Microstructuring of stainless steel implants by electrochemical etching

The effects of electrochemically enhanced etching on stainless steel coronary stent surfaces have been investigated in respect to their applicability as surface modifications prior drug-coating. Two methods have been investigated, one basing on grain boundary etching with diluted HNO₃ and the other one on hydrochloric acid etching. The etching current was in the range of 30-200 mA which accounts for 0.34-2.28 mA/mm2 surface. Grain boundary etching produced a micro-furrowed surface providing volume for the coating drug. The theoretical volume offered by the furrows was calculated on the basis of laser perthometry and was determined to be 0.146 mm3/cm2. With the hydrochloric acid etching method it was possible to generate an evenly rough, terraced surface. Both surfaces have been coated with Rapamycin in ethanol (20 mg/mL) and examined under SEM after dilatation. It was shown that a uniform drug layer is maintained after dilatation of the stent and little flaking is visible. Quantification of the amount of Rapamycin yielded 21.4 μg/mm2 for the electropolished stents, 36.6 μg/mm2 for the grain-boundary etched stents and 27.7 μg/mm2 for the hydrochloric acid etching after dilatation. For the grain boundary etched stents an improved drug adhesion was found, while the hydrochloric acid etchings resulted in a deterioration of the adhesion properties.