Mechanical and chemical characterisation of bioresorbable polymeric stent over two-year in vitro degradation.

Polymeric stent is a temporary cardiovascular scaffold, made of biodegradable poly (l-lactic) acid, to treat coronary artery stenosis, with expected resorption by the human body over two to three years. In this paper, the mechanical properties of a polymeric stent over two-year in vitro degradation were studied and characterised using atomic force microscopy and nanoindentation techniques, complemented with analyses of weight loss, gel permeation chromatography and differential scanning calorimetry. Atomic force microscopy assessed stent degradation at the surface, whilst nanoindentation was able to investigate the property at a greater depth into the material. No significant changes to the Young's modulus were observed with the atomic force microscopy due to bulk degradation nature of the polymer. Chemical analyses demonstrated a reduction of molecular weight and an increase of crystallinity, indicating degradation of the stents. Berkovich nanoindentation showed a trend of reduction in modulus over in vitro degradation, which was, however, not continuous due to the variations of measurements associated with the pyramidal indenter tip and the semi-crystalline structure of the polymer.