(C-100) Experimental and Numerical Investigation of the Crimping Process for 3D Printed Bioresorbable Vascular Scaffolds

With coronary and peripheral artery disease affecting 8-12 million and 18.2 million Americans respectively, vascular stents have become critical to the healthcare of many^{1, 2}. In recent years, this stenting process has seen significant drive toward the development of polymeric bioresorbable vascular scaffolds (BVS) in order to reduce stent migration rates and to further address restenosis and thrombosis concerns arising when utilizing traditional metal stents. With this shift toward polymeric scaffolds with new mechanical properties, new design and manufacturing paradigms are possible. We have in the past demonstrated our capability to rapidly 3D print BVS with clinically relevant radial strength within 13 minutes^{3, 4}. However, to fully harness the potential of this manufacturing technique, understanding the fundamental mechanics of BVS performance during crimping and later deployment processes in vivo is crucial and currently under-researched. The crimping process, or diameter reduction of the BVS prior to its deployment, serves as a good testing point for understanding BVS deformation, as it is the first time in its lifespan that the BVS must undergo significant deformation. To provide rapidly acquired design information, we have developed a simulation which is able to qualitatively reproduce experimentally validated trends in radial strength between varying BVS designs in a fraction of the time that would be required by experiment. This simulation can be used to give additional information on BVS responses to deformation, thereby allowing for more informed design of BVS for deformation and higher potential for in vivo BVS viability.

An ABAQUS/Standard simulation based on existing literature was created for several in-house developed BVS designs. This simulation contains a twelve plate surface assembly which displaces in the BVS’s radial direction. Via contact with these plates, the BVS displaces radially, shrinking in diameter in a simulated crimping procedure^5-8. Radial strength of the BVS is obtained by analysis of contact forces on each node of the rigid plate assembly. In both simulation and experiment, four designs of BVS were used: an “arrowhead” BVS designed for rigidity alongside three “wave” BVS designs of radial thicknesses 80 µm, 110 µm, and 155 µm which were designed for flexibility. Experimental results were obtained via 3D printing of a methacrylated poly(1,12 dodecamethylene citrate)-based resin utilizing an in-house micro-continuous liquid interface production printer with a lateral resolution of 7.1 µm x 7.1 µm and a layer thickness of 5 µm. BVS were postprocessed under UV flood and heated in an oven prior to being hydrated in phosphate-buffered saline and crimped with an RX650 Radial Force Tester (MSI; Flagstaff, AZ).

Simulation of BVS crimping yields accurate qualitative analysis of relative performance of designs. Differences in radial force with variation as small as 30 µm in radial thicknesses can reliably be reproduced via the use of simulation. Simulation analysis can be completed in under five hours, as opposed to an experimental process time of approximately 36 hours, resulting in a seven-fold increase in speed of design analysis. In both experiment and simulation, arrowhead BVS designs which were designed for rigidity showed higher radial strength than wave BVS designs which were designed for crimping flexibility; however, the arrowhead designs also exhibited much higher and more concentrated maximum Von Mises stresses for the same amounts of crimping as the wave designs, indicating that they are not well suited for applications where significant amounts of crimping are necessary. This work establishes a framework for numerically predicting the crimping behavior of 3D printed BVS and can potentially be more broadly applied to other 3D printed polymeric deployable devices. It enables quick design iteration without relying on time consuming trial-and-error approaches in developing BVS designs with the optimal clinical outcome.

The authors gratefully acknowledge funding from NIH grant #R01HL141933. Additionally, the authors gratefully acknowledge Dr. Henry Oliver Tenadooah Ware for his CAD work for the stents utilized on this project.

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