Mechanical Properties of Conducting Printed Nanosheet Network Thin Films Under Uniaxial Compression.

Thin film networks of solution processed nanosheets show remarkable promise for use in a broad range of applications including strain sensors, energy storage, printed devices, textile electronics, and more. While it is known that their electronic properties rely heavily on their morphology, little is known of their mechanical nature, a glaring omission given the effect mechanical deformation has on the morphology of porous systems and the promise of mechanical post processing for tailored properties. Here, this work employs a recent advance in thin film mechanical testing called the Layer Compression Test to perform the first in situ analysis of printed nanosheet network compression. Due to the well-defined deformation geometry of this unique test, this work is able to explore the out-of-plane elastic, plastic, and creep deformation in these systems, extracting properties of elastic modulus, plastic yield, viscoelasticity, tensile failure and sheet bending vs. slippage under both out of plane uniaxial compression and tension. This work characterizes these for a range of networks of differing porosities and sheet sizes, for low and high compression, as well as the effect of chemical cross linking. This work explores graphene and MoS2 networks, from which the results can be extended to printed nanosheet networks as a whole.

High-resolution micro-cavity filling sensing by fiber optic interferometry.

In the last decade, new potential applications of micro- and nano-products in telecommunication, medical diagnostics, photovoltaic, and optoelectronic systems have increased the interest to develop micro-engineering technologies. Injection molding of polymeric materials is a recent method being adapted for serial manufacturing of optic components and packaging at the micro- and nano-scale. Quality assurance of replication into small cavities is an important but underdeveloped factor that is needed to ensure high production efficiency in any micro-fabrication industry. In this work, we introduce a fiber-based interferometric measurement sensor to monitor the cavity filling of optical microstructures fabricated into a macroscopic molding die. The interferometer was capable of resolving melt front motion into the microcavity to the point of complete filling as verified by atomic force microscopy. Despite the low reflectivity of the transparent polymer and unoptimized reflected light collection optics, this system is capable of monitoring polymer movement during the course of filling and detecting the completion of the process. The simplicity and flexibility of the technology could allow eventual instrumentation of injection molds, embossing, and nanoimprint tooling suitably modified with a small optical window to accommodate light from an optical fiber. This would provide a solution to the challenging problem of monitoring local, nanometer scale filling processes.

Effect of cross-linking and hydration on microscale flat punch indentation contact to collagen-hyaluronic acid films in the viscoelastic limit.

The properties of the extracellular matrix (ECM) have profound impact upon cell behaviour. As an abundant protein in mammals, collagen is a desirable base material to engineer an ECM tissue scaffold, but its structural weakness generally requires molecular crosslinking or incorporation of additional ECM-based macromolecules such as glycosaminoglycans. We have performed microscopic indentation to test collagen films under dry and aqueous conditions prepared with different levels of physical and chemical crosslinking. Our technique isolates intrinsic properties of the poro-viscoelastic matrix in a regime minimizing the influence of drainage hydrodynamics and allows direct measurement of the effect of hydrating a specific sample. A doubling of the effective stress-strain stiffness under crosslinking could be directly correlated to structural changes in X-ray diffraction spectra, while electron microscopy revealed possible fibril bridging mechanisms explaining observed toughness. Overall, an intrinsic viscoelastic stress-strain response of collagen under various conditions of cross-linking was observed for both dry and wet conditions, with the latter most affected by indentation rate. Under creep testing, a three order of magnitude increase in dynamic compliance and factor three reduction in relaxation time was found going from the dry to hydrated state. When fitted to a simple viscoelastic model, crosslinking showed a tendency to decrease relaxation time in both states, but reduced dynamic compliance only in the hydrated case. This suggests a reduced role of virtual crosslinks under hydration. This is the first study reporting consistent mechanical testing of dry and hydrated ECM-derived biomaterials, accessing the intrinsic material mechanics under in vivo-like conditions. STATEMENT OF SIGNIFICANCE: This manuscript presents new insights into the effect of crosslinking on mechanical properties of dry and hydrated collagen intended for tissue scaffolding applications. A novel microscopic indentation technique allowed testing of the poro-viscoelastic matrix isolated in a regime minimizing the influence of drainage hydrodynamics, so direct comparison of the effect of hydration on the intrinsic material behaviour to could be made. A variety of experimental techniques including X-ray diffraction, infrared spectroscopy, and scanning electron and atomic force microscopy were used to augment the mechanical testing. The results of creep testing were numerically analysed using a four-component viscoelastic model. This is the first mechanical testing of dry and hydrated ECM-derived biomaterials, accessing the intrinsic material mechanics under in vivo-like conditions.

Self-assembly of graphene ribbons by spontaneous self-tearing and peeling from a substrate.

Graphene and related two-dimensional materials have shown unusual and exceptional mechanical properties, with similarities to origami-like paper folding and kirigami-like cutting demonstrated. For paper analogues, a critical difference between macroscopic sheets and a two-dimensional solid is the molecular scale of the thin dimension of the latter, allowing the thermal activation of considerable out-of-plane motion. So far thermal activity has been shown to produce local wrinkles in a free graphene sheet that help in theoretically understanding its stability, for example, and give rise to unexpected long-range bending stiffness. Here we show that thermal activation can have a more marked effect on the behaviour of two-dimensional solids, leading to spontaneous and self-driven sliding, tearing and peeling from a substrate on scales approaching the macroscopic. We demonstrate that scalable nanoimprint-style contact techniques can nucleate and direct the parallel self-assembly of graphene ribbons of controlled shape in ambient conditions. We interpret our observations through a simple fracture-mechanics model that shows how thermodynamic forces drive the formation of the graphene-graphene interface in lieu of substrate contact with sufficient strength to peel and tear multilayer graphene sheets. Our results show how weak physical surface forces can be harnessed and focused by simple folded configurations of graphene to tear the strongest covalent bond. This effect may hold promise for the patterning and mechanical actuating of devices based on two-dimensional materials.

Characteristics, interactions and coating adherence of heterogeneous polymer/drug coatings for biomedical devices.

With this rise in surgical procedures it is important to focus on the mobility and safety of the patient and reduce the infections that are associated with hip replacements. We examine the mechanical properties of gentamicin sulphate as a model antimicrobial layer for titanium-alloy based prosthetic hips to help prevent methicillin-resistant Staphylococcus aureus infection after surgery. A top layer of poly(lactic-co-glycolic acid) is added to maintain the properties of the gentamicin sulphate as well as providing a drug delivery system. Through the use of nanoindentation and micro-scratch techniques it is possible to determine the mechanical and adhesive properties of this system. Nanoindentation determined the modulus values for the poly(lactic-co-glycolic acid) and gentamicin sulphate materials to be 8.9 and 5.2GPa, respectively. Micro-scratch established that the gentamicin sulphate layer is strongly adhered to the Ti alloy and forces of 30N show no cohesive or adhesive failure. It was determined that the poly(lactic-co-glycolic acid) is ductile in nature and delaminates from the gentamicin sulphate layer of at 0.5N.

Controlling droplet-based deposition uniformity of long silver nanowires by micrometer scale substrate patterning.

We report control of droplet-deposit uniformity of long silver nanowires suspended in solutions by microscopic influence of the liquid contact line. Substrates with microfabricated line patterns with a pitch far smaller than mean wire length lead to deposit thickness uniformity compared to unpatterned substrates. For high boiling-point solvents, two significant effects were observed: The substrate patterns suppressed coffee ring staining, and the wire deposits exhibited a common orientation lying perpendicular over top the lines. The latter result is completely distinct from previously reported substrate groove channeling effects. This work shows that microscopic influence of the droplet contact line geometry including the contact angle by altered substrate wetting allows significant and advantageous influence of deposition patterns of wire-like solutes as the drop dries.

Interfacial characteristics and determination of cohesive and adhesive strength of plasma-coated hydroxyapatite via nanoindentation and microscratch techniques.

We investigate the chemical composition and mechanical properties of plasma-deposited hydroxyapatite on grit-blasted Ti-6Al-4V coupons as models of typical prosthetic hip implants. Nanoindentation is used to extract the mechanical properties of the hydroxyapatite (HA) coating and to evaluate the behavior of the material as a function of distance from the interface. A microscratch technique was used to determine parameters of cohesive and adhesive failure of the material that are critical in determining the functionality of these biomedical devices. This delamination method has not been studied in detail before and is usually considered to be unsuitable because of the thickness of the HA and the roughness of the substrate. However, through cross-section analysis of the scratch test, we can determine the point at which the HA delaminates from the substrate. It was concluded that spallation occurs locally, and there is no evidence of gross spallation, indicating that the coating is well adhered to the substrate.

Mechanical properties of self-assembled chitin nanofiber networks.

Chitin nanofibers are structural components of the insect cuticle, the exoskeleton of crabs, and mollusk shells. Chitin nanofibers have found broad use in biomedical applications. Here, we study structure-properties-processing relationships of 3 nm chitin nanofiber networks self-assembled from a chitin hexafluoroisopropanol solution.

Mechanical constraint and release generates long, ordered horizontal pores in anodic alumina templates.

We describe the formation of long, highly ordered arrays of planar oriented anodic aluminum oxide (AAO) pores during plane parallel anodization of thin aluminum 'finger' microstructures fabricated on thermally oxidized silicon substrates and capped with a silicon oxide layer. The pore morphology was found to be strongly influenced by mechanical constraint imposed by the oxide layers surrounding the Al fingers. Tractions induced by the SiO(2) substrate and capping layer led to frustrated volume expansion and restricted oxide flow along the interface, with extrusion of oxide into the primary pore volume, leading to the formation of dendritic pore structures and meandering pore growth. However, partial relief of the constraint by a delaminating interfacial fracture, with its tip closely following the anodization front, led to pore growth that was highly ordered with regular, hexagonally packed arrays of straight horizontal pores up to 3 µm long. Detailed characterization of both straight and dendritic planar pores over a range of formation conditions using advanced microscopy techniques is reported, including volume reconstruction, enabling high quality 3D visualization of pore formation.

Large magnetoresistance in few layer graphene stacks with current perpendicular to plane geometry.

A large magnetoresistance (MR) effect of few-layers graphene between two non-magnetic metal electrodes with current perpendicular to graphene plane is studied. A non-saturation and anisotropic MR with the value over 60% at 14 T is observed in a two-layer graphene stack at room temperature. The resistance of the device is only tens of ohms, having the advantage of low power consumption for magnetic device applications.

Memory and threshold resistance switching in Ni/NiO core-shell nanowires.

We report on the first controlled alternation between memory and threshold resistance switching (RS) in single Ni/NiO core-shell nanowires by setting the compliance current (I(CC)) at room temperature. The memory RS is triggered by a high I(CC), while the threshold RS appears by setting a low I(CC), and the Reset process is achieved without setting a I(CC). In combination with first-principles calculations, the physical mechanisms for the memory and threshold RS are fully discussed and attributed to the formation of an oxygen vacancy (Vo) chain conductive filament and the electrical field induced breakdown without forming a conductive filament, respectively. Migration of oxygen vacancies can be activated by appropriate Joule heating, and it is energetically favorable to form conductive chains rather than random distributions due to the Vo-Vo interaction, which results in the nonvolatile switching from the off- to the on-state. For the Reset process, large Joule heating reorders the oxygen vacancies by breaking the Vo-Vo interactions and thus rupturing the conductive filaments, which are responsible for the switching from on- to off-states. This deeper understanding of the driving mechanisms responsible for the threshold and memory RS provides guidelines for the scaling, reliability, and reproducibility of NiO-based nonvolatile memory devices.

Mediators of inflammation and regeneration.

Characterization of the molecular response under caries lesions requires a robust and reliable transcript isolation system, and analysis of data indicated that collection of extracted teeth in either liquid nitrogen/RNA-stabilizing solution facilitated this. Subsequent transcriptional analysis indicated higher general activity in carious pulps, while characterization of inflammatory mediators, including cytokines and S100 proteins, highlighted increasing expression levels associated with both microbial front progression and elevated cellular immune response. Analysis of the pleiotropic hormone adrenomedullin (ADM) indicated that transcript and protein levels are increased in pulpal tissue during caries, and that protein levels sequestered in dentin due to primary dentinogenesis are comparable with those of TGF-ß1. Expression analysis of a leucine-rich-repeat-containing protein (LRRC15/Lib) indicated that this highly conserved molecule was up-regulated during caries, is transcriptionally regulated by pro-inflammatory stimuli, and is relatively abundant in mineralized tissues.

Molecular confinement accelerates deformation of entangled polymers during squeeze flow.

The squeezing of polymers in narrow gaps is important for the dynamics of nanostructure fabrication by nanoimprint embossing and the operation of polymer boundary lubricants. We measured stress versus strain behavior while squeezing entangled polystyrene films to large strains. In confined conditions where films were prepared to a thickness less than the size of the bulk macromolecule, resistance to deformation was markedly reduced for both solid-glass forging and liquid-melt molding. For melt flow, we further observed a complete inversion of conventional polymer viscosity scaling with molecular weight. Our results show that squeeze flow is accelerated at small scales by an unexpected influence of film thickness in polymer materials.

Variable temperature thin film indentation with a flat punch.

We present modifications to conventional nanoindentation that realize variable temperature, flat punch indentation of ultrathin films. The technique provides generation of large strain, thin film extrusion of precise geometries that idealize the essential flows of nanoimprint lithography, and approximate constant area squeeze flow rheometry performed on thin, macroscopic soft matter samples. Punch radii as small as 185 nm have been realized in ten-to-one confinement ratio testing of 36 nm thick polymer films controllably squeezed in the melt state to a gap width of a few nanometers. Self-consistent, compressive stress versus strain measurements of a wide variety of mechanical testing conditions are provided by using a single die-sample system with temperatures ranging from 20 to 125 degrees C and loading rates spanning two decades. Low roughness, well aligned flat punch dies with large contact areas provide precise detection of soft surfaces with standard nanoindenter stiffness sensitivity. Independent heating and thermometry with heaters and thermocouples attached to the die and sample allow introduction of a novel directional heat flux measurement method to ensure isothermal contact conditions. This is a crucial requirement for interpreting the mechanical response in temperature sensitive soft matter systems. Instrumented imprint is a new nanomechanics material testing platform that enables measurements of polymer and soft matter properties during large strains in confined, thin film geometries and extends materials testing capabilities of nanoindentation into low modulus, low strength glassy, and viscoelastic materials.

Measuring glassy and viscoelastic polymer flow in molecular-scale gaps using a flat punch mechanical probe.

This paper investigates molecular-scale polymer mechanical deformation during large-strain squeeze flow of polystyrene (PS) films, where the squeeze flow gap is close to the polymer radius of gyration (R(g)). Stress-strain and creep relations were measured during flat punch indentation from an initial film thickness of 170 nm to a residual film thickness of 10 nm in the PS films, varying molecular weight (M(w)) and deformation stress rate by over 2 orders of magnitude while temperatures ranged from 20 to 125 degrees C. In stress-strain curves exhibiting an elastic-to-plastic yield-like knee, the response was independent of M(w), as expected from bulk theory for glassy polymers. At high temperatures and long times sufficient to extinguish the yield-knee, the mechanical response M(w) degeneracy was broken, but no molecular confinement effects were observed during thinning. Creep measurements in films of 44K M(w) were well-approximated by bulk Newtonian no-slip flow predictions. For extrusions down to a film thickness of 10 nm, the mechanical relaxation in these polymer films scaled with temperature similar to Williams-Landel-Ferry scaling in bulk polymer. Films of 9000K M(w), extruded from an initial film thickness of 2R(g) to a residual film thickness of 0.5R(g), while showing stress-strain viscoelastic response similar to that of films of 900K M(w), suggestive of shear-thinning behavior, could not be matched to a constitutive flow model. In general, loading rate and magnitude influenced subsequent creep extrusion depth of high-M(w) films, with deeper final extrusions for high loading rates than for low loading rates. The measurements suggest that, for high-resolution nanoimprint lithography, mold flash or final residual film thickness can be reduced for high strain and strain rate loading of high-M(w) thin films.

Room temperature mechanical thinning and imprinting of solid films.

The mechanical patterning of thin films has received recent attention due to significant potential for efficient nanostructure fabrication. For solid films, mechanically thinning wide areas remains particularly challenging. In this work, we introduce a new plastic ratchet mechanism involving small amplitude (<10 nm), oscillatory shear motion of the forging die. This isothermal mechanism significantly extends mass transport across surfaces, broadening the scope of nanoscale processing for a potentially wide class of solid ductile materials.

Plasticity, healing and shakedown in sharp-asperity nanoindentation.

Spatially localized stress fields produced by instrumented, sharp indentation probes are a route to testing the mechanical properties of materials at the smallest length scales. Here we provide direct experimental measurement of indentation plasticity with contact strain fields involving up to a few thousand atoms. We observe two types of nanoscale plasticity: on the pristine surface, high-resolution sensing shows an overall smooth, remarkably reversible indentation response interjected by sudden discrete drops in indenter load. The jumps often occur in pairs with pop-in motion during loading healed by a corresponding pop-out motion on the unload stroke to define a compact hysteresis loop. Despite the general reversibility, cyclic indentation at a single sample position leads to a subtle plastic ratchet and shakedown behaviour with displacements correlated to the underlying gold lattice constant. Our results concur with a previously established picture of thermally activated atomistic plasticity, but suggest a new mechanism at reduced scales that suppresses permanent mass transport.