Lubricious Coatings

Lubricity refers to a conformal coating’s capability to reduce friction during operation, preventing any hindrances to its function that could jeopardize patient well-being. Lubricious coatings offer essential protection for devices such as cardiac-assist devices (CADs), catheters, elastomers, cables guidewires and stents. In contrast to an uncoated device, lubricious films can reduce frictional forces by more than 90%. This relative ease of use is important for implants and similar devices that require navigation throughout the patient’s vascular system or other internal structure; otherwise, patients can suffer from abrasion generated between the device surface and blood vessel walls.

Coefficient of Surface Friction

The degree of physical resistance a device demonstrates is numerically expressed by a coating’s coefficient of friction (µ), which characterizes the level of resistance a surface exerts on substances moving across it. Static and dynamic coefficients describe the friction forces that must be overcome to a) initiate movement against a surface and b) maintain movement against that surface respectively. Ideally, a coating will impart identical static and dynamic coefficients of friction against a surface, eliminating a differential in force to initiate movement and to keep it moving. In syringe technology, a difference between these two is referred to as break-out force, a detrimental property to those who administer injections.

Where higher-level surface lubricity is sought, lower µ-values are the objective; they signify lessened frictional resistance, minimizing non-release, dry-sticking challenges that interfere with devices’ performance. For instance, a µ-value of 1 indicates an equal quantity of force is needed to either lift an object, or slide it across a level surface; these calculations compare an object’s weight to the total force required to make it move. Most everyday objects and materials have a coefficient between 0 and 1; values closer to 1 are not feasible for medical purposes. For medical devices, a µ-value ranging from 0.01 to 0.1 is ideal, but remains difficult to achieve for application to the expansive degree of metallic and polymeric substrates used for medical appliances, which require highly-specified levels of abrasion resistance and non-thrombogenic properties in addition to biocompatibility and lubricity. Appropriate safety standards also need to be met.

Much depends on the materials comprising the touching surfaces. Conformal coatings like Teflon™ (PTFE) and Parylene, which provide high-level lubricity, maintain that level for a prolonged operational duration, making them very useful for specialized medical applications.

Properties of Reliable Coating Lubricity

Lubricated surfaces have lower levels of friction. Wet hydrophilic coatings amass water as a source of lubricity, applied by liquid methods such as dipping or spraying the film substance onto substrates. Applied to catheters or guidewires, they temporarily minimize development of thrombosis. However, their lubricious function decreases with time, dissociating or dissolving from the matrix surface, leaving particulates in tissue or the bloodstream, endangering patient health. Thus, they are less reliable long-term than hydrophobic coatings.

Hydrophobic conformal coatings provide dry-film lubricity, and include such materials as Parylene and liquid PTFE. Applied by wet methods, PTFE has a lower co efficient of friction than Parylene. With the requirement to eliminate PFOAs in Teflon solutions, the industry encountered serious issues with the resulting Teflon films cracking and flaking. Given time, scientists were able to identify modifications to the process that would reduce particulate shedding of the final Teflon films. Shedding has been substantially eliminated with PTFEs, however, Parylene can offer better quality dry-film lubricity in many cases, essentially because of its vapor deposition polymerization (VDP) application process, which provides a generally more resilient and pinhole-free protective film that never suffered from the catastrophic issue encountered by PTFEs with the elimination of PFAS.

The coefficient of friction for PTFEs range from 0.05 to 0.2, and Parylenes fall in the range of 0.13 – 0.3. In addition to improving components’ lubricity, Parylene films also reduce overall surface tack, with micron-thin biocompatible coating protection, generating outstanding chemical, dielectric barrier and moisture protection for an exceptional range of medical devices and components. These Parylene coatings provide sufficient dry-film lubricity to lubricate implanted medical devices per specification without compromising the devices’ original elasticity.