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3D printing is a versatile technology for creating objects with custom geometries and compositions and is increasingly employed for fabricating hybrid solid–liquid composites (SLCs). These composites, comprising solid matrices with integrated liquid components, showcase unique properties such as enhanced flexibility and improved thermal and electrical conductivities. This review focuses on methods to fabricate SLCs directly by different 3D printing techniques, e.g. without needing to backfill or impregnate a porous matrix. The techniques of extrusion, vat photopolymerization and material jetting combined with microfluidics, inkjet printing, vacuum filling and ultraviolet light curing to produce SLCs are emphasized. We also discuss the development of feedstocks, focusing on emulsions and polymer capsules as fillers, and analyze current literature to highlight their significance. The review culminates in a perspective on new directions, highlighting the potential of bicontinuous interfacially jammed emulsion gels (bijels) to facilitate the printing of continuous liquid pathways, alongside the importance of understanding ink formulation and stability. Concluding with future perspectives, we underline the transformative impact of 3D‐printed SLCs in diverse applications, signaling a significant advancement in the field. 

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. 

Three-dimensional printing (3DP) of functional materials is increasingly important for advanced applications requiring objects with complex or custom geometries or prints with gradients or zones with different properties. A common 3DP technique is direct ink writing (DIW), in which printable inks are comprised of a fluid matrix filled with solid particles, the latter of which can serve a dual purpose of rheology modifiers to enable extrusion and functional fillers for performance-related properties. Although the relationship between filler loading and viscosity has been described for many polymeric systems, a thorough description of the rheological properties of three-dimensional (3D) printable composites is needed to expedite the creation of new materials. In this manuscript, the relationship between filler loading and printability is studied using model paraffin/photopolymer composite inks containing between 0 and 73 vol. % paraffin microbeads. The liquid photopolymer resin is a Newtonian fluid, and incorporating paraffin microbeads increases the ink viscosity and imparts shear-thinning behavior, viscoelasticity, and thixotropy, as established by parallel plate rheometry experiments. Using Einstein and Batchelor's work on colloidal suspension rheology, models were developed to describe the thixotropic behavior of inks, having good agreement with experimental results. Each of these properties contributes to the printability of highly filled ([Formula: see text]43 vol. % paraffin) paraffin/photopolymer composite inks. Through this work, the ability to quantify the ideal rheological properties of a DIW ink and to selectively control and predict its rheological performance will facilitate the development of 3D printed materials with tunable functionalities, thus, advancing 3DP technology beyond current capabilities. 

A modular platform for 3D printing fluid-containing structures is reported. Pickering emulsion-templated fluid-filled polymeric capsules were synthesized and incorporated into viscous liquids to produce inks for direct ink writing. Printed objects could be cured by solvent removal or irradiation with ultraviolet light to give monolithic structures containing capsules of fluid, with porosity dependent upon the curing method. 

Abstract We report the facile synthesis and 3D printing of a series of triblock copolymers consisting of soft and hard blocks and demonstrate that alkene pendant groups of the hard block can be covalently modified. The polymers are prepared using a salenCo(III)TFA/PPNTFA binary catalyst system and 1,2‐propanediol as a chain transfer agent, providing an efficient one‐pot, two‐step strategy to tailor polymer thermal and mechanical properties. Thixotropic inks suitable for direct ink write printing were formulated by dissolving the block copolymers in organic solvent and dispersing NaCl particles. After printing, porous structures were produced by removing solvent and NaCl with water to give printed structures with surfaces that could be modified via UV‐initiated thiol‐ene click reactions. Alternatively, a tetra‐thiol could be incorporated into the ink and used for cross‐linking to give objects with high solvent resistance and selective degradability.