Dry Film Lubricity of Parylene
Posted by Sean Horn
Friday, December 21, 2018 7:30
@ 7:30 AM
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. It is true that pre-synthesized liquid coatings are easier to apply. However, their conformal films are dimensionally thicker, making 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 chemical vapor deposition (CVD) application method synthesizes coating in-process. Rather than simply apply coating ON substrate surfaces, CVD 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. Equally important for many purposes is provision of good dry film lubricity, a condition which:
- allows assemblies to be more easily positioned in physically constricted functional environments, and
- reduces friction during operation,
- features particularly important for extended, efficient use of biomedical implants, or
- applications involving microelectricalmechanical systems (MEMS) and nanotechnology (NT).
Parylene Lubricity for Biomedical Devices
Dry film lubricity is very important for optimal performance of implanted biomedical appliances. 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 similar appliances, 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 implantable medical devices. The objective is improving access to the targeted treatment area, while minimizing tissue damage. Either wet hydrophilic or dry hydrophobic methods enhance friction reduction:
- Hydrophilic Coatings Including such materials as hydroxyethyl methacrylate, polyethylene oxide, or polyvinylpyrrolidone (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, 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. In addition, CVD 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 the performance of medical devices.
Hydrophobic parylene seldom flakes or dissociates during use, enhancing its utility for implants where development of particulate is a concern. XY’s greater resistance to corrosion and moisture makes its protective films less likely to dissociate/dissolve during use, improving patient safety. In comparison, PTFE’s liquid application technology is more particulate-disposed, prone to chipping and flaking more readily than parylene.
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.
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