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

Electrical stimulation of excitable tissue: design of efficacious and safe protocols.

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

Coronary stents: a materials perspective.

The performance of any material in the human body is controlled by two sets of characteristics: biofunctionality and biocompatibility. With the wide range of materials available in the mid-1990s, it is relatively easy to satisfy the requirements for mechanical and physical functionality of implantable devices. Therefore, the selection of materials for medical applications is usually based on considerations of biocompatibility. When metals and alloys are considered, the susceptibility of the material to corrosion and the effect the corrosion has on the tissue are the central aspects of biocompatibility. Corrosion resistance of the currently used 316L stainless steel, cobalt-chromium, and titanium-based implant alloys relies on their passivation by a thin surface layer of oxide. Stainless steel is the least corrosion resistant, and it is used for temporary implants only. The titanium and Co-Cr alloys do not corrode in the body; however, metal ions slowly diffuse through the oxide layer and accumulate in the...

Characteristics of Metals Used in Implants

Corrosion behavior of titanium nitride coated Ni-Ti shape memory surgical alloy.

Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

Fabrication of Al matrix in situ composites via self-propagating synthesis

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

The key to the biocompatibility of NiTi surgical devices resides in the improvement of the material's corrosion resistance. To protect the surface of NiTi from corrosion, an original PIRAC nitriding method was proposed. In the present work, the corrosion and electrochemical properties of NiTi samples annealed under a low pressure of highly reactive nitrogen at 900 and 1000°C were studied. The microstructure of PIRAC coatings was characterized employing XRD and SEM/EPMA and was found to consist of a hard TiN outer layer followed by a Ti2Ni layer. Electrochemical tests were performed in Ringer's solution, both upon immersion in deaerated solution and after long term exposure (400 h) under aeration at open circuit potential (OCP). PIRAC nitriding treatment was found to significantly improve the corrosion resistance of NiTi alloy. Samples nitrided at 900°C, 6 h and at 1000°C, 0.5–5 h were passive in a wide range of potentials with no signs of activation up to 1.1 V. A very low anodic current in the passive region and an exceedingly low metal ion release rate were measured on PIRAC coated samples. The relationship between the nitriding parameters and protective properties of PIRAC coatings has been established and the role of the outer and inner surface layers in the improvement of NiTi corrosion resistance is discussed.

Irena Gotman

Papers

Characteristics of Metals Used in Implants

Corrosion behavior of titanium nitride coated Ni-Ti shape memory surgical alloy.

Fabrication of Al matrix in situ composites via self-propagating synthesis

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

TiN coating improves the corrosion behavior of superelastic NiTi surgical alloy