Crystallization and Biocompatibility Enhancement of 3D-printing Poly(L-lactide) Vascular Stents with Long Chain Branching Structures
A series of adjustable long chain branching poly(L-lactide)s (b-PLAs) prepared by reactive processing of linear PLA using pyromellitic dianhydride (PMDA) and polyfunctional epoxy ether (PFE) as branching agent and its fabricated the vascular stents via 3D-printing. FTIR and GPC results indicated the chain branching reaction occurred and the average molecular weight increases obviously after the chain branching reaction. The rheological test of b-PLAs demonstrated the b-PLAs were composed by symmetric or asymmetric-star, or tree-like chains configuration. The volume fraction of branching structure of the chains increased from 0.03 to 0.3 with the branching agent content increasing. The effects of branching structure on melt crystallization behavior of b-PLAs investigated by means of DSC. The isothermal crystallization results showed that the half-time of crystallization (t1/2) of the samples decreased from 16.8 min for linear PLA to 2.3 min as the branching agent content 2 wt% at 106 °C. Also observed using polarized optical microscopy experiments, nucleation density of b-PLAs significantly increased with the volume fraction of branching structure increasing because enrichment of segments around branching structure facilitated nucleation, thus the b-PLAs samples have higher probability form primary nucleus than the linear PLA. Moreover, mechanical testing demonstrated that forming branching structure could be an effective modification of the mechanical properties for PLA. Microstructure with smaller spherulite size and higher crystallinity for b-PLAs improved its the tensile strength and modulus from 45.7 MPa and 1.63 GPa to 77.2 MPa and 3.41 GPa, respectively. Furthermore, the radial force performances of 3D printing b-PLAs vascular stents were enhanced from 4.8 N to 13.7 N by the branching structure of chains. The CCK8 assay results indicated that the osteoblasts activity of b-PLA is higher than that of linear PLA, and SEM results also indicated b-PLA covered and flattened with a better attachment morphology for the osteoblasts than PLA. Therefore, the b-PLAs with long chain branching structure could effectively facilitate cell growth, proliferation, differentiation.