Remodeling of Murine Branch Pulmonary Arteries Under Chronic Hypoxia and Short-Term Normoxic Recovery.

Chronic hypoxia plays a central role in diverse pulmonary pathologies, but its effects on longitudinal changes in the biomechanical behavior of proximal pulmonary arteries remain poorly understood. Similarly, effects of normoxic recovery have not been well studied. Here, we report hypoxia-induced changes in composition, vasoactivity, and passive biaxial mechanics in the main branch pulmonary artery of male C57BL/6J mice exposed to 10% FiO2 for 1, 2, or 3 weeks. We observed significant changes in extracellular matrix, and consequently wall mechanics, as early as 1 week of hypoxia. While circumferential stress and stiffness returned toward normal values by 2-3 weeks of hypoxia, area fractions of cytoplasm and thin collagen fibers did not return toward normal until after 1 week of normoxic recovery. By contrast, elastic energy storage and overall distensibility remained reduced after 3 weeks of hypoxia as well as following 1 week of normoxic recovery. While smooth muscle and endothelial cell responses were attenuated under hypoxia, smooth muscle but not endothelial cell responses recovered following 1 week of subsequent normoxia. Collectively, these data suggest that homeostatic processes were unable to preserve or restore overall function, at least over a brief period of normoxic recovery. Longitudinal changes are critical in understanding large pulmonary artery remodeling under hypoxia, and its reversal, and will inform predictive models of vascular adaptation.

Evolving structure-function relations during aortic maturation and aging revealed by multiphoton microscopy.

The evolving microstructure and mechanical properties that promote homeostasis in the aorta are fundamental to age-specific adaptations and disease progression. We combine ex vivo multiphoton microscopy and biaxial biomechanical phenotyping to quantify and correlate layer-specific microstructural parameters, for the primary extracellular matrix components (fibrillar collagen and elastic lamellae) and cells (endothelial, smooth muscle, and adventitial), with mechanical properties of the mouse aorta from weaning through natural aging up to one year. The aging endothelium was characterized by progressive reductions in cell density and altered cellular orientation. The media similarly showed a progressive decrease in smooth muscle cell density and alignment though with inter-lamellar widening from intermediate to older ages, suggesting cell hypertrophy, matrix accumulation, or both. Despite not changing in tissue thickness, the aging adventitia exhibited a marked thickening and straightening of collagen fiber bundles and reduction in cell density, suggestive of age-related remodeling not growth. Multiple microstructural changes correlated with age-related increases in circumferential and axial material stiffness, among other mechanical metrics. Because of the importance of aging as a risk factor for cardiovascular diseases, understanding the normal progression of structural and functional changes is essential when evaluating superimposed disease-related changes as a function of the age of onset.

Biomechanical characterization of murine pulmonary arteries.

The biomechanical properties of the major pulmonary arteries play critical roles in normal physiology as well as in diverse pathophysiologies and clinical interventions. Importantly, advances in medical imaging enable simulations of pulmonary hemodynamics, but such models cannot reach their full potential until they are informed with region-specific material properties. In this paper, we present passive and active biaxial biomechanical data for the right and left main pulmonary arteries from wild-type mice. We also evaluate the suitability of a four-fiber family constitutive model as a descriptor of the passive behavior. Despite regional differences in size, the biaxial mechanical properties, including passive stiffness and elastic energy storage, the biaxial wall stresses at in vivo pressures, and the overall contractile capacity in response to smooth muscle cell stimulation under in vivo conditions are remarkably similar between the right and left branches. The proposed methods and results can serve as baseline protocols and measurements for future biaxial experiments on murine models of pulmonary pathologies, and the constitutive model can inform computational models of normal pulmonary growth and remodeling. Our use of consistent experimental protocols and data analyses can also facilitate comparative studies in health and disease across the systemic and pulmonary circulations as well as studies seeking to understand remodeling in surgeries such as the Fontan procedure, which involves different types of vessels.

Biaxial mechanics of 3D fiber deposited ply-laminate scaffolds for soft tissue engineering part I: Experimental evaluation.

Tissue engineering strategies require the provision of a micromechanical state of stress that is conducive to the generation and maintenance of healthy mature tissue. Of particular interest, angle-ply biomimetic scaffolds augmented with cellular content have been proposed for annulus fibrosus (AF) engineering in order to repair the intervertebral disc. However, the influence of the inherent variability of fabricated constructs and physiological conditions on overall scaffold mechanics, micromechanical environment within the scaffold, and consequent cellular differentiation is relatively unknown. In this study, melt extrusion 3D fiber-deposition (3DF) was used to fabricate five different polycaprolactone angle-ply scaffold architectures which were subject to multiaxial tensile testing and linear elastic orthotropic constitutive fitting. All scaffold groups predicted stiffnesses similar to previously reported native AF moduli in biaxial and uniaxial tensile strain. However, no single scaffold group in this study simultaneously achieved all target AF mechanics in all loading regimes. In equibiaxial tension, the biaxial stiffness ratio of native AF (EEr = 0.55 to 0.62) was predicted between fiber angles of 30° and 35°, which is similar to the collagen orientation in native AF. In global equibiaxial loading, an apparent asymptote in the transverse moduli (EEx ranging -380 MPa to 700 MPa) was observed near the 40° fiber angle scaffolds in equibiaxial tensile strain, attributed to stiffening from the transverse loading. These results highlight that tissue engineering scaffold designs should target replication of physiologically-relevant native tissue mechanics and demonstrate the importance of designing constructs that are unaffected by anticipated variations in manufacturing and clinical application.

Heart valve scaffold fabrication: Bioinspired control of macro-scale morphology, mechanics and micro-structure.

Valvular heart disease is currently treated with mechanical valves, which benefit from longevity, but are burdened by chronic anticoagulation therapy, or with bioprosthetic valves, which have reduced thromboembolic risk, but limited durability. Tissue engineered heart valves have been proposed to resolve these issues by implanting a scaffold that is replaced by endogenous growth, leaving autologous, functional leaflets that would putatively eliminate the need for anticoagulation and avoid calcification. Despite the diversity in fabrication strategies and encouraging results in large animal models, control over engineered valve structure-function remains at best partial. This study aimed to overcome these limitations by introducing double component deposition (DCD), an electrodeposition technique that employs multi-phase electrodes to dictate valve macro and microstructure and resultant function. Results in this report demonstrate the capacity of the DCD method to simultaneously control scaffold macro-scale morphology, mechanics and microstructure while producing fully assembled stent-less multi-leaflet valves composed of microscopic fibers. DCD engineered valve characterization included: leaflet thickness, biaxial properties, bending properties, and quantitative structural analysis of multi-photon and scanning electron micrographs. Quasi-static ex-vivo valve coaptation testing and dynamic organ level functional assessment in a pressure pulse duplicating device demonstrated appropriate acute valve functionality.