Characterisation and constitutive modelling of biaxially stretched poly(L-lactic acid) sheet for application in coronary stents.

A poly(L-lactic acid) stent is exposed to a variety of processing techniques, temperatures and environmental conditions during its lifecycle, from the manufacturing process, to crimping through to deployment within the body. The effect of the biaxial stretching procedure and the effects of temperature and extension rate (post-processing) on the mechanical response of poly(L-lactic acid) are hereby investigated, and a constitutive model calibrated against experimental data is proposed. Dumb-bell specimens were punched from biaxially stretched sheets subjected to different processing histories, and tested under uniaxial tension at various temperatures (20, 37 and 55 °C) and extension rates (1, 5 and 10 mm/min). A Design of Experiments methodology was employed to identify the parameters that had the most significant effect on the mechanical response of the polymer. Results show that the elastic modulus and yield strength of the stretched sheets are strongly dependent on the aspect ratio of the biaxial deformation, along with the temperature during uniaxial deformation (post-processing). In contrast, these mechanical properties were not heavily dependent on extension rate (post-processing). A transversely isotropic, elastic-plastic constitutive model for finite element implementation is proposed, with the intention that it may be used as a design tool for developing high stiffness, thin-strut polymeric stents that contend with the performance of their metallic counterparts.