Comparison of mechanical behavior among the extrapulmonary arteries from rats

Results of comparative tests on pulmonary arteries from untreated Long-Evans rats are presented from three sections of the artery: the trunk, and the right and left main extrapulmonary arteries. Analyses were conducted looking for mechanical differences between the flow (longitudinal) and circumferential directions, between the right and left main arteries, and between each of the mains and the trunk. The mechanical properties of rat pulmonary arteries were obtained with a bubble inflation technique. A flat disk of rat pulmonary artery was constrained at the periphery and inflated, and the geometry of the resulting bubble of material recorded from six different angles. To analyze the data, the area under the stress-strain curve was calculated for each test and orientation. This area, related to the strain-energy density, was calculated at stress equal to 200kPa, for the purpose of statistical comparison. The mean values for the area show that the trunk is less compliant than the main arteries; this difference is supported by histological evidence. When comparing the circumferential and longitudinal properties of the arteries, differences are found for the trunk and left main arteries, but with opposite orientations being more compliant. The mean values for the two orientations for the right main artery are statistically identical. There was indication of significant difference in mechanical properties between the trunk and the main arteries. The left main artery in the circumferential orientation is highly compliant and appears to strongly influence the likelihood that significant differences will exist when included in a statistical population. These data show that each section of the extrapulmonary arterial system should not be expected to behave identically, and they provide the baseline mechanical behavior of the pulmonary artery from normotensive rats.

A microstructural hyperelastic model of pulmonary arteries under normo- and hypertensive conditions

This work represents the first application of a statistical mechanics based microstructural orthotropic hyperelastic model to pulmonary artery mechanics under normotensive and hypertensive conditions. The model provides an analogy between the entangled network of long molecular chains and the structural protein framework seen in the medial layer, and relates the mechanical response at macro-level to the deformation (entropy change) of individual molecular chains at the micro-level. A finite element approach was adopted to implement the model. Material parameters were determined via comparing model output to measured pressure-stretch results from normotensive and hypertensive trunks and branches obtained from a rat model of pulmonary arterial hypertension. Results from this initial study show that this model appears reasonable for the study of hyperelastic and anisotropic pulmonary artery mechanics. Typical tangent modulus values ranged from 200 to 800 kPa for normotensive arteries-this increased to beyond 1 MPa for hypertensive vessels. Our study also provokes the hypothesis that increase of cross-linking density may be one mechanism by which the pulmonary artery stiffens in hypertension.

Bubble-test method for synthetic and bovine vascular material

A measurement system has been designed and constructed at NIST for the study of the mechanical properties of synthetic and bovine vascular materials. The measurement technique was validated on latex, where good agreement was found with the Neo-Hookean model. Measurements were also made on expanded polytetrafluoroethylene, which is commonly used for vascular grafts. The measurements of this material were carried out over a pressure range greater than would be seen in vivo. However, the strains were still small enough to effectively apply the Neo-Hookean model to these data.

Comparison of strength properties of normotensive and hypertensive rat pulmonary arteries

A series of tests were conducted to quantify the difference in the mechanical properties of normo- and hypertensive pulmonary arteries. A bubble-test design was employed to measure the biaxial properties of a segment of artery. The test results compare the properties at multiple orientations of the trunk, right, and left pulmonary arteries from normal (Control) and monocrotaline-treated male Long-Evans wild rats that ranged in age from 8 to 17 weeks old, along with some preliminary results from hypoxic Long-Evans knock-out rats. Data show little difference between the stress-strain relationship of the control pulmonary arteries and that of the monocrotaline-treated pulmonary arteries. However, the preliminary results from the hypoxic pulmonary arteries show that the arterial material strains less before the onset of strain-stiffening behavior. The longitudinal orientation exhibits strain stiffening at lower strains than does the circumferential orientation. The differences between the left and right main arteries are minor. The trunk consistently demonstrates less stiffening in the region of larger strains for all conditions.

Static and quasi-static calibration of a bio-MEMS device

A bio-MEMS device that has been designed to stimulate cells by oscillatory actuation in the vertical direction has been calibrated. The displacement of this device was determined experimentally by a laser interferometer when actuated by a static voltage, and by an atomic force microsope when actuated quasi-statically at 0.1 Hz. Both experimental calibrations were compared to a simple model.

An Experimental Method for Measuring Mechanical Properties of Rat Pulmonary Arteries Verified With Latex

This paper describes a test method for measuring the mechanical properties of small, nonlinear membrane samples from a rat model for pulmonary hypertension. The size and nonlinearity of the pulmonary artery samples poses a challenge for developing a test method that will generate quality, reproducible data in the pressure range experienced by the hypertensive pulmonary artery. The experimental method described here has sufficient precision to yield a combined relative standard uncertainty of 4 %. The method is calibrated against 75 µm thick latex and the data agree well with the neo-Hookian model.

Thermal Conductivity Measurement of an Electron-Beam Physical-Vapor-Deposition Coating

An industrial ceramic thermal-barrier coating designated PWA 266, processed by electron-beam physical-vapor deposition, was measured using a steady-state thermal conductivity technique. The thermal conductivity of the mass fraction 7 % yttria-stabilized zirconia coating was measured from 100 °C to 900 °C. Measurements on three thicknesses of coatings, 170 μm, 350 μm, and 510 μm resulted in thermal conductivity in the range from 1.5 W/(m·K) to 1.7 W/(m·K) with a combined relative standard uncertainty of 20 %. The thermal conductivity is not significantly dependent on temperature.

Thermal Evaluation of Scorched Graphite-Epoxy Panels by Infrared Scanning

A simple measurement system is described for evaluating damage to graphite-epoxy panels, such as those used in high-performance aircraft. The system uses a heating laser and infrared imaging system to measure thermal performance. Thermal conductivity or diffusivity is a sensitive indicator of damage in materials, allowing this thermal measurement to show various degrees of damage in graphite-epoxy composites. Our measurements track well with heat-flux damage to graphite epoxy panels. This measurement system, including analysis software, could easily be used in the field, such as on the deck of an aircraft carrier or at remote air strips.

Thermal-Conductivity Apparatus for Steady-State, Comparative Measurement of Ceramic Coatings

An apparatus has been developed to measure the thermal conductivity of ceramic coatings. Since the method uses an infrared microscope for temperature measurement, coatings as thin as 20 μm can, in principle, be measured using this technique. This steady-state, comparative measurement method uses the known thermal conductivity of the substrate material as the reference material for heat-flow measurement. The experimental method is validated by measuring a plasma-sprayed coating that has been previously measured using an absolute, steady-state measurement method. The new measurement method has a relative standard uncertainty of about 10 %. The measurement of the plasma-sprayed coating gives 0.58 W·m(-1)·K(-l) which compares well with the 0.62 W·m(-1)·K(-l) measured using the absolute method.

Thermal Conductivity of Magnesium Oxide From Absolute, Steady-State Measurements

The thermal conductivity of polycrystalline magnesium oxide has been measured over the temperature range from 400 K to 1300 K using a modified guarded-hot-plate design. Three different thicknesses of specimens having 93 % of theoretical density were tested to verify the operation, accuracy, and reproducibility of our apparatus. The measured thermal conductivity ranges from 30 W · m^-1 · K^-1 down to 8 W · m^-1 · K^-1 and has an inverse-temperature functionality. The results agree well with literature values for this material.