An Overview of Silver Nanowire Polyol Synthesis Using Millifluidic Flow Reactors for Continuous Transparent Conductive Film Manufacturing by Direct Ink Writing

Fiber-polymer composites, consisting of the polymer matrix and functional synthetic/natural fibers, show tremendous promise due to their unique mechanical, thermal, and electrical properties. By printing such fiber-polymer composites using additive manufacturing (AM) techniques, the three-dimensional models can be fabricated with arbitrary free-form geometry and various functionalities. Direct ink writing (DIW) is an extrusion-based AM approach and has a diverse choice of printable matrix materials. Applications, including energy storage, mechanical reinforcements, and biomimetic materials, have been presented through DIW of fiber composites. Yet grand challenges such as the complicated process planning, low geometrical accuracy, filament shape instabilities, and minimal fiber extrusion capability still exist in the DIW of fiber-polymer composites. This dissertation aims to address these challenges in DIW of fiber-polymer composites and to develop a fundamental understanding of the complex interplay of ink properties, DIW printing process, and printed composite functionalities through testing the following hypotheses: Hypothesis I: The incorporation of an electric field between the printing nozzle and the substrate can induce an electrowetting effect to enlarge the printable ranges of materials and the working range of manufacturing process settings. Hypothesis II: The addition of water-washable gel can change the rheological properties of the fiber suspensions in a way that the shear-yielding and shear-thinning properties are enhanced to increase the printability of composites with higher geometrical accuracy. Hypothesis III: Sonication can remove the primary wall of natural fibers and enhances the printability of natural fibers in DIW while retaining its mechanical characteristics in the printed composite. The findings are beneficial to material science and manufacturing process, enabling significant improvements in DIW of fiber-polymer composites and future applications in functional devices fabrication in the following perspectives: a) Generalized mappings between the printed feature geometries and process parameters were developed. b) The influence of liquid ink rheological properties on printing stability was understood. c) Approaches for modifying synthetic/natural fiber for higher printability were established. d) Electric-field-assisted and temperature-controlled DIW process planning strategies were developed and evaluated. e) Novel applications of DIW printed fiber composites were demonstrated.

Direct Ink Writing (DIW), an extrusion-based 3D printing technique, offers a broad application space. As such, the technique continues to find use in biomedical, flexible electronic, ceramic, and energy device applications, among others. With this broad application space comes an expanding material library of inks with diverse rheological and microstructural properties. This begs the question: what constitutes a printable ink? How does one define printability? And how does one design for printability? Researchers currently have a broad understanding of what constitutes a printable ink. However, time and time again, inks with unique rheological properties and formulations are printed. Currently, ink synthesis and ink characterization for DIW take a linear analysis approach – one in which rheological and material specifications are an afterthought. To progress DIW while understanding the fundamental questions discussed above, it may be necessary to take a design approach. The design approach melds understanding of the yield stress fluid microstructure and resultant rheological properties, providing a wholistic view of the DIW fluid parameter space. Implementing design enables one to target rheological properties for a given fluid microstructure or implement new yield stress microstructures with predictable relevant rheological parameters. This ability ultimately accelerates DIW material implementation, reduces experimental time, and opens the door for novel microstructure exploration and ink development. In this work, we explore DIW printability through a series of cases studies which involve attractive glass and repulsion dominated yield stress fluids. Through the development of a DIW rheological database, we establish the utility of Ashby-like plots in transitioning DIW to a design-based engineering approach. Through the Ashby plots and the clustering of yield stress fluid microstructures, we investigate and identify why defining the concept of printability remains elusive. Ultimately, we propose microstructurally-dependent targeted rheological parameters and demonstrate the utility of design for DIW to accelerate DIW ink implementation.

Rheology and printability: A survey of critical relationships for direct ink write materials design

In this study, we conducted a comprehensive experimental campaign aimed at controlling the final properties of 3D printed cellulose acetate. We equipped a commercial printer with a peristaltic pump to be able to print in a continuous fashion by means of the Direct Ink Writing technique. We investigated the effect of ink concentration and printing parameters on the density, mechanical and functional properties of printed objects. Furthermore, water absorption tests demonstrated the hygroscopic behavior of cellulose acetate, with higher water content in samples with lower densities. The diffusion of water within the polymer network followed Fickian diffusion, with the diffusion coefficient influenced by the density of samples. Overall, this study highlights the importance of printing conditions in achieving desired properties in 3D printed cellulose acetate. The ability to fine‐tune the mechanical properties and water absorbance of 3D printed cellulose acetate makes it promising for applications in plant science and bioengineering.Highlights Cellulose acetate has been 3D printed via Direct Ink Writing. The shear‐thinning behavior allows for shape retention during printing. Density of printed samples is strongly controlled by printing parameters. Density of printed parts influences mechanical properties and water absorption.

Direct ink writing (DIW) of structural and functional ceramics: Recent achievements and future challenges

Advanced 3D-printed phase change materials

Hybridizing Direct Ink Write and mask-projection Vat Photopolymerization to enable additive manufacturing of high viscosity photopolymer resins

This study examines the fabrication and characterization of high‐strength‐to‐density ratio, short carbon fiber‐reinforced epoxy composite honeycomb structures using Direct Ink Writing (DIW) and engineered inks. The epoxy inks, with 10–40 wt% carbon fibers and other precursors, were tailored to desired viscoelastic and shear‐thinning properties for 3D printing, enabling precise DIW printing of bioinspired hexagonal honeycomb structures. These formulated inks in combination with DIW facilitated honeycomb production with variable cell sizes and geometries without the need for molds. DIW 3D printing ensured precise material deposition, controlled composition, superior surface finish, and shape fidelity when compared to conventional methods. The mechanical performance of these structures was tailored through varying the base material composition and by changing the infill densities from 30% to 50%. In‐plane compressive testing demonstrated nominal increases of 86% in stiffness and 204% in strength for specimens printed with 20 wt% carbon fibers as infill density increased from 30% to 50%. Specific stiffness and strength showed similar trends. Comparisons with scaling laws revealed higher‐than‐expected strength, exceeding values reported in published studies. These findings showcase DIW with engineered inks as a pathway to fabricate advanced honeycomb composite structures, enabling tailored designs and properties for innovative, lightweight, high‐strength structural design and applications.Highlights Developed carbon fiber‐reinforced viscoelastic inks (10–40 wt%) for DIW. Used viscoelastic inks with DIW to create honeycombs with diverse geometries. Attained superior print quality in honeycombs over PLA/fiber composites. Compression tests showed increased stiffness and strength with higher infill. Outstanding compressive performance exceeding strength expectations.

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in-depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects asaguideline toward possible futuristic innovations.

Poly(tetrafluoroethylene) (PTFE) is a unique polymer with highly desirable properties such as resistance to chemical degradation, biocompatibility, hydrophobicity, antistiction, and low friction coefficient. However, due to its high melt viscosity, it is not possible to three-dimensional (3D)-print PTFE structures using nozzle-based extrusion. Here, we report a new and versatile strategy for 3D-printing PTFE structures using direct ink writing (DIW). Our approach is based on a newly formulated PTFE nanoparticle ink and thermal treatment process. The ink was formulated by mixing an aqueous dispersion of surfactant-stabilized PTFE nanoparticles with a binding gum to optimize its shear-thinning properties required for DIW. We developed a multistage thermal treatment to fuse the PTFE nanoparticles, solidify the printed structures, and remove the additives. We have extensively characterized the rheological and mechanical properties and processing parameters of these structures using imaging, mechanical testing, and statistical design of experiments. Importantly, several of the mechanical and structural properties of the final-printed PTFE structures resemble that of compression-molded PTFE, and additionally, the mechanical properties are tunable. We anticipate that this versatile approach facilitates the production of 3D-printed PTFE components using DIW with significant potential applications in engineering and medicine.

There is an interest in soft robotics for various space applications within low gravity and tight space environments to navigate, observe and even service various components within space assets, spacecraft and planetary stations. As an example, in an emergency, such as a pipe system failure occurring in a spacecraft, a soft robot may be able to assist in navigating the structure to detect the location and means of failure. The manufacturing of soft robotic actuators in space can be crucial. In addition, the fabrication of soft robotic tooling in a single step plays a critical role in space. 3D printing systems allow the manufacturing of soft robotic actuator parts in a single step. Stereolithography (SLA) and digital light processing (DLP) systems are not convenient because of the space environment. However, direct ink writing (DIW) which is the most commonly available and affordable system compared to SLA or DLP can be utilized to produce 3D objects, in addition, the DIW process permits the ability to in situ produce composite structures during the printing process. Various inorganic particles can be inserted within the elastomer print to control the mechanical, electrical and magnetic properties on-demand to produce smart or multi-functional soft robotic actuators and structures.The current work focuses on the development of a highly flexible and low (or nearly no) cytotoxicity silicone-based elastomer via thiol-ene click reaction that allows rapid polymerization of an elastomer to prevent the collapse of the structure during the printing. The work also demonstrates the use of this novel UV-durable elastomer to produce structures with embedded electrical interconnects and sensors for in-situ measurement of temperature and strain. In this study Poly(mercaptopropylmethylsiloxane-co-dimethylsiloxane) (M-MPS) was first prepared through the cohydrolysis-condensation reaction of 3-Mercaptopropylmethyltrimethoxysilane (MPS) and dimethyldimethoxylsilane (DMDES) with water and hydrochloric acid. Then M-MPS was compounded with various molecular weight vinyl-terminated polydimethylsiloxane (VPS) (MW~800-25000) and different dosage (0.1- 4.0 wt%) of a photoinitiator (1:1 weight ratio of 2-Hydroxy-2-methyl-phenyl-propane-1-one (PI 1173) and phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide (PI819)). For example, Figure 1 depicts the change in the viscosity of (M-MPS)-VPS 6000 2 wt% when it exposes to UV light irradiation. When (M-MPS)-VPS 6000 2 wt% was exposed to UV light irradiation, it polymerized at ~2 sec which is enough time to retain the structure during printing. To elucidate the properties of synthesized elastomer, FTIR, NMR, SEM, and tensile test measurements were performed. Furthermore, the synthesized elastomer was loaded with conductive inorganic particles so as to utilize the printed material as a temperature and tactile sensor (and sensor array). In addition, other printing variables were evaluated to produce 2D/3D structures of the pure and composite elastomers, such as the effect of the power and wavelength of UV light source, loading different types of conductive powders, and thiol-ene ratio. Acknowledgments: Work supported by a subcontract under the NASA-EPSCOR program (project number is 80NSSC20M0218). The authors would like to thank our NASA technical monitor Mr. Curtis Hill and project manager Dr. Melanie Page due to their contributions. Material characterization and imaging work were made possible with the support of the West Virginia University Shared Research Facilities (SRF). Figure 1

In the healthcare industry, the selection of biocompatible materials suitable for 3D printing is markedly less extensive than what is typically available through conventional manufacturing processes. Liquid silicone rubber (LSR) is distinguished by its exceptional stability, excellent biocompatibility, and considerable flexibility, offering significant prospects for manufacturers of medical devices involved in 3D printing. The primary aim of this research is to examine the essential factors and their interconnections that affect the 3D printing process with a Direct Ink Writing (DIW) 3D printer, which is specifically tailored for the production of aortic heart valves made from UV-cured silicone. Additionally, this study aims to investigate how the size of the heart valve impacts cardiac performance. This study implements House of Quality (HOQ) and Interpretive Structural Modeling (ISM) techniques to evaluate the interrelations among the different factors identified in the 3D printing process. Liquid silicone is especially advantageous for Direct Ink Writing (DIW) due to its low-temperature curing properties and low viscosity, which enable precise printing for intricate designs. Two different sizes of aortic heart valves, namely 23 mm and 36 mm, will be manufactured using UV-cured silicone, with both sizes having the same leaflet thickness of 0.8 mm and 1.6 mm. An examination will be conducted to assess how the size of the valve influences its performance and functionality. A Mock Circulatory Loop experimental setup will be used to test the silicone-printed heart valves, focusing on their capacity to maintain unidirectional flow and inhibit backflow through the flexible leaflets that function in alignment with the cardiac cycle.

Fe-based amorphous alloy has excellent soft magnetic properties; traditionally, Fe-based amorphous alloy such as soft magnetic devices was fabricated by insulation enveloping and suppression molding methods. In this process, the aging of organic envelope materials and the crystallization of Fe-based amorphous alloy were usually occurring, accompanying with low magnetic induction and poor mechanical properties. The direct ink writing (DIW) technique can make complex-shaped parts and needs no heating treatment after forming, which can avoid the effect of traditional molding process. In the present study, varying mass fraction FeSiB/EP composite parts were prepared by the DIW technique with the Fe-based amorphous alloy powder and epoxy resin, in which microscopic morphology, magnetic properties, and mechanical properties of FeSiB/EP soft magnetic composites were studied. The results indicate that the slurry with iron powder mass fraction of 92.3, 92.6, and 92.8 wt% has good printing performance and self-support ability, which is suitable for DIW. The density of the printed parts is about 4.317, 4.449, and 4.537 g/cm3, which is almost similar with the iron powder. The tensile strength and elongation of printing parts are significantly improved compared with the pure epoxy resin. From the photos of microscopic morphology of printing parts, it can be seen that FeSiB powders are evenly dispersed in EP, no pores, and defects, with the proportion increasing of powders; the insulation coating thickness decreases; and the magnetic performance improves. The optimal sample is 92.8 wt% FeSiB/EP, in which saturation magnetic induction strength is 137.9759 emu/g and coercivity is 4.6523 A/m.

Direct ink writing of ATZ composites based on inks prepared by colloidal or hydrogel route: Linking inks rheology with mechanical properties

Additive manufacturing (AM) of high-performance structural ceramic components with comparative strength and toughness as conventionally manufactured ceramics remains challenging. Here, a UV-curing approach is integrated in direct ink writing (DIW), taking advantage from DIW to enable an easy use of high solid-loading pastes and multi-layered materials with compositional changes; while, avoiding drying problems. UV-curable opaque zirconia-based slurries with a solid loading of 51 vol% are developed to fabricate dense and crack-free alumina-toughened zirconia (ATZ) containing 3 wt% alumina platelets. Importantly, a non-reactive diluent is added to relieve polymerization-induced internal stresses, avoid subsequent warping and cracking, and facilitate the de-binding. For the first time, UV-curing assisted DIW-printed ceramic after sintering reveals even better mechanical properties than that processed by a conventional pressing. This is attributed to the aligned alumina platelets, enhancing crack deflection and improving the fracture toughness from 6.8 ± 0.3MPa m0.5 (compacted) to 7.4 ± 0.3MPa m0.5 (DIW). The four-point bending strength of the DIW ATZ (1009 ± 93MPa) is also higher than that of the conventionally manufactured equivalent (861 ± 68MPa). Besides homogeneous ceramic, laminate structures are demonstrated. This work provides a valuable hybrid approach to additively manufacture tough and strong ceramic components.