The proper delivery pressure for cardioplegic solution in neonatal cardiac surgery - an investigation of biomechanical and structural properties of neonatal and adult coronary arteries

When cardioplegic solution is injected into coronary arteries with a pump in order to ensure myocardial protection, it is necessary to determine the correct delivery pressure to avoid damage of the heart. Biomechanical and structural properties of the neonatal coronary artery wall should be taken into account when determining the delivery pressure. We investigated twelve coronary artery specimens without cardiac pathology retrieved from autopsies of neonates 9.3 ± 9.7 days old and compared them to adult specimens with no detected atherosclerosis. There was a rapid increase in the strain until the inner pressure reached 80 - 100 mmHg, whilst the increase of stress in the wall of the neonatal coronary arteries was less rapid. When the pressure exceeded 100 mmHg, the increase in the strain slowed down, whilst the wall stress and modulus of elasticity began to increase rapidly. Morphologic examination of tensile properties revealed prominent affection of the vascular wall of the neonates, with accentuated redistribution (loosening) of medial myocytes and the adventitial vasa vasorum. Collectively, a raised inner pressure applied to cardioplegic solution injected into the coronary artery of a neonate may increase the risk of structural damage to the vascular wall.

Equilibrated Surface Alloys and the Influence of Strain

Structural and thermodynamic properties of equilibrated ultrathin Pd-Au, Cu-Ag, and Co-Ag alloys on Ru(0001) were studied and are comparatively discussed. These studies provide a framework for a detailed understanding of the energetics of surface alloy formation.

Additionally, the differences between the two bulk immiscible systems (Cu-Ag and Co-Ag) and the bulk miscible Pd-Au system are discussed.

First-principles calculations of free energies of unstable phases: the case of fcc W

Ab initio molecular dynamics simulations are used to solve the long-standing problem of calculating the free energies of unstable phases, such as fcc W. We find that fcc W is mechanically unstable with respect to long-wavelength shear at all temperatures considered (T>2500 K), while the short-wavelength phonon modes are anharmonically stabilized. The calculated fcc-bcc enthalpy and entropy differences at T=3500 K (308 meV and 0.74k_{B} per atom, respectively) agree well with the recent values derived from analysis of experimental data.

First-principles theory of competing order types, phase separation, and phonon spectra in thermoelectric AgPbmSbTe(m+2) alloys

Using a first-principles cluster expansion, we shed light on the solid-state phase diagram and structure of the recently discovered high-performance Pb-Ag-Sb-Te thermoelectrics. The calculated bulk thermodynamics favors the formation of coherent precipitates of ordered Ag(m)Sb(n)Te(m+n) phases immiscible with rocksalt PbTe, such as AgSbTe2. The solubility is high for Pb in AgSbTe2 and low for (Ag,Sb) in PbTe (8% vs 0.6% at 850 K). The differences in the phonon spectra of PbTe and AgSbTe2 suggest that these precipitates enhance the thermoelectric performance by lowering thermal conductivity.

First-principles prediction of a ground state crystal structure of magnesium borohydride

Mg(BH(4))(2) contains a large amount of hydrogen by weight and by volume, but its promise as a candidate for hydrogen storage is dependent on the currently unknown thermodynamics of H2 release. Using first-principles density-functional theory calculations and a newly developed prototype electrostatic ground state search strategy, we predict a new T=0 K ground state of Mg(BH(4))(2) with I4[over ]m2 symmetry, which is 5 kJ/mol lower in energy than the recently proposed P6(1) structure. The calculated thermodynamics of H(2) release are within the range required for reversible storage.

Linking surface stress to surface structure: measurement of atomic strain in a surface alloy using scanning tunneling microscopy

Annealed submonolayer CoAg/Ru(0001) films form an alloy with a structure that contains droplets of Ag surrounded by Co [G. E. Thayer, V. Ozolins, A. K. Schmid, N. C. Bartelt, M. Asta, J. J. Hoyt, S. Chiang, and R. Q. Hwang, Phys. Rev. Lett. 86, 660 (2001)]. To understand how surface stress contributes to the formation of this structure, we use scanning tunneling microscopy to extract atomic displacements at the boundaries between regions of Co and Ag. Comparing our measurements to Frenkel-Kontorova model calculations, we show how stress due to lattice mismatch contributes to the formation of the alloy droplet structure. In particular, we quantitatively evaluate how competing strain and chemical energy contributions determine surface structure.

Role of stress in thin film alloy thermodynamics: competition between alloying and dislocation formation

Using scanning tunneling microscopy (STM) and first-principles local-spin-density-approximation calculations to study submonolayer films of Co (1-c)Ag (c)/Ru(0001) alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration c exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained Co(0.6)Ag (0.4) surface alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The agreement between STM measurements and our calculated phase diagram supports this interpretation.