Robocasting Research Articles

Development of hybrid polyvinylpyrrolidone/carboxymethyl cellulose/collagen incorporated oregano scaffolds via direct ink write printing for potential wound healing applications

Additive manufacturing can develop regenerative scaffolds for wound healing. 3D printing offers meticulous porosity, mechanical integrity, cell adhesion and cost-effectiveness. Herein, we prepared ink composed of carboxymethyl cellulose (CMC), polyvinylpyrrolidone (PVP), collagen, and oregano extract for the fabrication of tissue constructs. The blend was optimized to form a homogeneous ink and rheological characterization demonstrated shear thinning behavior. The scaffolds were printed using Direct Ink Write (DIW) at a flow speed of 4 mm3/s and a layer height of 0.18 mm. The fabricated scaffolds demonstrated an ultimate tensile strength (UTS) and toughness of 730 KPa and 2.72 MJ/m3, respectively. Scanning Electron Microscopy (SEM) revealed an average pore size of 300 ± 30 μm. Fourier transform infrared spectroscopy (FTIR) analysis confirmed that all materials were present. The contact angle of the composite scaffold was 68° ± 1°. Moreover, the scaffolds presented 82 % mass loss (degradation) in phosphate buffer saline (PBS) over 14 days. The composite scaffold exhibited inhibition zones of 9 mm and 12 mm against Staphylococcus aureus and Escherichia coli, respectively. The PVP/CMC/collagen/oregano 3D printed scaffolds exhibited excellent biocompatibility with the mesenchymal stem cells and humman dermal fibroblast cells, confirmed by water-soluble tetrazolium - 8 (WST-8) assay (test conducted for 7 days). The enhanced angiogenic potential of said scaffold was assesed by release of vascular endothelial growth factor followed by further validation through in-vivo CAM assay. Thus, confirming suitability for the potential wound healing application.

Effect of Oil Content on the Printability of Coconut Cream.

We developed a method to perform direct ink writing (DIW) three-dimensional (3D) printing of coconut-based products with high oil content by varying compositions of the coconut oil and the coconut cream. The addition of oils is particularly crucial in providing energy, developing neurological functions, and improving the palatability of food. Despite the potential merits of high oil-content foods, there have been limited studies on 3D printing of high oil-content foods. In particular, the effect of oil content on the printability of food inks has not been studied to date. 3D printing of food inks with high oil contents is challenging due to oil separation that leads to unpredictable changes in rheological properties. In this work, we surveyed the behavior of the mixture of the coconut oil and the coconut cream and identified the appropriate conditions for the food inks that show the printability in DIW 3D printing. We initially formulated coconut cream inks added with coconut oil that did not exhibit oil separation, and characterized the rheological properties of such inks. We successfully 3D-printed coconut cream with additional coconut oil and successfully fabricated 3D structures with inks containing 25% water with an additional 10% (w/w) of coconut oil. Texture profile analysis (TPA) suggested that the hardness index and the chewiness index of mesh-shaped 3D-printed coconut cream decreased due to an increase in the water content of the ink. Overall, this study offered an understanding of the stability of the food inks and demonstrated the fabrication of 3D colloidal food with controlled oil content, which can be applied to formulating foods with tunable oil content to cater to individual nutritional needs without compromising the stability of the inks.

Printing resolution effect on mechanical properties of porous boehmite direct ink 3D printed structures

Direct ink writing (DIW) is one of the facile and versatile manufacturing techniques for additive manufacturing of ceramics. 3D printed porous boehmite structures fabricated via DIW can offer significant benefits in terms of their high specific surface area and robust mechanical properties, rendering them suitable for applications like catalysis, adsorption, etc. The characteristics of the boehmite structures produced through 3D printing are significantly influenced by several parameters associated with the DIW process, including but not limited to solids concentration in the ink, resolution, and drying time. In this work, for the first time, we try to optimize the DIW parameters for 3D printing of porous boehmite structures to achieve better shape stability, resolution, and mechanical properties. A shear-thinning boehmite ink was prepared by peptization of boehmite powder in nitric acid and the rheological properties were optimized by varying concentrations of boehmite and nitric acid. Boehmite ink with a solid loading of 37 % and a pH of 4 was found to have the optimal rheological properties and printability. A Menger sponge porous structure was printed by DIW using the optimized ink with different sample sizes and resolutions. The resolution and size were optimized for better shape accuracy of printing and structural integrity. Crack-free boehmite structures were obtained by optimizing the drying time. Boehmite structures printed with higher resolution resulted in enhanced compressive strength and energy absorption, i.e., compressive strength and energy absorption of 0.5 mm resolution boehmite structure is enhanced by 208 % and 144 %, respectively, compared to the 1 mm resolution structure.

Using pyrolytic carbon-coated short carbon fiber to fabricate Csf/ZrB2–SiC composites by direct ink writing and precursor infiltration pyrolysis

Direct ink writing (DIW) offers significant advantages for efficient manufacturing of complex ceramic composite components. However, the mechanical properties of short carbon fiber-reinforced ceramic composites fabricated by DIW technology were not ideal, mainly be ascribed to the absence of a suitable coating on the fiber as an interface. In this work, Csf/ZrB2 green bodies were printed by an optimized extrusion process using pyrolytic carbon-coated short carbon fibers (Csf). High-performance Csf/ZrB2–SiC composites were obtained through the infiltration and pyrolysis of SiC precursor six times. With increasing Csf content, the dimensional deviations decreased and the mechanical properties increased. When the Csf content was 30 vol%, the dimensional deviation of the Csf/ZrB2–SiC composite was less than −0.5 % in the X and Y directions and about −2 % in the Z direction. A marked increase in the flexural strength and fracture toughness of the composite was also obtained, reaching 275.0 MPa and 5.26 MPa m1/2, respectively.

Bionic Artificial Skin Based on Self-Healable Ionogel Composites with Tailored Mechanics and Robust Interfaces.

Bionic artificial skin which imitates the features and functions of human skin, has broad applications in wearable human-machine interfaces. However, equipping artificial materials with skin-like mechanical properties, self-healing ability, and high sensitivity remains challenging. Here, inspired by the structure of human skin, an artificial skin based on ionogel composites with tailored mechanical properties and robust interface is prepared. Combining finite element analysis and direct ink writing (DIW) 3D printing technology, an ionogel composite with a rigid skeleton and an ionogel matrix is precisely designed and fabricated, realizing the mechanical anisotropy and nonlinear mechanical response that accurately mimic human skin. Robust interface is created through co-curing of the skeleton and matrix resins, significantly enhancing the stability of the composite. The realization of self-healing ability and resistance to crack growth further ensure the remarkable durability of the artificial skin for sensing application. In summary, the bionic artificial skin mimics the characteristics of human skin, including mechanical anisotropy, nonlinear mechanical response, self-healing capability, durability and high sensitivity when applied as flexible sensors. These strategies provide strong support for the fabrication of tissue-like materials with adaptive mechanical behaviors.

Direct Ink Writing of SiCN/RuO2/TiB2 Composite Ceramic Ink for High-Temperature Thin-Film Sensors.

Direct ink writing (DIW) of high-temperature thin-film sensors holds significant potential for monitoring extreme environments. However, existing high-temperature inks face a trade-off between cost and performance. This study proposes a SiCN/RuO2/TiB2 composite ceramic ink. The added TiB2, after annealing in a high-temperature atmospheric environment, forms B2O3 glass, which synergizes with the SiO2 glass phase formed from the SiCN precursor to effectively encapsulate RuO2 particles. This enhances the film's density and adhesion to the substrate, preventing RuO2 volatilization at high temperatures. Additionally, the high conductivity of TiB2 improves the film's overall conductivity. Test results indicate that the SiCN/RuO2/TiB2 film exhibits high linearity from room temperature to 900 °C, high stability (resistance drift rate of 0.1%/h at 800 °C), and high conductivity (4410 S/m). As a proof of concept, temperature sensors and a heat flux sensor were successfully fabricated on a metallic hemispherical surface. Performance tests in extreme environments using high-power lasers and flame guns verified that the conformal thin-film sensor can accurately measure spherical temperature and heat flux, with a heat flux sensor response time of 53 ms. In conclusion, the SiCN/RuO2/TiB2 composite ceramic ink developed in this study offers a high-performance and cost-effective solution for high-temperature conformal thin-film sensors in extreme environments.

Manipulating fungal growth in engineered living materials through precise deposition of nutrients

One main challenge of emerging fungal-based engineered living materials (ELMs) lies in achieving localized multi-material properties in these structures. Although three-dimensional (3D) printing can efficiently vary local composition and properties, it has not yet been demonstrated in fungal-based ELMs. This work thus explores the concept of using nutrients to manipulate fungal foraging behavior in 3D structures fabricated using direct ink writing (DIW) for the next generation of fungal-based ELMs. Using two fungal strains (Pleurotus ostreatus and Ganoderma lucidum), this study shows that the ink formulation used is suitable for both DIW and mycelium growth. Varying the nutrient content allows for either the inhibition or promotion of exploration and bridging of mycelium in different sections, the control of mycelium density in three dimensions and the fabrication of patterned surfaces. There is potential in fabricating patterned fungal-based ELMs and lab-on-a-chip systems to investigate the effects of other substances and microorganisms on the foraging behavior of mycelium.

Machine learning–enabled direct ink writing of conductive polymer composites for enhanced performance in thermal management and current protection

This study introduces a novel approach that leverages the synergy of machine learning (ML) and Direct Ink Writing (DIW) to optimize the manufacturing feasibility of conductive polymer composites (CPCs) films. The main research focus centers on precisely fine-tuning the printing parameters to strike the perfect balance between the high loading of dendritic copper fillers and their influences on processing and composite performance properties. This pioneering combination significantly enhances the precision of the final product without resorting to time-consuming procedures. ML algorithms contributed to identifying optimal printing variables such as speed, flow pressure, and filler concentration, which helped in identifying the optimal filler content and its performance capabilities. The resulting films exhibited thermo-resistive behavior with a noticeable resistivity increase by 6-7 orders of magnitude at elevated temperatures, specifically around 100 °C. Furthermore, the study highlights remarkable strain-sensing capabilities, simultaneously showcasing a substantial increase in composite modulus. These discoveries bear substantial significance for the development of exceptionally functional thermal interface materials suitable for use in sensors, current collectors, and energy storage devices. The method presented here offers a promising pathway for advancing the fabrication and performance optimization of conductive polymer composites, opening up diverse applications in emerging technologies.

Solar salt encapsulated into 3D printed activated carbon/alumina supports for thermal energy storage applications

The encapsulation of phase change materials (PCMs) into additive manufactured porous supports is attracting great interest for developing thermal energy storage (TES) materials with improved energy performance. Here, highly porous (86 %) self-supported 3D activated carbon/alumina supports are fabricated by direct ink writing (DIW) and, then, infiltrated with solar salt, a highly corrosive PCM with a melting temperature around 220 °C commonly employed in concentrated solar power plants. This novel, robust, chemically compatible, and lightweight infiltrated 3DTES exhibits good thermal energy storage efficiency (70 %) and thermal stability, high energy storage density (381 J g−1), and avoids the liquid leakage of the molten salt. Besides, the 3D activated carbon/alumina support promotes a better ability to absorb solar energy (79 %) and enhances the thermal conductivity of the solar salt (up to 64 %). These results validate the use of DIW for manufacturing innovative TES with an enhanced energy storage behaviour.

3D-printed β-TCP/Ti6Al4V composite scaffolds for bone regeneration: Process parameter optimization and evaluation

A β-TCP/Ti6Al4V composite scaffold with interconnected macro porous architecture was fabricated using Direct Ink Writing (DIW). Pluronic F-127 and de-ionized water was used as binder and solvent for ink preparation. The present work was carried out to study the rheological behavior of the composite bioceramic ink and to investigate DIW process parameters such as Ti6Al4V proportion, infill percentage and extrusion pressure. The Box-Behnken response surface methodology, ANOVA, sensitivity, desirability approach are used for the experimental, statistical and numerical optimization of the parameters suitable for DIW. The output responses such as dimensional error of the fabricated scaffold from the original dimensions and compressive strength are considered for multi-objective optimization. The result defined that the optimal values are solid loading 55 %v/v (40 %v/v of β-TCP, 15 %v/v of Ti6Al4V) and 45 %v/v of Pluronic gel, 98 % infill rate and 6.36 bar pressure. The dimensional error and compressive strength of the scaffold printed at the optimized conditions are found as 1.88 % and 19 MPa with macro and micro pores suitable for bone regeneration with satisfactory biocompatibility assed via MTT assay.

Reactive Processing of Furan-Based Monomers via Frontal Ring-Opening Metathesis Polymerization for High Performance Materials.

Frontal ring-opening metathesis polymerization (FROMP) presents an energy-efficient approach to produce high-performance polymers, typically utilizing norbornene derivatives from Diels-Alder reactions. This study broadens the monomer repertoire for FROMP, incorporating the cycloaddition product of biosourced furan compounds and benzyne, namely 1,4-dihydro-1,4-epoxynaphthalene (HEN) derivatives. A computational screening of Diels-Alder products is conducted, selecting products with resistance to retro-Diels-Alder but also sufficient ring strain to facilitate FROMP. The experiments reveal that varying substituents both modulate the FROMP kinetics and enable the creation of thermoplastic materials characterized by different thermomechanical properties. Moreover, HEN-based crosslinkers are designed to enhance the resulting thermomechanical properties at high temperatures (>200°C). The versatility of such materials is demonstrated through direct ink writing (DIW) to rapidly produce 3D structures without the need for printed supports. This research significantly extends the range of monomers suitable for FROMP, furthering efficient production of high-performance polymeric materials.

Fully 3D printed functional PDMS composites with designable structures and performances

Three-dimensional (3D) printing, as a layer-to-layer additive manufacturing technology, has received widespread attention for excellent designability. However, as for direct ink writing (DIW), current printing level is difficult to achieve high-precision printing of thermoset composites of different compositions. Therefore, fully 3D printing based on thermoset composites with high designability is proposed. The intralayer and interlayer of structure and materials prepared by this method are designable, and layer thicknesses as well as inter-layer patterns are adjustable. In this work, alumina (Al2O3) and short carbon fiber (SCF) are used as thermally conductive fillers, polydimethylsiloxane (PDMS) is conducted as thermoset matrix. Benefit from the high designability of our method, a series of Al2O3/SCF/Al2O3 (ASA) and SCF/Al2O3/SCF (SAS) composite samples with sandwich structures are fabricated and compared. The different materials and structural designs of these composite samples give them completely different properties in terms of thermal, electromagnetic shielding, and mechanical properties, making it possible to create customized designs for different scenarios. Taking thermal management materials (TMMs) as an example, we use this method to prepare ASA and SAS composites with sandwich structure, thermal conductivity of A40S30A40 and S30A40S30 reached 1.00 W/(m·K) and 1.55 W/(m·K) respectively. In all, customized and multifunctional applications make PDMS composites have a widespread prospect.

Effective Young’s modulus of highly porous 3D printed mono-material and coaxial structures

The dynamic elastic response of 3D macroporous scaffolds is crucial to determine their performance under operating conditions. In this work, the elastic behaviour of 3D printed bar-shaped scaffolds of γ-Al2O3 and γ-Al2O3/graphene nanoplatelets (GNP) composites (18 vol%), fabricated by Direct Ink Writing (DIW), is investigated. Dynamic Young’s modulus (E) is studied by the Impulse Excitation Technique (IET) and compared with theoretical results obtained by Finite Element Methods (FEM) and the multi-scale material simulator GeoDict®. In addition, Resonant Ultrasound Spectroscopy (RUS) is used to characterize E of single- and bi-material coaxial struts. While additions of GNP to γ-Al2O3 lead to an increase in E, it decreases in the coaxial struts due to the development of thermal residual stresses at the core/shell interface. The suitability of combining experimental and theoretical methods for the analysis of the elastic properties and the anisotropy associated with the lattice pattern is demonstrated.

Direct ink writing of woodpile-kind alumina phononic crystals for MHz regime

We report comprehensive work on the fabrication of alumina-based phononic crystals (PhCs) using direct ink writing (DIW), measurement of anisotropic properties, and identification of acoustic bandgaps. Woodpile-structure-kind PhCs of characteristic size 1.1 mm and 10 periods were fabricated from alumina and polyvinyl alcohol (PVA) solution, and processed by freezing under vacuum. Next, the anisotropic elastic constants of alumina were obtained from bulk specimens using resonant ultrasound spectroscopy. Band diagrams and transmission loss were computed using the finite-element method and good correlations were found between the bandgaps estimated by these methods. Multiple partial bandgaps and several local resonances were observed in many directions for frequencies below 2 MHz. This work is the first step toward developing ceramic PhC using additive manufacturing methods for high-temperature applications.

Mechanical evaluation of DIW-printed carbon nanofibers - alumina-toughened zirconia composites

Direct ink writing (DIW) offers cost-effective fabrication of intricate ceramic structures. However, the mechanical properties (reliability) of DIW-obtained components have been inadequate for structural applications. This study aimed to improve mechanical properties of DIW alumina-toughened zirconia (ATZ) composite filaments with and without carbon nanofibers (CNFs). Optimized debinding and sintering conditions were determined for Conventional Sintering (CS) and Spark Plasma Sintering (SPS). The influence of nozzle diameter, CNF content and sintering method on the fracture strain, Young’s modulus, flexural strength and Weibull modulus was carefully assessed via correlation analyses. CNFs, although intended to enhance properties, were often agglomerated, which negatively affected mechanical properties. Smaller nozzle diameters enhanced Young’s modulus, with CS-sintered ATZ filaments showing transformation-induced plasticity (TRIP). Mechanical properties rivaled or surpassed those of conventional techniques (e.g., cold isostatic pressing, slip casting) with a good level of mechanical reliability achieved. Overall, DIW showed promise for ceramic materials, particularly ductile Ce-TZP-based composites.

Material extrusion 3D-printing technology: A new strategy for constructing water-soluble, high-dose, sustained-release drug formulations

The advantage of low-temperature forming through direct ink writing (DIW) 3D printing is becoming a strategy for the construction of innovative drug delivery systems (DDSs). Optimization of the complex formulation, including factors such as the printing ink, presence of solvents, and potential low mechanical strength, are challenges during process development. This study presents an application of DIW to fabricate water-soluble, high-dose, and sustained-release DDSs. Utilizing poorly compressible metformin hydrochloride as a model drug, a core-shell delivery system was developed, featuring a core composed of 96 % drug powder and 4 % binder, with a shell structure serving as a drug-release barrier. This design aligns with the sustained-release profile of traditional processes, achieving a 25.8 % reduction in volume and enhanced mechanical strength. The strategy facilitates sustained release of high-dose water-soluble formulations for over 12 h, potentially improving patient compliance by reducing formulation size. Process optimization and multi-batch flexibility were also explored in this study. Our findings provide a valuable reference for the development of innovative DDSs and 3D-printed drugs.

3D printable carbon nanotubes-based composites with reconfigurable shapes and properties

In this work, we developed an efficient approach to fabricating composites of carbon nanotubes (CNT) with shape reconfiguration ability and Joule heating behavior by incorporating the direct ink writing (DIW) technique. Our 3D printable ink consisted of a high concentration of CNT stabilized by polyvinylpyrrolidone (PVP) in an aqueous polyvinyl alcohol (PVA) solution. The rheology of the ink was tuned by the amounts of CNT and PVP. The printed objects underwent the freeze-thaw process to form weak hydrogels induced by the crystallization of PVA. Subsequently, the hydrogels were immersed in a deep eutectic solvent (DES) bath consisting of choline chloride and glycerol. This process replaced the water in the hydrogels with DES and led to mechanically tough DES/CNT gels. We utilized the high stretchability of DES/CNT gel to achieve complex geometries that would be challenging to accomplish solely with DIW. After reshaping the DES/CNT gels, the target shape was fixed by further solvent exchange with ethanol and drying. The final CNT composites showed a high electric conductivity of up to 33.94 S/m and exhibited remarkable Joule heating performance using a low applied voltage (5 V). Additionally, we successfully achieved a spatial variation of mechanical stiffness, electrical conductivity, and Joule heating across the printed objects by a selective solvent removal process. In short, this work expands the scope of possible shapes and properties of CNT-based composites.

Hierarchically porous bone scaffold fabricated via direct foam writing with TCP/ZrO2 composite ink

Customizing bone scaffolds to mimic the hierarchically porous structure of natural bone presents a promising strategy for treating critical bone defects. Although low-cost direct ink writing (DIW) is often used to customize bone scaffolds, the limited print resolution made it difficult to print microporous structures. In this study, DIW was combined with the direct foam method to construct bone scaffolds with hierarchical and interconnected macro-micro porous structures. The foam stabilization mechanism of pure TCP was investigated to prepare TCP-enhanced ZrO2-based composite foam inks. Incorporating TCP endowed a richer interconnection structure to the composite foam. The printed bone scaffolds feature three scales of pore structure: interconnected macropores of 400–500 μm, bubble template pores of 50 μm, and open pores of <20 μm. The scaffold exhibited properties comparable to cancellous bone, such as 90 % porosity, 3.1 MPa compressive strength, and 1.0 GPa elastic modulus. In vitro experiments have demonstrated that cells can grow into the interior of the scaffold by following the connecting pores. And, incorporating TCP significantly enhances osteogenic protein and gene expression in bionic bone scaffolds. This facile and controllable approach offers an alternative solution to addressing the challenge of clinical implant shortages.

Three-Dimensional Printing of Yttrium Oxide Transparent Ceramics via Direct Ink Writing.

The utilization of 3D printing technology for the fabrication of intricate transparent ceramics overcomes the limitations associated with conventional molding processes, thereby presenting a highly promising solution. In this study, we employed direct ink writing (DIW) to prepare yttrium oxide transparent ceramics using a ceramic slurry with excellent moldability, solid content of 45 vol%, and shear-thinning behavior. A successfully printed transparent yttrium oxide ring measuring 30 mm in diameter, 10 mm in inner diameter, and 0.9 mm in thickness was obtained from the aforementioned slurry. After de-binding and sintering procedures, the printed ceramic exhibited in-line transmittance of 71% at 850 nm. This work not only produced complex yttria transparent ceramics with intricate shapes, but also achieved in-line transmittance that was comparable to that of the CIP method (79%), which can meet certain optical applications.

3D Printed Swordfish‐Like Wireless Millirobot

Inspired by the efficient swimming capabilities of swordfish, a novel wireless soft swordfish‐like robot with programmable magnetization has been developed, integrating direct‐ink‐writing (DIW) 3D printing and assembly technology. This 20 mm long robot features a streamlined form and magnetically programmable movements, enabling biomimetic locomotion patterns such as straight‐line swimming and turning swimming. The robot includes a silicone‐based torso (body, abdomen, and pectoral fin) and a crescent‐shaped tail fin made from a magnetically programmable polymer embedded with neodymium‐iron‐boron (NdFeB) particles. The tail fin, fabricated by multi‐material alternating printing to achieve a gradient magnetism distribution, is controlled by an external magnetic field to mimic the rapid oscillation of a swordfish's tail, achieving a swimming speed of 0.51 BL/ s. The tail fin's asymmetric oscillation amplitudes, adjusted by magnetic field control, allow the robot to transition seamlessly from high‐speed straight swimming to agile turning. The robot can perform tracking swimming along specific planned paths, such as “C” and “Z” shaped trajectories. Potential applications include environmental monitoring and targeted drug release. The multi‐material 3D printing technology enhances the robot's efficiency and sensitivity in simulating natural biological movements, extending to the design and development of various flexible devices and soft robots.