Shape memory properties provide a very attractive insight into materials science, opening unexplored horizons and giving access to unconventional functions in every material class (metals, polymers, and ceramics). In this regard, the biomedical field, forever in search of materials that display unconventional properties able to satisfy the severe specifications required by their implantation, is now showing great interest in shape memory materials, whose mechanical properties make them extremely attractive for many biomedical applications. However, their biocompatibility, particularly for long-term and permanent applications, has not yet been fully established and is therefore the object of controversy. On the other hand, shape memory polymers (SMPs) show promise, although thus far, their biomedical applications have been limited to the exploration. This paper will first review the most common biomedical applications of shape memory alloys and SMPs and address their critical biocompatibility concerns. Finally, some engineering implications of their use as biomaterials will be examined.

Shape Memory Materials for Biomedical Applications

Computer simulation of 3-D grain growth using a phase-field model

Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

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Gas-phase spectroscopy of biomolecular building blocks.

Density-functional theory (DFT) and its variations provide a fruitful approach to the computational modeling of the microscopic structures and phase behavior of soft-condensed matter. The methodology takes deep root in quantum mechanics but shares a mathematical similarity with a number of classical approaches in statistical mechanics. This review discusses different strategies commonly used to formulate the free-energy functional of complex fluids for either phenomena-oriented applications or as a generic description of the thermodynamic nonideality owing to various components of intermolecular forces. We emphasize the connections among different schemes of DFT approximations, their underlying assumptions, and inherent limitations. We also address extensions of equilibrium DFT to phenomenological theories for the dynamic properties of complex fluids and for the kinetics of phase transitions. In addition, we highlight the recent literature concerning applications of DFT to diverse static and time-dependent phenomena in complex fluids.

Density-Functional Theory for Complex Fluids

Mode-coupling theory is an approach to the study of complex behavior in the supercooled liquids which developed from the idea of a nonlinear feedback mechanism. From the coupling of slowly decaying correlation functions the theory predicts the existence of a characteristic temperature ${T}_{c}$ above the experimental glass transition temperature ${T}_{g}$ for the liquid. This article discusses the various methods used to obtain the model equations and illustrates the effects of structure on dynamics and scaling behavior over different time scales using a wave-vector-dependent model. It compares the theoretical predictions, experimental observations, and computer simulation results, and also considers phenomenological extensions of mode-coupling theory. Numerical solutions of the model equations to study the dynamics from a nonperturbative approach are also reviewed. The review looks briefly at recent observations from landscape studies of model systems of structural glasses and their relation to the mode-coupling temperature ${T}_{c}$. The equations for the mean-field dynamics driven by the $p$-spin interaction Hamiltonian are similar to those of mode-coupling theory for structural glasses. Related developments in the nonequilibrium dynamics and generalization of the fluctuation-dissipation relation for the structural glasses are briefly touched upon. The review ends with a summary of the open questions and possible future direction of the field.

Mode-coupling theory and the glass transition in supercooled liquids

Sr 2 CuO 2 (CO 3 ) was prepared at 1273 K and 0.01 MPa CO 2 partial pressure in a flowing gas of O 2 CO 2 using a mixture of SrCO 3 and CuO powders as a starting material. The compound has a tetragonal structure with lattice constants a = 7.8045(1), and c = 14.993(1) A , and its space group is 14. The formula per unit cell is 8 Sr 2 CuO 2 (CO 3 ), and measured and calculated densities are D m = 4.71 g/cm 3 , and D x = 4.81 g/cm 3 , respectively. The crystal structure was refined by Rietveld analysis on X-ray powder diffraction and neutron powder diffraction data. The final residuals ( R F ) were 4.31 and 4.27% for the X-ray and neutron data, respectively. The structure consists of deformed [CuO 6 ] octahedrons and layers of ordered triangular CO 3 groups. Sr atoms having eight near oxygen neighbors are between the [CuO 6 ] octahedrons and the CO 3 layers.

Preparation and crystal structure of Sr2CuO2(CO3)

The time evolution of three-dimensional cellular patterns is discussed on the basis of the oertex model which consists of a set of equations for the motion of vertices obtained from the full curvature-driven equation of motion of cell boundaries (faces) by a reduction in the degrees of freedom. Some difficulty in the earlier vertex models, originating from the fact that faces are non-coplanar, is now side-stepped by introducing virtual vertices as supplementary degrees of freedom. Equations of motion are then readily derived assuming that the whole process is purely dissipative. By solving these equations numerically, scaling properties concerning cellular pattern growth are obtained. In addition, geometrical features such as the three-dimensional versions of the Aboav-Weaire law and the Lewis law are verified with fair accuracy. We emphasize that our vertex model has the potentiality for efficient improvements of indefinite degree by introducing arbi- trary numbers of virtual vertices.

Computer modelling of three-dimensional cellular pattern growth

Synchrotron x-ray diffraction study establishes a novel structure sequence in ${\mathrm{SnI}}_{4}$ at high pressures to 153 GPa. A new crystalline phase responsible for the metallization is observed above 7.2 GPa. The pressure-amorphized ${\mathrm{SnI}}_{4}$ emerging above 15 GPa crystallizes at 61 GPa into another new structure having an apparent fcc lattice with a nearest-neighbor distance as short as 3.00 \AA{}. This provides evidence for the occurrence of molecular dissociation. Atomic arrangement in the nonmolecular phase and structural properties of the amorphous state are discussed.

Amorphization and Molecular Dissociation of Sn I 4 at High Pressure

An investigation of the precise determination of melting temperature in the modified Lennard-Jones system under pressure-free conditions [Y. Asano and K. Fuchizaki, J. Phys. Soc. Jpn.78, 055002 (2009)10.1143/JPSJ.78.055002] was extended under finite-pressure conditions to obtain the phase diagram. The temperature and pressure of the triple point were estimated to be 0.61 e/k B and 0.0018(5) e/σ3, and those of the critical point were 1.0709(19) e/k B and 0.1228(20) e/σ3, where e and σ are the Lennard-Jones parameters for energy and length scales, respectively, and k B is the Boltzmann constant. The potential used here has a finite attractive tail and does not suffer from cutoff problems. The potential can thus be a useful standard in examining statistical–mechanical problems in which different treatments for the tail would lead to different conclusions. The present phase diagram will then be a useful guide not only for equilibrium calculations but also for nonequilibrium problems such as discussions of the limits of phase (in)stability.

Kazuhiro Fuchizaki

Papers

Preparation and crystal structure of Sr2CuO2(CO3)

Computer modelling of three-dimensional cellular pattern growth

Amorphization and Molecular Dissociation of Sn I 4 at High Pressure

Phase diagram of the modified Lennard-Jones system

Equation of state of hexagonal boron nitride