Biodegradable microfibrillar polymer-polymer composites from poly(L-lactic acid)/poly(glycolic acid)
eXPRESS Polymer Letters
Stents have been used to treat coronary artery disease (the build-up of plaque in coronary arteries) for decades and they have evolved significantly. The stenting process begins with balloon angioplasty during which a balloon catheter is navigated to the target site and inflated to widen the vessel and flatten the plaque, as illustrated in Figure 1a -1c. A second balloon catheter with a stent over the balloon is then navigated to the target site and deployed via balloon expansion, Figure 1d
... sion, Figure 1d -1e). Finally the balloon is deflated and removed, leaving the stent in place to support the vessel, Figure 1f -1g. Early stents were made of medical grade stainless steel  and are known as bare metal stents (BMSs). Unfortunately the occurrence of restenosis (re-nar-rowing of the vessel via either an inflammatory reaction  or formation of a clot/thrombus ) was common after BMS implantation. For this reason attempts were made at coating stents with various compounds, e.g. gold, silicon carbide, titaniumnitride-oxide, etc. to make them more inert [3, 4] . These stents are known as coated metal stents (CMSs); their efficacy results were mixed -in some cases yielding higher restenosis rates than BMSs  . The next step in stent evolution was the development of drug-eluting stents (DESs) which gradually release pharmaceutical agents after implantation to mitigate restenosis. However, clinical studies revealed that late stent thrombosis rates were higher in the case of DESs when compared with Abstract. Biodegradable coronary stents have been under development for several years and a trend in biodegradable stent material development has emerged: reinforcement to enhance mechanical properties and creep resistance to improve vessel support. The aim of this work is to investigate the mechanical and viscoelastic characteristics of poly(L-lactic acid)/poly (glycolic acid) (PLLA/PGA) microfibrillar polymer-polymer composites (MFCs) at 37°C to determine the suitability of PGA fibrils as a reinforcement for polymeric, biodegradable stents. PLLA/PGA MFCs were produced via cold-drawing and subsequent compression moulding of extruded PLLA/PGA blend wires. Scanning electron microscopy revealed excellent fibril formation in the case of a 70/30 wt% PLLA/PGA MFC-the mean fibril diameter being 400 nm and aspect ratios exceeding 250. Tensile tests demonstrate Young's modulus and strength increases of 35 and 84% over neat PLLA in the case of a 70/30 wt% PLLA/PGA MFC. Creep resistance of the PLLA/PGA MFCs is lower than that of neat PLLA, as shown via relaxation. Dynamic mechanical thermal analysis demonstrates that it is the onset of glass transition of PGA that is the underlying cause for low creep resistance of the PLLA/PGA MFCs at 37°C.