Direct Ink Write (DIW) is a material extrusion Additive Manufacturing (AM) technique that selectively extrudes material in a layer-by-layer fashion to form 3-dimensional parts. DIW can process a breadth of materials due to the inherent flexibility of its feedstock and the simplicity of its process mechanics (e.g., pressure-induced dispensing through a nozzle, no heat transfer, etc.). As a result, the process has attracted great interest from researchers aiming to develop novel materials for AM. The breadth of materials being introduced for DIW speaks to a need for guidelines for materials design and for a set of analytical characterization techniques that can be used to understand how inks will behave in the DIW process. The DIW process can be separated into three distinct sub-functions: extrusion, solidification, and layer support. Rheological experiments are the primary method used to characterize inks under conditions that mimic the three sub-functions of DIW and predict their printability, e.g. whether they will be successful in DIW printing. A broad range of rheological experiments has been used by researchers to characterize inks and judge their suitability for DIW and this review categorically compiles the predominant techniques. Then, a range of rheology-printability relationships including those derived from observed experimental trends and rigorous analytical models are discussed along with the usefulness and limitations of these relationships. Additional application specific concepts such as bridging and printability maps are further addressed in the context of rheological requirements. Finally, generalizable rheological guidelines of successful DIW inks are provided. This review collectively evaluates rheology-printability relationships in a generalized manner, independent of specific material chemistry or DIW process design. Significant insight into a comprehensive holistic approach for DIW ink development is provided. Lastly, the outlook of this research area is discussed to identify critical gaps in the fundamental understanding of both ink and process design, in the context of a rheological viewpoint. 

Rheology and Printability: A Survey of Critical Relationships for Direct Ink Write Materials Design

In the human body, almost all cells interact with extracellular matrices (ECMs), which have tissue and organ-specific compositions and architectures. These ECMs not only function as cellular scaffolds, providing structural support, but also play a crucial role in dynamically regulating various cellular functions. This comprehensive review delves into the examination of biofabrication strategies used to develop bioactive materials that accurately mimic one or more biophysical and biochemical properties of ECMs. We discuss the potential integration of these ECM-mimics into a range of physiological and pathological in vitro models, enhancing our understanding of cellular behavior and tissue organization. Lastly, we propose future research directions for ECM-mimics in the context of tissue engineering and organ-on-a-chip applications, offering potential advancements in therapeutic approaches and improved patient outcomes. 

Biofabrication methods for reconstructing extracellular matrix mimetics

In this review, today's state of the art in the rheology of gels and transition through the yield stress of yielding liquids is discussed. Gels are understood as soft viscoelastic multicomponent solids that are in the incomplete phase separation state, which, under the action of external mechanical forces, do not transit into a fluid state but rupture like any solid material. Gels can "melt" (again, like any solids) due to a change in temperature or variation in the environment. In contrast to this type of rheology, yielding liquids (sometimes not rigorously referred to as "gels", especially in relation to colloids) can exist in a solid-like (gel-like) state and become fluid above some defined stress and time conditions (yield stress). At low stresses, their behavior is quite similar to that of permanent solid gels, including the frequency-independent storage modulus. The gel-to-sol transition considered in colloid chemistry is treated as a case of yielding. However, in many cases, the yield stress cannot be assumed to be a physical parameter since the solid-to-liquid transition happens in time and is associated with thixotropic effects. In this review, special attention is paid to various time effects. It is also stressed that plasticity is not equivalent to flow since (irreversible) plastic deformations are determined by stress but do not continue over time. We also discuss some typical errors, difficulties, and wrong interpretations of experimental data in studies of yielding liquids. 

Rheology of Gels and Yielding Liquids

The rational design of mechanical metamaterials endows them with unusual properties inaccessible by conventional materials. In this study, compression–twist coupling building blocks with embedded chirality are fabricated by multimaterial 4D printing of shape memory polymers. Each chiral unit can be programmed individually by mechanical compression at a selected high temperature, whose temporary shape is self‐maintained at the reduced temperature when the load is removed. With these building blocks, lattice‐like architectures can be constructed in planar and layer‐to‐layer assemblies. Such modular design overcomes the limitation of conventional material arrangement by allowing on‐demand re‐programmability of chirality‐defined structural heterogeneity. Using this strategy, metamaterials with tunable Poisson's ratio and shape‐morphing metastructures are developed by harnessing the cooperative deformation of spatially heterogenous tessellation that undergoes local shrinkage or expansion, depending on the encoded Poisson response. An untethered multimodal soft gripper is assembled by chiral metastructures with distinct morphing modes, capable of unscrewing up a bottle cap attributed by the programmed functionality at module‐level. The present modular chiral metamaterials with embedded chirality and flexible deployability may promote the applications of mechanical metamaterials in active shape‐morphing structures and functional soft machines.

4D Printing of Chiral Mechanical Metamaterials with Modular Programmability using Shape Memory Polymer

Over the last two decades, microfluidic devices have rapidly emerged as revolutionary platforms for research and medicine. Various kinds of materials and fabrication strategies have been used to manufacture microfluidic devices; however, most of these strategies face challenges including complexity, time consumption, and cost in recreating delicate and intricate structural organizations. Herein, a silicone composite (Si-Co) was developed and employed as a functional support bath in which a sacrificial ink was 3D printed using embedded printing, enabling freeform fabrication of complex-shaped microfluidic devices. Si-Co was a soft material that had high optical transparency and tunable mechanical properties and yield stress with self-recovering ability. Sacrificial ink filaments formed inside the Si-Co support bath were found to be stable and circular with a wide range of resolution reaching up to 50 µm in channel size. The strategy was exemplified by fabricating microfluidic devices for generation of stable microgels of various sizes. To confirm biocompatibility of Si-Co, induced-pluripotent stem cell-derived endothelial cells were lined up inside microfluidic channels to obtain vascular mimics. Additionally, proof-of-concept spheroid fabrication devices were generated. Overall, the presented strategy allows for facile, rapid, cost-effective, and high-resolution printing and presents potential for the development of microfluidic devices for various applications, such as but not limited to organ-on-a-chip devices, 3D bioprinting and drug testing. 

3D Embedded Printing of Microfluidic Devices using a Functional Silicone Composite Support Bath

Embedded extrusion printing in yield-stress-fluid baths

Support bath-assisted additive manufacturing has been extensively employed in constructing 3D complex soft biomaterials by extrusion- or inkjet-based printing. However, the conventional support bath (i.e., slurry of gelatin microparticles used in freeform reversible embedding of suspended hydrogels and nanoclay) can only provide static in-situ support to the deposited constructs, which is not feasible for vat photopolymerization printing since the printed constructs are mobile during the printing process. A dynamic support bath assisted vat photopolymerization printing method using yield stress fluid (YSF) is proposed in this study, which enables the dynamic accompanying supporting for the printing structures. As the moving platform rises continually, the YSF bioink was elevated with the cured construct near the curing region and remaining in a solid-like state to provide support against gravity and surface tension. While the YSF bioink was in a liquid-like state, due to the atmospheric pressure difference during the rising of moving platform, the YSF bioink can flow to fill up the vacuum and ensure continuous forming. Thus, this printing method achieved the vat photopolymerization printing of extremely soft cell-laden bioinks (Young’s modulus of 4.3 KPa) into 3D complex constructs with excellent biological functionality. 

Photopolymerization Bioprinting with a Dynamic Support Bath

In this study, the test performance of high-resolution digital off-axis holography for the non-spherical droplet characteristics and spatial distribution in the primary breakup region of a pressure-swirl nozzle were experimentally investigated, with spatial resolution of 4.5μm and 3.0μm. The field of interest (FOI) was restricted within 9 mm downstream of the nozzle outlet, where the primary and some secondary breakup processes take place. The liquid investigated here is RP-3 kerosene, and the injection pressure and liquid temperature were controlled in the range of 0.5 ∼ 1.0 MPa and 250 ∼ 291 K separately. Clear spray morphology and deformed droplet image were obtained through the holographic reconstruction process. Droplet extraction criteria were proposed to distinguish deformed droplets from ligaments. The overall drop size and ellipticity distribution were quantified and discussed. The results show that the overall drop size distributions near the nozzle are also typical single-peaked curves. The proportions of non-spherical droplet increase as the fuel pressure and temperature increase. The spatial Sauter mean diameter (SMD) distributions of spray at different positions downstream of the nozzle were calculated, with the grid size of 1 × 1 × 1 mm3, showing the doughnut-like annular SMD and droplet number distribution profiles of the hollow conical spray. As the temperature and fuel pressure decrease, SMD distribution becomes uneven and larger. Fine droplets exist in the region where the perforation and primary breakup process occur, and the SMD increases over the axial distance due to the breakup process of ligaments near the nozzle. Results also prove that digital off-axis holography presented here is expected to realize the 3D viewing of the conical liquid sheet and statistical quantification of the droplet where primary and secondary breakup processes mainly occurred. 

Conical liquid sheet morphology and 3D droplet distribution of aviation kerosene pressure-swirl spray with digital off-axis holography

Ideal tissue engineering scaffolds need interconnected pores and high porosity to enable cell survival, migration, proliferation, and differentiation. However, obtaining a high-resolution structure is difficult with traditional one-temperature control fused deposition modeling (FDM). In this study, we propose a dual-temperature control method to improve printability. A numerical model is developed in which the viscosity is a function of temperature and shear rate to study the influence of two different temperature control modes. Quantitative tests are used to assess filament formation and shape fidelity, including one-dimensional filament printing, deposition at corners, fusion, and collapse. By using dual-temperature control, the width of the deposited poly(ε-caprolactone) filament is reduced to 50 μm. The comparative results of both the experimental method and numerical simulation suggest that the dual-temperature control FDM can manufacture spatially arranged constructs and presents a promising application in tissue engineering. 

https://link.springer.com/content/pdf/10.1007/s42242-023-00239-1.pdf

Numerical simulation and printability analysis of fused deposition modeling with dual-temperature control

Corona discharge occasionally occurs to the contact box of switchgear due to condensation under high humidity and pollution conditions, which may result in a switchgear insulation fault. In this article, we develop an experimental platform for the condensation of contact boxes in switchgear. A metallographic microscope is used to measure the size and distribution characteristics of dewdrops on the surface of the contact box under various temperature, humidity, and pollution conditions. A simulation model for the distribution of dewdrops is established using fractal theory, and the simulation results match the measured results well. The corona onset voltage is predicted and verified by the UV imager’s measurements by substituting the electric field distribution around the contact box with different dewdrops into the streamer inception criterion. The research results indicate that higher humidity and temperature differences result in a larger dewdrop and a shorter condensation formation time, resulting in a lower corona onset voltage. The effect law is similar under high pollution conditions, but the dewdrop is larger, the condensation formation time is longer, and the corona onset voltage is significantly lower than under clean conditions. As a result, high humidity and pollution conditions must be avoided during the operation of switchgear.

Corona Onset Characteristics of Contact Box in Switchgear With Condensation Under High Humidity and Pollution Conditions

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