A biodegradable 3D woven magnesium-based scaffold for orthopedic implants

Porous Magnesium (Mg) is a promising biodegradable scaffold for treating critical-size bone defects, and as an essential element for human metabolism, Mg has shown sufficient biocompatibility. Its elastic moduli and yield strengths are closer to those of cortical bone than common, inert metallic implants, effectively reducing stress concentrations around host tissue as well as stress shielding. More importantly, Mg can degrade and be absorbed in the human body in a safe and controlled manner, thereby reducing the need for second surgeries to remove implants. The development of porous Mg scaffolds via conventional selective laser melting (SLM) techniques has been limited due to Mg’s low boiling point, high vapor pressures, high reactivity, and non-ideal microstructures in additively manufactured parts. Here we present an exciting alternative to conventional additive techniques: 3D weaving with Mg wires that have controlled chemistries and microstructures. The weaving process offers high throughput manufacturing as well as porous architectures that can be optimized for stiffness and porosity with topology optimization. Once woven, we dip-coat the weaves with polylactic acid (PLA) to enhance their strength and corrosion resistance. Following fabrication, we characterize their mechanical properties, corrosion behavior, and cell compatibility in vitro, and we use an intramuscular implantation model to evaluate their in vivo corrosion behavior and tissue response.

Kilohertz frame rate snapshot hyperspectral imaging of metal reactive materials

We demonstrate a kilohertz frame rate snapshot hyperspectral imaging system suitable for high-speed imaging, which we name snapshot hyperspectral imager for emission and reactions (SHEAR). This system splits the sensor of a single high-speed camera to simultaneously capture a conventional image and a spectrally sheared response of the scene under study. Given the small, point-source-like nature of burning metal micro-particles, the spectral response of the species is captured without the need for a slit, as is needed in conventional imaging spectrometers. We pair robust image registration techniques with sparse reconstruction algorithms to computationally disentangle overlapping spectra associated with many burning particles over the course of a combustion experiment. As a proof-of-concept experiment, representative physical vapor deposited Al:Zr composite particles are ignited, and their burn evolution is recorded at a frame rate of 2 kHz using this method. We demonstrate operation over two distinct wavelength ranges spanning hundreds of nanometers in wavelength and with sub-nanometer resolution. We are able to track hundreds of individual Al:Zr particles in a single high-speed video, providing ample statistics of burn time, temperature, and AlO emission timing in a high-throughput method. The demonstrated technology is high-throughput, flexible in wavelength, inexpensive, and relatively easy to implement, and provides a much needed tool for in situ composite metal fuel diagnostics.

Looking at education through the microscope

Advanced research microscopes in universities can be used to enhance the education of STEM students, as demonstrated by initiatives at Johns Hopkins University that give students the opportunity to get hands-on experience with sophisticated microscopes.

Gas Suppression via Copper Interlayers in Magnetron Sputtered Al-Cu2O Multilayers

The use of thin-foil, self-propagating thermite reactions to bond components successfully depends on the ability to suppress gas generation and avoid pore formation during the exothermic production of brazes. To study the mechanisms of vapor production in diluted thermites, thin film multilayer Al-Cu-Cu₂O-Cu foils are produced via magnetron sputtering, where the Cu layer thickness is systematically increased from 0 to 100 nm in 25 nm increments. The excess Cu layers act as diffusion barriers, limiting the transport of oxygen from the oxide to the Al fuel, as determined by slow heating differential scanning calorimetry experiments. Furthermore, by adding excess Cu to the system, the temperature of the self-propagating thermite reactions drops below the boiling point of Cu, eliminating the metal vapor production. It is determined that Cu vapor production can be eliminated by increasing the Cu interlayer thickness above 50 nm. However, the porous nature of the final products suggests that only metal vapor production is suppressed via dilution. Gas generation via oxygen release is still capable of producing a porous reaction product.

Nanotwinned metal MEMS films with unprecedented strength and stability

Silicon-based microelectromechanical systems (MEMS) sensors have become ubiquitous in consumer-based products, but realization of an interconnected network of MEMS devices that allows components to be remotely monitored and controlled, a concept often described as the "Internet of Things," will require a suite of MEMS materials and properties that are not currently available. We report on the synthesis of metallic nickel-molybdenum-tungsten films with direct current sputter deposition, which results in fully dense crystallographically textured films that are filled with nanotwins. These films exhibit linear elastic mechanical behavior and tensile strengths exceeding 3 GPa, which is unprecedented for materials that are compatible with wafer-level device fabrication processes. The ultrahigh strength is attributed to a combination of solid solution strengthening and the presence of dense nanotwins. These films also have excellent thermal and mechanical stability, high density, and electrical properties that are attractive for next-generation metal MEMS applications.

Irradiation-enhanced reactivity of multilayer Al/Ni nanomaterials

We have investigated the effect of accelerated ion beam irradiation on the structure and reactivity of multilayer sputter deposited Al/Ni nanomaterials. Carbon and aluminum ion beams with different charge states and intensities were used to irradiate the multilayer materials. The conditions for the irradiation-assisted self-ignition of the reactive materials and corresponding ignition thresholds for the beam intensities were determined. We discovered that relatively short (40 min or less) ion irradiations enhance the reactivity of the Al/Ni nanomaterials, that is, significantly decrease the thermal ignition temperatures (Tig) and ignition delay times (τig). We also show that irradiation leads to atomic mixing at the Al/Ni interfaces with the formation of an amorphous interlayer, in addition to the nucleation of small (2-3 nm) Al3Ni crystals within the amorphous regions. The amorphous interlayer is thought to enhance the reactivity of the multilayer energetic nanomaterial by increasing the heat of the reaction and by speeding the intermixing of the Ni and the Al. The small Al3Ni crystals may also enhance reactivity by facilitating the growth of this Al-Ni intermetallic phase. In contrast, longer irradiations decrease reactivity with higher ignition temperatures and longer ignition delay times. Such changes are also associated with growth of the Al3Ni intermetallic and decreases in the heat of reaction. Drawing on this data set, we suggest that ion irradiation can be used to fine-tune the structure and reactivity of energetic nanomaterials.

Combining nanocalorimetry and dynamic transmission electron microscopy for in situ characterization of materials processes under rapid heating and cooling

Nanocalorimetry is a chip-based thermal analysis technique capable of analyzing endothermic and exothermic reactions at very high heating and cooling rates. Here, we couple a nanocalorimeter with an extremely fast in situ microstructural characterization tool to identify the physical origin of rapid enthalpic signals. More specifically, we describe the development of a system to enable in situ nanocalorimetry experiments in the dynamic transmission electron microscope (DTEM), a time-resolved TEM capable of generating images and electron diffraction patterns with exposure times of 30 ns-500 ns. The full experimental system consists of a modified nanocalorimeter sensor, a custom-built in situ nanocalorimetry holder, a data acquisition system, and the DTEM itself, and is capable of thermodynamic and microstructural characterization of reactions over a range of heating rates (10(2) K/s-10(5) K/s) accessible by conventional (DC) nanocalorimetry. To establish its ability to capture synchronized calorimetric and microstructural data during rapid transformations, this work describes measurements on the melting of an aluminum thin film. We were able to identify the phase transformation in both the nanocalorimetry traces and in electron diffraction patterns taken by the DTEM. Potential applications for the newly developed system are described and future system improvements are discussed.

Fast X-ray microdiffraction techniques for studying irreversible transformations in materials

A pair of techniques have been developed for performing time-resolved X-ray microdiffraction on irreversible phase transformations. In one technique capillary optics are used to focus a high-flux broad-spectrum X-ray beam to a 60 µm spot size and a fast pixel array detector is used to achieve temporal resolution of 55 µs. In the second technique the X-rays are focused with Kirkpatrick-Baez mirrors to achieve a spatial resolution better than 10 µm and a fast shutter is used to provide temporal resolution better than 20 µs while recording the diffraction pattern on a (relatively slow) X-ray CCD camera. Example data from experiments are presented where these techniques are used to study self-propagating high-temperature synthesis reactions in metal laminate foils.

Monitoring Time-Dependent Deformation in Small Volumes

Time-dependent deformation (TDD) in small volumes was monitored using an alternating (AC) force technique and a Nanoindenter. The AC technique provides a continuous measure of contact stiffness during an indentation. By holding the force constant and monitoring the stiffness, the applied pressure and an effective strain rate were measured. Reasonable strain rate sensitivities were obtained for permanent indentations involving significant plastic strains. Time-dependent recovery due to residual strain energy is also reported. For reversible, elastic indentations, TDD was seen when the applied force was held constant. The deformation is attributed to atomic diffusion.

Acoustic Emissions During Indentation Tests

When a hard tip indents a sample, cracking, delamination and plasticity can all produce acoustic signals in the form of elastic waves. This paper describes how these acoustic emissions (AE) can be monitored using piezoelectric elements and a load-controlled indentation instrument. Signals emitted from thin films and bulk materials are shown and they are correlated with distinct jumps in displacement of the indenter tip. The benefits and difficulties of different methods of monitoring the emissions are also considered.

Variations in the mechanical properties of Alouatta palliata molar enamel

Teeth have provided insights into many topics including primate diet, paleobiology, and evolution, due to the fact that they are largely composed of inorganic materials and may remain intact long after an animal is deceased. Previous studies have reported that the mechanical properties, chemistry, and microstructure of human enamel vary with location. This study uses nanoindentation to map out the mechanical properties of Alouatta palliata molar enamel on an axial cross-section of an unworn permanent third molar, a worn permanent first molar, and a worn deciduous first molar. Variations were then correlated with changes in microstructure and chemistry using scanning electron microscopy and electron microprobe techniques. The hardness and Young's modulus varied with location throughout the cross-sections from the occlusal surface to the dentin-enamel junction (DEJ), from the buccal to lingual sides, and also from one tooth to another. These changes in mechanical properties correlated with changes in the organic content of the tooth, which was shown to increase from approximately 6% near the occlusal surface to approximately 20% just before the DEJ. Compared to human enamel, the Alouatta enamel showed similar microstructures, chemical constituents, and magnitudes of mechanical properties, but showed less variation in hardness and Young's modulus, despite the very different diet of this species.

The effect of prism orientation on the indentation testing of human molar enamel

Recent nanoindentation studies have demonstrated that the hardness and Young's modulus of human molar enamel decreases by more than 50% on moving from the occlusal surface to the dentine-enamel junction on cross-sectional samples. Possible sources of these variations are changes in local chemistry, microstructure, and prism orientation. This study investigates the latter source by performing nanoindentation tests at two different orientations relative to the hydroxyapatite prisms: parallel and perpendicular. A single sample volume was tested in order to maintain a constant chemistry and microstructure. The resulting data show very small differences between the two orientations for both hardness and Young's modulus. The 1.5-3.0% difference is significantly less than the standard deviations found within the data set. Thus, the variations in hardness and Young's modulus on cross-sectional samples of human molar are attributed to changes in local chemistry (varying levels of mineralization, organic matter, and water content) and changes in microstructure (varying volume fractions of inorganic crystals and organic matrix). The impact of prism orientation on mechanical properties measured at this scale by nanoindentation appears to be minimal.

An in vitro uniaxial stretch model for axonal injury

We have developed a unique uniaxial stretching device to study axonal injury and neural cell death resulting from brain tissue deformations common in traumatic head injuries. Using displacement control rather than force control, this device is capable of achieving strains >70% and strain rates up to 90 s(-1), well above those currently used for studying axonal injury. We have demonstrated that the deformation of the specimen was uniaxial, uniform and highly reproducible; the prespecified displacement profiles could be realized almost precisely; and adequate cell adhesion could be achieved readily. The entire device can fit into a biological safety cabinet to maintain sterility, and the specimens are convenient for cell culture. This device can be used to investigate a wide range of biomechanical issues involved in diffuse axonal injury.

Nanoindentation mapping of the mechanical properties of human molar tooth enamel

The mechanical behavior of dental enamel has been the subject of many investigations. Initial studies assumed that it was a more or less homogeneous material with uniform mechanical properties. Now it is generally recognized that the mechanical response of enamel depends upon location, chemical composition, and prism orientation. This study used nanoindentation to map out the properties of dental enamel over the axial cross-section of a maxillary second molar (M(2)). Local variations in mechanical characteristics were correlated with changes in chemical content and microstructure across the entire depth and span of a sample. Microprobe techniques were used to examine changes in chemical composition and scanning electron microscopy was used to examine the microstructure. The range of hardness (H) and Young's modulus (E) observed over an individual tooth was found to be far greater than previously reported. At the enamel surface H>6GPa and E>115GPa, while at the enamel-dentine junction H<3GPa and E<70GPa. These variations corresponded to the changes in chemistry, microstructure, and prism alignment but showed the strongest correlations with changes in the average chemistry of enamel. For example, the concentrations of the constituents of hydroxyapatite (P(2)O(5) and CaO) were highest at the hard occlusal surface and decreased on moving toward the softer enamel-dentine junction. Na(2)O and MgO showed the opposite trend. The mechanical properties of the enamel were also found to differ from the lingual to the buccal side of the molar. At the occlusal surface the enamel was harder and stiffer on the lingual side than on the buccal side. The interior enamel, however, was softer and more compliant on the lingual than on the buccal side, a variation that also correlated with differences in average chemistry and might be related to differences in function.

Preparation of multilayered materials in cross-section for in situ TEM tensile deformation studies

The success of in situ transmission electron microscopy experimentation is often dictated by proper specimen preparation and sample design procedures. We have developed a novel technique permitting the production of tensile specimens of multilayered films in cross-section for in situ deformation studies. Of primary, importance in the development of this technique is the production of an electron transparent micro-gauge section. This micro-gauge section predetermines the position at which plastic deformation, crack nucleation and growth, and failure are observed. In short, we report in detail, a unique combination of specimen preparation procedural steps and the design of a multilayer foil sample. The ability of these procedures to facilitate the success of in situ TEM tensile studies of layered materials in cross-section is demonstrated using a Cu-Zr multilayer foil.

Atomic force microscope measurements of the hardness and elasticity of peritubular and intertubular human dentin

An atomic force microscope was used to measure the hardness and elasticity of fully-hydrated peritubular and intertubular human dentin. The standard silicon nitride AFM tip and silicon cantilever assembly were replaced with a diamond tip and stainless steel cantilever having significantly higher stiffness. Hardness was measured as the ratio of the applied force to the projected indentation area for indentations with depths from 10-20 nm. The sample stiffness was measured by imaging specimens in a force-modulated mode. Hardness values of 2.3 +/- 0.3 GPa and 0.5 +/- 0.1 GPa were measured for the peritubular and intertubular dentin, respectively. Stiffness imaging revealed that the elastic modulus of the peritubular dentin was spatially homogeneous; whereas, there was considerable spatial variation in the elasticity of the intertubular dentin. The atomic force microscope can be used to measure the mechanical properties of fully hydrated calcified tissues at the submicron level of spatial resolution, thus augmenting more traditional depth sensing probes.

Hardness and Young's modulus of human peritubular and intertubular dentine

A specially modified atomic-force microscope was used to measure the hardness of fully hydrated peritubular and intertubular dentine at two locations within unerupted human third molars: within 1 mm of the dentine enamel junction and within 1 mm of the pulp. The hardness of fully hydrated peritubular dentine was independent of location, and ranged from 2.23 to 2.54 GPa. The hardness of fully hydrated intertubular dentine did depend upon location, and was significantly greater near the dentine enamel junction (values ranged from 0.49 to 0.52 GPa) than near the pulp (0.12-0.18 GPa). A Nanoindenter was used to estimate the Young's modulus of dehydrated peritubular and intertubular dentine from the unloading portion of the load displacement curve. The modulus values averaged 29.8 GPa for the peritubular dentine (considered to be a lower limit), and ranged from 17.7 to 21.1 GPa for the intertubular dentine, with the lower values obtained for dentine near the pulp.