Operationally, a tube is a hollow cylinder composed of glass, metal, plastic or a similar substance, designed to contain or transport something, typically liquids or gases.  When many people think of tubing, they envision its use in construction or mechanics.  Tubing of this nature is defined not only by its purpose and the stuff its made of, but also by two dimensions -- outside diameter (OD) and wall thickness (WT). 

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 (µ_s) occurs when an object moves across a stationary surface; kinetic friction (µ_k) 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   

           Conformal coatings are surface treatments applied to a wide range of products and devices used for aerospace, automotive, biomedical, consumer, military and numerous other purposes.  Their primary objective is providing a protective film that supports a selected device’s ease of use, operating function, and service life, through an exceptional variety of working environments.  Liquid Teflon (PTFE) and parylene are two of the more widely used hydrophobic conformal coatings. 

Selection of the material used to coat a medical device is very influenced by the operational environment it will encounter when implanted in the body.  Pertinent operational/performance factors typically include:

Biocompatible parylene conformal coatings provide superior protection for medical stents.  They represent an enabling technology consistently applied to medical devices of all types for 35 years, to diminish problems stemming from surface microporosity and consequent biofluid corrosion after implant.  Providing a reliable barrier to chemicals and moisture, parylene’s static and dynamic coefficients of friction are comparable to those of Teflon.

A metal alloy of nickel (Ni) and titanium (Ti), nitinol (NiTi) exhibits the properties of shape memory and superelasticity, which make it very useful for adaptation to conformal coatings.  However, like parylene, nitinol is often difficult and expensive to produce; the extreme reactivity of the alloy’s titanium component requires exceptionally tight compositional control during combination and manufacture.  

Application of parylene’s xylylene monomer employs a chemical vapor deposition (CVD) process implemented under a vacuum.  Unlike wet coating application methods – brushing, dipping, spraying, etc. – parylene CVD is not line-of-sight.  Because the vaporous monomer envelopes all sides of the assembly being coated, appropriate process control allows vacuum deposition of an entirely conformal coating, one that penetrates deep into any crevices, rivulets, or sharp edges and points that exist on the assembly’s surface.  The resultant parylene film is insulating, ultra-thin, and pinhole-free, exhibiting superior protective barrier qualities and very low moisture permeability.

Parylene Conformal Coatings

Implantable Medical Devices and the Uses of Parylene

Parylene and Conformal Coatings