite Resins

3D printing is seen as a game‐changing manufacturing process in many domains, including general medicine and dentistry, but the integration of more complex functions into 3D‐printed materials remains lacking. Here, it is expanded on the repertoire of 3D‐printable materials to include antimicrobial polymer resins, which are essential for development of medical devices due to the high incidence of biomaterial‐associated infections. Monomers containing antimicrobial, positively charged quaternary ammonium groups with an appended alkyl chain are either directly copolymerized with conventional diurethanedimethacrylate/glycerol dimethacrylate (UDMA/GDMA) resin components by photocuring or prepolymerized as a linear chain for incorporation into a semi‐interpenetrating polymer network by light‐induced polymerization. For both strategies, dental 3D‐printed objects fabricated by a stereolithography process kill bacteria on contact when positively charged quaternary ammonium groups are incorporated into the photocurable UDMA/GDMA resins. Leaching of quaternary ammonium monomers copolymerized with UDMA/GDMA resins is limited and without biological consequences within 4–6 d, while biological consequences could be confined to 1 d when prepolymerized quaternary ammonium group containing chains are incorporated in a semi‐interpenetrating polymer network. Routine clinical handling and mechanical properties of the pristine polymer matrix are maintained upon incorporation of quaternary ammonium groups, qualifying the antimicrobially functionalized, 3D‐printable composite resins for clinical use.     

Polymer Field‐Effect Transistors Fabricated by the Sequential Gravure Printing of Polythiophene,Two Insulator Layers,and a Metal Ink Gate

The mass production technique of gravure contact printing is used to fabricate state‐of‐the art polymer field‐effect transistors (FETs). Using plastic substrates with prepatterned indium tin oxide source and drain contacts as required for display applications, four different layers are sequentially gravure‐printed: the semiconductor poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), two insulator layers, and an Ag gate. A crosslinkable insulator and an Ag ink are developed which are both printable and highly robust. Printing in ambient and using this bottom‐contact/top‐gate geometry, an on/off ratio of >10⁴ and a mobility of 0.04 cm² V^?1 s^?1 are achieved. This rivals the best top‐gate polymer FETs fabricated with these materials. Printing using low concentration, low viscosity ink formulations, and different P3HT molecular weights is demonstrated. The printing speed of 40 m min^?1 on a flexible polymer substrate demonstrates that very high‐volume, reel‐to‐reel production of organic electronic devices is possible.     

3D Printing of Liquid Metals: Recent Advancements and Challenges

The development of flexible electronics (FEs) has rapidly accelerated in numerous fields due to their exceptional deformability, bending, and stretchability. Room-temperature gallium-based liquid metals (LMs) are considered as efficient conductive materials for FEs due to their outstanding electrical conductivity and intrinsic flexibility. Recently, 3D printing has become a promising technique for fabricating FEs. However, the poor printability due to high surface tension and fluidity offers huge challenges in the 3D printing of LMs. This review summarizes the effective strategies to address these challenges. It primarily focuses on three points: 1) how to improve the printability of LM and its wettability with the substrate, 2) how to select the appropriate printing method to improve the printing speed and ensure the resolution of printing structure, and 3) how to provide perfect encapsulation for LM-based FEs with 3D printing. Following a brief introduction, the mainstream printing technologies and recent developments in the 3D printing of LMs are provided, with an emphasis on the selection of printing method, improvement of printability, encapsulation, and conductivity activation. Then, the revolutionary changes attained after 3D printing of LMs are specifically focused upon. Finally, opinions and potential directions for this thriving discipline are explored.     

Printed and Electrochemically Gated,High‐Mobility,Inorganic Oxide Nanoparticle FETs and Their Suitability for High‐Frequency Applications

Solution‐processed or printed n‐channel field‐effect transistors (FETs) with high performance are not reported very often in the literature due to the scarcity of high‐mobility n‐type organic semiconductors. On the other hand, low‐temperature processed n‐channel metal oxide semiconductor (NMOS) transistors from electron conducting inorganic‐oxide nanoparticles show reduced‐performance and low mobility because of large channel roughness at the channel‐dielectric interface. Here, a method to produce ink‐jet printed high performance NMOS transistor devices using inorganic‐oxide nanoparticles as the transistor channel in combination with a 3D electrochemical gating (EG) via printed composite solid polymer electrolytes is presented. The printed FETs produced show a device mobility value in excess of 5 cm² V^?1 s^?1, even though the root mean square (RMS) roughness of the nanoparticulate channel exceeds 15 nm. Extensive studies on the frequency dependent polarizability of composite polymer electrolyte capacitors show that the maximum attainable speed in such printed, long channel transistors is not limited by the ionic conductivity of the electrolytes. Therefore, the approach of combining printable, high‐quality oxide nanoparticles and the composite solid polymer electrolytes, offers the possibility to fully utilize the large mobility of oxide semiconductors to build all‐printed and high‐speed devices. The high polarizability of printable polymer electrolytes brings down the drive voltages to ≤1 V, making such FETs well‐suited for low‐power, battery compatible circuitry.     

Ion‐Conductive,Viscosity‐Tunable Hexagonal Boron Nitride Nanosheet Inks

Liquid‐phase exfoliation of layered solids holds promise for the scalable production of 2D nanosheets. When combined with suitable solvents and stabilizing polymers, the rheology of the resulting nanosheet dispersions can be tuned for a variety of additive manufacturing methods. While significant progress is made in the development of electrically conductive nanosheet inks, minimal effort is applied to ion‐conductive nanosheet inks despite their central role in energy storage applications. Here, the formulation of viscosity‐tunable hexagonal boron nitride (hBN) inks compatible with a wide range of printing methods that span the spectrum from low‐viscosity inkjet printing to high‐viscosity blade coating is demonstrated. The inks are prepared by liquid‐phase exfoliation with ethyl cellulose as the polymer dispersant and stabilizer. Thermal annealing of the printed structures volatilizes the polymer, resulting in a porous microstructure and the formation of a nanoscale carbonaceous coating on the hBN nanosheets, which promotes high wettability to battery electrolytes. The final result is a printed hBN nanosheet film that possesses high ionic conductivity, chemical and thermal stability, and electrically insulating character, which are ideal characteristics for printable battery components such as separators. Indeed, lithium‐ion battery cells based on printed hBN separators reveal enhanced electrochemical performance that exceeds commercial polymer separators.     

Achieving the Upper Bound of Piezoelectric Response in Tunable,Wearable 3D Printed Nanocomposites

The trade‐off between processability and functional responses presents significant challenges for incorporating piezoelectric materials as potential 3D printable feedstock. Structural compliance and electromechanical coupling sensitivity have been tightly coupled: high piezoelectric responsiveness comes at the cost of low compliance. Here, the formulation and design strategy are presented for a class of a 3D printable, wearable piezoelectric nanocomposite that approaches the upper bound of piezoelectric charge constants while maintaining high compliance. An effective electromechanical interphase model is introduced to elucidate the effects of interfacial functionalization between the highly concentrated perovskite nanoparticulate inclusions (exceeding 74 wt%) and light‐sensitive monomer matrix, shedding light on the significant enhancement of piezoelectric coefficients. It is shown that, through theoretical calculation and experimental validations, maximizing the functionalization level approaches the theoretical upper bound of the piezoelectric constant d₃₃ at any given loading concentration. Based on these findings, their applicability is demonstrated by designing and 3D printing piezoelectric materials that simultaneously achieve high electromechanical sensitivity and structural functionality, as highly sensitive wearables that detect low pressure air (<50 Pa) coming from different directions, as well as wireless, self‐sensing sporting gloves for simultaneous impact absorption and punching force mapping.     

3D Printed Supercapacitor: Techniques,Materials, Designs,and Applications

Supercapacitors (SCs) offer broad possibilities in the rising domain of military and civilian owing to their intrinsic properties of superior power density, long lifetime, and safety features. Despite of low-cost, facile manufacture, and time-saving, 3D printing technology unleashes the potential of SCs in terms of achieving desirable capacitance with high mass loading, fabrication of well-designed complicated structures, and direct construction of on-chip integration systems. In this review, first, the representative printing technologies for SCs and advanced printable materials are scrutinized for SCs and advanced printable materials. Then the structure design principles of electrodes and devices are respectively highlighted and reported cases are systematically summarized. Next, configurations of the SCs and their applications in various areas are described in detail. Finally, the promising research directions for the future are discussed. The perspectives reviewed here are expected to provide a comprehensive understanding of 3D-printed SCs and guidance in realizing their promise in various applications.     

3D Laser Nanoprinting of Functional Materials

3D laser nanoprinting represents a revolutionary manufacturing approach as it allows maskless fabrication of 3D nanostructures at a resolution beyond the optical diffraction limit. Specifically, it endows the printed structures novel physical, chemical, or mechanical properties not observed at macroscopic scale. However, 3D laser nanoprinting typically relies on the photopolymerization process, indicating its limitation on the printable materials and functionalities. The capability to print diverse functional materials beyond polymer will enable a lot of new device applications in nanophotonics, microelectronics, and so on. One of the strategies is to use the 3D-printed polymer structures as skeletons for functional material deposition, while another is to mix the functional components with the photocurable molecules and print the nanocomposites. More recently, several laser nanoprinting techniques beyond photopolymerization are also developed. In this review, the cutting-edge technical innovation is summarized and a couple of examples are highlighted showing exciting applications of the printed structures in magnetic microrobots, photonics, and optoelectronics. Finally, the vision for existing challenges and future development in this field is shared.     

Jammed Emulsions with Adhesive Pea Protein Particles for Elastoplastic Edible 3D Printed Materials

3D printed materials are of great relevance to produce medicinal scaffolds and specialized foods. An approach to forming 3D printable materials is to use jammed oil droplets. Jammed oil droplets are highly viscous and can be extruded through the nozzle of a 3D printer, while after chemical cross-linking they acquire a self-standing ability. However, the molecules currently used to stabilize and cross-link the oil droplets have questionable biocompatibility. Therefore, this study aims to produce a 3D printable jammed emulsion using pea proteins. This jammed oil-in-water emulsion is remarkably stable and viscoelastic enough to be extruded through the printer nozzle. Adhesive pea protein particles formed by pH adjustment act as physical cross-links between the oil droplets, forming a scaffold with elastoplastic rheological properties that flows above critical stress while, without any additional treatment, exhibits the required self-standing properties for 3D printing. By understanding the properties of pea proteins and their behavior in bulk and on interfaces, pea protein-based 3D printable material is created for the first time.     

Diels–Alder Reversible Thermoset 3D Printing: Isotropic Thermoset Polymers via Fused Filament Fabrication

This study presents a new 3D printing process, the Diels–Alder reversible thermoset (DART) process, and a first generation of printable DART resins, which exhibit thermoset properties at use temperatures, ultralow melt viscosity at print temperatures, smooth part surface finish, and as‐printed isotropic mechanical properties. This study utilizes dynamic covalent chemistry based on reversible furan‐maleimide Diels–Alder linkages in the polymers, which can be decrosslinked and melt‐processed during printing between 90 and 150 °C, and recrosslinked at lower temperatures to their entropically favored state. This study compares the first generation of DART materials to commonly 3D printed high‐toughness thermoplastics. Parts printed from typical fused filament fabrication compatible materials exhibit anisotropy of more than 50% and sometimes upward of 98% in toughness when deformed along the build direction, while the first generation of DART materials exhibit less than 4% toughness reduction when deformed along the build direction. At room temperature, the toughest DART materials exhibit baseline toughness of 18.59 ± 0.91 and 18.36 ± 0.57 MJ m^?3 perpendicular and parallel to the build direction, respectively. DART printing will enable chemists, polymer engineers, materials scientists, and industrial designers to translate new robust materials possessing targeted thermomechanical properties, multiaxial toughness, smooth surface finish, and low anisotropy.     

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.     

Printing of Highly Integrated Crossbar Junctions

A new process is presented that combines nanoimprint lithography and soft lithography to assemble metal–bridge–metal crossbar junctions at ambient conditions. High density top and bottom metal electrodes with half‐pitches down to 50 nm are fabricated in a parallel process by means of ultraviolet nanoimprint lithography. The top electrodes are realized on top of a sacrificial layer and are embedded in a polymer matrix. The lifting of the top electrodes by dissolving the sacrificial layer in an aqueous solution results in printable electrode stamps. Crossbar arrays are noninvasively assembled with high yield by printing the top electrode stamps onto bare or modified bottom electrodes. A semiconducting and a quasi metal like conducting type of polymer are incorporated in the cross points to form metal‐polymer‐metal junctions. The electrical characterization of the printed junctions revealed that the functional integrity of the electrically addressed conductive polymers is conserved during the assembling process. These findings suggest that printing of electrodes represents an easy and cost effective route to highly integrated nanoscale metal‐bridge‐metal junctions if imprint lithography is used for electrode fabrication.     

Limpet Tooth-Inspired Painless Microneedles Fabricated by Magnetic Field-Assisted 3D Printing

Microneedle arrays show many advantages in drug delivery applications due to their convenience and reduced risk of infection. Compared to other microscale manufacturing methods, 3D printing easily overcomes challenges in the fabrication of microneedles with complex geometric shapes and multifunctional performance. However, due to material characteristics and limitations on printing capability, there are still bottlenecks to overcome for 3D printed microneedles to achieve the mechanical performance needed for various clinical applications. The hierarchical structures in limpet teeth, which are extraordinarily strong, result from aligned fibers of mineralized tissue and protein-based polymer reinforced frameworks. These structures provide design inspiration for mechanically reinforced biomedical microneedles. Here, a bioinspired microneedle array is fabricated using magnetic field-assisted 3D printing (MF-3DP). Micro-bundles of aligned iron oxide nanoparticles (aIOs) are encapsulated by polymer matrix during the printing process. A bioinspired 3D-printed painless microneedle array is fabricated, and suitability of this microneedle patch for drug delivery during long-term wear is demonstrated. The results reported here provide insights into how the geometrical morphology of microneedles can be optimized for the painless drug delivery in clinical trials.     

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.     

CelloMOF: Nanocellulose Enabled 3D Printing of Metal–Organic Frameworks

3D printing is recognized as a powerful tool to develop complex geometries for a variety of materials including nanocellulose. Herein, a one‐pot synthesis of 3D printable hydrogel ink containing zeolitic imidazolate frameworks (ZIF‐8) anchored on anionic 2,2,6,6‐tetramethylpiperidine‐1‐oxylradical‐mediated oxidized cellulose nanofibers (TOCNF) is presented. The synthesis approach of ZIF‐8@TOCNF (CelloZIF8) hybrid inks is simple, fast (≈30 min), environmentally friendly, takes place at room temperature, and allows easy encapsulation of guest molecules such as curcumin. Shear thinning properties of the hybrid hydrogel inks facilitate the 3D printing of porous scaffolds with excellent shape fidelity. The scaffolds show pH controlled curcumin release. The synthesis route offers a general approach for metal–organic frameworks (MOF) processing and is successfully applied to other types of MOFs such as MIL‐100 (Fe) and other guest molecules as methylene blue. This study may open new venues for MOFs processing and its large‐scale applications.     

On the Principles of Printing Sub‐micrometer 3D Structures from Dielectric‐Liquid‐Based Colloids

Dielectrophoresis‐assisted (DEP) on‐demand printing of dielectric‐liquid‐based colloidal gold under room conditions is demonstrated and employed to print 2D and 3D structures with sub‐micrometer feature sizes. The focus of the work is primarily on explaining the physics of the printing process, based on the formation of a controlled sequence of sub‐micrometer drops. The physics of 3D structure formation on the substrate is explained through the visualization and analysis of various time‐scales relevant to the printing process and the pinning of the contact line of the printed colloids. A parametric variation of the related variables, namely the applied voltage and the pulse length, is used to investigate the morphology and topography of a host of basic, printable 2D and 3D features. It is established that it is possible to obtain uniform particle deposits in 2D by filling up an initial coffee‐ring‐type non‐uniform deposit with a series of subsequently formed drops, all obtained during a single electric pulse. Finally, on‐demand production of multilayered, sub‐micrometer gold tracks is demonstrated, where the annealed tracks exhibit exceptionally low electrical resistivity for their sizes, only two times higher than that of bulk gold.     

3D Printing of Hydrogels for Stretchable Ionotronic Devices

In the booming development of flexible electronics represented by electronic skins, soft robots, and human–machine interfaces, 3D printing of hydrogels, an approach used by the biofabrication community, is drawing attention from researchers working on hydrogel-based stretchable ionotronic devices. Such devices can greatly benefit from the excellent patterning capability of 3D printing in three dimensions, as well as the free design complexity and easy upscale potential. Compared to the advanced stage of 3D bioprinting, 3D printing of hydrogel ionotronic devices is in its infancy due to the difficulty in balancing printability, ionic conductivity, shape fidelity, stretchability, and other functionalities. In this review, a guideline is provided on how to utilize the power of 3D printing in building high-performance hydrogel-based stretchable ionotronic devices mainly from a materials’ point of view, highlighting the systematic approach to balancing the printability, printing quality, and performance of printed devices. Various 3D printing methods for hydrogels are introduced, and then the ink design principles, balancing printing quality, printed functions, such as elastic conductivity, self-healing ability, and device (e.g., flexible sensors, shape-morphing actuators, soft robots, electroluminescent devices, and electrochemical biosensors) performances are discussed. In conclusion, perspectives on the future directions of this exciting field are presented.     

Additive Manufacturing of Batteries

Additive manufacturing, i.e., 3D printing, is being increasingly utilized to fabricate a variety of complex‐shaped electronics and energy devices (e.g., batteries, supercapacitors, and solar cells) due to its excellent process flexibility, good geometry controllability, as well as cost and material waste reduction. In this review, the recent advances in 3D printing of emerging batteries are emphasized and discussed. The recent progress in fabricating 3D‐printed batteries through the major 3D‐printing methods, including lithography‐based 3D printing, template‐assisted electrodeposition‐based 3D printing, inkjet printing, direct ink writing, fused deposition modeling, and aerosol jet printing, are first summarized. Then, the significant achievements made in the development and printing of battery electrodes and electrolytes are highlighted. Finally, major challenges are discussed and potential research frontiers in developing 3D‐printed batteries are proposed. It is expected that with the continuous development of printing techniques and materials, 3D‐printed batteries with long‐term durability, favorable safety as well as high energy and power density will eventually be widely used in many fields.     

Ultrathin Sticker‐Type ZnO Thin Film Transistors Formed by Transfer Printing via Topological Confinement of Water‐Soluble Sacrificial Polymer in Dimple Structure

To create ultrathin sticker‐type electronic devices that can be attached to unconventional substrates, it is highly desirable to develop printable membrane‐type electronics on a handling substrate and then transfer the printing to a target surface. A facile method is presented for high‐efficiency transfer printing by controlling the interfacial adhesion between a handling substrate and an ultrathin substrate in a systematic manner under mild conditions. A water‐soluble sacrificial polymer layer is employed on a dimpled handling substrate, which enables the topological confinement of the polymer residue inside and near the dimples during the etching and drying processes to reduce the interfacial adhesion gently, creating a high yield of transfer printing in a deterministic manner. As an example of an electronic device that was created using this method, a highly flexible sticker‐type ZnO thin film transistor was successfully developed with a thickness of 13 μm including a printable ultrathin substrate, which can be attached to various substrates, such as paper, plastic, and stickers.