Modelling Stretch Blow Moulding of Poly (l-lactic acid) for the Manufacture of Bioresorbable Vascular Scaffold.

Huidong Wei,Shiyong Yan,Gary Menary

doi:10.3390/polym13060967

Abstract

Stretch blow moulding (SBM) has been employed to manufacture bioresorbable vascular scaffold (BVS) from poly (l-lactic acid) (PLLA), whilst an experience-based method is used to develop the suitable processing conditions by trial-and-error. FEA modelling can be used to predict the forming process by the scientific understanding on the mechanical behaviour of PLLA materials above the glass transition temperature (Tg). The applicability of a constitutive model, the ‘glass-rubber’ (GR) model with material parameters from biaxial stretch was examined on PLLA sheets replicating the biaxial strain history of PLLA tubes during stretch blow moulding. The different stress–strain relationship of tubes and sheets under equivalent deformation suggested the need of re-calibration of the GR model for tubes. A FEA model was developed for PLLA tubes under different operation conditions, incorporating a virtual cap and rod to capture the suppression of axial stretch. The reliability of the FEA modelling on tube blowing was validated by comparing the shape evolution, strain history and stress–strain relationship from modelling to the results from the free stretch blow test.

Highlights

Bioresorbable vascular scaffolds (BVSs) from poly (l-lactic acid) (PLLA) was considered to be a new-generation cardiovascular medical device for its ability of decomposing into lactic acid and being absorbed inside the body after the remodelling of an artery [1,2,3].The bioresorbable behaviour offers a big advantage over the permanent metal scaffolds by providing the option of interventional treatment on the occasions of further formation of plaque [4]
For the SIM process at 77 ◦ C (T77SIMP6), an early onset of inflation process was observed with the accomplishment of forming within 1.3 s (Figure 4b). This instant strain change was replicated in the biaxial stretch test by a maximum strain rate of 10 s−1, where a more evident nonlinear increase of axial strain was observed for both tube blowing and biaxial tests
The different mechanical behaviour of PLLA tubes and sheets highlighted the effect of processing history of raw materials on the mechanical performance of products for subsequent manufacture and the need of experimental characterisation on the behaviour of tubes [26]

Summary

Introduction

Bioresorbable vascular scaffolds (BVSs) from poly (l-lactic acid) (PLLA) was considered to be a new-generation cardiovascular medical device for its ability of decomposing into lactic acid and being absorbed inside the body after the remodelling of an artery [1,2,3].The bioresorbable behaviour offers a big advantage over the permanent metal scaffolds by providing the option of interventional treatment on the occasions of further formation of plaque [4]. A concern on using PLLA BVSs was raised for the thick struts (of 150 μm) rather than metal scaffolds (of 80 μm) due to the weak mechanical performance [5]. This disadvantage resulted in a big profile of scaffolds, leading to difficult deployment and high risk of plaque formation by disturbing the blood flow [6], which significantly restricted the clinical applications [7,8]. The PLLA material experiencing SBM gains the orientation and crystallisation of the morphology, and the stiffness, strength and ductility are significantly improved, which can be further machined by a femtosecond laser [11,13,14]

Methods

Results

Conclusion