The Influence of Weaving Technologies on the Integral Characteristics of Synthetic Vascular Prostheses.

The aim of the study is to determine physical and structural properties of woven synthetic prostheses depending on the type of the weave. Materials and Methods: Ten vascular prostheses manufactured at the Science and Technology Park of the BNTU "Polytechnic" (Minsk, Republic of Belarus) have been analyzed. The prostheses differed in the type of weaving, duration and temperature of thermal fixation during crimping. Three samples had a single-layer structure and 7 samples had a double-layer structure. Tests for water permeability, resistance to radial bending, and porosity of the prostheses have been performed. Results: The single-layer woven prostheses have demonstrated a low level of water permeability: the best result was shown by sample No.1: 80 [77.1; 80.5] ml/min/cm2. A strong direct correlation was revealed for these prostheses: the larger the pore diameter, the greater permeability (r=0.778; p=0.05). The single-layer woven prostheses appeared to be most resistant to radial bending, samples No.1 and 3 had no deformations at the minimum radius of the cylinder (r<4 mm), sample No.2 showed deformation on the cylinder with r=5 mm. For the single-layer prostheses, a strong negative correlation was noted (r=‒0.97; p=0.04) between the density of the warp threads and the kinking radius.All double-layer prostheses have demonstrated higher water permeability and weak resistance to deformation during radial bending. Samples No.4 and 8 were found to have minimum and maximum water permeability of 276.5 [258.3; 288.4] and 538.8 [533.3; 564.3] ml/min/cm2, respectively. The minimum kinking radius (7 mm) was shown by samples No.9 and 10. The worst results were demonstrated by sample No.6, which was deformed with minimal bending. Conclusion: Samples with ordinary plain weave have a low level of water permeability and high resistance to radial deformation, which makes them look most promising for the application in vascular surgery.

Ab ovo: Factors Affecting the Radial Stiffness of Thoracic Aorta Stent-Grafts.

The aim of the investigation was to study the factors influencing the radial stiffness of the thoracic aorta stent-grafts with the stent elements made of nitinol tubes by laser cutting and thermal shape setting. Materials and Methods: The work used stent elements made by different technologies by two different manufacturers from a nitinol tube with a wall thickness of 0.5 mm (E1) and 0.4 mm (E2), with a final diameter of 20 mm. Height of cells E1 - 15 mm, E2 - 12.5 mm. The stents were manually attached to a tubular woven non-crimped base (PTGO Sever, Russia) with a 6/0 suture, resulting in either single or continuous stitches. In the RLU124 radial force tester (Blockwise Engineering LLC, USA), each of the four stent-grafts, as well as their individual stent elements, were compressed by 10 mm from the initial diameter. The dependence of the radial forces on deformation under loading and unloading was graphically presented. The temperature and enthalpy of phase transitions of nitinols into the austenite (Af) and martensitic (Mf) phases were studied using differential scanning calorimetry (DSC-3; Mettler Toledo, USA). All indicators were compared with the characteristics of two commercial models - Cronus (China) and E-vita Open Plus (Germany). Results: Four prototypes of SibHybrid stent-grafts were tested; those differed in their stent elements, distances between them, and the type of sutures (single or continuous). The stent elements of the models studied differed in the values of Af, Mf, and the enthalpy of phase transitions of nitinols. The hardest stent was the E2 prototype. The fixation of stent elements to the woven fabric in the graft increased the radial force by 4.0-5.5 times. During compression by 50 and 20% of the original diameter, the SibHybrid models developed radial force 4.5-6.0 times greater compared with the E-vita Оpen Plus model. The radial force values of SibHybrid models were almost the same as for the Cronus and models at 20% compression. Using continuous twining round suturing increased the radial force by about 10 N; accordingly, SibHybrid E2 had the highest radial force because it was fixed by a continuous suture. The density of the stent elements fixed on the fabric did not affect the radial force of the stent-graft as a whole. Conclusion: In the manufacture of stent elements from nitinol tubes, the main factor determining the radial stiffness is the technology of nitinol shape setting. With the standard technology of thermal shape setting, radial force can be changed by varying the height of the structure cell element and the cross-sectional area of the cell bars, as well as the suturing technique.

Electromagnetic wave band structure due to surface plasmon resonances in a complex plasma.

The dielectric properties of complex plasma containing either metal or dielectric spherical inclusions (macroparticles, dust) are investigated. We focus on surface plasmon resonances on the macroparticle surfaces and their effect on electromagnetic wave propagation. It is demonstrated that the presence of surface plasmon oscillations can significantly modify plasma electromagnetic properties by resonances and cutoffs in the effective permittivity. This leads to related branches of electromagnetic waves and to the wave band gaps. The conditions necessary to observe the band-gap structure in laboratory dusty plasma and/or space (cosmic) dusty plasmas are discussed.

Quantum-tunneling-enhanced charging of nanoparticles in plasmas.

The role of quantum tunneling effect in the electron accretion current onto a negatively charged grain immersed in isotropic plasma is analyzed, within the quasiclassic approximation, for different plasma electron distribution functions, plasma parameters, and grain sizes. It is shown that this contribution can be small (negligible) for relatively large (micron-sized) dust grains in plasmas with electron temperatures of the order of a few electronvolts, but becomes important for nanosized dust grains (tens to hundreds of nanometers in diameter) in cold and ultracold plasmas (electron temperatures at approximately tens to hundreds of Kelvin), especially in plasmas with depleted high-energy "tails" in the electron energy distribution.

Charge on the dust in the plasma.

The grain in the plasma gets charged until the net plasma flux to the grain surface vanishes. In the absence of any analytical formula, this ambipolar condition is utilized to compute the grain charge indirectly. The present work proposes an approximate analytical expression for the grain charge that is exact in the asymptotic limit and in which the relative error between analytical and numerically computed solutions is less than 3%. Using this formula, we show that, much like the plasma sheath, the Bohm criterion is satisfied near the dust surface.

Shielding of a moving test charge in a quantum plasma.

The linearized potential of a moving test charge in a one-component fully degenerate fermion plasma is studied using the Lindhard dielectric function. The motion is found to greatly enhance the Friedel oscillations behind the charge, especially for velocities larger than half of the Fermi velocity, in which case the asymptotic behavior of their amplitude changes from 1/r3 to 1/r2.5. In the absence of the quantum recoil (tunneling) the potential reduces to a form similar to that in a classical Maxwellian plasma, with a difference being that the plasma oscillations behind the charge at velocities larger than the Fermi velocity are not Landau damped.

Instability of the ionization-absorption balance in a complex plasma at ion time scales.

The stability of ion plasma perturbations is investigated in a homogeneous isotropic complex plasma, where a balance between plasma creation due to ionization and plasma loss due to the absorption on dust particles has been reached. The analysis is performed on the basis of a self-consistent fluid description including dust charge variations and ion-neutral friction. It is shown that the stability depends primarily on the nature of the ionization source. For an ionization source proportional to the electron density, an instability takes place at wave numbers below a certain threshold, and the instability mechanism is explained in detail. No instability is found for a constant ionization source.

Dynamics of two particles in a plasma sheath.

The stability and arrangements of two dust particles in a plasma are investigated in terms of the Hamiltonian of the system. It is shown that the Hamiltonian description of a non-Hamiltonian system can be used to predict qualitative features of possible equilibria in a variety of confinement potentials and can provide useful plasma diagnostics. The results compare favorably with those of simulations and are used to create experimental hypotheses. In particular, the symmetry-breaking transition of the particles as they leave the horizontal plane admits a Hamiltonian description which is used to elucidate the wake parameters.

Numerical simulations of potential distribution for elongated insulating dust being charged by drifting plasmas.

The potential distributions surrounding elongated insulating dust grains being charged by supersonic plasma flows are studied using the particle-in-cell method. The plasma flow introduces an asymmetry in the dust charging. This leads to a complex surface charge distribution on the dust, and to ion focusing in the wake region. We demonstrate that the charge and potential distributions on the dust surface and the wake behind the dust depend on the rod length and dust inclination angle with respect to the flow. The role of the surface charge distribution in the interactions between insulating rods in a plasma is discussed. Our simulations are carried out in two spatial dimensions, treating ions and electrons as individual particles.

Wake behind dust grains in flowing plasmas with a directed photon flux.

The wake behind conducting dust grains in a supersonic plasma flow with a directed photon flux is studied by the particle-in-cell method. The electron emission leads to a positive charge on the dust. The resulting plasma wake differs significantly from the case without photoelectrons. This wake is studied for different photon fluxes and different angles between the incoming unidirectional photons and the plasma flow velocity. The simulations are carried out in two spatial dimensions, treating ions and electrons as individual particles.

Superfluidlike motion of an absorbing body in a collisional plasma.

Motion of a small charged absorbing body (micrograin) immersed in a stationary weakly ionized high pressure plasma environment is considered. It is shown that the total frictional (drag) force acting on the grain can be directed along its motion, causing the grain acceleration. At some velocity, the forces associated with different plasma components can balance each other, allowing free undamped superfluid motion of the grain. The conditions when such behavior can be realized and the possibility of a superconductive grain current are discussed in the context of complex (dusty) plasmas.

Criticality in a Vlasov-Poisson system: a fermioniclike universality class.

A model Vlasov-Poisson system is simulated close to the point of marginal stability, thus assuming only the wave-particle resonant interactions are responsible for saturation, and shown to obey the power-law scaling of a second-order phase transition. The set of critical exponents analogous to those of the Ising universality class is calculated and shown to obey the Widom and Rushbrooke scaling and Josephson's hyperscaling relations at the formal dimensionality d=5 below the critical point at nonzero order parameter. However, the two-point correlation function does not correspond to the propagator of Euclidean quantum field theory, which is the Gaussian model for the Ising universality class. Instead, it corresponds to the propagator for the fermionic vector field and to the upper critical dimensionality d(c) = 2. This suggests criticality of collisionless Vlasov-Poisson systems corresponds to a universality class analogous to that of critical phenomena of a fermionic quantum field description.

Parametric instabilities in magnetized multicomponent plasmas.

This paper investigates the excitation of various natural modes in a magnetized bi-ion or dusty plasma. The excitation is provided by parametrically pumping the magnetic field. Here two ionlike species are allowed to be fully mobile. This generalizes our previous work where the second heavy species was taken to be stationary. Their collection of charge from the background neutral plasma modifies the dispersion properties of the pump and excited waves. The introduction of an extra mobile species adds extra modes to both these types of waves. We first investigate the pump wave in detail, in the case where the background magnetic field is perpendicular to the direction of propagation of the pump wave. Then we derive the dispersion equation relating the pump to the excited wave for modes propagating parallel to the background magnetic field. It is found that there are a total of twelve resonant interactions allowed, whose various growth rates are calculated and discussed.

Theory of dust and dust-void structures in the presence of the ion diffusion.

A dust void is a dust-free region inside the dust cloud that often develops for conditions relevant to plasma processing discharges and complex plasma experiments. A distinctive feature of the void is a sharp boundary between the dust and dust-free regions; this is manifested especially clear when dissipation in the plasma is small and discontinuity of the dust number density appear. Here, the structure of the dust void boundary and the distribution of the dust and plasma parameters in the dust structure bordering the void is analyzed taking into account effects of dissipation due to the ion diffusion on plasma neutrals. The sharp boundary between the dust and void regions exists also in the presence of the ion diffusion; however, only derivatives of the dust density, dust charge, electron density and electric field are discontinuous at the void boundaries, while the functions themselves as well as derivatives of the ion drift velocity and the ion density are continuous. Numerical calculations demonstrate various sorts of diffusive dust void structures; the possibility of singularities in the balance equations caused by the diffusion process inside the dust structures is investigated. These singularities can be responsible for a new type of shocklike structures. Other structures are typically self-organized to eliminate the singularities. Numerical computations in this case demonstrate a set of thin dust layers separated by high density thin dust clouds similar to the multiple-layer dust structures observed in the laboratory and in the upper ionosphere. The possibility for existence of a few equilibrium positions of the void boundary is discussed.

Rotational modes of oscillation of rodlike dust grains in a plasma.

Three dimensional rotatory modes of oscillations in a one-dimensional chain of rodlike charged particles or dust grains in a plasma are investigated. The dispersion characteristics of the modes are analyzed. The stability of different equilibrium orientations of the rods, phase transitions between the different equilibria, and a critical dependence on the relative strength of the confining potential are analyzed. The relations of these processes with liquid crystals, nanotubing, and plasma coating are discussed.

Low-pressure diffusion equilibrium of electronegative complex plasmas.

A self-consistent fluid theory of complex electronegative colloidal plasmas in parallel-plate low-pressure discharge is presented. The self-organized low-pressure diffusion equilibrium is maintained through sources and sinks of electrons, positive and negative ions, in plasmas containing dust grains. It is shown that the colloidal dust grain subsystem strongly affects the stationary state of the discharge by dynamically modifying the electron temperature and particle creation and loss processes. The model accounts for ionization, ambipolar diffusion, electron and ion collection by the dusts, electron attachment, positive-ion-negative-ion recombination, and relevant elastic and inelastic collisions. The spatial profiles of electron and positive-ion-negative-ion number densities, electron temperature, and dust charge in electronegative SiH4 discharges are obtained for different grain size, input power, neutral gas pressure, and rates of negative-ion creation and loss.

Ion-acoustic waves in a complex plasma with negative ions.

A self-consistent theory of linear waves in complex laboratory plasmas containing dust grains and negative ions is presented. A comprehensive model for such plasmas including source and sink effects associated with the presence of dust grains and negative ions is introduced. The stationary state of the plasma as well as the dispersion and damping characteristics of the waves are investigated. All relevant processes, such as ionization, diffusion, electron attachment, negative-positive ion recombination, dust charge relaxation, and dissipation due to electron and ion elastic collisions with neutrals and dust particles, as well as charging collisions with the dusts, are taken into consideration.

Kinetics of plasma flowing around two stationary dust grains.

The characteristics of plasma particle kinetics in the presence of ions flowing around two stationary dust grains aligned in the direction of the flow are studied using a three-dimensional molecular dynamics simulation code. The dynamics of plasma electrons and ions as well as the charging process of the dust grain are simulated self-consistently. Distributions of electron and ion number densities, and the electrostatic plasma potential are obtained for various intergrain distances, including those much less, of the order of, and more than the plasma electron Debye length.

Charge of a macroscopic particle in a plasma sheath.

Charging of a macroscopic body levitating in a rf plasma sheath is studied experimentally and theoretically. The nonlinear charge vs size dependence is obtained. The observed nonlinearity is explained on the basis of an approach taking into account different plasma conditions for the levitation positions of different particles. The importance of suprathermal electrons' contribution to the charging process is demonstrated.