2020 Virtual AIChE Annual Meeting

(28h) Effect of Tungsten Disulfide Nanotubes (WSNTs) on Flow-Induced Crystallization of Polylactide (PLA) for Thinner and Stronger Bioresorbable Vascular Scaffolds.

Authors

Di Luccio, T. - Presenter, California Institute of Technology
Ramachandran, K., California Institute of Technology
Shao, Z., Allegro Ophthalmics
Kornfield, J. A., California Institute of Technology
Shen, B., California Institute of Technology
Ruiz Bello, E. E., California Institute of Technology
Bioresorbable Vascular Scaffolds (BVSs) are transient implants designed to replace metal stents for the treatment of Coronary Heart Disease (CHD). BVSs, made from bioresorbable and biocompatible polymer polylactide (PLA), provide strength to the vessel wall to keep the artery open during the first critical 6 months from the implant and are completely resorbed in 2-3 years. However, the widespread adoption of current BVSs is hindered by their thickness (150μm), which is nearly twice that of metal stents (~80μm). A thinner scaffold is easier to implant and can access smaller and more complex lesions.

Towards a thinner yet strong BVS our approach is to enhance radial strength of PLA by promoting oriented crystallization. Here we use tungsten disulfide (WS2) nanotubes (WSNTs) as potential reinforcing agent of PLA. Cytotoxicity assays indicate that WSNTs and PLA-WSNT nanocomposites are well tolerated in vitro by cell lines (HUVEC and HASMC) that are relevant to vascular tissue. Moreover, WSNTs disperse homogeneously in PLA and their high aspect ratio is expected to increase radial strength of the scaffold during processing.

To understand the impact of WSNTs on oriented crystallization under flow, we subject PLA and PLA-WSNTs (0.05wt% and 0.1wt%) to a short-term shear flow (130°C, for 10-40s at a wall shear stress of 0.11-0.23MPa) and probe the evolution of microstructure by in situ rheo-optical experiments. We discovered that the presence of WSNTs favors the formation of "thread-like precursors" as demonstrated by a distinct upturn in the birefringence signal during flow. Thread-like precursors successively grow into "shish-kebabs" as detected by an increase in birefringence after cessation of flow. Both these effects are strongly enhanced by inclusions of WSNTs even at a concentration as low as 0.05wt%. Ex situ polarized light microscopy and X-ray scattering led to a surprising discovery: in both neat PLA and PLA-WSNTs, the flow-induced oriented semicrystalline morphology does not have cylindrical symmetry about the flow direction.

Our results show that a relatively small amount of WSNTs (0.05 wt%) significantly promote oriented crystallization of PLA by reducing the critical shear stress and duration required to produce thread-like precursors that eventually evolve into shish-kebabs. This highly oriented crystalline morphology would dramatically improve the strength of the polymer. Thus PLA-WSNTs might enable a future generation of thinner and stronger BVSs.