Tom G. Mackay

New polyurethane heart valve prosthesis: design, manufacture and evaluation

In light of the thrombogenicity of mechanical valves and the limited durability of bioprosthetic valves, alternative designs and materials are being considered for prosthetic heart valves. A new tri-leaflet valve, made entirely from polyurethane, has been developed. The valve comprises three thin polyurethane leaflets (approximately 100 microns thick) suspended from the inside of a flexible polyurethane frame. The closed leaflet geometry is elliptical in the radial direction and hyperbolic in the circumferential direction. Valve leaflets are formed and integrated with their support frame in a single dip coating operation. The dipping process consistently gives rise to tolerably uniform leaflet thickness distributions. In hydrodynamic tests, the polyurethane valve exhibits pressure gradients similar to those for a bioprosthetic valve (St Jude Bioimplant), and levels of regurgitation and leakage are considerably less than those for either a bi-leaflet mechanical valve (St Jude Medical) or the bioprosthetic valve. Six out of six consecutively manufactured polyurethane valves have exceeded the equivalent of 10 years function without failure in accelerated fatigue tests. The only failure to date occurred after the equivalent of approximately 12 years cycling, and three valves have reached 527 million cycles (approximately 13 years equivalent). The simplicity of valve manufacture, combined with promising results from in vitro testing, indicate that further evaluation is warranted.

Plane waves with negative phase velocity in Faraday chiral mediums

The propagation of plane waves in a Faraday chiral medium is investigated. Conditions for the phase velocity to be directed opposite to the direction of power flow are derived for propagation in an arbitrary direction; simplified conditions which apply to propagation parallel to the distinguished axis are also established. These negative phase-velocity conditions are explored numerically using a representative Faraday chiral medium, arising from the homogenization of an isotropic chiral medium and a magnetically biased ferrite. It is demonstrated that the phase velocity may be directed opposite to power flow, provided that the gyrotropic parameter of the ferrite component medium is sufficiently large compared with the corresponding nongyrotropic permeability parameters.

Polyurethane heart valves: Fatigue failure, calcification, and polyurethane structure

Six flexible-leaflet prosthetic heart valves, fabricated from a polyetherurethaneurea (PEUE), underwent long-term fatigue and calcification testing. Three valves exceeded 800 million cycles without failure. Three valves failed at 775, 460, and 544 million cycles, respectively. Calcification was observed with and without associated failure in regions of high strain. Comparison with similar valves fabricated from a polyetherurethane (PEU) suggests that the PEU is likely to fail sooner as a valve leaflet. Localized calcification developed in PEUE leaflets at the primary failure site of PEU leaflets, close to the coaptation region of the three leaflets. The failure mode in PEU valves had the appearance of abrasion wear associated with calcification. High strains in the same area may render the PEUE leaflets vulnerable to calcification. Intrinsic calcification of this type, however, is a long-term phenomenon unlikely to cause early valve failure. Both polymers performed similarly during static in vitro and in vivo calcification testing and demonstrated a much lesser degree of calcification than bioprosthetic types of valve materials. Polyurethane valves can achieve the durabilities required of an implantable prosthetic valve, equaling the fatigue life of currently available bioprosthetic valves.

Differentiation between gaseous and formed embolic materials in vivo : application in prosthetic heart valve patients

Background and Purpose Doppler emboli detection is an established technique, but the nature of the underlying embolic material remains unclear. The intensity and spectral distribution of emboli signals could help to distinguish between signals arising from formed and gaseous emboli. We undertook this study to develop and evaluate a differentiation algorithm based on the spectral characteristics of emboli signals. Subsequently the algorithm was applied to patients with mechanical prosthetic cardiac valves. Methods Emboli signals detected in patients with carotid disease, acute stroke, and atrial fibrillation were used as formed emboli data, and signals detected in patients undergoing cardiac catheterization studies were used as gaseous emboli data. For each embolus signal, the maximal amplitude, the sum of amplitudes, and the spectral intensity distribution were calculated. Two hundred emboli signals from each category were used to develop a differentiation algorithm, which was subsequently evaluated on 501 additional solid and 995 gaseous emboli signals. The same algorithm was used to analyze 5958 emboli signals detected in 60 patients with mechanical prosthetic valves. Results The best results were obtained with an algorithm based on both the maximal amplitude and the sum of amplitudes (sensitivity, 99%; specificity, 96.5%). On subsequent evaluation, the sensitivity and specificity of the algorithm were 99.6% and 89.8%, respectively. Of the 5958 emboli signals detected in prosthetic valve patients, 92.4% were classified as gaseous. Conclusions Differentiation between gaseous and formed emboli signals, as detected by transcranial Doppler in vivo, is feasible by means of spectral analysis. Application of the differentiation algorithm in prosthetic valve patients suggests that the embolic material in these patients is gaseous. The possibility of distinguishing between different formed embolic materials with this technique remains to be evaluated.

A limitation of the Bruggeman formalism for homogenization

Abstract The Bruggeman formalism provides an estimate of the effective permittivity of a particulate composite medium comprising two component mediums. The Bruggeman estimate is required to lie within the Wiener bounds and the Hashin–Shtrikman bounds. Considering the homogenization of weakly dissipative component mediums characterized by relative permittivities with real parts of opposite signs, we show that the Bruggeman estimate may not be physically reasonable when the component mediums are weakly dissipative; furthermore, both the Wiener bounds and the Hashin–Shtrikman bounds exhibit strong resonances.

Modeling Chiral Sculptured Thin Films as Platforms for Surface-Plasmonic-Polaritonic Optical Sensing

Biomimetic nanoengineered metamaterials called chiral sculptured thin films (CSTFs) are attractive platforms for optical sensing because their porosity, morphology, and optical properties can be tailored to order. Furthermore, their ability to support more than one surface-plasmon-polariton (SPP) wave at a planar interface with a metal offers functionality beyond that associated with conventional SPP-based sensors. An empirical model was constructed to describe SPP-wave propagation guided by the planar interface of a CSTF-infiltrated with a fluid which supposedly contains analytes to be detected-and a metal. The inverse Bruggeman homogenization formalism was first used to determine the nanoscale model parameters of the CSTF. These parameters then served as inputs to the forward Bruggeman homogenization formalism to determine the reference relative permittivity dyadic of the infiltrated CSTF. By solving the corresponding boundary-value problem for a modified Kretschmann configuration, the characteristics of the multiple SPP modes at the planar interface were investigated as functions of the refractive index of the fluid infiltrating the CSTF and the rise angle of the CSTF. The SPP sensitivities thereby revealed bode well for the implementation of fluid-infiltrated CSTFs as SPP-based optical sensors.

Towards a metamaterial simulation of a spinning cosmic string

The spacetime metric of a spinning cosmic string may be formally represented in flat spacetime by a nonhomogeneous bianisotropic medium. The constitutive parameters of this bianisotropic medium can be established using a noncovariant formalism, thereby paving the way for laboratory simulations of a spinning cosmic string using metamaterial technology.

Gravitation and electromagnetic wave propagation with negative phase velocity

Gravitation has interesting consequences for electromagnetic wave propagation in vacuum. The propagation of plane waves with phase velocity directed opposite to the time-averaged Poynting vector is investigated for a generally curved spacetime. Conditions for such negative-phase-velocity (NPV) propagation are established in terms of the spacetime metric components for general and special cases. Implications of the negative energy density of NPV propagation are discussed.

Towards gravitationally assisted negative refraction of light by vacuum

Mode converting signal launchers for efficient coupling signals between dielectric insular and image waveguides and TEM mode transmission lines. The launcher includes a conductive ground plane and an elongated high permittivity dielectric waveguide adjacent to the ground plane. A TEM mode transmission line provides an elongated conductive strip fixed adjacent to the dielectric waveguide such that a coupling region is formed having a length of at least two times the wavelength in the dielectric waveguide.

Negative phase velocity in a uniformly moving, homogeneous, isotropic, dielectric-magnetic medium

Homogeneous, isotropic mediums characterized by relative permittivity scalars and relative permeability scalars are well known to support the propagation of plane waves with negative phase velocity (NPV), provided that both R 0 and μR > 0), can support NPV propagation when they are viewed in a reference frame which is uniformly translated at a sufficiently high velocity. Representative numerical examples are used to explore the constitutive parameter regimes which support NPV propagation under the uniform-velocity condition.