Transformation of Nano-Size Titanium Dioxide Particles in the Gastrointestinal Tract and Its Role in the Transfer of Nanoparticles through the Intestinal Barrier.

In this work, the size transformation of the TiO2 nanofraction from pharmaceutical grade E171 powder was studied during its transit through the gastrointestinal tract (GIT). It was shown that pharmaceutical-grade TiO2 powder contained about 0.68% (w/w) of particles smaller than 240 nm in diameter. In the observed GIT transit process the TiO2 nanoparticles were agglomerated up to 150-200 nm in simulated salivary fluid, with gradual agglomerate enlargement up to 300-600 nm and more than 1 micron in simulated gastric fluid. In the intestinal fluid the reverse process occurred, involving a decrease of agglomerates accompanied by the formation of a small fraction with ~50 nm average size. This fraction can be further involved in the histohematic transport process. The acidity degree (pH) and mineral composition of solutions, as well as the transit speed along the gastrointestinal tract, influence the nature of the particle transformation significantly. The rapid passing between the gastrointestinal tract sections creates conditions for a decrease in part of the TiO2 particles, up to 100 nm, and may be associated with the violation of the structural and functional integrity of the intestinal mucus layer.

Remote epitaxy using graphene enables growth of stress-free GaN.

The properties of group III-Nitrides (III-N) such as a large direct bandgap, high melting point, and high breakdown voltage make them very attractive for optoelectronic applications. However, conventional epitaxy on SiC and sapphire substrates results in strained and defective films with consequently poor device performance. In this work, by studying the nucleation of GaN on graphene/SiC by MOVPE, we unambiguously demonstrate the possibility of remote van der Waals epitaxy. By choosing the appropriate growth conditions, GaN crystals can grow either in-plane misoriented or fully epitaxial to the substrate. The adhesion forces across the GaN and graphene interface are very weak and the micron-scale nuclei can be easily moved around. The combined use of x-ray diffraction and transmission electron microscopy demonstrate the growth of stress-free and dislocation-free crystals. The high quality of the crystals was further confirmed by photoluminescence measurements. First principles calculations additionally highlighted the importance of the polarity of the underlying substrate. This work lays the first brick towards the synthesis of high quality III-N thin films grown via van der Waals epitaxy.

Propagation of combustion waves in the shell-core energetic materials with external heat losses.

In this paper, the properties and stability of combustion waves propagating in the composite solid energetic material of the shell-core type are numerically investigated within the one-dimensional diffusive-thermal model with heat losses to the surroundings. The flame speed is calculated as a function of the parameters of the model. The boundaries of stability are determined in the space of parameters by solving the linear stability problem and direct integration of the governing non-stationary equations. The results are compared with the characteristics of the combustion waves in pure solid fuel. It is demonstrated that a stable travelling combustion wave solution can exist for the parameters of the model for which the flame front propagation is unstable in pure solid fuel and it can propagate several times faster even in the presence of significant heat losses.

Instability and Spontaneous Reconstruction of Few-Monolayer Thick GaN Graphitic Structures.

Two-dimensional (2D) semiconductors are a very hot topic in solid state science and technology. In addition to van der Waals solids that can be easily formed into 2D layers, it was argued that single layers of nominally 3D tetrahedrally bonded semiconductors, such as GaN or ZnO, also become flat in the monolayer limit; the planar structure was also proposed for few-layers of such materials. In this work, using first-principles calculations, we demonstrate that contrary to the existing consensus the graphitic structure of few-layer GaN is unstable and spontaneously reconstructs into a structure that remains hexagonal in plane but with covalent interlayer bonds that form alternating octagonal and square (8|4 Haeckelite) rings with pronounced in-plane anisotropy. Of special interest is the transformation of the band gap from indirect in planar GaN toward direct in the Haeckelite phase, making Haeckelite few-layer GaN an appealing material for flexible nano-optoelectronics.

[Investigation of the Influence of Angiogenesis on Tumor Growth with the Use of a Mathematical Model].

A mathematical model of tumor growth is developed taking into account angiogenesis. Malignant cells under metabolic stress produce vascular endothelium growth factor that stimulates angiogenesis, increasing nutrient influx in tumor. The model takes into account the migration and proliferation dichotomy in the malignant cells depending on nutrient concentration. Convective fluxes originated due to active tumor cell proliferation in compact dense tissue are also considered. The computational analysis of the model has demonstrated that diffusive tumor growth rate does not depend on angiogenesis while for non-invasive tumors angiogenesis can significantly alter tumor growth, although it is not able to stop it completely. The causes and significance of the result for estimation of the antitumor efficacy of antiangiogenic therapy are discussed.

Understanding Phase-Change Memory Alloys from a Chemical Perspective.

Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

Study of band inversion in the PbxSn1-xTe class of topological crystalline insulators using x-ray absorption spectroscopy.

Pb(x)Sn(1-x)Te and Pb(x)Sn(1-x)Se crystals belong to the class of topological crystalline insulators where topological protection is achieved due to crystal symmetry rather than time-reversal symmetry. In this work, we make use of selection rules in the x-ray absorption process to experimentally detect band inversion along the PbTe(Se)-SnTe(Se) tie-lines. The observed significant change in the ratio of intensities of L1 and L3 transitions along the tie-line demonstrates that x-ray absorption can be a useful tool to study band inversion in topological insulators.

[Evaluation of the placental morphofunctional state in perinatal HIV transmission].

The paper presents the results of investigating the mechanisms of placental insufficiency and transplacental infection of infants born to HIV-infected mothers who have received specific antiretroviral therapy mothers and who have not.

Interfacial phase-change memory.

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

Distortion-triggered loss of long-range order in solids with bonding energy hierarchy.

An amorphous-to-crystal transition in phase-change materials like Ge-Sb-Te is widely used for data storage. The basic principle is to take advantage of the property contrast between the crystalline and amorphous states to encode information; amorphization is believed to be caused by melting the materials with an intense laser or electrical pulse and subsequently quenching the melt. Here, we demonstrate that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state. We further show that the principal change in optical properties occurs during the distortion of the still crystalline structure, upsetting yet another commonly held belief that attributes the change in properties to the loss of long-range order. Furthermore, our results suggest a way to lower energy consumption by condensing phase change inducing energy into shorter pulses or through the use of coherent phonon excitation.

Expression of thrombospondin-1 gene mRNA and protein in the placenta in gestosis.

The expression of TSP-1 gene mRNA and TSP-1 protein in the placental tissue was studied during normal pregnancy and in gestosis. The formation of placental tissue in normal gestation was associated with expression of TSP-1 gene mRNA and of TSP-1 protein. Gestosis was associated with inflammatory reaction in the placenta characterized by increased counts of lymphocytes and macrophages in the villous stroma and involution degenerative changes in tissue. Disorders in placental villi maturation and branching in gestosis were paralleled by hyperexpression of TSP-1 gene mRNA by placental cells and hyperexpression of TSP-1 protein predominating in the stromal elements of terminal villi and near villous vessels.

Toward the ultimate limit of phase change in Ge(2)Sb(2)Te(5).

The limit to which the phase change memory material Ge(2)Sb(2)Te(5) can be scaled toward the smallest possible memory cell is investigated using structural and optical methodologies. The encapsulation material surrounding the Ge(2)Sb(2)Te(5) has an increasingly dominant effect on the material's ability to change phase, and a profound increase in the crystallization temperature is observed when the Ge(2)Sb(2)Te(5) layer is less than 6 nm thick. We have found that the increased crystallization temperature originates from compressive stress exerted from the encapsulation material. By minimizing the stress, we have maintained the bulk crystallization temperature in Ge(2)Sb(2)Te(5) films just 2 nm thick.

Initial structure memory of pressure-induced changes in the phase-change memory alloy Ge2Sb2Te5.

We demonstrate that while the metastable face-centered cubic (fcc) phase of Ge2Sb2Te5 becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. Upon higher compression, a body-centered cubic phase is obtained in both cases around 30 GPa. Upon decompression, the amorphous phase is retained for the starting fcc phase while the initial structure is recovered for the starting trigonal phase. We argue that the presence of vacancies and associated subsequent large atomic displacements lead to nanoscale phase separation and loss of initial structure memory in the fcc staring phase of Ge2Sb2Te5.

[Autowaves in a model of growth of an invasive tumor].

A mathematical model for the invasive tumour growth has been constructed, which takes cell division, death, and motility into account. The model includes local cell density and the distribution of nutrient (oxygen) concentration. Cancer cells die in the absence of nutrients; therefore, the distribution of oxygen in tissue substantially affects both the tumour proliferation rate and structure. The model adequately describes the experimentally measured rate of tumour proliferation. The existence of autowave solutions has been demonstrated, and their properties have been investigated. The results are compared with the properties of the Kolmogorov-Petrovskii-Piskunov and Fisher equations. It is shown that the nutrient distribution influences the speed selection and the convergence of the initial conditions to the automodel solution.

Regulatory mechanisms for apoptosis in placental tissue during normal pregnancy and gestosis-complicated pregnancy.

The localization of apoptosis and expression of proapoptotic and antiapoptotic factors by the placental tissue were compared during normal pregnancy and gestosis-complicated pregnancy. The degree of apoptosis did not differ in the third trimester of normal pregnancy and gestosis-complicated pregnancy. Increased expression of Fas, caspase-8, and caspase-3 in placental tissue during normal pregnancy was shown to contribute to the suppression of angiogenesis and growth of placental tissue. No differences were found in the expression of FasL (CD95L), caspase-2, caspase-9, and Mcl-1 by placental cells during normal pregnancy and gestosis-complicated pregnancy. Increased expression of TRAIL by trophoblast cells is a protective mechanism from apoptotic signals of maternal cytotoxic lymphocytes and NK cells during gestosis.

Expression of VEGF and VEGF-R3 receptor by placental endothelial cells in health and gestosis.

We carried out a comparative analysis of changes in VEGF secretion and expression of VEGF-R3 receptor by placental endothelial cells in health and gestosis and of changes in VEGF-R3 expression by EA.hy926 human endothelial cells during culturing with supernatants conditioned by placental explants from women with normal pregnancy and patients with gestosis. Reduced secretion of VEGF and expression of VEGF-R3 by placental endothelial cells in gestosis can be caused by functional deficiency of the endothelial cells and low viability of endothelial cells.

Analysis of growth kinetics and proliferative heterogeneity of Lewis lung carcinoma cells growing as unfed culture.

AIM: To analyze the growth kinetics and proliferative heterogeneity of Lewis lung carcinoma (LLC) cells during their growth in monolayer for 5 days without replacement of culture medium (unfed culture). METHODS: Cell biology methods, sandwich enzyme-linked immunosorbent assay for vascular endothelial growth factor (VEGF) detection (ELISA), enzymatic glucose-oxidase method for glucose measurements, mathematical modeling. RESULTS: Created mathematical model showed good fit to experimental data; that allowed to determine kinetic (model) parameters of LLC cells and predict the changes in number of proliferating and quiescent cells (proliferative heterogeneity) during their growth. It was shown that growth kinetics of viable LLC cells possesses non-monotonous character - during first three days of growth the number of cells raised exponentially, with following decrease after the maximal level was achieved. At the same time the decrease of number of viable cells/increase of number of dead cells has been observed upon complete depletion of culture medium by glucose content. Glucose dependence of cell transition rate from proliferation to resting state predicted by mathematical model possessed a pronounced two-phase character. At a wide range of relatively high glucose concentrations (> 1.0 mg/ml) the transition rate was close to zero. At concentrations lower than 0.7 mg/ml, the rate of transition swiftly increased resulting in sharp change in cellular composition. At an interval from 70 to 90 h, practically all proliferating cells transited to a resting state. The rate of quiescent cell death was relatively low, and this was in part caused by too low level of glucose consumption compared to proliferating cells. It was shown that during LLC cells growth VEGF production rate decreased monotonously in spite of the fact that the level of VEGF in incubation medium increased monotonously. Observed monotonous decrease of VEGF production rate could not be explained by VEGF degradation in incubation medium (our results displayed the stability of VEGF molecule during investigations). CONCLUSIONS: Weak dependence of cell transition rate from proliferating to resting state from glucose level (> 0.7 mg/ml) and low rate of cell death provided slow decrease of the pool of quiescent cells in the population, thus significantly increasing their chance to survive upon nutritional deficiency.

Pressure-induced site-selective disordering of Ge2Sb2Te5: a new insight into phase-change optical recording.

We demonstrate that , the material of choice in phase-change optical recording (such as DVD-RAM), can be rendered amorphous by the application of hydrostatic pressure. It is argued that this structural change is due to a very strong second-nearest-neighbor Te-Te interaction that determines the long-range order in the metastable cubic phase of and also to the presence of vacancies. This newly discovered phenomenon suggests that pressure is an important factor for the formation of the amorphous phase which opens new insight into the mechanism of phase-change optical recording.

In situ X-ray absorption fine structure detection of reversible photoinduced anisotropy in amorphous selenium.

Photoinduced anisotropy in amorphous selenium ( a-Se) has been studied by in situ x-ray absorption fine structure spectroscopy. It is found that upon irradiation with linearly polarized light the chainlike fragments in a-Se are oriented with their c-axis perpendicular to the polarization plane of the inducing light. This chain orientation is reversible, correlates with optical anisotropy, and acts as a prelude to photoinduced anisotropic crystallization. This optically controlled reorientation of molecular fragments in a solid can be a basis for future novel devices.