Abstract:The mechanical properties of ordinary materials degrade substantially with reduced density, due to the bending of their structural elements under applied load. We report a class of micro-architected materials that maintain a nearly constant stiffness per unit mass density, even at ultra-low density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with constituent materials ranging from polymers to metals and ceramics, is made possible by using projection microstereolithography, an additive micromanufacturing technique, combined with nanoscale coating and postprocessing. We found that these materials exhibit ultra-stiff properties across more than three orders of magnitude in density, regardless of the constituent material. One Sentence Summary:We report a class of micro-architected materials that change their stiffness linearly with reduced density.Main Text: Nature has found a way to achieve mechanically efficient materials by evolving cellular structures. Natural cellular materials, including honeycomb (1) (wood, cork) and foamlike structures, such as trabecular bone (2), plant parenchyma (3), and sponge (4), combine low weight with superior mechanical properties. For example, lightweight balsa has a stiffness-toweight ratio comparable to that of steel along the axial loading direction (5). Inspired by these naturally occurring cellular structures, manmade lightweight cellular materials fabricated from a wide array of solid constituents are desirable for a broad range of applications including structural components (6, 7), energy absorption (8, 9), heat exchange (10, 11), catalyst supports (12), filtration (13,14), and biomaterials (15,16). However, the degradation in mechanical properties can be drastic as density decreases (17,18). A number of examples among recently reported low-density materials include graphene elastomers (19), metallic micro-lattices (20), carbon nanotube foams (21), and silica aerogels (22,23). For instance, the Young's modulus of low-density silica aerogels (22, 23) decreases to 10 kPa (10 -5 % of bulk ) at a density of less than 10 mg/cm 3 (< 0.5% of bulk). This loss of mechanical performance is because most natural and engineered cellular solids with random porosity, particularly at relative densities less than 0.1%, exhibit a quadratic or stronger scaling relationship between Young's modulus and density as well as between strength and density. Namely, E/E s ~ (/ s ) n and  y  ys ~ (/ s ) n , where E is Young's modulus,  is density,  y is yield strength, and s denotes the respective bulk value of the solid constituent material property. The power n of the scaling relationship between relative material density and the relative mechanical property depends on the material's microarchitecture. Conventional cellular foam materials with stochastic porosity are known to...

Flexible, stretchable, and spanning microelectrodes that carry signals from one circuit element to another are needed for many emerging forms of electronic and optoelectronic devices. We have patterned silver microelectrodes by omnidirectional printing of concentrated nanoparticle inks in both uniform and high-aspect ratio motifs with minimum widths of approximately 2 micrometers onto semiconductor, plastic, and glass substrates. The patterned microelectrodes can withstand repeated bending and stretching to large levels of strain with minimal degradation of their electrical properties. With this approach, wire bonding to fragile three-dimensional devices and spanning interconnects for solar cell and light-emitting diode arrays are demonstrated.

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.

Pen‐on‐paper flexible electronics are fabricated using a conductive silver ink‐filled rollerball pen. This approach provides a low‐cost, portable route for fabricating conductive text, electronic art, interconnects for light emitting diode (LED) arrays, and three‐dimensional (3D) antennas on paper.

Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low density, exceptional mechanical properties, thermal stability, large surface area, and excellent electrical conductivity. Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications. However, these three-dimensional (3D) graphenes exhibit physicochemical property attenuation compared to their 2D building blocks because of one-fold composition and tortuous, stochastic porous networks. These limitations can be offset by developing a graphene composite material with an engineered porous architecture. Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing. The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing. The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightweight, highly conductive, and exhibit excellent electrochemical properties. In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g(-1)) and power densities (>4 kW·kg(-1)) that equal or exceed those of reported devices made with electrodes 10-100 times thinner. This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

Increasingly stringent design constraints imposed by compact wireless devices for telecommunications, defense, and aerospace systems require the miniaturization of antennas. Unlike most electronic components, which benefi t from decreased size, antennas suffer limitations in gain, effi ciency, system range, and bandwidth when their size is reduced below a quarter-wavelength. The electrical size of the antenna is measured by its ka value, where k is the wavenumber ( k = 2 π / λ , λ = wavelength at the operating frequency) and a is the radius of the smallest sphere that circumscribes the antenna. Antennas are considered to be electrically small when ka ≤ 0.5. Recent attention has been directed towards producing radiofrequency identifi cation (RFID) antennas by screen-printing, [ 1 ] inkjet printing, [ 2 ] and liquid metalfi lled microfl uidics [3][4][5] in simple motifs, such as dipoles and loops. However, these fabrication techniques are limited in both spatial resolution and dimensionality; yielding planar antennas that occupy a large area relative to the achieved performance.Omnidirectional printing of metallic nanoparticle inks offers an attractive alternative for meeting the demanding form factors of 3D electrically small antennas (ESAs). We have previously demonstrated that fl exible, stretchable, and spanning silver microelectrodes with features as small as ∼ 2 μ m can be patterned both in and out-of-plane (e.g., spanning arches) on fl at substrates by this approach. [ 6 ] Here, for the fi rst time, we demonstrate the conformal printing of silver nanoparticle inks on curvilinear surfaces to create electrically conductive meander lines. When interconnected with a feed line and ground plane, the resulting 3D ESAs exhibit performance properties that nearly match those predicted theoretically for these optimized designs.Antennas act as effective transducers between free space and guided waves over a range of frequencies, known as their impedance bandwidth. The impedance of most small antennas can be approximated by a single resistor-inductor-capacitor (RLC) circuit, and their bandwidth is inversely proportional to their radiation quality factor ( Q ), defi ned as the ratio of energy stored to energy radiated. [ 7 ] Because of this inverse relationship, a low Q serves to increase bandwidth, and therefore the data rate over a given wireless channel. However, a fundamental relation exists between the antenna size and Q . [ 8 , 9 ] As the maximum dimension decreases below a wavelength, the bound on the minimum attainable Q rapidly increases, a phenomenon commonly referred to as Chu's limit -an important fi gure of merit for antenna performance. While Thal recently derived a more accurate bounding limit, [ 10 ] both limits depend on the electrical size of the antenna. For decades, researchers have sought antenna designs that approach these fundamental lower limits. However, prior efforts, such as those based on genetic algorithms [ 11 , 12 ] lack fl exibility, since their output cannot be modifi ed in a straightfor...

Capacitance loss with the increase of mass loading represents an outstanding challenge for supercapacitors. Here we demonstrate for the first time a mm-thick, 3D printed graphene aerogel structure that can support pseudocapacitive MnO 2 to hundreds of mg/cm 2 without sacrificing its gravimetric and volumetric performance. The electrode simultaneously achieves high gravimetric, areal, and volumetric capacitances, which is impossible for conventional bulk electrodes. Most importantly, these findings validate the new concept of ''printing'' practically feasible pseudocapacitor electrodes and devices.

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