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1. Analytical Study of Shear-Thinning Fluid Flow in Direct Ink Writing Process

   https://doi.org/10.1115/MSEC2022-85747  

   Guo, Zipeng; Fei, Fan; Song, Xuan; Zhou, Chi (June 2022, ASME 2022 17th International Manufacturing Science and Engineering Conference)

   Abstract As a facile and versatile additive manufacturing technology, direct ink writing (DIW) has attracted considerable interest in academia and industry to fabricate three-dimensional structures with unique properties and functionalities. However, so far, the physical phenomena during the DIW process are not revealed in detail, leaving a research gap between the physical experiments and the underlying theories. Here, we presented a comprehensive simulation study of non-Newtonian ink flow during the DIW process. We used the computational fluid dynamics (CFD) method and revealed the shear-thinning behavior during the extrusion process. Different nozzle geometry models were adopted in the simulation. The advantages and drawbacks of each syringe-nozzle geometry were analyzed. In addition, the ink shear stress and velocity fields were investigated and compared in the case studies. Based on these investigations and analysis, we proposed an improved syringe-nozzle geometry towards high-resolution DIW. Consequently, the high-resolution and high shape fidelity DIW could enhance the DIW product performance. The results developed in this work offer valuable guidelines and could accelerate further advancement of DIW. 

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2. Machine Learning-Based Modeling of Electric-Field-Assisted Direct Ink Writing (EDIW) Process

   https://doi.org/10.1115/MSEC2023-106095  

   Chen, Yinong; Deshpande, Anupam Ajit; Joyee, Erina Baynojir; Pan, Yayue (June 2023, American Society of Mechanical Engineers)

   Abstract Direct ink writing (DIW) is an extrusion-based additive manufacturing technology. It has gained wide attentions in both industry and research because of its simple design and versatile platform. In electric-field-assisted Direct Ink Writing (eDIW) processes, an external electric field is added between the nozzle and the printing substrate to manipulate the ink-substrate wetting dynamics and therefore optimize the ink printability. eDIW was found effective in printing liquids that are typically difficult to print in the conventional DIW processes. In this paper, an eDIW process modeling system based on machine learning (ML) algorithms is developed. The system is found effective in predicting eDIW printing geometry under varied process parameter settings. Image processing approaches to collect experiment data are developed. Accuracies of different machine learning algorithms for predicting printing results and trace width are compared and discussed. The capabilities, applications and limitations of the presented machine learning-based modeling approach are presented. 

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3. Effects of Electrostatic Force on Conducting Polymer Jets in Electric Field-Assisted Direct Ink Writing

   https://doi.org/10.1115/1.4069220  

   Wang, Xinnian; Kim, Yongil; Bandini, Vitor; Pan, Yayue; Yarin, Alexander L (October 2025, Journal of Manufacturing Science and Engineering)

   Abstract The electronics industry is rapidly advancing toward the development of highly miniaturized sensors and circuits, driving an increasing demand for precise, localized manufacturing techniques. Extrusion-based additive manufacturing—particularly direct ink writing—has emerged as a promising method for fabricating microscale electronic components. Recent efforts have focused on producing fine-resolution structures capable of conformal deposition on complex or uneven surfaces. While prior studies have established theoretical models for the trajectory of non-conductive material jets under electric fields—demonstrating feasibility in printing high-resolution features—a theoretical framework for conductive ink behavior under similar conditions remains lacking. This study introduces a theoretical model to describe the behavior of conductive jet extrusion under varying electrostatic forces. The model is validated through high-speed physical and manufacturing experiments using poly(3,4-ethylene-dioxythiophene)-based ink. The results demonstrate that the application of an external electric field significantly broadens the printable window, enabling: (i) high-speed printing up to 1.7 m/s with successful deposition on rough textile substrates (average surface roughness Ra = 8 µm), and (ii) the formation of micro-sized lines with widths as small as ∼60% of the nozzle's inner diameter (e.g., 300 µm-wide lines printed using a 500 µm diameter nozzle). 

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4. Go with the flow: Rheological requirements for direct ink write printability

   https://doi.org/10.1063/5.0155896  

   Wei, Peiran; Cipriani, Ciera; Hsieh, Chia-Min; Kamani, Krutarth; Rogers, Simon; Pentzer, Emily (September 2023, Journal of Applied Physics)

   The rapid development of additive manufacturing, also known as three-dimensional (3D) printing, is driving innovations in both industry and academia. Direct ink writing (DIW), an extrusion-based 3D printing technology, can build 3D structures through the deposition of custom-made inks and produce devices with complex architectures, excellent mechanical properties, and enhanced functionalities. A paste-like ink is the key to successful printing. However, as new ink compositions have emerged, the rheological requirements of inks have not been well connected to printability, or the ability of a printed object to maintain its shape and support the weight of subsequent layers. In this review, we provide an overview of the rheological properties of successful DIW inks and propose a classification system based on ink composition. Factors influencing the rheology of different types of ink are discussed, and we propose a framework for describing ink printability using measures of rheology and print resolution. Furthermore, evolving techniques, including computational studies, high-throughput rheological measurements, machine learning, and materiomics, are discussed to illustrate the future directions of feedstock development for DIW. The goals of this review are to assess our current understanding of the relationship between rheological properties and printability, to point out specific challenges and opportunities for development, to provide guidelines to those interested in multi-material DIW, and to pave the way for more efficient, intelligent approaches for DIW ink development. 

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5. Direct Ink Write 3D Printing of Fully Dense and Functionally Graded Liquid Metal Elastomer Foams

   https://doi.org/10.1002/adfm.202410908  

   Pak, Spencer; Bartlett, Michael_D; Markvicka, Eric_J (September 2024, Advanced Functional Materials)

   Abstract Liquid metal (LM) elastomer composites offer promising potential in soft robotics, wearable electronics, and human‐machine interfaces. Direct ink write (DIW) 3D printing offers a versatile manufacturing technique capable of precise control over LM microstructures, yet challenges such as interfilament void formation in multilayer structures impact material performance. Here, a DIW strategy is introduced to control both LM microstructure and material architecture. Investigating three key process parameters–nozzle height, extrusion rate, and nondimensionalized nozzle velocity–it is found that nozzle height and velocity predominantly influence filament geometry. The nozzle height primarily dictates the aspect ratio of the filament and the formation of voids. A threshold print height based on filament geometry is identified; below the height, significant surface roughness occurs, and above the ink fractures, which facilitates the creation of porous structures with tunable stiffness and programmable LM microstructure. These porous architectures exhibit reduced density and enhanced thermal conductivity compared to cast samples. When used as a dielectric in a soft capacitive sensor, they display high sensitivity (gauge factor = 9.0), as permittivity increases with compressive strain. These results demonstrate the capability to simultaneously manipulate LM microstructure and geometric architecture in LM elastomer composites through precise control of print parameters, while maintaining geometric fidelity in the printed design. 

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