Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

비만아 부모의 돌봄 경험: q 방법론

3D bioactive composite scaffolds for bone tissue engineering.

Increased use of reconstruction procedures in orthopedics, due to trauma, tumor, deformity, degeneration and an aging population, has caused a blossom, not only in surgical advancement, but also in the development of bone implants. Traditional synthetic porous scaffolds are made of metals, polymers, ceramics or even composite biomaterials, in which the design does not consider the native structure and properties of cells and natural tissues. Thus, these synthetic scaffolds often poorly integrate with the cells and surrounding host tissue, thereby resulting in unsatisfactory surgical outcomes due to poor corrosion and wear, mechanical mismatch, unamiable surface environment, and other unfavorable properties. Musculoskeletal tissue reconstruction is the ultimate objective in orthopedic surgery. This objective can be achieved by (i) prosthesis or fixation device implantation, and (ii) tissue engineered bone scaffolds. These devices focus on the design of implants, regardless of the choice of new biomaterials. Indeed, metallic materials, e.g. 316L stainless steel, titanium alloys and cobalt chromium alloys, are predominantly used in bone surgeries, especially in the load-bearing zone of prostheses. The engineered scaffolds take biodegradability, cell biology, biomolecules and material mechanical properties into account, in which these features are ideally suited for bone tissue repair and regeneration. Therefore, the design of the scaffold is extremely important to the success of clinical outcomes in musculoskeletal surgeries. The ideal scaffolds should mimic the natural extracellular matrix (ECM) as much as possible, since the ECM found in natural tissues supports cell attachment, proliferation, and differentiation, indicating that scaffolds should consist of appropriate biochemistry and nano/micro-scale surface topographies, in order to formulate favorable binding sites to actively regulate and control cell and tissue behavior, while interacting with host cells. In addition, scaffolds should also possess a similar macro structure to what is found in natural bone. This feature may provide space for the growth of cells and new tissues, as well as for the carriers of growth factors. Another important concern is the mechanical properties of scaffolds. It has been reported that the mechanical features can significantly influence the osteointegration between implants and surrounding tissues, as well as cell behaviors. Since natural bone exhibits super-elastic biomechanical properties with a Young's modulus value in the range of 1–27 GPa, the ideal scaffolds should mimic strength, stiffness and mechanical behavior, so as to avoid possible post-operation stress shielding effects, which induce bone resorption and consequent implant failure. In addition, the rate of degradation and the by-products of biodegradable materials are also critical in the role of bone regeneration. Indeed, the mechanical integrity of a scaffold will be significantly reduced if the degradation rate is rapid, thereby resulting in a pre-matured collapse of the scaffold before the tissue is regenerated. Another concern is that the by-products upon degradation may alter the tissue microenvironment and then challenge the biocompatibility of the scaffold and the subsequent tissue repair. Therefore, these two factors should be carefully considered when designing new biomaterials for tissue regeneration. To address the aforementioned questions, an overview of the design of ideal biomimetic porous scaffolds is presented in this paper. Hence, a number of original engineering processes and techniques, including the production of a hierarchical structure on both the macro- and nano-scales, the adjustment of biomechanical properties through structural alignment and chemical components, the control of the biodegradability of the scaffold and its by-products, the change of biomimetic surface properties by altering interfacial chemistry, and micro- and nano-topographies will be discussed. In general, the concepts and techniques mentioned above provide insights into designing superior biomimetic scaffolds for bone tissue engineering.

Biomimetic porous scaffolds for bone tissue engineering

Nanostructuring of titanium-based implantable devices can provide them with superior mechanical properties and enhanced biocompatibity. An overview of advanced fabrication technologies of nanostructured, high strength, biocompatible Ti and shape memory Ni–Ti alloy for medical implants is given. Computational methods of nanostructure properties simulation and various approaches to the computational, “virtual” testing and numerical optimization of these materials are discussed. Applications of atomistic methods, continuum micromechanics and crystal plasticity as well as analytical models to the analysis of the reserves of the improvement of materials for medical implants are demonstrated. Examples of successful development of a nanomaterial-based medical implants are presented.

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Nanostructured titanium-based materials for medical implants: Modeling and development

Self-propagating high-temperature synthesis (SHS) of compacted blends of nano-size TiO 2 and micron-size Al powders was used to fabricate in situ alumina–TiAl/Ti 3 Al interpenetrating phase composites. Combustion wave propagation and pressureless or pressure-assisted thermal explosion (TE) modes have been investigated, and samples' temperature in the course of combustion synthesis has been accurately monitored. The ignition of self-sustained reaction in the system studied occurred at temperatures well above the melting point of Al, and temperature evolution in the samples was affected by interfacial barriers and by heat transfer to the surrounding ambience. The application of a moderate pressure (∼ 50 MPa) during thermal explosion at 950 °C yielded near fully dense (up to 98% TD) Al 2 O 3 –TiAl/Ti 3 Al composites with fine micron size interpenetrating ceramic and intermetallic networks. The aluminide component had a very fine submicron γ+α 2 lamellar microstructure. The TE procedure performed under uni-axial pressure between press rams (reactive forging) seems especially attractive due to the extremely short processing cycle (several minutes) and near-net-shape capability. This approach is compared with reactive hot pressing (RHP) where only partial conversion of reagents into products was observed after one hour exposure at 950 °C.

In situ processing of dense Al2O3–Ti aluminide interpenetrating phase composites

Materials with fine microstructures by advanced powder metallurgy

In the present research, near-fully dense in situ composites were fabricated from cold sintered B4C–3Ti and 2BN–3Ti powder blends with and without the addition of Ni. Two reactive synthesis techniques were employed: combustion consolidation (pressure-assisted thermal explosion) and reactive hot pressing (displacement reaction under pressure) RHP. In both approaches, the processing or preheating temperature (≤1100°C) was considerably lower than those typical of current methods used for the processing/consolidation of ceramic matrix composites. Microstructure characterization of the materials obtained was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive analysis (EDS). Mechanical properties were evaluated by measuring microhardness, fracture toughness and three-point bending strength. Full conversion of reagents into products was achieved in B4C–3Ti, B4C–3Ti–1.5Ni and 2BN–3Ti–1.5Ni blends during combustion consolidation, and a moderate external pressure of 150 MPa was sufficient to ensure full density of the final products. Unlike this, no thermal explosion occurred in 2BN–3Ti samples at 1100°C under pressure. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The RHP processing route yielded dense materials with finer microstructures, however full conversion of reagents into products has not been achieved. The addition of Ni to the powder blends was shown to enhance densification, as well as improve the fracture toughness of the composites synthesized.

Dense in situ TiB2/TiN and TiB2/TiC ceramic matrix composites: reactive synthesis and properties

Titanium and Ti–6Al–4V alloy samples were coated using a Powder Immersion Reaction Assisted Coating (PIRAC) nitriding method in order to modify their surface properties. Depending on the processing temperature, strongly adherent single(TiN)- or double(Ti2N/TiN)-layer coatings were obtained on both substrates. Several characteristics of PIRAC-coated Ti alloys relevant to their applications in total joint replacements were studied. Residual stresses in PIRAC coatings measured by sin2ψ X-ray diffraction method were found to be compressive in nature and were significantly lower than those reported for PVD TiN layers on similar substrates. In vitro fretting tests of PIRAC nitrided Ti–6Al–4V-to-Ti–6Al–4V couples simulating in vivo conditions at the interface of modular orthopedic implants demonstrated a major reduction in fretted areas, as well as a remarkable reduction of the corrosion potential drop at the initial stages of fretting as compared to the uncoated alloy. In addition, a 25% reduction of fretting-induced dissolved Ti ions concentration in testing solution was measured by EAAS. The results of the research suggest that titanium nitride PIRAC coatings can provide surgical titanium alloys with the longed-for fretting wear and corrosion resistant surface thereby minimizing the ion- and particulate-generating potential of modular orthopedic implants.

Titanium nitride coatings on surgical titanium alloys produced by a powder immersion reaction assisted coating method: residual stresses and fretting behavior

Elazar Y. Gutmanas

Papers

Nanostructured titanium-based materials for medical implants: Modeling and development

In situ processing of dense Al2O3–Ti aluminide interpenetrating phase composites

Materials with fine microstructures by advanced powder metallurgy

Dense in situ TiB2/TiN and TiB2/TiC ceramic matrix composites: reactive synthesis and properties

Titanium nitride coatings on surgical titanium alloys produced by a powder immersion reaction assisted coating method: residual stresses and fretting behavior