Lightweight architectures

The ability to rapidly fabricate 3D mesoscale architectures with optimized topology and composition would enable myriad applications, including photonics,^{Reference Yablonovitch40–Reference Yablonovitch43} structural materials,^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8,Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44,Reference Compton and Lewis45} catalyst supports,^{Reference Chai, Yoon, Yu, Choi and Sung46,Reference Lu, Nitz, Comotti, Weidenthaler, Schlichte, Lehmann, Terasaki and Schuth47} filtration media,^{Reference Moorthy and Beebe48,Reference Li, Zong, Zhang, Yang, Zhang, Li and Wei49} and heat exchangers.^{Reference Boomsma, Poulikakos and Zwick50} Nature is replete with examples of complex 3D architectures, such as butterfly wings,^{Reference Saranathan, Osuji, Mochrie, Noh, Narayanan, Sandy, Dufresne and Prum51} wood,^{Reference Gibson52} bone,^{Reference Launey, Buehler and Ritchie53} beaks,^{Reference Miserez, Schneberk, Sun, Zok and Waite54} and the dactyl club (an appendage with a claw at the terminal end) that certain shrimp possess^{Reference Weaver, Milliron, Miserez, Evans-Lutterodt, Herrera, Gallana, Mershon, Swanson, Zavattieri, DiMasi and Kisailus55} to name a few, with hierarchical features seamlessly integrated across many length scales. Their remarkable properties, structural color^{Reference Saranathan, Osuji, Mochrie, Noh, Narayanan, Sandy, Dufresne and Prum51} or high specific stiffness and strength, arise in part from mesoscopic features,^{Reference Wegst, Bai, Saiz, Tomsia and Ritchie56} including compositional periodicity,^{Reference Saranathan, Osuji, Mochrie, Noh, Narayanan, Sandy, Dufresne and Prum51} gradients,^{Reference Miserez, Schneberk, Sun, Zok and Waite54} fiber alignment,^{Reference Li, Weaver and Ortiz57} and other forms of hierarchical complexity.^{Reference Launey, Buehler and Ritchie53} Together, these attributes have important implications for the macroscopic response and the scaling laws associated with each structure.^{Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44}

Inspired by nature, researchers have focused on creating synthetic analogues via self- and directed-assembly methods. Specifically, programmable methods, such as 3D printing, are being increasingly used to create architected materials, opening up an entirely new design space.^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8,Reference Sun, Wei, Ahn, Seo, Dillon and Lewis29,Reference Buckmann, Stenger, Kadic, Kaschke, Frolich, Kennerknecht, Eberl, Thiel and Wegener39,Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44,Reference Compton and Lewis45,Reference Greiner, Richter and Bastmeyer58–Reference Zhang, Toman, Yu, Biyikli, Kirca, Chmielus and To62} Simple structures fabricated from a single material can exhibit enhanced or even fundamentally new properties. At a fixed material and bulk density, the careful design of architecture can enable control over light propagation or the mode of mesoscopic deformation (e.g., bending versus stretching of mesoscopic beams). 3D printed architectures can also serve as templates for patterning materials.

Polymer architectures created by ink- and light-based 3D printing approaches can be transformed into semiconducting,^{Reference Gratson, Garcia-Santamaria, Lousse, Xu, Fan, Lewis and Braun4} metallic,^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8} and ceramic^{Reference Meza, Das and Greer60} structures that are difficult to directly print at these length scales.^{Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44} One elegant strategy is to conformally coat printed polymer architectures via gas-phase or solution-based techniques, such as chemical vapor deposition (CVD),^{Reference Gratson, Garcia-Santamaria, Lousse, Xu, Fan, Lewis and Braun4} atomic layer deposition,^{Reference Meza, Das and Greer60} or electroless plating.^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8} This strategy allows structural control over features at finer resolution than those achieved by the printing method used (Figure 2). For instance, hollow semiconducting, metallic, and ceramic lattices with ultrathin walls have been produced by conformal coating followed by etching away the underlying polymeric features. Both optical and mechanical properties can be dramatically tailored by this combined approach.

Figure 2.

Templated mesoscale architectures. (a) Scanning electron microscope (SEM) images of hollow trilayer Si-SiO₂-Si woodpile structures (right image is of a single tube shown in left image); white box indicates region with enhanced contrast for visualization of the trilayer nanostructure. Adapted with permission from Reference 4. © 2006 Wiley. (b) The hierarchical structure of an ultralightweight nickel lattice: optical image (top); schematic image (bottom left) of the lattice region indicated by the black box, with the unit cell highlighted in yellow; and SEM images (bottom middle) of an individual hollow member and (bottom right) a cross section of the wall. Note: t, thickness; D, diameter; L, length of each member in the lattice; θ, inclination angle. Adapted with permission from Reference 8. © 2011 American Association for the Advancement of Science. (c) Schematic (top left) of a polymer microlattice composed of an octet-truss unit cell (the red box indicates a coplanar part of the unit cell that would be fabricated as part of the same layer during the sequential, layer-by-layer microstereolithography fabrication process).^{Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44} The bottom left SEM image shows the fabricated unit cell corresponding to the schematic, which is approximately 300 μm in size. The SEM image on the right shows a lattice consisting of repeating octet-truss unit cells. (d) SEM images (left) and compressive mechanical response of hollow ceramic lattices (right)^{Reference Meza, Das and Greer60} that exhibit different deformation mechanisms due to differences in the ratio of wall thickness to tube radius (i.e., [top] recoverable elastic buckling versus [bottom] irrecoverable brittle failure). The Roman numerals I, II, III, and IV indicate initiation of compression, yielding, initiation of unloading, and complete unloading, respectively. (c–d) Adapted with permission from References 44 and 60, respectively. © 2014 American Association for the Advancement of Science.

A seminal example is shown in Figure 2a, in which printed polymer lattices are transformed into hollow woodpile architectures composed of silicon for photonic applications. Silicon is an ideal material for photonic crystals, because it has a high refractive index (n ∼ 3.45). The woodpile structure is particularly well-suited for microfabrication^{Reference Lin, Fleming, Hetherington, Smith, Biswas, Ho, Sigalas, Zubrzycki, Kurtz and Bur42} and direct-write assembly^{Reference Gratson, Garcia-Santamaria, Lousse, Xu, Fan, Lewis and Braun4} methods. To fabricate these architectures, concentrated polyelectrolyte inks are printed as face-centered-tetragonal lattices.^{Reference Gratson, Xu and Lewis3,Reference Gratson, Garcia-Santamaria, Lousse, Xu, Fan, Lewis and Braun4} These structures are then coated with silica using a room-temperature CVD process, followed by polymer removal by etching. Finally, the hollow silica features are coated with silicon using a high-temperature CVD process. The final architecture, which is composed of an interconnected array of cylinders with a total wall thickness of ∼300 nm,^{Reference Gratson, Garcia-Santamaria, Lousse, Xu, Fan, Lewis and Braun4} exhibits a partial photonic bandgap.

Ultralight architected metals with densities on the order of 1 mg cm⁻³ have been recently fabricated by combining a light-based patterning method with conformal coating.^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8,Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44} Their wall thickness gives rise to an additional relevant length scale, which can introduce dramatic new mechanical effects, observable at the macroscale during mechanical loading and unloading. Electroless nickel plating is one approach that has been used to conformally coat polymer scaffolds with a thickness of approximately 100 nm.^{Reference Schaedler, Jacobsen, Torrents, Sorensen, Lian, Greer, Valdvit and Carter8} The hollow nickel-phosphorus structure left behind after the polymer is etched away exhibits unexpected elastic recovery from compressive strains, a result of the introduction of new modes of local deformation (e.g., elastic shell buckling) and associated stable relief cracks at the nodes.

Scaling relationships, determined previously for porous materials,^{Reference Gibson and Ashby63} relate their macroscopic properties to their relative density, ρ/ρ_s. The relative density is defined as the total bulk density of the porous structure, including void space (ρ), divided by the density of the solid material from which the structure is assembled (ρ_s). It is often desirable in engineering applications to use cellular solids that are as low-density as possible for a given function. Lower-density structures can be constructed by simply using lower relative densities, but desired properties often decrease by larger factors. For example, the relative stiffness of a typical stochastic foam scales as (E/E _s) ∝ (ρ/ρ_s)^{Reference Smay, Cesarano and Lewis2}, where E is Young’s modulus of the bulk structure and E _s is Young’s modulus of the solid material that the structure is made from.^{Reference Gibson and Ashby63} If an ordered cellular structure is designed to deform predominantly via bending, it exhibits a comparable scaling relationship.^{Reference Zheng, Lee, Weisgraber, Shusteff, DeOtte, Duoss, Kuntz, Biener, Ge, Jackson, Kucheyev, Fang and Spadaccini44} However, different motifs can force deformation to occur via stretching, rather than bending, of the members, as an example. This results in a linear scaling relationship between relative stiffness and relative density, or (E/E _s) ∝ (ρ/ρ_s). Thus, at its simplest, the ability to tune a structure can enable control of the deformation mechanism. This can determine the relevant scaling behavior, which also affects the failure modes of the system.

Recently, sufficiently long and thin hollow ceramic tubes with wall thicknesses less than 20 nm have been observed to locally deform via reversible elastic shell buckling, enabling these architectures to fully recover after compression of up to 50% strains (Figure 2d, top plot), in sharp contrast to the brittle response expected for ceramics (which is observed when the geometry is slightly changed—Figure 2d, bottom plot).^{Reference Meza, Das and Greer60} While related size effects at the nanoscale, such as size-dependent mechanical properties, have been reported previously,^{Reference Guo, Xie and Luo64,Reference Bian, Bassett, Wang and Hanrath65} 3D printing opens new avenues for achieving these benefits in larger engineering structures.

Beyond control over the deformation mechanism, architected materials can also be designed and fabricated to exhibit more exotic responses, such as negative stiffness^{Reference Duoss, Weisgraber, Hearon, Zhu, Small, Metz, Vericella, Barth, Kuntz, Maxwell, Spadaccini and Wilson61} or negative Poisson’s ratio.^{Reference Buckmann, Stenger, Kadic, Kaschke, Frolich, Kennerknecht, Eberl, Thiel and Wegener39} By controlling the arrangement of simple cylindrical features of submillimeter diameter in a “woodpile” structure via direct-write 3D printing, macroscopic structures can be produced that exhibit negative shear stiffness under certain conditions.^{Reference Duoss, Weisgraber, Hearon, Zhu, Small, Metz, Vericella, Barth, Kuntz, Maxwell, Spadaccini and Wilson61} Each layer consists of a number of parallel cylinders that are oriented orthogonal to those in the previous layer. In a simple cubic structure, every other layer is printed in the same position, whereas they are laterally offset by a half-period in a face-centered-tetragonal geometry. If the cylinders are arranged in a simple cubic structure, then the structures, when confined by a static force in one direction, tend to snap between different configurations as a shear stress is applied in a direction orthogonal to the static stress. Upon changing their arrangement from a simple cubic to a face-centered-tetragonal structure, a more conventional shear stiffness response is obtained.

Another macroscopic property that can be engineered through mesoscopic architecture is the Poisson ratio, ν = –ε_t/ε_l; this ratio relates the extent to which a material deforms laterally (transverse strain, ε_t) when a defined deformation is applied in a direction perpendicular to this (longitudinal strain, ε_l). Typically, when materials are pulled in tension along one axis, they contract in the orthogonal directions, giving rise to a positive Poisson’s ratio. However, mesoscopic architectures have been designed that produce a negative Poisson’s ratio at the macroscale, where tension along one axis leads to expansion along the orthogonal axes.^{Reference Lakes66,Reference Prall and Lakes67} These auxetic materials have been fabricated by direct-laser writing with submicron features.^{Reference Buckmann, Stenger, Kadic, Kaschke, Frolich, Kennerknecht, Eberl, Thiel and Wegener39} Only through 3D printing has the fabrication of nonstochastic negative Poisson’s ratio structures, previously assembled by hand,^{Reference Prall and Lakes67} reached sufficient resolution and throughput to be considered for practical use (e.g., as damping materials or indentation-resistant structural materials).

3D printing of composite architectures has also been recently demonstrated.^{Reference Compton and Lewis45,Reference Wang, Lau, Thomas and Boyce68–Reference Guo, Yang, Heuzey and Therriault72} In one approach, direct-writing with fiber-reinforced epoxy inks has been reported (Figure 3).^{Reference Compton and Lewis45} During printing, the ink experiences a high shear environment within the nozzle that induces alignment of anisotropic fillers along the print path (Figure 3a). Macroscopic cellular architectures, such as the honeycomb structure shown in Figure 3b, can be rapidly fabricated with different print paths to yield different spatial distributions of fiber orientation. The uniform local alignment of the fibers (Figure 3c) results in anisotropic mechanical properties (Figure 3d), akin to those observed in natural materials, such as balsa wood.^{Reference Gibson52} Alternate approaches combine stiff and soft elements with disparate mechanical properties to engineer lightweight materials with excellent toughness, impact resistance, and specific mechanical properties.^{Reference Wang, Lau, Thomas and Boyce68–Reference Guttag and Boyce71} For example, nacre-like architectures^{Reference Gao, Ji, Jager, Arzt and Fratzl73} of alternating stiff and soft polymer features have been printed.^{Reference Dimas, Bratzel, Eylon and Buehler69} These mesoscale architectures mimic that of nacre, which exhibits mechanical properties that are superior to those of the individual soft and stiff constituents, a result of crack diversion, breaking of mineral bridges, topological interlocking of the stiff components, and viscoelastic behavior of the soft phase during deformation.^{Reference Wegst, Bai, Saiz, Tomsia and Ritchie56} The possibilities for designing and fabricating architected materials with unprecedented performance are nearly endless.

Figure 3.

Hierarchical bioinspired composites. (a) Schematic view of shear-induced fiber alignment during direct ink writing. (b) Optical image of fiber-reinforced epoxy hexagonal lattice inspired by balsa wood (the hexagonal unit cell is 6 mm in size, from wall to parallel wall). (c) Scanning electron microscope image showing fiber alignment along the (out-of-plane) print path (scale bar is 20 μm). (d) Stiffness as a function of density of three-dimensional (3D) printed carbon fiber-epoxy structures, commercial 3D printed polymers, and balsa wood. The location of the region labeled “tensile bars” indicates that the carbon fiber-epoxy material developed in Reference 45 can have up to an order of magnitude higher stiffness than materials used in conventional 3D printing techniques. The stiffness of the printed cellular structures is comparable to that of balsa wood as well as to solid materials printed by standard 3D printing techniques, but at an intermediate density. The dashed lines indicate the analytical scaling predictions for the stiffness of the printed cellular structures (in this case, triangular lattices) based on four different characteristic defect sizes. Adapted with permission from Reference 45. © 2014 Wiley. Note: E, Young’s modulus of the structure; E _s, Young’s modulus of the solid (cell wall); e, the characteristic imperfection size defined by the ratio of the amplitude of cell wall waviness to the cell wall thickness; SLA, stereolithography; FDM, fused deposition modeling; SLS, selective laser sintering.

3D electronics

The ability to rapidly fabricate 3D electronics would enable many applications, including displays, solid-state lighting, microbatteries, sensors, and wearable and biomedical devices with embedded circuitry in arbitrary form factors. High-resolution, planar patterning of functional materials has been possible for some time. Roll-to-roll^{Reference Jain, Klosner, Zemel and Raghunandan74} and inkjet printing^{Reference Sirringhaus, Kawase, Friend, Shimoda, Inbasekaran, Wu and Woo75} have been used to create functional features such as polymeric transistors over large areas, and “dip-pen” nanolithography^{Reference Piner, Zhu, Xu, Hong and Mirkin76} (which uses the tip of an atomic force microscope to place collections of molecules in ordered patterns on a substrate via capillary transport) has been used to create functional features with nanoscale precision. However, it is far more challenging to design functional materials that can be patterned in three dimensions.

One enabling advancement is the ability to print electrically conductive features in flexible, stretchable, spanning, and conformal motifs (Figure 4a–d).^{Reference Ahn, Duoss, Motala, Guo, Park, Xiong, Yoon, Nuzzo, Rogers and Lewis7,Reference Adams, Duoss, Malkowski, Motala, Ahn, Nuzzo, Bernhard and Lewis77} These features can serve as interconnects for photovoltaic microcells and LED arrays or as the conductive features necessary to produce functional antennae (Figure 4e–f). With the use of direct-writing, concentrated silver inks (>70 wt% Ag) have been patterned at room temperature at resolutions approaching 1 μm (Figure 4a–d).^{Reference Ahn, Duoss, Motala, Guo, Park, Xiong, Yoon, Nuzzo, Rogers and Lewis7} Their viscoelastic properties allow the ink to be printed out of plane and across gaps (Figure 4a) to form self-supporting features, such as bridges, that connect functional devices (Figure 4b–d). Upon thermal annealing at 150–550°C, the electrical resistivity of the printed silver interconnects dramatically reduces. These inks have also been patterned conformally on both concave and convex glass hemispheres to produce customized, 3D electrically small antennas.^{Reference Adams, Duoss, Malkowski, Motala, Ahn, Nuzzo, Bernhard and Lewis77} An antenna is electrically small when its electrical size, defined as the product of the wavenumber k and the radius a of the smallest possible sphere that can encompass the entire antenna, is less than 0.5. Efficient hemispherical design (Figure 4e) requires patterning of conductive features on curvilinear surfaces. Figure 4f highlights the antenna performance, which is given by the voltage standing wave ratio (VSWR) as a function of frequency, an indicator of how well energy is coupled from source to antenna. A lower VSWR indicates a stronger coupling between the wave and the antenna (a VSWR value of 1 corresponds to perfect coupling), and the width defines the impedance bandwidth. The antenna efficiency is greater than 70%, at a bandwidth that matches the value predicted by computer simulation.

Figure 4.

Three-dimensional (3D) electronic devices. (a–b) Schematic and optical images of direct-writing of spanning silver electrodes wiring up a 4 × 4 light-emitting diode microcell array, respectively. (c–d) Scanning electron microscope images of silver interconnect arches that have been printed to span over an electrode junction and onto a gold contact pad, respectively. Adapted with permission from Reference 7. © 2009 American Association for the Advancement of Science. (e–f) Optical image and performance of a 3D electrically small antenna, respectively, in which the voltage standing wave ratio (VSWR) is plotted as a function of frequency. The indicated bandwidth of 15.2% corresponds to a VSWR ratio of 5.83:1. The parameters k and a are the wavenumber and the radius of the smallest sphere that encompasses the antenna, respectively; when their product, which defines the electrical size of the antenna, is less than 0.5, the antenna is electrically small. Adapted with permission from Reference 77. © 2011 Wiley.

Meniscus-confined electrodeposition is another approach for directly writing conductive features.^{Reference Hu and Yu78} For example, a precursor electrolyte solution, such as CuSO₄, is brought near a conductive substrate in a micropipette, allowing the meniscus to make contact. Upon applying a small voltage, copper metal is electrodeposited from the precursor solution, or “ink,” onto the substrate. Arbitrary conductive features can be patterned by translating the micropipette in a prescribed manner. While this approach obviates the need for post-processing, it is limited by slow writing speeds (about 250 nm s⁻¹ for Cu wires of approximately 1 µm in diameter) and the need to maintain continuous electrical contact with a conductive substrate during printing.

Recently, direct-writing of metals that are liquid at room temperature, specifically, eutectic gallium and indium (EGaIn), has been pursued to create interconnects for flexible, stretchable electronics.^{Reference Chiechi, Weiss, Dickey and Whitesides79–Reference Khan, Trlica and Dickey82} Immediately upon printing, an ultrathin, passivating oxide layer forms on the outer surface of the liquid metal when exposed to air. This imparts sufficient mechanical stability to allow this low-viscosity liquid to retain its shape without wetting and spreading, resulting in freestanding 3D shapes with a resolution of 10 μm or less. However, since these electrodes are inherently a liquid under ambient conditions, the printed features must be fully encapsulated within the functional device of interest.

Multimaterial 3D printing of mesoscale architectures is an emerging area of interest (Figure 5). Recent advances include printing of rechargeable lithium- (Li-) ion microbatteries^{Reference Sun, Wei, Ahn, Seo, Dillon and Lewis29} and LED arrays.^{Reference Kong, Tamargo, Kim, Johnson, Gupta, Koh, Chin, Steingart, Rand and McAlpine83} Specifically, 3D microbatteries have been fabricated^{Reference Sun, Wei, Ahn, Seo, Dillon and Lewis29} by co-printing Li₄Ti₅O₁₂ (LTO) as an anode material and LiFePO₄ (LFP) as a cathode material in interdigitated, high-aspect-ratio motifs. These active materials were chosen because of their minimal volume change during charge/discharge cycles.^{Reference Zaghib, Armand and Gauthier84,Reference Zhang, Shyy and Sastry85} In both cases, inks were designed with LTO or LFP nanoparticles in solution, with total solid loadings of approximately 60 wt%. These concentrated viscoelastic inks exhibit a strong shear thinning response and readily flow through nozzles as small as 30 μm in diameter. The anode and cathode were printed with high fidelity in the form of high-aspect-ratio, interdigitated features (Figure 5a–b). Subsequent heating to 600°C removes the organic processing aids and partially sinters the nanoparticles. The anode and cathode remain porous to ensure superior ion and electron transport during operation.^{Reference Sun, Wei, Ahn, Seo, Dillon and Lewis29}Figure 5c shows the areal capacity of the batteries for up to 30 charge/discharge cycles with minimal capacity fade. Their areal energy and power densities are among the highest reported to date for microbatteries.^{Reference Sun, Wei, Ahn, Seo, Dillon and Lewis29} Moreover, rechargeable microbatteries of nearly arbitrary size, geometry, and capacity can be fabricated by this programmable method that are well-matched to the specific needs and physical dimensions of each targeted application.

Figure 5.

Three-dimensional printed microbatteries and light-emitting diodes (LEDs). (a–b) Schematic view and scanning electron microscope image of printed lithium-ion microbatteries, respectively. (c) Areal capacity of printed microbatteries as a function of charge–discharge cycle showing minimal degradation of performance over the first 30 cycles. (a–c) Adapted with permission from Reference 29. © 2013 Wiley. (d–e) Schematic views of printed quantum dot light-emitting diodes (QD-LEDs), which show an individual LED and an LED array (with the two colors illustrating the tunability of the emission wavelength based on the size of the quantum dots), respectively. (f) Current density–voltage curves and forward luminance output of a printed green QD-LED, demonstrating the functionality of the QD-LEDs fabricated in this highly customizable approach (scale bar of inset optical image is 2 mm). (d–f) Adapted with permission from Reference 83. © 2014 American Chemical Society. Note: LTO, Li₄Ti₅O₁₂; LFP, LiFePO₄; EGaIn, eutectic gallium indium; TPD, N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine; PEDOT:PSS, poly(ethylenedioxythiophene) polystyrene sulfonate; NP, nanoparticle.

Multimaterial mesoscale architectures require not only that each material meets specific functional requirements, but also that the printed materials are compatible with one another. These challenges had to be overcome in the recent fabrication of 3D printed LEDs (Figure 5d–f).^{Reference Kong, Tamargo, Kim, Johnson, Gupta, Koh, Chin, Steingart, Rand and McAlpine83} Specifically, quantum dot light-emitting diodes (QD-LEDs) were printed, which required the development and co-printing of several different functional inks to produce light emission, charge transport, conductive leads, and UV-curable adhesion layers. The multilayer architectures necessitated the use of orthogonal solvents that would not “mix” with underlying patterned features. While the brightness of the printed QD-LEDs is more than an order of magnitude lower than the best reported to date via solution-processing, this approach constitutes an impressive demonstration of multimaterial 3D printing of optoelectronic devices at the mesoscale.

The future possibilities of 3D printing as a platform for creating customized, fully integrated devices are vast. Additionally, hybrid strategies can be implemented in which 3D printing is combined with automated pick-and-place methods to create more complicated electronic devices.^{Reference Lewis and Ahn86} The development of new functional inks, flexible printing platforms, and software that integrates computer-aided design with electronic device layout capabilities in three dimensions is essential to transforming how electronic devices are designed and integrated into our daily lives.

Future perspective

3D printing is rapidly advancing beyond patterning complex forms to directly embedding function. Nearly arbitrary, 3D mesoscale architectures can be printed with minimum feature sizes ranging from ∼100 nm to 100 μm from multiple classes of materials. By digitally designing and building 3D mesoscale architectures, one can fundamentally control their macroscale properties. While multimaterial integration is still in its infancy, one can envision a “model to make” strategy that begins with multiscale modeling, coupled with computer-aided design, which is then transformed into complex physical embodiments through the synthesis and patterning of functional, structural, and even biological inks, to yield 3D architectures with as yet unimagined performance.