Computational Analysis of the Effects of Fiber Deformation on the Microstructure and Permeability of Blood Oxygenator Bundles.

Mechanical loads on the polymeric fibers of oxygenating bundles are commonly present due to bundle press-fitting during device assembly and blood pressure load. However, computational fluid dynamics (CFD) simulations for fiber bundle optimization neglect possible changes in microstructure due to such deformations. The aim of this study is to investigate the impact of fiber deformability on bundle microstructure and fluid dynamics mainly in terms of permeability. Fibers from commercial mats typically used for blood oxygenators were mechanically tested and based on these experimental data, a material model was developed to simulate the structural deformations the fibers undergo under press-fitting and blood pressure loads. Then, CFD simulations were performed on deformed bundle repetitive units to investigate permeability under varying loading conditions. The effects of different bundle geometric parameters on the variation of bundle permeability due to press-fitting were evaluated. Bundle press-fitting results in significant changes in microstructure that are reflected in a bundle permeability more than halved for a 15% press-fitting. This impact on permeability is present in all the simulated fiber bundles and becomes more pronounced as the pitch between fibers and thus bundle porosity decreases. Instead, the analyses on pressurized bundle show only small deformations caused by pressure load, with permeability changes below 1%. While blood pressure effects could be neglected, bundle press-fitting turns out to have a significant impact on bundle microstructure and permeability. Neglecting such microstructure variations during CFD simulations could also lead to incorrect assessment of the local fluid dynamics within the bundle.

Assessment of a fully-parametric thoraco-lumbar spine model generator with articulated ribcage.

The present paper describes a novel user-friendly fully-parametric thoraco-lumbar spine CAD model generator including the ribcage, based on 22 independent parameters (1 posterior vertebral body height per vertebra + 4 sagittal alignment parameters, namely pelvic incidence, sacral slope, L1-L5 lumbar lordosis, and T1-T12 thoracic kyphosis). Reliable third-order polynomial regression equations were implemented in Solidworks to analytically calculate 56 morphological dependent parameters and to automatically generate the spine CAD model based on primitive geometrical features. A standard spine CAD model, representing the case-study of an average healthy adult, was then created and positively assessed in terms of spinal anatomy, ribcage morphology, and sagittal profile. The immediate translation from CAD to FEM for relevant biomechanical analyses was successfully demonstrated, first, importing the CAD model into Abaqus, and then, iteratively calibrating the constitutive parameters of one lumbar and three thoracic FSUs, with particular interest on the hyperelastic material properties of the IVD, and the spinal and costo-vertebral ligaments. The credibility of the resulting lumbo-sacral and thoracic spine FEM with/without ribcage were assessed and validated throughout comparison with extensive in vitro and in vivo data both in terms of kinematics (range of motion) and dynamics (intradiscal pressure) either collected under pure bending moments and complex loading conditions (bending moments + axial compressive force).