3D printing opens new avenues for designing and fabricating architected materials in a scalable manner. Specifically, we fabricate tailored architectures by direct writing of soft, hard, and composite inks. Using digital assembly, we programmably encode the mechanical responses of interest, including bending, snap-through instabilities, specific stiffness, and Poisson&’s ratio. This talk will highlight multiple examples of novel architectures and their resulting mechanical properties created by this approach.

In this study, we suggest a new methodology to construct a 3D metal/elastomer hybrid microstructure by stiffness patterning and folding instability. Thin, layered elastomer undergoes the unstable process in the presence of the compression, creating the wrinkle or fold structure. Here, we adopted stiffness patterning method on the surface of the elastomer to control the position of fold structure. Subject to a compressive surface strain, the stiffness-patterned substrate could produce selective-deformation depending on substrate stiffness. Thus the folded structure appears on the softer part of the substrate. Using this approach, we constructed 3D metal-elastomer hybrid microstructure by site-selective folding instability combined with a wet transfer method. The folded metal-elastomer hybrid microstructure is reversibly stretched and compressed to large levels (200 %) of strain without damage in the metal structure. Moreover, our method opens the way to provide a new kind of microfabrication design like optical film or mechanical sensor.

Cellular solids are instrumental in creating lightweight, strong, and damage-tolerant engineering materials. By extending their feature size down to the nanoscale, we are able to simultaneously exploit the architecture and size effects to substantially enhance the structural integrity of architected meta-materials. We show that hollow-tube alumina nanolattices of 3D kagome geometry with and without pre-fabricated notches always fail at the same load when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. For notches with (a/w) > 1/3, the samples fail at lower peak loads and this is attributed to the increased compliance as fewer unit cells span the un-notched region. Finite element simulations of the kagome tension samples show that the failure is governed by tensile loading for (a/w) < 1/3 but as (a/w) increases, bending begins to play a significant role in the failure. This work explores the flaw sensitivity of hollow alumina kagome nanolattices in tension, using experiments and simulations, and demonstrates that the discrete-continuum duality of architected structural meta-materials gives rise to their flaw insensitivity and fracture tolerance even when made entirely of intrinsically brittle materials.

Sacrificial templating offers a versatile method of synthesizing new materials with hierarchically porous structures for use as catalysts and absorbents. This process facilitates precise tunability of pore size in order to produce materials with rationally designed pore structures spanning all porosity regimes (macroporous, mesoporous, and microporous). Such materials offer the ability to selectively engineer a catalyst or absorbent in order to obtain desirable transport properties during chemical processing. This work demonstrates the ability to produce three dimensionally ordered mesoporous materials (3DOm) including silica&’s, carbons, and transition metal oxides using various colloidal templating techniques. These materials demonstrated the ability to tune surface area and pore size via templating; while simultaneously stabilizing metastable oxide phases such as a-TiO₂ and t-ZrO₂ at elevated temperatures. The versatility of these hierarchically porous materials is demonstrated via the selective separation of sugars and the photocatalytic decomposition of methylene blue dye. To characterize the samples, precursor solutions were analyzed using DLS and SAXS prior to templating; while templated materials were characterized using N₂ and Ar adsorption, SEM, HRTEM, XRD, SAXS, and XPS. The discussion will conclude with a review of these findings in order to highlight the appropriate selection of chemical precursor&’s, processing conditions, and potential pitfalls to consider when synthesizing future materials via templating.

3D printing opens new avenues for designing and fabricating architected materials in a scalable manner. We use a 3D extrusion-based printing method, known as direct ink writing, to fabricate soft, reversibly-deformable architected materials. Specifically, silicone-based inks with tailored rheology are patterned into structures comprised of precise beam geometries. The aspect ratio and the tilting angle affect the mechanical response of the beams, which can include such phenomena as snap-through instabilities and multistability, leading to nonlinear wave propagation. Beam geometries can be programmably tuned to encode spatially-varying mechanical responses. Due to the soft architecture, the static and dynamic response of the architected material can be further tuned by applying static deformation. This talk will highlight multiple examples of novel architectures and mechanical responses created by this approach.

Recent advances in topological optimization methodologies for design of internal material architecture, coupled with the emergence of micro- and nanoscale fabrication processes, 3D imaging, and micron scale testing methodologies, now make it possible to design, fabricate, and characterize lattice materials with unprecedented control. This talk will describe a collaborative effort that employs scalable 3D textile manufacturing, location specific joining, and vapor phase alloying to produce metallic lattices with a wide range of internal architectures, alloy compositions, and mechanical and functional properties. Topology optimization allows properties to be decoupled and tailored for specific applications. Dramatic enhancements in permeability have been balanced with modest reductions in stiffness, and are being used to develop heat exchanger materials with high thermal transport, low impedance, low thermal gradients and high temperature strength. In a parallel effort, architectural designs to maximize both structural resonance and inter-wire friction are also being employed to develop metallic lattices capable of mechanical damping at elevated temperatures. These examples will be used to highlight the benefits to be gained by the design, manufacturing and characterization of metallic lattice materials with a wide range of tailorable properties.

3D printing has opened the doors to design freedom such that just about anything that can be imagined and sketched up in CAD can be fabricated from plastics, metals and even ceramics. The plastic systems which have been around for more than 25 years, have been limited by the fact that it is difficult to incorporate reinforcement into the resins so mechanical properties are generally limited to neat resin stiffness and strengths. Additionally the layer by layer process of making parts often leads to weakness in the laminations. Metal AM parts on the other had have mechanical properties similar to wrought metal properties of the same composition but it is generally very expensive to build and finish metal AM parts limiting their application.
Electroplating additive manufactured organic resin parts can be a great complement to SLA, SLA, FDM and other 3D printing methods. Coating of copper and nickel can provide metallic properties such as heat dissipation, electromagnetic and optical reflectivity, barriers against harsh environments, improve thermal stability as well as stiffen and strengthen parts in a predictable way. This presentation will review the mechanics and methodology for applying the coatings for different purposes as well as simple design modifications, easily done with print on demand additive manufacturing, that facilitate making functional accurate metal clad plastic parts. Additionally, the issues encountered in working with the different AM systems will be covered along with how these can be dealt with and processed successfully.

The collective physical properties of nanoparticle (NP) assemblies depend strongly on interparticle distance. Controlled tuning of interparticle spacings therefore offers the possibility to optimize the response of NP solids for applications including optical, magnetic and electronic devices.
In this series of projects, a variety of lipophilic, highly flexible, dendritic ligands (generations 0 to 4) was designed to bind colloidal nanocrystals and induce self-assembly properties. After ligand exchange, by controlling the solvent evaporation rate, the corresponding dendron-capped nanoparticle hybrids were found to self-organize into hexagonal close-packed (hcp) superlattices. The interparticular spacing can be progressively varied from 2.2 to 6.3 nm with increasing the dendritic generation, covering a range that is intermediate between commercial ligands and DNA-based ligand shells.
Dual mixtures of dendronized hybrids resulted in unprecedented superlatices (where both components have same size inorganic core, but different dendritic covering) which are isostructural with NaZn₁₃ and CaCu₅ crystals.
The synthetic and self-assembly details as well as latest results will be discussed.

Non-close packed colloidal lattices made by self-assembly can be used to fabricate a variety of large-scale nanostructures (honeycomb, nano-pillars) by nanosphere lithography, and devices such as photonic crystals and biosensors [1]. This method has benefits over close-packed arrays due to greater applicability with tunable interparticle distance, and less susceptibility to particle size variation and solution contaminants. Multiple methods exist for self-assembly of close-packed polystyrene microspheres [2], but achieving control over microsphere spacing presently requires chemical and/or mechanical modifications of the substrate or the particles [1, 3]. We present an ultrasonic mist deposition approach that leads to robust, large-scale ordered structures of microspheres, with interparticle spacing above four particle diameters. Starting with a solvent-microsphere colloid, a piezoelectric element is used to generate a mist that passes through a gravity-buoyancy barrier to preferentially select mist particles containing individual microspheres before being transported onto a water surface. Upon landing on the water, at specific interparticle spacing an isotropic pair potential between the microspheres is established due to the combination of zeta potential, capillary, and surface tension forces at the water-air interface. Unlike the work of [4], the colloidal lattice is formed at a water-air interface and exhibits pair potential interactions without an applied electric field, suggesting a new mechanism for interparticle interaction at the observed spacing. Use of thisnew mist method, as opposed to traditional Langmuir-Blodgett or spin coating methods, enhances the fraction of microspheres deposited at the appropriate spacing to enter the ordered lattice potential well, while enabling straightforward extension to large scale deposition. No surface modifications to the microspheres or substrate are required, furthering the flexibility and ease of this approach. This work was supported by the SUTD-MIT International Design Center.
1. Jian-Tao Zhang, et al. Periodicity-Controlled Two-Dimensional Crystalline Colloidal Arrays. Langmuir vol. 27, 15230-15235 (2011).
2. Junhu Zhang, et al. Colloidal Self-Assembly Meets Nanofabrication: From Two-Dimensional Colloidal Crystals to Nanostructure Arrays. Advanced Materials vol. 22, 4249-4269 (2010).
3. Xiao Li, et al. Modulating Two-Dimensional Non-Close-Packed Colloidal Crystal Arrays by Deformable Soft Lithography. Langmuir vol. 26, 2930-2936 (2010).
4. Ke-Qin Zhang & Xiang Y. Liu. In situ observation of colloidal monolayer nucleation driven by an alternating electric field. Nature vol. 429, 739-743 (2004).

A great deal of effort has been devoted to stack 2D planar nanostructures for the amplification of its own photonic functionality or the achivement of newly derived photonic behavior, such as chirality. In our experiment, we demonstrated to stack 3D nanostructures layer upon layer via repetitive execution of e-beam lithography and 3D assembly of nanoparticles via IAAL (Ion-Assisted Aerosol Lithography).
Our multi-layer 3D nanostructures were fabricated through following 3-step procedures. (1) We generated cross-shaped patterns with the size of 1.5mu;m#8553;1.5mu;m and the interval of 1.5mu;m on the silicon substrate via e-beam lithography. (2) Positively charged metal nanoparticles generated by spark discharge were blown onto the pattered silicon substrates where negative voltage was applied. nanoparticles assembled on the exposed silicon substrate and grew in the lateral directions, due to the nanoscopic electrostatic lens induced by the pre-deposited cations on e-beam resist. (3) 3D nanostructures were sintered and strengthened by e-beam irradiation to endure the spin-coating of e-beam resist for the upper layer patterns. Then, e-beam resist was spin-coated upon the nanostructure of the previous layer and patterns for the next layer which was aligned according to previous layer was fabricated via e-beam lithography. Through repetition of procedure (1)-(3), multi-layer 3D nanostructures were manufactured. Finally, e-beam resist was eradicated through O₂ plasma ashing.
In order to demonstrate the photonic enhancement of multi-layer nanostructures, compared to mono-layer nanostructures, we measured the SERS signals of thiophenol molecueles chemisorbed onto the bi-layer nanostructures and the mono-layer nanostructures. It was observed that the peak intensities of the bi-layer nanostructures were significantly increased, compared to those of the mono-layer nanostructures. The bi-layer nanostructures exhibited 4-5 fold higher peak intensities, compared to the mono-layer nanostructures.

This work examines the effect of corrugated architecture on stress transfer in metal matrix composite materials. Due to the fact that during unbending a corrugation exhibits a transition from bending dominated behaviour towards stretching dominated behaviour it is possible to construct a composite material that transitions from minimal stress transfer initially to large amounts of stress transfer upon further deformation. During the loading of a corrugation reinforced composite, the corrugation will unbend leading to an evolving reinforcement alignment and therefore evolving conditions for the transfer of stress. In this work, the stress transfer in composites with plastically deforming components is analyzed using an adapted shear lag model and Finite Element Modeling. Experimental investigations into strain distribution are also included.

Recently, block copolymers (BCPs) have been investigated extensively for nanolithography applications to provide highly controlled structures with a simple and cost-effective method. Among various structures, vertically aligned pillars are one of the most prominent architectures for the emerging application, such as vertical transistor, light reflection controller, and chemical and biological sensor. Because of the easiness of surface modification, silica nano-pillars are promising candidate to realize these applications. In this presentation, we demonstrate a facile route to fabricate a well ordered vertically aligned silica nano-pillars making the most of self-assembled BCPs.
Polystyrene-block-polydimethylsiloxane (PS-b-PDMS; 31k-b-14.5k) was dissolved in cyclohexane, followed by spin coating on the Si substrate which was coated with PS brush in advance. The solvent annealing was conducted in chloroform vapor within 2 hours. The reactive ion etching (RIE) and the sequential calcination could easily provide silica nanostructures. The morphology of the obtained structures was observed by scanning electron microscope (SEM), whereas the composition was verified by Fourier transform infrared spectroscopy (FT-IR). Grazing-incidence small angle X-ray scattering (GISAXS) measurements were also conducted to investigate the morphological transition as well as the ordering of BCPs during annealing process. With increasing the annealing time, micro phase separation was induced and cylindrical PDMS became the dominant phase, leading to the optimum condition of 1.5 hours. Well aligned nano-pillars could be found throughout the substrate of 1 cm². Over 2 hours, the structure gradually lost the regularity. In addition, we also investigated the effect of the vapor pressure, and drying speed of the solvent on the morphology of the samples. GISAXS measurements well agree the observation of SEM images. Some optical properties derived from fine structure of the fabricated nano-pillars will be presented.

Lizards living in desert have remarkable ability to collect and transport water through their skin to the mouth. The lizards drink water using specialized scaled integuments, semi-tubular capillary system, which consists of honeycomb-like scales as water reservoir, and interscalar channels as water transporter. Inspired from these unique integuments, we suggest that the lizard&’s water drinking system can be major components of open-closed hybrid microfluidic systems. Thus, we utilized a novel controlled fracture technique for a microfluidic system.
Although fractures or cracks are typically difficult to control precisely due to their randomness, crack formation can be controlled under the delicate conditions. In this study, we controlled the stress distribution throughout the surface at the predefined position. As expected, controlled cracks were created from the predefined notches as initiating points. When the extent of tensile strain became increased, the number of cracks formation was also increased. At the 75% of prestretched condition, the cracks were formed completely at almost of prepatterned notches. From these results, we suggested that the controlled fracture was successfully fabricated from the initiating notches and guiding line under the prestrained condition.
By introducing fluorescent dye to visualize the surfaces of fabricated system, we observed that the dye was firstly retained on the crack induced open channels and subsequently absorbed into folded closed channels by the capillary force. For the strains from 45% to 60%, interestingly, there was only dye absorption into folded channels starting from the crack. The dye was detected over all of the closed channels at 75% complete crack formation.
In conclusion, we have presented a simple, but highly reproducible technique to fabricate a bio-inspired hybrid microfluidic system by simply varying the prestrained condition of prepattened elastomer. This hybrid system is collectively applicable to control the fluid flow in the microfluidic system.

Nanostructures can often be viewed as mathematically distinct and measurable because of the repetition in the geometry of their underlying structures. Far from being random, these patterns reflect the order of established scientific rules, and rigorous analogues between mathematical objects and subsets of nanostructures can be made. Under the aegis of physical and chemical laws, nanostructures form discrete and measurable geometric patterns ranging from repeating lattices to complicated polygons. Established work from several areas of pure mathematics (such as graph theory and Ramsey theory) can be used to analyze and predict potential properties from well-defined nanostructures. Specifically, carbon has distinct allotropes that build upon the basic honeycomb-like lattice structure of graphene. Because these allotropes have clear commonalities in terms of geometric properties, this paper details several approaches to the use of graph theory to enumerate structures and properties of nanocarbons. Graph theoretic treatment of the lattice that forms the foundation of graphene is completed, and parameters for obtaining other forms of nanocarbon based on these graph theoretic principles are established. Graph theoretic methods of understanding, analyzing, and predicting behavior of nanostructures are detailed and established as a potentially innovative method of understanding carbon nanostructures.

A combined theoretical and experimental study on α-Ag₂WO₄crystals is presented. Structural and optical properties and the formation of Ag nanoparticles on α-Ag₂WO₄ induced by electron irradiation have been studied. α-Ag₂WO₄ samples was prepared by co-precipitation method. The samples were characterized by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) characterization. Their optical properties were investigated by ultravioletminus;visible (UV-vis) spectroscopy and photoluminescence (PL) measurements at room temperature. Thermodynamic equilibrium shape of the α-Ag₂WO₄crystal is built with the calculated surface energies through a Wulff construction. The (110) and (011) orientation is the dominating surfaces in the Wulff shape. It suggests that the Wulff shape of α-Ag₂WO₄ is closely related to the chemical environment around, and a larger increase in surface energies for the ((010)/ surface rather than those in the other surfaces allows us to find a good agreement between experimental and theoretical morphology. We have revealed a correlation between the exposed morphology and presence of hydroxyl anions, and have provided an explanation to the puzzle that arises from. We believe that these findings can be used as an important tool to the understanding of controlled structural and physical characteristics in nano- and micro-crystals.

This paper describes a method of 3D printing of three-dimensional micro-architectures with embedded functional nanoscale materials such as silicon nanowires and enzymes . A three-dimensional CAD model is first sliced into a series of closely spaced horizontal planes. These two-dimensional image slices are sequentially transmitted to the reflective liquid-crystal-on-silicon chip, which is illuminated with UV light from a light emitting diode array. Each image is projected through a reduction lens onto the surface of the photosensitive resin containing encapsulated nanomaterials. The exposed liquid cures, forming a layer in the shape of the two-dimensional image, and the substrate on which it rests is lowered, reflowing a thin film of liquid over the cured layer. A microfluidic material delivering system is used to flush the first liquid and deliver the second liquid on top of the first layer. The image projection is then repeated with the next image slice forming the subsequent layer.he substrate travels in the z-direction between each build layer, perpendicular to the fluid surface of the carrier fluid bath, and is controlled by a motorized linear translation stage. The unique advantage of 3D printing technique is the capability of 3D printing a large array of multi-materials or materials with graded encapsulated material concentrations and functionalities.

Statement of Purpose: 3D hydrogel cell cultures provide a unique environment for studying cell interactions by more closely mimicking in vivo extracellular matrix. However, many tissues, particularly connective tissue and cartilage, exhibit anisotropic mechanical properties that are not replicated by hydrogels. Recent advances in additive manufacturing combined with visible-light photopatterning and low-toxicity photoinitiators have enabled a new field of additive biomanufacturing for high-resolution spatial patterning of scaffolds. Using these advantages, superposition of microarchitected lattices within the hydrogel can mimic tissue anisotropies, providing new, synthetic environments for cell cultures. Specifically, a microarchitected lattice can confer an orthotropic Poisson's ratio to the surrounding hydrogel.
Methods: The hydrogel constructs consist of two parts: 1) mechanical lattice and 2) hydrogel filler. The mechanical lattice was fabricated using Projection Microstereolithography, a digital mask-based 3D printer. To create 3D structures, digital models are converted to a series of images and projected into a UV-curable resin in coordination with precise movements of the substrate. In this way, 3D structures are built from a sequence of 2D images.
The mechanical lattice was fabricated from UV-cured PEGDA monomer. After the lattice was fabricated, the hydrogel resin was added to the lattice and cured in place using a UV lamp. Prior to photopolymerization, 10um polystyrene beads at a concentration of 1% (w/w) were added to the hydrogel resin for visualization.
Mechanical properties were measured on an Instron 5943 and strain measurements were calculated optically during compression.
Results: The results show a relationship between the microarchitecture and Poisson's ratio of the sample. Polystyrene beads acting as displacement markers throughout the patterned volume will give local displacement values along the imaging plane, generating a spatial displacement map. The microarchitecture alone has been tested and showed an orthotropic Poisson's ratio of up to 1 along a single plane.
Conclusions: Recent advances in additive biomanufacturing enable the creation of hydrogel environments with unique mechanical properties. Hydrogels with embedded microarchitecture can allow control over mechanical properties such as Poisson's ratio, providing new environments for studying cell interactions as well as tissue engineering applications.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
LLNL-ABS-669732

The Interface between neural electrodes and neural tissue plays an important role in neural recording and stimulation. Low impedance electrodes are required for development of high signal-to-noise ratio neural recording devices. To design a low impedance interface, we need to understand the electrical and physical process contributing to the impedance. Conducting polymers (CPs) have been widely used in biomedical application especially neural interfaces owning to (1) their organic nature; (2) both ionic and electronic conductivity; and (3) ability to be functionalized with bioactive molecules. Here we report fabrication of Conducting polymer poly (pyrrole) (PPy) microcups using microsphere templates. PPy-coated poly (lactic-co-glycolic) acid (PLGA) microspheres deposited on gold electrodes could decrease the impedance of electrodes by increasing the effective electrode surface area. To access the electrical properties of PPy microcups, electrochemical impedance spectroscopy (EIS) was employed. An equivalent circuit model including a coating capacitance in parallel with a pore resistance and interface impedance in series was proposed and fitted to EIS experimental measurements. Curve fitting results indicated that the proposed model supported the experimental data with high consistency. In this study, 4wt% PLGA with 2wt% benzyltriethylammonium chloride (BTEAC) with respect to PLGA was dissolve in chloroform and electrosprayed on gold substrate. PLGA microspheres were coated with PPy using electrochemical deposition with 0.5 mA/cm² current density for 8 minutes. EIS measurements were performed by applying an AC sine wave with 10 mV rms amplitude over the frequency range of 1-10⁴ Hz. The equivalent circuit model consisted of several electrical elements such as double layer capacitance, charge transfer resistance, Warburg impedance, coating capacitance and resistance, and solution resistance. In summary, fabrication and measurement techniques with the corresponding equivalent circuit model have been developed to characterize the electrical properties of electrode-electrolyte interface. This proposed model provided a tool for better understanding of complex electrode-electrolyte interface. Furthermore, we demonstrated that the experimental and fitted results are consistent with high accuracy. In future, we aim to improve the conducting polymer microcup fabrication process using the predicted data from our proposed model.

Several hollow hierarchical structures have been synthesized through templating and etch methods. This study focuses on single ligaments of the structures by developing tubes with metal-foam sandwich walls that are designed to resist buckling, a common failure mode for hierarchical lattices. A high-melting point metal is used for the sandwich walls while the foam core is made of a nanoporous metal. Tubes are fabricated by using a series of synthesis methods beginning with the electrodeposition of two metals onto a thermally stable metal tube. The deposited metals are subsequently homogenized to generate a binary alloy and the nanoporous foam is made by dealloying the diffused metals. Finally, a sputtered layer of metal adds the outer sandwich panel. The geometry and porosity of the foam is further modified through heat treatments that coarsen the ligament size of the foam and give control over the available surface area. These structures not only have the potential to improve the deformation behavior of more complex hierarchical structures, but also multi-functionalize them through the extremely high surface area offered by nanoporosity.

An unprecedented deagglomeration and uniform distribution of Al₂O₃ nano-dispersoids in the Aluminum matrix is achieved through a novel two step contact-non contact ultrasonic casting technique that combines stirring, contact type ultrasonication and non contact type ultrasonication. It is known that contact type ultrasonic casting leads to pushing of nano-dispersoids and reagglomeration during solidification. The microstructure consists of three distinguished regions: grain boundary segregation zone; particle depleted zone and particle reinforced zone. Non contact type ultrasonic casting possess less time for complete deagglomeration during solidification. The microstructure consists of mixture of small agglomerates and deagglomeated particles. Unlike in contact type ultrasonic casting, no such definite zones are observed. During this process, engulfing of particles is dominated over pushing and hence grains without particulate reinforced zone are not formed. Thus, both processes in individual provide neither complete deagglomeration nor distribution. Hence in the present work, a novel two step contact-non contact ultrasonication coupled with electrical stirring is introduced for the first time ever. The contact type ultrasonication along with stirring helps in not only effective deagglomeration but also uniform distribution of nano-dispersoids throughout the melt and on the other hand, the non contact type ultrasonication avoids pushing and reagglomeration during solidification. The microstructure achieved during this process contains neither depleted zones and nor (or) least grain boundary segregation. Particles are uniformly distributed in the matrix. The method presented in this work can be promising way of synthesizing bulk MMNCs with highest deagglomeration and highest uniform distribution of nano-dispersoids.

Planar optical microcavities (MCs) are light confining structures employed in optics, photonics, and quantum information processing studies. The efficient and fast tuning of MC resonances can be extremely advantageous in photonics devices and quantum optics experiments. On other hand, surface acoustic waves (SAWs) have emerged in the past few years as a powerful and non-destructive tool to achieve modulation of the optical properties of semiconductor light emitting nanostructures like quantum wells and quantum dots.
In this work, we simulate the dynamic optical response of highly piezoelectric MCs under the modulation induced by SAWs. We design MCs that resonate in the visible/NIR optical range and are grown on sapphire substrates to allow both reflectivity and transmission experiments. We work with structures mainly based on ZnO/SiO2 and LiNbO3/SiO2 double Bragg mirrors (DBRs) which sandwich SiO2 (or polymer) active layers. By placing aluminum interdigital transducers (IDTs) on the samples surfaces, we calculate the modulation of the optical resonances of the MCs induced by the strain and piezoelectric fields of the SAW across the structures. We show how the strong interaction between photons in the cavity and SAW phonons leads to the folding of the dispersion relation of these systems and, consequently, to new optical resonances appearing in the spectrum. These resonances depend strongly on the SAW power which allows for the tuning of the MCs resonances over wide ranges of energies. In this way, we discuss the application of highly piezoelectric MCs in experiments where nanostructures with different transition energies are coupled to different resonances of confined electromagnetic field.

Developing a reliable sealant or sealing system remains one of the top priorities in research on planar solid oxide fuel cell (SOFC) technology. Recent studies on SOFC glass-based sealants have focused on self-healing glass seals, such as those made of compliant alkali-containing silicate glass. However, thermochemical and thermomechanical properties of self-healing alkali-silicate glasses are usually weaker than those of alkaline-earth silicate glasses. One way to overcome such weaknesses is to use fillers to modify glass properties. For example, Al₂O₃ filler can strengthen alkali-silicate glass network structure. The objective of this study is to find suitable fillers for applications in solid oxide fuel cell sealing glass. This was accomplished by adding various amounts of Al₂O₃ and 8 mol % Y₂O₃-stabilized zirconia (8YSZ) powders as fillers to the alkali-containing borosilicate sealing glass prepared in this work. The effect of these fillers on the viscosities, electrical conductivities, and infrared spectroscopy of the sealants were investigated. It was concluded that long-term sealing behavior could be improved by simultaneous addition of Al₂O₃ and 8YSZ fillers.

Recent advancements in manufacturing have provided unprecedented abilities to realize engineered materials with defined pore architectures. This has presented new opportunities in design, with topology optimization (TO) in particular being particularly well-suited to explore this now expanded design space. Topology optimization is a systematic design methodology for optimizing material distribution across a design domain. Material may be (computationally) added or removed freely, meaning architectural connectivity and shape may evolve during the design process. This design freedom enables discovery of new material architectures, often predicted to approach theoretical bounds, but typically comes at the price of reduced manufacturability and increased cost. This talk will discuss the integration of manufacturing into the TO design process so that designed architectures respect manufacturing constraints (and thereby limit re-design) and leverage new manufacturing capabilities. Various manufacturing processes are considered (e.g., additive manufacturing, 3D weaving) and are presented in the context of material architecture design for optimized mechanical, thermal, and/or fluid permeability properties.

Research on architected materials is growing rapidly due to the possibility to create materials with exotic properties, such as negative Poisson&’s ratio, negative refraction or extremely low mass density. These unusual properties arise from the geometry and structure of the material, rather than its bulk composition. Today&’s architected materials can be classified into two types: homogeneous (with periodic structures) and heterogeneous (with non-periodic structures). Most studies on architected materials focus on periodic arrangements of structures, and state of the art microstructural optimization is used to achieve desired material properties. However, designing complex structural assemblies to obtain spatially varying properties is much more challenging.
In our work, we present a complete framework for the fabrication of deformable objects with spatially tailored mechanical properties. First, we precompute a database of structures indexed by their mechanical properties, e.g., elastic modulus or Poisson&’s ratio. To achieve a large coverage in the mechanical properties space, we introduce a numerical optimization method for sampling structures with desired responses. Second, we synthesize the interior microstructure of a specific object by tiling microstructures drawn from the precomputed families. In a final step, we evaluate the resulting structure using a fully integrated FE analysis and compare results with the mechanical behavior obtained by quasi-static compression and tension tests. For the experimental testing we fabricate macro-scale samples using selective laser sintering as well as micro-scale samples using two-photon lithography. Our results indicate a good fit between the predicted and the measured mechanical properties. Furthermore, we show the possibility to change the non-linear behavior of the base material to a linear behavior by choosing a proper set of microstructure tiling geometry.

Along with their composition and atomic structure, the micro-architecture of biological materials strongly influences their mechanical properties. Nacre and bones are fascinating examples wherein intricate design motifs are employed to achieve outstanding strength. Nacre has high toughness as a result of its lamellar brick and mortar structure. Compact bone has a hierarchical assembly of lamellar tubes with varying orientation angle, leading to high fracture toughness. Toughening mechanisms arise at the nanoscale from the sliding of collagen fibrils and the formation and breakage of sacrificial bonds. In particular, interfacial friction plays a primary structural role in load transmission.
We use interleaved assemblies of paper sheets as a macroscale model system to study the contact mechanics and the origin of toughness in lamellar materials. We create a joint by interleaving the pages of a pair of identical paperback books (inspired by the “phonebooks problem” as seen in the TV show ‘MythBusters&’), and measure their strength and toughness by tensile testing. Through modeling and experiments, we identify how these properties scale with the architectural parameters of the joint: namely the number of sheets, the sheet thickness, the overlap among pages and the free lengths. Large joint strengths (> MPa) are obtained when the free length (defined as the length between the testing grip and the interleaved joint) is decreased and hence the page angles become larger.
The strength and toughness of these reversible, self-locking joints are highly dependent on the contact area among the overlapping pages. This seemed initially counter intuitive because friction force is known to be independent of contact area. In the regime of intermediate overlapping area, the peak tensile stress initially increases to 500 kPa, followed by a linear decline similar to strain softening, and exhibiting oscillatory stick-slip. We explain this in light of the micro-mechanics of friction by defining pressure dependent interfacial shear strength between 2 pages in contact. At large normal loads, micro-scale contact asperities between 2 pages flow plastically until they self-conform, the junction area becomes effectively equal to the apparent area, and the friction force saturates. This effect leads to high apparent plasticity during tensile loading, and eventual separation of the joint without fracture of the pages. In the large contact area regime, the books are inseparable and the pages start breaking in an effectively brittle regime. In sum, we show that, without the use of adhesives, the effective strength and ductility of laminated materials can be tuned simply by the architecture of interleaving and the micromechanics of friction. The insights from this study suggest the possibility for topological design of materials with unique mechanical properties arising from interlayer friction, and encourage investigation of these effects at small length scales.

An architecture comprised of many layers of hierarchical features had contributed to the overall stability of all life forms and objects. The smallest structural length-scale and largest length-scale of a material often differ by more than 5 orders of magnitude. Failure to fill this gap often results in a material with largely coupled mechanical properties, such as strength, ductility and density. For example, ultralight materials with volume fractions reduced to <1% than their parent material, have significantly degraded compressive and tensile strength 10^-4% of their parent material. Here we present hierarchical metallic materials with three dimensional features nested across over five orders of magnitude in length-scale, with configurable deformation modes separated by at least one order of magnitude at each level of hierarchy. We show that at very low relative densities the optimized materials with fractal hierarchy exhibit near linear scaling between strength-to-density. Remarkably, these meta-materials, with bend/stretch dominated hierarchical combinations, exhibit an elastomeric like, super elasticity that is super compressible and stretchable, with high specific strength and tensile strain greater than 20%.

The aim of this research is to introduce an analytical method that can rapidly calculate the dynamic response of a general multi-phased microarchitectured material (i.e., how the material&’s lattice deforms over time) when it is subjected to an ambient thermal load. A number of tunable three-phase microarchitectured material lattices that consist of compliant beam-based elements made of two different homogenous materials surrounded by void space have been designed such that the lattice can be made to achieve a desired bulk thermal expansion coefficient. Although analytical tools have been used to characterize and optimize the steady state deformation of these material lattices such that they achieve their final desired expansion when subjected to a change in ambient temperature, none of these tools consider the rate at which the different-material beam elements absorb the heat or the inertial effect of the mass within the lattice. Such considerations are critical to incorporate within the characterization and optimization tools for designing such microarchitectured materials because the material may not satisfy its intended functional requirements if its lattice contracts or expands in undesired ways before it reaches the final deformed state or if it rings with undesired vibrations induced by the thermal load. Whereas the final steady state deformation of traditional periodic microarchitectured material lattices can be calculated from the thermal response of a single repeated unit cell within the lattice, considerations of heat absorption and lattice inertia require characterizing the thermal response of every unit cell within the entire lattice simultaneously. Thus, an analytical method that can be used to rapidly characterize and optimize such lattices while considering these important time-dependent effects is not only necessary to the field, but such a method is significantly more complex and difficult to create. This paper introduces such a method and verifies is predictions via finite element analysis. Such a method could not only be used to characterize and optimize lattice designs that achieve tunable thermal expansion coefficients, it could also be used to design lattices that utilize internal dynamic vibrations to create desired bulk damping properties.

The tessellation of a large number of unit cells comprising slender beams leads to highly regular cellular materials, which can be designed, for instance, to achieve micro-architectured lattice materials with specific mechanical properties. In this paper, recent progress in developing materials with densities below 1000 kg/m³ and large specific strength, i.e. ratio of strength to density, will be discussed. On the one hand, it has been demonstrated that metallic and ceramic structures based on hollow beams with mass densities as low as about 1 kg/m³ can be realized. On the other hand, the mechanical properties, in particular the strength of such lattice materials is governed by the volume fraction of the constituent material. Also, the nodal connectivity of the truss structure has a pronounced influence.
In our work, we have designed specific micro-architectures, which are fabricated by applying 3D direct laser writing [1]. The structures consist of polymeric trusses with typical diameters in the range of 1 µm. The structures are coated with ceramic films by atomic layer deposition with thicknesses between 10 and 100 nm. Nanomechanical tests, employed to specifically designed test structures, have shown that this approach takes advantage of mechanical size effects for both the polymer and the ceramic films, as the strength of the materials at small scale exceeds the one of the bulk counterparts. With respect to the mechanical behavior of the truss structures, it will be shown that the specific strength can be further increased by specific heat treatments for improving the properties of the polymer as well as optimizing the ratio between thickness of the ceramic film to the diameter of the polymer truss.
It is also found that the elastic deformation behavior as well as the failure of the structure depends sensitively on details of the truss architecture and the loading conditions. For a detailed comparison between failure under compressive and tensile loading, push-to-pull structures were designed and tested. It turns out that certain truss designs show a pronounced asymmetry between tension and compression while others are less affected by the loading direction. Overall, it is found that tensile loading leads to lower strengths and brittle failure, presumably initiated by crack formation at stress concentrations at the nodes. Finally, the potential of further developments and some limitations in practical application of the materials are critically discussed.
[1] J. Bauer, S. Hengsbach, I. Tesari, R. Schwaiger, and O. Kraft “High-strength cellular ceramic composites with 3D microarchitecture” PNAS (2014)

Composites play an important role as structural materials in a broad range of fields due to their potential to combine beneficial properties of their constituents. In Nature, composites display fascinating architectures at multiple length scales, each of which can influence different properties. They are increasingly being used as an inspiration in developing novel materials. One particular principle of interest is the combination of hard building blocks, that constitute a majority phase, and a soft matrix phase, thus mimicking the microstructure of nacre, with its exceptional fracture toughness.
In this work we present a geometrical concept known as topological interlocking (Estrin et al., 2011), which can be utilised to vary the geometry of the hard building blocks and potentially produce structures with improved properties compared to a traditional brick-and-mortar design. The concept of topological interlocking is based on periodic assemblies of identical, discrete elements with specifically designed geometries, where each block is kinematically held in place by its neighbours. Our previous investigations have demonstrated that a ceramic plate segmented into interlocked elements can withstand flexural deflections ten times larger than those of a solid plate of the same thickness from the same material (Krause et al., 2012). It is anticipated that hybrid materials obtained by adding a soft phase to assemblies of topologically interlocked blocks will have superior mechanical properties.
With the aid of additive manufacturing, which has gained much attention due to its broad and far reaching potential applications, we can now readily access further design freedom which allows for the fabrication of complex geometries with fine features at micrometre resolution. Employing the state of the art 3D printing technology as a simple and efficient rapid manufacturing technique, we are able to print multiple polymeric materials with intricate inner architectures and widely contrasting mechanical properties within the same build. As a result, the ability to develop and investigate complex polymer composite assemblies, based upon the two design principles - topological interlocking and mimicking nacre - has become viable.
In this talk we will present results of a study detailing the various experimental and computational modelling work done to develop and fabricate a range of nacre-inspired, multi-material topologically interlocked structures using 3D-printing techniques.
References
Estrin, Y., A.V. Dyskin, and E. Pasternak (2011), Topological interlocking as a material design concept. Materials Science and Engineering C, 31, pp. 1189-1194.
Krause, T., Molotnikov, A., Carlesso, M., Rente, J., Rezwan, K., Estrin, Y., Koch, D. (2012), Mechanical Properties of Topologically Interlocked Structures with Elements Produced by Freeze Gelation of Ceramic Slurries, Advanced Engineering Materials 14, pp. 335-341.

Inverse opals are most widely used as photonic crystals for ultraviolet, optical and infrared applications.These highly interconnected porous structures are also attractive for applications such as sensors, fuel cells, filters, and catalysts. At the same time, engineers are aiming for lightweight structures with optimized mechanical strength, often inspired by nature&’s cellular materials with foam-like structures such as sponges, trabecular bone or plant parenchyma. The resultant optimized strut-based structures have shown high stiffness- and compressive strength-to-weight ratios, but can suffer from strut buckling and a lack of mass production techniques. Here we show that mechanical metamaterials based on ceramic inverse opaline structures with densities in the range of 330-910 kg/m³ are not only suitable as photonic crystals but also show better stiffness- and compressive strength-to-weight ratios compared to micro-fabricated optimized strut-based structures, but lower than carbon nanoframes fabricated by interference lithography. Pure silica inverse opal structures and silica inverse opals whose pores have been internally coated by a thin TiO₂-layer have been fabricated and their structural and mechanical behaviour was investigated. Our experimental results, supported by numerical simulations, show that these arch-shaped porous structures can outperform both strut-based and honeycomb structures due to their nearly isotropic mechanical response.

New weight efficient materials are needed to enhance the performance of vehicle systems allowing increased speed, maneuverability and fuel economy. Additive manufacturing combined with electroforming will be used to generate buckling optimized micro truss structures. A multi length-scale hybrid approach is applied to guide the optimization of the additive manufactured materials. The goal of the research was to expand material space for strength versus density allowing for reduced vehicle weight while maintaining strength. Fused Deposition Modeling (FDM) was used to take advantage of manufacturing flexibility, and electrodepositing was used to generate a high specific strength, bio-inspired hybrid material. The nano through meso scale characteristics/behaviors are analyzed with techniques ranging from EBSD to full scale composite panel testing to validate the optimized hybrid cores.
Microtension samples (3mm x 1mm with a 250µm x 250µm gage) were used to investigate the electrodeposited coatings in both the transverse (TD) and growth (GD) directions. Three bath chemistries were tested: copper, traditional nickel sulfamate (TNS) nickel, and nickel deposited with a platinum anode (NDPA). NDPA shows tensile strength upwards of 1600 MPa, significantly beyond the expected values reported in literature. This strengthening was linked to grain size refinement into the sub-30nm range, in addition to grain texture refinement resulting in only 17% of the slip systems for nickel being active. Anisotropy was observed in both nickel deposits which was linked to texture evolution inside of the coating. Information gained from microsample testing guided the selection of 15µm layer of copper deposition followed by a bulk layer of platinum deposited nickel.
Classical formulas for structural collapse were used to guide a experimental parametric study to establish a weight/volume efficient strut topology. Length, diameter and thickness were all investigated to determine the optimal column topology. It was discovered that the most optimal topology exists when Eulerian buckling, shell micro buckling and yielding failure modes all converge on a single geometric topology.
Three macro scale sandwich topologies (pyramidal, tetrahedral, and strut reinforced tetrahedral (SRT) were investigated with respect to strength per unit weight. The topologies were optimized across length scales using, texture on the nano-scale microsamples on the micro-scale and the parametric column study on the meso scale. The results validated additive manufacturing as a viable method for removing geometric constraints observed by other manufacturing methods. The SRT was the most optimized topology showing the highest strength per unit weight. The final topology sits in a best-of-both area of material space exceeding the commercially available honeycombs strength per relative density by 1670% and strength per weight by 507%.

Recent advances in direct manufacturing open the door for promising materials such as ordered structural foams. Reduced bending in closed cell stochastic foams, compared to open cell, gives rise to substantially higher stiffness and strength. Closed cell ordered foams similarly outperform lattice materials and can achieve theoretical bounds for stiffness (Hashin and Shtrikman, 1963). Mechanical models for the stiffness of foams (Gibson, 1989 & Grenestedt, 1999) and theoretical bounds are used as metrics to compare a variety of ordered foam topologies. Representative volume element (RVE) finite element modeling (FEM) is used to calculate strain energy distributions to identify features common in high performance designs. Three classes of materials are identified: maximal performance designs with a total stiffness that approaches theoretical bounds at low relative densities, a high performance stretch dominated group, and a compliant group with high mesoscale configurational entropy. A variety of historical and novel topologies are considered, including open cell designs, that largely represent the performance of stiff mesoscale ordered materials, eliciting a wide range of isotropy, over the range of low to intermediate relative densities. One design in particular achieves theoretical bounds for isotropic stiffness in the low density limit and maintains greater than 96% of the theoretical upper bounds though 40% relative density. This material is composed of two highly anisotropic substructures whose relative density can be scaled independently allowing the isotropy to be tailored. In this light, this material is amenable to optimization to produce functionally graded multicellular designs. Test specimens consisting of single unit cells were fabricated using a Stratasys uPrint SEtrade; Plus 3-D printer out of an ABS material (ABSplustrade; - P430). Approximately 100 ~0.5mm thick layers composed each specimen. A sacrificial support material is removed using a lye bath and agitator. Compression experiments were conducted on four specimens using a servo-hydraulic press both parallel (n=3) and perpendicular (n=1) to the print direction. Experiments agree quantitatively with finite element (FE) models exhibiting peak strengths that are ~75% of Voigt bound. This suggests, assuming no strain hardening in the ABS plastic, ~75% of the material has yielded at peak load. No significant difference in stiffness or peak strength was observed between compression parallel and perpendicular to the print direction however delamination of printed layers was prominent in the latter. Single unit cell structures and the corresponding periodic material are predicted to behave similarly in FE models suggesting little edge effects. Attempts are being made to fabricate test specimens out of 304 steel/Brass composite and TiC. The TiC material has the potential to outperform existing materials by nearly an order of magnitude in terms of specific stiffness.

A continuum model is developed for hexagonal lattices, composed of a set of masses connected by linear axial and torsional springs, with nonlinearity arising solely from geometric effects. For a set of lattice parameters, these lattices exhibit complex deformation patterns under uniform loading conditions due to instabilities. A continuum model accounting for these instabilities is developed from explicit expressions for the potential energy functional of the lattice unit cell. This functional is non-convex, captures the bistable nature of the lattice and is used to derive a constitutive model for the lattice. Finite element analysis of continuum media illustrate the formation of microstructure patterns with discontinuous displacement gradients, similar to the features observed in nonlinear elasticity and finite deformation plasticity. A comparison of discrete lattice simulations and finite element analysis under general loading conditions is presented, illustrating that our continuum model captures the effective behavior due to instabilities arising in the lattice. Furthermore, the developed model allows the explicit computation of tangent stiffnesses, which are utilized to investigate the wave propagation characteristics of the lattices under different states of applied stress, and in particular near the onset of instabilities. Of interest, is the extension of the pattern-forming predictions to the dynamic regime, which is relevant to wave guiding and load path design under high amplitude mechanical loads.

Artificially structured composite materials that enable manipulation and control of sound waves in air have received significant interest in recent years, not only because of their rich physics, but also for their broad range of applications. Here, we report a new class of tunable and switchable acoustic metamaterials composed of three-dimensional stretchable chiral helices in air arranged on a two-dimensional square lattice. Through a combination of Finite Element analyses and desktop-scale experiments we demonstrate that the deformation of the helices can be exploited to control the propagation of pressure waves in the surrounding air. In fact, our results indicate that the bandgap of the acoustic metamaterial can be tuned and even suppressed by increasing the amount of applied deformation, paving the way to the design of a new class of acoustic switches with on/off capabilities.

From semiconductors to metallurgy, defects play a key role in material properties. A direct consequence of defects in a lattice is the existence of localised states of vibration. These defect modes have been studied from multiple angles, but the tuning of a material's properties through the excitation of defect states remains unexplored. In this work, we show that the excitation of a defect mode can result in extreme mechanical properties, such as negative or infinite stiffness. In addition, the excitation parameters, the frequency and the amplitude, provide a high degree of control on the mechanical response. This allows to selectively tune the incremental stiffness at arbitrary points in a material's force-displacement relation, a feature that does not exist in other systems with tunable stiffness. The stiffness tunability originates in the nonlinear coupling between the oscillation of the defect mode, the force at the boundary of the material, and the deformation. This talk will provide a description of the stiffness tuning mechanism, as well as numerical and experimental demonstrations of this phenomenon in a one-dimensional lattice of steel spheres.

In recent years, advances in three-dimensional lithographic techniques have allowed for the fabrication of architected materials with precisely controlled topology and micro- to nanoscale periodicity. Many such architected materials possess photonic properties and photonic bandgaps within particular wavelength ranges, making them photonic crystals. Broadly tunable photonic crystals in the near- to mid-infrared region could find use in spectroscopy, non-invasive medical diagnosis, chemical and biological sensing, and military applications, but so far have not been widely realized. We report the fabrication and characterization of three-dimensional (3D) tunable photonic crystals composed of polymer nanolattices with an octahedron unit-cell geometry. These photonic crystals exhibit a strong peak in reflection in the mid-infrared that shifts substantially and reversibly with application of compressive uniaxial strain. A strain of ~40% results in a 2.2mu;m wavelength shift in the pseudo-stop band, from 7.3mu;m for the as-fabricated nanolattice to 5.1mu;m when strained. We found a linear relationship between the overall compressive strain in the photonic crystal and the resulting stopband shift, with a ~50nm blueshift in the reflection peak position per percent increase in strain. These results suggest that architected nanolattices can serve as efficient three-dimensional mechanically-tunable photonic crystals, providing a foundation for new opto-mechanical components and devices across infrared and possibly visible frequencies.

Thermoelectric material research is driven by the goal to minimize the thermal transport, which yields high thermoelectric figure of merit. In semiconductor materials heat is mainly carried by high frequency phonons. As the propagation of phonons in atomic lattices bares similarities to the propagation of elastic waves in microscale systems, the atomic structure can be used as a design guideline for novel microlattice materials.
For instance, phononic superlattices are commonly used to alter phonon propagation. The basic principle involes the stacking of layers with a mismatch in their characteristic impedance. If the wavelength of incoming elastic-waves, or phonons, is in the order of the stacking period, interference between the incoming and reflected waves occurs and yields a band gap in the frequency spectrum. In this work we exploit the analogy between phonons in atomic superlattices and the propagation ultrasound in micro scale superlattice, to create a novel material for ultrasound control. We show the occurence of band gaps that can be tailored to a specific frequency between 1 and 20 MHz. We use the specific lattice geometry of Si/Ge superlattices to guide the design of a micro phononic superlattice. Moreover we employ a 2PP 3D-Lithography method to create fully 3D polymeric lattice geometries with minimum feature sizes of less than 10 mu;m. The overall mechanical properties are controlled by means of varying the independent trusses that constitute the unit cell. We use a numerical model including fluid structure interaction to predict the occurrence of band gaps. Finally, the numerical model is validated using an experimental setup with high frequency ultrasonic transducers.

Topology optimization is the most power tool for designing materials with unprecedented topological complexity, such as multiphase cellular materials. This technique allows for efficient development of feasible features within a multiphase design domain, potentially leading to unique material properties. Although stiffness optimization of lightweight cellular materials and structures made of a single constituent phase has been extensively investigated, the application of topology optimization to more complex objective functions and multiphase cellular material systems is still in its infancy. In this presentation, we will discuss a systematic optimal design approach for multiphase periodic hybrid cellular materials with ideal combinations of high stiffness, low density, and high loss coefficient, resulting in maximum vibration damping under wave propagation. In the suggested framework for the multiphase microstructure, each point within the design domain can take a combination of phases and we adopt a penalization approach to achieve a clear phase separation. We utilize classic homogenization theory to model the effective stiffness of the unit cell and the Bloch-Floquet approach to obtain the damping capacity of the microstructure. The effects of wave frequency, damping properties of each individual phase, and existence of each phase on the optimal design will be discussed. The proposed approach can optimize the damping performance for a range of frequencies, as opposed to single frequency, thus leading to more robust designs.

Acoustic metamaterials utilizing local resonances are known to produce low frequency band gaps that break the limit of typical Bragg scattering mechanisms, however these band gaps tend to be quite narrow. We present on a new type of acoustic meta-structure that can simultaneously produce both low frequency and wide band gaps, by coupling a truss lattice with periodic local resonators to induce different local resonator modes. Analytical models that reduce the mode shapes to a 1D approximation inform the lattice geometry design. Specifically, by analyzing beam deformations of the individual truss elements, we can modify simple geometries to create low-stiffness geometries. Numerical modeling results show the ability to control band gap width and center frequency by varying the geometry, resonator material, resonator volume fraction/spacing, and truss dimensions. By using novel 3D printing techniques, we can fabricate functional composite polymeric-metallic structures, as well as easily fabricate complex geometries. The presence of low and wide band gaps is shown through finite element modeling and transmission experiments with laser vibrometers on 3D printed meta-structures. We show tunability of the band gaps by optimizing the lattice geometry. These acoustic meta-structures are designed for applications in structural vibration mitigation and sound absorption, but manufacturing on different size scales could open up applications in higher frequency ranges, e.g. for frequency filtering in MEMS devices.

Phononic crystals are periodic elastic structures which exhibit a range in frequency where elastic wave propagation is barred. The ability to design structures with such phononic bandgaps has been of growing interest in recent years due to their potential as sound filters, acoustic mirrors, acoustic wave guides, and vibration solators and in transducer design. Typical structures take the form of 2D or 3D arrays of inclusions of one or more materials in a matrix with contrasting properties. The position and width of the gap can be tailored by the selection of (a) constituent materials with contrasting densities and contrasting speeds of sound, (b) lattice topology and (c) volume fraction of inclusions. Many solid-air structures have been pursued through experiments and/or simulations , but most of them are characterized by a high porosity ( > 40%), limiting their potential applications in many scenarios. Here, we introduce a novel design for 2D phononic crystals comprising arrays of mutually orthogonal elongated voids. Importantly, our study shows that in such structures a bangap exists even for porosities as low as 5%. We also show that the smallest structure ligament size controls the appearance and evolution of the bandgaps and experimentally verify our findings.

One of the primary vibroacoustic challenges faced by sandwich panel designers is reducing the acoustic power transmitted through these structures without adding significant mass or compromising stiffness. In this study, sandwich beams with a variety of truss cores are investigated experimentally and theoretically to understand the structure-property relationships between core architecture and vibroacoustic performance. Sandwich beams are fabricated with HRL&’s polymer truss, formed from an interpenetrating array of self-propagating waveguides cured under ultraviolet light. The speed of transverse waves is measured through each beam as a function of excitation frequency and the wave speeds are used to determine the coincidence frequency of each beam—a key to understanding acoustic transmission through the structure. Quasi-static measurements are performed to determine the homogenized elastic and shear modulus of the truss structure and these values are incorporated into a previously-developed consistent higher-order model used to describe sandwich panel vibration. Symmetric and antisymmetric facesheet motion is mathematically decoupled, and the relation between excitation frequency and wavenumber in antisymmetric facesheet motion is solved implicitly to determine the transverse wave speed as a function of excitation frequency. Model predictions are compared against experimental measurements, demonstrating that homogenized truss properties are sufficient inputs to the consistent higher-order approach and that the model provides an accurate tool for acoustic and dynamic property prediction. Using this tool, an acoustically optimal sandwich beam—maximizing the coincidence frequency—is designed, fabricated, and characterized. This beam demonstrates subsonic transverse wave speeds in the frequency range of interest, indicating superior low-frequency acoustic performance while retaining comparable mechanical properties.

Acoustic metamaterials provide the remarkable ability to control, direct, and manipulate sound waves. Within this infant field, a promising approach to fabricate locally resonant acoustic metamaterials is the use of resonators composed of a heavy core surrounded by a rubber shell dispersed in an epoxy matrix. At their resonant frequency, the cores vibrate 180° out-of-phase with the matrix, and a band gap in transmission is observed making these materials excellent sound absorbers. A significant challenge in the study and adoption of these materials is the lack of techniques to easily fabricate resonators with a wide range of sizes and properties. Here, we present a robust yet simple technique to fabricate resonators with diameters ranging from 50 µm to 5 mm from core-shell drops generated in microfluidic and millifluidic devices. We started by fabricating resonators with core diameters ranging from 50 µm to 1 mm from double emulsion drops composed of a concentrated ceramic suspension in the core (inner drop) surrounded by a UV-crosslinkable rubber shell (outer drop) using microcapillary microfluidic devices. The double emulsion drops were collected and exposed to UV to crosslink the shell material forming resonators with resonant frequencies ranging from 100 kHz to 25 kHz based on the core mass. Even lower resonant frequencies were obtained by fabricating resonators with core diameters ranging from 1.2 mm to 1.7 mm from core-shell drops extruded in air from a coaxial nozzle at rates up to 6 drops/minute. The effects of core density were studied by utilizing suspensions composed of ceramic particles of increasing density including silica, alumina, and lead zirconate titanate (PZT). The transmission properties of the acoustic metamaterials made with resonators with different core diameters, core materials, and level of ordering within the matrix, were measured using a shaker/accelerometer setup capable of testing from 1 kHz to 12 kHz. For example, acoustic metamaterials composed of randomly dispersed 1.4 mm alumina-core resonators at a 30 vol% concentration in epoxy showed a well defined band-gap at 8.5 kHz. A finite element model was also developed to capture the acoustic transmission physics of these materials. Overall this technique offers a robust path for the fabrication of acoustic resonators and locally resonant acoustic metamaterials.

In the oil and gas industry, acoustic transducers have been found to provide valuable geological sonic information such as compressional wave velocity, shear wave velocity, and rock formation slowness. These data can be used to indicate lithology, determine porosity, detect over-pressured formation zones, and check well to well correlation. One category of such acoustic transducers is equipped with piezoelectric elements. Conventional piezoelectric transducers are packaged by epoxy resin. Because of the liquid nature of uncured epoxy resin, it is difficult to position the piezoelectric elements accurately.
The introduction of polyether ether ketone (PEEK) as the packaging material solved this issue. PEEK is a semi-crystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained to high temperatures. The packaging with PEEK comprises three PEEK tubes, inter, center, and outer. Positions for piezoelectric elements are machined out on the center tube. Due to the ease of machining on solid form, architectures of the composite acoustic transducers can be devised with great flexibility and creativity. Both the shapes and the positions of the piezoelectric elements can be varied by design. These designs can be modeled with finite element methods (FEM) and the best design for the oil drilling application can be finalized and fabricated.
COMSOL Multiphysics^® solves problems in a programming environment that integrates relevant physics. In this case, it includes electrical circuit, solid mechanics, acoustics, and piezoelectricity. Here a compete model and procedure to study the performance of an architected composite acoustic transducer is provided. The displacement analysis gives insights into the resonance modes of the piezoelectric elements. The acoustics analysis gives the necessary information on the acoustic performance of the transducers, such as acoustic pressure spatial distribution, acoustic pressure frequency response, transmitting voltage response, and directivity. These are important criteria to judge the effectiveness of an architected transducer.

The ability to independently control fiber alignments and structural geometry is critical for design of optimal three-dimensional (3D) fibrous structures. The rachis (shaft) of a bird feather is an example of such an optimized natural fibrous structure and synthetic realization of such structures using high performance fibers will add a new paradigm to composite structure design. We present a novel method to 3D-assemble carbon fiber structures, containing no seams or adhesive joints, using a confluence of several textile methodologies. A variety of complex structural shapes with tailored fiber topologies are demonstrated to be achievable. These optimized structures are shown to have unprecedented static and dynamic strength as well as damage tolerance and ductility. Moreover, structure made with high temperature brittle matrices such as SiC are also shown to have a high toughness due to the 3D interlocking of fibers.

Total hip replacements are used to restore joint motion for patients with compromised joint mobility. Current stems used in total hip arthroplasty are made from fully solid materials and are much stiffer than the natural bone. The drastic difference in stiffness leads to reduced stress on the surrounding bone. Since bone is a living tissue sensitive to mechanotransduction, it will remodel due to the changed stress state. The remodeling manifests as a reduction in bone quality surrounding the implant, and is termed stress shielding. The reduction in bone stock can lead to serious complications such as peri-prosthetic fracture, and greatly increases the risk of a revision surgery.
Recently a microarchitectured high strength fully porous biomaterial fabricated through Selective Laser Melting (SLM) has been introduced with variable mechanical properties for load bearing orthopedic applications. The architecture is tuned to be mechanically biocompatible with the host bone. Applied to a hip replacement implant, this microarchitecture tuning allows for a reduction in stress shielding while respecting bone ingrowth and manufacturing requirements.
To demonstrate the potential to minimize bone loss via material micro-architecture tailoring in porous hip implants, complementary experimental and computational investigations have been undertaken. While each of these studies is characterized by its own biomechanical metrics and test conditions, they both show an epistemic convergence of reduced stress shielding. The in-vitro experiments are conducted on six composite femurs that are divided and implanted with either an optimal porous implants or a fully solid control stem, both with equivalent implant macro geometry. Both the intact and implanted femurs are loaded in a quasi-physiological loading state with acetabular forces only. The surface strain on the medial calcar is recorded using Digital Image Correlation (DIC). The change of strain between intact and implanted femur is used as a proxy for stress shielding. The femurs with fully solid implant showed a larger region of reduced strain post implantation as compared to the porous implant with tailored microarchitecture. The latter shows 27% decrease of surface strain change which serves as an indication of stress shielding reduction. On the other hand, the computational work on a fully physiological implanted femur accounts for bone remodeling and compares a five year stress shielding projection for both the optimized microarchitectured implant and the fully solid control. The computational results show a 6% bone loss in the optimum implant down from 29% of a fully solid implant after five years. The reduction of surface strain experimentally measured, and the reduction in projected stress shielding via physiological simulations, seemingly indicate that the tuned fully porous implant can significantly reduce stress shielding in THA.

Recent advancements in additive manufacturing have enabled fabrication of macro-scale cellular materials with unprecedented dimensional control over the unit cell and sub-unit-cell features. These topologically architected materials have structural hierarchy spanning six orders of magnitude in length scale. A key representative topology that has received significant attention over the past few years is a single-phase hollow microlattice, with unit cell in the millimeter range and wall thickness of the order of hundreds of nanometers.
Previous studies showed that the damping performance of these microlattices is superior to that of any existing metallic material, making them ideal candidates for vibration isolation applications. The caveat is that the buckling-related structural mechanisms responsible for this energy dissipation require relative densities well below 1%, thus limiting the strength, stiffness, and energy absorption per unit volume that these materials can provide.
This de#64257;ciency can be obviated by fabricating hollow lattices with a sandwich wall architecture comprising an elastomeric core and metallic face sheets. In this topology, the face sheets and the sandwich architecture would increase stiffness and strength, whereas the elastomeric core would provide constrained-layer damping. This intrinsic damping mechanism can be used by itself (in lattices of any relative density) or combined with the buckling-related structural damping discussed above (albeit, exclusively in ultralight lattices). Here we present an analytical model for stiffness and damping (both intrinsic and structural) of hybrid hollow microlattices. This model is verified by Finite Elements analyses, and ultimately adopted in optimal design studies to identify hybrid microlattices with ideal combinations of high stiffness and damping and low density.

Energy absorbing materials are widely used in our life ranging from protective packaging of delicate components to crash mitigation in automobiles and aircraft to vibration isolators. Existing energy absorbers include metallic foams, thin walled tubes, carbon nanotube based materials and polymeric composites and exploit mainly plastic and viscous dissipation mechanisms intrinsic to their bulk materials. Energy absorption through plastic deformation results in irreversible degradation of the microstructures of materials, which limits their use over time. In addition, energy absorption through viscous dissipation is strongly affected by the rate of the applied load. Additional dissipative phenomena have been proposed, such as the zipping and unzipping of van der Waals interactions and sliding interactions in carbon nanotube based materials. However, there are often challenges in these systems with consistency of properties under repeated loading, as well as inherent scaling and environmental challenges associated with the use of nanomaterials. Therefore, there is a need for reusable energy absorbing material, whose response is reversible and unaffected by the rate of the applied load, the loading history, and the scale of the system. Here, we report a shape-recoverable system that absorbs energy without degradation by harnessing nonlinear behaviors of porous elastic structures based on rational design of geometry.
By combining numerical calculations and a customized additive manufacturing technique, we have harnessed snap-though instabilities in elastic beams to design reusable energy-absorbing structures. This strategy offers a unique range of advantages, as it can be applied to structures with various length scales (from micro to macro) and provides a simple modular design scheme, so that the mechanical response can be tuned by controlling geometric parameters guided by a “phase diagram”. Moreover, the loading process is fully reversible, allowing the structures to be consistently reused many times, with the energy absorption unaffected by the loading rate. Furthermore, since the findings of this work are independent of material properties, one could combine this structural design with different classes of materials, e.g., to produce stimuli-responsive structures capable of recovering when exposed to an environmental cue, or to obtain enhanced total energy dissipation by introducing material-dependent dissipative mechanisms, at other length scales, in addition to the material-independent structural dissipation. Thus, our findings can open new opportunities for designing energy absorbing materials and provide a new class of structures that can be utilized for a wide range of applications, including reusable bumpers, protective cases for sensitive equipment, and position controllers in soft robotics.
Reference: S. Shan^dagger;, S. H. Kang^dagger;, J. R. Raney^dagger;, P. Wang, L. Fang, F. Candido, J. Lewis, and K. Bertoldi, Adv. Mater. (2015). (dagger;: equal contribution)

Active materials like shape memory, ferroelectric and magnetostrictive alloys obtain their characteristic properties due to phase transformations. In these materials, phase transformations occur by changing the packing arrangement of the atoms in a process that resembles multistable mechanisms switching between stable configurations. A similar behavior has been observed in folded proteins in which a change in configuration (e.g. from folded to unfolded) provides the mechanism through which biological materials obtain remarkable properties such as combinations of strength and toughness, superelasticity and shock energy dissipation, among others.
We extend the notion of phase transformations to periodic cellular materials by introducing materials whose unit cells have multiple stable configurations. Each stable configuration of the unit cell corresponds to a phase, and transitions between these phases are interpreted as phase transformations for the material. In this work, we present a cellular material that exhibits phase transformation and use finite element simulations and analytical models to understand the underlying mechanics of the unit cell that allows the material to exhibit phase transformations. We study the quasi-static behavior of this material and analyze its wave propagation characteristics using Bloch- periodicity analyses implemented in a finite element framework, allowing us to evaluate the tenability of the material from the bandgaps perspective as the material goes from one stable configuration to the other. Preliminary results show that this material is characterized by long serrated loading and unloading plateaus, it exhibits hysteresis under cycling load and the phase transformation alters the preferred directions of wave propagation at certain frequencies. An attractive feature of these materials is that these behaviors are obtained without inelastic deformation of the base material.

The emergence of novel methods for the assembling of cellular structures with periodic lattice topologies from materials of very high specific strength is leading to a series of new cellular materials with combinations of strength and density that open up many potential applications. However, the resistance to crack growth and other modes of failure associated with very high specific surface area of these materials continues to impede progress. This presentation reviews recent advances in lattice manufacturing and the use of novel techniques for controlling their failure modes including the use of surface modification methods to both increase the strength and reduce susceptibility to environmental effects. Prospects for significant further advances in strong ultralight structures/materials will be identified and the most promising future opportunities discussed.

Mechanical investigations on materials with finite dimensions, such as thin films, micropillars, and nanowires, demonstrate an increase in the strength of a material with decreasing dimension, often approaching the theoretical strength when the feature size drops to the nanometer regime. However, it is challenging to assemble micropillars into a bulk nanostructured material without creating defects and weak points. Dealloying is a useful self-organization processing method technique that allows bulk nanostructured materials to be fabricated using a bottom-up approach. In this work, we used liquid metal dealloying (LMD) to fabricate high-strength Ta-Cu bicontinuous nancomposites, immersing parent alloys of Ti-Ta in a bath of molten Cu to dissolve Ti out of the alloy while allowing the refractory component to diffuse along the metal/liquid interface and reorganize into a porous network. After the dealloying process is completed, a dense bicontinuous composite with a characteristic ligament diameter is formed upon cooling. The new material is polycrystalline, consisting of individual, single-crystal bcc refractory micropillar networks (the “dealloyed phase”), and a rapidly solidified Cu phase (the “matrix phase"). We can tune the structure to have ligaments as small as 50 nm and as large as 10 µm by changing kinetic parameters (time and temperature) of the LMD reaction. Here we report the structure and mechanical properties of Ta/Cu nanocomposites with ligaments spanning the range 50 nm - 10 µm using a combination of Vickers microhardness, nanoindentation, and compression testing. The materials show high strength values and large plastic strains, with an increase in strength with decreasing feature size. Each phase in these materials is topologically continuous in three-dimensions, distinctly different from multilayer materials and secondary-phase strengthened materials. At the length scales we are studying, dislocations pile up at the interface between the two phases, but the tortuosity of the material inhibits complete delamination during failure.

We have developed a novel approach for rapidly fabricating large-area hierarchical material lattices based on electrohydrodynamic film patterning (EHD-FP). Using a controlled electric field and patterned electrodes, EHD-FP enables the rapid fabrication of hierarchical material films 20-40 µm in thickness with features at multiple length scales, including micron to sub-micron features. EHD-FP can be readily adapted to high speed, continuous roll-to-roll processing to create large area patterns and materials.
Hierarchical materials are materials which concurrently realize functional features on multiple length scales (sub-micron up to a millimeter level). This allows large void space in a material structure to be filled with load bearing members, adding compliance to the material without significantly increasing the density. Hierarchical materials have the potential to be transformational for a variety of applications, enabling components which simultaneously offer the best performance attributes of ceramics, metals, and plastics. Researchers have pursued hierarchical material structures for almost 20 years, leveraging inspiration from both nature and architecture, but few have shown success in synthetically re-creating these structures at multiple length scales with techniques that scale to high-volume, low-cost production.
We present our EHD-FP fabrication and mechanical testing results for two-dimensional (2D) hierarchical Kagome lattices made with ultraviolet (UV) curable polymers. Using the EHD-FP process, hierarchical films are easily and rapidly created with low viscosity UV cross-linked polymers. Our mechanical test results provide confirmation of some of the underlying mechanics for hierarchical materials showing a ~2X improvement in strength and modulus over bulk constituents.
This material is based upon work supported by the Defense Advanced Research Projects Agency under Contract No. HR0011-14-C-0036.

Graphene aerogels with high porosity, superior mechanical elasticity, and excellent electrical conductivity have become new candidate materials especially for nanoelectronics, sensors, catalysis, energy storage, and biomedicine applications. Current progress has produced bulk 3D assemblies of graphene by the self-assembly or gelation of the graphene oxide (GO) suspension via hydrothermal reduction, chemical reduction, or direct crosslinking of the GO sheets. Other templating methods like chemical vapor deposition (CVD) coatings and freeze-casting have already been employed to control over the pore morphology of 3D graphene monoliths. However, the architecture of these graphene networks remains largely random, precluding the ability to tailor transport and other physical properties of the material, which limit their performance compared to the potential of an engineered architecture. Here, we report the fabrication of periodic graphene aerogel mesostructures, possessing an engineered architecture via a 3D printing technique known as Direct Ink Writing. The 3D printed graphene aerogels are lightweight, highly conductive and exhibit super-compressibility (up to 90% compressive strain). Moreover, the Young&’s moduli of the 3D printed graphene aerogels show an order of magnitude improvement over bulk graphene materials with comparable geometric density and possess large surface areas. We also show their potential application in energy storage area (i.e., micro-supercapacitors). Adapting the 3D printing technique to graphene aerogels realizes the possibility of fabricating a myriad of complex aerogel architectures for a broad range of applications.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
IM release number: LLNL-ABS-670332

Sub-micron sized hybrid particles comprising polymers such as polystyrene or poly(methyl methacrylate) and silica or organosiloxanes/organosilsesquioxanes are interesting building blocks for the production of nanocomposites. Examples of products comprising such particles range from optical coatings to chromatographic materials and chemical sensors. Furthermore, such particles can yield hollow or porous silica particles through removal of the polymer material using e.g. pyrolysis, or hollow or porous polymer particles through selective removal of silica. This further expands the number of potential applications and products. Particularly, particles with a programmable composition, size and architecture are of interest, since they form key building blocks for advanced composites with a tailored set of functionalities.
In spite of the large application potential of sub-micron sized hybrid polymer silica particles, they are not yet widely used in commercial applications. This is mainly due to the limitations of the synthesis routes developed to date. Most of these are cost intensive, multi-step and multi-pot routes. In some cases, tailored block-copolymers are required and some processes are intrinsically difficult to perform on large scale.
Here, we present a facile and versatile platform approach for the synthesis of sub-micron sized hybrid particles in a one-pot emulsion based process using low cost, commercially available starting materials.We designed a one pot synthesis approach based on simultaneous radical polymerization of monomers like styrene and methyl methacrylate (MMA) and silica segregation, and prepared hybrid particles [1]. The required surfactants is formed in situ through hydrolytic conversion of the 'silica' momomer, i.e. an organotrialkoxysilane [2]. We studied the chemical composition of these particles using Fourier-transform infrared spectroscopy, X-ray diffraction and thermogravimetric analysis. Furthermore, we performed a detailed study on the architecture of the particles, using advanced electron microscopy (SEM, TEM, STEM) analysis. We demonstrate that particle size, composition and architecture are programmable in this synthesis route. Particle architectures range from homogeneous to multi-domain and core-shell particles. To demonstrate the versatility of this synthesis route, we prepared particles based on combinations of MMA, styrene and a variety of silica precursors. In addition, we prepared particles with reactive rest groups suited for further functionalization.
[1] M. Segers, R. van Zandvoort, M. Sliepen, N. Arfsten, M. Verheijen, H. Keul, P. Buskens, M. Möller, Chemistry of Materials2014, 26, 5718-5724.
[2] M. Segers, N. Arfsten, P. Buskens, M. Möller, RSC Advances2014, 4, 20673-20676.

Building nano-structured functional three-dimensional (3D) materials from two-dimensional (2D) units has been proved very difficult [1], and new experimental approaches have been devised to overcome these difficulties. Foams are good examples of 3D structures that can be built from 2D units, with diversified technological applications ranging from liquid and/or gas adsorbents to energy storage (supercapacitors) [2-5]. In this work we report the fabrication of a foam-like 3D nanostructured material of ultra-low density, high porosity, composed of rGO (reduced graphene oxide) and ZrO₂ (zirconia) nanoparticles. These structures were obtained using a simple wet chemistry method and have well-defined foam morphology. For the cases where zirconia is the minority component (25 wt%), we observe a flaky microstructure and the presence of zirconia nanoparticles seems to prevent the flakes from tearing off by blocking crack propagation, thus increasing foam resistance to fracture. DFT and molecular dynamics simulations support these interpretations. However, a marked change in the foam morphology was observed when the concentration of zirconia is increased (50 wt%). In this case the foams present a complete mesh-like inter-connected structure with the flakes coiling around the zirconia particles at different levels, depending on the local zirconia content. The foam morphology has a direct correlation to the oil/organic solvent adsorption capacity. The compositions containing zirconia showed an increment up to 40% in the absorption of gasoline in comparison with pure rGO sheets [6].
References:
[1] Li, D. & Kaner, R.B. Graphene-based materials. Science320, 1170-1171 (2008).
[2] Zou, J. et al. Ultralight multiwalled carbon nanotube aerogel. ACS Nano4, 7293-7302 (2010).
[3] Banhart, J. Metal foams: production and stability. Adv. Eng. Mater.8, 781-794 (2006).
[4] Reddy, E.S. & Schmitz, G.J. Superconducting foams. Supercond. Sci. Technol.15, L21-L24 (2002).
[5] Banhart, J. Manufacture, characterization and application of cellular metals and metal foams. Prog. Mater. Sci.46, 559-632 (2001).
[6] T.-R. Shan, B. D. Devine, T. W. Kemper, S. B. Sinnott, and S. R. Phillpot, Charge-optimized many-body potential for the hafnium/hafnium oxide system. Phys. Rev. B81, 125328 (2010).

The presentation will focus on a system for classification of structure of periodic trusses. The system employs concepts from crystallography and geometry to describe nodal locations and connectivity of struts. Through a series of illustrative examples of progressively increasing complexity, a rational taxonomy of truss structure is developed. Its conceptual evolution begins with elementary cubic trusses, increasing in complexity with non-cubic and compound trusses as well as supertrusses, and, finally, with complex trusses. The conventions and terminology adopted to define truss structure yield concise yet unambiguous descriptions of structure types and of specific (finite) trusses. The utility of the taxonomy is demonstrated by bringing into alignment a disparate set of ad hoc and incomplete truss designations previously employed in a broad range of science and engineering fields. Effects of structure type on elastic properties are probed through a series of numerical simulations of prototypical trusses subjected to shear, uniaxial compression and hydrostatic loading. The latter results demonstrate the merits of compound trusses: ones comprising two or more interpenetrating elementary trusses.

Truss cores offer an attractive alternative to commercially available honeycomb materials for use in lightweight sandwich structures especially in the low core density regime (<0.1 g/cc), or where an open-celled architecture is desired for multi-functionality (e.g. structure and fluid flow). We report the design, structural optimization, and mechanical performance of hollow truss cores fabricated via electroplating nano-grained nickel onto sacrificial polymer templates. The performance of these hollow nickel truss cores is compared to honeycombs under transverse compression, transverse shear, and mixed compression and shear loading utilizing both finite element analyses and experimental results from an enhanced Arcan apparatus. We identify truss cores with compression and shear strengths exceeding those of honeycomb and present analytical equations for guiding the design of optimal hollow trusses. Mass reduction and/or strength improvements enabled by optimal hollow truss cores in structural sandwich panels are presented.
Copyright 2015 HRL Laboratories LLC

Mechanical defects in materials, which exists in different sizes and shapes, cause a redistribution of stresses which needs to be taken account in design for applications. A flawless material can be weakened by magnitudes depending on whether there is an edge crack, center crack, or even circular crack. Our goal is to design a composite that can mitigate stress concentration with a given crack and have improved strength and toughness compared to its bulk materials. Current engineering materials have tradeoffs between toughness and stiffness. On the other hand, natural composites actually have the ability to combine the properties of stiffness and toughness effectively. For example, bone and abalone shells, have intricate geometries and different material phases that allow them to achieve both high stiffness and toughness. We study the effects of designing a composite architecture composed of two materials with vastly different mechanical properties, such as a stiff and soft material. Here, a greedy optimization algorithm is employed to explore how to design composites with improved strength and toughness, and 3D printing is used to overcome manufacturing limitations to allow us to create more diverse geometries. Our research shows that, using an algorithmic design, we can generate an optimized composite morphology, 3D-print, and experimentally validate our results through mechanical tests and digital image correlation (DIC). This provides a proof of concept that through computational modeling, complementary material properties can be spatially optimized to synthesize improved composites, advancing towards biological materials&’ characteristics of high toughness and high stiffness.

Additive manufacturing of ceramics enables fabrication of architected materials and complex shaped structures that are not feasible with conventional ceramic processing routes. We report UV curable pre-ceramic monomers that are exposed with UV light in a commercial stereolithography system or through a patterned mask, forming a three-dimensional polymer structure that can have complex shape and cellular architecture. These polymer structures can be pyrolyzed to a ceramic with virtually no porosity and defects. Silicon oxycarbide microlattice and honeycomb materials fabricated with this approach exhibit strength of 9 - 163 MPa at densities of 0.05 - 0.8 g/cm³ - 10X higher than ceramic foams of similar density. The materials exhibit good oxidation performance and survive 3h at 1700°C in air with less than 1 mg/cm² weight change. Additive processing of such materials is of interest for thermal protection systems, porous burners, biomedical devices and lightweight, loadbearing sandwich structures.

Global interest and acceptance of additive manufacturing (AM) and 3D-printing (3D) technologies within academic research, industrial manufacturing, and consumer markets is growing exponentially. With this increased interest and attention, also comes an elevated awareness of the limitations among the wide variety of AM and 3DP technologies and platforms. Chief among these limitations are; 1) Materials: The variety of materials compatible with AM and 3DP platforms and co-printing compatible with each other; and 2) Manufacturing: scalability, speed, and platform material versatility. In this presentation, we provide a comprehensive update of ongoing work in the Shah Tissue Engineering and Additive Manufacturing (TEAM) laboratory towards further developing a near-universal 3D-ink system for 3D-printing a broad range of functional and structural ceramic, metallic, polymeric, composite, and biological tissue materials. These liquid 3D-inks can be produced through simple mixing of three components: a solvent mixture, elastomeric polymer, and particles. The composition of the solvent and elastomeric polymer are conserved across 3D-inks, while the comprising particle material defines individual inks. Inks can be fabricated in small or large volumes (greater than 1 liter) and stored for extended periods of time (> 1 year) prior to use. The liquid inks are rapidly 3D-printed (linear deposition speeds exceeding 150 mm/s) via simple extrusion, at room-temperature, to form solid, self-supporting, user defined objects comprised of upwards of thousands of layers (many centimeters in size), which may be handled immediately, using a single 3D-printing platform. Many of the resulting 3D-printed materials can be utilized as printed, while others can undergo additional post-printing-processing (PPP) to transform or impart additional functionality to the object. This presentation will include additional discussion on the ongoing development of “3DP-Enabled Materials”; materials with distinct nano/microstructures that are directly derived from the 3DP process and impart highly advantageous mechanical, physical, and biological properties. In this context, we will provide a comprehensive update on the expanse of 3D-inks we have been developing for energy, structural, and medical applications, which will include but will not be limited to 3D-printed graphene (3DG), hyperelastic bone (HB), metals and alloys, planetary soils, natural tissues, and engineered particle systems and their use in functional devices.

In order to combine high strength with low density, cellular materials with specific 3D microarchitecture have been fabricated by applying 3D direct laser writing and atomic layer deposition [1]. The material is constructed as alumina-polymer composites, consisting of polymer structures with a specific microarchitecture and trusses with typical diameters of about 1.0µm, coated with thin layers of alumina between 10 and 100nm. Using this approach, the material benefits from the mechanical size effect with an increasing strength due to the nanometer-sized building blocks. With the use of a novel technique of push-to-pull tensile tests, the mechanical characterization of the composite material is possible [2]. The specimens for these tests are also manufactured using 3D direct laser writing and enable tensile experiments of the nanocomposites.
It is the aim of this work to improve the strength of the nanocomposites by optimizing the properties of the polymer microstructures after the laser writing step by applying specific annealing treatments at different temperatures as well as for different durations. Using polymer push-to-pull structures, the influence of the thermal treatments on the mechanical properties of the photopolymer was examined. For instance, after 45min at 200°C, the tensile strength increases by a factor of 11 compared to the samples without heat treatment. For the same annealing time at a higher temperature (250°C/45min), the strength increases to a lesser extent. Overall, these experiments show that depending on the specific heat treatment, the strength, stiffness and engineering toughness of the polymer material can be optimized for the use in an architected material. With respect to their strength as well as their deformation and failure behavior the structures are examined using in situ nanomechanical testing methods. On the basis of these experiments, the role of the different material properties, i.e. stiffness, strength and toughness, for the behavior of polymer-based truss-structures will be discussed.
References
[1] J. Bauer, S. Hengsbach, I. Tesari, R. Schwaiger, O. Kraft (2014) High-strength cellular ceramic composites with 3D microarchitecture. - Proc. Natl. Acad. Sci. 111 (7): 2453-2458, USA.
[2] J. Bauer, A. Schroer, R. Schwaiger, I. Tesari, C. Lange, L. Valdevit, O. Kraft (2015) Push-to-pull tensile testing of ultra-strong nanoscale ceramic-polymer composites made by additive manufacturing. - Extreme Mechanics Letters 3: 105-112.

Nanoporous or hollow silica/organosiloxane spheres are of interest for a wide variety of applications ranging from antireflective coatings and light scattering materials to catalyst carriers and materials for encapsulation and controlled release of active ingredients. In spite of this large potential, however, such particles are currently not widely applied. The main reasons for this are the high cost and technical complexity of the synthesis routes developed to date. Most synthesis routes are based on deposition of silica on template materials through hydrolytic conversion of molecular precursors such as tetraethoxysilane, and subsequent removal of the template. Latter, however, is expensive and technically difficult to realize without negatively affecting the dispersion stability. In a substantial part of the approaches developed to date, di- or triblock copolymers are applied. These are typically not commercially available, and costly to produce. Furthermore, after synthesis substantial amounts of surfactants are present in the particle dispersion, which strongly limits the formulation window for use of such particles in coatings or other matrices. Again, removal of this excessive amount of surfactant is costly. Here, were present a one-pot synthesis route that can be performed in a conventional emulsion reaction, and involves only commercially available, low-cost starting materials. The resulting dispersion of hollow or nanoporous particles does not contain surfactants.
For the synthesis of hollow organosiloxane/organosilsesquioxane spheres, we dispersed organotrialkoxysilanes (R-TMS) in aqueous ammonia [1]. Through hydrolytic conversion of R-TMS, a surfactant was formed that stabilized the emulsion of Ph-TMS in water, making the use of conventional surfactants obsolete. Through hydrolysis and condensation, an organosiloxane shell was formed at the outside of the Ph-TMS droplets, which progressively grows inwards. After removal of low molecular weight and unreacted core material through solvent extraction, hollow organosiloxane/organosilsesquioxane spheres were formed. Structure and composition of these particles was determined. We managed to produce particles with a diameter between 200 nm and 700 nm, a size distribution less than 5%, and a tunable shell thickness of minimum 50 nm. In addition, we extensively investigated the formation and composition of the surface active species formed through hydrolytic conversion of R-TMS, and the selective shell growth process.
The lessons from the above mentioned study on the surface active species and shell growth process were applied to rationally modify the synthesis procedure with the aim of producing nanoporous particles. Through addition of specific solvents, we managed to produce nanoporous spheres with a diameter between 200 nm and 700 nm and a size distribution less than 5%. Porosity was studied in detail.
[1] M. Segers, N. Arfsten, P. Buskens, M. Möller, RSC Advances 2014, 4, 20673-20676.

Electrospinning can integrate nano/micro-scale structures to build functioning three dimensional fiber architectures. In this study, engineered electrospun scaffolds with fibers oriented with designed curvature in three dimensions (3D) including the looped structure were developed based on the principle of electrostatic repulsion. This unique structure is different from most electrospun scaffolds with fibers oriented mainly in one direction or in one plane. Here we illustrate that 3D electrospun recycled polystyrene fibers could closely mimic the unique architectures of multi-direction and multi-layer nano-spiderweb. In contrast to virgin PS of the same molecular weight, the recycled PS (Dart Styrofoam) are known to contain zinc stearate, which acts as a surfactant. Zeta potential measurements show that recycled polystyrene fibers are 50% more charged than standard polystyrene fibers. Addition of Zn stearate to the PS solution also produced fibers with a similar increase in charge. Optical images of the fibers indicated that the fibers with Zn stearate had larger curvature, than the virgin PS fibers, indicating that their modulus was lower. The ratio of the moduli of the fibers was in agreement with the model from previous studies [C.Pai-2011]. The modulus of the fibers was shown to decrease with increasing fiber diameter, in agreement with previous observations, only for the virgin PS. In contrast the modulus remained constant, at the bulk value, for the recycled PS over the range of 750-2500nm. The surfactant, which is known to decrease the surface tension, may have also been effective at decreasing the confinement of the PS, where chain stretching was shown to occur, in response to the high surface tension at the air interface. Three dimensional flexible architecture with complex structures are shown to be necessary in order to block the motion of Cimex lectularius. These insects are very difficult to trap since they do not respond to lures, and are resistant to chemical solutions. Here we show how an electrospun network of surfactant modified polymer fibers can be used to immobilize the insects. The polymer network is carefully designed such that the diameter of the fibers fits exactly between the setae on the legs of the insect. The spacing between fibers is engineered to be on the micro scale such that the leg of the insect can penetrate. In order for them to be trapped, the mechanical response of the fibers has to be specifically tailored so that it is elastically deformed, without fracturing or flowing. We utilized optical microscope, Scanning Electron Microscope (SEM) and AFM nanoindentation to characterize the dimension and mechanical response of the fiber respectively. Carefully controlling and tailoring the electrospinning parameters we can now utilize architected 3D nanofiber to create an environmental-friendly Cimex immobilization device which can lead to annihilation solution for all the other harmful insects.

Materials with tunable thermal expansion have broad applications, from dental fillings with near-zero thermal expansion to induce minimum toothache, to precision alignment of instruments deployed in environments with large temperature variations. Previous studies on these structures were mostly focused on theoretically design and prediction.
In this invited talk, we report the fabrication of complex, three-dimensional mechanical metamaterials of tunable thermal expansion using a low cost, high throughput 3D microfabrication technique, namely projection stereolithography (PuSL). The PuSL system is reliable and capable of manufacturing a variety of highly complex, three-dimensional structures from micro- to mesoscales with microscale architecture and submicron precision. Our experimental results indicate such mechanical metamaterials exhibit significant negative thermal expansion over a large range of temperature variations, thanks to the structural interaction of material components with distinct thermal expansion ratios. Our experimental results match consistently with the theoretical and computational models. We expect that our studies may pave ways for zero-thermal-expansion materials.

Origami Metamaterials are folding structures that provide new routes
to functional sheets and materials. We show that the simplest building blocks of origami-based materials
- rigid, degree-four vertices - are generically multistable. Despite their simplicity, we have only recently
established the rules that govern the folding geometry of generic vertices, i.e. what are the mountain
valley arrangements, and which folds can be closed. Moreover, when classifying these vertices,
a large number of non-generic vertices arise, only one of which is the well know flat-foldable
vertex used in traditional origami. When such vertices are dressed with spring like interactions,
as is physically relevant for folded materials, many of such vertices are bistable, but by careful design,
we can also create monostable, tristable, quadstable, pentastable and hexastable states. The latter
are very rare --- in a random search less than one in a million have six stable states --- but we
precisely show that almost symmetric vertices virtually guarantee such high number of stable states.
This points to a general theme in mechanical metamaterials, where weakly broken symmetries lead to
the most interesting mechanics. Finally, we show how to program the stability
features of a single vertex into periodic fold tessellations. The resulting metasheets provide a previously
unanticipated functionality—tunable and switchable shape and size via multistability.

Controlling the nonlinear response of mechanical structures paves the way toward designing artificial materials with adaptive and tunable mechanical properties. We demonstrate a novel structure, built out of slender flexible beams arranged in an open cellular pattern, in which specific regions are singled out for buckling under a uniform external stress. Our design exploits a recently introduced class of topological mechanical lattices -- periodic structures with a topological characterization of the unit cell which guarantees the presence of stress states at specific edges or domain walls. These lattices are mechanical analogues of exotic quantum systems which display topologically protected electronic states. The nontrivial topology manifests itself not in the connectivity of the constituent phases, but rather in the structure of vibrational modes of the lattice. The resulting buckling regions are indistinguishable from their surroundings as far as material parameters or connectivity of their constituent elements are concerned. Furthermore, the localized response is robust against a wide range of structural deformations. We have demonstrated the phenomenon through numerical calculations of the linear response of the proposed structures, as well as experiments on 3D-printed prototypes. Besides illustrating the power of topological concepts in structural design, our approach has potential applications in tuning mechanical response without affecting secondary physical properties, for instance in thermomechanical or optomechanical metamaterials.

“As the extension, so the force” wrote R. Hooke in 1678 when he appreciated for the first time how solids
deform. This law of linear elasticity applies to all materials and as such constitutes the foundation of
mechanics. Yet, in the past few years, the emergence of articial periodic composites ---mechanical metamaterials--- has challenged our understanding of how solids manage stresses and deformations.
In this talk, I will present a class of non-periodic mechanical metamaterials in 3D. Our structures consist of cubic
lattices of anisotropic unit cells that can be stacked in a complex combinatorial fashion. We design and
3d-print this flexible complex ordered mechanism, in which we combine elastic hinges and defects to tailor the
mechanics of the material. Finally, we use this large design space and the aperiodicity to encode smart
functionalities such as surface patterning and texture morphing and multistability.

The mechanics of fibrous materials is both complex and intriguing. Here, we explore one class of such materials, made of the disordered entanglement of a single long coiled fiber. This architected material is of fundamental and technological interests. From a theoretical point of view, this system can be seen as a disordered material, akin to glasses, but with very long (possibly infinite) ranged correlations transmitted along the single fiber. From an applied point of view, this material is a potential replacement for sintered materials, because with comparable porosities, entangled structures are highly ductile while sintered powders are intrinsically brittle.
We study the mechanics of this material using a combination of experiments and simulations. Experimentally, we produced cylindrical samples by entangling a single fiber made of either a superelastic shape-memory alloy, a viscoelastic polymer or a ductile metal. The samples were subjected to cycles of compressions and tensions, tracking the local deformations in the samples with optical cameras and x-ray tomography. Numerically, we employed Kirchhoff&’s elastic rod theory to simulate the same mechanical cycles on both numerical substitutes of the experimental samples (i.e. cylindrical samples of geometry and density comparable to the experiments) and on idealized samples with a cubic and periodic geometry.
We will discuss our main observations on the mechanical properties of the monofilamentous systems. First, stress-strain curves show a prolonged regime of non-linear elasticity with a marked hysteresis between loading and unloading. Also, tracking the sample shape variations, we observed surprisingly large and reversible extensions of the volume, both in tension and compression. This material is thus reversiby dilatant in both directions of deformation. In terms of Poisson's function, this system shows a unique highly non-linear variation of its Poisson's function, which varies continuously from above 1/2 in compression to less than 0 in tension. We show through the simulations that this unusual behavior arises from an interplay between the elongation of the coiled wire and rearrangements due to steric effects, which, unlike in traditional discrete media, are hysteretically reversible when the architecture is made of an elastic fiber.

Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hishy;eshy;rshy;ashy;shy;rshy;chical design of 3-dimensional nano-architectures. Such structural meta-materials exhibit superior thermoshy;mechanical proshy;shy;shy;pershy;shy;ties at exshy;treshy;meshy;ly low mass densities (lighter than aerogels), making these solid foams ideal for many scientific and techshy;noshy;loshy;gishy;cal applications. The dominant deshy;forshy;mation mechanisms in such “meta-materials”, where individual constituent size (nanometers to microns) is compashy;rable to the characteristic microstructural length scale of the constituent solid, are essentially unshy;known. To harness the lucrative properties of 3-dimensional hierarchical nanostructures, it is critical to assess mechanical properties at each relevant scale while capturing the overshy;all structural complexity.
We present the fabrication of 3-dimensional nano-lattices whose constituents vary in size from several nanometers to tens of microns to millimeters. We discuss the deformation and mechanical properties of a range of nano-sized solids with different microstructures deformed in an in-situ nanomechanical instrument. Attention is focused on the interplay between the internal critical microstructural length scale of materials and their external limitations in revealing the physical mechashy;nisms which govern the mechanical deformation, where competing material- and structure-induced size effects drive overall properties.
We focus on the deformation and failure in metallic, ceramic, and glassy nano structures and discuss size effects in nanomaterials in the framework of mechanics and physics of defects. Specific discussion topics include: fabrication and characterization of hierarchical 3-dimensional architected meta-materials for applications in biomedical devices, ultra lightweight batteries, and damage-tolerant cellular solids, nano-mechanical experiments, flaw sensitivity in fracture of nano structures.

Metamaterials are rapidly arising at the frontier of science and engineering due to their exotic and unusual properties obtained by structure rather than chemistry. Using origami, the ancient art of paper folding, programmable metamaterials have been previously created from two-dimensional sheets through folding along predefined creases. In particular, origami patterns have proven to be promising in the design of solar panels for space deployment, flexible medical stents, 3D cell laden microstructures and flexible electronics. Moreover, it has been shown that cellular metamaterials can be designed by stacking folded layers. However, most of the proposed origami-inspired mechanical metamaterials are based on the miura-ori fold pattern which has only a single degree of freedom and is intrinsically 2D, and therefore limits the design of fully 3D metamaterials. Here, inspired by snapology - a type of modular unit-based origami in which paper ribbons are used to create complex geometric extruded polyhedra- we designed and fabricated fully 3D reprogrammable metamaterials whose shape and volume can be actively controlled in multiple degrees of freedom by embedded pneumatic actuation. This mechanical metamaterials shows a 100-fold volume reduction by fully collapsing, can change their microstructure to multiple distinct shapes, and have a tunable load carrying capability spanning four orders of magnitude. Since the transformation modes described for these materials operate independently of the object's macro-scale external geometry, this transformable metamaterial can be machined into any desired architecture and therefore has the potential to simplify the creation of entirely new types of deployable and reconfigurable devices. In fact, by exploiting origami's scale-free geometric character, this approach can be extended to the micro- and nano-scale, as well as to the meter-scale, opening avenues for applications ranging from tunable wave guides to deployable shelters

Most conventional materials expand in transverse directions when they are compressed uniaxially. Here, we exploit structural hierarchy to design a new class of two dimensional metamaterials with negative Poisson&’s ratio that contract in transverse directions under uniaxial compressive loads. Auxetic behavior is achieved through mechanical instabilities (i.e. buckling) introduced by structural hierarchy over a wide range of applied compression. This unusual behavior is demonstrated through a combination of experiments and finite element simulations which exhibits an excellent qualitative and quantitative agreement. The work provides new insights into the role of structural organization and hierarchy in designing two dimensional auxetic metamaterials, and new opportunities for developing energy absorbing materials, tunable membrane filters, and acoustic dampeners.

Generic 4-vertex meshes are rigid to folding. However, specific 4-vertex tilings can exhibit folding motion, as is for instance the case in the well known Miura-ori fold pattern. These fold patterns exhibit rigid folding motion by means of degenerate constraints. Here we systematically show how to construct heterogeneous 4-vertex meshes that have a minimal number of degenerate constraints, yet are still rigidly foldable. We exploit a previously overlooked symmetry that relates the folding angles of a generic 4-vertex to several associated conjugate vertices. Additionally, 3D printed, freely hinging sheets reveal that these heterogeneous 4-vertex meshes have not just one, but anywhere between two and exponentially (scaling with the length of the sides of the sheet) many one-degree-of-freedom folding branches. We show how these folding branches emerge from the generic 4-vertex, which itself is a one-degree-of-freedom system with two discrete folding branches. Finally, we establish a local foldability condition that incorporates the combinatorics of the individual vertices to count the total number of folding branches of a given sheet.

Shape reconfigurable materials have the capability of significant controlled morphological change from their current shape into a desired shape and maintaining the deformed shape. This property of the matter could pave the way for manufacture of highly adaptable components that are fully reconfigurable based on functionality. Foreseeable applications include deployable structures, multi-purpose structures, energy absorption and vibration control. For example, such material can be compressed significantly to be transported with considerably less cost; or it can be used to make a tool with multiple stable shapes for different purposes. Several routs to this goal have already been proposed including origami folding, shape memory morphing, shape-shifting jamming materials and modular self-assembly.
Here, a novel concept is proposed for achieving high volumetric and morphological change in 2D and 3D lattice materials based on multistable structural unit cells. Passive volumetric change in vertical and horizontal directions is independently achieved through multiaxial contraction of shape reconfigurable unit cells that are arranged according to a Cartesian tiling to form a square periodic material. Each unit consists of hinged beams that allow multiple stable states with different levels of volumetric contraction. Unlike origami folding, which is often based on a single degree-of-freedom rigid folding mechanism and becomes energetically inefficient to achieve additional degrees of freedom through soft bending modes, the proposed method offers a diverse set of possible reconfigurable topologies through various folding strategies and folding levels.
Prototypes of the proposed material are manufacture studied analytically, numerically and experimentally. The 2D material prototype is manufacture using laser cutting of sheets of propylene. The 3D material prototype is manufactured through assembly of unit cells that are fabricated partly via laser cut propylene segments and 3D printed cores. The analytical calculation is done using a variation of the beam-deflection method. The experimental study is performed using Instron machine. Moreover, a parametric study is performed to obtain the optimal design. The material was experimentally shown to achieve considerable shape change. Moreover, deformation was shown to be able to be introduced locally, including highly deformed zones with increased density surrounded by less deformed zones, allowing a wide range of deformed configuration based on the location of highly deformed zones. Specifically, a square-shaped 2D material was transformed into a variety of shaped including I-shape, X-shape and the circular shape, among other forms. The future step would be to equip the unit cells of the structure with servo motors to enable autonomous shape reconfigurability at the unit cell and consequently bulk material levels.

In nature, plants, insects and animals incorporate intricate folding patterns into their anatomical structures to perform complex functions in varied environments. These complex 3D structures, ranging from skin to insect wings to elaborate leaf arrangements, self-assemble from an inherently patterned 2D sheet to a 3D structure. Drawing inspiration from this mode of self-assembly, we discuss the self-folding of patterned metallic sheets using both differential stress and surface forces. The advantageous characteristics of the work include: (a) the incorporation of micropatterns on the sheet using conventional photolithography which is highly versatile in creating a variety of patterns of different shapes and sizes; (b) the hands-free and wire-free self-folding of the sheets using forces derived from the release of residual stress or the minimization of surface tension; and (c) the self-sealing of edges of self-folded origami sheets by solder coated edges enhancing the overall rigidity and strength of the structures. Consequently, the self-folding of patterned and sealed “egg-crate” like structures is realized. Patterns can be incorporated in the form of “smart” patches on the walls of the egg-crates which can be selectively functionalized with biomolecules. Apart from the intellectual appeal of these hands-free, self-sealing sheets, we envision applicability of these egg-crate like microstructures in lab on a chip assays as functionalized microwells and as light weight mechanical metamaterials.

Most gas barrier thin films suffer from cracking or plastic deformation when stretched,
leading to significant loss of barrier. In an effort to make a stretchable gas barrier, which
maintains low permeability when exposed to cyclic strain, layer-by-layer assemblies of tannic
acid (TA) and polyethylene oxide (PEO) were prepared. A 40-bilayer TA/PEO assembly
maintained its oxygen transmission rate (6X lower than the 1.6-mm-thick rubber substrate) after
being stretched 100%. The superior elasticity of the TA/PEO assembly originates from its unique
microstructure, which can be tailored by altering deposition pH. At low pH (le; 7), TA forms a
rigid network through self-association that imparts high modulus to the TA/PEO assembly. By
increasing the ionization of TA at higher pH (> 7), the formation of this rigid network is
inhibited due to reduced self-association and lower TA content in the film. TA/PEO thin films
assembled at pH 8.25 not only exhibit a permeability that is two orders of magnitude lower than
pure PEO, but also display better gas barrier retention than fully polymeric assemblies (e.g.,
PAA/PEO). No loss of gas barrier was observed after a single 100% stretch, which is a
remarkable improvement over PAA/PEO, whose OTR increased by 70%. Although repeated
extension to 100% strain leads to somewhat reduced gas barrier in the TA/PEO assembly, likely
due to breaking of H-bonds, the level of reduction is relatively small. The TA/PEO assembly
introduced here is a significant step forward, offering a practical solution for improving the gas
barrier of elastomeric pressurized objects, such as tires or blimps.

Heterogeneous thin films with features at the nanoscale (typically 1-100 nm) often display unique properties that can be practically exploited. For example as ultra-hard coatings and new materials with enhanced optical and/or magnetic properties have been proposed. However, for these materials to reach their full potential means being able to accurately and precisely control the size, position and nature of the features, often at the atomic scale. This in turn means being able to characterise the structure of thin films at this level.
The latest generation of corrected scanning transmission electron microscope (STEM) instruments can now provide images showing the atomic structure of materials with approximately 0.1 nm resolution. When combined with a high angle annular dark field detector, which removes the effect of Bragg diffraction from the image, means that for suitable samples atomic resolution images can be produced with chemical contrast (scales to Z^1.7 where Z is the atomic number). This makes it the ideal technique to establish the nanoscale and atomic ordering.
Using immiscible silver and copper alloy films deposited under controlled and varying conditions it is possible to engineer thin films that self-organize into various nanoscale structures. Using this as a model system it will be shown how corrected STEM can be used to determine information on thin film nanostructure that is either very difficult or impossible to obtain with other techniques. This includes continuous multilayer, multi domain or solid solution structures, material heterogeneity, crystal structure and orientation. The use of modelling to aid in determining material structure will also be discussed.

Crack propagation in microelectronics interconnect structures depends on how the metallization features are arranged in the dielectric stack and how they affect the local effective modulus of the material in which the crack is propagating. Local metallization structures can significantly alter the environment in which a crack initiates and provide resistance to cracking during packaging processes such as chip joining. Hence, probing the local mechanical properties as a function of depth for complex multilevel chip is critical.
High speed, constant strain rate nanoindentation testing can achieve this goal; however, detailed knowledge of the dynamic response of the testing system is required for accurate and precise measurements. During these experiments depth and contact stiffness are measured continuously as the load is rapidly increased. Various properties, such as the hardness and modulus, at specific locations and depths in the sample can be calculated from these direct measurements. A single indentation test must be performed in a few seconds to allow a sufficient number of tests to be performed in a reasonable time so that these results can be studied in the form of 4D (spatial locations, depth and stiffness) maps. Such maps measured from different arrangements of dielectric layers and copper metallization features in a low dielectric constant interconnect structure will be presented to demonstrate the capability of the technique. This technique for volumetric mapping of mechanical properties of sub-surface features of integrated circuits can be correlated to mechanical failures in the devices.

Recently there has been tremendous research on self-assembled vertically aligned nanocomposite thin films with two immiscible components hetero-epitaxially grown on single crystal substrates^1-4. These structures have the advantages of utilizing both component functions and tuning material properties with high interface-to-volume ratio, hetero-epitaxial strain, or cation valence state.
Here we report about the characterization of self-assembled vertically aligned non-magnetic zirconium oxide (ZrO₂) and ferromagnetic perovskite lanthanum strontium manganese oxide (La_2/3Sr_1/3MnO₃, LSMO) pillar-matrix nanostructures with a film thickness of about 50 nm, which are epitaxially grown on (001) single-crystalline lanthanum aluminum oxide (LaAlO₃) substrate by pulsed laser deposition. With the application of electron energy-loss spectroscopy (EELS) in a probe-aberration-corrected JEOL ARM200CF, atomic resolution elemental distribution, including La, Sr, Mn, and Zr, and the Mn valence state variation at the interface between LSMO and ZrO₂ were examined. Minor amounts of Zr were found to occupy Mn positions within the matrix. The Zr concentration reaches a minimum near the pillar-matrix interface accompanied by oxygen vacancies. Substantial La and Mn diffusion into the pillar was revealed at atomic resolution and a concomitant change of the Mn valence state was observed.
In addition, Mn-rich walls were found connecting adjacent pillars. The crystal lattices on either side of the wall are displaced by an anti-phase shift. The Mn valence state in the channel was found to be identical compared to the matrix. The wall plane is of {110} or {130} type. The role of the pillars and walls regarding elastic strain and local electric fields will be discussed.
The spin, charge, and orbital ordering in LSMO are extremely sensitive to local structural and elemental variations. Thus, these results provide the basis for understanding the origin of the anomalous magnetic anisotropy and modifications to the electric transport properties of LSMO by introducing non-magnetic ZrO₂ pillars.⁵
References
1. A.P. Chen et al., Nanotechnology 22 (2011) 31
2. A. Imai et al., ACS Nano 7 (2013) 11079
3. H. Zheng et al. Nano Lett. 6 (2006) 1401
4. H.J. Liu et al., ACS Nano 6 (2012) 6952
5. D. Zhou et al., APL Materials 2 (2014) 127301
Acknowledgements
The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007-2013] under grant agreement no 312483 (ESTEEM2). Financial support for the ARM200CF project by the Max Planck Society is gratefully acknowledged.

Mechanical instabilities such as wrinkling or folding have long been known for one universal mechanism for the pattern formation in nature as a part of morphogenesis with various scales. In the laboratory level, these instability patterns on a surface have been applied to various applications. Moreover, micro- and nano-scale instability patterns have become new way to obtain novelty to existing devices or materials. However, these patterns have intrinsic randomness so that various applications have bounded utilities. In this research, we suggested a new strategy to control surface structures originated from instability patterns under biaxial compressive stresses. To address strain localization zone, we constructed moduli-patterned elastic membranes using scrapping-mediated micromolding process. We observed that a typical wrinkle-to-fold transition occurred only the softer region. By modifying the boundary conditions and geometry, we demonstrated control over the final folded morphology having spatiality and directionality. Moreover, to confirm the strain localization behavior, we calculated stress distribution and displacements of moduli-patterned substrate using a finite element analysis (FEA). The local strain in the softer region was 3-fold higher than a stiffer region, resulting in selective pattern formation. The ideas introduced here should find application in the many situations where elastic inhomogeneous materials subjected to stresses impacts two-dimensional pattern formation.

The conditioning of novel porous ceramic structures and their surfaces for applications in important fields of energy, environmental and chemical processing is of pivotal interest. While ceramic materials feature a high corrosion and high-temperature resistance, they are far more demanding to be manufactured and to be conditioned than metals or polymers. With the aid of recently established new ceramic manufacturing techniques, porous structures with pore sizes in the micro- (< 2nm), meso- (2 - 50 nm) and macropore (> 50 nm) range were generated for targeted applications. The characterisation of the transport paths within the opaque, three-dimensional structures, however, is highly challenging and requires imperatively the establishment of novel analytical methods. Thus, we have combined two powerful techniques; nuclear magnetic resonance (NMR) for spatial analysis of the liquid phase distribution and dynamic flow processes, whereas X-ray tomography (X-CT) is applied for quantitative analysis of the ceramic structure. Our preliminary results obtained from the systematic combination of these two powerful methods will be presented and discussed in this talk.

Recently we optimized 3D woven lattice materials that were manufactured using textile techniques so as to maximize fluid permeability with minimal reductions in mechanical stiffness. Specific wires were removed from a fully dense “standard” structure, in which all wire positions are occupied, to form a less dense “optimized” structure as suggested by topology optimization. Here we demonstrate that these standard and optimized woven structures also offer superior performance in heat transfer applications due to their excellent thermal conductivity, high specific surface area, effective fluid mixing, and 3D flow capabilities. The 3D flow capabilities prove particularly advantageous when the weaves are combined with flow manifolds to create unique flow patterns within the weaves.
To characterize three critical design variables - pressure drops, heat transfer, and temperature uniformity - we first fabricated 3 x 1 x 1 inch blocks using the standard and optimized woven architectures, and we studied three simple flow patterns: axial, focused bifurcated and full bifurcated using water and air as coolants. Next we combined thin sections of the two woven Cu blocks with a unique flow manifold to create a 3D flow pattern within the weaves. The properties measured for all four flow patterns are compared to those for common heat dissipation media and we find that while the weaves can exhibit a slightly larger resistance to flow, they offer superior heat transfer performance and better temperature uniformity. These properties make them ideal candidates for applications requiring maximum heat transfer and minimal temperature variation, with limited constraints on pressure drop or pumping power. Finite element modeling is also used to assess pressure drops and heat transfer for the various flow patterns and the standard and optimized architectures.

Materials with negative thermal expansion (NTE) have broad applications, from dental fillings with near-zero thermal expansion to induce minimum toothache, to precision alignment of instruments deployed in environments with large temperature variations. Previous studies on NTE structures were mostly focused on theoretically design and prediction. Here, with projection micro-stereolithography, we experimentally fabricate multi-material structures that exhibit significant negative thermal expansion over a large range of temperature variations. The negative thermal expansion is induced by the structural interaction of material components with distinct thermal expansion ratios. Our experimental results match consistently with the theoretical and computational models. We expect that our studies may pay ways for zero-thermal-expansion materials by mixing the NTE structures with positive-thermal-expansion materials.

The development of new techniques in additive micro-manufacturing in recent years has enabled the fabrication of material microstructures with essentially arbitrary designs, including those with multiple constituent materials and void space in nearly any geometry. With an essentially open design space, the onus is now on the engineer to design composite materials which are optimal for their purpose. These new materials, called meta-materials or materials with architected microstructures, offer the potential to mix and match properties in a way that exceeds that of traditional composites.
We concentrate on the thermal and elastic properties of isotropic meta-materials, and design microstructures with combinations of Young's modulus, Poisson's ratio, thermal conductivity, thermal expansion, and mass density which are not found among naturally-occurring or traditional composite materials. We also produce designs with thermal expansion far below other materials. We can achieve the bounds on material properties predicted by theory.
We use homogenization theory to predict the material properties of a bulk meta-material comprised of a periodic lattice of unit cells, then use topology optimization to rearrange two constituent materials and void space within the unit cell in order to extremize an objective function which yields the combinations of properties we seek. This method is quite general and can be extended to consider additional properties of interest. We construct the design space to satisfy material isotropy directly (2D), or to satisfy cubic symmetry (3D), from which point an isotropy constraint function is easily applied. This construction can also be used to increase computational efficiency of the homogenization. We also develop and use filtering, nonlinear interpolation, and thresholding methods to render the design problem well-posed and to ensure our designs are manufacturable.
We have written two computer implementations of this design methodology. The first is for creating two-dimensional designs, which can run on a serial computer in approximately half an hour. The second is a parallel implementation to allow optimization in three dimensions with a large number of parameters. When running on a high-performance computing cluster, it allows for solutions in a few hours despite the greatly increased computational cost.

Cellular materials with programmable elastic properties are suitable for a range of high-end applications in aerospace, automotive, and acoustics due to their enhanced mechanical properties, such as shear resistance, indentation resistance, and damping properties. Topology optimization has been established as a proven numerical design tool for the creation of materials with extremal static and dynamic properties. The method has also proven efficient for generating more exotic materials such as isotropic negative Poisson&’s ratio materials and materials with isotropic negative thermal expansion coefficients.
Previous works have suffered from three key limitations: (1) Lack of suitable, scalable manufacturing methods, (2) restriction to two-dimensional (2D) designs, and (3) restriction to small deformations of the material, i.e. the linear regime.
We show how recent advances in topology optimization allow overcoming these key constraints. First we present material architectures with programmable Poisson&’s ratios within the range of -0.8 to 0.8, which remain nearly constant over large deformations of 20% or more [1-2]. A combination of topology and shape optimization is used to take manufacturing constraints into account. This allows scalable 3D printing of the architectures in a silicone rubber using direct ink writing.
Second we present an approach to obtain three-dimensional (3D), manufacturable cellular materials. The approach has successfully been applied to obtain an isotropic material with a Poisson&’s ratio of -0.5, which was manufactured using selective laser sintering (SLS) [3]. We also illustrate how the approach can be applied to design three-phase, 3D cellular materials with a negative thermal expansion.
The presented improvements may be generalized to other properties than Poisson&’s ratio and thermal expansion coefficient. The suggested approaches all constitute important steps towards systematically designing manufacturable cellular materials with programmable properties.
References:
[1] Clausen, A., Wang, F., Jensen, J. S., Sigmund, O. & Lewis, J. A. Topology Optimized Architectures with Programmable Poisson&’s Ratio for Large Deformations. (Submitted).
[2] Wang, F., Sigmund, O. & Jensen, J. S. Design of materials with prescribed nonlinear properties. J. Mech. Phys. Solids 69, 156-174 (2014).
[3] Andreassen, E., Lazarov, B. S. & Sigmund, O. Design of manufacturable 3D extremal elastic microstructure. Mech. Mater. 69, 1-10 (2014).

Electrochemical nanoporous actuators have low weight, large specific surface areas and low voltage operating capabilities, making them attractive for application in small scale electromechanical devices. The actuation strain of nanoporous actuators at the macroscopic scale is a manifestation of microscopic (atomistic) phenomena occurring at the ligament surface. The key feature that couples the atomistic mechanisms to the macroscale is the architectural morphology at the mesoscopic scale. The goal of this work is to bridge these length scales by investigating the actuation and mechanical properties of nanoporous actuators through a multiscale modeling approach. We evaluate and compare the performance of metallic and graphene actuators having different nanoporous architectures, and capture their response in simple scaling relations.
Nanoporous metals and graphene nanofoams immersed in an aqueous electrolyte undergo considerable dimensional changes when a bias voltage is applied. The primary actuation mechanism is the electric double-layer charging of the internal surface in combination with a large surface to volume ratio. The induced charge changes the interatomic bond length, causing the material to expand or contract macroscopically. To capture these sub-atomic features we modified existing atomistic models for gold (i.e., the surface embedded atom method, SEAM) and graphene (i.e., the adaptive intermolecular reactive empirical bond order potential, AIREBO) and calibrated the potentials to density functional theory (DFT) calculations.
Our results provide a fundamental understanding of the physical mechanisms that couple the electrochemical injection of charge at the nanoscale to the overall actuation stroke and work output of gold and graphene nanofoams. Key mechanisms that set the nanofoams of graphene apart from those of gold are the ratio between the inherent graphene and gold stiffness, the strain-charge ratio of the graphene ribbons and gold surfaces and the different architectural morphologies that control bending versus stretching dominated mechanical properties. The results can be used as guidelines for the synthesis of nanoporous actuators having optimal actuator strokes and work densities.

Systems in space are vulnerable to large temperature changes when travelling into and out of the Earth's shadow. Variations in temperature can lead to undesired geometric changes in sensitive applications requiring very fine precision, such as sub-reflector supporting struts. To suppress such failures, materials with a low coefficient of thermal expansion (CTE) over a wide range of temperatures are needed. Besides low CTE, these materials should also provide desirable stiffness, strength and extraordinarily low mass.
Bi-material lattices have been proposed as a potential class of low CTE materials. [1] Under an applied load, the member of a lattice generally responds with either bending or stretching deformation. Those that axially deform lead to lattices that are structurally efficient and thus suitable in applications where low mass is a priority. In 3D, a tetrahedron is generally the basic unit cell that makes up most stretch-dominated lattices, as it is the simplest rigid polyhedron that can be constructed from inextensional pin-jointed struts. If the CTE of a tetrahedron is carefully tailored, then a stretching dominated lattice can be designed to obtain a low, or even zero, CTE structure. In this work, we present a systematic strategy that uses tetrahedron tessellation to develop 3D low thermal expansion lattices with low mass, besides high stiffness and strength. We show zero CTE can be achieved in several stretch-dominated lattices by tailoring the compensation of the thermal expansion and joint rotation at the member nodes. For certain topologies, such as the Octet Truss, the process can lead to a zero CTE in a specific direction at the cost of a large thermal expansion in the others. Thermal and mechanical anisotropy can thus become beneficial if the application requires it.
In this work, we use a combination of experiments and theoretical analysis to test proof-of-concept lattices that exploit tetrahedron tessellation to achieve zero CTE. Selected lattice samples are fabricated by a simple snap-fit and vacuum brazing method [2] from Al 6061 and Ti-6Al-4V alloy sheets. We use digital image correlation to test their thermo-mechanical properties, which are in turn compared to those obtained via asymptotic homogenization. The results are plotted into four quadrant charts that map mass, stiffness, strength and CTE, and enable a comparison with other structural concepts in literature. The strategy presented here show that stretch-dominated lattices can be tuned to display zero CTE without jeopardizing their structural efficiency.
1. Steeves, C.A., et al., Concepts for structurally robust materials that combine low thermal expansion with high stiffness. Journal of the Mechanics and Physics of Solids, 2007. 55(9): p. 1803-1822.
2. Dong, L., V. Deshpande, and H. Wadley, Mechanical response of Ti-6Al-4V octet-truss lattice structures. International Journal of Solids and Structures, 2015. 60: p. 107-124.

Aligned liquid crystal elastomers (LCEs) are capable of undergoing large reversible shape change in response to thermal stimuli and have potential applications as thermal sensors, actuators for deployment of micro devices and reconfigurable topologies. Recent advances in LCE patterning tools have demonstrated sub-millimetre control of director orientation, enabling the preparation of materials with arbitrarily complex director fields. However, without design tools to connect the 2D director pattern with the activated 3D shape, LCE design relies on intuition and/or trial and error. Here we present a design methodology based on topology optimization to predict the director profiles in monolithic LCEs to generate folding. The distributions of order/disorder and director orientations are optimized to match the thermally actuated deformation with a target deformation inspired from origami. Predicted director patterns were evaluated with nonlinear mechanical analysis to determine large folding capability and then fabricated and experimentally tested for comparison. The optimal design exhibits a strategy to counteract the mechanical frustration due to anti-clastic bending, observed in twisted nematic approaches for folding, by reducing the order of contraction in the center of the fold line. Interfaces between folds were also designed using insights from the optimal single hinge and were used to create multi-hinge networks. The application of topology optimization to LCE design provides an initial step toward leveraging the engineering potential of the programmable anisotropy and heterogeneity of LCEs and is an important step towards leveraging the unique opportunities afforded by LCE patterning into functional designs.

We present a material design framework that maximizes thermal rectification in bulk materials, the first report of which was by Starr in 1936 [1]. Analogous to their electrical counterparts, thermal rectifiers allow preferential heat flow in a particular direction and enable logic calculations using heat carriers as well as superior thermal management in microelectronics. Despite investigations of bulk thermal rectification and the more recent interest in its nanoscale applications [2], the material properties required to optimize this rectification have not yet been characterized. This is due in part to the large unknown parameter space that is related to the choice of materials, their arrangements, and operating temperature ranges. Optimizing material properties for bulk thermal rectification should lead to experimental convenience, reduced fabrication cost, and more readily mass production. Therefore, we present a systematic investigation of material systems and mechanisms that maximize the bulk thermal rectification R=J+/J-, where J+ and J- denote heat fluxes from opposite sides of a system, e.g., left to right and right to left, respectively. Considering that the material thermal conductivity κ is a function of both position x and temperature T, we develop a mathematical framework to determine the dependence of R for the system on its constituent conductivities and subsequently identify the behavior of κ(x,T) which maximizes R. Using this framework, we evaluate strategically designed configurations of commonly available material systems over a wide temperature range. The model enables improved material selection and design in order to optimize bulk thermal rectification.
[1] C. Starr, J. Appl. Phys., vol. 7, no. 15, pp. 15-19, 1936.
[2] N. A. Roberts and D. G. Walker, Int. J. Therm. Sci., vol. 50, pp. 648-662, 2011.

Flexible, lightweight battery component is desirable for a variety of applications in portable, wearable and bendable electronics and lightweight electric vehicles to buildings with arbitrarily shaped surfaces. Current efforts have been largely limited to the trade-off between mechanical flexibility and device functionality due to inability to control how the active material is arranged in three-dimensions. Here, we present a new design of a 3D flexible Li battery by using a multi-porous tube-like current collector fabricated from additive manufacturing. The micro-scale architected current collector with integration of nanoscale active materials is assembled into a cable-type battery. Active materials such as transition metal oxides, silicon nanowires, or LiFePO4 can be integrated on the surface of the ligaments, serving as the Li storage medium. A counter electrode core and gel-like polymer electrolyte is further integrated into the architected electrode. We discuss the electrochemical performance at free and strained states and possible material failure mechanism. The metallic tube-like current collector can sustain large strain in the form of bending, twisting and stretching with full recovery with a range of designed flexible architectures.

Free glycerol removal from crude biodiesel is of a great importance due to its negative effects on biodiesel storage and usage. Traditional purification processes suffer from a number of problems such as significant product loss. Enhancing the biodiesel purification yield, selective adsorption of free glycerol from crude biodiesel mixture might be a promising approach in biodiesel dry washing. Therefore, a core@shell architecture, consisting of high surface area, high pore volume mesoporous core templates covered with a size-selective zeolite is proposed. Various types of mesoporous silica spheres with different sizes (commercial silica gel spheres: 20-45 µm and 3 µm and HMS spheres: ~1.5 µm) were used as core templates, which subsequently covered with a polycrystalline silicalite-1 shell. Applying typical characterization techniques, complete coverage and uniformity of the shell were assured. Performing a batch-wise glycerol adsorption test, purification yield and free glycerol removal efficiency of synthesized core@shell adsorbent were evaluated and compared to those purified by using conventional adsorbents including bare mesoporous silica gel spheres, conventional zeolites and physical mixture of the constitutive building materials of the core@shell adsorbent. Although mesoporous silica gel showed slightly higher glycerol adsorption capacity, the study revealed that the mesoporous adsorbents tend to trap a significant amount of bulkier molecules, e.g. FAME in their large pore network (d_pore= 7nm). In contrast, microporous silicalite-1 shell with minimized effective diffusion length hindered FAME's diffusion into the mesopores by its intrinsic sieving ability, maintaining a larger number of adsorption sites for glycerol. In addition, the core@shell adsorbents benefited from large pore volume of mesoporous silica as core compartment, providing a multi-layer glycerol adsorption. This feature of the synthesized core@shell material significantly enhanced the dry washing performance in terms of purification yield and adsorption capacity, in comparison to other conventional adsorbents (remaining glycerol to FAME ratio in adsorbent improved from around 0.65 for silica gel up to 5.2 for core@shell particles).