Search

Significant efforts have been devoted to enhancing the sensitivity and working range of flexible pressure sensors to improve the precise measurement of subtle variations in pressure over a wide detection spectrum. However, achieving sensitivities exceeding 1000 kPa−1 while maintaining a pressure working range over 100 kPa is still challenging because of the limited intrinsic properties of soft matrix materials. Here, we report a magnetic field-induced porous elastomer with micropillar arrays (MPAs) as sensing materials and a well-patterned nickel fabric as an electrode. The developed sensor exhibits an ultrahigh sensitivity of 10,268 kPa−1 (0.6–170 kPa) with a minimum detection pressure of 0.25 Pa and a fast response time of 3 ms because of the unique structure of the MPAs and the textured morphology of the electrode. The porous elastomer provides an extended working range of up to 500 kPa with long-time durability. The sophisticated sensor system coupled with an integrated wireless recharging system comprising a flexible supercapacitor and inductive coils for transmission achieves excellent performance. Thus, a diverse range of practical applications requiring a low-to-high pressure range sensing can be developed. Our strategy, which combines a microstructured high-performance sensor device with a wireless recharging system, provides a basis for creating next-generation flexible electronics.

Dislocation dynamic simulations are intended as a tool to understand and predict the mechanical behavior of metallic materials, but its prediction has never been directly verified by experiments due to differences in specimen strain rate and size. In this work, a comprehensive experimental framework is proposed to attempt direct comparison between experiments and discrete dislocation dynamics (DDD) modelling. By integrating high-throughput sample fabrication and a customized testing apparatus, the sample size and strain rate typically employed in DDD simulations are explored experimentally. Constitutive properties such as stress-strain response are measured, and microstructural information is obtained from transmission electron microscopy (TEM) imaging, electron backscatter diffraction (EBSD), and TEM-based orientation mapping. Magnesium and copper were selected, as case studies, to demonstrate the newly developed experimental procedure. Measured stress-strain responses for Mg are consistent with those obtained with a miniaturized Hopkison bar experiments. By exploiting the validated workflow, the effect of strain rate on micropillar heterogeneous deformation and associated dislocation plasticity were revealed. The work establishes a methodology for the systematic study of not only metals but also other materials and structures at the microscale and high strain rates.

Summary Animals' teeth have evolved to provide food procurement, mastication, and protection. These functions, directly linked to survival of many living animal species, require superior hardness and abrasion resistance in the animal's dentition system. Such resistance typically emerges from damage tolerance and sharpness preservation during the organism’s life span. An example is the sea urchin tooth, which through gradients in mechanical properties together with exploitation of microstructural features achieves such functionality. Using contact mechanics, dimensional analysis, and a novel in situ scanning electron microscopy experimental methodology, conditions for tooth deformation and wear, via a self-sharpening mechanism consisting in plate chipping, were imaged and quantified. Nonlinear finite-element modeling of the self-sharpening mechanism provided insight into the synergy between constituent material properties and tooth microstructural elements. The findings reported here should inspire the design of novel tools used in machining operations, e.g., cutting and grinding, as well as in mining and tunnel boring.

Summary During the last decade, two-dimensional (2D) materials have emerged as versatile building blocks for the next generation of engineered materials. However, the intrinsically brittle behavior of 2D materials has thus far delayed their adoption in applications such as sensors and structural materials. Herein, we demonstrate a strategy for toughening graphene oxide (GO) through synergistic interfacial interactions between GO monolayers and ultrathin layers of strongly interacting poly(vinyl alcohol) (PVA). By creating GO-PVA and PVA-GO-PVA nanolaminates, we demonstrate a 2-fold increase in GO toughness, which translates into dramatic increases in energy dissipation and piercing resistance. Atomistic simulations show that this remarkable behavior arises from a polymer chain crack-bridging mechanism, resulting from a synergistic combination of interdomain reinforcements across the GO monolayer and extensive GO-polymer interfacial hydrogen-bonding interactions. The reported findings highlight the potential for achieving engineered 2D materials with superior mechanical properties by incorporating deformation and failure-resistant mechanics arising from tailored chemical interactions between constituents.

The fi eld of in situ nanomechanics is greatly benefi ting from microelectromechanical systems (MEMS) technology and integrated microscale testing machines that can measure a wide range of mechanical properties at nanometer scales, while characterizing the damage or microstructure evolution in electron microscopes. This article focuses on the latest advances in MEMS-based nanomechanical testing techniques that go beyond stress and strain measurements under typical monotonic loadings. Specifi cally, recent advances in MEMS testing machines now enable probing key mechanical properties of nanomaterials related to fracture, fatigue, and wear. Tensile properties can be measured without instabilities or at high strain rates, and signature parameters such as activation volume can be obtained. Opportunities for environmental in situ nanomechanics enabled by MEMS technology are also discussed.

A number of experimental techniques have been used to characterize the mechanical properties and wear of biomaterials, from nanoindentation to scratch to atomic force microscopy testing. While all these experiments provide valuable information on the mechanics and functionality of biomaterials (e.g., animals’ teeth), they lack the ability to combine the measurement of force and sliding velocities with high resolution imaging of the processes taking place at the biomaterial-substrate interface. Here we present an experiment for the in situ scanning electron microscopy characterization of the mechanics of friction and wear of biomaterials with simultaneous control of mechanical and kinematic variables. To illustrate the experimental methodology, we report the wear of the sea urchin tooth, which exhibits a unique combination of architecture and material properties tailored to withstand abrasion loads in different directions. By quantifying contact conditions and changes in the tooth tip geometry, we show that the developed methodology provides a versatile and promising tool to investigate wear mechanisms in a variety of animal teeth accounting for microscale effects.

Electrospun carbon nanofibers, produced from polyacrylonitrile (PAN) nanofiber precursors, with their superior mechanical properties, are promising candidates for manufacturing advanced polymer composites. Here, we report a series of tensile tests performed in situ under scanning electron microscope (SEM)/transmission electron microscope (TEM) observation, which show that the modulus and strength of electrospun carbon nanofibers can be enhanced through a simple mechanical constraint during the carbonization step in the electrospinning process. The constrained carbon nanofibers of diameter less than 150 nm were nanomanipulated inside the SEM onto a specialized microelectromechanical systems (MEMS) based testing platform and subsequently tested in uniaxial tension until failure. It was identified that both the strength and modulus of the constrained carbon nanofibers with sub-150 nm diameters are on average higher compared to their unconstrained counterparts by ∼22% and ∼31% respectively. Also, by evaluating the internal graphitic order of the constrained carbon nanofibers using TEM-based diffraction methods, we identified that the mechanical constraint during carbonization results in a better degree of orientation in the graphitic crystallites along the fiber axis. Finally, we use Weibull statistics for the deconvolution of the effects of diameter and the mechanical constraint on the tensile properties of carbon nanofibers. The Weibull analysis also showed that the comparatively superior strength of CCNFs is primarily due to better alignment of crystallites with the fiber axis.

Nanoelectromechanical (NEMS) systems fabricated using atomically thin materials have low mass and high stiffness and are thus ideal candidates for force and mass sensing applications. Transition metal dichalcogenides (TMDCs) offer certain unique properties in their few-layered form – such as piezoelectricity and a direct band gap (in some cases) – and are an interesting alternative to graphene based NEMS. Among the demonstrated methods for displacement transduction in NEMS, cavity-interferometry provides exquisite displacement sensitivity. Typically, interferometric measurements are complemented with Raman spectroscopy to characterize the number of layers in 2D materials, and the measurements necessitate high vacuum conditions to eliminate viscous damping. Here, we report an experimental setup that facilitates both Raman spectroscopy and interferometric measurements on few-layered Tungsten Disulfide (WS2) resonators in high vacuum (

In vitro and ex vivo intracellular delivery methods hold the key for releasing the full potential of tissue engineering, drug development, and many other applications. In recent years, there has been significant progress in the design and implementation of intracellular delivery systems capable of delivery at the same scale as viral transfection and bulk electroporation but offering fewer adverse outcomes. This review strives to examine a variety of methods for in vitro and ex vivo intracellular delivery such as flow-through microfluidics, engineered substrates, and automated probe-based systems from the perspective of throughput and control. Special attention is paid to a particularly promising method of electroporation using micro/nanochannel based porous substrates, which expose small patches of cell membrane to permeabilizing electric field. Porous substrate electroporation parameters discussed include system design, cells and cargos used, transfection efficiency and cell viability, and the electric field and its effects on molecular transport. The review concludes with discussion of potential new innovations which can arise from specific aspects of porous substrate-based electroporation platforms and high throughput, high control methods in general.

Kirigami structures provide a promising approach to transform flat films into 3D complex structures that are difficult to achieve by conventional fabrication approaches. By designing the cutting geometry, it is shown that distinct buckling-induced out-of-plane configurations can be obtained, separated by a sharp transition characterized by a critical geometric dimension of the structures. In situ electron microscopy experiments reveal the effect of the ratio between the in-plane cut size and film thickness on out-of-plane configurations. Moreover, geometrically nonlinear finite element analyses (FEA) accurately predict the out-of-plane modes measured experimentally, their transition as a function of cut geometry, and provide the stress-strain response of the kirigami structures. The combined computational-experimental approach and results reported here represent a step forward in the characterization of thin films experiencing buckling-induced out-of-plane shape transformations and provide a path to control 3D configurations of micro- and nanoscale buckling-induced kirigami structures. The out-of-plane configurations promise great utility in the creation of micro- and nanoscale systems that can harness such structural behavior, such as optical scanning micromirrors, novel actuators, and nanorobotics. This work is of particular significance as the kirigami dimensions approach the sub-micrometer scale which is challenging to achieve with conventional micro-electromechanical system technologies.

Naturally occurring biological materials with stiff fibers embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. In particular, biological composite materials with helicoidal architecture have been shown to exhibit enhanced damage tolerance and increased impact energy absorption. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. We also introduce additional architectures, i.e., single unidirectional lamina and mono-balanced architectures, for comparison purposes. Compared with associated mono-balanced architectures, helicoid architectures exhibit less fiber reorientation values and lower values of strain stiffening. The explanation for this difference is addressed in terms of the measured in-plane deformation, due to uniaxial tensile of the laminae, correlated to lamina misorientation with respect to the loading direction and lay-up sequence.

Graphene oxide (GO), whose highly tunable surface chemistry enables the formation of strong interfacial hydrogen-bond networks, has garnered increasing interest in the design of devices that operate in the presence of water. For instance, previous studies have suggested that controlling GO's surface chemistry leads to enhancements in interfacial shear strength, allowing engineers to manage deformation pathways and control failure mechanisms. However, these previous reports have not explored the role of ambient humidity and only offer extensive chemical modifications to GO's surface as the main pathway to control GO's interfacial properties. Herein, through atomic force microscopy experiments on GO-GO interfaces, the adhesion energy and interfacial shear strength of GO were measured as a function of ambient humidity. Experimental evidence shows that adhesion energy and interfacial shear strength can be improved by a factor of 2-3 when GO is exposed to moderate (∼30% water weight) water content. Furthermore, complementary molecular dynamics simulations uncovered the mechanisms by which these nanomaterial interfaces achieve their properties. They reveal that the strengthening mechanism arises from the formation of strongly interacting hydrogen-bond networks, driven by the chemistry of the GO basal plane and intercalated water molecules between two GO surfaces. In summary, the methodology and findings here reported provide pathways to simultaneously optimize GO's interfacial and in-plane mechanical properties, by tailoring the chemistry of GO and accounting for water content, in engineering applications such as sensors, filtration membranes, wearable electronics, and structural materials.

Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe2 ) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe2 monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications.

Combining experimental and computational studies of nanocomposite interfaces is highly needed to gain insight into their performance. However, there are very few literature reports, combining well-controlled atomic force microscopy experiments with molecular dynamic simulations, which explore the role of polymer chemistry and assembly on interface adhesion and shear strength. In this work, we investigate graphene oxide (GO)-polymer interfaces prevalent in nanocomposites based on a nacre-like architectures. We examine the interfacial strength resulting from van der Waals and hydrogen bonding interactions by comparing the out-of-plane separation and in-plane shear deformations of GO-polyethylene glycol (PEG) and GO-polyvinyl alcohol (PVA). The investigation reveals an overall better mechanical performance for the anhydrous GO-PVA system in both out-of-plane and in-plane deformation modes, highlighting the benefits of the donor-acceptor hydrogen bond formation present in GO-PVA. Such bond formation results in inter-chain hydrogen bond networks leading to stronger interfaces. By contrast, PEG, a hydrogen bond acceptor only, relies primarily on van der Waals inter-chain interactions, typically resulting in weaker interactions. The study also predicts that water addition increases the adhesion of GO-PEG but decreases the adhesion of GO-PVA, and slightly increases the shear strength in both systems. Furthermore, by comparing simulations and experiments, we show that the CHARMM force field has enough accuracy to capture the effect of polymer content, water distribution, and to provide quantitative guidance for achieving optimum interfacial properties. Therefore, the study demonstrates an effective methodology, in the Materials Genome spirit, toward the design of 2D materials-polymer nanocomposites system for applications demanding mechanical robustness.

Bighorn sheep ( Ovis canadensis ) rams hurl themselves at each other at speeds of ∼9 m/s (20 mph) to fight for dominance and mating rights. This necessitates impact resistance and energy absorption mechanisms, which stem from material-structure components in horns. In this study, the material hierarchical structure as well as correlations between the structure and mechanical properties are investigated. The major microstructural elements of horns are found as tubules and cell lamellae, which are oriented with (∼30⁰) angle with respect to each other. The cell lamellae contain keratin cells, in the shape of pancakes, possessing an average thickness of ∼2 µm and diameter of ∼20–30 µm. The morphology of keratin cells reveals the presence of keratin fibers and intermediate filaments with diameter of ∼200 nm and ∼12 nm, respectively, parallel to the cell surface. Quasi-static and high strain rate impact experiments, in different loading directions and hydration states, revealed a strong strain rate dependency for both dried and hydrated conditions. A strong anisotropy behavior was observed under impact for the dried state. The results show that the radial direction is the most preferable impact orientation because of its superior energy absorption. Detailed failure mechanisms under the aforementioned conditions are examined by bar impact recovery experiments. Shear banding, buckling of cell lamellae, and delamination in longitudinal and transverse direction were identified as the cause for strain softening under high strain rate impact. While collapse of tubules occurs in both quasi-static and impact tests, in radial and transverse directions, the former leads to more energy absorption and impact resistance. Statement of Significance Bighorn sheep ( Ovis canadensis ) horns show remarkable impact resistance and energy absorption when undergoing high speed impact during the intraspecific fights. The present work illustrates the hierarchical structure of bighorn sheep horn at different length scales and investigates the energy dissipation mechanisms under different strain rates, loading orientations and hydration states. These results demonstrate how horn dissipates large amounts of energy, thus provide a new path to fabricate energy absorbent and crashworthiness engineering materials.

Graphene oxide (GO) shows promise as a nanocomposite building block due to its exceptional mechanical properties. While atomistic simulations have become central to investigating its mechanical properties, the method remains prohibitively expensive for large deformations and mesoscale failure mechanisms. To overcome this, we establish a coarse-grained (CG) model that captures key mechanical and interfacial properties, and the non-homogeneous effect of oxidation in GO sheets. The CG model consists of three types of CG beads, representing groups of pristine sp2 carbon atoms, and hydroxyl and epoxide functionalized regions. The CG force field is parameterized based on density functional-based tight binding simulations on three extreme cases. It accurately quantifies deterioration of tensile modulus and strength at the expense of improving interlayer adhesion with increasing oxidation of varying chemical compositions. We demonstrate the applicability of the model to study mesoscale phenomena by reproducing different force vs. indentation curves in silico, corroborating recent experimental observations on how chemistry near contact point influences properties. Finally, we apply the model to measure the fracture toughness of pristine graphene and GO. The critical stress intensity factor ( K c ) of graphene is found to be the highest, and epoxide-rich GO also possesses higher K c compared to hydroxyl-rich GO.

In PNAS, Cao et al. (1) employ track-etched membranes for the intracellular delivery of various molecular cargoes via electroporation in both adherent and suspended cells. While the concept of utilizing such membranes for electroporation is not novel (2, 3), its extension to suspended cells and the wide parametric space covered in the article are noteworthy. The objectives of this letter are to correct a minor lapse in the article’s introduction, as well as to put the results in a broader context. In ref. 1, the authors cite that the delivery efficiency of a plasmid, using track-etched membranes and localized electroporation (2), was less than 50% (reference 16 in ref. 1). However, an efficiency … [↵][1]1To whom correspondence may be addressed. Email: espinosa{at}northwestern.edu. [1]: #xref-corresp-1-1

Graphene oxide (GO) sheets have been used as a model system to study how the mechanical properties of two-dimensional building blocks scale to their bulk form, such as paper-like, lamellar-structured thin films. Here, we report that the modulus of multilayer GO films can be significantly enhanced if some of the sheets are drastically weakened by introducing in-plane porosity. Nanometer-sized pores are introduced in GO sheets by chemical etching. Membrane-deflection measurements at the single-layer level show that the sheets are drastically weakened as the in-plane porosity increases. However, the mechanical properties of the corresponding multilayer films are much less sensitive to porosity. Surprisingly, the co-assembly of pristine and etched GO sheets yields even stiffer films than those made from pristine sheets alone. This is attributed to the more compliant nature of the soft porous sheets, which act as a binder to improve interlayer packing and load transfer in the multilayer films., Graphene oxide sheets have been used as a model system to study how the mechanical properties of individual 2D building blocks scale to their bulk form. Here the authors show that the modulus of multilayer graphene oxide films can be enhanced if some of the sheets are weakened by introducing in-plane porosity.

MEMS-based tensile testing platforms are very powerful tools for the mechanical characterization of nanoscale materials, as they allow for testing of micro/nano-sized components in situ electron microscopes. In a typical configuration, they consist of an actuator, to deliver force/displacement, and a load sensor, which is connected to the sample like springs in series. Such configuration, while providing a high resolution force measurement, can cause the onset of instability phenomena, which can later compromise the test validity. In the present paper such phenomena are quantitatively discussed through the development of an analytical model, which allows to find a relationship between the rise of instability and the sensor stiffness, which is the key parameter to be optimized.

This paper describes a systematic study on the nanoscale toughening of monolayer graphene oxide (GO) by an ultra-thin polymer adlayer, which impedes the propagation of cracks during intraplanar fracture. Using molecular dynamics simulations, the crack-bridging capabilities of a library of five hydrogen-bonding-capable polymers are explored against an epoxide-rich GO substrate. The best crack-bridging effect is found in polymers with functional groups that can both donate/accept hydrogen atoms and have better capability to form cooperative hydrogen bonds. Aligning the chains of poly(acrylic acid) orthogonally to the crack propagation direction significantly enhances the fracture toughness of monolayer GO (by 310%) in comparison to that for an adlayer with randomly arranged chains (180% enhancement). Notably, van der Waals interactions, which are seldom highlighted in the fabrication of strong GO-polymer interfaces, are found to also provide significant crack-bridging capabilities when the polymers possess large side groups. These results pave the way for a set of design criteria that can help in remediating the intrinsically brittle mechanical behavior of two-dimensional materials, a barrier that currently restricts their potential applications.

Localized electroporation has evolved as an effective technology for the delivery of foreign molecules into cells while preserving their viability. Consequently, this technique has potential applications in sampling the contents of live cells and the temporal assessment of cellular states at the single-cell level. Although there have been numerous experimental reports on localized electroporation-based delivery, a lack of a mechanistic understanding of the process hinders its implementation in sampling. In this work, we develop a multiphysics model that predicts the transport of molecules into and out of the cell during localized electroporation. Based on the model predictions, we optimize experimental parameters such as buffer conditions, electric field strength, cell confluency, and density of nanochannels in the substrate for successful delivery and sampling via localized electroporation. We also identify that cell membrane tension plays a crucial role in enhancing both the amount and the uniformity of molecular transport, particularly for macromolecules. We qualitatively validate the model predictions on a localized electroporation platform by delivering large molecules (bovine serum albumin and mCherry-encoding plasmid) and by sampling an exogeneous protein (tdTomato) in an engineered cell line.

Kirigami with a variety of cut patterns have been recently investigated as means to transform 2D surfaces into 3D structures, offering a number of functionalities. In this work, we show that a single Kirigami motif, defining two inner panels connected by hinges, can generate a rich variety of out-of-plane symmetric and asymmetric deformation modes. The out-of-plane responses, caused by local instabilities when a far field tensile load is applied, manifest themselves at the inner plates, exhibiting a combination of one- and two-dimensional rotations (tilt and twist), effectively morphing the planar geometries into complex 3D surfaces. By conducting numerical analyses and experiments, the relationship between changes in the cut geometry, and out-of-plane responses is identified. In particular, two types of instabilities emerge, coincident bifurcation points for both inner plates, responsible for two-dimensional tilts, and sequential bifurcation points, which produce one-dimensional tilts with distinct buckling thresholds. Nonlinear parametric finite element analysis (FEA) is used to explore the space of possible configurations and characterize the behavior of the structure as a function of its parameters, revealing a variety of deformation modes and structural responses of interest. Numerical predictions are experimentally validated using paper Kirigami and the shadow Moire technique. The combined numerical–experimental approach reveals a plethora of programmable and controllable deformation modes with intrinsic nonlinear behaviors, enabled by tunable imperfections, which promise to influence a variety of applications ranging from light modulation to fluid flow control.

In a hypervelocity impact (HVI) event, fiber fabrics and the fabric-reinforced polymers (FRP) would undergo shock compression, large deformation and fragmentation. The smooth particle hydrodynamics (SPH) approach was applied to assess the shielding performance of the fabric and its composite structure in a Whipple shield. In the fabric model, a fiber is built by SPH particles to properly reproduce the spreading feature of fragmented fabric under HVI. The simulations display that an aluminum panel, serving as the bumper of a Whipple, has the better performance in debris spreading than fabric layers. In the stuffed layer of a Whipple, the widely used plain weave fabric has the similar performance as the 3D weave both in debris spreading and speed retarding. The fabric model is further developed and extended to FRP by building fiber and polymer materials separately based on specific geometries. The computations illustrate that the FRP/Aluminum hybrid laminate can efficiently reduce the shock peak under HVI and meanwhile produce large deformation for kinetic energy absorption, in good agreement with experimental measurements. It applies to the rear wall of a Whipple which should resist the HVI of a debris cloud, forming a high but short shock pulse. The further optimization of the hybrid laminate was made by using a corrugated aluminum plate, a gap and a Kevlar fabric layer, leading to the considerable reduction of the laminate areal mass in a prescribed thickness.

The mechanical properties of graphene oxide (GO) are of great importance for applications in materials engineering. Previous mechanochemical studies of GO typically focused on the influence of the degree of oxidation on the mechanical behavior. In this study, using density functional-based tight binding simulations, validated using density functional theory simulations, we reveal that the deformation and failure of GO are strongly dependent on the relative concentrations of epoxide (-O-) and hydroxyl (-OH) functional groups. Hydroxyl groups cause GO to behave as a brittle material; by contrast, epoxide groups enhance material ductility through a mechanically driven epoxide-to-ether functional group transformation. Moreover, with increasing epoxide group concentration, the strain to failure and toughness of GO significantly increases without sacrificing material strength and stiffness. These findings demonstrate that GO should be treated as a versatile, tunable material that may be engineered by controlling chemical composition, rather than as a single, archetypical material.

The characterization of nanomaterials under high strain rates is critical to understand their suitability for dynamic applications such as nanoresonators and nanoswitches. It is also of great theoretical importance to explore nanomechanics with dynamic and rate effects. Here, we report in situ scanning electron microscope (SEM) tensile testing of bicrystalline silver nanowires at strain rates up to 2/s, which is 2 orders of magnitude higher than previously reported in the literature. The experiments are enabled by a microelectromechanical system (MEMS) with fast response time. It was identified that the nanowire plastic deformation has a small activation volume (10b(3)), suggesting dislocation nucleation as the rate controlling mechanism. Also, a remarkable brittle-to-ductile failure mode transition was observed at a threshold strain rate of 0.2/s. Transmission electron microscopy (TEM) revealed that along the nanowire, dislocation density and spatial distribution of plastic regions increase with increasing strain rate. Furthermore, molecular dynamic (MD) simulations show that deformation mechanisms such as grain boundary migration and dislocation interactions are responsible for such ductility. Finally, the MD and experimental results were interpreted using dislocation nucleation theory. The predicted yield stress values are in agreement with the experimental results for strain rates above 0.2/s when ductility is pronounced. At low strain rates, random imperfections on the nanowire surface trigger localized plasticity, leading to a brittle-like failure.

Understanding the complicated failure mechanisms of hierarchical composites such as fiber yarns is essential for advanced materials design. In this study, we developed a new Monte Carlo model for predicting the mechanical properties of fiber yarns that includes statistical variation in fiber strength. Furthermore, a statistical shear load transfer law based on the shear lag analysis was derived and implemented to simulate the interactions between adjacent fibers and provide a more accurate tensile stress distribution along the overlap distance. Simulations on two types of yarns, made from different raw materials and based on distinct processing approaches, predict yarn strength values that compare favorably with experimental measurements. Furthermore, the model identified very distinct dominant failure mechanisms for the two materials, providing important insights into design features that can improve yarn strength.

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura-Ori tubes. The origami-architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson's ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson's ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.

Novel 2D materials, e.g., graphene oxide (GO), are attractive building blocks in the design of advanced materials due to their reactive chemistry, which can enhance interfacial interactions while providing good in-plane mechanical properties. Recent studies have hypothesized that the randomly distributed two-phase microstructure of GO, which arises due to its oxidized chemistry, leads to differences in nano- vs meso‑scale mechanical responses. However, this effect has not been carefully studied using molecular dynamics due to computational limitations. Herein, a continuum mechanics model, formulated based on density functional based tight binding (DFTB) constitutive results for GO nano-flakes, is establish for capturing the effect of oxidation patterns on the material mechanical properties. GO is idealized as a continuum heterogeneous two-phase material, where the mechanical response of each phase, graphitic and oxidized, is informed from DFTB simulations. A finite element implementation of the model is validated via MD simulations and then used to investigate the existence of GO representative volume elements (RVE). We find that for the studied GO, an RVE behavior arises for monolayer sizes in excess to 40 nm. Moreover, we reveal that the response of monolayers with two main different functional chemistries, epoxide-rich and hydroxyl‑rich, present distinct differences in mechanical behavior. In addition, we explored the role of defect density in GO, and validate the applicability of the model to larger length scales by predicting membrane deflection behavior, in close agreement with previous experimental and theoretical observations. As such the work presents a reduced order modeling framework applicable in the study of mechanical properties and deformation mechanisms in 2D multiphase materials.

Recent major improvements to the transmission electron microscope (TEM) including aberration-corrected electron optics, light-element-sensitive analytical instrumentation, sample environmental control, and high-speed and sensitive direct electron detectors are becoming more widely available. When these advances are combined with in situ TEM tools, such as multimodal testing based on microelectromechanical systems, key measurements and insights on nanoscale material phenomena become possible. In particular, these advances enable metrology that allows for unprecedented correlation to quantum mechanics and the predictions of atomistic models. In this Perspective, we provide a summary of recent in situ TEM research that has leveraged these new TEM capabilities as well as an outlook of the opportunities that exist in the different areas of in situ TEM experimentation. Although these advances have improved the spatial and temporal resolution of TEM, a critical analysis of the various in situ TEM fields reveals that further progress is needed to achieve the full potential of the technology.

Numerous experimental and computational studies have established that the hierarchical structures encountered in natural materials, such as the brick-and-mortar structure observed in sea shells, are essential for achieving defect tolerance. Due to this hierarchy, the mechanical properties of natural materials have a different size dependence compared to that of typical engineered materials. This study aimed to explore size effects on the strength of bio-inspired staggered hierarchical composites and to define the influence of the geometry of constituents in their outstanding defect tolerance capability. A statistical shear lag model is derived by extending the classical shear lag model to account for the statistics of the constituents' strength. A general solution emerges from rigorous mathematical derivations, unifying the various empirical formulations for the fundamental link length used in previous statistical models. The model shows that the staggered arrangement of constituents grants composites a unique size effect on mechanical strength in contrast to homogenous continuous materials. The model is applied to hierarchical yarns consisting of double-walled carbon nanotube bundles to assess its predictive capabilities for novel synthetic materials. Interestingly, the model predicts that yarn gauge length does not significantly influence the yarn strength, in close agreement with experimental observations.

This paper presents evidence that strongly adhered carbonaceous surface impurities, intrinsic impurities that accompany multiwall carbon nanotubes (MWCNTs) synthesized by arc-discharge, are a component that cannot be ignored in experiments involving single nanotubes and their interfaces with a second surface. At the interface that forms between a carbon nanotube and a graphitic surface, these impurities can significantly alter the adhesion properties of the underlying nanotube and can cause over 30% scatter in computed interaction energies, similar in magnitude to the scatter reported in experimental measurements involving individual CNTs. Also presented is high-resolution TEM evidence that commonly used purification techniques that are effective at removing larger impurity particles from as-produced arc-discharge MWCNT samples do not remove these strongly adhered carbonaceous surface impurities.

Time-dependent mechanical characterization of nanowires is critical to understand their long-term reliability in applications, such as flexible-electronics and touch screens. It is also of great importance to develop a theoretical framework for experimentation and analysis on the mechanics of nanowires under time-dependent loading conditions, such as stress-relaxation and fatigue. Here, we combine in situ scanning electron microscope (SEM)/transmission electron microscope (TEM) tests with atomistic and phase-field simulations to understand the deformation mechanisms of single crystal silver nanowires held under constant strain. We observe that the nanowires initially undergo stress-relaxation, where the stress reduces with time and saturates after some time period. The stress-relaxation process occurs due to the formation of few dislocations and stacking faults. Remarkably, after a few hours the nanowires rupture suddenly. The reason for this abrupt failure of the nanowire was identified as stress-assisted diffusion, using phase-field simulations. Under a large applied strain, diffusion leads to the amplification of nanowire surface perturbation at long wavelengths and the nanowire fails at the stress-concentrated thin cross-sectional regions. An analytical analysis on the competition between the elastic energy and the surface energy predicts a longer time to failure for thicker nanowires than thinner ones, consistent with our experimental observations. The measured time to failure of nanowires under cyclic loading conditions can also be explained in terms of this mechanism.

Molecular dynamics simulations on nanoindentation of circular monolayer molybdenum disulfide (MoS2) film are carried out to elucidate the deformation and failure mechanisms. Typical force-deflection curves are obtained, and in-plane stiffness of MoS2 is extracted according to a continuum mechanics model. The measured in-plane stiffness of monolayer MoS2 is about 182 ± 14 N m-1, corresponding to an effective Young's modulus of 280 ± 21 GPa. More interestingly, at a critical indentation depth, the loading force decreases sharply and then increases again. The loading-unloading-reloading processes at different initial unloading deflections are also conducted to explain the phenomenon. It is found that prior to the critical depth, the monolayer MoS2 film can return to the original state after completely unloading, while there is hysteresis when unloading after the critical depth and residual deformation exists after indenter fully retracted, indicating plasticity. This residual deformation is found to be caused by the changed lattice structure of the MoS2, i.e. a phase transformation. The critical pressure to induce the phase transformation is then calculated to be 36 ± 2 GPa, consistent with other studies. Finally, the influences of temperature, the diameter and indentation rate of MoS2 monolayer on the mechanical properties are also investigated.

A continuum model for GO membranes is developed in this study. The model is built representing the membrane as a two-dimensional, heterogeneous, two-phase continuum and the constitutive behavior of each phase (graphitic or oxidized) is built based on DFTB simulations of representative patches. A hyper-elastic continuum model is employed for the graphene areas, while a continuum damage model is more adequate for representing the behavior of oxidized regions. A finite element implementation for GO membranes subjected to degradation and failure is then implemented and, to avoid localization instabilities and spurious mesh sensitivity, a simple crack band model is adopted. The developed implementation is then used to investigate the existence of GO nano-representative volume elements.

MEMS-based tensile testing devices are powerful tools for mechanical characterization of nanoscale materials. In a typical configuration, their design includes an actuator to deliver loads/displacements to a sample, and a sensing unit for load measurement. The sensing unit consists of a flexible structure, which deforms in response to the force imposed to the sample. Such deformation, while being necessary for the sensing function, may become a source of instability. When the sample experiences a load drop, as it may result from yield, necking or phase transitions, the elastic energy accumulated by the sensor can be released, thus leading to loss of the displacement-controlled condition and dynamic failure. Here, we report a newly-developed MEMS testing system where the sensor is designed to constantly keep its equilibrium position through an electrostatic feedback-control. We show design, implementation, and calibration of the system, as well as validation by tensile testing of silver nanowires. The implemented system allows capture of softening events and affords significant improvement on the resolution of stress–strain curves.

This brief report describes a novel tool for microfluidic patterning of biomolecules and delivery of molecules into cells. The microdevice is based on integration of nanofountain probe (NFP) chips with packaging that creates a closed system and enables operation in liquid. The packaged NFP can be easily coupled to a micro/nano manipulator or atomic force microscope for precise position and force control. We demonstrate here the functionality of the device for continuous direct-write parallel patterning on a surface in air and in liquid. Because of the small volume of the probes (~3 pL), we can achieve flow rates as low as 1 fL/s and have dispensed liquid drops with submicron to 10 µm diameters in a liquid environment. Furthermore, we demonstrate that this microdevice can be used for delivery of molecules into single cells by transient permeabilization of the cell membrane (i.e., electroporation). The significant advantage of NFP-based electroporation compared with bulk electroporation and other transfection techniques is that it allows for precise and targeted delivery while minimizing stress to the cell. We discuss the ongoing development of the tool toward automated operation and its potential as a multifunctional device for microarray applications and time-dependent single-cell studies.

A biologically inspired, multilayer laminate structural design is deployed into nanocomposite films of graphene oxide-poly(methyl methacrylate) (GO-PMMA). The resulting multilayer GO-PMMA films show greatly enhanced mechanical properties compared to pure-graphene-oxide films, with up to 100% increases in stiffness and strength when optimized. Notably, a new morphology is observed at fracture surfaces: whereas pure-graphene-oxide films show clean fracture surfaces consistent with crack initiation and propagation perpendicular to the applied tensile load, the GO-PMMA multilayer laminates show terracing consistent with crack stopping and deflection mechanisms. As a consequence, these macroscopic GO-PMMA films become defect-tolerant and can maintain their tensile strengths as their sample volumes increase. Linear elastic fracture analysis supports these observations by showing that the stiffness modulation introduced by including PMMA layers within a graphene oxide film can act to shield or deflect cracks, thereby delaying failure and allowing the material to access more of its inherent strength. Together, these data clearly demonstrate that desirable defect-tolerant traits of structural biomaterials can indeed be incorporated into graphene- oxide-based nanocomposites.

Understanding atomic interactions between constituents is critical to the design of high-performance nanocomposites. Here, we report an experimental-computational approach to investigate the adhesion energy between as-produced arc discharge multiwalled carbon nanotubes (MWCNTs) and graphene. An in situ scanning electron microscope (SEM) experiment is used to peel MWCNTs from graphene grown on copper foils. The force during peeling is obtained by monitoring the deflection of a cantilever. Finite element and molecular mechanics simulations are performed to assist the data analysis and interpretation of the results. A finite element analysis of the experimental configuration is employed to confirm the applicability of Kendall's peeling model to obtain the adhesion energy. Molecular mechanics simulations are used to estimate the effective contact width at the MWCNT-graphene interface. The measured surface energy is γ = 0.20 ± 0.09 J·m(-2) or γ = 0.36 ± 0.16 J·m(-2), depending on the assumed conformation of the tube cross section during peeling. The scatter in the data is believed to result from an amorphous carbon coating on the MWCNTs, observed using transmission electron microscopy (TEM), and the surface roughness of graphene as characterized by atomic force microscopy (AFM).

Recently there has been a major thrust to develop novel nanomaterials exhibiting unique properties. These nanostructures are envisioned as building blocks for the next generation of electronic and energy harvesting systems. In this context, identification of their size-dependent properties is essential, but due to challenges in nanoscale experimentation, unambiguous characterization of mechanical and electromechanical behavior has been elusive. However, in the past few years, in-situ experimentation has emerged as a powerful technique to overcome these challenges. Furthermore, the coupling of such experimental findings to atomistic simulations has shown to have the potential to reveal mechanistic insights, essential for the deep understanding of nanoscale behavior. Here, we present a summary of a few significant results obtained by our group using this approach. Specifically, we discuss size effects that influence the mechanical and electromechanical properties of metallic and semiconducting nanowires.

In this paper, we describe an innovative modular microfluidic platform allowing filtering, concentration and analysis of peptides from a complex mixture. The platform is composed of a microfluidic filtering device and a superhydrophobic surface integrating surface enhanced Raman scattering (SERS) sensors. The microfluidic device was used to filter specific peptides (MW 1553.73 D) derived from the BRCA1 protein, a tumor-suppressor molecule which plays a pivotal role in the development of breast cancers, from albumin (66.5 KD), the most represented protein in human plasma. The filtering process consisted of driving the complex mixture through a porous membrane having a cut-off of 12–14 kD by hydrodynamic flow. The filtered samples coming out of the microfluidic device were subsequently deposited on a superhydrophobic surface formed by micro pillars on top of which nanograins were fabricated. The nanograins coupled to a Raman spectroscopy instrument acted as a SERS sensor and allowed analysis of the filtered sample on top of the surface once it evaporated. By using the presented platform, we demonstrate being able to sort small peptides from bigger proteins and to detect them by using a label-free technique at a resolution down to 0.1 ng μL−1. The combination of microfluidics and nanotechnology to develop the presented microfluidic platform may give rise to a new generation of biosensors capable of detecting low concentration samples from complex mixtures without the need for any sample pretreatment or labelling. The developed devices could have future applications in the field of early diagnosis of severe illnesses, e.g. early cancer detection.

Whale baleen is a keratin-based biological material; it provides life-long (40-100 years) filter-feeding for baleen whales in place of teeth. This study reveals new aspects of the contribution of the baleen's hierarchical structure to its fracture toughness and connects it to the unique performance requirements, which require anisotropy of fracture resistance. Baleen plates are subjected to competing external effects of hydration and varying loading rates and demonstrate a high fracture toughness in transverse loading, which is the most important direction in the filtering function; in the longitudinal direction, the toughness is much lower since delamination and controlled flexure are expected and desirable. The compressive strength is also established and results support the fracture toughness measurements: it is also highly anisotropic, and exhibits a ductile-to-brittle transition with increasing strain rate in the dry condition, which is absent in the hydrated condition, conferring impact resistance to the baleen. Using 3D-printing prototypes that replicate the three principal structural features of the baleen plate (hollow medulla, mineralized tubules, and sandwich-tubular structure) are created, and the role of its structure in determining its mechanical behavior is demonstrated. These findings suggest new bioinspired engineering materials.

Electromechanical coupling is a topic of current interest in nanostructures, such as metallic and semiconducting nanowires, for a variety of electronic and energy applications. As a result, the determination of structure-property relations that dictate the electromechanical coupling requires the development of experimental tools to perform accurate metrology. Here, a novel micro-electro-mechanical system (MEMS) that allows integrated four-point, uniaxial, electromechanical measurements of freestanding nanostructures in-situ electron microscopy, is reported. Coupled mechanical and electrical measurements are carried out for penta-twinned silver nanowires, their resistance is identified as a function of strain, and it is shown that resistance variations are the result of nanowire dimensional changes. Furthermore, in situ SEM piezoresistive measurements on n-type, [111]-oriented silicon nanowires up to unprecedented levels of ∼7% strain are demonstrated. The piezoresistance coefficients are found to be similar to bulk values. For both metallic and semiconducting nanowires, variations of the contact resistance as strain is applied are observed. These variations must be considered in the interpretation of future two-point electromechanical measurements.

Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270 °C and carbonized in nitrogen at 800 °C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300 nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262 GPa and strength of 7.3 GPa measured for a 108 nm diameter fiber. In particular, through TEM evaluation of the structure of each individual nanofiber immediately prior to testing, we elucidate a dependence of mechanical properties on the molecular orientation of the graphitic structure: the strength and stiffness of the fibers increases with a higher degree of orientation of the 0 0 2 graphitic planes along the fiber axis, which coincides with decreasing fiber diameter.

Designing lightweight high-performance materials that can sustain high impulsive loadings is of great interest for marine applications. In this study, a finite element fluid–structure interaction model was developed to understand the deformation and failure mechanisms of both monolithic and sandwich composite panels. Fiber (E-glass fiber) and matrix (vinylester resin) damage and degradation in individual unidirectional composite laminas were modeled using Hashin failure model. The delamination between laminas was modeled by a strain-rate sensitive cohesive law. In sandwich panels, core compaction (H250 PVC foam) is modeled by a crushable foam plasticity model with volumetric hardening and strain-rate sensitivity. The model-predicted deformation histories, fiber/matrix damage patterns, and inter-lamina delamination, in both monolithic and sandwich composite panels, were compared with experimental observations. The simulations demonstrated that the delamination process is strongly rate dependent, and that Hashin model captures the spatial distribution and magnitude of damage to a first-order approximation. The model also revealed that the foam plays an important role in improving panel performance by mitigating the transmitted impulse to the back-side face sheet while maintaining overall bending stiffness.

In this study, we developed a finite element fluid–structure interaction model to understand the deformation and failure mechanisms of both monolithic and sandwich composite panels. A new failure criterion that includes strain-rate effects was formulated and implemented to simulate different damage modes in unidirectional glass fiber/matrix composites. The laminate model uses Hashin’s fiber failure criterion and a modified Tsai–Wu matrix failure criterion. The composite moduli are degraded using five damage variables, which are updated in the post-failure regime by means of a linear softening law governed by an energy release criterion. A key feature in the formulation is the distinction between fiber rupture and pull-out by introducing a modified fracture toughness, which varies from a fiber tensile toughness to a matrix tensile toughness as a function of the ratio of longitudinal normal stress to effective shear stress. The delamination between laminas is modeled by a strain-rate sensitive cohesive law. In the case of sandwich panels, core compaction is modeled by a crushable foam plasticity model with volumetric hardening and strain-rate sensitivity. These constitutive descriptions were used to predict deformation histories, fiber/matrix damage patterns, and inter-lamina delamination, for both monolithic and sandwich composite panels subjected to underwater blast. The numerical predictions were compared with experimental observations. We demonstrate that the new rate dependent composite damage model captures the spatial distribution and magnitude of damage significantly more accurately than previously developed models.

Polymer composite yarns containing a high loading of double-walled carbon nanotubes (DWNTs) have been developed in which the inherent acrylate-based organic coating on the surface of the DWNT bundles interacts strongly with poly(vinyl alcohol) (PVA) through an extensive hydrogen-bond network. This design takes advantage of a toughening mechanism seen in spider silk and collagen, which contain an abundance of hydrogen bonds that can break and reform, allowing for large deformation while maintaining structural stability. Similar to that observed in natural materials, unfolding of the polymeric matrix at large deformations increases ductility without sacrificing stiffness. As the PVA content in the composite increases, the stiffness and energy to failure of the composite also increases up to an optimal point, beyond which mechanical performance in tension decreases. Molecular dynamics (MD) simulations confirm this trend, showing the dominance of nonproductive hydrogen bonding between PVA molecules at high PVA contents, which lubricates the interface between DWNTs.