Graphene is a two-dimensional material that offers a unique combination of low density, exceptional mechanical properties, large surface area and excellent electrical conductivity. Recent progress has produced bulk 3D assemblies of graphene, such as graphene aerogels, but they possess purely stochastic porous networks, which limit their performance compared with the potential of an engineered architecture. Here we report the fabrication of periodic graphene aerogel microlattices, possessing an engineered architecture via a 3D printing technique known as direct ink writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit supercompressibility (up to 90% compressive strain). Moreover, the Young's moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications.

We present a new class of architected materials that exhibit rapid, reversible, and sizable changes in effective stiffness.

Three-dimensional printing of multi-material parts relies upon efficient mixing of the ink components and a rapid response to composition changes. However, at low Reynolds numbers and large Peclet numbers, mixing disparate viscosity and density inks poses a challenge. In this study, the performance of active micromixers for disparate non-Newtonian inks is evaluated using both This article is protected by copyright. All rights reserved.2 experiments and computational fluid dynamics simulations. The mixing efficiencies are compared with scaling relationships for active micromixers. Using detailed simulation results, multiple factors are identified that can impact the micromixer response time during a composition change. Lastly, an active micromixer is proposed and evaluated to efficiently mix arbitrary multi-material ink compositions and produce fine composition gradients within printed parts.

The mixing of materials during additive manufacturing is a major benefit which allows one to compositionally and spatially tailor material properties, for example to locally control the reactivity in fuel: oxidizer systems known as thermites. This work characterizes an active mixing printhead used in conjunction with a 3D printing process known as Direct Ink Writing. Besides compositional control, a major benefit of this approach is that it offers a safe method for working with these materials which can otherwise be hazardous once mixed. Custom fuel and oxidizer inks are fed at fixed volumetric rates into an active mixing head, and both the rotational speed of the mixing impeller as well as the fuel:oxidizer ratio are varied. Upon ignition, the propagation speed increases with the rotational speed of the mixer and plateaus above a critical value of approximately 750 RPM. The critical mixing speed is corroborated by computational fluid simulations and an analytical expression that considers the inks' complex fluid behavior. Additionally, varying the composition results in a wide range of propagation speeds with peak reactivity corresponding to a fuel-rich formulation ( = 1.5). A test article incorporating a fast and slow-burning region demonstrates how spatial composition can manipulate the reactivity. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

multimaterial DIW printer uses two separate ink dispensers joined at a single nozzle junction to simultaneously 3D print two viscoelastic inks through a single nozzle. Some models have been established to help control the printing of viscoelastic inks, but they have not been applicable for use in structures with complex compositional gradients. [18,22,23] As illustrated in Figure 1A, the toolpath of the printer is directly coupled with a desired composition (Φ) in order to determine the dispensing rate of each material. A distinction between the programmed dispensing rate of an ink (Q i ) and the measured flow rate out of the nozzle (Q Ni ) must be made when considering how to obtain an accurate compositional profile. Ideally, the increases and decreases in the measured ink flow rate synchronously follow the programmed dispensing rate. In reality, a number of factors introduce significant differences between the ideal ink dispensing rates and the realized outputs. Achieving accurate compositional 3D printing involving gradient transitions requires that we account for more complex behaviors, such as hydraulic compliance and non-Newtonian ink response, that result in significant differences between the programmed Q i and the realized Q Ni ( Figure 1B). For binary compositional changes within the same component, a combination of ink retraction and over extrusion can be used to account for these nonidealities. [22] For gradient compositional changes, the microfluidic circuit analogy (MCA) can be used to model and inform the ink dispensing profile that is required to achieve accurate compositional control ( Figure 1C).Here, we first describe the MCA model that is used to prescribe the ink-dispensing profile for improved compositional deposition accuracy in 3D-printed geometries with compositional gradients. We outline a calibration procedure to extract the MCA model parameters, which are then used in the MCA model to quantitatively guide the ink-dispensing profile for the desired compositional gradients to be printed. We finally validate the model in a DIW system using viscoelastic polydimethylsiloxane (PDMS) inks with time-dependent compositional changes. The ability to print multiple materials with accurate compositional profiles in a programmable manner enables the fabrication of new functional materials that were previously inaccessible.We use a model based on the microfluidic circuit analogy to determine the correct ink dispensing profiles required for 3D printing of structures with compositional gradients requires accurate dispensing control to achieve desired profiles. Here, empirical data are used with a model based on the microfluidic circuit analogy (MCA) to project dispense rate profiles that yield improved compositional accuracy in the printed part. Since minor variation in the experimental setup for each printing session can result in significant changes, a calibration procedure is developed to measure the system response. This calibration enables the extraction of the empirical MCA model parameters speci...

shown that architecture could be used to manipulate the reactivity of aluminum/ copper oxide (Al/CuO) thermites by tailoring the flow of gases and entrained particles using structure. [23] A similar behavior had previously been seen in porous-Sibased energetic materials. [21] This is an exciting result in that it enables one to tailor the energy release rate in such materials without defaulting to the conventional approach of changing the formulation.To further expand upon the previous results, we seek to develop AM formulations which can enable a wide range of architectures to be printed. With this, we can design test articles to further understand and quantify to what extent architecture can be used to control reactivity. However, the direct printing of thermite raises safety concerns since, as-mixed, the materials can lead to a rapid reaction in the event of accidental ignition. A far safer approach would be to formulate the precursor materials separately, and then to directly mix during the printing process.In this work, we formulated an Al and a CuO precursor ink separately. One factor in the choice of these precursors was to pick two systems that could be formulated with similar rheological properties, as disparate rheological properties could lead to potential issues during mixing operations. Al and CuO powder feedstock materials were formulated using micron-sized particles of the materials incorporated into an aqueous hydrogel matrix to render an extrudable prethermite "ink." The rheology of these high solids loaded prethermite inks had to be such that the inks could be extruded through a nozzle (i.e., shear thinning) as wet filaments. The formulation parameters can be seen in Table 1, and some considerations are discussed later in the text. Once formulated, the inks were loaded into a syringe and mounted on the printer. A schematic of the printing setup is shown in Figure 1. The basic components of this setup are highlighted, and include two mounts for syringes, linear extrusion motors, and a precise xyz positioning stage (Aerotech). After extrusion, parts are dried in air and the as-deposited printed filaments retain the properties possessed by the dry powder feedstock material. The formulation and printability of the individual materials are investigated; we subsequently show that the two materials can be mixed on-the-fly to render an ignitable thermite ink.Additive manufacturing (AM) has recently shown great promise as a means to tailor a wide range of material properties, both quasi-static and dynamic. An example of controlling the dynamic behavior is to tailor the chemical energy release rate in composite energetic materials such as thermiteswhich are a subset of pyrotechnics that use a metal fuel and a metal oxide as an oxidizer. Since these materials are most hazardous once finely mixed, the approach taken here is to formulate the fuel and oxidizer separately such that they can be mixed on-the-fly. Herein, the development, formulation, and characterization of two respective aqueous 3D printable in...

Figure 1: The four-(black), fi ve-(white) and six-(grey) connected (4 4 .6 2)(4 6 .6 4)(4 8 .6 6 .8) network of 3 Two MOFs were prepared from zinc(II) sulphate and 1,3,5benzenetricarboxylic acid (H 3 BTRI) using the same method although the concentration of starting materials in the solvent system was varied. [Zn 6 (μ 3-(OH) 2 (BTRI) 4 (DMF) 2.5 (H 2 O) 2 ]·[Zn(H 2 O) 3 (DMF) 3 ]·3.1H 2 O [1] and [Zn 2 (HBTRI)(BTRI)(H 2 O) 3 ]·DMA·3H 2 O [2] are both anionic networks. The counter ions are located in channels in the structure. Scanning electron microscopy (SEM) reveals that 2 "self-heals" upon dehydration and rehydration. 1 demonstrates the six-and threeconnected (4.6 2)(4 2 .6 10 .8 3)-sab net while 2 demonstrates the fi ve-and three-connected (6 3)(6 10)-kdd net 2. Topological analysis allows for a simple comparison between different MOFs.

In this paper, inks for transparent elastomers that are formulated by matching the refractive index of silica and polysiloxanes are described. The transparent inks transmit up to ≈90% of 700 nm light through 1 cm and remain transparent when solidified. The inks and solidified materials exhibit a thermochromic effect. This thermochromic effect can be controlled by the refractive index mismatch. Transparency may increase or decrease as temperature increases, depending on the refractive index mismatch of the base polysiloxane and silica. It is found that the rheological properties of the ink depend on the distribution of silica particles, which is dictated by silica functionality, weight content, and processing. Siloxane precursors that vary in chemical functionality are introduced to tailor the mechanical properties of the printed elastomers, which obtain stretchability >65% along with a tensile modulus of 1.9 MPa. After optimizing siloxane chemistry and ink processing, the authors are able to print transparent elastomers. Potential applications are demonstrated for printed structures by printing encapsulation structures for light‐emitting diodes, semitransparent dye‐filled structures, a microfluidic mixing device, and a multimaterial structure that exhibits temperature‐dependent camouflage.