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 threedimensional (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.

Nanoparticles (NPs) are materials considered to be 1–100 nm in size and are available in different dimensional shapes, geometrical sizes, physical morphologies, mechanical robustness, and chemical compositions. Irrespective of the dimensions (i.e., zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D)), NPs have a tendency to become entangled together, forming aggregations due to high attraction, making it hard to realize their full potential from their ordered counterparts. Many challenges exist to attain high-quality stabilized dispersion and long-range ordered assembly of NPs. Three-dimensional printing (3DP), also known as additive manufacturing (AM), is a technique dependent on layer-by-layer material addition for building 3D structures and encompasses a few categories based on the feedstock material types and printing mechanisms. One benefit from the 3DP procedures is their capability to produce anisotropic microstructural/nanostructural characteristics for desired mechanical reinforcement, transport phenomena, energy management, and biomedical implants. This paper briefly overviews relevant 3DP methods with an embedded nature to assemble nanoparticles without interference with external fields (e.g., magnetic or electrical). Our focus is the shear-field-induced nanoparticle alignment, covering material jetting-, electrohydrodynamic-, filament melting-, and ink writing-based 3DP. A concise summary of photopolymerization and its “optical tweezer” effects on nanoparticle confinement also inspires creative approaches in generating ordered nanostructures. The nanoparticles and polymers involved in this review are diverse, consisting of metallic, ceramic, and carbon nanoparticles in matrices or on surfaces of varying macromolecules. A short statement of challenges (e.g., low resolution, slow printing speed, limited material options) for 3DP-enabled nanoparticle orders provides some perspectives toward the enormous potential of 3DP in directing NPs assembly and fabricating high-performance polymer/nanoparticle composites.

No abstract

Anisotropic polymer/nanoparticle composites display unique mechanical, thermal, electrical, and optical properties depending on confirmation and configuration control of the composing elements. Processes, such as vapor deposition, ice-templating, nanoparticle self-assembly, additive manufacturing, or layer-by-layer casting, are explored to design and control nanoparticle microstructures with desired anisotropy or isotropy. However, limited attempts are made toward nanoparticle patterning during continuous fiber spinning due to the thin-diameter cross section and 1D features. Thus, this research focuses on a new patterning technique to form ordered nanoparticle assembly in layered composite fibers. As a result, distinct layers can be retained with innovative tool design, unique material combinations, and precise rheology control during fiber spinning. The layer multiplyingenabled nanoparticle patterning is demonstrated in a few material systems, including polyvinyl alcohol (PVA)-boron nitride (BN)/PVA, polyacrylonitrile (PAN)-aluminum (Al)/PAN, and PVA-BN/graphene nanoplatelet (GNP)/ PVA systems. This approach demonstrates an unprecedentedly reported fiber manufacturing platform for well-managed layer dimensions and nanoparticle manipulations with directional thermal and electrical properties that can be utilized in broad applications, including structural supports, heat exchangers, electrical conductors, sensors, actuators, and soft robotics.

From wool to Kevlar, one‐dimensional (1D) fiber has experienced the transition from clothing materials to structural applications in the past centuries. However, the recent advancements in tooling engineering and manufacturing processes have attracted much attention from both academia and industry to fabricate novel, versatile fibers with unique microstructures and unprecedented properties. This mini‐review focuses on the fabrication techniques of porous, coaxial, layer‐by‐layer and segmented fibers with continuous solution and melt fiber spinning methods. In each section of this review article, the unique structure‐related applications, including intelligent devices, healthcare devices, energy storage systems, wearable electronics and sustainable products, are discussed and evaluated. Finally, the combination of additive manufacturing for 1D fiber patterning in two‐dimensional and three‐dimensional devices, in addition to challenges in the reviewed fiber microstructures, is briefly introduced in the conclusion. © 2021 Society of Industrial Chemistry.