Implantable devices place a special set of requirements and challenges on their coatings. The moisture and broad mixture of chemicals that are found inside of the body are challenging in and of themselves. However, the body also has needs from the coatings that are placed within it. They need to be non-irritating and inert enough to be harmless. For most applications, the best choice is USP Class VI compliant parylene coatings.
USP Class VI Certification
The most important factor in finding the best implantable coating is whether or not it holds a biocompatibility certification. The industry's go-to standard is the United States Pharmacopeia's Class VI certification which is typically also covered by products that also comply with the European ISO's 10993 standard.
To achieve Class VI certification, a coating material starts by proving itself to be inert when injected into the body of a test animal. First, it gets mixed with four different carriers -- saline solution, alcohol saline, vegetable oil and polyethylene glycol -- and, as appropriate, injected either intravenously or into a body cavity (intraperitoneally). If, after three days, the animal is still alive and shows significant reactions, the material goes to the next phase of certification.
Next, it gets mixed with the same carriers and injected into multiple sites on two animals. To pass (and earn Class I, II, III or IV certification), the animal must show no sign of reaction after three days of daily checks.
Finally, to earn the certification, strips of the material get surgically implanted in two animal's muscle tissue. Assuming that the material causes no significant reaction after five or seven days, it earns its USP Class VI certification.
A coating that has its biocompatibility certification in place has two key benefits. First, once it is tested, you should not have to test it again, saving you from an expensive delay. Second, the certification gives you a sense of assurance that it is an appropriate choice for your implantable device or item.
Parylene Coating for Implantable Devices
Parylene isn't only the gold standard for conformal coatings. It is also the best option for implantable devices. On just about every metric, it perfectly suits the needs of biological applications. Its USP Class VI certification also means that it is safe to use.
More than almost any other conformal coating, parylene dimer is chemically inert. It resists both the acids and bases that are typically encountered when implanted. At the same time it also has excellent resistance to moisture and to corrosive materials -- like salt. While it is unlikely to encounter industrial solvents when implanted, it can also withstand a full range of organic chemicals.
The unique mechanism of parylene adhesion also makes it a good choice in implantable devices. It generally adheres to itself. This means that it forms a tight seal around implantable devices, protecting them from the body and the body from them.
Vapor based deposition means that a parylene coating is more truly conformal than that created by any other compound. Since parylene coats any part of a device that air can touch, it covers exposed surfaces underneath parts and even areas inside of the item that are not otherwise covered. Given that, over time, bodily fluids can also permeate any of those cracks or flaws, parylene is a safe option.
Parylene coatings aren't just truly conformal. They are also unique in their ability to create a truly conformal coating in thicknesses that are measured in microns. While other coatings can approach parylene's performance, they typically are anywhere from 10 to hundreds of times thicker when applied. Given that size is often a concern in implantable device, parylene's thinness is another significant benefit.
Even though it dries, parylene has a high degree of lubricity -- similar to PTFE (teflon). Its dry film lubricity makes it an excellent match for implantable devices since it reduces the risk of irritation or inflammation when the device is positioned. In fact, many needles are coated with parylene to make injections both easier and potentially less painful.