Electrolyte solution for electrolytic polishing of Nitinol needles

Abstract

A low-toxicity electropolishing process has been developed to remove the oxide layer from the surface of Nitinol needles. Low concentrations of citric and sulfamic acids are mixed with moderate concentrations of sulfuric acid and used as the electrolyte solution. The process can be easily integrated into current suture needle manufacturing processes and processes requiring electropolishing of Nitinol-containing medical devices.

Description

Technical Field

[0001] The technical field to which the present invention relates is related to the surface of nickel-titanium (nitinol) alloys such as those found in medical devices, and in particular to the electropolishing of nitinol surgical needles.

Background Art

[0002] Nitinol is classified as a shape memory / superelastic alloy that has found interesting applications in a wide range of engineering, from aerospace to biopharmaceuticals. The latter applications are due to its biocompatibility in addition to its inherent properties. Unique properties such as shape memory and pseudoelasticity make nitinol an excellent candidate in many functional designs such as superelastic suture needles. However, the complexity of manufacturing and processing this alloy poses an obstacle to large-scale industrial processes. The present invention presents a solution for the manufacturing scale electropolishing process of nitinol-based medical devices and in particular nitinol suture needles.

[0003] Electropolishing the surface of nickel-titanium alloy wire (e.g., nitinol) currently requires the use of flammable and toxic solvents (e.g., see "Electropolishing fixture and electrolyte solution for nitinol stent polishing and method of using the same", European Patent Application No. 1255880 (A1)), so it is flammable, or highly corrosive (i.e., fluoride) and alcohol-based (e.g., see "Electropolishing in organic solutions", U.S. Patent Application No. 20060266657), and a solution is needed.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In this technical field, there is a need for a new non-hazardous electropolishing solution for nickel-titanium, including medical devices. The present invention presents an electropolishing solution and process for electropolishing nickel-titanium alloys using a non-alcoholic non-flammable aqueous solution.

Means for Solving the Problems

[0005] One aspect of the present invention relates to an electrolytic polishing solution suitable for removing an oxide layer from a nickel-titanium surface containing a non-alcoholic aqueous solution, about 25 to 50 wt% sulfuric acid, about 0.5 to 10 wt% citric acid, and about 0. to 2 wt% sulfamic acid.

[0006] Another aspect of the present invention relates to a process for electrolytically polishing a metal surface, providing a metal, providing an electrolytic polishing device including at least one anode, at least one cathode, and a tank for containing a sufficient amount of the novel electrolytic solution of the present invention to immerse the metal, contacting the anode with the metal, immersing the metal in the electrolytic cell, subjecting the metal to a current of 1 to 5 amperes for a period of time to polish the metal.

[0007] Typically, the electrolytic solution is maintained at a temperature of 40 to 80 °C in the process, and the current in the range of 1 to 5 amperes is maintained for about 10 to 30 seconds.

[0008] These and other aspects and advantages of the present invention will become more apparent from the following description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] This patent or application document contains at least one drawing executed in color. Copies of this patent with color drawings will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.

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DETAILED DESCRIPTION OF THE INVENTION

[0010] A method and an electrolyte solution for electropolishing nickel-titanium alloy (Nitinol) needles are developed to ensure removal of the oxide layer(s) on the surface of the needles formed during previous manufacturing processes. Low-concentration citric acid and sulfamic acid mixed with medium-concentration sulfuric acid in a non-alcoholic solution have been found to provide an excellent electrolyte solution for the non-flammable electropolishing process. This process can be easily incorporated into current manufacturing processes using existing equipment.

[0011] Electropolishing of a metal surface involves passing an electric current through the metal surface immersed in a bath using an electrolyte. The metal surface is connected to the positive electrode (anode) of a power source, and the negative electrode is connected to a special electrode (cathode) located within the bath of the electrolyte.

[0012] Regarding the useful concentration of sulfuric acid in the electrolyte solution, the minimum concentration of sulfuric acid is about 25 wt%. The higher the concentration, the more corrosive the solution becomes. The recommended concentration is less than about 50 wt%, preferably in the range of about 30 - 45 wt%, more preferably about 35 - 40 wt%, and most preferably about 38 wt% sulfuric acid.

[0013] The useful range of citric acid concentration in the electrolyte solution is about 0.5 - 10 wt%, preferably about 0.8 - 5 wt%, more preferably about 1 - 2 wt%, and most preferably about 1 wt% citric acid.

[0014] The useful range of the concentration of sulfamic acid in the electrolytic solution is about 0.2 to 2% by weight, preferably about 0.5 to 1.5% by weight, more preferably about 0.8 to 1.2% by weight, and most preferably about 1% by weight of sulfamic acid.

[0015] Optionally, it is desirable to add a substance that contributes to Ni 2+ ions to the electrolytic solution. Such Ni 2+ ion contributing substances include nickel salts such as nickel(II) nitrate, nickel(II) chloride, nickel(II) phosphate, and nickel(II) sulfate and their hydrates. A preferred form is nickel(II) sulfate hexahydrate. The useful range of any of these nickel salt concentrations in the electrolytic solution is about 0.2 to 2% by weight, preferably about 0.5 to 1.5% by weight, more preferably about 0.8 to 1.2% by weight, and most preferably about 1% by weight. Most preferred is about 1% by weight of nickel sulfate hexahydrate.

[0016] In use, the novel electrolytic solution of the present invention is used in a process for electropolishing a metal surface, to provide a metal, and to provide an electropolishing device comprising at least one anode, at least one cathode, and a tank for containing the electrolytic solution in an amount of solution sufficient to immerse the metal. bringing the anode into contact with the metal, immersing the metal in the electrolytic cell, subjecting the metal to a current of 1 to 5 amperes for a period of time to polish the metal.

[0017] The temperature range used for the electrolyte solution during the electropolishing process is about 40 to 80°C. A typical temperature covering the current electropolishing process of stainless steel needles is about 60°C.

[0018] Typical currents suitable for the electropolishing process of the present invention include the use of a current of about 1 to 5 amperes (amps or A). One skilled in the art will understand that the current can vary based on the metal being processed and its size. For the electropolishing time of a nitinol needle with a diameter of 50 mils (0.050 inches), there is about 3 A over a single-needle process time of 10 to 30 seconds. One skilled in the art will recognize that the electropolishing process is complete when the color of the metal changes from dark black / dark brown to silver.

[0019] Conveniently, all electropolishing parameters used with the novel electrolyte solution of the present invention are within the current process parameters for the electropolishing of stainless steel needles. As a result, the process can be easily implemented on a typical electropolishing line without major changes to the capital equipment.

[0020] Figure 1 shows how a typical 40-mil nitinol tapered needle appears before being treated with the novel electrolyte solution of the present invention. Referring to Figure 1, a dark black / dark brown oxide coating starting from the tip of the needle to a position on the stem of the needle is seen.

[0021] Figure 2 shows the effect of electropolishing on a 50-mil wire by the proposed process. The straight portion of the wire (region B) is left outside the polishing solution and used as a control. As shown in the photograph, the blue oxide was completely removed after 30 seconds at 50°C in an electrolyte solution containing 38 wt% sulfuric acid, 1 wt% citric acid, and 1 wt% sulfamic acid (region A). The current was 3 A for the electropolishing process.

Examples

[0022] The following examples show how the novel electrolyte solution functions on nitinol needles, and its significance for oxide removal is demonstrated by the adhesion between the polished needles and silicone lubrication, which is measured by needle penetration measurements.

[0023] As demonstrated by the following needle penetration test, the electropolished surface of the Nitinol needles fabricated from the examples of the present invention has good adhesion to silicone coating. The coating performance of medical devices can be tested in various friction or adhesion tests. In the case of surgical needles, the coating performance and integrity are evaluated using a penetration test device. The coated surgical needle is held using an automatic locking forceps or a similar holding device. The coated needle is then passed through a medium representative of typical human tissue. Approximately half of the length of the needle is passed through the medium and then withdrawn before the next pass. The test medium is typically a type of synthetic rubber (Duraflex™, manufactured by Monmouth Rubber and Plastic Corporation, Monmouth, NJ). A typical test involves using 10 needles each passed through the medium 20 times individually. The maximum force at each pass is recorded and used as an indicator of coating performance. Typically, the penetration force increases with each successive pass in a series as the coating wears off the needle. Further details of the equipment and method can also be found in U.S. Patent No. 5,181,416.

[0024] Example 1 of the present invention: Preparation of electropolishing of electrolyte solution 1 of the present invention and 40-mil tapered point Nitinol needles using the solution of the present invention 38.77 g of 98% sulfuric acid solution (Sigma Aldrich) was mixed with 1 g of citric acid (Sigma Aldrich) and 1 g of sulfamic acid (Sigma Aldrich) and 59.23 g of water at ambient temperature for 1 hour. This solution resulted in an aqueous solution of approximately 38 wt% sulfuric acid, 1 wt% citric acid, and 1 wt% sulfamic acid. A 140-mil tapered point Nitinol needle was used as the anode through which a current of 3 A flowed in this electrolyte solution at 60 °C for 30 seconds. As shown in Figure 3, the oxide on the surface of the needle (not shown) was removed, and the needle was made silver as a result of electropolishing.

[0025] Example 2 of the present invention: Preparation of the electrolyte solution 2 of the present invention and electropolishing of a 40-mil tapered point nitinol needle using this solution 37.76 g of 98% sulfuric acid solution (Sigma Aldrich) was mixed with 1 g of citric acid (Sigma Aldrich), 1 g of sulfamic acid (Sigma Aldrich), 1 g of nickel (II) sulfate hexahydrate (Sigma Aldrich) and 59.24 g of water at ambient temperature for 1 hour. This solution resulted in an aqueous solution containing approximately 38 wt% sulfuric acid, 1 wt% sulfamic acid, 1 wt% citric acid and 1 wt% nickel (II) hexahydrate sulfate. A 140-mil tapper point nitinol needle was used as the anode, through which a current of 3 A flowed through this electrolyte solution at 60 °C for 15 seconds. As shown in Figure 4, the dark purple oxide on the surface of the needle (not shown) was removed, and the needle was silvered as a result of electropolishing. It should be noted that compared with Example 1 of the present invention, only half the time (15 seconds vs. 30 seconds) was required to complete the removal of the oxide from the nitinol needle.

[0026] Comparative Example 1: Preparation of a conventional electrolyte solution containing only sulfuric acid 38.77 g of 98% sulfuric acid solution was mixed with 61.23 g of water at ambient temperature for 1 hour. This solution resulted in an aqueous solution containing approximately 38 wt% sulfuric acid. One 40-mil tapered point nitinol needle was used as the anode, through which a current of 3 A flowed through this electrolyte solution at 60 °C for 30 seconds. No sign of color change was observed on the nitinol needle (not shown). The treatment time was further carried out for 2 minutes, and the color of the needle remained unchanged, indicating that the oxide layer on the surface of the nitinol needle could not be removed using only sulfuric acid in the electrolytic solution.

[0027] Comparative Example 2: Preparation of a conventional electrolyte solution containing only sulfuric acid and citric acid A 98% sulfuric acid solution of 38.77 g and 1 g of citric acid were mixed with 60.23 g of water at ambient temperature for 1 hour. An aqueous solution containing approximately 38 wt% sulfuric acid and 1 wt% citric acid was obtained with this solution. One 40-mil tapered point nitinol needle was used as the anode, and a current of 3 A was passed through this electrolyte solution at 60 °C for 30 seconds. No sign of color change was observed in the nitinol needle. The treatment time was further carried out for 2 minutes, and the color of the needle was changed to a slightly darker blue as shown in Figure 5, which indicates that the removal of oxides on the surface of the nitinol needle is not efficient using sulfuric acid and citric acid in the electrolytic solution.

[0028] Osmosis test example: Coating and testing of nitinol needles One set of 10 electropolished 40-mil tapered point nitinol needles was coated with the silicone solution described in Example 1a and coated with an equal number of unpolished nitinol needles in the manner described in Example 2a of US Patent Publication No. 2018 / 0353990. One set of conventional stainless steel needles having the same geometry (CT-1) was also coated with the same silicone solution. All 6 sets of needles were subjected to an osmosis test, and the results are summarized in Table 1.

[0029] [Table 1]

[0030] Referring to Table 1, the oxides on the surface of the nitinol needles (obtained from previous process steps) affect the adhesion of the silicone coating layer to the needles. The adhesion of the nitinol needles to the surface between the silicone lubricating layers becomes better, as exemplified by the improvement in the osmosis performance of the polished needles (Examples 1 and 2 of the present invention) by oxide removal by electropolishing. The osmosis performance of the electropolished nitinol needles treated with the novel electrolytic solution of the present invention (Examples 1 and 2 of the present invention) is equivalent to that of conventional stainless steel needles having no oxide layer and having the same silicone coating.

[0031] In summary, a low-cost and less harmful non-flammable electrolytic solution was developed to remove the oxide layer on the surface of nitinol needles. Low concentrations of citric acid and sulfamic acid were added to medium-concentration sulfuric acid. This solution can be easily added to current electrolytic polishing devices.

[0032] As described above, the present invention has been illustrated and described in terms of its detailed embodiments. However, those skilled in the art will understand that various changes can be made to the form and details of the present invention without departing from the spirit and scope of the claimed invention.

[0033] 〔Embodiment〕 (1) An electrolytic polishing solution suitable for removing an oxide layer from a nickel-titanium surface, a) about 25 to 50% by weight of sulfuric acid, and b) about 0.5 to 10% by weight of citric acid, and c) about 0.2 to 2% by weight of sulfamic acid, comprising a non-alcoholic aqueous solution, the electrolytic polishing solution. (2) The non-alcoholic aqueous solution is a) about 30 to 45% by weight of sulfuric acid, and b) about 0.8 to 5% by weight of citric acid, and c) about 0.5 to 1.5% by weight of sulfamic acid, the electrolytic polishing solution according to Embodiment 1. (3) The non-alcoholic aqueous solution is a) about 35 to 40% by weight of sulfuric acid, and b) about 1 to 2% by weight of citric acid, and c) about 0.8 to 1.2% by weight of sulfamic acid, the electrolytic polishing solution according to Embodiment 1. (4) The non-alcoholic aqueous solution is a) about 38% by weight of sulfuric acid, and b) about 1% by weight of citric acid, and c) about 1% by weight of sulfamic acid, the electrolytic polishing solution according to Embodiment 1. (5) The non-alcoholic aqueous solution is a) about 25 to 50% by weight of sulfuric acid, and b) about 0.5 to 10% by weight of citric acid, and c) about 0.2 to 2% by weight of sulfamic acid, and d) about 0.2 to 2% by weight of nickel salts and their hydrates, the electrolytic polishing solution according to Embodiment 1.

[0034] (6) The non-alcoholic aqueous solution is a) about 35 to 40% by weight of sulfuric acid, and b) about 1 to 2% by weight of citric acid, and c) about 0.8 to 1.2% by weight of sulfamic acid, and d) about 1 to 2% by weight of nickel salts and their hydrates, the electrolytic polishing solution according to Embodiment 1. (7) The non-alcoholic aqueous solution is a) about 37% by weight of sulfuric acid, and b) about 1% by weight of citric acid, and c) about 1% by weight of sulfamic acid, and d) about 1% by weight of nickel sulfate hexahydrate, the electrolytic polishing solution according to Embodiment 1. (8) A process for electrolytically polishing a metal surface, comprising d) providing a metal, e) providing an electrolytic polishing device including at least one anode, at least one cathode, and a tank for containing the electrolytic solution of the amount sufficient to immerse the metal according to Embodiment 1, f) contacting the anode with the metal, g) immersing the metal in the electrolytic cell, h) subjecting the metal to a current of 1 to 5 amperes for a certain time to polish the metal. (9) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 2. (10) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 3.

[0035] (11) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 4. (12) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 5. (13) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 6. (14) The process according to Embodiment 8, wherein the electrolytic solution has the composition according to Embodiment 7. (15) The process according to Embodiment 8, wherein the electrolytic solution is maintained at a temperature of 40 to 80 °C.

[0036] (16) The process according to Embodiment 8, wherein the current is maintained for about 10 to 30 seconds. (17) A medical device manufactured by the process according to any one of Embodiments 8 to 16.

Claims

1. An electrolytic polishing solution for electrolytic polishing in the manufacture of a medical device having a nickel-titanium alloy surface, comprising: a) 25 to 50% by weight of sulfuric acid; b) 0.5 to 10% by weight of citric acid; c) 0.2 to 2% by weight of sulfamic acid, and containing a non-alcoholic aqueous solution.

2. The electrolytic polishing solution according to Claim 1, wherein the medical device is a surgical needle.

3. The electrolytic polishing solution according to Claim 2, which is for electrolytic polishing before coating the surgical needle.

4. The electrolytic polishing solution according to Claim 3, wherein the coating is a silicone coating.

5. The electrolytic polishing solution according to any one of Claims 1 to 4, which is for removing an oxide layer from the nickel-titanium alloy surface.

6. The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution comprises: a) 30 to 45% by weight of sulfuric acid; b) 0.8 to 5% by weight of citric acid; c) 0.5 to 1.5% by weight of sulfamic acid; The balance being water only. The electrolytic polishing solution according to any one of Claims 1 to 5.

7. The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution comprises: a) 35 to 40% by weight of sulfuric acid; b) 1 to 2% by weight of citric acid; c) 0.8 to 1.2% by weight of sulfamic acid; The balance being water only. The electrolytic polishing solution according to any one of Claims 1 to 5.

8. The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution comprises: a) 38% by weight of sulfuric acid; b) 1% by weight of citric acid; c) 1% by weight of sulfamic acid; The balance being water only. The electrolytic polishing solution according to any one of Claims 1 to 5.

9. The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution comprises: a) 25 to 50% by weight of sulfuric acid; b) 0.5 to 10% by weight of citric acid; c) 0.2 to 2% by weight of sulfamic acid; d) 0.2 to 2% by weight of nickel salt and its hydrate; The balance being water only. The electrolytic polishing solution according to any one of Claims 1 to 5.

10. The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution comprises: a) 35 to 40% by weight of sulfuric acid; b) 1 to 2% by weight of citric acid; c) 0.8 to 1.2% by weight of sulfamic acid, and d) 1 to 2% by weight of nickel salts and their hydrates, and the balance water, consisting only of the electrolytic polishing solution according to any one of claims 1 to 5. **Claim 11** The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution a) 37% by weight of sulfuric acid, and b) 1% by weight of citric acid, and c) 1% by weight of sulfamic acid, and d) 1% by weight of nickel sulfate hexahydrate, and the balance water, consisting only of the electrolytic polishing solution according to any one of claims 1 to 5. **Claim 12** A process for electrolytically polishing a metal surface, comprising: d) providing a metal; e) providing an electrolytic polishing device including at least one anode, at least one cathode, and a tank for containing an amount of the electrolytic polishing solution sufficient to immerse the metal according to any one of claims 1 to 11; f) bringing the anode into contact with the metal; g) immersing the metal in the tank; h) subjecting the metal to a current of 1 to 5 amperes for a period of time to polish the metal. **Claim 13** The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution a) 30 to 45% by weight of sulfuric acid, and b) 0.8 to 5% by weight of citric acid, and c) 0.5 to 1.5% by weight of sulfamic acid, and the balance water, consisting only of or a) 35 to 40% by weight of sulfuric acid, and b) 1 to 2% by weight of citric acid, and c) 0.8 to 1.2% by weight of sulfamic acid, and the balance water, consisting only of or a) 25 to 50% by weight of sulfuric acid, and b) 0.5 to 10% by weight of citric acid, and c) 0.2 to 2% by weight of sulfamic acid, and d) 0.2 to 2% by weight of nickel salts and their hydrates, and the balance water, consisting only of or a) 35 to 40% by weight of sulfuric acid, and b) 1 to 2% by weight of citric acid, and c) 0.8 to 1.2% by weight of sulfamic acid, and d) 1 to 2% by weight of nickel salts and their hydrates, and the balance water, consisting only of the process according to claim 12. **Claim 14** The electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution a) 38% by weight of sulfuric acid, and b) 1% by weight of citric acid, and c) 1% by weight of sulfamic acid, and the balance water, consisting only of the process according to claim 12. **Claim 15** the electrolytic polishing solution consists only of the non-alcoholic aqueous solution, and the non-alcoholic aqueous solution consists of a) 37% by weight of sulfuric acid, b) 1% by weight of citric acid, c) 1% by weight of sulfamic acid, d) 1% by weight of nickel sulfate hexahydrate, and the balance of water only, the process according to claim 12. **Claim 16** The process according to any one of claims 12 to 15, wherein the electrolytic polishing solution is maintained at a temperature of 40 to 80 °C. **Claim 17** The process according to any one of claims 12 to 15, wherein the current is maintained for 10 to 30 seconds.

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