7 December 2023
When it comes to solving non-release, dry sticking challenges, most design and manufacturing engineers will have experience with PTFE, and frequently turn to it to solve their problems.
Traditionally PTFE has a lower coefficient of friction than Parylene. However Parylene has a comparable coefficient of friction with modern PFOA (C8)-free PTFE coatings. In many cases application-specific considerations make Parylene a better choice.
PTFE, which is a relatively hard coating around R58, can be prone to chipping and flaking in certain conditions. The removal of PFOA (C8) from all PTFE used in medical device applications adds further adhesion challenges. Reports of poor adhesion and delamination on PTFE coated stainless steel guide wires and other mandrel type products have led to a number of voluntary recalls.
Parylene is a softer polymer (R80). When used in low friction applications, it will provide a higher yield, or reusability, and does not chip or flake in the same way PTFE does. Therefore, where particulate is a risk Parylene may be a better choice.
Parylene’s unique vapor-deposition process provides for a truly conformal ultra-thin coating. For lubricity in mandrels the typical Parylene coating thickness is ≈ 4µm and as low as 0.15µm for elastomers. By comparison, PTFE tends to be applied by spray or dip, which can be prone to pooling, bridging, and edge effect. Therefore Parylene is likely a better solution where dimensional tolerance is tight, and with parts that are more topographically complex.
While there is good adhesion in both PTFE and Parylene mandrels, the selection of PTFE would seem a better option due to its hardness characteristics. However, wear characteristics depend on the operational application, as well as operator use; comparative performance testing provides the best data to make a long term choice for a specific application.
While Parylene in low friction applications will provide a higher yield in high friction, it will wear. For example, if tungsten were used for radiopacity in a peebax tip, when the peebax shrinks to a Parylene mandrel it will tear like sandpaper. PTFE rod in this case might be a better solution.
In laser welding applications, Parylene significantly outperforms PTFE, which cracks and flakes within short periods of use.
Parylene’s properties are largely unaffected by any sterilization method. PTFE is unsuitable for Gamma and does not perform too well with Autoclave. Where Gamma is used ETFE will be a better choice for catheter lumen and in this case a Parylene-coated introducer mandrel will compliment ETFE in release-ability. PTFE will not release from ETFE effectively.
Manufacturing and Coating Process
While PTFE is non-toxic, both hydrofluoric acid and carbon dioxide are among the toxic byproducts of its manufacturing process. It also releases hydrogen chloride and other toxic substances at high temperatures. As PTFE cures at high temperature it is not suitable for heat sensitive substrates such as Nitinol.
Parylene is a green chemistry. It is chemically inert and non-toxic. It produces no leechable ingredients, being free of catalytic, plasticizer and solvent residues. Parylene is applied at room temperature, so it will be the natural choice with heat-sensitive components and substrates.
Find out more about Parylene coating process.
The up-front cost of PTFE pre-coated spool wire is cheaper, and this often drives the decision in stainless steel single-use mandrels. Though for longer bare wires, non-rounds, and shaped parts, this difference is diminished. In many cases where there is potential reuse, Parylene will yield better results by reducing the overall unit cost.
The choice for a hydrophobic medical coating will depend on the application and its specific considerations. Both Parylene and PTFE have their pros and cons. Where both may initially seem like a good fit, comparative performance testing, using the intended application, will lead to better long-term decisions, as opposed to evaluating “paper-only” specifications.
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