Solid-Phase Orientation (Die Drawing) of Polymer Tubes to Enhance Mechanical Properties of Bioresorbable Vascular Scaffolds

Abstract

Die-drawing, also known as solid phase orientation, is a technique used to enhance the mechanical properties of polymers for non-medical applications [1]. Arterius has adapted and optimised this technology to manufacture state-of-the-art bioresorbable polymeric scaffolds (stents) based on polylactic acid (PLA) for vascular and non-vascular applications. The mechanical properties of the polymer are improved by orientating the polymer chains and introducing crystallinity to the polymer. As the PLA polymer is drawn over a mandrel and through a die, close to the glass transition temperature (Tg > 55 °C), the polymer chains align, gradually increasing orientation with the bi-axial draw, thus promoting significantly improved mechanical properties. In-house testing of the die-drawn tubing compared to the extruded tubing showed an increase in tensile modulus up to 79.7%, yield strength increases of up to 121.5%, ultimate tensile strength increases of up to 267.9% and elongation at break increase of up to 1,022.8%. The die-drawing process also introduces crystallinity into the PLLA tubing, initially the amorphous extruded tube has a crystallinity of around 1.3% which increased to around 40% crystallinity after the die-drawn process. Another attribute of the orientated polymer tubing is the radial strength it provides to the scaffold once it has been laser cut into an innovative closed-cell design, which is comparable to market-leading metallic stents.