Characterisation and modelling of Poly (L-Lactic acid) for stretch blow moulding of bioresorbable vascular scaffolds

  • Huidong Wei

Student thesis: Doctoral ThesisDoctor of Philosophy


Bioresorbable scaffolds (BVS) made from biodegradable polymers offer an alternative to metal stents with a great advantage of treating coronary heart disease. As one of the most widely used base material, poly (1-lactic acid) (PLLA) shows excellent biodegradable and biocompatible properties whilst it is a rigid polymer for a good candidate material.

In contrast to the metal product, the mechanical performance of PLLA BVS is much weaker due to the brittle behaviour and reduced stiffness. The processing technique, stretch blow moulding is introduced in the manufacturing stage to tailor the morphology of PLLA by strain induced orientation and crystallisation. The mechanical behaviour of PLLA above glass transition temperature (Tg) in stretch blow moulding is poorly studied and there are a lot of trial and errors for the medical device industry to develop the suitable and optimal processing conditions. The aim of this work is to gain the knowledge on mechanical behaviour of PLLA in stretch blow moulding (SBM) by characterisation and modelling of the material at conditions relevant to SBM.

The thermal and mechanical behaviour of PLLA were characterised to acquire the processing window and viscoelasticity at different conditions. The high rate deformation by biaxial stretch testing was employed to mimic the blow moulding process. A constitutive model-glass (GR) rubber model was calibrated by the mechanical data to capture the nonlinear viscoelasticity of PLLA at biaxial stretch.

A direct investigation on the deformation behaviour of PLLA was conducted in a free stretch blow test on PLLA tubes by developing a test rig. In the experimental setup, isothermal conditions was provided by a water bath and strain measurement by digital image correlation (DIC). The influence from processing temperature, pressure and sequence of operation on the forming process was studied on the customised rig by a qualitative and quantitative way, which was highlighted by the strong nonlinear strain evolution during forming process with very high maximum strain rate.

The applicability of the constitutive model of PLLA was examined by a replicative biaxial stretch testing with the strain history from blowing process. The different strain hardening properties highlighted the influence of processing history of PLLA before biaxial deformation on the mechanical behaviour above Tg, thus emphasizing the direct investigation of deformation of PLLA tube parison by free stretch blow. The constitutive model was subsequently modified and implemented into finite element analysis (FEA) for the purpose of process simulation and validation of free stretch blow, where the good agreement demonstrated the potential of material modelling for the application of improving the design and manufacture of BVS.
Date of AwardDec 2019
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsEC/Horizon 2020 Marie Skłodowska-Curie actions
SupervisorGary Menary (Supervisor)

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