In Vitro Evaluation of Paclitaxel-Eluting fully Bioabsorbable Stents with PLLA Coating

A drug-eluting fully bioabsorbable stent which is made of poly(L-lactic acid) (PLLA) and coated with a drug using PLLA as coating substrate was developed. Drug release behavior from the stent was evaluated in vitro in terms of different molecular weights of substrate PLLA and different coating methods. Four different types of coating method including, PLLA spray coating on the PLLA stent followed by immersion in the drug solution (type 1), spray coating of PLLA/drug mixture (type 2), spray coating of PLLA/drug mixture followed by the heating process (type 3), and spray coating of PLLA/drug mixture followed by immersion in the drug solution (type 4), were employed. The inner and surface structures of the spray coated layer varied depending on the molecular weights of PLLA and/or the drug coating methods, and consequently led to the change in the amount of drug contained as well as the drug releasing behavior. Spraying a pure PLLA onto stent created a dense PLLA coating layer, which prevented the invasion of the drug deep in the layer, and the drug was just only adhered to the pure PLLA surface, resulted in the significant reduction of drug content on the stent surface. On the contrary, the spray coating of PLLA/drug mixture provided a porous layer, which allowed for the drug to invade deep into the layer and resulted in increase of drug content. Porosity and the size of the pore tended to decrease with increasing PLLA molecular weight, and drug release rate also decreased accordingly. Additional heat process facilitated PLLA crystallization, leading to significant reduction of the drug release rate.