Fluid-Structure Analysis of a Collapsible Axial Impeller Pump Cage Design for the Fontan Circulation
Authors:   , Bisirri Evan Alexander , Throckmorton Amy 
Published: July 2021

Of the thousands of children born each year with congenital heart defects, a small cohort have severe malformations of their cardiac chambers leading to a single ventricle physiology. The current treatment standard is a series of palliative surgeries ending with the Fontan procedure which is characterized by connection of the superior and inferior vena cava to the pulmonary arteries. Though lifesaving, this physiology is plagued by a host of complications such as decreased cardiac output, ventricular dysfunction, and thromboembolism. Current pharmacologic and surgical treatments only slow a patient's progression to premature congestive heart failure. Heart transplantation is a viable option, if patients are able to survive the donor organ waiting period. Mechanical circulatory support devices could provide a promising new treatment strategy, though current blood pumps are not designed to support the unique anatomy and physiology of the Fontan circulation. To address the clinical need, this research utilized computational modeling to evaluate the protective cage (stent) of an intravenously deployed collapsible axial flow pump aimed at improving Fontan hemodynamics. The pump functions by rotating a triple bladed impeller housed within the protective cage of four twisted filaments terminating at a set of diffuser blades. Cage expansion relies on the properties of its material composition whether through stored elastic energy or shape memory effects. Paramount to the success of this device is sustention of the expanded geometry. ANSYS CFX and ANSYS Mechanical were employed to characterize the deformation of cages constructed with nitinol and Bionate 80A while steel was used as a reference material. Eight one way coupled fluid structure interaction simulations per material have been designed to examine the cage surface pressure distribution, stress, strain, and resultant deformation for inlet flow rates of 2 and 4 L/min at impeller rotational speeds of 2000, 3000, and 4000 RPM. Free flow conditions featuring the cage alone were also tested to understand the impact of the impeller domain. To develop the simulation meshes, over 70 computational fluid dynamics simulations were carried out as part of a grid independence study. Mesh quality metrics, including aspect ratio, warping factor, and Jacobian ratio, were satisfied for both fluid and structural meshes. Results were extracted from a set of 24 complete simulations. As theoretically expected, the steel cage produced the lowest maximum deformation on a scale less than a millimeter, while the nitinol cage deformed by several millimeters. Despite nitinol's slight deformation increase, the overall cage geometry was minimally altered. In contrast, the Bionate 80A cage produced drastic deformation on the scale of several centimeters. The rotating impeller simulations generated a wide surface pressure range of 4-34 mmHg compared to the free flow simulations, which resulted in a 19-21 mmHg range. The free flow condition produced a uniform pressure distribution over the cage surface, while the rotating conditions caused high peak pressures at the diffuser blades yet an overall decrease in average pressure. These dramatic differences resulted in a minimum increase of 1.5 times the maximum deformation for the free flow simulations compared to the rotating impeller variants. Nitinol's comparable deformation resistance to steel in combination with improved biocompatibility and its shape memory properties establish nitinol as a promising material for further exploration in this application. The Bionate 80A cage deformation was too large, thus alternative materials with greater stiffness should be sought. To further increase deformation resistance it may prove advantageous to either increase filament thickness or to add a fifth filament. Future work should focus on validation via benchtop testing, and additional simulation conditions, such that deformation trends associated with flow rate and impeller rotational speed can be readily identified.