ELECTROCHEMICAL POLISHING OF MATRIX STENTS OF THE 316LVM STEEL USING MICROSECOND PULSES

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.

As a pretreatment before ECP to remove burr formed after laser cutting, acid etching was carried out in a solution of the composition HF (1 %), HNO 3 (9 %), and H 2 O (90 %) at 45 °C for 15 min in an ultrasonic bath. After acid etching, the stents were washed in water [27]. The roughness of the surface of the initial samples was Ra 0.22 μm.
ECP of the samples was performed in an electrolyte of the following composition: 50 % -phosphoric acid, 25 %sulfuric acid, 20 % -glycerol, 5 % -distilled water. The electrolyte temperature was maintained within 33 ± 5 °С. The required value of the current density was set by changing the voltage. To process the samples, we used experimental equipment consisting of an adjustable constant voltage source [29], a block of the system for forming pulses of a given duration, a bath with devices for heating, cooling, and stirring the electrolyte (Figure 2). The measurement of the surface roughness of the samples before and after processing was carried out using an MII-4 micro interferometer. Electron microscopic photographs of the surface of the samples before and after processing were obtained on a VEGA II LMV scanning electron microscope. Optical photographs of the surface of thin sections of the samples before and after processing were obtained using an Altami MET1 metallographic microscope. The shape of the current pulses was monitored with an S8-46/1 oscilloscope. The mass was measured with an Ohaus PA214C weigh-scales. The control of corrosion resistance was carried out using a PI-50-Pro-3 pulse potentiostat-galvanostat in a 0.9 % sodium chloride solution.
For the treatment duration t, the values of 2, 4, 6, 8, 10, and 12 min were taken. The amplitude of current density for the pulsed ECP was 1.3 A/cm 2 , and for the ECP at a direct current was of 0.65 A/cm 2 . The accepted values of the current density ensure the equality of the amount of technological current for the investigated processes of pulsed ECP (PECP) and ECP at direct current ( Figure 3). The duration of the no-current pause between pulses, equal to half of the period T, is compensated by twice the amplitude of the current I PECP at a pulsed ECP relative to the ECP on a direct current I ECP . Results and discussion. To establish the pulse duration that provides the best surface quality (the smallest surface roughness), the samples were preliminarily processed with the following characteristics: pulse duration τ = 10, 20, 40, 80, 160 and 320 μs; duty cycle S = 2; peak current density of pulses i = 1.3 А/cm 2 ; processing time t = 8 min.
The obtained dependence of the change in the surface roughness of the samples ΔRa on the pulse duration τ during the pulsed ECT is shown in Figure 4. The maximum improvement in surface quality is observed at a pulse duration of 40 µs. High processing quality at this value of the pulse duration is achieved by creating the most favorable conditions under which comparable and high rates of anodic dissolution and surface passivation are provided, which makes it possible to completely remove dissolution products and restore the concentration of active components in the processing zone and ensure the maximum rate of smoothing of the surface microrelief with minimal material removal. Later in this work, when processing samples in the pulsed mode, the value of the pulse duration was taken to be 40 μs. Figure 5 shows the results of experimental studies of the effect of treatment duration t on the change in surface roughness and removal of stent material in the process of pulsed ECP and ECP at direct current. From the presented dependencies it can be seen that when using both the pulsed ECP mode and the traditional ECP using direct current, an approximately equal change in the roughness parameter Ra takes place. In this case, the removal of material ∆m from the treated surfaces of the stent for the pulsed mode is, on average, 2.5 times less than for the constant current mode. Less removal with an equal amount of process current is explained by the passivation of the treated surface during the no-current pause between pulses. Accordingly, when a pulse following the pause occurs, electrochemical dissolution does not begin instantly, but with a certain delay, commensurate with the duration of the acting pulse, which has the main effect on the extraction. Intensive smoothing of the microrelief at low metal removal in the pulsed mode is associated with the accelerated dissolution of primarily polarized microprotrusions on the treated surface as compared to passivated micro depressions. Quantitatively, the efficiency of ECP of stent surfaces using a pulsed mode can be estimated by the ratio of the achieved change in roughness to the mass of metal removed during processing. As shown in Figure 6 of the diagram, it can be seen that in general, both for the pulsed mode and for the mode at constant current, with an increase in the duration, the processing efficiency decreases. At the same time, the efficiency of ECP with the use of the pulsed mode is on average 2.3 times higher than with the traditional ECP using direct current. In the process of stent implantation using a high-pressure balloon (up to 12 atm), there is a possibility of vascular wall perforation. Therefore, the edges of the stent frame elements must be rounded [31]. On the other hand, excessive rounding of the edges, along with a decrease in the cross-section of the frame elements, leads to a significant decrease in the stiffness of the stent and, accordingly, to the loss of its ability to reinforce the vascular wall. In modern stent production processes, the main edge rounding is provided by shot blasting, prior to electrochemical polishing [5]. Therefore, directly at the stage of electrochemical polishing, significant rounding of the edges is undesirable (more than 20 microns). The use of the pulsed mode allows reducing of the rounding of edges during the polishing of stents (Figure 7). The value of the radius of rounding of the edges r after processing in the pulsed mode is on average 15 % less than when processing with direct current. This is achieved due to significantly less metal removal in the process of pulsed ECP compared to DC ECP.
From those presented in Figures 5-7 experimental dependencies, it follows that to ensure a high quality of the stents surface under the condition of small removal with a slight rounding of the edges (10-15 µm), the required duration of the pulse ECP is 6-8 min. Figure 8 shows polarization curves obtained in 0.9 % sodium chloride solution for initial stent samples and stent samples after pulsed ECP and direct current ECP for 6 min. It was found that the type of technological current used in the process of ECP of stents does not affect the protective properties of the surface. Pulsed ECP, as well as traditional ECP using direct current, leads to a significant increase in the pitting potential ϕ of the studied stent samples -from 770 to 1370 mV.
Micrographs of the initial stent samples and samples after a 6-min pulse ECP are shown in Figure 9. The surface of the original stent samples is characterized by the presence of scratches and other defects formed as a result of preliminary mechanical, sandblasting, and chemical etching. The edges have laser-cut burrs. As a result of the pulsed ECP, the relief of microroughness is smoothed out, and a smooth surface that does not contain defects is formed. In this case, the rounding of the edges is insignificant (10-15 µm). Conclusion. The use of pulsed modes in the process of ECP for products of small cross-section and stiffness, for which excessive metal removal is not allowed, such as coronary stents, allows high-quality surface finishing with high efficiency, at which the intensity of smoothing the microroughness of the treated surface, referred to the total metal removal, increases significantly in comparison with traditional ECP using direct current. The efficiency of ECP in the pulsed mode is, on average, 2.3 times higher than with traditional ECP using direct current.
According to the results of the studies performed, it was found that the high quality of the surface of stents made of the 316LVM corrosion-resistant steel under the condition of low removal (11-14 %) with a slight rounding of the edges is achieved in the process of a pulsed ECP with a pulse duration of 40 μs, a current density of 1.3 А/cm 2 with a duration processing 6-8 min. Processing with such characteristics leads to an improvement in the surface roughness Ra of 0.12-0.13 μm, the formation of a smooth surface, the removal of scale and burrs formed during laser cutting, as well as an increase in the pitting potential in comparison with the initial state from 770 to 1370 mV.