Biorheology

Residual strains in the unloaded human aorta are fundamental for understanding the in vivo stress and strain distribution across the aortic wall, including the intima, media, and adventitia, which is crucial for aortic function.

To report the detailed pattern of axial variation in layer-specific residual stretches and its relationship to age, gender, and circumferential location.

Circumferential and axial strips from the anterior and posterior quadrants of nine aortic levels from twenty-one autopsy subjects were photographed before and after dissection into layers to determine layer-specific zero-stress states. Image analysis was used to assess variations in opening angles, residual stretches, and thickness at each level and direction.

Age-related changes included a significant decline in the axial opening angle of the intact wall, driven by the adventitia. Its axial external residual stretch declined strikingly with age in the descending thoracic and abdominal aorta, outweighing the smaller increase in intact wall external residual stretch. Similarly, the slight age-related increase in intact wall internal residual stretch was more than offset by the significant decrease in circumferential internal residual stretch of the intima in the arch and descending thoracic aorta. The posterior quadrant was thicker and experienced lower external residual strains than the anterior. Gender-related differences were minimal, aside from greater wall thickness in males.

The age-related differences identified, distinguished by direction and layer, are consistent with previous observations on the orientation and age-related depletion of elastin across aortic layers, reinforcing its role as the primary determinant of residual strains.

The viscosity of blood analog fluid (BAF) influences the hemodynamics during testing of medical devices and implants in cardiovascular systems mimicking physiologic flow conditions. BAF, typically composed of water, glycerin, and Xanthan gum, is used to simulate blood's non-Newtonian shear-thinning behavior. Additionally, BAF may include microsphere particles for flow visualization in Laser Doppler Velocimetry (LDV) experiments, though their impact on viscosity remained an under-investigated area. Hypothesis: Addition of particles in the form of polymer microspheres in a BAF solution influences the rheological properties of the fluid.

Three different test fluids comprising of 16 ml BAF solution with the varying concentrations of polymer microspheres were created: 1% (weight/volume; w/v) and 2% w/v represent test samples, whereas 0% w/v (no microsphere) represents a control sample. The viscosities of BAF for with and without polymer microspheres were measured using a concentric cylinder viscometer. Recorded viscosity data was then optimized by fitting the Carreau model.

Seeding of polymer microspheres in BAF results in significant changes in shear-thinning properties, such as zero- and infinite-viscosity ( μ 0 and μ ∞ , respectively), in relation to viscosity of real blood or BAF without microspheres. It is evident that effect of microsphere seeding in BAF is relatively more pronounced on μ 0 as compared to μ ∞ .

Polymer microspheres in BAF significantly alters its shear-thinning properties and must be considered for better hemodynamic evaluations in medical device testing.