In a previous blog post on permanent hydrophilic coatings, I noted that really all hydrophilic coatings have some sort of bioerosion, degradation, and/or resorption rates in vivo. For most coatings of this nature, those rates are high, which means they are not always suitable for implantation.
Honestly, that's a rather broad and general statement about the utility of implantable hydrophilic coatings. In reality it goes back to a question I like to ask a lot on this blog: What is your application?
Let me break that out into some more specific thought questions:
What kind of device do you want to coat?
WHY do you want to coat it?
Do you want it to be slippery? Non-thrombogenic? Closely associated with water to prevent fogging or misting?
Given your answer to the question above, exactly WHEN do you want the coating to possess those properties during the life of the device? Pre-implantation? During implantation? During explantation? The whole time?
Given that last answer, how long is that time period? Minutes? Hours? Days? Weeks? Forever?
In many cases, for example in the case of an implantable cardiac pacemaker, surgeons may be complaining that it is difficult to squish the leads into place during the procedure. A lubricious coating might help with that. However, once the device is implanted, who cares about what happens to the coating as long as it is biocompatible? Or maybe I should ask: do you care what happens?
If you do care, then you need to ask yourself why. Is there some other function a slippery, non-thrombogenic, water-loving coating will serve a purpose after implanting that pacemaker?
Most of the time, when clients come to me asking for permanent hydrophilic coatings, it actually turns out that they do not need them to be permanent. They just need them to fulfil a temporary role, which the coating can do easily, and then when it goes away it is of no consequence.
The Qmed blog has an insightful article on differentiating between leachables and extractables in medical devices. Though the article does not specifically mention lubricious hydrophilic coatings, it is still an important consideration.
Something to know: All hydrophilic coatings contain multiple ingredients, some of which are not completely bound within. Even crosslinked coatings that purport to be chemically resistant still contain unreacted products from whatever reactions are used in the crosslinking. The article at Qmed makes us aware that these sorts of leftovers can either leach out or be extracted out, and there is a difference.
The author notes that leaching occurs under "normal" conditions of use, i.e. what will come out of your coating when you place it in the body at 37C for some length of time? Extraction is what happens during exaggerated conditions, i.e. what will come out of the coating at 50C in an acidic water bath, or an oil bath?
Extractables can give clues to what the leachables might be. They can also tell you how stable your material is, chemically. The presence of an extractable is not necessarily a show-stopper. Unwanted leachables might be harder to explain, however. Either way, both of these things will make up part of the larger picture of biocompatibility for your material.
An archive of the webinar we recently announced on hydrophilic and hydrophobic coatings is now available to everyone for download. Please stop by and grab the file and listen to it.
Click here to download the hydrophilic coatings webinar.
The webinar was sponsored by Biocoat and Specialty Coating Systems. Half of the presentation by Josh Simon is actually about hydrophilic coatings and the second half by Lonny Wolgemuth talks about hydrophobic coatings. Remember, hydrophilic means "water loving". Hydrophobic means "water fearing". The webinar sets the record straight on which is which and why you would want to use some in specific applications.
For both coatings, lubricity is discussed, i.e. how slippery they are respectively, as well as some basic mechanical properties and medical device applications.
All in all, I am told this is a pretty good overview of coatings, and it is a nice place to start if you are just beginning your research on this area for possible future products or medical devices.
A press release in Medical News Today
talks about a new type of possible antimicrobial coating in the form of positively charged short chain peptides. In a nutshell, the article talks about the possibilities of immobilizing these peptides onto the surface of a medical device, to create a medical device coating, so they stay active and continue to act against microorganisms. My question is: Since they are peptides, would they set off a chronic immune response at the surface? Depending on the type of implant, that may not be desirable.
Nevertheless, as was discussed at the last BioInterface meeting
, the concept of creating a surface that inhibits bacterial colonization and formation of biofilms is a good one. It is unclear to me upon reading the article if the peptides inhibit biofilm formation or if they are actively bacteriocidal. If they are bacteriocidal, it would be interesting to know what kind of half-life they have at the surface, i.e. how long they remain active. The drawbacks of releasing antibacterial agents from surfaces is that they eventually run out and the local concentration of antimicrobial agent can drop below the minimum inhibitory concentration (MIC) thereby promoting drug resistance. Immobilizing an antimicrobial agent on the surface could get around this, depending on how long it stays active.
I wonder if a surface like this could be combined with other agents that inhibit bacterial attachment?
The market for biomaterials is described in a new report that can be found here
. The "World Biomaterials Market" report seems to list all the major players in biomaterials, breaking it down into polymers, natural materials, metals, and other. As you may surmise, hyaluronic acid is covered in the report, as are the major companies that sell hydrophilic coatings, like Surmodics, Biocoat
, and AST.