Physical Coatings

(by J.J.G. Bos)

Categorizing Physical Coatings
PTFE layers
Polyethylene layers
Nylon surfaces
Silanized coatings

I have taken the liberty of naming coatings that directly operate from their physical surface properties "Physical Coatings". They thus do not include drugs for e.g. interaction with the blood. Some people would prefer to use the term "inert coating", rather than "physical coating", but that suggests that the body (especially the blood) does not respond biochemically to the presence of such a surface. Well, it does and often violently. This underlines the down-side of physical coatings: They are not tailored to optimize the biochemical interaction with its in-vivo surrounding. The up-side is that physical coatings are generally much easier to handle, both during manufacture and use.

Hydrogel coatings that do not utilize drugs can be considered physical; they operate on absorption of body fluids and, in case of the lubricious versions, use mobility of the hydrated hydrophilic groups for reduction of friction. The Sci-Notes page of this website contains links to more elaborate information regarding hydrogels. I will therefore continue with some other well-known types of physical coatings.


Physical coatings (beside hydrogels) that are commonly applied to invasive medical devices are: PTFE (teflon), polyethylene, polypropylene, nylon, polyurethane (linear), and various silicone-based films. Application and research in the field of materials such as parylene and "diamond-like carbon" are quite exciting, and there is lots of information available on the web from various manufacturers. However, this little chapter will limit its focus on some properties of PTFE, polyethelene (PE), nylon 12, and silanized coatings, with emphasis on in-vitro and in-vivo friction.

PTFE Teflon layers are extremely popular for use with invasive devices, especially as lining of guiding-catheter lumens and as coating for guide wires. The PTFE will give rise to surface-induced thrombus generation when used in blood, but the thrombi hardly adhere. So, the worst effect is that only small thrombi can be expected to come off the PTFE layer. It is well-known that the smaller the thrombus, the less the harm. PTFE therefore causes much less risk regarding thrombus formation than e.g. nylon-12. Clinical practice has shown that the effect can be lived with. This then leaves us with the major merit of PTFE: Friction reduction. Where lubricious hydrogels show breakdown regarding friction-reduction when local pressure-induced dehydration occurs during use, PTFE simply keeps on going. Wet or dry, it does not matter much.

Biggest down-side of PTFE as coating material is that it does not adhere well to other polymers, even though it is possible to modify the PTFE surface in order to improve bonding (e.g. with help of a plasma treatment). But even that leaves lots of worries. However, PTFE is very successfully coated onto metals (think of the frying pan), and is therefore an extremely popular choice for application to guide wires. The coating can even be ordered in a color of your liking. The coating-process involves sintering onto the metal, which thus includes application of very high temperatures. Too high for polymers, but no bother for metals.

The left graph below shows the static friction coefficient of some other materials against PTFE. The coefficient varies between approximately 0.07 and 0.11. Low-friction hydrogel coatings are able to go (well) below these numbers, provided they do not (locally) dehydrate upon use. However, material integrity of the PTFE layer is far more superior, i.e. there is very little chance of particle generation upon use. This in contrast with hydrogel coatings.

Polyethylene (PE) is known for its good biocompatibility, just like PTFE. It generates less thrombus than e.g. nylons and linear polyurethanes. Especially high-density polyethylene (HDPE) shows low friction with numerous counter-surfaces. Typical values for static friction are 0.10 for HDPE and up to 0.18 for LDPE. HDPE lining is used in balloon-catheter lumens in order to reduce friction with the (PTFE-coated) guidewire. The poor mechanical properties of HDPE do not make it suitable for thin-wall catheter manufacture, thus explaining the dual layer approach: Nylon on top for mechanical strength and stability, and PE as lining on the inside for reduced friction. PTFE is very hard to use in this particular application. There is one major drawback with polyethylene: It does not always respond well to gamma sterilization. The figure in the middle (above) shows some static friction data of low-density polyethylene (LDPE) against a selection of other surfaces. Cetry's EMPEH coating easily beats the PE lining (refer to the PRODUCTS pages for more information on the EMPEH coating).

Nylon exhibits excellent mechanical properties and is thus mostly used for high-performance endovascular catheters, such as diagnostic, PTA, and PTCA catheters. Biocompatibility is quite good, but it is often compromised by the presence of radiopaque salts that are mixed into the nylon material (see the SEM photographs in the Sci-Note on hydrogels to get the idea). Thrombi formed at the nylon surface upon contact with blood, adhere extremely well. This in turn results in fast increase of its coefficient of friction. Nylon catheters are usually coated or lined with another material, in order to obtain low-friction properties (read e.g. the above paragraph on polyethylene lining of nylon tubing for balloon catheters). Nylon can also be used as lining or countersurface of stainless steel, e.g. for hip joints. The graph at the right above shows static friction data for nylon-12 against a selection of other surfaces. Friction is generally higher than that of polyethylene or PTFE, and that makes it unsuitable for low-friction application in coaxial catheter systems.

There exists a whole range of coating materials for medical applications. A very important subrange is that of silicones. Low-friction silicone gels are applied to injection needles and tips of guidewires (a section of the wire that is hard to coat with PTFE). Apart from friction reduction, it also reduces the chance of oxidation (rust); realize that EtO sterilization is a very wet and oxidizing process, which may easily start rust-seeds on the steel. With a shelf life of several years, some rust stains might become visible on some sorts of steel.

Low-friction medical silicone coatings generally consist of cross-linked molecular chains combined with smaller unbound molecules for lubrication. These silicone coatings are genuine gels. The reaction mechanism, typical for most silicones, results in excellent adhesion to glass, but also to steel, nylon, plasma-treated polyethylene, etc. Curing happens under reaction with moisture and air; pot life is excellent. These coatings are generally highly biocompatible, with thrombus generation approximately equal to that of high-density polyethylene. Static friction is typically slightly higher than for PTFE, but it strongly depends on the state of curing, coating thickness, and properties of the free molecular content in the silicone gel. Too thick or too thin silicone layers on e.g. nylon will give rise to a higher coefficient of friction. The coating is usually applied by means of dipping or flushing techniques, followed by curing on air.