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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. 

Solid–liquid composites (SLCs) combine the properties of solids and liquids, enhancing functionalities and expanding potential applications. Traditional methods for creating SLCs often face challenges such as low mass transfer efficiency, difficulty in controlling separation behavior, and substantial waste production. Herein, we report a new approach to solve these challenges by using disulfide-based responsive polymeric capsule shells to make liquid-filled monoliths for carbon capture. The capsules are prepared through interfacial polymerization and contain either non-polar poly(α-olefin)432 or highly polar 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][TFSI]) at 74–82 wt%. Upon gentle heating, the dynamic disulfide bonds of the isolated capsules undergo bond exchange, leading to the fusion of capsule shells into free-standing monoliths that retain >89 wt% of their liquid core and remain stable for at least two weeks. These monoliths demonstrate CO2 absorption rates and capacities comparable to their capsule counterparts; further, in response to radiofrequency (RF), they reach the CO2 desorption temperature in only ∼31 s. This innovative system addresses the limitations of conventional SLC fabrication techniques, offering a versatile and practical approach to fusing polymer capsule shells for applications across separation, energy storage, and carbon capture applications.