In-vitro calcification study of polyurethane heart valves.

Tri-leaflet polyurethane heart valves have been considered as a potential candidate in heart valve replacement surgeries. In this study, polyurethane (Angioflex(®)) heart valve prostheses were fabricated using a solvent-casting method to evaluate their calcification resistance. These valves were subjected to accelerated life testing (continuous opening and closing of the leaflets) in a synthetic calcification solution. Results showed that Angioflex(®) could be considered as a potential material for fabricating prosthetic heart valves with possibly a higher calcification resistance compared to tissue valves. In addition, calcification resistance of bisphosphonate-modified Angioflex(®) valves was also evaluated. Bisphosphonates are considered to enhance the calcification resistance of polymers once covalently bonded to the bulk of the material. However, our in-vitro results showed that bisphosphonate-modified Angioflex(®) valves did not improve the calcification resistance of Angioflex(®) compared to its untreated counterparts. The results also showed that cyclic loading of the valves' leaflets resulted in formation of numerous cracks on the calcified surface, which were not present when calcification study did not involve mechanical loading. Further study of these cracks did not result in enough evidence to conclude whether these cracks have penetrated to the polymeric surface.

Effects of fluid flow shear rate and surface roughness on the calcification of polymeric heart valve leaflet.

Surface defects, blood flow shear rates and mechanical stresses are contributing factors in the calcification process of polymeric devices exposed to the blood flow. A number of experiments were performed to evaluate the effect of surface defects such as roughness and cracks and flow shear rate on the calcification process of a polyurethane material used in the design of prosthetic heart valves. Results showed that polyurethane surface gets calcified and the calcification is more pronounced at the lower shear rates. Roughness and cracks both increase the calcification levels. The results also suggest very little diffusion of calcium to the subsurface indicating that calcification of a polyurethane material, is a surface phenomenon. Based on a simple peeling test, the bond strength between the calcified layer and polyurethane was found to be extremely weak, suggesting that the bonding is in the form of Van-der-Waals. A limited set of experiments with polycarbonate showed that polycarbonate is less prone to calcification compared to polyurethane (p values less than 0.05), indicating its potential application in medical devices exposed to blood flow.

Effect of pulsatile blood flow on thrombosis potential with a step wall transition.

It is well known that thrombus can be formed at stagnation regions in blood flow. However, studies of thrombus formation have typically focused on steady state flow. We hypothesize that pulsating flow may reduce persistent stagnation at the sites of low shear stress by decreasing exposure time. In this study, a step-wall transition, which is commonly found on implantable devices, is used as a test bed causing a recirculation vortex. Stagnation at such a step is considered using computational fluid dynamics studies and flow visualization experiments. Parametric studies were performed with varying step height, pulsatility, and velocity. The percentage of time along the wall with shear stresses below a threshold for thrombosis and the total length of wall that maintains contact with stagnant flow throughout the cardiac cycle are calculated. Persistent stagnation occurs at the corner of a step-wall transition in all cases and is observed to decrease with a decrease in step height, an increase in mean velocity, and an increase in pulsatility. Under steady flow conditions, a flow reattachment point resulting from recirculation is observed with expanding steps, whereas a flow separation point is observed with contracting steps. Pulsatility decreases persistent stagnation at the flow separation point with contracting steps, whereas it completely eliminates persistent stagnation at the flow reattachment point with expanding steps. The results of this work conclusively show that stagnation can be reduced by increasing pulsatility and flow velocity and by decreasing step height.

In vitro and computational thrombosis on artificial surfaces with shear stress.

Implantable devices in direct contact with flowing blood are associated with the risk of thromboembolic events. This study addresses the need to improve our understanding of the thrombosis mechanism and to identify areas on artificial surfaces susceptible to thrombus deposition. Thrombus deposits on artificial blood step transitions are quantified experimentally and compared with shear stress and shear rate distributions using computational fluid dynamics (CFD) models. Larger steps, and negative (expanding) steps result in larger thrombus deposits. Fitting CFD results to experimental deposit locations reveals a specific shear stress threshold of 0.41 Pa or a shear rate threshold of 54 s(-1) using a shear thinning blood viscosity model. Thrombosis will occur below this threshold, which is specific to solvent-polished polycarbonate surfaces under in vitro coagulation conditions with activated clotting time levels of 200-220 s. The experimental and computational models are valuable tools for thrombosis prediction and assessment that may be used before proceeding to clinical trials and to better understand existing clinical problems with thrombosis.