The human body is like a fortress. It protects your organs on the inside while rejecting foreign invaders and pushing them outside. Typically, that’s exactly what you’d want it to do, but sometimes, we need foreign materials to be accepted, like in the case of transplants, rods in damaged bones and pacemakers. So, when designing antibacterial coatings for medical devices, it’s important that they meet guidelines for biocompatibility including cleanliness, consistency and stability. Parylene has many advantages that make it a great choice for implanted medical devices.
Figure 1. Image of an infant with a cochlear implant
Predictable, Safe, and Stable
Parylene has a long history of use as a protective coating for medical device biocompatibility and conforms to the USP Class VI and ISO 10993 standards. It is transparent, pin-hole free, and conforms precisely to any surface’s features. Parylene’s thickness is critically controlled and extremely consistent. It coats very thinly, on the order of microns, providing deep crevice penetration, and adds very little weight or volume. On the whole, Parylene works extremely well in most medical applications.
When a material is biocompatible it means the materials won’t interact with living tissue negatively, aren’t toxic, and are physiologically non-reactive. Parylene biobased coatings don’t evoke an immune response, are biologically stable and chemically inert, in that it survives being exposed to the chemicals found in the body, and those chemicals won’t react with parylene. The types of biocompatibility-related testing that parylene has passed is shown in Table 1 below:
Table 1. Parylene coating and dimer testing based on industry literature
|ASTM Hemolysis Complete (Direct and Indirect)||ISO 10993-4||C & N||Meets Requirements|
|ISO Partial Thromboplastin Time||ISO 10993-4||C & N||Meets Requirements|
|ISO Lee & White Clotting Time – Human Blood (Direct)||ISO 10993-4||C & N||Meets Requirements|
|ISO Lee & White Clotting Time – Human Blood (Indirect)||ISO 10993-4||C & N||Meets Requirements|
|ISO In Vitro Hemocompatibility (Direct)||ISO 10993-4||C & N||Meets Requirements|
|ISO In Vitro Hemocompatibility (Indirect)||ISO 10993-4||C & N||Meets Requirements|
|ISO Cytotoxicity Test – Neutral Red Uptake 4 Concentrations||ISO 10993-5||C & N||Meets Requirements|
|ISO MEM Elution Cytotoxicity||ISO 10993-5||C & N||Extracts Confirm Suitability|
|ISO Implant/Muscle/2Weeks||ISO 10993-6||C & N||Classified as Non-Irritant|
|ISO Implant/Muscle/13Weeks||ISO 10993-6||C & N||Classified as Non-Irritant|
|ISO Implant/Muscle/26Weeks||ISO 10993-6||C & N||Classified as Non-Irritant|
|ISO Klingman Maximization/2 Extracts/35 Animals/Concurrent (+) controls||ISO 10993-10||C & N||Meets Requirements|
|ISO Rabbit Pyrogen-Material Mediated||ISO 10993-11||C & N||Meets Requirements|
|USP Physiochemical/Plastics||USP||C & N||Meets Criteria|
|USP Physiochemical Test For Plastics – Non-Volatile Residue||USP||C & N||Meets Criteria|
|USP Class VI Test Parylene C||USP||C||Meets Criteria|
|USP Class VI Test Parylene N||USP||N||Meets Criteria|
|RoHS Compliance Parylene Type C||EU||C||Compliant|
|RoHS Compliance Parylene Type N||EU||N||Compliant|
|Reach Compliance Testing Per Regulation 1907/2006 Parylene C||ECHA||C||Passes|
|Reach Compliance Testing Per Regulation 1907/2006 Parylene N||ECHA||N||Passes|
In a hospital setting, antimicrobial coatings for medical devices and instruments are vital for keeping them free of contamination. Parylene can withstand sterilization by E-beam, gamma ray, EtO, and autoclave, with the effects shown in Table 2 below. It protects against chemicals, moisture, and bodily fluids. Overall, Parylene protects the device from the body and the body from the device, helping to prevent premature and critical use device failure in the long term.
Table 2. Effects of various sterilization methods on parylene biocompatibility
|Sterilization Method||Parylene N||Parylene C|
|Dielectric Strength||WVT||Tensile Strength||Tensile Modulus||COF||Dielectric Strength||WVT||Tensile Strength||Tensile Modulus||COF|
* 5% values aren’t likely to be statistically significant. NA = not applicable; COF = coefficient of friction; WVT = water vapor transmission.
A Low-Friction Biocompatible Lubricant
Lubricity is the measure of the reduction in the coefficient of friction (CoF) and/or wear by a biocompatible lubricant. In biomedical materials and implants, the wear performance may be related to their apparent CoF in the presence of biological fluids.
Parylene is a low-friction polymer coating that allows for easy sliding and serves as a dry lubricant. That slickness is important in many medical applications because increased friction typically means a procedure is more painful and takes longer to accomplish. Parylene is about as slippery as TeflonTM and has proven to be extremely useful for stents, syringes, catheters, needles, and other medical implants.
Parylene is used as a highly flexibility and low friction coating that resists contamination and discoloration on catheters, medical seals, and related products that use medical grade silicone and rubber. It has also been used to coat stylets and mandrels. A stylet is a malleable metal wire used to guide an endotracheal tube during a difficult intubation. Parylene has been used extensively with vascular therapy devices, such as stents, angioplasty catheters, guiding catheters and wires. A mandrel is used in the manufacturing of precision medical tubing, such as forming catheter tubing.
Inhospitable To Bacteria, Microbes, And Mold
Parylene inhibits the growth of bacteria and fungi. In fact, that’s one of the criteria for conformal biobased coatings to meet the qualification to IPC-CC-830 – Qualification and Performance of Electrical Insulating Compound for Printed Wiring Assemblies and tested in accordance with IPC-TM-650, Test Method 22.214.171.124 – Fungus Resistance – Conformal Coating. IPC-CC-830 says that “the cured conformal coating shall not contribute to or be attacked by biological growth.”
Fungus resistant materials are desired because the presence of fungi can cause infections that lead to serious health problems. Other reasons included in the IPC-HDBK-830 – Guidelines for Design, Selection, and Application of Conformal Coatings are:
- Microorganisms digest organic materials as a normal metabolic process, thus degrading the substrate, reducing surface tension, and increasing moisture penetration.
- Enzymes and organic acids, produced during metabolism, diffuse out of cells and onto the substrate and cause metal corrosion, glass etching, hardening of grease, and other physical and chemical changes to the substrates.
- The physical presence of microorganisms produces living bridges across components that may result in electrical failures.
- The physical presence of fungi can produce aesthetically unpleasant situations in which users will reject the equipment.
Wrapping It Up
The US FDA has approved parylene with a Class VI biocompatibility rating suitable for human implantable devices, based on its performance in the USP Class VI grade and ISO 10993 group of standards. Not only does parylene meet FDA biocompatibility standards, it’s also chemically inert, highly conformable with very well controlled thickness, resistant to flaking, and is capable of withstanding the effects of multiple sterilization processes. With all of these capabilities and characteristics, parylene biobased polymer coatings are just what the doctor ordered.