When it comes solving non-release, dry sticking challenges most design and manufacturing engineers will have experience with PTFE and turn to it to solve their problems.
PTFE has a lower co efficient of friction than Parylene. However in many cases application specific considerations make Parylene a better choice.
PTFE which is a relatively hard coating around R58, it can be prone to chipping and flaking in certain conditions. The removal of PFOA (C8) from all PTFE used in Medical Device applications added further adhesion challenges. Reports of poor adhesion and delamination on stainless steel guide wires and other mandrel type products led to a number of voluntary recalls.
As a softer polymer R80 Parylene in low friction applications 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 vapour 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. PTFE tends to be applied by spray or dip and 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 complex in their topography.
Where there is good adhesion in both PTFE and Parylene mandrels, the PTFE being a harder coating, on the face of it would seem a better option. However wear characteristics depend on the application and operator use; comparative performance testing will provide the best information 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 wielding applications Parylene significantly out performs PTFE which cracks and flakes within a short period 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, 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 inhert 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.
Watch our video to find out more about Parylene coating process.
Up front cost PTFE pre coated spool wire is cheaper and this tends to drive 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 higher results 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 seem a good fit a long term decision would benefit from comparative performance testing using the intended application rather than a paper only decision.
Written by Jason Delaney Technical Sales Manager at Curtiss-Wright.
Curtiss-Wright is a NYSE listed global diversified industrial company that provides niche highly engineered products and services to industries including Medical, Aerospace, Defence and Energy.
Our Parylene Coating Services was set up in 1991. With founding expertise extending back as part of Union Carbide’s development team who commercialised Parylene over 40 years ago.
Today our ISO13485 certified coating facilities in North America and Europe provide specialist Parylene coatings mainly for the Medical Device and Aerospace Industries.
Our proprietary Dimers, the precursor for Parylene are over 99.7% pure. These are tested to the requirements of ISO10993 and USP Class VI for bio compatibility and long term implant use. Drug and device master files are held with the FDA.
Our aim is to support R&D and process engineering in their work with life saving devices. We welcome new development and evaluation projects and provide a collaborative low cost and fast turnaround service.