Contributing to good performance for internal medical appliances, lubricity is a conformal coating’s ability to lower operational friction that might retard its function and endanger patient health. Lubricious coatings offer essential protection for appliances like cardiac-assist devices (CADs), catheters, elastomers, guidewires, and stents. Compared to an uncoated device, lubricious films can reduce frictional forces by more than 90%, dramatically decreasing potential harm caused by excessive insertion-force or internal puncture damage. 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 quantifies:
- the magnitude of resistance a surface exerts on substances moving across it, or
- the minimum force necessary for an object to slide on a surface, divided by the forces pressing them together.
Static friction (µs) occurs when an object moves across a stationary surface; kinetic friction (µk) results for two objects simultaneously in motion, moving across each other. Conformal coatings are used in both circumstances, especially for medical implants with moving MEMS/nano-tech components.
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
Materials like PTFE and parylene generate dry-film, hydrophobic lubricity, which offers numerous advantages to coating surfaces/substrates. In addition to extended performance life for both the coating and the substrate, benefits include decreased retention of dirt, improved corrosion/moisture resistance, and self-cleaning. Greater resistance to corrosion and moisture makes them less likely to dissociate/dissolve during use, enhancing better patient safety.
Parylene Dry Film Lubricity
Parylene film provides excellent dry lubricant protection, abrasion and wear resistance for surgical instruments and other medical devices. Hydrophobic PTFE has a generally lesser µ-value (0.2-0.3) than parylene (0.25-0.4), but its wet-technology is more particulate-disposed, prone to chipping and flaking more readily than parylene. In comparison to PTFE, parylene’s chemical vapor deposition (CVD) method of film-application generates a coating typically more resilient, pinhole-free and protective, particularly under harsh operating conditions.
Except in the case of coiled guidewires, parylene lubricity enhances the entry and removal of medical implements within the body. Lubricity is also aided by the micro-thin coating layers parylene can produce, minimizing the actual physical dimensions of an implant, offering lower kinetic resistance when in contact with internal surfaces. Hydrophobic parylene offers a superior option for improved lubricity for metal substrates, selected elastomers like silicone rubber, tubing and wire, without risk of particulates.
Of the parylenes, type AF-4 has the lowest coefficient of friction. However, most parylene types provide reliable dry-film lubricity for medical appliances, improving their operational flexibility.
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