Three-dimensional (3D) hierarchical materials are important to a wide range of emerging technological applications. We report a method to synthesize complex 3D microengineered materials, such as microlattices, with nearly fully dense truss elements with a minimum diameter of approximately 20 μm and having high aspect ratios (up to 20:1) without using any templating or supporting materials. By varying the postprocessing conditions, we have also introduced an additional control over the internal porosity of the truss elements to demonstrate a hierarchical porous structure with an overall void size and feature size control of over five orders of magnitudes in length scale. The method uses direct printing of nanoparticle dispersions using the Aerosol Jet technology in 3D space without templating or supporting materials followed by binder removal and sintering. In addition to 3D microlattices, we have also demonstrated directly printed stretchable interconnects, spirals, and pillars. This assembly method could be implemented by a variety of microdroplet generation methods for fast and large-scale fabrication of the hierarchical materials for applications in tissue engineering, ultralight or multifunctional materials, microfluidics, and micro-optoelectronics.

Keywords

Here, we use the Aerosol Jet printing technique to deposit aerosolized microdroplets containing metal nanoparticles to assemble the nanoparticles in the form of highly intricate microscale 3D networks, such as microscaffolds/microlattices with nearly fully dense truss elements without the use of any supporting materials. The structural features are shown spanning over five orders of magnitudes in length scale. Furthermore, we were able to controllably engineer the internal porosity and surface topography of the truss elements of the scaffolds through varying sintering conditions (for example, sintering profile and/or power source), thus introducing a hierarchy in porosities within the structure, in addition to that from free-form printing.

Solution-based noncontact processes ( 16 ), such as inkjet printing, have been used to print molten lead-free solder materials ( 17 ) and piezoelectric and metal nanoparticles ( 18 , 19 ). Microwires or micropillars having features of the order of 100 μm were realized in these works ( 18 , 19 ). Several process parameters (substrate, solvent, and droplet velocity) have also been investigated to address the fabrication constraints and requirements in the printing processes ( 20 ). Another solution-based nanoparticle printing method, namely, the Aerosol Jet technique, uses a focused sheath of gas to transfer the aerosolized microdroplets containing the nanoparticles. Such a transfer of material allows nanoparticle solutions with a viscosity of up to 1000 centipoise (cP) to be printed using this method. A wide range of feedstock materials can thus be printed; for example, inks and dispersions containing nanoparticles of polymers ( 21 ), metals ( 13 ), metal oxides ( 22 ), and other ceramics ( 23 ) have been used to print using the Aerosol Jet method.

Free-form fabrication by printing of nanoparticle solutions and inks followed by sintering is a relatively recent area that has gained importance in the fabrication of a diverse set of electronic devices and materials ( 11 , 12 ). Printing of nanoparticles over the surfaces of 3D components, such as hemispherical domes ( 11 ) or pillars ( 13 ), has been demonstrated for antenna applications. A continuous dispense of nanoparticle dispersion from a micronozzle was used to make nonintersecting structures/motifs in 3D ( 12 ). Other nozzle-based methods ( 14 ) use nanoparticles dispersed in viscoelastic inks, which leads to a significant matrix filler between the particles, limiting the part density. Three-dimensional conductive polymer-metal hybrid structures were fabricated by adding silver nitrate to a photocurable oligomer in the presence of suitable photoinitiators and exposing them to a digital light system ( 15 ).

Materials with three-dimensional (3D) architectures, such as micro/nanolattices and scaffolds, are of high current interest ( 1 ) because they can exhibit extraordinary material properties and functionalities in diverse applications, such as biomedical implants ( 2 ), porous membranes ( 3 ), load-bearing structures ( 4 ), microfluidic devices ( 5 ), fast-charging and high-capacity batteries ( 6 ), and supercapacitors ( 7 ). Such microengineered materials have been realized by a variety of techniques and methods, such as two-photon lithography ( 4 ), projection microstereolithography of polymers ( 8 , 9 ), and the use of a polymer opal templating ( 10 ), followed by deposition of suitable materials, such as metals or ceramics over the polymer template ( 4 , 8 , 10 ). The available techniques to deposit over the template (for example, atomic layer deposition, electroless deposition, or electrodeposition) allow a variety of ceramics and metals to be used in these methods. The polymer is then removed by burnout or by chemical dissolution after the structure is built up. The final product consists of hollow tubes and their composites for truss elements that form the complex 3D scaffold. Although the template fabrication and the material deposition steps are additive, the removal of the underlying template adds a fabrication step, requires the use of chemicals, and creates waste.

RESULTS AND DISCUSSION

Pointwise spatial printing inspired by nature First, we describe the free-form printing used to obtain the complex hierarchical structures described in this study. The printing process was inspired by similar methods occurring in nature that resulted in complex or elegant 3D architectures. For example, petal-shaped gypsum crystals typically found in the West African deserts and called “Desert Roses” are formed by precipitation and condensation of aerosolized liquid droplets (in a fog) that carry dissolved sulfur compounds. The crystal is then formed in 3D by evaporation of the solvents by the desert heat (24, 25). This process is depicted schematically in Fig. 1A. We adopted this strategy to fabricate 3D microarchitectures in a controlled manner that mimicked the natural process shown in Fig. 1A. We use silver nanoparticles dispersed in an aqueous ethylene glycol solution as a source of building material (see Materials and Methods). An aerosolized fog containing droplets from this solution is then generated from the Aerosol Jet technique and carried toward the heated substrate to build a 3D microarchitecture/microscaffold of silver nanoparticles with truss elements in the air that do not require any supporting material or templating (Fig. 1B). The principle of operation of the Aerosol Jet technique is given in section S1. We use a marked increase in the evaporation rate of the solvent droplets when their diameter is below 50 μm at moderate temperatures to solidify the deposited material (nanoparticle with binders) before the arrival of the successive droplet. A high particle loading ratio (up to about 70% by volume at the time the droplets exit the print head) is used, and the nanoparticles are sintered in the presence of an energy source, such as heat or photonic power. It is noted that this approach is independent of the microdroplet generation method, although we used the Aerosol Jet printing for finer droplet sizes and, hence, features. The achieved microarchitectures and microlattices consist of a network of interconnecting spatial truss elements at various angles to the horizontal that are as thin as 20 μm in diameter and have open void sizes ranging from 100 μm to 1 mm from the printed architectures. To achieve these fine features, precise process control is highly critical. In the next section, we present physical models to predict the required process parameters for the 3D architectures. Fig. 1 Analogy of the natural growth of Desert Rose and the 3D buildup of nanoparticles by pointwise printing to realize microarchitectures. (A) An illustration of the Desert Rose formation process by condensation of sulfur-containing fog along with the elevated temperature of the desert climate. Desert Rose photo courtesy of O. Apostolidou (reprinted with permission). (B) In a process inspired by that shown in (A), we used successive condensation of droplets of nanoparticle ink in the spatial dimension followed by solvent evaporation and sintering to create controlled 3D microarchitectures with hierarchical porosity. The scanning electron microscopy (SEM) image resembles a petal-shaped structure (right). The truss element diameter is about 40 μm.

Physical model and control parameters Building a 3D structure in the air with nearly fully dense truss elements without the sacrificial materials relies on accurate fabrication parameter settings, such as platen temperature and droplet loading ratio. Determination of the critical angle of growth and the stability of growing elements are the two main parameters that constrain the geometry of the 3D microscaffolds. To gain an understanding of the two parameters, we first studied the kinematics of the droplet precipitation process to predict the lowest angle of growth in space for the nanoparticle assembly for different droplet heights (measured after drying) as function of the droplet diameter (Fig. 2). The formulation of the model and the resulting equations are derived in section S2. A condensed ink droplet deposited over a ledge and the free-body diagram being considered for the analysis are shown in Fig. 2A. For spherical droplets with a radius of about 20 ± 5 μm and a dried droplet height of 10 to 20 μm, the predicted critical angle of growth is about 35° ± 7° (Fig. 2B). The second significant parameter is the solvent rate of evaporation, which dictates how fast the precipitated droplet dries out. It is desirable to have fast solvent evaporation for the nanoparticle microdroplets above the previously solidified droplet. The evaporation rate and microdroplet half-life time (that is, when the droplet loses half of its volume to evaporation) have been estimated as a function of time and temperature (see section S3 for the details of the derivation and Fig. 2C). It is revealed that for temperature ≤100°C, the drying time increases markedly with an increase in the droplet diameter. The abovementioned results and analyses were verified experimentally by assembling nanoparticles as pillars and successively lowering the angle of growth to find the critical angle at which the droplet would eventually fall down instead of forming the pillar, as shown in Fig. 2D. The critical angle in these experiments was about 37°, in agreement with that predicted in Fig. 2B. Further, the pillars could be realized only at temperatures higher than about 90°C, again in reasonable agreement with the prediction of the evaporation model. The models developed could thus be used to determine the required platen temperature and the stable microdroplet diameters to build a free-standing microarchitecture to fit the specific microdroplet generation method. Next, we use the method to create the unique 3D microarchitectures in two simple manufacturing steps, namely, the deposition of the nanoparticle ink droplets and final sintering. Fig. 2 An illustration of physical models developed to study the stability and control of pointwise printing and their experimental verification. (A) A simplified free-body diagram of a critical droplet at the edge of a structure and the illustration of a designed experiment to verify the models. (B) The predicted angle of growth as a function of droplet radius for different measured dried droplet heights. (C) The half-life time of the microdroplet was numerically calculated by evaporation rate estimation as a function of substrate temperature. (D) SEM images showing a series of inclined pillars fabricated at different angles to verify the model predictions. A magnified side view of the last inclined pillar establishes the critical angle (smallest angle to the horizontal for the assembled pillar by this method) at 37° that matches reasonably well with the model prediction. Scale bar, 250 μm.

Three-dimensional hierarchical architectures Figure 3 (A to C) shows microscale networked lattice structures printed using the above schema having octahedral (Fig. 3, A and C) and hexagonal (Fig. 3B) unit cells, with fullness fractions of 27, 9.5, and 14.5% and structural periodicities of 100, 300, and 200 μm, respectively. The overall length scale of the structures is of the order of a few millimeters. The as-printed microscale scaffolds (before sintering) consisted of nanoparticles dispersed in binders. However, after sintering, the binders are expected to escape, leaving the sintered nanoparticles to form the 3D network in space. Figure 3D shows the scaffold after sintering with the sintered nanoparticles, showing the near-full density of the truss elements. The micrographs in Fig. 3 thus demonstrate scaffold structures with high complexity at length scales described above with nearly fully dense truss elements. Next, we show that nanoparticle sintering conditions can be varied to obtain very low porosities (near-full density) to higher porosities in the truss elements if desired, which gives rise to the hierarchical structures that span over several orders of magnitudes in length scale. We note that hierarchical porosities are observed in naturally developed materials and biological systems that help them tolerate strain (26, 27). The method described in this study offers an excellent opportunity to artificially fabricate such strain-tolerant structures by tailoring porosities from nanoscales (from nanoparticle sintering) to hundreds of micrometers in scale (from the Aerosol Jet printing). For example, Fig. 3 clearly shows the scaffold with a porosity of the order of 100 to 300 μm through printing in 3D. Because the truss elements are made from nanoparticles and binders when the printing occurs, different sintering conditions can remove the binders and stop the crystal growth before complete pore closure to give them variable densities. The porosities could also be manipulated by using different nanoparticle sizes. Fig. 3 Pointwise-printed 3D microarchitectures with different network topologies. (A) An open octahedral microarchitecture with truss elements having a diameter of about 35 μm. Scale bars, 50 μm. (B) Pointwise-fabricated microarchitecture with a combination of octahedral and hexagonal structures. Scale bars, 50 μm. (C) Top surface of an octahedral scaffold structure at different magnifications and (D) truss elements of the 3D-printed scaffold after binder escape and nanoparticle sintering and possible grain growth. Figure 4 shows a schematic and the micrographs of nanoparticle porosities for silver nanoparticles having diameters in the range of 30 to 50 nm. The pore sizes and length scales of the order of 100 nm are evident. We also used silver flakes of about 1 μm in size and sintered both types of nanoparticles under different conditions. The detailed study of the sintering conditions can be found in section S4, where different levels of porosities are observed for different sintering conditions and two particle sizes, as shown in fig. S4. Fig. 4 The pointwise printing in combination with controlled sintering technique. The printed material is designed to contain the first level of macroporosities. Coalescence of sintered nanoparticles starts to form the second level of porosity and could be stopped before pore closure and grain growth. Scale bars, 100 nm. To further demonstrate the hierarchical length scales achieved by this technique, we fabricated a variety of micro-architected structures, as shown in Fig. 5. Printed silver lattices with octahedral and hexagonal architectures are shown in Fig. 5A. The truss elements have diameters in the range of 30 to 55 μm, whereas the periodicity of pores from printing is of the order of about 300 μm. The overall scale of the scaffold is over a few millimeters. Figure 5 (B and C) shows features at length scales of hundreds to tens of micrometers, respectively. Figure 5D shows a variety of microporous structures that could be achieved by sintering of nanoparticles having a characteristic length scale from several tens of nanometers to a few micrometers. The variety of motifs and the geometry of fabricated structures in Fig. 5 thus demonstrate the precision of pointwise printing in the form of controlled porous microarchitectures that span over five orders of magnitude in size scale. Fig. 5 Pointwise-printed hierarchical materials with 3D microarchitectures having features that span over five orders of magnitudes in length scale. (A) SEM images of octahedral and hexagonal microlattices at a bulk view at a length scale of millimeters. (B) High aspect ratio truss elements forming the architecture of sintered structures introducing the first level of porosity. (C) The engineered surface of truss elements of the lattice induced by different sintering profiles featuring a high level of porosity to near dense materials. (D) The final order of controlled surface features and porosity for different sintering temperature profiles that show several microscale to nanoscale voids.