Certifying Magnets - A process



  • Aren't there different types of mammalian cell cultures as well? I would imagine we would need specific ones (no point in using the HeLa cell line now is there?).
  • Yes, there are many types of mammalian cell types. I figured using the 3t3 line and the HepaRG line was sufficient. It really covers your bases.
    HeLa per se is not your best bet, but it is the most common. One way or the other tho, you are using immortalized cell lines. Otherwise they just die.
    This is why labs move on to implanting in rats. Because you can't make normal cells happy (easily) in a lab setting. 
  • Cough ANIMAL TRIALS cough
  • edited December 2016
    Yeah, I know about the animal trials (hence my comment on it being the gold standard. Short of human trials anyway). Just thinking about the in vitro stage that is right before that that's all.


    Do we have any new methods of testing different materials yet? ^^
  • There are two different levels of testing. There's biocompatibility testing and then there's integrity testing. Testing for biocompatibility is based on the iso standards. But this is really just meant to show if the material is ok.. won't reject etc. Once that's been determined, the only other testing is to make sure the coating is going to do it's job.

    I use two tests for this. The simplest is a saline soak. Some people advise adding a detergent to lower tension. I think that was Amals idea originally. The point of this is to show if there are any breaches of the coating. It doesn't show is what would happen in the body; we've already determined that via the ISO tests. The point of it is just to confirm the coating is intact.
    There are a few problems of course. In the case of the TiN magnets, sometimes they have incredibly small pinholes. They will break down.. but it might take a year. There is a method to speed up this process. It basically consists of running a charge through the fluid. This will actively push in into holes and speed up breakdown if it's going to happen.

    The second type of test I do is permiability testing. Just because a coating is compatible and intact doesn't mean it won't allow fluid to enter an implant or device. I use a nickel metal object which I coat. Then I soak it in a solution of dimethylglyoxime. If fluid passes through the coating, the nickel turns pink. This test is good when first trying out a new kind of coating but isn't something you'd use on an object you intend to implant.
  • edited July 2017
    One method you can use to determine the integrity of a magnets coating is the method I am intending on using in my batch. Depending on the material you use for your coatings you can use acidic or alkaline solutions such as diluted bleach. The solutions pH has to be basic enough not to degrade the outer biocompatible layer but powerful enough to quickly degrade one of the inner coatings.

    To accomplish this you can easily use Au (gold,) or Cu (copper) undercoats. You can test for Au and Cu chemically using the strannous test for gold and copper testing tabs. For the outer biocompatible coating I would recommend Ti (titanium) as it forms an oxide film when it comes into contact with oxygen from the air, the bleach, or our blood. This film is incredibly chemically resistant, and is also found in TiN, however Ti's film is more resistant

    If you have these undercoats and a sufficient outer coating you can soak each magnet in about 1/2 fluid ounce of bleach. After that time you can perform the chemical test to determine if the solution degraded the copper/gold.
  • edited July 2017
    Have you tried the stannous chloride test or are you just going by what I said before?

    That test is very sensitive like I originally stated in a different thread but the gold needs to be in solution which is tough to achieve if there are any other metals involved.


    Keep in mind that most of my knowledge is self taught so I could have this wrong but I think anything above gold on the reactivity chart will drop the gold out of solution or prevent it from being dissolved in the first place.  If the gold isn't in solution, the stannous chloride will not detect it even if it had been dissolved and precipitated back out of solution.

    I think parylene over gold would work with the bleach and acid to test with the stannous but almost any other metal will cause problems if you are trying to use the stannous chloride test for gold.  I haven't tried it and don't have a way to test it myself.  I have used the stannous test before but it was either to check that I had dissolved gold or to check that I precipitated all of the gold back out of solution.

    If you have actually tried your ideas and seen it work, then disregard this.  Like I said, I could be wrong.  I suggest doing the test on a coated magnet that is know defective even if it means damaging the outer coating purposely to make sure you can really detect a bad one.

    EDIT:  Now that I think about it a little more, the stannous test might not even work with the parylene.  If the thin layer of gold is dissolved and exposes the copper or other layer under it, the gold might just precipitate back out of solution and then the test will not detect it.  I haven't tried it myself though.  Just going by the difficulty I had when I was trying to recover and refine gold from old circuit boards or gold ore before.  In that case, I had to dissolve everything before the gold stayed in solution.

    Another problem came from having too much of whatever active solution was used to dissolve the gold.  The solution can redissolve the gold that the stannous chloride solution precipitated causing the purple or black color to disappear.  Usually, if that was the problem, the color would show up as the drops of stannous was added but quickly vanish again.

    I'm not trying to be difficult but don't want my previous comments about the stannous chloride test to be taken as the perfect test for this purpose.  I haven't tried it so I don't know for sure if it will even work and it might lead to a false sense that everything is good.  I just thought it might be something worth looking into.  Again, if you have confirmed this test, disregard this post.
  • edited July 2017
    "deposited nickel can be chemically separated from the copper substrate by immersion in an aqueous solution containing CrO3 (250 g/l) and sulfuric acid (15 cc/l) at room temperature for around 4 days"

    This follows the same principle as diluting the bleach to get an alkaline solution basic enough to degrade the copper but not the nickel, it's just on the other side of the spectrum, basic. This same method can be put towards gold and any other metal, but that wouldn't be necessary in the first place.

    If you coated an N52 in Au+Ti you would only have to leave the magnet in 1/2 ounce of bleach for 2-3 days as the titanium will not degrade. If there is any holes in the coating even a few microns thick the alkaline solution will quickly degrade the gold which could then be tested for using the strannous test.
    There wouldn't be any other metals in the solution.

    I would personally recommend staying away from gold due to the fact it can't bond well with other coatings. Gold has a very slick surface, is very soft and prone to failure. 
  • No. If you place a TiN coated magnet in bleach it's destroyed rapidly.

  • Ah. Diluted. We'll have to see. If you dilute enough that it doesn't etch or damage the metal, I don't believe it would break down the other metals. It's worth a try.
  • edited July 2017
    These metal layers are only microns thick. They don't always responds the way you expect. Even an unflawed titanium layer will break down in bleach. Your thinking that you're going to get a nice titanium oxide layer. The problem is the entire depth of the later oxidizes and cracks aways from the underlying layers. Look o the mechanical characteristics of titanium oxide.
  • edited July 2017
    I'm not singularly relying on the titanium's oxide layer for the chemical resistance. Titanium, as a metal, is very chemically resistant on it's own. Can you backup your claim that even unflawed titanium, with an oxide layer from the oxygen in the diluted bleach, would break down?

    If I remember correctly the pH that gold begins to degrade in is approximately 7.7, and bleach is very alkaline at pH 11-13. I can then dilute this solution with something as basic as water (Pun intended) until it is pH 8.5-9.25 (or lower, if necessary) This pH level will degrade the gold and the titanium will be unaffected even if left in the solution for 2+ days.

    I would like to point out that blood has a pH of between 7.35 and 7.45 (For comparison water is 7 and baking soda is 8.3)
  • In undiluted bleach? Yes. Thin film deposited titanium acts very different then a chunk of titanium. Here's an example http://www.sciencedirect.com/science/article/pii/S0921509303013923

    It doesn't directly relate to what we're doing.. its a different alloy but one which we would similarly expect resistance to corrosion from. I've done these tests many times and you can watch the titanium surface break down under a scope in real time. In this particular application, it will break down. The reason? I don't know and it probably varies. However, you can watch the surface etch over time. It's not just delamination after the subsurface breaks down.
  • Are all m36 doomed to fail eventually ?
  • No. But we have no way of testing for which ones will. Ok, I need to be clearer. If you go ahead with this process without accounting for the stuff I'm talking about.. there will be a good percentage which fail and some that don't. But testing for the presence of the undercoating won't tell us which will fail. This may be a gamble people are willing to take but obviously isn't good enough for a product.
  • edited July 2017
    Why would testing for the presence of the undercoat not show if the top layer was entirely intact to prevent the degradation of the undercoat?

    If your concern is it will be difficult to discern which magnets failed that can be easily solved by using small amount of the solution and testing each magnet individually.

    I understand your stance on if the coating will degrade, but even if theoretically you're right we can adjust the pH and the time left in the solution as needed. We could even reduce the pH to 8 and leave it in the solution for a week in a half which would most definitely be a low enough pH for the Ti to not be effected but still degrade the Au.

    If a thin film deposited titanium has different mechanical characteristics than it would be logical that gold is also effected. If this is true we can easily find the "sweet spot" between the degradation points of the Au and Ti.

    The article you referenced doesn't provide much information on the thickness of the coating, the pH of the solution, or the duration it was left in the solution. The alloy is also different from pure Ti or TiN and you can reasonably expect it would degrade at a lower pH.

    You mentioned you've done these tests multiple times, do you happen to have any data? I would love to see it. What was the thickness of the coating, the pH of the solution, and the time it was in the solution?
Sign In or Register to comment.