Electrochemical polishing of matrix stents of the 316LVM steel using microsecond pulses

Abstract

With the development of minimally invasive treatment technology, coronary stents made of corrosion-resistant steel are in demand for restoring the patency of blood vessels. The effectiveness of coronary stenting depends on various factors, but the quality of the surface of the stents is a major factor. The higher the quality of the surface of the stent is, the less negative the effect on the circulatory system, arterial walls, and the higher the biocompatibility of the stent is. The complex shape, small cross-section, size, and low rigidity of coronary stents are the main reasons for the inability to ensure high surface quality using mechanical finishing methods. Therefore, electrochemical methods are used to polish stents. For electrochemical polishing (ECP) of stents, an electric mode based on direct current is traditionally used. The disadvantages of direct current ECP are excessive metal removal and the need to use electrolytes of complex compositions, often containing toxic components. As an alternative to the traditional ECP with the use of direct current, we have proposed a method of pulsed ECP using pulses of microsecond duration for polishing stents. The use of pulsed current allows one to achieve a significant increase in the efficiency of the SEC process, when, due to the localization of the anodic dissolution, the smoothing speed of the microroughness of the treated surface, referred to the total metal removal, increases significantly. The paper presents a comparative analysis of ECP modes using direct and pulse current to change the surface roughness, removal, radius of curvature of the edges, and corrosion resistance on the example of stents made of the 316LVM stainless steel. Based on the results of the studies, technological regimes of pulsed ECP were established that provide the highest quality polishing of the stent surface with a small metal removal with a slight rounding of the edges.

Electropolishing and Shaping of Micro-Scale Metallic Features

“… Micro-photographs of stents before ( a – c ) and after ( d – f ) EP showing scratches, oxide and slag [ 56 ]; ( g ) XPS analysis of mechanically polished Nitinol implant [ 162 ] (open access article distributed under the Creative Commons Attribution License). …”

“…This is in contrast to other manufacturing processes, such as micro-milling, UV lithography, laser polishing, and chemical etching, none of which can provide the same accuracy in shaping micro-features, due to the limitations of tool size and wear, the rectilinear nature of light, and surface defects and stresses that are induced at tool–micro-feature interfaces [ 47 , 48 , 49 , 50 , 51 , 52 ]. Moreover, electrochemical polishing is capable of removing material at controlled rates using various waveforms: pulsed current (PC) and pulse reverse pulse current (PPR) can help maintain the structural integrity of micro-scale features [ 53 , 54 , 55 , 56 , 57 ]. The critical applications for the electrochemical polishing of micro-structures include TEM sample preparation [ 29 , 58 , 59 ], AM parts [ 12 , 60 , 61 , 62 ], biomedical stents and implants [ 7 , 17 , 54 , 56 , 63 ], and RF MEMS or MOEMS [ 64 , 65 , 66 , 67 , 68 ], which will be explained later in this review.…”

“…Moreover, electrochemical polishing is capable of removing material at controlled rates using various waveforms: pulsed current (PC) and pulse reverse pulse current (PPR) can help maintain the structural integrity of micro-scale features [ 53 , 54 , 55 , 56 , 57 ]. The critical applications for the electrochemical polishing of micro-structures include TEM sample preparation [ 29 , 58 , 59 ], AM parts [ 12 , 60 , 61 , 62 ], biomedical stents and implants [ 7 , 17 , 54 , 56 , 63 ], and RF MEMS or MOEMS [ 64 , 65 , 66 , 67 , 68 ], which will be explained later in this review. There is also potential for electropolishing to be used as a shaping and forming process in the manufacturing of micro-mold tool inserts in order to boost the performance of micro-injection molding by preventing demolding problems.…”