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PARYLENE - A SUPERIOR COATING FOR BIOMEDICAL APPLICATIONS

THE DEPOSITION PROCESS OF PARYLENE

THE MOST IMPORTANT FEATURES OF PARYLENE

MEDICAL APPLICATIONS FOR PARYLENE

PARYLENE - A SUPERIOR COATING FOR BIOMEDICAL APPLICATIONS

Protective coating of a biomedical surface may be required for a number of reasons, including physical isolation from moisture, chemicals and other substances, surface passivation, electrical insulation, biocompatibility, immobilization of microscopic particles and reduction of friction.

For all these applications, an extremely consistent coating thickness is critical and the coating must have as little impact as possible on component dimensions as well as on its main features.

The demand made on medical coatings vary greatly, depending on the form and function of the substrate; in most cases, these requirements can not be met with conventional coatings.

Parylene is a vacuum-deposited polymer that can be applied to nearly any biomedical substrate. The coating is stable, causes minimal or no change in the component response characteristics and isolates surfaces electrically and chemically from body fluids, moisture and ionic contaminants. It is deposited with equal thickness on all surfaces, in crevices and on edges.

Since it is formed from a pure molecular precursor (a monomer gas), the Parylene based coatings have no contaminating inclusions. Because the conversion from monomeric gas to polymer film is direct, no solvents, plasticizers, catalysts or any other ingredients are used. The resulting film has very low thrombogenic properties and low potential to trigger an immune response. Parylene has been shown to be highly resistant to the potentially damaging effects of corrosive body fluids, electrolytes, chemicals, proteins, enzymes and lipids. The film also forms an effective barrier against the passage of contaminants from a coated substrate to the body or surrounding environment.

Unlike liquid coatings applied by spraying, dipping or brushing, Parylene does not pull away from edges, bridge between adjacent surfaces or exhibit meniscus forces. And because buildup is consistent and even, physical and electrical protection can be achieved with a substantially thinner layer of Parylene as compared to conventional coatings, which often require heavier applications to overcome coating imperfections. Since there is no cure cycle with Parylene, substrates are not subject to cure forces, solvents, liquid-phase effects, or elevated temperatures. Because substrates are placed under vacuum prior and during coating process, any volatiles that might be present are extracted.

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THE DEPOSITION PROCESS OF PARYLENE

Parylene is the generic name for the members of a unique polymer series - poly-p-xylylene. The basic member of the series is Parylene N - a completely linear, highly crystalline material.. The other members of the series also commercially available are Parylene C and Parylene D; they are originated from the same monomer modified only by the substitution of one or two chlorine atoms for aromatic hydrogens.

The Parylene polymers are deposited from the vapor phase by a process that is not line-of-site and all sides of an object to be encapsulated are uniformly impinged by the gaseous monomer and uniformly coated. This characteristic is responsible for the truly conformal nature of the coating.

The deposition process consists of three steps:

• Sublimation under vacuum at approximately 120 deg.C of the stable crystalline dimer di-p-xylylene, to produce vapors of this material.
• Pyrolysis of the vapors at approximately 650 deg.C to form gaseous p-xylylene,the reactive monomer.
• Deposition and simultaneous polymerization of the p-xylylene to form poly (p-xylylene) or Parylene.

The monomer enters the deposition chamber, where it simultaneously adsorbs and polymerizes on the surface of the component to be coated. There are no liquid phases involved and the substrate temperature is very close to the temperature of the room.

A diagram of the parylene process

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THE MOST IMPORTANT FEATURES OF PARYLENE

• Uniform thickness and true conformality - the established process guarantees precise control of thickness and inherent uniformity, especially critical in micro-electronics applications; no bridging, thin-outs, puddlings, run-offs which are common problem with other coating materials. Since it is based on a gas, Parylene can penetrate spaces which typical conventional coatings can not cover: small recesses, crevices and holes and even the edges and the inside spaces of very fine tubes.
• Pinhole free - tough coatings as thin as 0.1 microns can be achieved without any voids. Chemical, fungus and bacteria resistance - Parylene resists attack from exposure to most acids, bases and solvents. It is an excellent inhibitor to the growth of fungus and bacteria.
• Superior barrier properties - Parylene provides exceptional corrosion protection from moisture, salt spray, corrosive vapors and other hostile environments. Its water vapor transmission rate has been found to be significantly lower than most conventional coatings. With respect to migrating ionic species, Parylene coatings have been proven to act as barrier to extractable metals which otherwise will contaminate substrates.
• Impressive mechanical strength - since it has high tensile and yield strength, Parylene is used for encapsulating microcircuits because it increases the pull strength of wire and lead bonds, face bonded chips and conductor bridges and therefore contributes significantly to device integrity. Since its specific gravity is low, the Parylene layers are typically lighter than most other functional coatings.
• High dielectric characteristics - its extremely high dielectric strength combined with its electrical stability in various media provide unique insulating property. The dielectric constant and dielectric losses are low and unaffected by absorption of water vapors. Its volume and surface resistivities are advantageously high because of the purity, low affinity to moisture and in particular its freedom from trace ionic impurities present in conventional coatings.
• Thermal stability - Parylene coatings remain stable at continuous temperatures as high as 130 deg.C in air, or 220 deg.C in the absence of oxygen. It has good mechanical properties from -200 to 275 C.
• Stress-free - since the polymerization of the film takes place on the substrate surface at room temperature, there is no thermal or mechanical stress introduced during application, hence original performance parameters of coated subjects are basically unaffected.
• Particle immobilization - assures circuit integrity, preventing mobility of loose solder, wire particles or other mobile debris left from manufacture. Pressed powder parts, ferrites, ceramics, corrosive metals, glass and epoxy particulates can be positively stabilized.
• Dry film lubricant -inherent excellent dry lubricity, as indicated by coefficient of friction measurements make Parylene a valuable asset as a dry film lubricant, particularly as a coating for surgical instruments. Compared to fluoropolymers, Parylene has also the ability to provide wear and abrasion resistance.
• Sterilization - due to their thermal and chemical resistance, Parylene coatings can survive the conditions of many common sterilization techniques (e.g. autoclave, radiation, ethylene oxide).
• Biocompatibility and biostability - biomedical applications are being accomplished due to the Parylene ability to offer very thin, pin-hole free ,uniform and conformal coatings which have resistance to many chemicals and are also compatible with bodily tissues and fluids. Parylene is effective in sealing the microporosity of substrates that could otherwise trap and retain contaminants. Deposition of a thin layer of Parylene over a cytotoxic surface can render it atraumatic to cells. Parylene is not vulnerable to corrosive aqueous implantation environment, hence it is regarded as likely candidate for implantation devices.

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MEDICAL APPLICATIONS FOR PARYLENE

Due to its combination of unique process and polymer properties, Parylene has become a logical choice to be used in numerous critical biomedical applications:

• Coating of biomedical devices and components in order to render their materials of construction compatible with body tissues and fluids and at the same time to maintain their original function.
• Dielectric protection of implantable devices to eliminate leakage currents
• General insulation resistance for the encapsulation of electronic circuitry to isolate active devices from body fluids, moisture and ionic contaminants.
• As a moisture and fluid barrier for implantable artificial organs

Parylene coatings have found suitability in such diverse applications: Pressure sensors, Cardiac assist devices, Prosthetic components, Bone pins, Electronic circuits, Catheters and Stylettes, Stents, Ultrasonic transducers, Bone growth stimulators, Brain probes, Needles, Pressure sensors, Feeder tubes, Cochlear and electronic implants, Cannulae and mandrels used in construction of catheters and instruments for Laparoscopic surgery.

It should be noted that most of devices and components need to pass the industry testing standards and obtain regulatory approvals in order to be used in biomedical applications.

Implantable Bioelectronic devices, including stimulating microelectronic probes and miniature wires connecting these electrodes with a source of control and power, must be protected from the corrosive ionic environment of extracellular fluids, if they are to be used in humans. These protective coatings have to be biocompatible and not release potentially toxic materials. In short, the coating must:

• resist water and ions-notably sodium
• adhere to the probe surface
• provide insulation along the interface
• be as thin as possible
• frequently withstand high electric fields
• be nontoxic and sterilizable
• be flexible and have a low coefficient of friction
• function reliably

The choice of coating materials is dictated by many factors including biocompatibility, mechanical characteristics, electrical properties, biostability in saline environments.

Parylene coating, has been shown to be an effective, inert coating for implantable bioelectronics.

Both Parylene N and Parylene C are certified to comply with the USP biological testing requirements for classification VI (acute systemic toxicity, intracutaneous toxicity and implantation).

Data on the biological properties of Parylenes has been obtained in studies at Battelle Memorial Institute, John Hopkins Hospital, University of California at San Diego, Carnegie-Mellon University, University of Michigan and several other institutions.

Typical Properties of Commercial Parylenes versus other coatings
Property	Parylene N	Parylene C	Epoxies	Silicones	Urethanes
ELECTRICAL PROPERTIES
Dielectric Strength, short time, volts/mil ( on .001" thick film)	7,000	5,600	-	-	-
Volume resistivity @ 23deg.C 50% RH, ohm-cm	1017	1016	1012-1017	1015	1015
Surface resistivity @ 23deg.C 50% RH, ohms	1013	1014 -	-	-	-
Dielectric Constant 60 Hz 1,000 Hz 1,0000,000 Hz	2.65 2.65 2.65	3.15 3.10 2.95	3.15-5.0 3.5-4.5 3.3-4.0	2.75-3.05 - 2.6-2.7	4.0-7.5 4.0-7.5 6.5-7.1
Dissipation Factor 60 Hz 1,000 Hz 1,0000,000 Hz	0.0002 0.0002 0.0006	0.020 0.019 0.013	0.002-0.01  0.002-0.02 0.003-0.05	0.007-0.001  - 0.001-0.002	0.015-0.017  0.05-0.06 -
PHYSICAL AND MECHANICAL PROPERTIES
Secant (Young) Modulus,psi	350,000	400,000	350,000	900	1,000-10,000
Tensile Strength,psi	6,000-11,000	10,000	10,800	800-1,000	175-10,000
Yield Strength,psi	6,100	8,000	-	-	-
Elongation to break,%	20-250	200	3-6	100	100-1,000
Density,g/cc	1.10-1.12	1.289	1.11-1.40	1.05-1.23	1.10-2.5
Index of refraction	1.661	1.639	1.55-1.61	1.43	1.50-1.6
Water absorption/24 hrs.,%	less than 0.10 %	less than 0.10 %	0.08-0.15	.12 (7 days)	0.02-1.5
Rockwell Hardness	R85	R80	M80-M110	40-45 Shore A	10-250 Sh
Coefficient of Friction Static Dynamic	.25 .25	.29 .29	- -	- -	- -
THERMAL PROPERTIES
Melting Temperature, deg.C	405	280	cured	cured	170 or cured
Heat distortion Temperature,degC	-	-	up to 220	up to 300	-
Linear Coeff. of expansion, 10-5 /degC	6.9	3.5	4.5-6.5	25-30	10-20
Thermal Conductivity, 10-4cal/sec/sqcm/cm	3	-	4-5	3.5-7.5	5
BARRIER PROPERTIES
Moisture transmission, g/mil/100in/24 hrs	1.6	0.21	1.79-2.38	4.4-7.9	2.4-8.7
Gas permeability, cc-mil/100sqin/24 hrs. to: Nitrogen Oxygen Carbon Dioxide Hydrogen	15 55 420 540	0.6 5 14 110	4 5-10 8 110	- 50,000 300,000 45,000	80 200 3,000 -

* Vitek Research Corporation has developed techniques which insure high degree of adhesion between Parylene coating and the surfaces of biomedical devices.

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