Newtonian Liquid-Assisted Material Extrusion 3D Printing: Progress, Challenges and Future Perspectives

Additive manufacturing has grown exponentially, led by the three-dimensional (3D) printing of materials ranging from plastics and ceramics to metals. 3D printing methods are categorized into different processes such as vat photopolymerization, material jetting, binder jetting, powder bed fusion, material extrusion, directed energy deposition, and sheet lamination, which require inks in different forms and with different rheological properties. Material extrusion has emerged as a popular and versatile method owing to its ability to print different materials. However, material extrusion in air requires ink with specific rheological characteristics such as shear thinning and finite yield stress (a Bingham plastic fluid). In recent years, material extrusion in embedding media possessing Newtonian or non-Newtonian characteristics has attracted interest for the fabrication of micro- to centimeter-scale objects in a layer-by-layer or freeform manner. The use of embedding media offers 3D printing of low-viscosity and slow-curing inks that are unsuitable for material extrusion in air. Different types of non-Newtonian (e.g., microparticulate gels and thixotropic polymer solutions) and Newtonian (e.g., water and ethanol) embedding media have been used. In this review, we focus on the existing research developments in Newtonian liquid-assisted material extrusion of soft materials, such as polyelectrolytes, hydrogels, and polymer solutions. We compared the advantages and disadvantages of non-Newtonian and Newtonian embedding media. The review begins with the role of liquid media in the interfacial stability and solidification or gelation of printed ink by different mechanisms, followed by the effect of printing parameters on print fidelity in detail. In addition, we provide a brief overview of various material systems used as Newtonian liquid-embedding media. Finally, this review highlights the challenges of this method, followed by future perspectives on the possible innovations and strategies.