Symposium ST03—Design, Synthesis and Characterization of Architected Materials for Structural Applications

Rapid progress in additive manufacturing methods has led to the creation of a new class of ultralight, stiff and strong 3D architected metamaterials comprised of a network of struts resembling a periodic truss structure. The performance of these micro-architected metamaterials is ultimately be limited by their tolerance to damage and defects, yet experimental investigations of their fracture toughness have remained elusive primarily due to manufacturing limitations. Using octet-truss metamaterial specimens comprising in excess of a million unit-cells we show that not only is stress intensity factor, as used in conventional elastic fracture mechanics, is insufficient to characterise fracture in these metamaterials but also that conventional fracture test methods using two-dimensional specimens with through-thickness cracks are inappropriate. Via a combination of finite element (FE) calculations and asymptotic analyses we extend the application of the ideas of fracture mechanics to metamaterials with a view to developing a continuum theory and measurement protocol for the failure of such architected metamaterials.

Introduction and Aim
Microarchitected lattices have attracted significant interest as lightweight, resource-efficient structural materials. One of their notable advantages is the ability to attain high weight-specific mechanical properties by manipulating how material is distributed in space. Here, we will present numerical and experimental results to reveal how the buckling of struts, hitherto neglected, affects the crack tip field, the location of fracture initiation and propagation, and their concomitant effects on the fracture toughness of regular triangular lattices.

Hitherto, the fracture toughness of lattices had mainly been studied numerically, assuming idealized geometry and material properties, and specimen size that is free from the influence of edge effects. The above can all have a significant impact upon the deformation of stretch-dominated lattices with slender struts (low relative density) even in the absence of material nonlinearities, but few studies have examined how local geometric instability affects fracture initiation and the subsequent crack growth in 2D prismatic lattices.

Methods
The influence of strut buckling in elastic-brittle triangular lattices is assessed numerically, through a Boundary Layer Method (BLM) for infinite-sized specimens, and results compared to those from simulations of finite-sized, Single Edge Notched Tension (SENT) specimens. In the former, crack-tip displacements field from LEFM were imposed as periodic boundary condition to a Representative Volume Element, and the latter follows the standard procedure described in BS8571:2018. Eigenvalue analysis was performed and it will be shown that stress-bifurcation (buckling) in the struts can occur under certain conditions prior to fracture. The effects of mixed-mode loading are also evaluated using the BLM through a range of boundary conditions with varying Mode I and Mode II contributions. Interaction between buckling and intrinsic size-effects of finite-sized specimens is addressed for lattices with 16<W/l₀<128 cells across the width, and crack-sizes of 0.15<a/W<0.65, where W is specimen-width, a is the crack-length and l₀ is the characteristic length of the unit cell. For validation, experimental results using SENT specimens, manufactured by laser-cutting of PMMA sheets, will also be presented.

Results
It is demonstrated that stress bifurcation in struts near the crack-tip occurs below a critical transition relative density. The latter is sensitive to the nonlinear strain of the strut material and shows good agreement with predictions by elastic buckling calculations. In infinite-sized lattices, the fracture initiation in pure Mode I loading is insensitive to buckling, however increased Mode II contribution leads to earlier onset of buckling and fracture at loads up to 70% lower than initially predicted. These phenomena are observed at practically relevant relative densities (up to ρ=0.2), e.g. a reduction of 68% at ρ=0.1. Finite-sized SENT specimens reveal fracture initiation in the post-buckling regime, with toughness values up to 60% below that predicted by standard scaling law. In deeply notched finite-sized specimens (a/W>0.48), buckling and fracture are first observed at the remote edge from the crack. Experimental results confirm the occurrence of strut buckling before fracture in SENT specimens and validate the numerically obtained fracture toughness scaling law.

Conclusions
It is shown that elastic strut buckling is an active mechanism in the fracture initiation of lattice materials, and can lead to material failure at smaller loads than predicted by current analyses. Finite deformation of individual struts is shown to significantly affect the bulk mechanical properties of the lattices.

Technological advances in additive manufacturing techniques have enabled the fabrication of novel three-dimensional nano- and micro-architected lattice materials, which facilitate the exploration of yet unreachable property combinations. Specifically designed lattice materials have great potential in particular in the field of lightweight materials, since high specific strength and stiffness combined with low densities and remarkable robustness can be achieved. For the fabrication of geometrically complex structures with small feature size, light-based additive manufacturing methods, such as 3D direct laser writing (3D-DLW), are the most promising ones. In combination with various post-processing steps, such as annealing, pyrolysis, and the application of coatings, the overall behaviors of the printed structures can be further adapted. However, the failure processes are not yet clear, in particular since the writing parameters themselves might affect the materials properties very locally affecting individual components of the architectures.
In this work, we fabricated tetrahedral microlattices via 3D-DLW using two different photoresists (IP-Dip, IP-S). The writing parameters were varied during the writing process of the microlattices to induce local changes of the material properties. The polymeric structures were then converted into glassy carbon through pyrolysis, which is accompanied by significant shrinkage of the lattices depending on the selection of temperatures. The mechanical behaviors of both the lattices and materials was investigated by compression tests using a nanonindenter. Raman Spectroscopy and X-ray Photoelectron Spectroscopy (XPS) were applied to further characterize the structural variations. A variation of the 3D-DLW parameters and the post-processing steps showed a significant influence on the materials structures and therefore the deformation and failure behavior of the microlattices. Understanding the relation between the chemical morphology and the mechanical behavior will contribute to our understanding and prediction of properties of architectures fabricated through 3D-DLW, which will further enhance the applicability of high-strength mechanical metamaterials.

Bending-dominated lattices offer superior energy absorption capacities and one of the most commonly used topologies is the hexagonal lattice. This architecture is frequently used as the core of sandwich panels, for example, because its simple geometry can be manufactured easily. The rapid development of additive manufacturing, however, allows us to produce much more complex architectures and this raises the question: could other bending-dominated topologies outperform the hexagonal lattice? This study reports the behaviour of a truncated-hexagonal lattice: a 2D prismatic bending-dominated topology with mechanical properties superior to those of the hexagonal lattice.
The truncated-hexagonal lattice is a semi-regular tesselation where each vertex is connected to two dodecagons and a triangle. It has a nodal connectivity of three (like the hexagonal lattice) and consequently, its behaviour is bending-dominated. We derived analytical expressions for the elastic modulus and strength of the truncated-hexagonal lattice when subjected to (i) uniaxial compression and (ii) shear. These analytical expressions were then validated using finite element simulations. Both analytical and numerical work assumed that the lattice is made from an elastic, perfectly plastic solid and that the bars are sufficiently slender to behave as Euler-Bernoulli beams.
The results showed that a truncated-hexagonal lattice is particularly stiff for a bending-dominated lattice. In fact, its Young’s and shear modulii are both 85% stiffer than those of the hexagonal lattice. This result is independent of orientation since the truncated-hexagonal lattice has in-plane isotropic elastic properties. The truncated-hexagonal lattice fails by plastic collapse when it is loaded in compression or in shear. Its plastic collapse strength is 11% higher than that of the hexagonal lattice.
In summary, we conclude that the stiffer and slightly stronger truncated-hexagonal lattice would be a good replacement to the hexagonal lattice in applications where energy absorption is critical. Experimental work is currently underway to corroborate the analytical and numerical results reported above.

Advances in fabrication technologies are enabling the production of architected materials with unprecedented properties. While most of these materials are characterized by a fixed geometry, an intriguing avenue is to incorporate internal mechanisms capable of reconfiguring their spatial architecture, therefore enabling tunable functionality. Inspired by the structural diversity and foldability expored in origami, researchers have started to create reconfigurable materials comprising assemblies of plates and hinges. Here our focus is on achieving architected materials with a finite range of encoded behaviors. We achieve this by harnessing nonlinear mechanical behavior that arises either through the underlying 3D geometry, or by direct assuming a nonlinear description for the folding behavior of the hinges, resulting in multistability. Guided by numerical analysis and physical prototypes, we systematically explore the mobility of the designed structures through the use of local actuation, in an effort to uncover and classify their potential behavior. By doing so, we find two transitions in the elastic behavior of these materials depending on the ratio between the stiffness of the plates and hinges. Given that the underlying principles are scale-independent, our strategy can be applied to design the next generation of reconfigurable structures and materials, ranging from transformable meter-scale architectures to nanoscale tunable photonic systems.

Diatoms are single-celled algae that produce, and live within, 3-D, microscopic, nanostructured, porous silica glass-based shells (called “frustules”). Diatom frustules have evolved over hundreds of millions of years to provide resistance to being crushed by predators. Indeed, micromechanical tests and simulations have indicated that Fragilariopsis kerguelensis frustules are capable of accommodating local compressive and tensile stresses in excess of 600 MPa and 500 MPa, respectively (C. E. Hamm, et al., Nature, 421, 841-843 (2003)). The robust nature of silica-based F. kerguelensis diatom frustules has led us to question how such microscale structures would behave if these structures were comprised of metallic or oxide/metal composite materials (i.e., materials that are ordinarily more resistant to fracture than bulk monolithic silica).

To address this question, SiO₂-based F. kerguelensis diatom frustules were reactively converted into 3-D replicas comprised of Ti and MgO/Ti composites. Live F. kerguelensis diatoms were graciously provided by Prof. Rebecca Robinson (University of Rhode Island). Diatom culturing was conducted in L1 medium at 4°C using 13 h light/11 h dark cycles. After harvesting and then washing of the diatom cells in water and ethanol, the cells were heated in air at 600^oC for 4 h to remove organic material to yield SiO₂ frustules. A series of gas/solid reactions was then used to convert these SiO₂ frustules into MgO/Ti and Ti replicas. Reactive conversion experiments were conducted on two types of specimens: i) a shallow bed of frustules (to allow for X-ray diffraction analysis at each stage of reactive conversion), and ii) isolated individual frustules on inert polished substrates (to allow for evaluation of the preservation of the overall frustule morphology and surface features at each stage of conversion via electron microscopy and to allow for in-situ mechanical tests). The latter specimens were oriented with the valve or the girdle band of the frustule or frustule replica facing upwards (to allow for compression tests with both orientations). Secondary electron microscopic images of individual frustules confirmed that the 3-D frustule morphology was preserved at each stage of reactive conversion. The micro/nanochemical evolution during such shape-preserving reactive conversion was evaluated using X-ray diffraction analysis, energy-dispersive X-ray analysis, and selective area electron diffraction analysis (via transmission electron microscopy).

In-situ mechanical tests were conducted with a Hysitron PI-88 system installed in a scanning electron microscope to allow for direct observation of the deformation behavior of SiO₂ F. kerguelensis frustules, and of Ti and MgO/Ti replicas of such frustules. Single and partial loading-unloading load functions were applied to these diatom frustules and frustule replicas. Load-displacement plots indicated that the SiO₂ frustules exhibited fully elastic and brittle behavior, whereas appreciable plasticity was detected for the Ti frustule replicas. While the SiO₂ frustules and Ti frustule replicas exhibited some similarities in apparent crack initiation locations and in crack propagation behavior, higher failure loads were accommodated by the Ti frustule replicas prior to catastrophic failure than for the SiO₂ frustules. The MgO/Ti frustule replicas exhibited higher stiffness, and sustained appreciably higher loads prior to catastrophic failure, than the Ti frustule replicas, while retaining notable plasticity. The use of such in-situ micromechanical tests and simulations to evaluate structural motifs responsible for such mechanical behavior will be discussed.

Biology creates complex porous structures to realize materials that are mechanically strong and lightweight, and have added chemical or physical functionalities, like bone or wood. Inspired by these remarkable materials, engineers have been adding pores to metals to make strong and lightweight multifunctional structures. Porous metals fabricated by traditional methods, however, have mechanical strengths far below their theoretical limits [1]. High-resolution 3D printing is one manufacturing technique that has realized nanolattices that approach the theoretical strength of their constituent materials through topological design, structural optimization, and size effects in nanomaterials. 3D printing, however, has inherent trade-offs between resolution and printing time that severely limit the scale-up of 3D printed nanolattices, which has restricted almost all nanolattice mechanical property measurements to compression by micro/nanoindentation [2]. The tensile properties of nanolattices, therefore, have been mostly unexplored. To truly take advantage of the remarkable properties of nanolattices, it is essential to realize new methods for fabricating macroscopic nanolattices and to understand how their chemistry and physical properties affect their tensile properties.
In this work, we demonstrate a crack-free self-assembly approach to fabricate 2 cm long multifunctional metallic nanolattices with 100 nm periodic features and 30 nm grain size, which translates to 1,000X more unit cells in the loading direction than prior studies. Benefiting from the scalability of this fabrication approach, we grow the first large-area multifunctional nickel nanolattices and measure the tensile properties with macroscopic testing equipment. We show that they can achieve an ultra-high 227 MPa tensile strength at a density less than aluminum, which is 3 times stronger than what has been reported for porous metals with the same relative density at any scale. We also found that the key to enhancing the tensile strength is to combine a lattice geometry that reduces stress concentrations with control of the nanostructure and grain size which reduce the size of inherent defects, such as voids, and increase the metal strength. The resulting nickel nanolattices have excellent structural photonic coloration, approach the theoretical limit of their upper tensile strength, and realize a new combination of strength and relative density for porous metals and nanolattices. The combination of high strength and high specific strength makes these nickel nanolattices ideal for resisting bending loads: they can resist the same load with 50% smaller volume than porous titanium, 50% lower mass than porous iron, and, importantly, 10X less volume than other nanolattices. The new ways developed in this work to fabricate metal nanolattices and characterize their performance will further the design and fabrication of lightweight high-strength multifunctional porous metals.

1. Zhao, B., et al., A review on metallic porous materials: pore formation, mechanical properties, and their applications. The International Journal of Advanced Manufacturing Technology, 2018. 95(5): p. 2641-2659.
2. Zhang, X., et al., Design, Fabrication, and Mechanics of 3D Micro-/Nanolattices. Small, 2020. 16(15): p. e1902842.

Electrically-mediated actuation schemes offer great promise beyond popular pneumatic and suction-based approaches in soft robotics, but they often rely on bespoke materials and manufacturing approaches that constrain design flexibility and widespread adoption. Following the recent introduction of a class of architected materials called handed shearing auxetics (HSAs), we present a 3D printing method for rapidly fabricating HSAs and HSA-based soft robots that can be directly driven by servo motors. To date, HSA fabrication has been limited to laser cutting of extruded Teflon (PTFE) tubes. Our work expands the HSA materials palette to include flexible and elastomeric polyurethanes. Herein, we investigate the influence of material composition and geometry on printed HSAs' mechanical behavior. In addition to individual HSA performance, we evaluate printed HSAs in two robotic platforms - four degree-of-freedom (DoF) platform and a soft gripper - to confirm that printed HSAs perform similarly to the original PTFE HSA designs. Finally, we demonstrate new soft robotic capabilities with 3D printed HSAs, including fully 3D printed HSA actuators, higher force generation in multi-DoF devices, and demonstrations of soft grippers with internal HSA endoskeletons. We anticipate our methods will expedite the design and integration of novel HSAs in electrically-driven soft robots and facilitate broader adoption of HSAs in the field.

Introduction/Aims
Solid foams are structurally efficient materials, and as such, are particularly attractive in weight sensitive applications, e.g. in packaging and insulation. The typical base materials of commercial foams are metallic or petrochemical, both of which pose some disadvantages due to energy intensity of manufacturing, and reliance on non-renewable resources respectively. Natural fibres can be exploited to create and reinforce biodegradable cellular materials, e.g. to increase rigidity, strength and energy absorption.

In this work we consider the use of cellulose fibres extracted from bamboo culms, and used in two configurations: as reinforcement for close-celled cellulose-matrix foams, and as a structural open-celled fibre network. Topology optimisation is used to improve the reinforcement efficiency of such fibres, with the aim of obtaining comparable mechanical properties to commercial foams. We present results on the effect of fibre length and distribution on the macroscopic mechanical response and deformation mechanisms in compression and bending.

Methods
This study uses relatively simple manufacturing techniques to fabricate closed and open-celled foams. Cellulose fibres are extracted from bamboo culms and used as the primary source of raw material, to reinforce a pulp slurry that is subsequently drained and solidified, and to create a fibre network that bound at the joints through microfibril bridges created by partial surface hydrolysis.

The material properties of fibres, the fibre/matrix interfacial strength, and critical fibre length are obtained through microtensile tests. The mechanical response of the resultant foams is reported in uniaxial compression and 3-point bending. These tests are coupled with strain mapping through digital image correlation (DIC), and microstructural investigation of deformation mechanisms through scanning electron microscopy (SEM).

Results
In close-celled foams, the internal microstructure consisted of interpenetrating ellipsoidal voids, with an aspect ratio of 1.5, and the compressive response was typical of elastic-brittle foams. Fibre reinforcement was exploited by two mechanisms: the delay of fracture propagation during uniaxial compression, and shear transfer during bending. The former resulted in an increase in the energy absorption/density ratio with fibre volume fraction, and the latter in an increase in the flexural rigidity and strength. A critical fibre length is reported for effective reinforcement in both cases. Bending stiffness and strength were largely determined by this parameter, but the reinforcement efficiency in compression was more sensitive to preferential fibre distribution than length.

In fibrous networks, surface treatment through partial hydrolysis was used to control the number of inter-fibre bridges. The latter increase density but reduce stiffness, due to a low joint strength. The effect of fibre length was shown in a linear compressive stiffness/density relationship, indicating that the network response was dominated by fibre stretching.

Conclusions
The response of these first cellular materials created from bamboo fibres showed that the mechanical properties are within the range of current foams. The results showed that there is a critical fibre length and distribution for optimal reinforcement efficiency. These findings indicate a potential route towards the manufacturing of sustainable lightweight materials for structural applications.

Architected cellular solids and lattices (2D and 3D) have been widely studied and integrated into many applications in mechanics. Past research has mainly focused on uniform, periodic lattices. However, lattices can be architected to possess non-uniform struts and non-periodic arrangements with interesting mechanical properties. Previous work has explored variations of non-uniformity, including linearly-graded or harmonically-graded lattices that result in staged failure. Yet designing more general non-uniform structures that optimize desired mechanical properties is challenging due to the large number of parameters. Machine learning algorithms such as neural networks can help in the design process by generating all possible geometric patterns and finding candidates that match the desired properties. However, this approach is limiting, as it is only feasible for low dimensions and can only be experimentally validated for a few design candidates that have been identified. Here, we discuss the use of a framework based on Bayesian optimization that incorporates multi-fidelity data originating from both experimental and numerical results in the pursuit of optimized properties. In this approach, the Bayesian optimization is a closed-loop design process based on Gaussian process regression which guides the choice of design variables for each iteration of experiments. We implemented this approach for the design of both 3D (body-centered cubic lattice) and 2D structures (square cellular solids), seeking to optimize for desirable combinations of mechanical properties (such as toughness, yield stress, etc.). As a “low-fidelity” data source we use numerical simulations in ABAQUS to simulate compression of architected lattices with non-uniform strut thicknesses. As a “high-fidelity” data source we perform experimental compression tests on structures that have been 3D printed using stereolithography (SLA). These data sources are fed into the multi-fidelity Bayesian optimization algorithm. The algorithm guides the choice of the next experiments/numerical simulations until sets of optimal parameters are found.

An architected material allows new opportunities in material design and synthesis by enabling new properties that are not observed in natural materials or from a bulk material that the “material” is made of based on the interplay between material and geometry. We have investigated liquid crystalline elastomer-based architected materials that harness energy absorption mechanisms across multiple length scales for extreme energy absorption. Moreover, the architected material shows strain-adaptive energy absorption behaviors with power-law relation between energy absorption and strain rate whose exponent can be tuned via controlling microscale mesogen alignment by couple nonlinear geometric and nonlinear material properties. We will report our characterization results of energy absorption behaviors of architected liquid crystalline elastomers over 6 orders of magnitudes strain rate ranges and numerical modeling results to investigate its strain-rate adaptive energy absorption mechanisms based on viscoelastic property characterization data of liquid crystalline elastomers over 20 orders of magnitudes frequency ranges. The findings from our study can contribute to realizing extremely lightweight and high energy absorbing materials, which will be beneficial for various applications, including automotive, aerospace, and personal protection.

Acknowledgements: This research is supported by the Army Research Office (Grant Number W911NF-17-1-0165) and the Johns Hopkins University Whiting School of Engineering start-up fund.

Tuning and programming the multiphysical properties of advanced materials is of critical importance for developing the next generation of adaptable multifunctional metamaterials. This study demonstrates the tunability of thermo-mechanical properties of graphene sheets by inspiring from kirigami mechanical metamaterials. The theoretical investigation, multiscale simulation, and experimentations show that the stress-strain response, the buckling-induced 3D patterns, as well as the thermal conductivity of nano-architected kirigami metamaterials can be tuned by altering geometrical parameters and cutting patterns. The thermal conductivity of kirigami-inspired graphene metamaterial can be further regulated and managed by an external mechanical strain. By analyzing and comparing the results from atomic simulation and continuum-based simulation, the effect of the length scale is discussed to explore the connection across the scales. Analytical models are also developed to predict the thermo-mechanical properties of heterogeneous kirigami and offer an explicit tool for the rapid engineering of graphene-based metamaterials to achieve desired stress-strain response and thermal conductivity. This work facilitates the applications of programmable kirigami-inspired nano-metamaterials in wearable nanoelectronics and nanoscale heat transport systems.

In solar harvesting for energy sustainability, the porphyrin compounds have been found to exhibit both photovoltaic (PV) and photothermal (PT) effects due to their unique structures. Both chlorophyll and chlorophyllin belong to the porphyrin compounds that are structurally characterized with a large ring molecule consisting of four pyrroles, denoted as the porphyrin ring. The optical properties of these compounds are typically characterized with strong absorptions near the UV and NIR regions, and high average visible transmittance. The porphyrin compounds are well-known for dye-sensitized solar cells (DSSC), bulk-heterojunction solar cells (BHJSC), and perovskite solar cells (PSC). Some of the porphyrin compounds have also been shown to exhibit strong photothermal effects. Upon sufficient solar harvesting, the photon energy can be converted to electricity or thermal energy by PV or PT through the same porphyrin thin film in an alternative fashion. Therefore, a PT- and PV- dual-modality can be designed and engineered for energy applications. In this study, the spectral-selective porphyrin thin films are applied on the window inner surface for thermal insulation in a single pane design. Upon solar irradiation, the window surface temperature can increase appreciably by the PT coating resulting in significantly reduced heat transfer, characterized by a low U-factor. Therefore, thermal insulation can be achieved optically without any interfering medium as conventionally required in the double- or triple- glazing. The building skin with a large surface area with the PT coating can also serve as an ideal substrate for PV panel for energy generation, with the dual modality that can be altered seasonably. We report the experimental results on the PT- and PV dual modality building skin based on porphyrin thin films for energy saving and generation. Also reported are the porphyrin compounds synthesis, thin film deposition, optical absorption characterization, and the photothermal/photovoltaic measurements.

The use of architecture in materials has been reported to enable novel combinations of mechanical properties such as high stiffness- and strength-to-density ratios. Furthermore, interesting size effects can appear when architected nanometer-scale features are achieved such as in pyrolytic carbon struts, which have recently been shown to exhibit rubber-like response. While this size effect has been explored in the quasi-static response of 3D beam-based architected materials, no work has explored its implications under extreme conditions. Exploiting these size effects in the dynamic regime has potential to enable ultralightweight impact-resistant materials for applications such as ballistic impact, blast loading, and micrometeoroid shielding in space.
We propose a method to fabricate and test nano-architected carbon lattice materials at supersonic ballistic impact speeds of up to ~1 km/s, allowing for in situ and post-mortem characterization. Using beam-based architectures as the periodic building-blocks, we fabricate these materials using a two-photon lithography process followed by pyrolysis, yielding a carbon-based nano-architected material. Using ∼2 μm tetrakaidekahedral unit cells, with beam diameters ranging from 300-800 nm, we explore the effects of relative density (i.e., fill fraction) on the impact response. Employing 14 μm-diameter projectiles via the laser-induced particle impact test (LIPIT), we analyze the effects of varying impact velocities from 50 – 1,100 m/s, attaining nominal strain rates on the order of 10⁶ s−1 to 10⁷ s⁻¹. We observe three distinct response regimes: (i) elastic impact, (ii) cratering and particle rejection, and (iii) cratering and particle capture, depending on the relative density of the lattice material as well as the impact energy. Our experiments demonstrate impact energy dissipation of up to 1.1 MJ/kg in these materials, outperforming traditional impact-resistant materials such as steel, aluminum, PMMA, and Kevlar. By analogy to planetary impact, we also introduce predictive tools for crater formation in these materials using dimensional analysis. These results uncover an extreme-condition regime over which nano-architecture can enable the design of new generations of ultra-lightweight, impact-resistant materials.

How can we overcome the formability of materials beyond original materiality ? To answer this question, we have adopted “simple geometric patterned structures” that significantly enhance flexibility and stretchability of brittle or stiff materials. Geometrical design such as auxetics, origami and kirigami provides us many examples of the formation of delicate and, detailed patterns leading to the effective distribution of stresses. Instead of process control and complex structure design, we employed the simple juxtaposition of unit design leading to simple, cheap and easily processed flexible, stretchable and deployable structures for deformable devices. In addition, newly developed computational approach expands our design dimension to the next level. We believe that geometrical modulation by controlled size, shape, and symmetry adds another dimension unleashing the limitation of conventional design space of structure materials

Phononic crystals represent a very promising route for tuning the properties of next-generation nanoelectronics, thermoelectrics, and ultralight materials. These consist of periodic arrays embedded in an elastic medium, arranged in a specific lattice symmetry [1,2,3]. Nanofabrication techniques can now produce nanoscale phononic crystals with dimensions <<100 nm (referred to as metalattices) which make it possible to engineer new elastic and transport properties [4,5]. To fabricate metalattices, nanospheres are first assembled into a colloidal crystal with face centered cubic order. This base structure can be tuned from monolayer to microns in thickness, with sphere sizes from the nanometer (nanoscale opals) to microns (opals) [1,6]. The interstitial space between the nanospheres of the colloidal crystal is then infiltrated with another material, forming a metalattice structure for periodicities in the sub-100nm. To understand the mechanical properties of metalattices, studies to date have focused only on one component of the metalattice—either the template or the etched-out structures, and always for periodicities >100 nm. In this work, we present a nondestructive method to accurately extract, for the first time, the mechanical and structural properties of metalattices with much smaller feature sizes. Specifically, we probe silicon metalattices fabricated from sphere diameters of 14 nm and 30 nm, with periodicities of 19 nm and 42 nm, respectively. These metalattices contain feature sizes that are an order of magnitude smaller than opals that have been characterized to date. We use an ultrafast laser pulse to heat a set of transducer gratings, which impulsively launch surface acoustic waves (SAW) in the metalattice. The wavelength of the SAW can be tuned by varying the transducer grating periodicity, which also changes the SAW penetration depth into the metalattice and the silicon substrate. We then monitor the SAW frequency from the time-dependent change in extreme ultraviolet (EUV) light diffracted off the grating. This method allows us to simultaneously extract the acoustic dispersion, as well as the Young’s modulus, thickness and filling fraction of the metalattice. Interestingly, the extracted mechanical and structural properties agree well with macroscopic predictions, while the transport properties of the same metalattices do not agree with bulk models. Additionally, the measured metalattice thicknesses agree with scanning electron microscopy images and the extracted Young’s moduli agree with nanoindentation measurements, while achieving higher accuracy. Finally, this technique represents the only approach to date to nondestructively validate the filling fraction of deep-nanoscale metalattices. It also has advantages over destructive electron imaging because it probes over large areas and does not suffer from material contrast issues. These results can enable precise fabrication, characterization and understanding of materials with tailored mechanical and transport properties.
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As concrete is a brittle material, it must generally be reinforced for use in building applications. Steel-reinforced concrete is a composite in which the majority of the compressive strength stems from the concrete and the steel reinforcement provides the tensile strength. Typically, the steel reinforcement often takes the form of a 1-D reinforcing bar or 2-D cage. However, the reinforcement geometry is not usually optimized for the particular loading scenario that the structure will experience. Additionally, the placement of the concrete itself may be optimized for particular applications, which may reduce the CO2 emissions for concrete structures.

The minimum amount of steel reinforcement in a concrete structure is dictated by the loads it must withstand. Further, building codes specify a maximum amount of steel allowed within a structure; concrete structures typically do not have a steel reinforcing ratio greater than 6%. As our goal is to optimize the amount and placement of both the steel reinforcement and the concrete within the overall structure, this is a constrained, multi-material topology optimization problem. Our approach is innovative, as we use the specific loading structure to inform where the steel and concrete should be placed and use digital manufacturing techniques to create that optimal steel reinforcing structure.

We use the Topology Optimization (TopOpt) Grasshopper tool developed at the Technical University of Denmark to inform the optimal steel placement within the concrete structures. TopOpt’s objective is to minimize the structure’s compliance, with constraints on the material volume and the void shape. TopOpt assumes that the concrete has a compressive stiffness that is 10 times greater than its tensile stiffness. This material anisotropy serves to ensure the concrete and steel are placed properly within the geometry. The loading scenario considered here is a flexural beam undergoing four-point bending. When using these topology optimization results to inform the 3-D reinforcement geometry design, the result is a steel volume fraction of 6%.

Digital fabrication technologies are well-suited to creating these complex reinforcement structures. We used an Omax 2626 to waterjet cut these architected, steel reinforcement geometries, placed this optimized steel reinforcement geometry into a 3” x 3” x 11” mold, and casted concrete around it. In this first structure, the placement of steel is optimized, but the placement of concrete is not. We created a second concrete geometry, wherein both the placement of steel and the concrete is optimized. This geometry contained 30% less concrete than the first geometry. Finally, we created a non-optimized, straight, steel rebar geometry and used it to reinforce a 3” x 3” x 11” concrete beam (for comparison purposes).

In four-point bending tests, we found that the beam with optimized steel reinforcement reached a peak load that is 1.28 times greater than the peak load reached by the beam with straight rebar reinforcement, despite the two beams having the same steel reinforcing ratio. Additionally, the beam with optimized steel reinforcement reached a toughness that is 1.38 times greater than the toughness reached by the beam with straight rebar reinforcement. When we compared the beam with optimized steel reinforcement and the beam with both optimized steel reinforcement and optimized concrete placement, we saw that the peak load is reduced by 18%, and the toughness is reduced by 26%.

These results suggest that optimizing the placement of steel reinforcement within concrete structures can have a significant improvement on the overall mechanical properties. Additionally, optimizing the placement of the concrete—and reducing the amount of concrete used—allowed us to create a structure with a 30% weight reduction, which may be beneficial in seismic applications.

Lithium-ion batteries (LIBs) with high energy and power density are crucial for energy storage applications such as vehicle electrification. Traditional LIBs have planar electrodes wherein their performance can be optimized for energy or power, but not both simultaneously. Three-dimensional (3D) electrodes have been shown to mitigate these trade-offs by using 3D architecture to enable fast ion transport in thick battery electrodes.¹ However, there is a need for more scalable, large-area fabrication methods for 3D electrodes. In this work, we expand a computational model² to analyze the feasibility of combining acoustic focusing with additive manufacturing to fabricate line-patterned battery electrodes. Acoustic focusing uses acoustic forces to control particle placement in a fluid, and its ability to align particles on a micron-scale make it a promising technique for 3D electrode fabrication. In our model, we solve for trajectories of particles in a fluid using analytical models of the focusing forces, which we compare to experimental results. We apply this model to battery-relevant materials to determine how focusing parameters such as viscosity, particle size, and volume fraction impact electrode geometry for optimizing battery energy and power performance. Our initial results suggest that lithium cobalt oxide (LiCoO₂) and lithium nickel manganese cobalt oxide (LiNi_0.33Mn_0.33Co_0.33O₂) are highly compatible with acoustic focusing.

1. C. L. Cobb and M. Blanco, J. Power Sources, 249, 357-366 (2014).
2. R. R. Collino, T. R. Ray, L. M. Friedrich, J. D. Cornell, C. D. Meinhart and M. R. Begley, Mater. Res. Lett., 6, 191–198 (2018).

Cellular materials, including stochastic foams and ordered lattices, have been used extensively for their weight efficient properties. The nearly-constant crushing stress of stochastic foams make them ideal for energy absorption applications. Notwithstanding, their strengths are low relative to those of ordered lattices. Although ordered lattices can attain higher strengths, they exhibit internal buckling and concurrent strain softening during compressive loading, making them undesirable for energy absorption applications. Although the performance of single-material foams and lattices has been studied extensively, recent advancements in additive manufacturing have opened new possibilities of multi-material lattice structures.

This talk will outline the design considerations for bi-material lattices that have potential for tuning the balance between high strength and high straining capability. To begin, a primitive structural motif that exhibits the desired behavior is identified and its compressive response is analyzed. A 2D multi-cell lattice based on the primitive motif is designed and several material variants are fabricated and tested. Analysis of test results addresses effects of finite node dimensions, constraints on member rotation at the nodes, free edges, and friction with the loading platens as well as limits dictated by rupture of tensile members or buckling of compressive members. The study culminates with guidelines on design of bi-material lattices with high strength and high straining capability.

Topology-optimized architected materials can feature unique mechanical behaviors, including unusual density-stiffness scaling. However, such materials have then been largely limited to periodic lattices. Here, we use machine learning and molecular dynamics to explore the effect of disorder as a new degree of freedom. We develop a Bayesian optimization framework that prescribes targeted topologies, which are subsequently validated by molecular dynamics simulations, which, in turn, generate new data to refine our machine learning model. This iterative learning pipeline is used to accelerate the discovery of light yet stiff disordered phases. We show that, at constant average local topology, disordered phases behave differently than their crystalline counterparts. This establishes disorder as a promising degree of freedom to discover new exotic phases with unusual mechanical properties.

Numerous three dimensional (3D) nanofabrication methods have been proposed for novel applications in mechanical metamaterials. However, highly periodic 3D fabrication in large area and volume has limited success. Here I present our recent efforts to expand the limit in size of highly periodic 3D nanostructures through Proximity field nanoPatterning (PnP) which uses conformal phase masks with outstanding scalability and easiness of the large area patterning. After brief overview of 3D nanofabrication technique and potential application fields, mechanical behaviors of nanocomposites based on 3D nanostructures will be discussed. Current applications are stretchable light scatter, interphase boundary nanocomposites (ceramic-polymer, ceramic-metal). The change of optical and mechanical behavior by changing the constitute materials and the geometry and thickness of cellular materials will prove the importance of structural motif in submicron, ordered porous materials and composites.

3D lightweight, architectured cellular structures that are fabricated via additive manufacturing methods are currently receiving much interest due to the tunable properties that can be realized by simply tuning the architecture. In addition, the use of nanoscale materials that have excellent mechanical properties as building blocks to create a large scale bulk material via hierarchical structuring allows for realization of a bulk material that display the excellent properties of the constitutive nanoscale materials. In this study, a simple bulk fabrication based on directional freeze-drying method was developed for synthesis of fully recoverable ultralight 3D porous nanocomposite that is composed of Ag nanowire/cellulose nanofiber(CNF). Freeze-drying method is a one-step process via ice crystal formation followed by sublimation of ice crystal which leaves behind the Ag nanowire/CNF nanocomposite walls to form the 3D porous structure. Compression tests on vertically and horizontally aligned Ag nanowire/CNF nanocomposite walls showed highly anisotropic mechanical properties. An optimized concentration of Ag nanowires resulted in the compressive strength of 100 kPa at a relative density of 0.96%, which has 1.5 times higher strength when normalized by the material density compared to that of metal microlattices fabricated by lithography methods. Horizontally aligned 3D porous structure showed perfect restoration while maintaining sufficient electrical conductivity that is suitable for flexible electronics and 3D strain sensors. Proposed freeze-drying methodology was also used to fabricate other 3D nanocomposite using polyvinylalcohol and Ag nanowire as a hard core that was then infiltrated with PDMS to create a hard core-soft matrix composite. The compression tests of the developed 3D bulk nanocomposite indicated excellent resilience and recoverability.

Architected materials can exhibit mechanical properties that do not occur with ordinary solids. By integrating hierarchy and size effects, micro-architected metamaterials fabricated by two-photon lithography with metallic or ceramic coating can be ultra-strong but lightweight. However, the attainment of both strength and ductility is generally mutually exclusive. Inspired by the Pantheon dome in Rome, which can withstand high load while keeping low density, micro-architected domes with a gradient helix are designed and deposited in a hierarchical nanostructured aluminum film with ultrahigh strength and considerable plasticity. Despite a thick coating usually causes catastrophic collapse, the thick-walled metallic dome shows recoverability during compression. The compressive strength significantly increases compared to current ductile-like micro-lattices. Detailed experimental and computational works reveal the graceful (non-catastrophic) failure due to the helical twisting and plastic flow in the supra-nano material. It is a promising method of suppressing brittle failure via a combination of architectural and material design. It can be used to impart enhanced functionality, making programmable stiffness, and tailored energy absorption all possible.

Architected materials hold much promise in achieving unprecedented structural properties that can significantly benefit applications are required to be stiff, strong and energy absorbent, with minimal mass, such as those in the aerospace, automobile and sports industries. The recent advent of additive manufacturing has sped up the pace of research in this field considerably by allowing complex, optimized designs to be realized easily and reliably. Nevertheless, many non-polymeric materials remain incompatible with most 3D printing schemes, and the few that are, require specialized equipment and treatments that remain far too costly for many industries to adopt. Here, we attempt to address this innovation gap by developing materials that are compatible with commercial stereolithography 3D printers originally designed for photosensitive polymers. Such printers are cost-effective, scalable and can produce structures that are mechanically isotropic with a fine resolution. Two strategies were adopted. First, non-polymeric powders were mixed into photosensitive resin and subjected to SLA 3D printing. Second, 3D printed polymeric structures were dip-coated in a mixture of non-polymeric powder and resin. In both cases, subsequent de-binding and sintering steps were required to remove the polymers and coalesce the non-polymeric powders. Lattices with solid struts and hollow tubes, made out of metal, ceramic and composites were successfully demonstrated. The microstructures of these lattices were examined in detail through SEM and micro-CT scans and related to their mechanical properties using simple analytical relationships.

Although much attention has been paid to the development of photothermal materials and designs that can convert solar irradiation into exploitable thermal energy, it remains some obstacles such as limited light absorbing band and narrow incident angle. This study proposes a black gold-coated hierarchical nanoturf (Au/h-nanoturf) membrane incorporated with randomly distributed high aspect ratio (AR) nanostructures and micro-through holes. Thanks to structural advantages, this large area membrane exhibited good absorption of the broadband solar light spectrum. The h-nanoturf is further combined with a microcone array to enhance solar absorption extended to the near-infrared range as well as the omnidirectional incident direction of the light. The fundamental mechanism of the strong omnidirectional broadband absorption performance of the h-nanoturf was thoroughly analyzed with computational electrodynamics simulations. Consequently, we employed the Au/3D h-nanoturf with microscale hole (µ-hole) membrane to fabricate an advanced solar-vapor generator.

In this review talk, we present our experimental and conceptual progress on three-dimensional (3D) microstructured chiral mechanical metamaterials, which we have introduced (T. Frenzel et al., Science 358, 1072 (2017)) and summarized (I. Fernandez-Corbaton et al., Adv. Mater. 31, 1807742 (2019)) previously. This includes recent results for the dynamic as well as for the (quasi-)static regime.

In the dynamic regime, we have previously discussed the possibility of chiral metamaterial phonons and acoustical activity, the elastic counterpart of optical activity (T. Frenzel et al., Nature Commun. 10, 3384 (2019)). In 3D metamaterials, the magnitude of the polarization rotatory power as well as the operation frequency can be tailored by rational design. However, acoustical activity has been highly anisotropic in these simple-cubic lattices and essentially restricted special propagation directions. More recently, we have designed architectures enabling nearly isotropic chiral phonons and nearly isotropic acoustical activity in 3D over a large bandwidth. This includes the possibility of 3D chiral quasi-crystalline mechanical metamaterials (Y. Chen et al., Phys. Rev. Lett. 124, 235502 (2020)) as well as of chiral triclinic (Y. Chen et al., submitted (2020)) and chiral simple-cubic truss lattices (Y. Chen et al., in preparation (2020)). The different underlying principles behind obtaining isotropic elastic behavior in 3D are discussed and compared.

In the (quasi-) static regime, 3D chiral mechanical metamaterials allow for further behaviors “forbidden” in classical Cauchy elasticity such as, e.g., conversion of strain to twist. These behaviors are intimately connected to a loss of scale invariance, i.e., the mechanical behavior depends on the number of unit cells in the metamaterial specimen. We build on our previous conceptual work on tailoring a characteristic chiral length scale. When divided by the lattice constant, the characteristic length scale turns into the characteristic number (P. Ziemke et al., Extreme Mech. Lett. 32, 100553 (2019)). We have designed more practical architectures that we have manufactured by multi-focus multi-photon 3D laser nanoprinting with as many as more than one hundred thousand unit cells total and more than three hundred billion voxels. Specifically, we can tailor the twist/strain to increase linearly up to a characteristic number of unit cells, beyond which the twist/strain asymptotically decays inversely with the number of unit cells. We achieve characteristic numbers as large as about ten experimentally. Conceptually, the characteristic number can probably be made arbitrarily large. This statement is backed up by numerical simulations on different levels of approximations as well as by a simple and intuitive analytical model (T. Frenzel et al., in review (2020)).

Mechanical energy harvesting (MEH) is the technology that utilizes ambient mechanical energy available in nature such as sound, vibrations, and all human-derived kinetic motions to convert into useful electricity. MEH is regarded as a great platform to provide sustainable power solution to operate wireless sensors, one of the key essentials for Smart City, particularly without the need to replace after a certain amount of time. Although attractive, insufficient power generation is still the issue to solve in MEH. Metamaterials, artificially engineered structures, have proved useful for input mechanical energy manipulation, thus providing a new paradigm of solution to enhancing power generation in MEH. Here, I will summarize a collection of advances that push the boundaries to achieve a new paradigm of energy harvesting systems using various metamaterial designs ranging from phononic crystals, elastic and acoustic gradient index (GRIN) metamaterials, locally-resonant acoustic materials, to elastic instability-based mechanical metamaterials. These metamaterials exhibit unconventional material properties such as negative mass density, negative bulk modulus, and programmable stiffness, thus enabling intriguing phenomenon including band gaps, negative refractions, and shape and pattern transformation capabilities. All these unusual and intriguing phenomena can contribute to tailoring and amplifying input mechanical energies, ultimately leading to drastic enhancement in overall power generation of MEH systems.

Two-photon lithography is a well-established technique for the creation of 3D polymer structures with sub-micron architecture. Recent work in the past few years has focused on the combination or integration of two-photon lithography with other fabrication processes for creating 3D nanoarchitectured structures from other materials such as ceramics and metals. Toward this goal, two fabrication schemes will be presented for creating 3D nanoarchitectured mechanical metamaterials from metals using electroforming, also called template-assisted electrodeposition. In the first example, two-photon lithography is used to create a 3D photoresist structure that serves as a template for fabricating a 3D nanoarchitectured metal matrix composite with a ceramic nanolattice reinforcement through electroforming and atomic layer deposition. Structural characterization and initial in situ micromechanical testing data will be shown, including studies at high temperatures. In the second example, the resolution of 3D electroforming is improved to below 200 nm by pyrolysis of two-photon lithography templates. We fabricate metal nanolattices with strut diameters down to 150 nm and test the resolution of the technique by creating free-standing single metal nanowires/pillars with diameters down to 60 nm. In situ micromechanical testing of these lattices will be presented.

In this work, we present preliminary results towards fabrication of 3D architectures using near field electrospinning as a novel additive manufacturing technology. Electrospinning is a technique of drawing a nanofibril string from a charged polymer droplet under a high electric field. In far-field electrospinning, the distance between the spinneret and the collector is quite large (5-10 cm), which leads to bending instability along the nanofiber jet resulting in random distribution of the deposited fibers on the collector. Keeping the electric field constant, reducing the distance between spinneret and collector (typically ≤ 1 cm) results in a highly focused electric field which leads to a precise deposition of the fibers. Such low-distance electrospinning is termed as near-field electrospinning (NFES). NFES has enabled the fabrication of several 2D geometries of various materials ranging from polymers to ceramics for a wide range of applications including piezoelectric nanogenerators, biomedical devices, wearable sensors, FETs, photodetector, and MEMS structures. Due to the ability of precise nanofiber deposition, several researchers have attempted to develop NFES as an additive nanomanufacturing tool. However, these studies lack fundamental analysis of the NFES and involve additional complex components. Here we focus on exploring the fundamentals of NFES towards developing it as a high-speed additive nanomanufacturing tool. Currently we are characterizing several process parameters, such as electric field, printing speed, viscosity, conductivity, and dielectric constant of the solution towards enabling the nanofibers to stack onto each other leading to the fabrication of 3D structure. The outcome of this work will enable NFES to fabricate high aspect ratio 3D structures of stacked nanofibers with various geometries at a high processing speed, compared to other additive nanomanufacturing technologies. Such structures can be useful for various applications such as electrodes for energy devices and scaffolds in tissue engineering.
We have used polyethylene oxide (PEO) as the polymer, and a rotating drum as the high-speed moving collector to make 3D walls of stacked nanofibers. We have studied the effect of different solvents and collector material on the 3D stacking of the nanofibers. The results show that collector conductivity and vapor pressure of the solvent play a crucial role on the stacking of the fibers. The experiments so far have resulted in a 3D stacked wall of PEO with a maximum height of ~250 µm and diameter of ~10 µm, while using a chromium/gold substrate and dichloromethane as the solvent.
Our ongoing work is to optimize the processing parameters to narrow down the diameter of nanofiber and to achieve the straight wall of stacked fibers with high aspect ratio. We plan to use these NFES-driven 3D stacked walls of nanofibers as scaffolds for tissue engineering. Furthermore, we target to achieve 3D structures of various other materials such as carbon and metal oxides using this NFES-driven additive nanomanufacturing process.

Most architected materials to date have enabled properties such as extreme stiffness and strength, at levels unattainable independently by their constituent materials, with few works focusing on the compliant, extensible regime pertinent to applications such as flexible electronics and tissue engineering. While exceptional mechanical properties such as extreme resilience and high deformability have been realized in many three-dimensional (3D) architected materials using beam-and-junction-based architectures, stress concentrations and constraints induced by the junctions limit their mechanical performance. Compliant architected materials that overcome these limitations, in combination with conductive components or constituent materials, could open a new path for the applicability or architected materials.

We present a new hierarchical design concept for flexible 3D architected materials in which fibers are interwoven to construct effective beams, such as the ones in classical monolithic-beam lattice architectures. Via in situ tension and compression experiments of additively manufactured polymeric woven and monolithic lattices at the microscale, we demonstrate the superior ability of woven architectures to achieve high tensile and compressive strains (>50%)—without failure events—via smooth reconfiguration of woven microfibers in the effective beams and junctions. Cyclic compression experiments reveal that woven lattices accrue less damage compared to lattices with monolithic beams. We employ numerical studies of woven beams with varying geometric parameters to present new design spaces for the development of architected materials with tailored compliance that is unachievable by similarly configured monolithic-beam architectures. This woven hierarchical design offers a pathway to make traditionally stiff and brittle materials more deformable by introducing a new building block for 3D architected materials, with complex nonlinear mechanics, applicable to flexible electronics applications.

The fundamental topology of cellular structures -- the location, number, and connectivity of nodes and compartments -- can profoundly impact their acoustic, electrical, chemical, mechanical, and optical properties, as well as heat, fluid and particle transport. Approaches harnessing swelling, electromagnetic actuation as well as mechanical instabilities in cellular materials have enabled a variety of interesting wall deformations and compartment shape alterations, but the resulting structures generally preserve the defining connectivity features of the initial topology. Achieving topological transformation presents a distinct challenge for existing strategies: it requires complex reorganization, repacking, and coordinated bending, stretching, and folding, particularly around each node where elastic resistance is highest due to connectivity. Here we introduce a two-tiered dynamic strategy to achieve systematic reversible transformations of the fundamental topology of cellular microstructures that can be applied to a wide range of materials and geometries. Our approach only requires exposing the structure to a liquid whose composition is selected to have the ability to first infiltrate and plasticize the material at the molecular scale, and then, upon evaporation, to form a network of localized capillary forces at the architectural scale that zip the edges of the softened lattice into a new topological structure, which subsequently re-stiffens and remains kinetically trapped. Reversibility is induced by applying a mixture of liquids separately acting at the molecular and architectural scales -- thus offering modular temporal control over the sequence of the softening-evaporation-stiffening actions -- restoring the original topology or providing access to intermediate modes. Guided by a generalized theoretical model connecting cellular geometries, material stiffness and capillary forces, we demonstrate programmed reversible topological transformations of various lattice geometries and responsive materials, undergoing fast global or localized deformations. We then harness dynamic topologies for developing active surfaces with information encryption, selective particle trapping and bubble release, and tunable mechanical, chemical and acoustic properties.

Traditional lattice metamaterials have greatly expanded the range of achievable material properties; however, they are generally physically continuous throughout their volume, and thus cannot take advantage of contact interactions or multi-body behavior. Here we present the new concept of an interpenetrating lattice, where two or more lattices interlace through the same volume without any direct connection to each other. Interpenetrating lattices greatly expand design possibilities, allowing single material printers of all types to print composite metamaterials irrespective of material or length scale. While the geometry defining interpenetrating lattices has been studied since the days of Euclid, additive manufacturing allows us to turn these mathematical concepts into physical objects with remarkable properties including reduced transmission of thermal, electrical, shock and vibration loads, increased toughness, multi-stable/negative stiffness behavior, and unusual energy transduction. In this first study on interpenetrating lattices, we reveal remarkable mechanical properties including improved toughness, multi-stable/negative stiffness behavior, and electromechanical coupling.

Carbon-carbon (C/C) composites are utilized by the aerospace industry primarily as structural materials due to their high thermal stability, low density, and good mechanical properties. Because of the aerospace industry relevance and unique properties of these materials this work will explore opportunities to manufacture complex parts via AM. Direct ink writing (DIW), an AM technique, offers a unique advantage in that it is amenable to printing of chopped fiber reinforcements and continuous fiber reinforcement. In DIW, complex structures are formed from the deposition of shear-thinning inks (suspensions or pastes) extruded through a nozzle. An added benefit of this process is that alignment of the fiber reinforcement is achieved during processing, allowing access to conventional fiber composite configurations. Here we present a route for the production of C/C composites by DIW of phenolic-based carbon fiber composite inks. Molecular gelators were employed as rheological modifiers to facilitate DIW of an otherwise low-viscosity liquid phenolic resin and chopped carbon fiber with an acid catalyst to accelerate low temperature curing. The rational for ink formulation and assessment of the subsequent rheological behavior will be discussed as they pertain to printability in addition to its effects on the conversion of these materials to C/C.

Owing to their outstanding properties including ultralow density, high specific surface area, and low thermal conductivity, aerogels are being considered for several applications including separation, thermal insulation, tissue scaffold, electromagnetic and sound absorption, sensors, supercapacitors, and catalysis. Generally, conventional aerogels are synthesized from a gel (either hydrogel or organogel) including multiple solvent exchange steps followed by solvent removal via oven, freeze, or supercritical drying. The complexity of this process not only raises the cost of fabrication but also significantly increases the duration of the process. We present a sustainable approach to prepare aerogels from sol-gel electrospun hybrid organic-inorganic nanofibers of polyvinylpyrrolidone (PVP)-titania (TiO₂). This facile and robust methodology resulted in the fabrication of a 3D self-supported cellular structure with elasticity, low density (~10 mg.cm^-3), and hierarchical porosity consisting of primary (1-5 µm) and secondary pores (10-60 µm). XPS analysis and SEM demonstrate that titania networks developed during sol-gel electrospinning and uniformly distributed inside the nanofibers. Moreover, FTIR analysis provides evidence that PVP and TiO₂ components chemically interact during the sol-gel processing through Ti-O-C linkage. The PVP-TiO₂ nanofibrous aerogel exhibit high mechanical flexibility, hierarchical porosity, and low density demonstrating their possible applications in diverse fields like filtration, CO₂ capture, and antimicrobial.

Recent advances in additive manufacturing have enabled the production of nanoarchitected material, consisting of truss structures with sub-micron geometric features. These materials have achieved unprecedented specific stiffness and strength as well as extraordinary energy absorption and recoverable compressibility. As additive manufacturing techniques continue to improve, scaling of nanoarchitected materials will reach the component level. Modeling of these parts will require a multiscale approach to span the range of length scales between the sub-micron features of the trusses and the component scale. While numerous homogenization schemes have been developed for multiscale modeling of architected materials, most are limited to capturing material and geometric effects in the linear and early yield regimes. While these methods may be sufficient for predicting the specific stiffness and strength, they are not useful in designing materials for energy absorption and large deformation applications.

In this work, we present a multiscale model for architected materials that captures material nonlinearities including yield, hardening, and failure as well as geometric nonlinearities including buckling, post-buckling softening, and densification. We employ an FE^2 approach where the component-scale deformation is modeled by finite element (FE) analysis and the microscale deformation is modeled by another FE simulation of the representative unit cell (RUC). A homogenization scheme is used to determine the stress-strain constitutive relation for the component-scale FE simulation through numerous evaluations of the RUC. The model is integrated into a computational framework for multiscale modeling that handles resource management and fault tolerance to allow many RUC evaluations to occur in parallel, enabling efficient use of high-performance computing resources.

We propose a composite light sail composed of nanometer-thick corrugated layers of alumina and molybdenum disulfide that can be used to accelerate a gram-scale probe to relativistic velocities for interstellar space travel. Our mechanical calculations suggest that this sail can sustain radiation pressures of tens of Pa, necessary to achieve a velocity of roughly a fifth the speed of light. In addition to our numerical analysis, we are demonstrating the manufacturability of this concept by producing prototype sections of the composite material using standard microfabrication techniques. Our results represent one of the first evaluations of sail robustness and fabricability of ultrathin light sails and highlight the need for additional research on the optical and mechanical properties of thin Al₂O₃ and MoS₂ films, particularly at elevated temperatures.

The Breakthrough Starshot Initiative aims to send an ultralight spacecraft to Proxima Centauri B within the next half century by accelerating it using a highly reflective light sail and an array of Earth-bound high-power lasers¹. A sail that will accomplish this must have a total mass of approximately 1 g and an area of 1-10 m² to minimize the size of the laser system, a high reflectivity over the entire doppler-shifted laser wavelength range to minimize the required laser power, a high emissivity at mid-infrared wavelengths for efficient radiative cooling, a shape that provides beam-riding stability over long distances, and mechanical robustness to maintain its form without tearing or fracturing. Several recent studies have examined the optics and dynamic stability of sails, proposing spherical designs², planar photonic heterostructures³, and textured photonic metasurfaces⁴. However, few studies have incorporated mechanical robustness and manufacturability assessments into their proposals.

Therefore, we propose a novel sail that addresses mechanical robustness, manufacturability, beam-riding stability, optical characteristic, and mass property considerations. Our design involves a ~2 m circular, spherically curved sheet comprised of several interlocking gores, each of which is composed of roughly 70 layers (~50 nm) of highly reflective (>50%) MoS₂ sandwiched between two 5-nm optically transparent Al₂O₃ films. We have incorporated a micron-scale hexagonally patterned corrugated rib structure into the sail that increases its bending stiffness⁵ by 10-100× and reduces its tensile stiffness⁶ by 2-3× relative to planar films, preventing it from clinging to itself and making it more robust against deformation and fracture. Our stress calculations, performed using a thin-walled pressure vessel model, confirm that the sail can tolerate the laser-induced acceleration forces with failure margins of 10-30%. Planned next steps include using finite element methods to validate our calculations and implementing an energy balance to quantify the temperature variation of the sail throughout its acceleration and the impact of thermal effects on its mechanical response.

Finally, to demonstrate this concept, we are fabricating small prototype sections of this sail. We use photolithography and deep reactive ion etching on a silicon wafer to create a hexagonal rib mold, use atomic layer deposition to conformally-coat the mold with a thin Al₂O₃ film, transfer a thin MoS₂ flake to the surface of the film, deposit a second Al₂O₃ film, and release the result using XeF₂ etching. We intend to extend this process to larger areas using chemical vapor deposition to directly synthesize MoS₂ upon the Al₂O₃. Taken together, our modeling and fabrication results indicate the possibility of laser-driven relativistic interstellar travel.


1) Atwater Nat. Mater. 17(2018)861
2) Manchester Astrophys J. Lett. 837(2017)L20
3) Ilic Nano Lett. 18(2018)5583
4) Salary Laser Photonics Rev. 14(2020)1900311
5) Davami Nat. Commun. 6(2015)10019
6) Jiao Extreme Mech. Lett. 34(2020)100599

The field of architected materials has been invigorated by a revolution in additive fabrication methods with micro- and nanoscale precision, enabling the creation of architected materials that can exploit the enhanced properties of nanomaterials. Despite many developments in this area, the mechanical properties of these nanoarchitected materials are impaired by designs that i) have little mechanical tunability and ii) have a poor distribution of load carrying elements. This work investigates the fabrication and mechanical characterization of a new class of highly tunable, tensegrity-inspired nanoarchitected materials. These are created in a two-step process involving two-photon lithography (TPL) to create a polymeric framework followed by pyrolysis to convert the material to amorphous carbon. This procedure relies on the size-dependent shrinkage of beams during pyrolysis, which enables nanoarchitectures to be controllably prestressed by varying the size of different elements in the structure. We demonstrate that these amorphous carbon nano-tensegrities have highly tunable mechanical properties that can be varied independently of their relative density, allowing more optimal strength and stiffness per unit mass.

Designing architected metamaterials is overwhelming for many computational approaches because of the large number and complexity of flexible elements that constitute their architecture—especially if such elements don’t repeat in periodic patterns or collectively occupy irregular bulk shapes. In this work, we introduce the freedom and constraint topologies (FACT) methodology, which leverages simplified assumptions regarding the topology, geometry, and constituent properties of such materials to enable their design with ~6 orders of magnitude greater computational efficiency than other approaches (e.g., topology optimization).

FACT utilizes a comprehensive library of intuitive geometric shapes, which embody the mathematics of screw theory, constraint-based design, and projective geometry. One set of shapes, called freedom spaces, represent all the ways a compliant system can deform with high compliance. Another set of shapes, called constraint spaces, represent the region of space within which flexible elements should be placed for exhibiting the compliant deformations of their complementary freedom spaces. By obeying simple systematic rules, designers (or an automated code) can rapidly search the full design space of compliant topology solutions, embodied by the complementary geometric shapes of the complete FACT library.

In this work, we will explain how FACT can be used specifically to design directionally compliant metamaterials, transmission-based architected materials such as materials that achieve desired Poisson’s ratios, and materials with Poisson’s ratio’s that fluctuated spatially and temporally.

Materials with heterogeneous microstructures have been shown to exhibit a superior combination of strength, ductility and toughness compared to both homogeneous nanostructured and coarse-grained materials. However, only limited progress has been made in the synthesis of heterogenous microstructure materials with robust control of key microstructural parameters. Here we report a novel technique to synthesize metallic films with precisely defined bimodal microstructures using magnetron sputtering, wherein the size, volume fraction and connectivity of nanocrystalline (< 100 nm) and coarse domains (> 1000 nm) can be explicitly controlled. Using this technique, we synthesized pure iron films in which the nanocrystalline and coarse-grained domains were configured to be in different spatial arrangements like series and parallel. Along with the spatial arrangements, the volume fraction was also varied by changing the size of these domains. The microstructure of these films was characterized using EBSD and TEM, and tensile testing was performed using MEMS based testing stages. As anticipated, the spatial arrangement (series versus parallel) of the nanocrystalline and coarse domains markedly altered the stress-strain response, with the parallel arrangement leading to co-deformation and higher strength. In addition, films containing higher volume fraction of nanocrystalline domains showed higher strength. More interestingly, the orientation of the coarse domains with respect to the loading axis also had a significant influence on the mechanical behavior due to the constraints it imposed on dislocation motion. These results indicate that this synthesis method can be used to systematically tailor the mechanical behavior of metallic films in a repeatable manner.

Premature failure of materials in extreme thermal and mechanical environments has been a challenge for decades. Extreme temperatures can enable enhanced efficiencies, e.g. in jet engines, or they may be required for normal operation, e.g. in hypersonic vehicles and spacecraft. The ceramic coating layer used for protection are prone to cracks and delamination due to inherent property mismatch between the layers. This talk describes the development of durable nanoarchitected alumina coatings with high impact and thermal shock resistance.

The coating was designed to have a gradient spinodal shell structure, its uniform stress distribution and minimize stress concentrations to prevent cracking and the stiffness of the spinodal shell was tailored by setting a preferential surface formation direction. A spinodal structure with gradient stiffness is chosen to ensure better mechanical interfacing with the metal substrate. The mechanical resilience of the material is achieved by using an architecture that exploits shell buckling as the dominant deformation mechanism. The thermal shock resistance was enhanced by maintaining the relative density below 0.04. The performance of the coating was investigated through numerical and experimental investigations. High strain rate and sequential thermo-mechanical simulations were performed to evaluate the stress and temperature distribution in an extreme environment. Nanoindentation and laser thermal shock experiments were performed to explore the failure mechanism. Through this work we demonstrate a new approach to create thermal protection coatings. Instead of searching for new advanced materials, we use the existing materials and use nanoarchitecture to make them resilient to extreme thermal and mechanical loads.

Natural materials exhibit remarkable toughness and damage tolerance, properties that stem from their hierarchical architecture built up from the nano- and microscale. Many of the underlying mechanisms behind these novel mechanical properties are not well understood at microstructural length scales. Replicating the architectural complexity of natural materials is not only challenging in terms of fabrication but is also time-consuming and expensive. Therefore, it is important to systematically develop and study complex nano- and microarchitectures to understand their effect on mechanical properties and thereby develop effective material design strategies.

In this study, we analyze crack propagation and energy absorption in dense microscale nanoarchitected Bouligand beams inspired by the dactyl club of a mantis shrimp. We developed a fabrication process utilizing two-photon lithography to create polymeric beams with increasing architectural complexity, starting from a beam with unidirectional fibers and moving to a herringbone structured beam with out-of-plane wavy fibers. We extended these methods to include a secondary ceramic phase between the polymeric fibers to understand the effect of material modulus mismatch on crack propagation at the microscale, a feature commonly witnessed in tough natural materials. We present the results of in-situ nanomechanical testing of microscale three-point bend specimens with nanoarchitected beams.

The results of this study help to develop a fundamental understanding of the effect of architectural tortuosity on crack propagation, energy dissipation, and toughening mechanisms at the microscale. Moreover, it reveals the microscale fracture properties of nanoarchitected materials, importantly illustrating how fracture processes change when the sample size is comparable to the process zone size and how that can affect macroscale properties. This combined understanding of the effect of both architecture and specimen length scale on toughness is crucial and will pave the way to successfully engineer tougher materials with bioinspired strategies.

A prevalent focus of materials research is the development of ultra-lightweight, multifunctional materials that exhibit unparalleled mechanical properties. Within the last decade, technological advances in additive manufacturing (AM) have enabled the fabrication of novel three-dimensional nano- and micro-architected lattice materials, allowing researchers to investigate previously unexplored phenomena and property spaces. Current printing techniques are mainly restricted to polymer-based systems, which narrow the functionalities and mechanical robustness of these architected lattice materials. While a handful of nano-architected ceramic and metallic systems have been realized by way of pyrolysis, namely SiOC, nanoporous Ni, and glassy carbon, new printable material systems are limited by the chemistry of the resin blend and require further research. Thus, alternative approaches such as the deposition of metallic and ceramic coatings on polymer-based lattice scaffolds offer a flexible and feasible route for expanding the mechanical and functionality space of these advanced materials.
Several novel nano- and micro-lattice materials have been developed through means of various light-based AM methods and coating deposition approaches, namely atomic layer deposition and plating techniques. However, such methods are limited to materials that undergo specific chemical reactions. Thus, magnetron sputtering, a technique that allows for the deposition of a nearly unlimited selection of metals, alloys, and ceramics can be utilized to further expand the synthesis space of these materials. Several recent publications have employed planar magnetron sputtering configurations to deposit a wide range of materials on nano- and micro-lattice structures, from high entropy alloys to bulk metallic glasses. Nonetheless, sputtering is a momentum-driven line-of-sight process, and thus achieving uniform coatings on fine-featured lattice materials remains a prominent challenge. As such, our work is based on inverted cylindrical magnetron sputtering, a novel coating approach that allows for an unprecedented 360° line-of-sight. With enhanced line-of-sight, it is expected that higher ionization rates and bombarding energy can be achieved as compared to planar cathodes under the same sputtering conditions. Overall, inverted cylindrical magnetron sputtering can lead to a greater understanding of the effect of cathode geometry and deposition parameters on coating uniformity on complex 3-D topologies.