A New 3D Printing Strategy by Harnessing Deformation, Instability, and Fracture of Viscoelastic Inks

Hybrid Formative-Additive Manufacturing

Material extrusion additive manufacturing (AM) provides extensive design flexibility and exceptional material versatility, enabling the fabrication of complex, multifunctional objects ranging from embedded electronics to soft robotics and vascularized tissues. The bottom-up creation of these objects typically requires discretization into layers and voxels. However, the voxel size, determined by the nozzle diameter, limits extrusion rate, creating a conflict between resolution and speed. To address these inherent scalability challenges, the study proposes a hybrid formative-additive manufacturing technology that combines the respective strengths of each method-speed and quality with complexity and flexibility. The approach involves 3D-printing complex geometries, multimaterial features, and bounding walls of bulky, lower-resolution volumes, which are rapidly filled via casting or molding. By precisely controlling the materials' rheological properties-while maintaining similar solidified properties and high interfacial strength-several typical AM flaws, such as bulging and internal voids, are eliminated, achieving exponentially faster production speeds for objects with varying feature sizes.

Flexible and Durable Direct Ink Writing 3D-Printed Conductive Fabrics for Smart Wearables

Functional fabrics have broad applications in smart wearables, offering diverse functions, such as sensing, energy harvesting, and actuation. The use of 3D printing to deposit functional materials onto textile fabrics has emerged as a transformative approach in smart wearable development due to the advantages it offers. However, achieving the desired functionalities while maintaining the fabric's flexibility, wearing comfort, washability, and durability of the printed material remains a challenge. In this study, direct ink writing (DIW) 3D printing technology was employed to print polybutylene succinate (PBS) solutions containing carbon nanotubes (CNTs) onto two types of fabrics. Various properties of the printed fabrics were assessed to examine the influence of printing solutions, fabric structures, and postprinting processes on printing performance. The printed fabrics exhibited excellent electrical conductivity, mechanical strength, gauge factor, and stability under repeated strains. These characteristics highlight their potential for use in smart wearable devices such as strain- and motion-detecting sensors. Analysis of the printed fabric morphologies revealed that factors such as fiber content, yarn structure, and surface roughness of the substrate fabric, along with the rheological properties and surface tension of the printing solution, played key roles in determining the wetting and penetration behaviors of the solution on the substrate. The solution's ability to penetrate and bond with fibers provided the printed fabrics with enhanced washability and abrasion resistance, demonstrating the advantages of DIW printing technology in developing textile-based sensors for smart wearables. Additionally, by using biobased and biodegradable nontoxic Cyrene as the solvent for processing, the printed fabric is safer for smart wearables, and the process is more environmentally friendly than commonly used toxic solvents for PBS.

New 3D Ink Formulation Comprising a Nanocellulose Aerogel Based on Electrostatic Repulsion and Sol-Gel Transition

New 3D printing aerogel materials are environmentally friendly and could be used in environmental protection and biomedical fields. There is significant research interest in 3D printing cellulose-based aerogels since cellulose materials are biocompatible and are abundant in nature. The gel-like nature of the cellulose water suspension is suitable for 3D printing; however, the complexity and resolution of the geometry of aerogels are quite limited, mainly due to the inks' low viscosity that fails to maintain the integrity of the shape after printing. To address this limitation, a carefully optimized formulation incorporating three key ingredients, i.e., nanofibrils (TEMPO-CNFs), 2,2,6,6-tetramethyl-1-piperidinyloxy modified cellulose nanocrystals (TEMPO-CNC), and sodium carboxymethyl cellulose (CMC), is utilized to enhance the viscosity and structural stability of the ink. This combination of cellulose derivatives utilizes the electrostatic repulsive forces between the negatively charged components to form a stable and uniformly distributed suspension of cellulose materials. Our ink formulations improve printability and shape retention during 3D printing and are optimal for DIW printing. We print by employing an all cellulose-based composite ink using a modified direct ink writing (DIW) 3D printing method, plus an in situ freezing stage to form a layer-by-layer structure, and then follow a freeze-drying process to obtain the well-aligned aerogels. We have investigated the rheological properties of the ink formulation by varying the concentration of these three cellulose materials. The obtained aerogels exhibit highly ordered microstructures in which the micropores are well-aligned along the freezing direction. This study demonstrates a strategy for overcoming the challenges of 3D printing cellulose-based aerogels by formulating a stable composite ink, optimizing its rheological properties, and employing a modified DIW printing process with in situ freezing, resulting in highly ordered, structurally robust aerogels with aligned microporous architectures.

Numerical Prediction and Experimental Validation of Deposited Filaments in Direct Ink Writing: Deposition Status and Profile Dimension

The deposition status and profile dimension of deposited filaments have an impact on the quality of the printed parts fabricated by direct ink writing (DIW). Previous works often failed to realize the full quantitative characterizations of the detailed influence of the process parameters on the deposition status and profile dimension. Herein, we predict and analyze the deposition status and profile dimension by proposing an improved three-dimensional (3D) numerical model. The prediction accuracy of the proposed numerical model is verified through filament deposition experiments. The maximum relative errors of width and height between the experimental and simulation results of cross-sections are 10.13% and 7.37%, respectively. The effect of process parameters on the deposition status and profile dimension has been quantified. Critical process parameters are identified as the dimensionless nozzle velocity (V*) and the dimensionless height (H*). Three deposition statuses named over-deposition, pressed deposition and freeform deposition are characterized depending on the combination of V* and H*. The current work demonstrates an effective approach for the prediction of the deposition status and profile dimension of the deposited filaments along with the investigation of the effects of process parameters in DIW based on numerical simulations.

Direct Ink Writing 3D Printing Polytetrafluoroethylene/Polydimethylsiloxane Membrane with Anisotropic Surface Wettability and Its Application in Oil-Water Separation

Biological surfaces with physical discontinuity or chemical heterogeneity possess special wettability in the form of anisotropic wetting behavior. However, there are several challenges in designing and manufacturing samples with anisotropic wettability. This study investigates the fabrication of PTFE/PDMS grid membranes using Direct Ink Writing (DIW) 3D printing for oil-water separation applications. The ink's rheological properties were optimized, revealing that a 60% PTFE/PDMS composite exhibited the ideal shear-thinning behavior for 3D printing. Our research investigated the interplay between various printing parameters like the extrusion air pressure, layer thickness, feed rate, and printing speed, which were found to influence the filament dimensions, pore sizes, and hydrophobic properties of the grid membrane. Two distinct grid structures were analyzed for their wettability and anisotropic hydrophobic characteristics. The grid membranes achieved up to 100% oil-water separation efficiency in specific configurations. Separation efficiency was shown to be dependent on factors like intrusion pressure, grid architecture, and the number of layers. This study underscores the potential of DIW 3D printing in creating specialized surfaces with controlled wettability, particularly superhydrophobicity and anisotropy, paving the way for advanced environmental applications such as efficient oil-water separation.

3D Printable Polymeric Composite Material with Enhanced Conductivity Achieved by Affinity Matching

Polymer-based conductive composites are lightweight, low-cost, and easily processable materials with important applications in various fields. However, achieving highly conductive and 3D printable polymer-based conductive composites remains challenging. In this study, we successfully developed a highly conductive composite suitable for direct ink writing 3D printing using unsaturated polyester resin as the polymer matrix and graphene nanosheets as conductive fillers and rheological modifiers. Due to the well-matched affinity between graphene nanosheets and unsaturated polyester, the graphene nanosheets aggregate within the unsaturated polyester, forming a 3D conductive network. Moreover, the shearing force during direct ink writing 3D printing induces the orientation of the 2D graphene nanosheets, significantly enhancing their conductivity along the printing direction. At room temperature, the unsaturated polyester resin/graphene composite shows a high conductivity of 69.9 S m-1 while maintaining excellent 3D printability. Structures printed using this material exhibit improved heat dissipation and electromagnetic shielding performance. The reported unsaturated polyester resin/graphene nanosheet composites demonstrate outstanding electrical and heat conductivity and excellent processability, making them promising candidates for applications in electromagnetic shielding, printed electronics, and other fields.

Foams with 3D Spatially Programmed Mechanics Enabled by Autonomous Active Learning on Viscous Thread Printing

Foams are versatile by nature and ubiquitous in a wide range of applications, including padding, insulation, and acoustic dampening. Previous work established that foams 3D printed via Viscous Thread Printing (VTP) can in principle combine the flexibility of 3D printing with the mechanical properties of conventional foams. However, the generality of prior work is limited due to the lack of predictable process-property relationships. In this work, a self-driving lab is utilized that combines automated experimentation with machine learning to identify a processing subspace in which dimensionally consistent materials are produced using VTP with spatially programmable mechanical properties. In carrying out this process, an underlying self-stabilizing characteristic of VTP layer thickness is discovered as an important feature for its extension to new materials and systems. Several complex exemplars are constructed to illustrate the newly enabled capabilities of foams produced via VTP, including 1D gradient rectangular slabs, 2D localized stiffness zones on an insole orthotic and living hinges, and programmed 3D deformation via a cable-driven humanoid hand. Predictive mapping models are developed and validated for both thermoplastic polyurethane (TPU) and polylactic acid (PLA) filaments, suggesting the ability to train a model for any material suitable for material extrusion (ME) 3D printing.

A comprehensive review of the application of 3D-bioprinting in chronic wound management

INTRODUCTION

Chronic wounds require more sophisticated care than standard wound care because they are becoming more severe as a result of diseases like diabetes. By resolving shortcomings in existing methods, 3D-bioprinting offers a viable path toward personalized, mechanically strong, and cell-stimulating wound dressings.

AREAS COVERED

This review highlights the drawbacks of traditional approaches while navigating the difficulties of managing chronic wounds. The conversation revolves around employing natural biomaterials for customized dressings, with a particular emphasis on 3D-bioprinting. A thorough understanding of the uses of 3D-printed dressings in a range of chronic wound scenarios is provided by insights into recent research and patents.

EXPERT OPINION

The expert view recognizes wounds as a historical human ailment and emphasizes the growing difficulties and expenses related to wound treatment. The expert acknowledges that 3D printing is revolutionary, but also points out that it is still in its infancy and has the potential to enhance mass production rather than replace it. The review highlights the benefits of 3D printing for wound dressings by providing instances of smart materials that improve treatment results by stimulating angiogenesis, reducing pain, and targeting particular enzymes. The expert advises taking action to convert the technology's prospective advantages into real benefits for patients, even in the face of resistance to change in the healthcare industry. It is believed that the increasing evidence from in-vivo studies is promising and represents a positive change in the treatment of chronic wounds toward sophisticated 3D-printed dressings.

A new 3D printing strategy by enhancing shear-induced alignment of gelled nanomaterial inks resulting in stronger and ductile cellulose films

Cellulose nanofibrils (CNFs) are derived from biomass and have significant potential as fossil-based plastic alternatives used in disposable electronics. Controlling the nanostructure of fibrils is the key to obtaining strong mechanical properties and high optical transparency. Vacuum filtration is usually used to prepare the CNFs film in the literature; however, such a process cannot control the structure of the CNFs film, which limits the transparency and mechanical strength of the film. Here, direct ink writing (DIW), a pressure-controlled extrusion process, is proposed to fabricate the CNFs film, which can significantly harness the alignment of fibrils by exerting shear stress force on the filaments. The printed films by DIW have a compact structure, and the degree of fibril alignment quantified by the small angle X-ray diffraction (SAXS) increases by 24 % compared to the vacuum filtration process. Such a process favors the establishment of the chemical bond (or interaction) between molecules, therefore leading to considerably high tensile strength (245 ± 8 MPa), elongation at break (2.2 ± 0.5 %), and good transparency. Thus, proposed DIW provides a new strategy for fabricating aligned CNFs films in a controlled manner with tunable macroscale properties. Moreover, this work provides theoretical guidance for employing CNFs as structural and reinforcing materials to design disposable electronics.

State of the art, challenges, and future prospects for the multi-material 3D printing of plant-based meat

The emergence of innovative plant-based meat analogs, replicating the flavor, texture, and appearance of animal meat cuts, is deemed crucial for sustainably feeding a growing population while mitigating the environmental impact associated with livestock farming. Multi-material 3D food printing (MM3DFP) has been proposed as a potentially disruptive technology for manufacturing the next generation of plant-based meat analogs. This article provides a comprehensive review of the state of the art, addressing various aspects of 3D printing in the realm of plant-based meat. The disruptive potential of printed meat analogs is discussed with particular emphasis on protein-rich, lipid-rich, and blood-mimicking food inks. The printing parameters, printing requirements, and rheological properties at the different printing stages are addressed in detail. As food rheology plays a key role in the printing process, an appraisal of this subject is performed. Post-printing treatments are assessed based on the extent of improvement in the quality of 3D-printed plant-based meat analogs. The meat-mimicking potential is revealed through sensory attributes, such as texture and flavor. Furthermore, there has been limited research into food safety and nutrition. Economically, the 3D printing of plant-based meat analogs demonstrates significant market potential, contingent upon innovative decision-making strategies and supportive policies to enhance consumer acceptance. This review examines the current limitations of this technology and highlights opportunities for future developments.

Extrusion-based 3D printing of soft active materials

Active materials are capable of responding to external stimuli, as observed in both natural and synthetic systems, from sensitive plants to temperature-responsive hydrogels. Extrusion-based 3D printing of soft active materials facilitates the fabrication of intricate geometries with spatially programmed compositions and architectures at various scales, further enhancing the functionality of materials. This Feature Article summarizes recent advances in extrusion-based 3D printing of active materials in both non-living (i.e., synthetic) and living systems. It highlights emerging ink formulations and architectural designs that enable programmable properties, with a focus on complex shape morphing and controllable light-emitting patterns. The article also spotlights strategies for engineering living materials that can produce genetically encoded material responses and react to a variety of environmental stimuli. Lastly, it discusses the challenges and prospects for advancements in both synthetic and living composite materials from the perspectives of chemistry, modeling, and integration.

Multiscale 3D printing via active nozzle size and shape control

Three-dimensional (3D) printers extruding filaments through a fixed nozzle encounter a conflict between high resolution, requiring small diameters, and high speed, requiring large diameters. This limitation is especially pronounced in multiscale architectures featuring both bulk and intricate elements. Here, we introduce adaptive nozzle 3D printing (AN3DP), a technique enabling dynamic alteration of nozzle diameter and cross-sectional shape during printing. The AN3DP nozzle consists of eight independently controllable, tendon-driven pins arrayed around a flexible, pressure-resistant membrane. The design incorporates a tapered angle optimized for extruding shear-thinning inks and a pointed tip suitable for constrained-space printing, such as conformal and embedded printing. AN3DP's efficacy is demonstrated through the fabrication of components with continuous gradients, eliminating the need for discretization, and achieving enhanced density and contour precision compared to traditional 3D printing methods. This platform substantially expands the scope of extrusion-based 3D printers, thus facilitating diverse applications, including bioprinting cell-laden and hierarchical implants with bone-like microarchitecture.

Direct Ink Writing of Low-Concentration MXene/Aramid Nanofiber Inks for Tunable Electromagnetic Shielding and Infrared Anticounterfeiting Applications

MXene inks offer a promising avenue for the scalable production and customization of printing electronics. However, simultaneously achieving a low solid content and printability of MXene inks, as well as mechanical flexibility and environmental stability of printed objects, remains a challenge. In this study, we overcame these challenges by employing high-viscosity aramid nanofibers (ANFs) to optimize the rheology of low-concentration MXene inks. The abundant entangled networks and hydrogen bonds formed between MXene and ANF significantly increase the viscosity and yield stress up to 103 Pa·s and 200 Pa, respectively. This optimization allows the use of MXene/ANF (MA) inks at low concentrations in direct ink writing and other high-viscosity processing techniques. The printable MXene/ANF inks with a high conductivity of 883.5 S/cm were used to print shields with customized structures, achieving a tunable electromagnetic interference shielding effectiveness (EMI SE) in the 0.2-48.2 dB range. Furthermore, the MA inks exhibited adjustable infrared (IR) emissivity by changing the ANF ratio combined with printing design, demonstrating the application for infrared anticounterfeiting. Notably, the printed MXene/ANF objects possess outstanding mechanical flexibility and environmental stability, which are attributed to the reinforcement and protection of ANF. Therefore, these findings have significant practical implications as versatile MXene/ANF inks can be used for customizable, scalable, and cost-effective production of flexible printed electronics.

Enhancing Electrical Conductivity of Stretchable Liquid Metal-Silver Composites through Direct Ink Writing

Structure-property-process relationships are a controlling factor in the performance of materials. This offers opportunities in emerging areas, such as stretchable conductors, to control process conditions during printing to enhance performance. Herein, by systematically tuning direct ink write (DIW) process parameters, the electrical conductivity of multiphase liquid metal (LM)-silver stretchable conductors is increased by a maximum of 400% to over 1.06 × 106 S·m-1. This is achieved by modulating the DIW print velocity, which enables the in situ elongation, coalescence, and percolation of these multiphase inclusions during printing. These DIW printed filaments are conductive as fabricated and are soft (modulus as low as 1.1 MPa), stretchable (strain limit >800%), and show strain invariant conductivity up to 80% strain. These capabilities are demonstrated through a set of electromagnetic induction coils that can transfer power wirelessly through air and water, even under deformation. This work provides a methodology to program properties in stretchable conductors, where the combination of material composition and process parameters leads to greatly enhanced performance. This approach can find use in applications such as soft robots, soft electronics, and printed materials for deformable, yet highly functional devices.

The status and challenging perspectives of 3D-printed micro-batteries

In the era of the Internet of Things and wearable electronics, 3D-printed micro-batteries with miniaturization, aesthetic diversity and high aspect ratio, have emerged as a recent innovation that solves the problems of limited design diversity, poor flexibility and low mass loading of materials associated with traditional power sources restricted by the slurry-casting method. Thus, a comprehensive understanding of the rational design of 3D-printed materials, inks, methods, configurations and systems is critical to optimize the electrochemical performance of customizable 3D-printed micro-batteries. In this review, we offer a key overview and systematic discussion on 3D-printed micro-batteries, emphasizing the close relationship between printable materials and printing technology, as well as the reasonable design of inks. Initially, we compare the distinct characteristics of various printing technologies, and subsequently emphatically expound the printable components of micro-batteries and general approaches to prepare printable inks. After that, we focus on the outstanding role played by 3D printing design in the device architecture, battery configuration, performance improvement, and system integration. Finally, the future challenges and perspectives concerning high-performance 3D-printed micro-batteries are adequately highlighted and discussed. This comprehensive discussion aims at providing a blueprint for the design and construction of next-generation 3D-printed micro-batteries.

Versatile xanthan gum-based support bath material compatible with multiple crosslinking mechanisms: rheological properties, printability, and cytocompatibility study

Extrusion-based bioprinting is a promising technology for the fabrication of complex three-dimensional (3D) tissue-engineered constructs. To further improve the printing accuracy and provide mechanical support during the printing process, hydrogel-based support bath materials have been developed. However, the gel structure of some support bath materials can be compromised when exposed to certain bioink crosslinking cues, hence their compatibility with bioinks can be limited. In this study, a xanthan gum-based composite support material compatible with multiple crosslinking mechanisms is developed. Different support bath materials can have different underlying polymeric structures, for example, particulate suspensions and polymer solution with varying supramolecular structure) and these properties are governed by a variety of different intermolecular interactions. However, common rheological behavior can be expected because they have similar demonstrated performance and functionality. To provide a detailed exploration/identification of the common rheological properties expressed by different support bath materials from a unified perspective, benchmark support bath materials from previous studies were prepared. A comparative rheological study revealed both the structural and shear behavior characteristics shared by support bath materials, including yield stress, gel complex moduli, shear-thinning behavior, and self-healing properties. Gel structural stability and functionality of support materials were tested in the presence of various crosslinking stimuli, confirming the versatility of the xanthan-based support material. We further investigated the effect of support materials and the diameter of extrusion needles on the printability of bioinks to demonstrate the improvement in bioink printability and structural integrity. Cytotoxicity and cell encapsulation viability tests were carried out to confirm the cell compatibility of the xanthan gum-based support bath material. We propose and demonstrate the versatility and compatibility of the novel support bath material and provide detailed new insight into the essential properties and behavior of these materials that serve as a guide for further development of support bath-based 3D bioprinting.

Gradient matters via filament diameter-adjustable 3D printing

Gradient matters with hierarchical structures endow the natural world with excellent integrity and diversity. Currently, direct ink writing 3D printing is attracting tremendous interest, and has been used to explore the fabrication of 1D and 2D hierarchical structures by adjusting the diameter, spacing, and angle between filaments. However, it is difficult to generate complex 3D gradient matters owing to the inherent limitations of existing methods in terms of available gradient dimension, gradient resolution, and shape fidelity. Here, we report a filament diameter-adjustable 3D printing strategy that enables conventional extrusion 3D printers to produce 1D, 2D, and 3D gradient matters with tunable heterogeneous structures by continuously varying the volume of deposited ink on the printing trajectory. In detail, we develop diameter-programmable filaments by customizing the printing velocity and height. To achieve high shape fidelity, we specially add supporting layers at needed locations. Finally, we showcase multi-disciplinary applications of our strategy in creating horizontal, radial, and axial gradient structures, letter-embedded structures, metastructures, tissue-mimicking scaffolds, flexible electronics, and time-driven devices. By showing the potential of this strategy, we anticipate that it could be easily extended to a variety of filament-based additive manufacturing technologies and facilitate the development of functionally graded structures.

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

A Guide to Printed Stretchable Conductors

Printing of stretchable conductors enables the fabrication and rapid prototyping of stretchable electronic devices. For such applications, there are often specific process and material requirements such as print resolution, maximum strain, and electrical/ionic conductivity. This review highlights common printing methods and compatible inks that produce stretchable conductors. The review compares the capabilities, benefits, and limitations of each approach to help guide the selection of a suitable process and ink for an intended application. We also discuss methods to design and fabricate ink composites with the desired material properties (e.g., electrical conductance, viscosity, printability). This guide should help inform ongoing and future efforts to create soft, stretchable electronic devices for wearables, soft robots, e-skins, and sensors.

Hydrogel Extrusion Speed Measurements for the Optimization of Bioprinting Parameters

Three-dimensional (3D) bioprinting is the use of computer-controlled transfer processes for assembling bioinks (cell clusters or materials loaded with cells) into structures of prescribed 3D organization. The correct bioprinting parameters ensure a fast and accurate bioink deposition without exposing the cells to harsh conditions. This study seeks to optimize pneumatic extrusion-based bioprinting based on hydrogel flow rate and extrusion speed measurements. We measured the rate of the hydrogel flow through a cylindrical nozzle and used non-Newtonian hydrodynamics to fit the results. From the videos of free-hanging hydrogel strands delivered from a stationary print head, we inferred the extrusion speed, defined as the speed of advancement of newly formed strands. Then, we relied on volume conservation to evaluate the extrudate swell ratio. The theoretical analysis enabled us to compute the extrusion speed for pressures not tested experimentally as well as the printing speed needed to deposit hydrogel filaments of a given diameter. Finally, the proposed methodology was tested experimentally by analyzing the morphology of triple-layered square-grid hydrogel constructs printed at various applied pressures while the printing speeds matched the corresponding extrusion speeds. Taken together, the results of this study suggest that preliminary measurements and theoretical analyses can simplify the search for the optimal bioprinting parameters.

Functional PDMS Elastomers: Bulk Composites, Surface Engineering, and Precision Fabrication

Polydimethylsiloxane (PDMS)-the simplest and most common silicone compound-exemplifies the central characteristics of its class and has attracted tremendous research attention. The development of PDMS-based materials is a vivid reflection of the modern industry. In recent years, PDMS has stood out as the material of choice for various emerging technologies. The rapid improvement in bulk modification strategies and multifunctional surfaces has enabled a whole new generation of PDMS-based materials and devices, facilitating, and even transforming enormous applications, including flexible electronics, superwetting surfaces, soft actuators, wearable and implantable sensors, biomedicals, and autonomous robotics. This paper reviews the latest advances in the field of PDMS-based functional materials, with a focus on the added functionality and their use as programmable materials for smart devices. Recent breakthroughs regarding instant crosslinking and additive manufacturing are featured, and exciting opportunities for future research are highlighted. This review provides a quick entrance to this rapidly evolving field and will help guide the rational design of next-generation soft materials and devices.

Material Extrusion Filament Width and Height Prediction via Design of Experiment and Machine Learning

The dimensions of material extrusion 3D printing filaments play a pivotal role in determining processing resolution and efficiency and are influenced by processing parameters. This study focuses on four key process parameters, namely, nozzle diameter, nondimensional nozzle height, extrusion pressure, and printing speed. The design of experiment was carried out to determine the impact of various factors and interaction effects on filament width and height through variance analysis. Five machine learning models (support vector regression, backpropagation neural network, decision tree, random forest, and K-nearest neighbor) were built to predict the geometric dimension of filaments. The models exhibited good predictive performance. The coefficients of determination of the backpropagation neural network model for predicting line width and line height were 0.9025 and 0.9604, respectively. The effect of various process parameters on the geometric morphology based on the established prediction model was also studied. The order of influence on line width and height, ranked from highest to lowest, was as follows: nozzle diameter, printing speed, extrusion pressure, and nondimensional nozzle height. Different nondimensional nozzle height settings may cause the extruded material to be stretched or squeezed. The material being in a stretched state leads to a thin filament, and the regularity of processing parameters on the geometric size is not strong. Meanwhile, the nozzle diameter exhibits a significant impact on dimensions when the material is in a squeezing state. Thus, this study can be used to predict the size of printing filament structures, guide the selection of printing parameters, and determine the size of 3D printing layers.

Learning to write with the fluid rope trick

Direct ink writing, a versatile method of 3D and 4D printing, requires the precise placement of a nozzle just above the print surface to prevent fluid instabilities that cause deviations from the prescribed print path. But what if one could harness the instability associated with the spontaneously folding or coiling of a thin stream of viscous fluid, i.e. use the "fluid rope trick" to write specified patterns on a substrate? Here we use Deep Reinforcement Learning to derive control strategies for the motion of the extruding nozzle and thus the fluid patterns that are deposited on the surface. The method proceeds by having a learner (nozzle) repeatedly interact with the environment (a viscous filament simulator), and improves its strategy using the results of this experience. We demonstrate the outcome of the learned control instructions using experiments to manipulate a falling viscous jet and create cursive writing patterns and Pollockian paintings on substrates.

3D Printing of Bioactive Gel-like Double Emulsion into a Biocompatible Hierarchical Macroporous Self-Lubricating Scaffold for 3D Cell Culture

The interconnected hierarchically porous structures are of key importance for potential applications as substrates for drug delivery, cell culture, and bioscaffolds, ensuring cell adhesion and sufficient diffusion of metabolites and nutrients. Here, encapsulation of a vitamin C-loaded gel-like double emulsion using a hydrophobic emulsifier and soy particles was performed to develop a bioactive bioink for 3D printing of highly porous scaffolds with enhanced cell biocompatibility. The produced double emulsions suggested a mechanical strength with the range of elastic moduli of soft tissues possessing a thixotropic feature and recoverable matrix. The outstanding flow behavior and viscoelasticity broaden the potential of gel-like double emulsion to engineer 3D scaffolds, in which 3D constructs showed a high level of porosity and excellent shape fidelity with antiwearing and self-lubricating properties. Investigation of cell viability and proliferation using fibroblasts (NIH-3T3) within vitamin C-loaded gel-like bioinks revealed that printed 3D scaffolds offered brilliant biocompatibility and cell adhesion. Compared to scaffolds without encapsulated vitamin C, 3D scaffolds containing vitamin C showed higher cell viability after 1 week of cell proliferation. This work represented a systematic investigation of hierarchical self-assembly in double emulsions and offered insights into mechanisms that control microstructure within supramolecular structures, which could be instructive for the design of advanced functional tissues.

Shape-Tunable 4D Printing of LCEs via Cooling Rate Modulation: Stimulus-Free Locking of Actuated State at Room Temperature

Liquid crystal elastomers (LCEs) have garnered considerable attention in the field of four-dimensional (4D) printing due to their large, reversible, and anisotropic shape-morphing capabilities. By utilizing direct ink writing, intricate LCE structures with programmable shape morphing can be achieved. However, the maintenance of the actuated state for LCEs requires continuous and substantial external stimuli, presenting challenges for practical applications, particularly under ambient conditions. This study reports a straightforward and effective physical approach to lock the actuated state of LCEs through rapid cooling while preserving their reversible performance. Rapid cooling significantly reduces the mobility of the lightly cross-linked network in LCEs, resulting in a notably slow recovery of mesogen alignment. As a result, the locked LCE structures retain their actuated state even at room temperature. Moreover, we demonstrate the ability to achieve tunable shapes between the original and actuated states by modulating the cooling rate, i.e., varying the temperature and type of cooling medium. The proposed method opens up new possibilities to achieve stable and tunable shape locking of soft devices for engineering applications.

3D Printing of Interpenetrating Network Flexible Hydrogels with Enhancement of Adhesiveness

3D printing of hydrogels has been widely explored for the rapid fabrication of complex soft structures and devices. However, using 3D printing to customize hydrogels with both adequate adhesiveness and toughness remains a fundamental challenge. Here, we demonstrate mussel-inspired (polydopamine) PDA hydrogel through the incorporation of a classical double network (2-acrylamido-2-methylpropanesulfonic acid) PAMPS/(polyacrylamide) PAAm to achieve simultaneously tailored adhesiveness, toughness, and biocompatibility and validate the 3D printability of such a hydrogel into customized architectures. The strategy of combining PDA with PAMPS/PAAm hydrogels leads to favorable adhesion on either hydrophilic or hydrophobic surfaces. The hydrogel also shows excellent flexibility, which is attributed to the reversible cross-linking of PDA and PAMPS, together with the long-chain PAAm cross-linking network. Among them, the reversible cross-linking of PDA and PAMPS is capable of dissipating mechanical energy under deformation. Meanwhile, the long-chain PAAm network contributes to maintaining a high deformation capability. We establish a theoretical framework to quantify the contribution of the interpenetrating networks to the overall toughness of the hydrogel, which also provides guidance for the rational design of materials with the desired properties. Our work manifests a new paradigm of printing adhesive, tough, and biocompatible interpenetrating network hydrogels to meet the requirements of broad potential applications in biomedical engineering, soft robotics, and intelligent and superabsorbent devices.

3D Printing of Ionogels with Complementary Functionalities Enabled by Self-Regulating Ink

Shaping soft and conductive materials into sophisticated architectures through 3D printing is driving innovation in myriad applications, such as robotic counterparts that emulate the synergic functions of biological systems. Although recently developed multi-material 3D printing has enabled on-demand creation of intricate artificial counterparts from a wide range of functional viscoelastic materials. However, directly achieving complementary functionalities in one ink design remains largely unexplored, given the issues of printability and synergy among ink components. In this study, an easily accessible and self-regulating tricomponent ionogel-based ink design to address these challenges is reported. The resultant 3D printed objects, based on the same component but with varying ratios of ink formulations, exhibit distinct yet complementary properties. For example, their Young's modulus can differ by three orders of magnitude, and some structures are rigid while others are ductile and viscous. A theoretical model is also employed for predicting and controlling the printing resolution. By integrating complementary functionalities, one further demonstrates a representative bioinspired prototype of spiderweb, which mimics the sophisticated structure and multiple functions of a natural spiderweb, even working and camouflaging underwater. This ink design strategy greatly extends the material choice and can provide valuable guidance in constructing diverse artificial systems by 3D printing.

Recent Progress and Perspectives of Direct Ink Writing Applications for Mass Transfer Enhancement in Gas-Phase Adsorption and Catalysis

Conventional adsorbents and catalysts shaped by granulation or extrusion have high pressure drop and poor flexibility for chemical, energy, and environmental processes. Direct ink writing (DIW), a kind of 3D printing, has evolved into a crucial technique for manufacturing scalable configurations of adsorbents and catalysts with satisfactory programmable automation, highly optional materials, and reliable construction. Particularly, DIW can generate specific morphologies required for excellent mass transfer kinetics, which is essential in gas-phase adsorption and catalysis. Here, DIW methodologies for mass transfer enhancement in gas-phase adsorption and catalysis, covering the raw materials, fabrication process, auxiliary optimization methods, and practical applications are comprehensively summarized. The prospects and challenges of DIW methodology in realizing good mass transfer kinetics are discussed. Ideal components with a gradient porosity, multi-material structure, and hierarchical morphology are proposed for future investigations.

Printability of Poly(lactic acid) Ink by Embedded 3D Printing via Immersion Precipitation

Immersion precipitation three-dimensional printing (ip3DP) and freeform polymer precipitation (FPP) are unique and versatile methods of 3D printing to fabricate 3D structures based on nonsolvent-induced phase separation via direct ink writing (DIW). Immersion precipitation involves complex dynamics among solvents, nonsolvents, and dissolved polymers, and the printability of 3D models in these methods requires further understanding. To this end, we characterized these two methods of 3D printing using polylactide (PLA) dissolved in dichloromethane (7.5-30% w/w) as model inks. We analyzed the rheological properties of the solutions and the effect of printing parameters on solvent-nonsolvent diffusion to achieve printability. The PLA inks exhibited shear-thinning properties, and their viscosities varied over three orders of magnitude (10^-1∼10² Pa·s). A processing map was presented to understand the ideal ranges of the concentration of PLA in inks and the nozzle diameter to ensure printability, and the fabrication of complex 3D structures was fabricated with adequate applied pressure and nozzle speed. The processing map also highlighted the advantages of embedded 3D printing over solvent-cast 3D printing based on solvent evaporation. Lastly, we demonstrated that the porosity of the interface and inner structure of the printed objects was readily tailored by the concentration of the PLA and the porogen added to the ink. The methods presented here offer new perspectives to fabricate micro-to-centimeter objects of thermoplastics with nanometer-scale inner pores and provide guidelines for successful embedded 3D printing based on immersion precipitation.

2.5D printing of a yield-stress fluid

We report on direct ink writing of a model yield-stress fluid and focus on the printability of the first layer, the one in contact with the supporting substrate. We observe a diversity of deposition morphologies that depends on a limited set of operational parameters, mainly ink flow rate, substrate speed and writing density, and also on material properties (e.g., yield-stress). Among these morphologies, one of them does not depend on fluid properties (as long as the fluid displays some yield-stress) and consists of flat films whose thickness is controllable in a significant range, about $$0.1{-}1$$ 0.1 - 1  mm, and tunable in real time during printing. We thus demonstrate the ability to print films with thickness gradients and prove that the printing fidelity is mainly due to a competition between yield-stress and capillarity.

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

Liquid flowing around a solid edge, i.e., overflow, is a commonly observed flow behavior. Recent research into surface wetting properties and microstructure-controlled overflow behavior has attracted much attention. Achieving controllable macroscale liquid dynamics by manipulating the micro-nanoscale liquid overflow has stimulated diverse scientific interest and fostered widespread use in practical applications. In this review, we outline the evolution of overflow and present a critical survey of the mechanism of surface wetting properties and microstructure-controlled liquid overflow in multilength scales ranging from centimeter to micro and even nanoscale. We summarize the latest progress in utilizing the mechanisms to manipulate liquid overflow and achieve macroscale liquid dynamics and in emerging applications to manipulate overflow for sustainable development in various fields, along with challenges and perspectives.

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate's hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics.

3D Printed Hybrid Aerogel Gauzes Enable Highly Efficient Hemostasis

Hemostatic materials have played a significant role in mitigating traumatic injury by controlling bleeding, however, the fabrication of the desirable material's structure to enhance the accumulation of blood cells and platelets for highly efficient hemostasis is still a great challenge. In this work, directed assembly of poly(vinyl alcohol) (PVA) macromolecules covering the rigid Kevlar nanofiber (KNF) network during 3D printing process is utilized to fabricate hydrophilic, biocompatible, and mechanically stable KNF-PVA aerogel filaments for effective enriching blood components by fast water absorption. As such, KNF-PVA aerogel gauzes demonstrate remarkable water permeability (338 mL cm-2 s-1 bar-1 ), water absorption speed (as high as 9.64 g g-1 min-1 ) and capacity (more than ten times of self-weight), and ability to enrich micron-sized particles when contacting aqueous solution. All these properties favor efficient hemostasis and the resulting KNF-PVA aerogel gauzes significantly outperform the commercial product Quikclot Gauze (Z-Medica) during in vivo experiments with the rat liver laceration model, reducing the hemostasis time by half (60 ± 4 s) and the blood loss by two thirds (0.07 ± 0.01 g). These results demonstrate a robust strategy to design various aerogel gauzes for hemostasis applications.

Rotational multimaterial printing of filaments with subvoxel control

Helical structures are ubiquitous in nature and impart unique mechanical properties and multifunctionality¹. So far, synthetic architectures that mimic these natural systems have been fabricated by winding, twisting and braiding of individual filaments^1-7, microfluidics^8,9, self-shaping^1,10-13 and printing methods^14-17. However, those fabrication methods are unable to simultaneously create and pattern multimaterial, helically architected filaments with subvoxel control in arbitrary two-dimensional (2D) and three-dimensional (3D) motifs from a broad range of materials. Towards this goal, both multimaterial^18-23 and rotational²⁴ 3D printing of architected filaments have recently been reported; however, the integration of these two capabilities has yet to be realized. Here we report a rotational multimaterial 3D printing (RM-3DP) platform that enables subvoxel control over the local orientation of azimuthally heterogeneous architected filaments. By continuously rotating a multimaterial nozzle with a controlled ratio of angular-to-translational velocity, we have created helical filaments with programmable helix angle, layer thickness and interfacial area between several materials within a given cylindrical voxel. Using this integrated method, we have fabricated functional artificial muscles composed of helical dielectric elastomer actuators with high fidelity and individually addressable conductive helical channels embedded within a dielectric elastomer matrix. We have also fabricated hierarchical lattices comprising architected helical struts containing stiff springs within a compliant matrix. Our additive-manufacturing platform opens new avenues to generating multifunctional architected matter in bioinspired motifs.

Fluid-Mediated Fabrication of Complex Assemblies

This Perspective accounts for recent progress in the directed control of interfacial fluid flows harnessed to assemble architected soft materials. We are focusing on the paradigmatic problem of free-surface flows in curable elastomers. These elastomers are initially liquid and cure into elastic solids whose shape is imparted by concomitant and competing phenomena: flow-induced deformations and curing. Particular attention is given to the role of capillary forces in these systems. Originating from the cohesive nature of liquids and thus favoring smooth interfaces, capillary forces can also promote the destabilization of interfaces, e.g., into droplets. In turn, such mechanical instabilities tend to grow into regular patterns, e.g., forming hexagonal lattices. We discuss how the universality, robustness, and ultimate regularity of these out-of-equilibrium processes could serve as a basis for new fabrication paradigms, where instabilities are directed to generate target architected solids obtained without each element laid in place by direct mechanized intervention.

Natural Materials for 3D Printing and Their Applications

In recent years, 3D printing has gradually become a well-known new topic and a research hotspot. At the same time, the advent of 3D printing is inseparable from the preparation of bio-ink. Natural materials have the advantages of low toxicity or even non-toxicity, there being abundant raw materials, easy processing and modification, excellent mechanical properties, good biocompatibility, and high cell activity, making them very suitable for the preparation of bio-ink. With the help of 3D printing technology, the prepared materials and scaffolds can be widely used in tissue engineering and other fields. Firstly, we introduce the natural materials and their properties for 3D printing and summarize the physical and chemical properties of these natural materials and their applications in tissue engineering after modification. Secondly, we discuss the modification methods used for 3D printing materials, including physical, chemical, and protein self-assembly methods. We also discuss the method of 3D printing. Then, we summarize the application of natural materials for 3D printing in tissue engineering, skin tissue, cartilage tissue, bone tissue, and vascular tissue. Finally, we also express some views on the research and application of these natural materials.

Copper inks for printed electronics: a review

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

3D printing of soft fluidic actuators with graded porosity

New additive manufacturing methods are needed to realize more complex soft robots. One example is soft fluidic robotics, which exploits fluidic power and stiffness gradients. Porous structures are an interesting type for this approach, as they are flexible and allow for fluid transport. In this work, the infill foam (InFoam) method is proposed to print structures with graded porosity by liquid rope coiling (LRC). By exploiting LRC, the InFoam method could exploit the repeatable coiling patterns to print structures. To this end, only the characterization of the relationship between nozzle height and coil radius and the extruded length was necessary (at a fixed temperature). Then by adjusting the nozzle height and/or extrusion speed the porosity of the printed structure could be set. The InFoam method was demonstrated by printing porous structures using styrene-ethylene-butylene-styrene (SEBS) with porosities ranging from 46% to 89%. In compression tests, the cubes showed large changes in compression modulus (more than 200 times), density (-89% compared to bulk), and energy dissipation. The InFoam method combined coiling and normal plotting to realize a large range of porosity gradients. This grading was exploited to realize rectangular structures with varying deformation patterns, which included twisting, contraction, and bending. Furthermore, the InFoam method was shown to be capable of programming the behavior of bending actuators by varying the porosity. Both the output force and stroke showed correlations similar to those of the cubes. Thus, the InFoam method can fabricate and program the mechanical behavior of a soft fluidic (porous) actuator by grading porosity.

3D direct-write printing of water soluble micromoulds for high-resolution rapid prototyping

Direct-write printing has contributed tremendously to additive manufacturing; in particular extrusion based printing where it has extended the range of materials for 3D printing and thus enabled use across many more sectors. The printing inks for direct-write printing however, need careful synthesis and invariably undergo extensive preparation before being able to print. Hence, new ink synthesis efforts are required every time a new material is to be printed; this is particularly challenging for low storage modulus (G') materials like silicones, especially at higher resolutions (under 10 µm). Here we report the development of a precise (< 10 µm) 3D printable polymer, with which we 3D print micromoulds which are filled with standard silicones like polydimethylsiloxane (PDMS) and left to cure at room temperature. The proof of concept is demonstrated using a simple water soluble polymer as the mould material. The approach enables micrometre scale silicone structures to be prototyped with ease, away from the cleanroom.

Three-Dimensional Printing of Customized Scaffolds with Polycaprolactone-Silk Fibroin Composites and Integration of Gingival Tissue-Derived Stem Cells for Personalized Bone Therapy

Regenerative biomaterials play a crucial role in the success of maxillofacial reconstructive procedures. Yet today, limited options are available when choosing polymeric biomaterials to treat critical size bony defects. Further, there is a requirement for 3D printable regenerative biomaterials to fabricate customized structures confined to the defect site. We present here a 3D printable composite formulation consisting of polycaprolactone (PCL) and silk fibroin microfibers and have established a robust protocol for fabricating customized 3D structures of complex geometry with the composite. The 3D printed composite scaffolds demonstrated higher compressive modulus than 3D printed scaffolds of PCL alone. Furthermore, the compressive modulus of PCL-Antheraea mylitta (AM) silk scaffolds is higher than that of the PCL-Bombyx mori (BM) silk scaffolds at their respective ratios. Compressive modulus of PCL-25AM silk scaffolds (73.4 MPa) is higher than that of PCL-25BM silk scaffolds (65.1 MPa). Compressive modulus of PCL-40AM silk scaffolds (106.1 MPa) is higher than that of PCL-40BM silk scaffolds (77.7 MPa). Moreover, we have isolated, characterized, and integrated human gingival mesenchymal stem cells (hGMSCs), an effective autologous cell source, onto the 3D printed scaffolds to evaluate their bone regeneration potential. The results demonstrated that PCL-silk microfiber composite scaffolds of Antheraea mylitta origin showed much higher bioactivity than the Bombyx mori ones because of arginine-glycine-aspartic acid (RGD) sequences present in the Antheraea mylitta silk fibroin protein favoring cell attachment and proliferation. By day 14, the metabolic activity of hGMSCs was highest in PCL-40AM (4.5 times higher than that at day 1). In addition, to show the translational potential of this work, we have fabricated a patient defect-specific model (mandible) using the CT scan obtained by the micro-CT imaging to understand the printability of the composite for fabricating complex structures to restore maxillofacial bony defects with precision when applied in a clinical scenario.

Embedded Core-Shell 3D Printing (eCS3DP) with Low-Viscosity Polysiloxanes

Flexible core-shell 3D structures are essential for the development of soft sensors and actuators. Despite recent advancements in 3D printing, the fabrication of flexible 3D objects with internal architectures (such as channels and void spaces) remains challenging with liquid precursors due to the difficulty to maintain the printed structures. The difficulty of such fabrication is prominent especially when low-viscosity polysiloxane resins are used. This study presents a unique approach to applying direct ink writing (DIW) 3D printing in a three-phase system to overcome this limitation. We performed core-shell 3D printing using a low-viscosity commercial polysiloxane resin (Ecoflex 10) as shell inks combined with a coaxially extruded core liquid (Pluronic F127) in Bingham plastic microparticulate gels (ethanol gel). In the process termed embedded core-shell 3D printing (eCS3DP), we highlighted the dependence of the rheological characteristics of the three fluids on the stability of the printed core-shell filament. With the core liquid with a sufficiently high concentration of Pluronic F127 (30 w/w%; σ_y = 158.5 Pa), the interfacial instability between the shell liquid and core liquid was suppressed; the removal of the core liquid permitted the fabrication of perfusable channels. We identified the printing conditions to ensure lateral attachments of printed core-shell filaments. Interestingly, judicious selection of the rheological properties and flow rates of three phases allowed the formation of droplets consisting of core liquids distributed along the printed filaments. eCS3DP offers a simple route to fabricate 3D structures of a soft elastomeric matrix with embedded channels and should serve as a useful tool for DIW-based fabrication of flexible wearable devices and soft robotic components.

Recent Developments and Implementations of Conductive Polymer-Based Flexible Devices in Sensing Applications

Flexible sensing devices have attracted significant attention for various applications, such as medical devices, environmental monitoring, and healthcare. Numerous materials have been used to fabricate flexible sensing devices and improve their sensing performance in terms of their electrical and mechanical properties. Among the studied materials, conductive polymers are promising candidates for next-generation flexible, stretchable, and wearable electronic devices because of their outstanding characteristics, such as flexibility, light weight, and non-toxicity. Understanding the interesting properties of conductive polymers and the solution-based deposition processes and patterning technologies used for conductive polymer device fabrication is necessary to develop appropriate and highly effective flexible sensors. The present review provides scientific evidence for promising strategies for fabricating conductive polymer-based flexible sensors. Specifically, the outstanding nature of the structures, conductivity, and synthesis methods of some of the main conductive polymers are discussed. Furthermore, conventional and innovative technologies for preparing conductive polymer thin films in flexible sensors are identified and evaluated, as are the potential applications of these sensors in environmental and human health monitoring.

High-throughput fabrication of soft magneto-origami machines

Soft magneto-active machines capable of magnetically controllable shape-morphing and locomotion have diverse promising applications such as untethered biomedical robots. However, existing soft magneto-active machines often have simple structures with limited functionalities and do not grant high-throughput production due to the convoluted fabrication technology. Here, we propose a facile fabrication strategy that transforms 2D magnetic sheets into 3D soft magneto-active machines with customized geometries by incorporating origami folding. Based on automated roll-to-roll processing, this approach allows for the high-throughput fabrication of soft magneto-origami machines with a variety of characteristics, including large-magnitude deploying, sequential folding into predesigned shapes, and multivariant actuation modes (e.g., contraction, bending, rotation, and rolling locomotion). We leverage these abilities to demonstrate a few potential applications: an electronic robot capable of on-demand deploying and wireless charging, a mechanical 8-3 encoder, a quadruped robot for cargo-release tasks, and a magneto-origami arts/craft. Our work contributes for the high-throughput fabrication of soft magneto-active machines with multi-functionalities.

Direct Ink Writing: A 3D Printing Technology for Diverse Materials

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in-depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects as a guideline toward possible futuristic innovations.

A High-Fidelity Preparation Method for Liquid Crystal Elastomer Actuators

Three-dimensional (3D) structural actuators based on monodomain liquid crystal elastomers (mLCEs) show a wide range of potential applications. A direct ink writing technique has been developed to print LCE structures. It is still a challenge to print high-precision 3D-mLCE actuators. Here, a method of wet 3D printing combined with freeze-drying is proposed. The coagulation bath is designed to restrain the nascent fiber disturbance of the capillary wave and weight by adjusting the ink viscosity and printing speed to control the LC molecular order, enabling uniform (B = 1.02) fibers with a high degree of orientational alignment (S = 0.45) of the mesogens. Furthermore, dynamic disulfide bond formation was used as the cross-linking point, which can allow the LCE network structure to be continuously cured to ensure adjacent layers are effectively bonded and, in combination with freeze-drying, produce the 3D-mLCE actuators of fidelity architecture (98.37 vol %) by printing. The actuators have excellent actuating strain (45.12%), and the dynamic disulfide bond makes them programmable. Finally, a printed bionic starfish and a printed bionic hand can easily grab regular and irregular objects. This work provides a feasible scheme for fabricating complex 3D-mLCEs with reversible changes in shape.

3D Printing of Stretchable, Adhesive and Conductive Ti3C2Tx-Polyacrylic Acid Hydrogels

Stretchable, adhesive, and conductive hydrogels have been regarded as ideal interfacial materials for seamless and biocompatible integration with the human body. However, existing hydrogels can rarely achieve good mechanical, electrical, and adhesive properties simultaneously, as well as limited patterning/manufacturing techniques posing severe challenges to bioelectronic research and their practical applications. Herein, we develop a stretchable, adhesive, and conductive Ti3C2Tx-polyacrylic acid hydrogel by a simple pre-crosslinking method followed by successive direct ink writing 3D printing. Pre-polymerization of acrylic acid can be initiated by mechanical mixing with Ti3C2Tx nanosheet suspension, leading to the formation of viscous 3D printable ink. Secondary free radical polymerization of the ink patterns via 3D printing can achieve a stretchable, adhesive, and conductive Ti3C2Tx-polyacrylic acid hydrogel. The as-formed hydrogel exhibits remarkable stretchability (~622%), high electrical conductivity (5.13 S m−1), and good adhesion strength on varying substrates. We further demonstrate the capability of facilely printing such hydrogels into complex geometries like mesh and rhombus patterns with high resolution and robust integration.

On-Demand Programming of Liquid Metal-Composite Microstructures through Direct Ink Write 3D Printing

Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle-like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high-power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.