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By measuring the geometry and crystallography of the three interfaces that meet at grain boundary thermal grooves, it is possible to determine the anisotropy of the surface free energy. Previously, the surface energy of MgO at 1400°C in air was approximated by a truncated double Fourier series with coefficients that were determined by fitting the observations to Herring's condition for local equilibrium at a triple junction. The purpose of this paper is to describe an alternative analysis of the same data set that is not limited by an assumed functional form of the surface energy. In this case, the space of surface characters is discretized and each orientation is associated with a capillarity vector (according to the Cahn–Hoffmann definition). The set of capillarity vectors that most closely satisfies the condition for local equilibrium at each triple junction is then determined by an iterative method. The relative surface free energies derived from this analysis are more anisotropic than those derived from the series fit and more consistent with the observed faceting of MgO in air at 1400°C. The relative surface energies of the low index planes are γ110/γ100 = 1.07±0.04 and γ111/γ100 = 1.17±0.04.

Geometric and crystallographic data obtained from a well annealed magnesia polycrystal have been used to specify the five macroscopic degrees of freedom for 4665 grain boundaries. The results indicate, that for this sample, the five parameter grain boundary character space is fully occupied. A finite series of symmetrized spherical harmonics has been used to approximate the misorientation dependence of the relative grain boundary energy. Best fit coefficients for this series were determined by assuming that the interfacial tensions at each triple junction are balanced. The grain boundary energy function shows Read-Shockley behavior at small misorientations and a broad minimum near the Σ3 misorientation. Furthermore, misorientations about the ‹100› axis create boundaries with relative energies that are less than those created by misorientations about the ‹110› or ‹111› axes.

Measurement of the geometry of triple junctions between grain boundaries in polycrystalline materials generates large sets of dihedral angles from which maps of the grain boundary energy may be extracted. A preliminary analysis has been performed for a sample of magnesia based on a three-parameter description of grain boundaries. An extended form of orientation imaging microscopy (OIM) was used to measure both triple junction geometry via image analysis in the SEM and local grain orientation via electron back scatter diffraction. Serial sectioning with registry of both in-plane images and successive sections characterizes triple junction tangents from which true dihedral angles are calculated. We apply Herring's relation at each triple junction, based on the assumption of local equilibrium at the junction. By limiting grain boundary character to a (three parameter) specification of misorientation for the preliminary analysis, we can neglect the torque terms and apply the sine law to the three boundaries. This provides two independent relations per triple junction between grain boundary energies and dihedral angles. Discretizing the misorientation and employing multiscale statistical analysis on large data sets allows (relative) grain boundary energy as a function of boundary character to be extracted from triple junction geometry. A similar analysis of thermal grooves allows the anisotropy of the surface energy to be measured in MgO.

The purpose of the present work is to model corrosion of implant materials (such as nitinol or stainless steel) in simulated environments both under static and dynamic conditions. The present model seeks to imitate the potentiodynamic testing protocol defined as in ASTM F2129, which is used to evaluate the corrosion susceptibility of small implant devices. The model is currently limited to one dimension and hence the effect of physical features, such as defects and crevices, on corrosion is not the focus of this study.

The multiplicity of distinct grain boundary configurations in polycrystals has made it difficult to determine the relative frequency with which each configuration is adopted. As a result, the physiochemical properties of each boundary and the influence of the distribution of boundaries on macroscopic materials properties are not well understood. Using a semiautomated system, we have measured all five macroscopically observable degrees of freedom of 4.1 × 106 boundary plane segments making up 5.2 × 106μm2 of grain boundary interface area in a magnesia polycrystal. Our observations demonstrate that not all grain boundary configurations occur with the same frequency and that the relative free energies of the different interfacial configurations influence the population distribution. Furthermore, the results indicate that relative grain boundary energies can be estimated based on the free surface energies.

We show here that the boundaries of individual grains in dense polycrystals prefer certain crystallographic habit planes, almost as if they were independent of the neighboring crystals. In MgO, SrTiO3, MgAl2O4, TiO2, and aluminum, the specific habit planes within the polycrystal correspond to the same planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations decrease the apparent complexity of interfacial networks and suggest that the mechanisms of solid-state grain growth may be analogous to conventional crystal growth. The results also indicate that a model for grain-boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.

Drug-polymer composite coatings, composed of styrene-isobutylene-styrene (SIBS) tri-block copolymers, are frequently used in controlled drug release biomedical device applications. In this work, we used atomic force microscopy to characterize the effects of different drug loadings and polymer chemistries (i.e., block copolymer ratio) on the variation of surface structures and compositions of SIBS-tetracycline (SIBS-TC) cast composites including tetracycline (TC) drug amount, drug phase size distribution, and drug and polymer phase morphologies. We tested the structural variations by fabricating and characterizing two types of composite specimens, that is, SIBS15 and SIBS30, composed of 15 and 30 Wt % of polystyrene (PS), respectively. The differences in the distribution of TC drug, PS, and polyisobutylene (PIB) polymer phase structures observed in SIBS15 and SIBS30 resulted in more drug at the surface of SIBS30 compared to SIBS15. To support the experimental findings, we have determined the Hildebrand solubility parameter of TC using molecular dynamics (MD) computation and compared it to the polymer components, PS and PIB. The MD results show that the solubility parameter of TC is much closer to that of PS than PIB, which demonstrates a higher thermodynamic stability of TC-PS mixtures.

Drug-eluting stent (DES) implantation is a common treatment for atherosclerosis. The safety and efficacy of these devices will depend on the uptake and distribution of drug into the vessel wall. It is established that the composition of atherosclerotic vessels can vary dramatically with patients' age and gender. However, studies focused on elucidating and quantifying the impact of these variations on important drug transport properties, such as diffusion (D) and partition (k) coefficients, are limited. We have developed an improved tissue mimic or artificial plaque to probe the effect of varying concentrations of plaque constituents on drug transport in vitro. Based on these artificial plaques, we have quantified the impact of gelatin (hydrolyzed collagen) and lipid (cholesterol) concentration on D and k using two model drugs, tetracycline and fluvastatin. We found that for tetracycline, increasing the collagen concentration from 0.025 to 0.100 (w/w) resulted in a fivefold decrease in diffusivity, whereas there was no discernible impact on solubility. Increasing the lipid concentration up to 0.034 (w/w) resulted in only minor changes to transport properties of tetracycline. However, fluvastatin exhibited nearly a fivefold increase in k and 10-fold decrease in D with increased lipid concentration. These results were in reasonable agreement with existing models and exhibited behavior consistent with previous observations on drugs commonly used in DES applications. These observations suggest that variations in the chemical characteristics of atherosclerotic plaque can significantly alter the release rate and distribution of drug following DES implantation.

A critical metrology issue for pharmaceutical industries is the application of analytical techniques for the characterization of drug delivery systems to address interrelationships between processing, structure, and drug release. In this study, cast coatings were formed from solutions of poly(styrene- b -isobutylene- b -styrene) (SIBS) and tetracycline in tetrahydrofuran (THF). These coatings were characterized by several imaging modalities, including time- of-flight secondary ion mass spectrometry (TOF-SIMS) for chemical imaging and analysis, atomic force microscopy (AFM) for determination of surface structure and morphology, and laser scanning confocal microscopy (LSCM), which was used to characterize the three-dimensional structure beneath the surface. The results showed phase separation between the drug and copolymer regions. The size of the tetracycline phase in the polymer matrix ranged from hundreds of nanometers to tens of microns, depending on coating composition. The mass of drug released was not found to be proportional to drug loading, because the size and spatial distribution of the drug phase varied with drug loading and solvent evaporation rate, which in turn affected the amount of drug released.

Drug delivery from biodegradable polymer coatings is becoming an important area of research for applications including medical devices. In this work, we probe the impact of polymer chemistry and solvent evaporation rate on drug morphology and the subsequent elution from biodegradable polymer systems. Two different formulations of poly(lactic-glycolic-acid) (PLGA) polymers are used in combination with tetracycline (TC) to fabricate coatings using two different solvent evaporation rates. We have conducted elution studies to quantify the release behavior of tetracycline (TC) at the first 2-3 days of dissolution. Significantly we have correlated the drug release kinetics with the drug microstructure of TC/PLGA coating using atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). These results suggest that polymer chemistry affects the rate of water absorption and drug dissolution which in turn alters the rate of TC release during the early stages of drug elution.

To improve functionality and performance, controlled drug-release coatings comprised of drug and polymer are integrated with traditional medical devices, e.g., drug eluting stents. Depending on manufacturing conditions, these coatings can exhibit complex microstructures. Previously, a thermodynamically consistent model was developed for microstructure evolution in these systems to establish relationships between process variables, microstructure, and the subsequent release kinetics. Calculations based on the model were, in general, consistent with experimental findings. However, because of assumptions regarding the evaporation of solvent during fabrication, the model was unable to capture variations through the coating thickness that are observed experimentally. Here, a straightforward method is introduced to incorporate solvent evaporation explicitly into the model. Calculations are used to probe the impact of solvent evaporation rate and drug loading on the microstructure that forms during manufacturing and subsequent drug release kinetics. The predicted structures and release kinetics are found to be consistent with experimental observations. Further, the calculations demonstrate that solvent evaporation rate can be as critical to device performance as the amount of drug within the coating. For example, changes of a factor of five in the amount of drug released were observed by modifying the rate of solvent evaporation during manufacturing.

In recent years, controlled release coatings, comprised of drug-polymer composites, have been integrated with medical devices, improving device functionality and performance. However, relationships between material properties, manufacturing environment, composite (micro)structure, and subsequent release kinetics are not well established. We apply a thermodynamically consistent model to probe the influence of drug-polymer chemistry (phobicity), drug loading, and evaporation rate on microstructure development during fabrication. For these structures, we compute release profiles for exposure to polymer-insoluble media and media in which the polymer readily dissolves. We find that with increasing drug-polymer phobicity, structural heterogeneities form at lower loadings and more rapid rates. The heterogeneities remain isolated and compact at low loadings and become interconnected as the drug to polymer ratio approaches 1.0. Release into polymer-insoluble media was dramatically enhanced by heterogeneities, resulting in up to a fourfold increase in drug release. In polymer-soluble media, however, heterogeneities diminished release. Although reductions of only 30% were typically observed, the absolute changes were much larger than observed in polymer-insoluble media. Our results suggest that improved comprehension and quantification of the physico-chemical properties in controlled release systems will enable the microstructure to be tailored to achieve desired responses that are insensitive to manufacturing variations.

A phase eld approach is developed to model wetting and heterogeneous crystal nucleation of an undercooled pure liquid in contact with a sharp wall. We discuss various choices for the boundary condition at the wall and determine the properties of critical nuclei, including their free energy of formation and the contact angle as a function of undercooling. We nd for particular choices of boundary conditions, we may realize either an analog of the classical spherical cap model or decidedly non-classical behavior, where the contact angle decreases from its value taken at the melting point towards complete wetting at a critical undercooling., 4 figures

Hematopoietic stem cell (HSC) therapy can significantly lower instances of infection in chemotherapy patients by accelerating the recovery of white blood cells in the body. However, therapy requires that HSCs be stored at cryogenic temperatures to retain the cells’ ability to proliferate. Currently, cells are stored in polymeric blood bags that are subject to fracture at the extremely low storage temperatures, which leads to cell contamination, thereby reducing their effectiveness. Therefore, we have developed an analytical model to predict the accumulation of stresses that ultimately lead to crack initiation and bag fracture during cryogenic storage. Our model gives explicit relationships between stress state in the container and thermoelastic properties of the container material, container geometry, and environmental factors that include temperature of the system and pressure induced by excess gas evolving from the stored medium. Predictions based on the model are consistent with experimental observations of bag failures that occurred during cryogenic storage applications. Finally, the model can provide guidance in material selection and bag design to fabricate bags that will be less susceptible to fracture.

The grain boundary plane distributions in MgO, SrTiO3, MgAl2O4, and Al are compared at lattice misorientations with a coincident site density of greater than or equal to 1/9. In most situations, the most frequently adopted grain boundary orientation is a habit plane of low index and low surface energy that depends on the particular material. Cases where the most common boundary orientation is a plane of high planar coincident site density instead of a characteristic habit plane are rare. In fact, in most cases, the distributions of grain boundary planes at misorientations with high lattice coincidence are not substantially different from the distributions at other, more general misorientations. The results indicate that a model for grain boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.

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Measurement of the geometry of triple junctions between grain boundaries in polycrystalline materials is used to generate large sets of dihedral angles from which maps of the grain boundary energy are extracted. A preliminary analysis has been performed for samples of magnesia and aluminum based on a three-parameter description of grain boundaries. An extended form of orientation imaging microscopy (OIM) was used to measure both triple junction geometry via image analysis in the SEM and local grain orientation via electron back scatter diffraction. Serial sectioning with registry of both in-plane images and successive sections characterizes triple junction tangents from which true dihedral angles are calculated. If there is local equilibrium at each triple junction, we may apply Herring's relation. By limiting grain boundary character to a (three parameter) specification of misorientation for the preliminary analysis, we can neglect the torque terms and apply the sine law to the three boundaries. This provides two independent relations per triple junction between grain boundary energies and dihedral angles. By discretizing the misorientation and employing multiscale statistical analysis on large data sets, (relative) grain boundary energy as a function of boundary character can be extracted from triple junction geometry. The results are discussed with respect to current understanding of grain boundary structure based on their crystallography. The results suggest that a three parameter characterization of grain boundaries (lattice disorientation) is not an adequate description of boundary character. A full analysis including torque terms and a five parameter boundary description is under development.