Robocasting Research Articles

Fully 3D printed dielectric elastomer actuators based on silicone and its composites

AbstractDielectric elastomer actuator (DEA) is one of the most promising types of soft actuation technology, which has great potential in the fields of wearable devices and soft robotics. It consists of a dielectric elastomer layer, which is an electroactive polymer that can produce large deformation, and compliant electrodes to bring charges to certain locations. In this article, direct ink writing (DIW) technology, an emerging 3D printing method, was used to realize the preparation of the electrode‐elastomer‐electrode stack of the DEA. The dielectric and electrode materials were designed with suitable rheological properties to fulfill the need for the extrusion process. The formulated silicone material not only presented excellent dielectric and mechanical properties, but also good printability. Extrudable electrodes were prepared based on silicone composites with the characteristics of mechanical compliance and high conductivity. The fully printed DEA achieved a maximum actuation strain of 11.11%, a fast response time of 0.76 s and excellent electromechanical repeatability. DEA arrays were also achieved, possessing the ability to carry out on‐demand actuation, allowing each actuator to be activated singly or work in groups. Thanks to the design freedom of the DIW technology, this strategy is able to manufacture fine and complex structures with precise active zones, paving a way for the fabrication of next‐generation smart devices.Highlights Printable silicone ink was formulated with good dielectric property and softness. Carbon black/silicone composites were obtained with high conductivity and compliant nature. The silicone composites were printed into thin films to act as electrodes. Fully 3D printed dielectric elastomer actuators (DEA) were achieved by direct ink writing. DEA arrays with on‐demand actuation were realized by well‐defined printing.

Rheology and Printability of a Porcelain Clay Paste for DIW 3D Printing of Ceramics with Complex Geometric Structures.

The modeling of ceramics with complex geometric structures currently depends on the handcrafted mode, with long cycles, high costs, and low efficiency; additive manufacturing (AM) technology can solve this problem well. Herein, the porcelain clay paste was successfully prepared for the direct ink writing (DIW) 3D printing process of ceramics with complex geometric structures, and the effects of sodium citrate (SC) content on the rheological properties and DIW 3D printability of the porcelain clay paste were investigated in detail. The SC has a vital role in the rheological behavior of porcelain clay paste. Adding SC increases the absolute zeta potential and decreases the viscosity of the paste, while a high SC content will lead to a low storage modulus of the paste. The porcelain clay paste with an SC content of 0.05% and a paste solid content of 75% possesses suitable rheological properties and a storage modulus for DIW 3D printing. The as-prepared porcelain clay paste has high DIW 3D printability at a pressure of 0.5 MPa, and a 3D-printed green body with a well-densified structure can be achieved. After being sintered, the 3D-printed ceramic exhibits high densification and mechanical properties. A green body with complex geometric structures is quickly and precisely modeled by the DIW 3D printing process with the resultant porcelain clay paste as the raw material. This work provides a practical approach to rapidly fabricating ceramics with complex geometrical structures.

In situ self-crosslinking binder system − enhances the mechanical performance gain of composite energetic materials

To improve the mechanical properties of microscale composite energetic materials, as well as to investigate their detonation performance, we used Toluene Diisocyanate (TDI) as a self-crosslinking curing agent based on an oil-in-water emulsion binder. We then prepared performance test samples with various microscale morphologies using the Direct Ink Writing (DIW) technique. We compared the mechanical properties of composite energetic materials, such as nanoindentation, quasi-static compression, and adhesion, and examined their infrared spectra, micromorphology, pore distribution, mechanical sensitivity, detonation performance, and combustion performance. The results indicated that with the in situ self-crosslinking binder system, the elastic modulus and hardness of the material were increased to 5.213 GPa and 0.100 GPa, respectively, which corresponds to an increase of approximately 638 % and 370 % compared to the original binder system. In the compressive strength test, the maximum stress increased by 104 %, and the maximum strain increased by 311 %, while the adhesion strength was enhanced by about 37.8 %. The infrared spectroscopy analysis demonstrated changes in the molecular structure due to in situ self-crosslinking. The improved surface morphology and more uniform pore size distribution also led to enhanced detonation performance and safety performance, with a detonation velocity increase of about 7.8 %, an increase in the critical detonation corner by 10°, a reduction in the critical detonation thickness by about 60.8 %, and a decrease in the critical detonation size to 0.3 × 0.3 mm. The impact sensitivity was reduced by 12.5 %, and the friction sensitivity was decreased by 30 %. Additionally, the material exhibited faster burn rates and larger flames. These findings provide significant scientific evidence for the regulation of mechanical properties and detonation performance of microscale composite energetic materials.

Design and fabrication of a fast-response and low-energy input micro igniter

In this study, we innovatively combine direct ink writing (DIW) and physical vapor deposition (PVD) to fabricate a novel micro igniter with fast-response and low-energy input characteristics. The igniter comprises a 5×5 mm² metal film bridge with gold contact pads. The metallic resistance is fabricated using a series of photolithography, metal deposition, and lift-off processes. After evaluating titanium and chromium as the resistive filament, titanium was selected due to its superior reactivity when in contact with CuO leading to reduced ignition energy. Onto the resistive titanium layer, either thermite multilayer films and/or energetic inks are deposited to achieve a head-to-head comparison of ignition characteristics. We show that igniters powered by (CuO/Ti)5 bilayers and Al/CuO/PVDF as energetic ink demonstrate the equivalent ignition performance as reactive (CuO/Ti)5 multilayered igniter. The ignition delay and ignition energy are below 1 ms and 2 mJ, respectively, while the flash intensity is approximately one decade higher than what is typically achieved with conventional micro-igniters. Furthermore, its ignition behaviour can be readily adjusted by altering the constituents of the fuel/oxidizer or adjusting the ratio of multilayer/inks, thereby allowing for easy adaptation of the micro-igniter to meet the diverse requirements of various applications.

Laser Sintering Direct Ink Write Silver Nanoflake Ink for On-Demand Manufacturing of Electronics in Space

Laser Sintering Direct Ink Write Silver Nanoflake Ink for On-Demand Manufacturing of Electronics in Space

PDMS microspheres as rheological additives for PDMS-based DIW inks

Direct Ink Writing holds vast potential for additive manufacturing with broad material compatibility as long as appropriate rheological properties are exhibited by the material of choice. Additives are often included to attain the desired rheological properties for printing, but these same additives can yield products with undesirable mechanical properties. For example, silica fillers are used to create silicone inks appropriate for printing but yield cured structures that are too stiff. In this work, we investigate the applicability of PDMS microspheres as a rheological and thixotropic additive for PDMS based DIW inks. We utilize a facile oil-in-water emulsion method to reproducibly obtain small (∼5 μm) PDMS microspheres, which are then incorporated into PDMS-based inks. More traditional inks with fumed silica and thixotropic additive were compared with inks containing PDMS microspheres at equal volume loadings to determine whether the PDMS microspheres could impart the desired rheological properties for DIW. Inks including PDMS microspheres exhibited surprising thixotropic effects, which enabled prints with fidelity analogous to traditional ink employing silica filler, while producing mechanically softer prints.

Microstructure and properties of high-entropy-superalloy microlattices fabricated by direct ink writing

Ni-Co-Fe-based high-entropy superalloys (HESAs) are fabricated into microlattices via a three-step process: (i) layer-by-layer extrusion of inks containing elemental powders (Ni, Co, Fe, Cr, Ti) and TiAl3 powders; (ii) sintering to densify and homogenize the struts; (iii) aging to achieve a γ/γ’ microstructure. The struts of the microlattices show a nearly pore-free and fully-homogenized microstructure. Increasing the Ti concentration from 4 at% (Al9Co26Cr7Fe16Ni38Ti4) to 9 at% (Al8Co25Cr7Fe15Ni36Ti9) leads to a significant increase in the volume fraction of strengthening γ’ precipitates, from 51 to 78 %. Furthermore, in the Ti-rich composition, the γ' precipitates exhibit a sharp-edged cubic morphology with larger sizes and higher lattice misfit (0.63 %) with respect to the γ matrix. As a result, Ti-rich HESA microlattices show higher strength at ambient temperature than Ti-poor ones, while retaining high compressive ductility (> 60 %). They also demonstrate superior specific strengths when compared to bulk Inconel 617 and other representative bulk HESA, up to 1000 °C. The combination of low-density (2.29–3.15 g/cm3), high strength at elevated temperatures, and high processability positions HESA microlattices from direct ink writing as promising candidates for structural components in extreme operating conditions. The versatility of the process is demonstrated by printing and sintering three miniature HESA objects with complex shapes (hollow turbine blade, gyroid heat exchanger, and compressor wheel).

Programmed multimaterial assembly by synergized 3D printing and freeform laser induction

In nature, structural and functional materials often form programmed three-dimensional (3D) assembly to perform daily functions, inspiring researchers to engineer multifunctional 3D structures. Despite much progress, a general method to fabricate and assemble a broad range of materials into functional 3D objects remains limited. Herein, to bridge the gap, we demonstrate a freeform multimaterial assembly process (FMAP) by integrating 3D printing (fused filament fabrication (FFF), direct ink writing (DIW)) with freeform laser induction (FLI). 3D printing performs the 3D structural material assembly, while FLI fabricates the functional materials in predesigned 3D space by synergistic, programmed control. This paper showcases the versatility of FMAP in spatially fabricating various types of functional materials (metals, semiconductors) within 3D structures for applications in crossbar circuits for LED display, a strain sensor for multifunctional springs and haptic manipulators, a UV sensor, a 3D electromagnet as a magnetic encoder, capacitive sensors for human machine interface, and an integrated microfluidic reactor with a built-in Joule heater for nanomaterial synthesis. This success underscores the potential of FMAP to redefine 3D printing and FLI for programmed multimaterial assembly.

DIW-Printed Thermal Management PDMS Composites with 3D Structural Thermal Conductive Network of h-BN Platelets and Al2O3 Nanoparticles.

Electronic devices play an increasingly vital role in modern society, and heat accumulation is a major concern during device development, which causes strong market demand for thermal conductivity materials and components. In this paper, a novel thermal conductive material consisting of polydimethylsiloxane (PDMS) and a binary filler system of h-BN platelets and Al2O3 nanoparticles was successfully fabricated using direct ink writing (DIW) 3D printing technology. The addictive manufacturing process not only endows the DIW-printed composites with various geometries but also promotes the construction of a 3D structural thermal conductive network through the shearing force during the printing process. Moreover, the integrity of the thermal conductive network can be optimized by filling the gaps between the BN platelets with Al2O3 particles. Resultingly, the configuration of the binary fillers is arranged by the shearing force during the DIW process, fabricating the thermal conductive network of oriented fillers. The DIW-printed BN/Al2O3/PDMS with 45 wt% thermal conductive binary filler can reach a thermal conductivity of 0.98 W/(m·K), higher than the 0.62 W/(m·K) of the control sample. In this study, a novel strategy for the thermal conductive performance improvement of composites based on DIW technology is successfully verified, paving a new way for thermal management.

Multi-material Direct Ink Writing 3D Food Printing using Multi-channel Nozzle

This paper describes a method for multi-material 3D food printing using a multi-channel nozzle in direct ink writing (DIW) with rapid ink switching. Existing methods for multi-material food printing rely on independently controlled syringes, posing challenges in (1) aligning more than one type of food inks and (2) creating a seamless continuous food filament with different materials. To overcome this limitation, we developed a method using a Y-junction nozzle for seamless ink switching. The use of two inks possessing different rheological properties may result in the backflow of the higher-yield-stress fluid into the channel of the lower-yield-stress fluid, which was addressed by designing the channel sizes to limit the backflow. We successfully demonstrated continuous printing of two food inks exhibiting a yield stress difference on the order of > 102. Real-time switching of the materials can delay deposition at the intended location, which was overcome by introducing an offset distance to the toolpath, resulting in a 74% reduction in print errors. Lastly, we demonstrated the Y-junction nozzle for co-laminar extrusion of two food materials, enabling anisotropic arrangements in 3D food objects. Our approach applies the Y-junction nozzle for multi-material 3D food printing, with broad applicability to various edible inks.

A vibrating membrane ejector for direct ink writing of printed electronics

Direct ink writing (DIW) represents a technical branch of additive manufacturing technology, particularly suitable for prototyping or small-batch printing production of printed electronic components. However, the existing print heads required for near or sub-100 μm line width are quite sophisticated, limiting the accessibility and application of the DIW method. This paper reports the use of the vibrating membrane ejector (VME) as a new option for the print head of DIW. The structure of the VME-based print head was specially designed for this purpose. Finite element modeling and analysis of the VME’s vibration characteristics were performed to provide insights into the ejection conditions and behaviors. The factors influencing the size of printed structures were identified and analyzed through the printing of a metal–organic complex silver (Ag) ink. After optimizing several operational parameters to limit the spreading effects and suppress the satellite droplets, the DIW printed line width has reached about 100 μm. The effectiveness of the VME-based print head was further demonstrated through the DIW fabrication of interdigitated electrodes and microstrip transmission lines. This highlights the versatility of the VME-based print head as a practical tool for device prototyping and ink development in the field of printed electronics.

Framework for the Generation of 3D Fiber Composite Structures from 2D Observations

Synthetic generation of realistic materials for testing of process–structure–property (PSP) relationships in computational materials science has gained significant traction over the past two decades. Generation tools continue to lag in some aspects of realism, leading to uncertainty and errors in simulating material response. The experimental collection of information to guide generation of 3D synthetic structures remains costly, time consuming, and challenging. These challenges are compounded by limitations of stereology, which permits estimation of 3D microstructural statistics from 2D observations under restrictive assumptions on constituent morphologies and size distributions and can be difficult to apply to microstructure metrics like constituent orientation distribution and clustering. This work seeks to overcome these challenges by introducing a framework for learning probable 3D microstructure statistics by minimizing the loss determined by matching statistics obtained from 2D observations via synthetic microstructure generation software (e.g. Dream.3D) and stereological principles. This framework is applied to short carbon fiber composite structures printed from an additive manufacturing process, Direct Ink Writing, with the hope that the framework could generalize to other particulate structures as well as allow for simulation and design of materials structure.

Acoustofluidics enables direct ink writing of vascular scaffolds with intrinsically patterned porous microstructures

Vascular scaffolds with well-organized pore characteristics are crucial for promoting infiltration, adhesion, growth, tissue-specific arrangement, and substance-exchanges of biological cells. However, their controllable fabrication is extremely challenging, especially for those with specially designed three-dimensional (3D) tubular and porous microstructures. In this paper, a new methodology based on acoustofluidics-assisted direct ink writing (DIW) technique is developed to fabricate vascular scaffolds with controlled porosity distributions and well-defined porous microstructures. Calcium carbonate (CaCO3) particles and alginate are uniformly mixed as the printing ink, which is printed along a rotating rod to form a vessel-like structure. Distributions of these CaCO3 particles in the printed structure are precisely controlled using bulk acoustic waves. The particles within the vessel-like structure are dissolved in situ using hydrochloric acid, resulting in the formation of 3D vascular scaffolds with intrinsically patterned porous microstructures. The scaffolds printed by the acoustofluidics-assisted DIW method exhibit a 69% improvement in pore connectivity compared to conventional 3D-printed scaffolds. Fibroblast cells are successfully seeded onto these scaffolds, achieving distinctive distributions along the patterned porous microstructures. This innovative methodology, with its excellent microfabrication capabilities, holds great promise for applications in tissue engineering, drug release, composite formation, microfluidic chips, and biosensors.

3D-printed polyether-ether ketone/carboxymethyl cellulose scaffolds coated with Zn–Mn doped mesoporous bioactive glass nanoparticles

Patient-specific fabrication of scaffold/implant requires an engineering approach to manufacture the ideal scaffold. Herein, we design and 3D print scaffolds comprised of polyether-ether-ketone (PEEK) and sodium-carboxymethyl cellulose (Na-CMC). The fabricated scaffold was dip coated with Zn and Mn doped bioactive glass nanoparticles (Zn–Mn MBGNs). The synthesized ink exhibit suitable shear-thinning behavior for direct ink write (DIW) 3D printing. The scaffolds were crafted with precision, featuring 85% porosity, 0.3 mm layer height, and 1.5 mm/s printing speed at room temperature. Scanning electron microscopy images reveal a well-defined scaffold with an average pore size of 600 ± 30 μm. The energy dispersive X-ray spectroscopy analysis confirmed a well dispersed/uniform coating of Zn–Mn MBGNs on the PEEK/Na-CMC scaffold. Fourier transform infrared spectroscopy approved the presence of PEEK, CMC, and Zn–Mn MBGNs. The tensile test revealed a Young's modulus of 2.05 GPa. Antibacterial assays demonstrate inhibition zone against Staphylococcus aureus and Escherichia Coli strains. Chick Chorioallantoic Membrane assays also present significant angiogenesis potential, owing to the antigenic nature of Zn–Mn MBGNs. WST-8 cell viability assays depicted cell proliferation, with a 103% viability after 7 days of culture. This study suggests that the PEEK/Na-CMC scaffolds coated with Zn–Mn MBGNs are an excellent candidate for osteoporotic fracture treatment. Thus, the fabricated scaffold can offer multifaceted properties for enhanced patient outcomes in the bone tissue regeneration.

Mechanical Behaviour, Contact Pose Estimation, and Finite Element Analysis of Vision Based Tactile Sensors Fabricated by Molding and Direct Ink Writing: A Comparative Study

This work pioneers the application of direct ink writing (DIW) to fabricate elastomeric additively manufactured vision based tactile sensor (VBTS). DIW cuts down the fabrication time by 76%, allowing design precise control and reducing the complexities of the process compared to the state‐of‐the‐art (SOTA) molding techniques. Successful fabrication of DIW sensor is verified in three stages. Firstly, the mechanical characteristics of the DIW sensor are at par with those of SOTA molded Ecoflex in terms of depth of compression, compression rate, and the number of cycles. Secondly, using robotic pose estimation as a demonstration, the force enables deformation in the DIW sensor shows comparable normality estimation performance to that of the SOTA Ecoflex with a mean absolute error of less than 0.6°. Thirdly, finite element analysis (FEA) of DIW and SOTA Ecoflex sensors using Yeoh model shows similar stress and strain distributions as another evidence of DIW deformability and durability signaling sensor's successful fabrication.

Additive manufacturing of synthetic rubber ink with high solid content reinforced by networked silica

Abstract Silica is a reinforcing filler commonly used in the production of environmentally friendly tires, as tires reinforced with silica have lower rolling resistance, which translates into reduced energy consumption and improved fuel economy. However, achieving the optimal dispersion of silica within the rubber matrix is crucial for maximizing its reinforcing effects. In this study, a three- dimensionally networked silica (NS) was introduced in various amounts to rubber inks to improve their tensile strength and increase miscibility to enable their use in additive manufacturing. The results show that synthetic rubber ink with a high content of styrene–butadiene rubber (90%) and reinforced by NS possesses adequate viscosity for use in the direct ink write (DIW) process. NS was confirmed to have an impact on the rheological properties and printability of the rubber ink as well as to improve the tensile strength of the printed parts. Different formulations were tested to study and facilitate the vulcanization process and identify the optimal curing conditions as well as the print parameters to use in DIW printing. The successful printing and vulcanization of various printed structures demonstrates the potential for using the developed printable ink in additive manufacturing. This study opens up new possibilities for creating rubber products (such as tire treads) with adequate flexibility and high tensile strength.

Integrating intrinsic and configural superiority into advanced impact-resistant organohydrogels via cryo-assisted direct ink writing

The growing safety consciousness towards life and engineering poses demand for efficient anti-impact materials. Decent advances have been made in material development and structure design, whereas the superiority synergism of materials and structures remains challenging. This work develops an advanced impact-resistant shear stiffening gel-polyvinyl alcohol-chitosan (SSG-PVA-CS) organohydrogel by integrating 3D printed customized architectures with strain rate enhanced matrix materials. Following rheological optimization of components, the precursor ink presents shear thinning and yield stress fluid behaviors enabling the direct ink writing (DIW) printing techinique. The printability is systemically explored through finite element modeling assisted flow dynamics analysis and experimental validating, and a quantitative phase diagram is constructed which determines the dependence of parameters on the printing mode and quality. The organohydrogel presents favorable mechanical strength, cyclic durability and rate enhancement effect, which imparts basis of impact resistance to the matrix. The rational structure design effectively reduces the volume density within 0.60 to 0.71 g/cm3 while maintaining considerable attenuation ratios of 73.09 % to 83.55 %, with structure-dependent dynamic mechanical behaviors clarified via finite element analysis. This study provides a reliable approach to fabricate impact-resistance materials with tailored architectures and mechanically functional matrix materials, unlocking paths to break through the performance bottleneck.

Direct Ink Writing on a Rotating Mandrel - Additive Lathe Micro-Manufacturing

Abstract Among the promising techniques within Additive Manufacturing (AM), Direct Ink Writing (DIW) stands out for its ability to work with a wide range of materials, including polymers, ceramics, glass, metals, and cement. However, DIW encounters a significant challenge in creating complex tubular structures, such as vascular scaffolds with micro scale features. To address this challenge, our research investigates a novel method known as Additive Lathe Direct Ink Writing (AL-DIW). AL-DIW entails the precise dispensing of ink onto a rotating mandrel to fabricate intricate hollow tubular structures with overhanging geometries. In this research, we present a series of test cases involving tubular structures, comprising straight-line patterns, curved line designs, and complex stent configurations, to underscore the efficacy of this technique in crafting hollow tubular geometries with micro-scale features. This study not only highlights the capabilities of AL-DIW but also contributes to the broader advancement of additive manufacturing techniques for various applications.

Highly efficient electric-thermal conversion of Silver/PVP composites for micro initiators by direct ink writing

Traditional film initiator fabrication techniques were limited by material and process constraints, resulting in restricted ignition capability, durability issues, and complexity in manufacturing. In this study, Metal/polymer composite materials were applied in film initiators in order to enhance the mechanical properties, longevity, and thermal attributes. Silver/polyvinylpyrrolidone composite film bridge (SPB) initiators were fabricated by direct ink writing (DIW). By adjusting the silver loading level and sintering temperature in the Ag nanoparticles/polyvinylpyrrolidone (Ag NPs/PVP) ink formulation, precise control over the structure and resistivity of the SPB was achieved. Compared to single silver film bridge (SFB) initiators prepared by magnetron sputtering (MS), SPBs demonstrated superior heating performance, higher current carrying capacity, and energy utilization under constant current excitation. In capacitor discharge tests, SPBs exhibited superior electric explosion characteristics and plasma properties, with a 25.53% higher energy utilization efficiency compared to SFBs. The peak plasma electron temperature of SPBs reached 7610 K, which was 1065 K higher than that of SFBs. This makes Silver/polyvinylpyrrolidone composite highly promising practical alternatives for high-efficiency, stable, and reliable ignition processes, with potential widespread applications in the aerospace field.

A green and facile direct ink writing technique for preparation calibration standards in laser ablation inductively coupled plasma mass spectrometry analysis

BackgroundLaser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a powerful tool for microanalysis of solid materials. Nevertheless, one limitation of the method is the lack of well-characterized homogeneous reference materials (RMs), such as BaF2 crystal and BaCO3 ceramics samples, making direct quantification difficult. This work presents a novel Direct Ink Writing (DIW) method to produce RMs for microanalysis. The Mg, Cr, Fe, Co, Ni, Cu, Y, Mo, Pr, Gd, Dy, Ho, Er, Tm, Yb, and Lu solutions were gravimetrically doped into BaCO3 by mixing with the dispersant and then cured with DIW techniques. (94) ResultsBaCO3 powder was combined with a dopant analyte to produce a printable slurry, aided by the use of a dispersant and cellulose. The resulting mixture was then printed using DIW equipment. The retention rates of the doped elements were investigated by internal and external standard method, and the results showed that they were completely dispersed in the solid material. After further optimization, it was found that there was no significant heterogeneity among the printed samples. LA-ICP-MS was used to analyze printed samples, to evaluate micro-scale homogeneity. The mass concentration of the doped element was determined by ICP-MS, verify its move closer to nominal value. Compared with the traditional reference materials preparation methods, the DIW technology greatly increased the sample homogeneity and the accuracy of the desired concentration. (132) SignificanceAs far as we know, there are few reports on the application of DIW method to prepare calibration standards. In brief, it is proved that the proposed method of preparing calibration standard by DIW technique to quantify analytes is valid and robust. This procedure provides great potential for LA-ICP-MS in-situ analysis in the field of well-prepared products, such as ceramic and crystal samples.(63)