5.1 Microstructure Analysis
As many articles have discussed the toxicity of the stainless steel gradually released after a period of time in vivo. These may be due to strength of the corrosion resistance and anti-abrasion ability [52]. Electrochemistry is a quite efficient way to increase its corrosion resistance and anti-abrasion ability.
The widely used of 316L is in the coronary stent, Humbeeck et al [53], done a series of tests about the surface roughness via electrochemistry.
The result of the roughness experiment showed in the data in the previous section. 5V-20 min treatment has the most porous structure with Ra 99.959 nm and unclear grain boundaries were observed. The porous also appeared as multiple layers compared to the lower voltage samples. In the contact angle experiment, the angle of all the samples are determined to be almost identical, with the exception of 5V-20 min treatment has a lower contact angle than others. Nevertheless, it is interesting to find out the 5V-40 min treatment resulted a greater contact angle than the 5V-20 min treatment. Prolong the voltage time may cause the number of porous to increase, the numerous pores
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then act like the lotus leafs with nano-scale fibers which can repel liquid easily.
Comparing the SEM data with those taken by Humbeeck [53], the electrochemically treated samples appear to be far smoother than the other techniques used, such as laser cut or acid-pickling.
Electrochemical polishing is a method of brightening and smoothering the surface of metals [54, 55] by immersing the parts in an electrolyte and applying direct positive current to the sample. The main electrical parameters of electrochemical polishing process are the anodic potential, the anodic current density and the applied voltage. The nature and rate of any electrochemical reaction are both determined by the electrode potential.
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5.2 Hemocompatibility
In the first 20 minutes, the treated samples of the 316L stainless steel performed astonishing result in the blood compatibility experiment. The red blood cells were not deformed or denatured, but the effect was not obvious to the fibrins. However, the blood cells started to denature after 5V-40 min treatment, and significant differences cannot be seen among various treatment except the 5V-40 min one showed good blood compatibility better than 5V-20 min ones. It still may due to its main limitation is the tendency to corrode when in contact with blood [56]. 316L stainless steel contains non-negligible amounts of chromium (16%~18%) and nickel (10%-14%); the release of these metallic ions into human tissue and fluids must be regarded as a likely source of long-term problems owing to their known carcinogenic and toxic effects.
Particularly, chromium and cobalt have been shown to (a) concentrate in the nucleus and mitochondria [57, 58], (b) interact with DNA and RNA, (c) inhibit oxidative metabolism and (d) induce neoplasmic cell formation.
In the fibrinogen test, the 2V and 5V are the two samples that have the most fibrinogens detected. The 2V treated sample still have a smoother surface and the 5V sample have the most porosity, and these criteria caused the fibrins
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cannot be easily adhered to the surface of the treated 316L stainless steel. As in the platelet adhesion test, the 5V sample was consistent detected the most platelets, with the same reason as the fibrinogen. Greater amount of the platelets in sample are also detected when 1V was applied. However, the roughness between the 1V and 2V samples were not significantly different, since the smooth surface of these samples prevent the platelet to adhere to the sample surface. Interestingly, the 3V sample showed the most obvious result among other sample in the platelet activation test. Subsequently, from the AFM data, 3V sample also showed to have the narrow and deep grooves which may stimulate the platelets to release more P-selectin [59].
Clark et a1. [60, 61] and Wojciak-Stothard et al. [62] have shown that a complex situation, varying with cell type as well as with several of the topographical measures. Cells react to a single cliff and demonstrated good reactivity cliff height in the range of l-20 pm according to Clark et a1. [61].
Clark et al. [60, 61] showed that on the groove and ridge topography that the extent of reaction is related to the groove width and depth and possibly also due to the number of adjacent grooves. There is a general evidence that the extent of orientation increases with groove depth up to about 25 pm from topographies of about 1 pm in height. Below this degree of relief results are
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less available, partly owing to the difficulty of quantifying the etch depth in earlier years, and perhaps partly due to the previous reasoning that there would not be any effect on the cells. Wojciak-Stothard et al. [62, 63] have demostrated that P388Dl macrophage-like cells react with cliff height down to dimensions at least as small as 44 nm. Other cell types so far studied do not appear to be as reactive, nevertheless epitena, epithelia, fibroblasts and endothelia would still react to depths as shallow as 70 nm.
When the grooves or ridges are appreciably wider than the cells, the impact on orientation are not remarkable, although cells may still align to one edge as according to Clark et al [64]. As the width of grooves and ridges is reduced to the width of the cells and less, effects on orientation become more remarked. When more than two ridge grooves meet, lines of actin condensation from a cell will mark out each discontinuity [62]. Clark et al. [65]
showed that baby hamster kidney (BHK) cells will react to groove/ridge topography with a pitch (repeat) at 260 nm. These grooves were relatively deep at 500 nm. No work has yet been discovered about the minimum width of topography to which a cell can react. Conversely, there are many reports suggested that rough, often grossly rough surfaces could aid cell adhesion, for instance that of Lydon and Gray [66].
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