Image-Based Evaluation of In Vivo Degradation for Shape-Memory Polymer Polyurethane Foam

Abstract

Shape-memory polymer (SMP) polyurethane foams have been applied as embolic devices and implanted in multiple animal models. These materials are oxidatively degradable and it is critical to quantify and characterize the degradation for biocompatibility assessments. An image-based method using high-resolution and magnification scans of histology sections was used to estimate the mass loss of the peripheral and neurovascular embolization devices (PED, NED). Detailed analysis of foam microarchitecture (i.e., struts and membranes) was used to estimate total relative mass loss over time. PED foams implanted in porcine arteries showed a degradation rate of ~0.11% per day as evaluated at 30-, 60-, and 90-day explant timepoints. NED foams implanted in rabbit carotid elastase aneurysms showed a markedly faster rate of degradation at ~1.01% per day, with a clear difference in overall degradation between 30- and 90-day explants. Overall, membranes degraded faster than the struts. NEDs use more hydrophobic foam with a smaller pore size (~150–400 μm) compared to PED foams (~800–1200 μm). Previous in vitro studies indicated differences in the degradation of the two polymer systems, but not to the magnitude seen in vivo. Implant location, animal species, and local tissue health are among the hypothesized reasons for different degradation rates.