Dry-Film Lubricity of Parylene

Chemically inert Parylene (Poly-para-xylylene/XY) conformal film is often selected because its micron-thin protective films generate precise coating uniformity, regardless of substrate topography. To this extent, XY far exceeds the capacities of liquid materials – resins of acrylic, epoxy, silicone or urethane – for a wide range of coating assignments. While pre-synthesized liquid coatings are easier to apply, their conformal films are dimensionally thicker, which can make them difficult to position in constricted operating spaces. Liquids are also generally less resistant to contaminant incursion and other problems that interfere with reliable performance of printed circuit boards (PCBs) and most other contemporary electronics, including biomedical implants.

Parylene’s unique vapor deposition polymerization (VDP) method generates the coating film in a gas process rather than simply applying a liquid coating onto substrate surfaces. The process assures gaseous XY molecules penetrate the surface as well, producing an exceptional, pinhole-free level of component security. In addition to strong moisture barrier properties and good electronic insulation, Parylene’s dielectric constant is low, with minimal levels of added mass, outgassing and environmental impact.

Good dry-film lubricity is equally vital for various purposes, enabling assemblies to be maneuvered more effortlessly in tight functional spaces. Reduced friction during operation is a feature particularly important for elastomers, cables and extended, efficient use of biomedical implants. It is also very useful for applications involving micro-electrical-mechanical systems (MEMS) and nanotechnology.

Parylene Lubricity for Biomedical Devices

Dry-film lubricity is very important for optimal performance of implanted biomedical systems. Lubricity diminishes overall operational friction on an implant’s surface, improving function and, in the case of biomedical devices, patient health. Used for implants and related components, lubricious coatings can be found as standard coating requirements for cardiac-assist devices (CADs), catheters, elastomers, guidewires and stents. Their presence dramatically lowers frictional forces acting on insertion and operation of these devices by 90% or more. Abrasion and internal puncture damage caused from necessary placement of medical equipment within the patient’s vascular system or other internal structure is significantly reduced with use of lubricious XY film. In addition, Parylene coatings maintain that level of lubricity for an extended operational period, improving the medical device’s functional performance, while prolonging its operational life.

Conformal film lubricity results from appropriate application of low-friction materials on the matrix surface of medical devices and components. Either wet hydrophilic or dry hydrophobic methods enhance friction reduction:

• Hydrophilic Coatings Including such materials as hydroxyethyl methacrylate, polyethylene oxide, or poly-vinyl pyrrolidone (PVP), hydrophilic coatings accumulate and collect water. Generally applied by liquid dip/spray techniques, they can lower the incidence of thrombosis when treatment dictates use of a catheter or guidewire. However, hydrophilic lubricity tends to diminish over time, dissolving from substrate surfaces, leaving particulates in blood or tissue. Hydrophobic substances like Parylene are superior.
• Hydrophobic Coatings Along with liquid Teflon^™ (PTFE), Parylene hydrophobic conformal films generate reliable dry-film lubricity. Despite PTFE’s lower co-efficient of friction (µ — Teflon/0.2 – 0.3, Parylene/0.25 – 0.4), XY often provides enhanced dry-film lubricity. Of Parylene varietals, Parylene HT^® has the lowest coefficient of friction. However, most Parylene types provide reliable dry-film lubricity for medical surfaces, improving their operational flexibility. In addition, VDP processes provide a thinner, stronger, more conformal pinhole-free protective film. Parylene’s micron-thin biocompatible films also lower overall surface tack, with superior chemical, dielectric barrier and moisture security, without compromising performance.

Hydrophobic Parylene does not flake or decompose during use, enhancing its utility for implants where development of particulate is a concern. In comparison, PTFE’s liquid application technology is more particulate disposed, prone to chipping and flaking, which can pose significant danger to the patient.

Further, Parylene benefits include lower contaminant retention and self-cleaning capacities, particularly important when confronted by the often-harsh operating conditions typical of the human body. XY conformal films represent a superior option for improved lubricity for metal substrates, selected elastomers like silicone rubber, tubing and wire, maintaining pinhole-free elasticity and resilience in response to changing bodily conditions. By minimizing the implant’s physical dimensions, micron-thin Parylene coating layers further support lubricity, providing bare minimum kinetic resistance when interfacing with inner-body surfaces.