Quantitative and Qualitative Evaluation of Shape Memory Polymer Polyurethane Foam Biodegradation

Abstract

Our laboratory has developed oxidatively degradable shape memory polymer (SMP), polyurethane foam vascular occlusion devices: a peripheral embolization device (PED) that employs a large-pore foam plug and neurovascular embolization device (NED) that uses a small-pore foam-over-wire. Measuring the mass loss and mass loss rate of our devices is critical in evaluating the systemic release rates of degradation products. Traditionally, histology has been used to estimate mass loss in scaffolds, allowing for simultaneous evaluation of degradation and the biologic response to the implant. From histopathologic analyses, it has been determined that the degradation of the material is quantifiable through trans-sectional area measurements. We have developed a method to estimate the error of the measured area loss in histological slides relative to predicting the volumetric mass loss of the whole device. SMP foam models were created to mimic the device geometry and were degraded in Blender to simulate cell-mediated degradation events. Potential in vivo mass loss scenarios based on histopathologic analyses were created, including: 1) random degradation (no volumetric bias), 2) axially-biased degradation, and 3) radially-biased degradation. Additional scenarios were created with variable cell (sphere) sizes to model heterogeneity in cell-material interface. The mass loss of all models was estimated using a varied number of sections. Device geometry and the stage of mass loss influenced the sampling error. These errors are pragmatic constraints for the in vivo method used to estimate mass loss. The in vivo image-based sectional estimation method was used to estimate and characterize the mass loss of NEDs and PEDs. High magnification and high-resolution images of struts and membranes were used to estimate total relative mass loss over time. The NED foams degraded a magnitude faster than the PED. Of note, implant species, implant location, and foam composition were markedly different. To better understand the significant difference in mass loss rates across the devices, we performed homogeneous oxidative degradation studies at varying ROS concentrations with the SMP foams from the PED and NED. The mass loss rates of the samples, including specific analysis of membranes and struts, were captured gravimetrically and with microscopic imaging.