Materials for implantation

As MattGuy pointed out. We had no thread on materials used for coating implants... until now.

This thread is intended to gather, share, and evaluate information around biocompatible/bioactive/bioinert/biodegradable materials.

For a starting point I added a link to the Standard Handbook of Biomedical Engineering & Design (McGraw-Hill Handbooks) in the textbook section of the wiki. Chapter 11 contains information about materials.

It lists a few nice bioinert materials, summarized with pros and cons:
PTFE - we all know it as a material with excellent bioinertness. big con: it's hard to work with because it has to be sintered, which makes it a poor choice for DIY applications.
HDPE - a thermoplast with very good bioinertness. it serves as reference material for implant-material-tests iirc. A couple of sources sell the material in certified implant-grade quality (including Dow corning). Pros: thermoplast with rather low melting point (around 135°C) so it can be easily molded, bend, cast, vaccuformed etc.. It can be autoclaved at 120°C  cons: we have yet to find a source selling the desired quality in low quantities/without 3-digit minimum orders.
FEP - a less commonly known material, very similar to PTFE, but it has a melting point below the decomposing temperature. Pro: So it's thermoplast so it can be melted, cast, heat-welded, vacuformed etc. very big Pro: Bola sells this material as tubes and foils in various diameters and thicknesses, minimum order is 50 Euro, and at least the tubes are USP class VI  certified. Foils are available from 500μm down to 25μm thickness. it can sterilized with a number of methods, including autoclaving. cons: minor drawback, melting point is at 253 to 282°C careful processing is required. It's still below the curie temperature of neodymium magnets (310°C)

Other candidates would be PMMA and cellulose, i haven't researched those 2.
From what i can conclude at the moment, HDPE is the easiest to work with material, altho it's a bit hard to source. FEP is easily available from Bola, but requires temperature regulated equipment to deal with it. Both materials can be used to build thin coatings (like used for magnetic implants), as well as thick shields for electronics (i'd probably prefer HDPE over FEP for those).
The FEP tubes may be of special interest for small implants, like custom made rfid tags, all other circuits/implants fitting in a cylindrical-like shape.

As for biodegradeable  PLA seems to be a good choice, I can't tell much about it tho. Maybe someone else can.

Parylene C, and ePTFE may qualify as bioactive materials. Again, I don't have much info on these.

As for conductive/electrode materials quoting from a book from L. A. GEDDES and R. ROEDER , Criteria for the Selection of Materials for Implanted Electrodes.(it's a bit out of context i know)
From their study they ranked the materials as candidates in the following order:
gold, platinum– 8% tungsten, platinum–10% iridium, platinum, gold–nickel–chromium, stainless steel,
titanium, gold–palladium–rhodium, nickel–chromium–molybdenum, platinum–10% rhodium, platinum–10% nickel,
platinized platinum, tantalum, zirconium, rhenium, and tungsten.
The materials found to be unsatisfactory were: silver, silver–silver chloride, copper, and iron.

Gold, Platin - 10% iridium, and stainless steel seems to be the most common choice, all of those seem to be more or less available at different prices from various lab-suppliers. Gold is probably the easiest to work with as it can be easily soldered, but of course quite expensive. Stainless steel requires special treatment for soldering, but it can be done. It's cheap which makes it a good choice for longer leads. All three materials are sometimes offered with optional ptfe coatings. In case of bare wires, I'd recommend the earlier mentioned FEP tubing to insulate the wires when desired.


  • @ThomasEgi what would a person be looking for when sourcing any of these materials? From what you've mentioned it looks like HDPE would be the way to go, I've found a few sources selling HDPE pellets for 3d printers, one even sells by the pound but I'm unsure what the difference is between medical grade and standard. Maybe @Saal could help out here
  • The difference between standard & medical grade is basically that standard can utilize a certain percentage of recycled (reground) polyethylene, whereas the medical grade (what I work with) must be 100% pure virgin resin (as in straight from the refinery-->molding & assembly). I haven't personally worked with ":implant grade" HDPE, but I would imagine that if there is such a thing the only difference would be that it's refined, processed, molded & packaged in a cleanroom environment and undergoes sterilization (likely of the radiation variety).

    About 40% of the parts we produce consist entirely of HDPE swabsticks for chlorhexidine gluconate/isopropyl alcohol surgical site antiseptic. HDPE is stupidly easy to work with, as @ThomasEgi has said, and it is entirely possible to set up a benchtop injection molding press for a few hundred dollars, provided you have the assistance of a skilled machinist (lathes & the like are a necessity). One of the things we actually do is mold product labels directly into the thermoplast to save our customers some labor, and depending on the item to be bioproofed we could probably do something similar here, negating the need for any hand coating, which carries a high possibility of introducing pathogens/impurities/compromised coating.
  • edited December 2013
    Without the mentioned injection molding things get a bit more difficult.
    I tried to coat a simple led throwie earlier today using regular 4mm HDPE welding wire. My first attempt to simply melt it in a stainless steel shot glass and dipping the circuit in failed due to the viscosity. For my 2nd attempt I created a sheet from multiple parallel wires by heating and pressing them between silicone coated baking paper. Then I rolled the sheet up to form a tube, closed one end, put the circuit inside, melted and closed the other end. It looks like a genuine piece of poop in white. Things learned from that experiment:
    1. dipping a circuit in molten thermoplastics has a good chance of damaging it.
    2. creating a primitive and ugly coating is possible without expensive tools.
    3. from what I HDPE does not form a sealing bond with stainless steel. So it is not suited for implants with electrodes. I suspect FEP also has this issue. Using silicone may be the only option.

    for implants without any electrodes or wires leaving the coating, a pre made capsule and cap could provide a convenient way to seal up smaller circuits. A tube with thin enough walls to press the ends flat should work fine too.

    Liquid silicone rubber/elastomers is yet another material have a close look at. Those may allow to coat things by just submerging them into the coating. Which would be as easy as hanging the pcb to 3 non-degradeable sutures, pull it out again, wait for the material to dry and cut the sutures off.
    Any thought on that? The description seems to be very promising in terms of coating all sorts of stuff, magnets, circuits, etc.
  • @Cassox and I have been discussing this a little bit. I work in a lab that focuses on creating materials that are non fouling.  One of our projects is basically altering the silicon with another molecule to reject build up or bacterial attachment on a molecular level.

    Even that one point where the suture is would be an initial area for attachment. What we have started doing is applying multiple spray coats with an airbrush. This allows you to coat the entire surface, and also reduce the thickness of the coating while maintaining the protective properties.
    I bought the airbrush on amazon, it was under 20 dollars. The only drawback is the need for a solvent that you can use to thin the silicone so that it is sprayable.
  • edited December 2013
    The product I linked above can at least be applied with a brush the way it is. In the description they state "It can be diluted with an aliphatic or
    aromatic solvent." which sounds like it can be diluted down to sprayable levels.
    It is 130 bucks plus shipping and handling. Shouldn't be all too hard to group-source that much. Even if the airbrush experiments fail it's still a good material for coating objects of all sorts.
  • edited December 2013
    The silicone ThomasEgi linked above has a viscosity (before the xylene evaporates) of 800 cps, according to a nice looking chart I found that's the same as high fructose corn syrup at 27°C, and latex paint is close at 750 cps, that has to be near sprayable. They say the xylene will evaporate as is, I'd think even if you had to add a little more it should still all evaporate with the thinner layers from the spray gun. Throwing it in a box for a day and testing the air with a xylene test tube might work to double check that though. Great idea with the spray gun too! The only concerns would be contamination or microscopic defects in the coating, I'm not sure how big of a concern those would be with the right gun and multiple layers...also no clue how to test for either of them.
  • We have found that contamination does not seem to be an issue when coating in a fume hood. Sterilize all your surfaces and you should be fine. Even a DIY fume hood would sufficient for this work, and should actually be considered when doing any of these sort of projects.

    For ideal removal of the solvent, placing the item into a vacuum desiccator over night. They aren't expensive, and would improve solvent removal as well as crosslinking off your polymer of choice
  • Well that sounds like a pretty practical setup then. Do you think the airbrush layers are thin enough to make it practical for coating magnets? I'm more interested in larger applications now, sensors and uC's but...I'm still having trouble sourcing a decent coated magnet so it'd be a two-fer for me. Either way I'll be setting it up to test after I get my magnet & RFID implants and ruler tattoo.
  • What about using certain gemstones, like cubic zirconia, corundum, quartz, or even diamonds, that are polished and cut into a capsule format to hold RFIDs?  
  • Altho some gemstones have certain biocompatibility. In general they have undesirable mechanical properties and are priced ways above our heads.
    Like, a small piece of FEP tubing which can hold an RFID chip costs less than 2 bucks (in certified quality). A cm² of synthetically grown polycristalline raw diamond goes for about 300 and more iirc (thickness somewhere between 0.5 and 1mm). And that's only a flat, unpolished surface. Polishing polycristalline diamonds is literraly, superhard and takes weeks on machines running 24/7. Cutting those things is only possible lasers. Growing a capsule on a substrate would be possible. but in order to insert the circuits, you'd have to have to glue on a cap, which leaves you to a different material again.

    So long story short. Gemstones aren't really a good choice, especially not if you work on a small budget.
  • edited December 2013
    Well, could you grow the crystal around the circuit, perhaps?

  • Side note, you need to check if your surface ummmm connects with the coating.
    For optimal coating, of any type, functionalizing the surface will greatly increase your coating strength and mechanical stability. This allows you to have a thinner coating with a greater mechanical strength.
    Functionalization will not reduce the qualities of your magnet.
    For circuitry, a tie coat may be ideal. Basically, primer painting.
  • for circuits, there are existing sprays to protect the circuit from stuff like humidity, oxygen etc. those are easy to use, cheap, and commonly available. I'm not sure to what extent it'll dissolve due to the solvents from the silicone coating. i'd recommend to use multiple layers of coating, each with an individual charge of silicone.

    @TheGreyKnight: diamonds are grown in a microwave powered plasma, feed by multi kW magnetrons (basically a huge , egg-shaped microwave oven). Not exactly the environment you want to put circuits in.
  • If your coating is fully cross-linked, like, give it a few days, subsequent exposure to solvents that are used in coatings won't mess with it's integrity. 
  • @ThomasEgi True that. And Artificial Corundum is made by dipping a seed crystal in molten alumina... So, that's out of the question as well.

    But will the next layers properly bind to the previous ones?
  • edited December 2013
    @TheGreyKnight, you could ask the guys producing that stuff.

    Another set of materials we can evaluate are produced by Masterbond.
    One of the more interesting products offered is a silver filled, electrically conductive one component epoxy.
    Since they advertise it to stick to metals, that may be a nice way to connect hard to solder metals (as used for electrodes) to the rest of the circuitry. There are also non-silver versions of this product which could be useful for coating in general.
    I'm not sure how well this stuff is suited for long term implantation, but at least for glue-connecting electrodes to wires it seems to be quite useful.

    /edit: this 2 component epoxy explicitly mentions implants, so it's probably worth some investigation

    /edit2: all their medical products listed
  • edited January 2018
    After a bit more research on materials, certifications and stuff I learned a couple of things.

    USP Class VI, is like a basic biological safety rating for material, class VI being the strictest class which includes toxicity tests and a 5 day implantation test.
    That does not qualify the material for long term implantation or makes any statement about stuff like carcinogenicity or genotoxicity. But at least the materials are of certain purity/quality and have a history of a certain degree of biocompatibility. So depending on the material, this can be a valid option, but it requires careful evaluation.
    There is a big number of materials out there that comply to the USP Class VI.

    When looking for long term implantable materials, the ISO 10993 should be your attention. It's a big set of tests which needs to be run on a material depending on its intended use. The following list shows the tests to run for each cathegory.
    When you find a material that's certified to comply with ISO 10993, that does not mean it's suited for long-term implantation, unless it explicitly says so. otherwise it could just mean it's safe for skin-contact.
    From what i found out there are a about 10 companies producing implant grade silicone for <30 days of implantation. And 3 of them produce materials that can be used for unrestricted implantation duration. Namely: Dow, Applied Silicone and NuSil
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