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.     

Dynamic Photomask‐Assisted Direct Ink Writing Multimaterial for Multilevel Triboelectric Nanogenerator

Triboelectric nanogenerator (TENG) devices are extensively studied as a mechanical energy harvester and self‐powered sensor for wearable electronics and physiological monitoring. However, the conventional TENG fabrication involving assembling steps and using the single property of matrix material suffers from simple devices shape and a single level of mechanical response for sensing and energy harvesting. Here, the printed multimaterial matrix for multilevel mechanical‐responsive TENG with on‐demand reconfiguration of shape is reported. A multimaterial 3D printing approach by using dynamic photomask‐assisted direct ink writing printing together with a two‐stage curing hybrid ink is first developed. Multimaterial structures with location‐specific properties, such as tensile modulus, failure stress, and glass transition temperature for controlled deformation, crack propagation path, and sequential shape memory, are directly printed. The printed multimaterial structure with sequential deformation behavior is used to fabricate a multilevel‐TENG (mTENG) device for multiple level mechanical energy harvesters and sensors. It is demonstrated that the mTENG can be embedded in shoe insoles to achieve both comfortable wearing and motion state monitoring. This work provides a new approach to combine multimaterial 3D printing with TENG devices for functional wearable electronics as energy harvester and sensors.     

3D Printed Anatomical Nerve Regeneration Pathways

A 3D printing methodology for the design, optimization, and fabrication of a custom nerve repair technology for the regeneration of complex peripheral nerve injuries containing bifurcating sensory and motor nerve pathways is introduced. The custom scaffolds are deterministically fabricated via a microextrusion printing principle using 3D models, which are reverse engineered from patient anatomies by 3D scanning. The bifurcating pathways are augmented with 3D printed biomimetic physical cues (microgrooves) and path‐specific biochemical cues (spatially controlled multicomponent gradients). In vitro studies reveal that 3D printed physical and biochemical cues provide axonal guidance and chemotractant/chemokinetic functionality. In vivo studies examining the regeneration of bifurcated injuries across a 10 mm complex nerve gap in rats showed that the 3D printed scaffolds achieved successful regeneration of complex nerve injuries, resulting in enhanced functional return of the regenerated nerve. This approach suggests the potential of 3D printing toward advancing tissue regeneration in terms of: (1) the customization of scaffold geometries to match inherent tissue anatomies; (2) the integration of biomanufacturing approaches with computational modeling for design, analysis, and optimization; and (3) the enhancement of device properties with spatially controlled physical and biochemical functionalities, all enabled by the same 3D printing process.     

3D Printing of Multifunctional Hydrogels

3D printing technology has been widely explored for the rapid design and fabrication of hydrogels, as required by complicated soft structures and devices. Here, a new 3D printing method is presented based on the rheology modifier of Carbomer for direct ink writing of various functional hydrogels. Carbomer is shown to be highly efficient in providing ideal rheological behaviors for multifunctional hydrogel inks, including double network hydrogels, magnetic hydrogels, temperature‐sensitive hydrogels, and biogels, with a low dosage (at least 0.5% w/v) recorded. Besides the excellent printing performance, mechanical behaviors, and biocompatibility, the 3D printed multifunctional hydrogels enable various soft devices, including loadable webs, soft robots, 4D printed leaves, and hydrogel Petri dishes. Moreover, with its unprecedented capability, the Carbomer‐based 3D printing method opens new avenues for bioprinting manufacturing and integrated hydrogel devices.     

3D‐Printed All‐Fiber Li‐Ion Battery toward Wearable Energy Storage

Conventional bulky and rigid power systems are incapable of meeting flexibility and breathability requirements for wearable applications. Despite the tremendous efforts dedicated to developing various 1D energy storage devices with sufficient flexibility, challenges remain pertaining to fabrication scalability, cost, and efficiency. Here, a scalable, low‐cost, and high‐efficiency 3D printing technology is applied to fabricate a flexible all‐fiber lithium‐ion battery (LIB). Highly viscous polymer inks containing carbon nanotubes and either lithium iron phosphate (LFP) or lithium titanium oxide (LTO) are used to print LFP fiber cathodes and LTO fiber anodes, respectively. Both fiber electrodes demonstrate good flexibility and high electrochemical performance in half‐cell configurations. All‐fiber LIB can be successfully assembled by twisting the as‐printed LFP and LTO fibers together with gel polymer as the quasi‐solid electrolyte. The all‐fiber device exhibits a high specific capacity of ≈110 mAh g^?1 at a current density of 50 mA g^?1 and maintains a good flexibility of the fiber electrodes, which can be potentially integrated into textile fabrics for future wearable electronic applications.     

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.     

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.     

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.     

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.     

Particle‐Free Emulsions for 3D Printing Elastomers

3D printing is a rapidly growing field that requires the development of yield‐stress fluids that can be used in postprinting transformation processes. There is a limited number of yield‐stress fluids currently available with the desired rheological properties for building structures with small filaments (≤l00 µm) with high shape‐retention. A printing‐centric approach for 3D printing particle‐free silicone oil‐in‐water emulsions with a polymer additive, poly(ethylene oxide) is presented. This particular material structure and formulation is used to build 3D structure and to pattern at filament diameters below that of any other known material in this class. Increasing the molecular weight of poly(ethylene oxide) drastically increases the extensibility of the material without significantly affecting shear flow properties (shear yield stress and linear viscoelastic moduli). Higher extensibility of the emulsion correlates to the ability of filaments to span relatively large gaps (greater than 6 mm) when extruded at large tip diameters (330 µm) and the ability to extrude filaments at high print rates (20 mm s^?1). 3D printed structures with these extensible particle‐free emulsions undergo postprinting transformation, which converts them into elastomers. These elastomers can buckle and recover from extreme compressive strain with no permanent deformation, a characteristic not native to the emulsion.     

3D Printed High‐Loading Lithium‐Sulfur Battery Toward Wearable Energy Storage

Wearable electronic devices are the new darling of consumer electronics, and energy storage devices are an important part of them. Here, a wearable lithium‐sulfur (Li‐S) bracelet battery using three‐dimensional (3D) printing technology (additive manufacturing) is designed and manufactured for the first time. The bracelet battery can be easily worn to power the wearable device. The “additive” manufacturing characteristic of 3D printing provides excellent controllability of the electrode thickness with much simplified process in a cost‐effective manner. Due to the conductive 3D skeleton providing interpenetrating transmission paths and channels for electrons and ions, the 3D Li‐S battery can provide 505.4 mAh g^?1 specific capacity after 500 cycles with an active material loading as high as 10.2 mg cm^?1. The practicality is illustrated by wearing the bracelet battery on the wrist and illuminating the red light‐emitting diode. Therefore, the bracelet battery manufactured by 3D printing technology can address the needs of the wearable power supply.     

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.     

Helical 3D‐Printed Metal Electrodes as Custom‐Shaped 3D Platform for Electrochemical Devices

3D‐printing represents an emerging technology that can revolutionize the way object and functional devices are fabricated. Here the use of metal 3D printing is demonstrated to fabricate bespoke electrochemical stainless steel electrodes that can be used as platform for different electrochemical applications ranging from electrochemical capacitors, oxygen evolution catalyst, and pH sensor by means of an effective and controlled deposition of IrO₂ films. The electrodes have been characterized by scanning electrode microscopy and energy dispersive X‐ray spectroscopy before the electrochemical testing. Excellent pseudocapacitive as well as catalytic properties have been achieved with these 3D printed steel‐IrO₂ electrodes in alkaline solutions. These electrodes also demonstrate Nernstian behavior as pH sensor. This work represents a breakthrough in on‐site prototyping and fabrication of highly tailored electrochemical devices with complex 3D shapes which facilitate specific functions and properties.     

Facile Handling of 3D Two-Photon Polymerized Microstructures by Ultra-Conformable Freestanding Polymeric Membranes

Micro-nano-fabrication on objects with complex surfaces is essential for the development of technologies in the growing fields of flexible electronics and photonics. Various strategies are devised to extend the fabrication from conventional planar substrates to curved ones, however, significant challenges still exist, especially in the framework of 3D printing and additive manufacturing. In this study, a novel technique is presented to realize 3D micro-structures on arbitrary complex surfaces providing an extreme level of conformability. This method relies on the fabrication of micro-structures via two-photon polymerization on polymeric nano-membranes that can be efficiently transferred to a specific target. Ultra-thin polymeric films are exploited as the support to suspend and transfer the printed micro-structures on the predefined surface. The nanofilm can finally be easily removed, apart from the region underneath the printed elements where it serves as a few tens of nanometers adhesive. The repeatability and feasibility of the proposed process are investigated and shown to provide large flexibility of choice on the printed structures, materials used, transfer procedures, and targeted substrate geometries. By integration with standard fabrication processes, the described technique offers a great potential for the development of next-generation multidimensional/multi-material micro-nano-technologies.     

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.     

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.     

Charge-Programmable Photopolymers for 3D Electronics via Additive Manufacturing

Charge-programmed 3D printing enables the fabrication of 3D electronics with lightweight and high precision via selective patterning of metals. This selective metal deposition is catalyzed by Pd nanoparticles that are specifically immobilized onto the charged surface and promises to fabricate a myriad of complex electronic devices with self-sensing, actuation, and structural elements assembled in a designed 3D layout. However, the achievable property space and the material-performance correlation of the charge-programmed printing remain unexplored. Herein, a series of photo-curable resins are designed for unveiling how the charge and crosslink densities synergistically impact the nanocatalyst-guided selective deposition in catalytic efficiency and properties of the 3D printed charge-programmed architectures, leading to high-quality 3D patterning of solid and liquid metals. The findings offer a wide tunability of the structural properties of the printed electronics, ranging from stiff to extreme flexibility. Capitalizing on these results, the printing and successful application of an ultralight-weight and deployable 3D multi-layer antenna system operating at an ultrahigh-frequency of 19 GHz are demonstrated.     

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.     

Flexible Hybrid Electronics: Direct Interfacing of Soft and Hard Electronics for Wearable Health Monitoring

The interfacing of soft and hard electronics is a key challenge for flexible hybrid electronics. Currently, a multisubstrate approach is employed, where soft and hard devices are fabricated or assembled on separate substrates, and bonded or interfaced using connectors; this hinders the flexibility of the device and is prone to interconnect issues. Here, a single substrate interfacing approach is reported, where soft devices, i.e., sensors, are directly printed on Kapton polyimide substrates that are widely used for fabricating flexible printed circuit boards (FPCBs). Utilizing a process flow compatible with the FPCB assembly process, a wearable sensor patch is fabricated composed of inkjet‐printed gold electrocardiography (ECG) electrodes and a stencil‐printed nickel oxide thermistor. The ECG electrodes provide 1 mVp–p ECG signal at 4.7 cm electrode spacing and the thermistor is highly sensitive at normal body temperatures, and demonstrates temperature coefficient, α ≈ –5.84% K^–1 and material constant, β ≈ 4330 K. This sensor platform can be extended to a more sophisticated multisensor platform where sensors fabricated using solution processable functional inks can be interfaced to hard electronics for health and performance monitoring, as well as internet of things applications.     

Self‐Healing and Stretchable 3D‐Printed Organic Thermoelectrics

With the advent of flexible and wearable electronics and sensors, there is an urgent need to develop energy‐harvesting solutions that are compatible with such wearables. However, many of the proposed energy‐harvesting solutions lack the necessary mechanical properties, which make them susceptible to damage by repetitive and continuous mechanical stresses, leading to serious degradation in device performance. Developing new energy materials that possess high deformability and self‐healability is essential to realize self‐powered devices. Herein, a thermoelectric ternary composite is demonstrated that possesses both self‐healing and stretchable properties produced via 3D‐printing method. The ternary composite films provide stable thermoelectric performance during viscoelastic deformation, up to 35% tensile strain. Importantly, after being completely severed by cutting, the composite films autonomously recover their thermoelectric properties with a rapid response time of around one second. Using this self‐healable and solution‐processable composite, 3D‐printed thermoelectric generators are fabricated, which retain above 85% of their initial power output, even after repetitive cutting and self‐healing. This approach represents a significant step in achieving damage‐free and truly wearable 3D‐printed organic thermoelectrics.