Review of Fiber-Based Three-Dimensional Printing for Applications Ranging from Nanoscale Nanoparticle Alignment to Macroscale Patterning

The field of additive manufacturing (AM) has witnessed spectacular growth in the past 4 decades because of its revolutionary processing mechanism in combining bottom-up and top-down approaches. Many have speculated that it will challenge traditional fabrication methods as the fourth industrial revolution. Among the subfields of three-dimensional (3D) printing, extrusion-based direct ink writing (DIW) is known for its vast material choices, high design flexibility, and acceptable cost efficiency to print many urgently demanded material systems, such as hydrogels or aerogels, nanoparticle suspensions, composite mixtures, liquid crystals, and liquid metals. Furthermore, the DIW's ability to construct complex architectures or hierarchies also contributes to broader applications across different fields, including intelligent robotics, energy generation and storage devices, biomedical implants, and sustainability systems. This review provides a comprehensive summary of recent advances in DIW development, focusing on engineered patterns at different scales, namely, nanoparticle alignment, one-dimensional (1D) fiber microstructure manipulation, and macroscale two-dimensional (2D)/3D spatial patterning. It highlights the hierarchies from nanoscale particle orientations to macroscale long-range-ordered structures. Finally, technical barriers and significant challenges prohibiting DIW for broader applications or impeding fundamental research to industrial commercialization are discussed.