PDMS Coating, the right way and the wrong way

As many of you know, I tend to talk a lot of ish about PDMS (silicone) coatings.  Part of this comes from the fact that in a lot of implant situations there are other options. But another reason is the methodology used to coat the device in silicone.

Sometimes, silicone is your best option. There will be no TiN coatings on circuit boards, for instance. So what do you do?
Well, I'm kind of bringing this up now because of a project we have been working on, that trandermal. Some part of this device is going to have to be coated in silicon, it's just an unfortunate effect of design. I want to show the differences between the silicone that we and body mod artists have normal access to, and how a non production method coating looks, super up close and personal.

This here is a comparison of a magnet coated in silicon from another source, and one that I did in the lab just the other day.

The top two images are the standard mold type coating. The bottom two are using a spray coating technique I worked up back at university. Both shots were taken at 400x
A couple of things to note. The top coating has no dark line. Why? This is because the coating is so thick that you can't see the edge of the coating and the magnet inside at the same time. Added bulk can be an issue. The roughness of the edges is caused by the removal from the mold, tho it is possible that these were in a sheet and just punched out. Regardless, what you are seeing is a raggedy little edge, prime for bacterial and platelet adhesion and other types of fouling. Not happy. Luckily, silicone is pretty inert, so it is fairly resistant to these issues. My question is, why make it worse?

Now to the bottom images. These are the two roughest spots I could find. You'll notice the characteristic pockmarking that you will find in silicone. It is technically a polymer mesh. The coating is significantly more thin. Some of the blur that we are getting in these images is due to the thin, smooth coating. It's hard to focus on the entirety a thin smooth area that goes around a curve with a microscope. I actually took a bunch of pictures, working my way through the layers, and may turn them into a gif... The point being is that with this method, you significantly reduce the "edges" another thing I like to complain about when talking about implant design.


  • Like I said, just getting the info out here, so that when I start talking about the next steps, people don't give me grief. Not that that will stop any of you ;)
  • Would it be possible to do a combination of both processes? Mold coating, and then a spray coat to eliminate the ragged edges?
  • Sure, but you would possibly have bonding issues if the first coating was completely cured before the second was applied.

    Also, the bulking is an issue. Part of whats cool about the second process is how thin it is. For instance, the magnets that are coated in silicon are roughly double thickness due to their coating process. If you need thicker you just build more coats of spray coat. 

    However, it is something to consider, depending on what you you want to implant. I need to do more tests on surface strength with this version...

  • for whatever reason I see neither of the referenced pictures. I'd also be interested in adding dip-coating to the list. From what I was able to figure out it's one of the more promising do-it-at-home approaches for medium sized implants (like smallish electronic stuff). For such things it'd be required to have the electronics inside a good metallic EM shield. The free space between circuit board and shield could get filled by casting epoxy or something more bioinert with UV curing (something the silicone can bond to) . Starting from there , pdms coating / mold / spray / dip ?
  • if you mouse over the "This" in my first post, you should see that it is a link. I can post the picture here but it is quite large and I think that making it small reduces the point of being able to see it under a microscope.

    dip coating is great, but you will get thickness variations towards the bottom, depending on how fast your polymer crosslinks. not really an issue unless you run into the problems mentioned below.

    Finding something that the PDMS just sticks to is actually more particular than one  imagines. So far, since i've started working on this, i've had varying degrees of success with a variety of surfaces. electrical tape, and silicon, yes. quartz and thermopastic, no. metal and glass only if treated. Actually, functionalizing your surface works well in many cases, but you should note that usually this requires acid and or heat.
    This can be worked around by completely surrounding the material in some cases, but it needs to stay in place. This is, i would say, the best reason to use molding. Dip coating or spraying on some surfaces just has your polymer run off unless you paint it on in super fine incremental layers.

    In these cases, as weird as it sounds, putting down a tie coat / primer layer is actually a good idea. This doesn't necessary mean paint per se. Polymers with a high amount of amines on the side chains work well...  If you vacuum your tie coat after application, you remove the volitiles and covering in pdms at this point works very well and the body is protected from the material underneath regardless.
  • Thx, I totaly missed that tiny link. One could also use a mechanism to rotate the object while curing to get a more even thickness.
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