Peter D. Lee

A model of solidification microstructures in nickel-based superalloys: predicting primary dendrite spacing selection

Abstract A combined cellular automaton-finite difference (CA-FD) model has been developed to simulate solute diffusion controlled solidification of binary alloys. Constitutional and curvature undercooling were both solved to determine the growth velocity of the solid/liquid interface. A modified decentered square/octahedron (in two or three dimensions) growth technique was implemented in the cellular automaton to account for the effect of crystallographic anisotropy. The resulting model is capable of simulating the growth of equiaxed and columnar dendritic grains in 2D and 3D, with the directions either aligned or inclined with the grid. The algorithm used can also be used on coarser grids, with a concomitant loss in resolution, allowing simulation of sufficiently large numbers of dendrites in 3D to investigate the distribution of spacings, as well as average behavior. Simulations were performed for directional solidification with a range of withdrawal velocities and nucleation conditions, but a constant thermal gradient. The simulations capture the full microstructural development and primary spacing selection by both branching and overgrowth mechanisms. The model illustrates that there is a range of possible stable spacings, and that the final spacing is history dependent. It was also found that a minimum deviation from the steady state dendrite spacing is required before the spacing adjustment mechanisms are activated. The influence of perturbing the withdrawal velocity upon the stability of the spacing was also investigated. It was found that perturbations significantly reduce the range of stable primary dendrite spacing.

Hierarchical porous materials for tissue engineering

Biological organisms have evolved to produce hierarchical three-dimensional structures with dimensions ranging from nanometres to metres. Replicating these complex living hierarchical structures for the purpose of repair or replacement of degenerating tissues is one of the great challenges of chemistry, physics, biology and materials science. This paper describes how the use of hierarchical porous materials in tissue engineering applications has the potential to shift treatments from tissue replacement to tissue regeneration. The criteria that a porous material must fulfil to be considered ideal for bone tissue engineering applications are listed. Bioactive glass foam scaffolds have the potential to fulfil all the criteria, as they have a hierarchical porous structure similar to that of trabecular bone, they can bond to bone and soft tissue and they release silicon and calcium ions that have been found to up-regulate seven families of genes in osteogenic cells. Their hierarchical structure can be tailored for the required rate of tissue bonding, resorption and delivery of dissolution products. This paper describes how the structure and properties of the scaffolds are being optimized with respect to cell response and that tissue culture techniques must be optimized to enable growth of new bone in vitro.

Optical Imaging of Voltage and Calcium in Cardiac Cells & Tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Hydrogen porosity in directional solidified aluminium-copper alloys : In situ observation

Abstract Using a temperature gradient stage and real time micro-focus radiography the formation of porosity was observedin situ during the solidification of aluminium-copper alloys. Pore morphology was characterized both in the final structure and as a function of temperature during solidification, providing a qualitative insight into the relative importance of the competing physical processes. The effect of solidification velocity, thermal gradient and alloy composition on both the growth kinetics and final structure of the porosity was quantified. Statistical analysis was used to compare the trends in these results to both diffusion controlled and shrinkage driven pore growth.

Simultaneous Voltage and Calcium Mapping of Genetically Purified Human Induced Pluripotent Stem Cell–Derived Cardiac Myocyte Monolayers

Rationale: Human induced pluripotent stem cell–derived cardiomyocytes (iPSC-CMs) offer a powerful in vitro tool to investigate disease mechanisms and to perform patient-specific drug screening. To date, electrophysiological analysis of iPSC-CMs has been limited to single-cell recordings or low-resolution microelectrode array mapping of small cardiomyocyte aggregates. New methods of generating and optically mapping impulse propagation of large human iPSC-CM cardiac monolayers are needed. Objective: Our first aim was to develop an imaging platform with versatility for multiparameter electrophysiological mapping of cardiac preparations, including human iPSC-CM monolayers. Our second aim was to create large electrically coupled human iPSC-CM monolayers for simultaneous action potential and calcium wave propagation measurements. Methods and Results: A fluorescence imaging platform based on electronically controlled light-emitting diode illumination, a multiband emission filter, and single camera sensor was developed and utilized to monitor simultaneously action potential and intracellular calcium wave propagation in cardiac preparations. Multiple, large-diameter (≥1 cm), electrically coupled human cardiac monolayers were then generated that propagated action potentials and calcium waves at velocities similar to those commonly observed in rodent cardiac monolayers. Conclusions: The multiparametric imaging system presented here offers a scalable enabling technology to measure simultaneously action potential and intracellular calcium wave amplitude and dynamics of cardiac monolayers. The advent of large-scale production of human iPSC-CMs makes it possible to now generate sufficient numbers of uniform cardiac monolayers that can be utilized for the study of arrhythmia mechanisms and offers advantages over commonly used rodent models.

Palette of fluorinated voltage-sensitive hemicyanine dyes

Optical recording of membrane potential permits spatially resolved measurement of electrical activity in subcellular regions of single cells, which would be inaccessible to electrodes, and imaging of spatiotemporal patterns of action potential propagation in excitable tissues, such as the brain or heart. However, the available voltage-sensitive dyes (VSDs) are not always spectrally compatible with newly available optical technologies for sensing or manipulating the physiological state of a system. Here, we describe a series of 19 fluorinated VSDs based on the hemicyanine class of chromophores. Strategic placement of the fluorine atoms on the chromophores can result in either blue or red shifts in the absorbance and emission spectra. The range of one-photon excitation wavelengths afforded by these new VSDs spans 440–670 nm; the two-photon excitation range is 900–1,340 nm. The emission of each VSD is shifted by at least 100 nm to the red of its one-photon excitation spectrum. The set of VSDs, thus, affords an extended toolkit for optical recording to match a broad range of experimental requirements. We show the sensitivity to voltage and the photostability of the new VSDs in a series of experimental preparations ranging in scale from single dendritic spines to whole heart. Among the advances shown in these applications are simultaneous recording of voltage and calcium in single dendritic spines and optical electrophysiology recordings using two-photon excitation above 1,100 nm.

Melt-derived bioactive glass scaffolds produced by a gel-cast foaming technique

Porous melt-derived bioactive glass scaffolds with interconnected pore networks suitable for bone regeneration were produced without the glass crystallizing. ICIE 16 (49.46% SiO(2), 36.27% CaO, 6.6% Na(2)O, 1.07% P(2)O(5) and 6.6% K(2)O, in mol.%) was used as it is a composition designed not to crystallize during sintering. Glass powder was made into porous scaffolds by using the gel-cast foaming technique. All variables in the process were investigated systematically to devise an optimal process. Interconnect size was quantified using mercury porosimetry and X-ray microtomography (μCT). The reagents, their relative quantities and thermal processing protocols were all critical to obtain a successful scaffold. Particularly important were particle size (a modal size of 8 μm was optimal); water and catalyst content; initiator vitality and content; as well as the thermal processing protocol. Once an optimal process was chosen, the scaffolds were tested in simulated body fluid (SBF) solution. Amorphous calcium phosphate formed in 8h and crystallized hydroxycarbonate apatite (HCA) formed in 3 days. The compressive strength was approximately 2 MPa for a mean interconnect size of 140 μm between the pores with a mean diameter of 379 μm, which is thought to be a suitable porous network for vascularized bone regeneration. This material has the potential to bond to bone more rapidly and stimulate more bone growth than current porous artificial bone grafts.

Characterizing the hierarchical structures of bioactive sol-gel silicate glass and hybrid scaffolds for bone regeneration

Bone is the second most widely transplanted tissue after blood. Synthetic alternatives are needed that can reduce the need for transplants and regenerate bone by acting as active temporary templates for bone growth. Bioactive glasses are one of the most promising bone replacement/regeneration materials because they bond to existing bone, are degradable and stimulate new bone growth by the action of their dissolution products on cells. Sol–gel-derived bioactive glasses can be foamed to produce interconnected macropores suitable for tissue ingrowth, particularly cell migration and vascularization and cell penetration. The scaffolds fulfil many of the criteria of an ideal synthetic bone graft, but are not suitable for all bone defect sites because they are brittle. One strategy for improving toughness of the scaffolds without losing their other beneficial properties is to synthesize inorganic/organic hybrids. These hybrids have polymers introduced into the sol–gel process so that the organic and inorganic components interact at the molecular level, providing control over mechanical properties and degradation rates. However, a full understanding of how each feature or property of the glass and hybrid scaffolds affects cellular response is needed to optimize the materials and ensure long-term success and clinical products. This review focuses on the techniques that have been developed for characterizing the hierarchical structures of sol–gel glasses and hybrids, from atomic-scale amorphous networks, through the covalent bonding between components in hybrids and nanoporosity, to quantifying open macroporous networks of the scaffolds. Methods for non-destructive in situ monitoring of degradation and bioactivity mechanisms of the materials are also included.

Hydrogen porosity in directionally solidified aluminium-copper alloys: A mathematical model

Abstract A combined continuum and stochastic model of diffusion-controlled growth was developed to simulate the formation of porosity during the solidification of aluminium alloys. The whole population of pores was tracked, rather than just the average values. A finite difference solution of the diffusion equations was used, combined with a stochastic model of nucleation. The growth of each individual pore was simulated, assuming the shape to be spherical until it impinged on the developing dendrites, at which point the growth was modelled as a hemispherically capped segmented cone, with the growth radius limited by the liquid space between the dendrites. A previously published model by one of the authors was used to predict the dendritic spacing as a function of the thermal conditions. The model was compared with in situ observations of the formation of porosity during the solidification of aluminium–copper alloys, where the size, distribution and morphological evolution of pores were measured as a function of temperature/time. The predicted development of the porosity, including the distribution in size and morphology, compared well with that observed experimentally. The qualitative agreement between the model predictions and experimental results supports the hypothesis that the effect of hydrogen and its diffusion must be incorporated into any accurate model of pore formation in aluminium alloys.

Titanium foams for biomedical applications: a review

Abstract Metals are the oldest of biomedical implant materials and metallic alloys remain the material of choice for applications involving hard tissue replacement. Ti alloy scaffolds are deemed the best among all the metallic alloys. Recently, porous Ti alloy scaffolds have received increasing attention over other metallic counterparts, including monolithic alloys, due to advantages associated with an open porous structure. The main advantages of open porous structures are a low Youngs moduli and enhanced bone ingrowth leading to better fixation with the host tissue. In this paper, the authors first review the suitability of Ti for biomedical applications and then explore the methods for producing highly porous Ti foams. The methods are assessed based on their ability to produce a macro-micro-structure appropriate for biomedical applications. The article concludes with a future outlook on porous Ti production.