Conformal Coating Education Center

Parylene for Bio-Medical

Whenever implantable devices come into contact with the human body, long term protection against body fluids, enzymes, proteins, and lipids is vital. Bio-medical surfaces typically require coating to protect from moisture, chemicals, and other potentially harmful substances.

In this Section:


Parylene for use in Bio-Medical implantable devices

Where it's Used

Whenever implantable devices come into contact with the human body, long term protection against body fluids, enzymes, proteins, and lipids is vital. Bio-medical surfaces typically require coating to protect from moisture, chemicals, and other potentially harmful substances.

A downfall for wet chemistry, liquid coatings such as silicones, acrylics, epoxy, or urethanes is that they do not meet bio-compatibility requirements and cannot be applied with precise control. On the contrary, parylene does not out-gas and is very effective against the passage of contaminants from both the body to substrate or substrate to body.

Parylene can be applied a couple of mils thick to a few hundred angstroms in thickness, it is pin hole free, applied at room temperature, contains no additives, insoluble in most solvents, very lubricious, has a very high dielectric strength, and is biocompatible and bio-stable.

Parylene C is currently being used is a number of well documented bio-medical implantable devices. Parylene C has been proven to be a terrific biocompatible material. It is USP Class VI implantable plastic material and conforms to material ISO-10993 Biological Evaluations for Medical Applications. Parylene C is also probably the longest proven protective biocompatible material.

Sources:
Parylene Technology for Neural Probes Applications. Changlin Pang. California Institute of Technology. 2008.

S. Nancy, "Literature Review: Biological Safety of Parylene C," Medical Plastics and Biomaterials, vol. 3, pp. 30-35, March 1996.

B. Humphrey, "Using Parylene for Medical Substrate Coating," Medical Plastics and Biomaterials, Janurary 1996.


Parylene and Sterilization

For Medical Devices

Parylene (XY -- poly(para-xylylene)) organic polymers are highly regarded through a wide range of industries – aerospace/defense, automotive, commercial, industrial, medical – for their utility as conformal coatings. Chemically inert, colorless, linear/polycrystalline and optically clear, XY coatings provide exceptional barrier protection, dielectric reliability, and insulation for printed circuit boards (PCBs) and similar electronic assemblies whose components must maintain performance through all operating conditions. Parylene conformal films safeguard function in the presence of biogases, biofluids, chemicals, moisture/mist, salt compounds, and temperature fluctuations.

Applied by a unique chemical vapor deposition (CVD) process, parylene assumes a gaseous consistency, allowing it to seep deep within a substrate surface while simultaneously forming an effective outer layer of protective film. The result is a truly conformal, pinhole-free coating, absent extraction issues, leaching and outgassing. CVD synthesizes coating in-process, allowing the deposited film to:

  • assume virtually any component configuration,
  • a distinct advantage over liquid coatings such as acrylic, epoxy, silicone or urethane,
  • applied by wet methods like brushing, immersion or spray.

CVD lets XY penetrate and coat small cracks, crevices, and openings along, within and under the assembly’s surface, reaching even hidden component areas, places where liquid coating materials – brushed, dipped or sprayed – cannot effectively approach.

Clean, self-contained CVD requires no additional chemicals to complete the process, depositing uniform XY film thickness, even on irregular surfaces. Vapor methods assure reliable coating deposition on an expanded range of devices and products, with longer-lasting security and performance, minimizing the need for repair and potential of assembly failure.

steriizer

Sterilization Processes for Parylene Conformal Films
Sterilization procedures for industrial processes destroy, permanently deactivate or otherwise remove all lifeforms existing on or within products, many designated for biomedical use. The objective is to establishing exceptional product safety, eradicating all sources of existing or potential contamination. Sterile pharmaceutical products include medical implants of all kinds, which need to function without fail in often turbulent biomedical environments.

  • Sterilization of implants – cannulae, cardiac assist devices (CADs), catheters/probes, electronic circuitry, needles, prosthetics, stents and similar devices -- purges any existing contamination, improving options for patient health after placement in the body.
  • XY conformal coatings protect the implant’s sterile environment within while generating reliable barrier protection that shields the device from the effects of biofluids/gases that can cause assemblies to fail prematurely.

As with XY’s superior comparative worth in relation to liquid coatings, it also registers well for sterilization. Chief among the coating’s benefits is the ability to withstand common sterilization techniques -- steam autoclave, electron beam (e-beam), ethylene oxide (EtO), gamma radiation and hydrogen peroxide ((H2O2)) plasma.

  • Autoclave moist-heat sterilization devices subject XY-coated PCBs/implants to high pressure saturated steam, measured at °C/pound or square inch, according to process time (minutes). These factors -- pressure, temperature -- are recorded throughout the entire automatically controlled/timed process, to assure appropriate component sterilization; autoclave procedures typically generate lower equipment/product damage.
  • Electron beam sterilization’s shorter exposure time generates less breakdown and long-term aging for XY films, sterilizing low-density, uniformly coated devices quickly and effectively. E-beam also modifies polymers, improving the switching speed of semiconductors.
  • A colorless liquid, ethylene oxide has a low boiling point of 10.8°C (55.44°F); rendered inflammable when mixed with CO2, sterilization relies on appropriately applied concentration – mg./lit. – in relation to time exposure, measured in hours.
  • Gamma radiation is a cold sterilization method with considerable penetration power; lethal to DNA and other vital cell constituents, it requires little thermal energy, an advantage for sterilizing heat-sensitive materials/products.
  • Compatible with most (>95%) medical devices and materials tested, hydrogen peroxide plasma sterilization is suitable for devices and materials -- corrosion-susceptible metal alloys, electrical devices, some plastics -- unable to tolerate high temperatures and humidity. H2O2 plasma generates free radicals hydroxyl and hydroperoxyl, which eliminate contaminant microorganisms.

The selection of the appropriate sterilization process is crucial to the success of the assignment. Selection criteria emphasize the type of device being sterilized, its purpose, and the type of XY coating being used. CVD allows parylene application to most vacuum-stable materials -- ceramics, fabrics, granular materials, metals, paper, plastics. Across type, XY varietals withstand the sterilization methods described above; different XY material types respond better to specific sterilization techniques.

  • Highly elastic, N. the most basic para-xylylene type, provides excellent penetration of minute assembly compartments. Unlike many other film materials, N withstands radiation sterilization via e-beam or gamma methods. However, autoclave sterilization is less recommended, because of lower resistance to heat.
  • Type C is less elastic than N, but high moisture resistance enhances its value for biomedical applications. C maintains chemical structure under radiation sterilization via e-beam/gamma. Yet, C’s low T5 point (125°C -- maximum operational surface temperature) is less than the temperature applied for most autoclave sterilization. XY-C also can anneal during high-temperature steam sterilization, increasing film crystallinity.
  • Parylene F has high thermal stability, sufficient to withstand steam autoclave temperatures required for creating multi-use category smart catheters/probes -- devices that offer lower cost and longer function, but require repeated sterilization cycles. While N and C adapt well to other sterilization methods, autoclave sterilization can reach temperatures that challenge their survival, we recommend parylene F in those situations.

Parylene withstands all common sterilization methods, but matching XY type with sterilization process is necessary. While autoclave is not recommended for N or C, radiation techniques are successful. H2O2 plasma sterilization treatment slightly alters C’s dielectric strength but does not for N. Radiation dosage and procedural duration always need to be monitored for optimal results. With high thermal stability, F better withstands autoclave temperatures recommended for sterilizing multi-use implants.


Tubing and Parylene Coating

Points to Consider

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).

Used as much for structural purposes as conveyance, these tubes can be square or rectangular in shape, as well as round. Tubing of this scale is also employed for automotive, energy, engineering, and precision/pressure uses. Depending on its purpose, larger-scale tubing may benefit from conformal coating, but this is more widespread for piping, where liquid films of epoxy or silicone are common.

Tubing
Conformal coating is more frequently applied for smaller, specialized tubing, used for biomedical, computer or electronics purposes. In these cases, tubing is also used as a means of material conveyance, such as:

  • carrying medicines through implanted devices within the body, or
  • liquid cooling for computers.

Size measurement for this tubing differs somewhat from that used for construction and other larger-scale applications. While OD measurements remain necessary, inner diameter (ID) measures replace WT. The proliferation of microelectromechanical systems (MEMS) and nanotechnology (NT) increases the need for reliable conformal coating of specialized tubing measured in microns or nanometers.

Liquid conformal coatings – resins of acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – are impractical for both tubing uses and MEMS/nano applications:

  • Their effective coating layers -- 0.025–0.127 millimeters for AR/ER/UR and .051–0.203 mm. for SR – are far too thick MEMS/NT devices.
  • Liquid deposition methods – brushing, immersion (dipping), spraying – cannot be adapted for adequate MEMS/NT coverage.

As important, liquid applications are not reliable for coating tubing of any size. Applied as wet material, brushing is slow and will not evenly coat inner tubing surfaces. Both dipping and spraying may provide a more evenOD coat and better internal coverage, but cannot be relied upon to provide a uniform and effective protective layer for ID surfaces.

In contrast, parylene (XY) conformal coating can be successfully applied to tubing regardless of size. Its specialized chemical vapor deposition (CVD) method creates pinhole-free conformal films of exceptional uniformity. Powdered XY dimer material is transformed into a gas under a vacuum, during CVD. Its vaporous condition allows XY to travel to, enter and coat anywhere on the substrate the gaseous XY reaches, including the tube’s inside surfaces.

  • Unlike liquid coatings, whose wet application methods prevent film materials from reaching a substrate’s obscure or hidden surfaces, parylene covers the entire assembly.
  • Vaporously penetrating a substrate’s surface, XY provides a protective layer both within the object’s surface, as well as to its interior, offering a further stratum of conformal protection.
  • This applies to tubing, regardless of size or shape. Gaseous migration of parylene into the interior of the tube allows uniform ID coating. Moreover, parylene’s ultra-thin films, typically from 0.013–0.051 mm., are manageable at levels less than one micron (1 μm/1,000 nms), encouraging MEMS/NT tubing uses. XY’s extremely thin conformal films add minimal build-up to either tubing’s OD/ID.

Truly conformal parylene generates a slippery, easily cleaned finished surface of exceptional dry lubricity, permanently bonding to film surfaces, both external and internal. It does not attract debris. XY’s ultra-thin coatings offer additional advantages for tubing:

  • adherence to the range of existing tubing materials/topographies,
  • biological/chemical inertness,
  • bubble-free conformability/flexibility at film thicknesses > 0.5 μms.,
  • low-friction lubricity,
  • penetration of extremely small crevices/spaces, and
  • reliable dielectric/moisture barrier properties, with
  • resistance to bacteria/fungus growth, heat, radiation and solvents.
  • Strong, resilient coatings eliminate pathways for corrosive compound entry.

These combined properties make biocompatible, dry lubricant XY the conformal film bonds without fail to tubing’s inner and outer surfaces, at thickness levels that add virtually nothing to its final dimensions.


Improving Bio-Compatibility with Parylene

Parylene Bio-compatibility

As a biomaterial, parylene offer numerous possibilities in the fields of biomedical implants, biophysical studies, biosensors and tissue engineering. For instance, biological microelectromechanical systems (BioMEMS) offer accurate, rapid medical diagnoses, copying standard laboratory services onto miniaturized devices that can be inserted safely into the human body.

A problem limiting greater effective miniaturization has been finding reliable coating materials that sustain bio-compatibility, while providing coating thicknesses suitably thin and strong to maintain ongoing functioning. Parylene provides bio-compatible conformal coatings for these purposes that are pinhole free, chemically inert, with low cytotoxicity and resistance to swelling in liquid environments. It has proven useful for such practical BioMEMS' devices as low-volume blood analysis cartridge systems and home pregnancy tests. In addition, parylene's elastometric properties sustain long-term implant usage within the body. A coating thickness of 8 µm generates sufficient protection from corrosive elements and debris.

The Possibilities of High Aspect Ratio Microstructures (HARMS) and Surface Modification/Functionalization (SM/F).

Persistent demand for greater functionality and improved performance include calls for faster delivery of treatment and enhanced diagnostic accuracy. In this respect, these developing technologies show considerable promise:

  • HARMS generates a larger range of performance functionalities, more efficiently and with higher throughput.
  • SM/F processes have been helpful for spinal interbody implants; applied in conjunction with HARMS, they functionalize implants, customizing its operational parameters to prevent or promote the adhesion of cells and proteins, as required by the treatment.

Moreover, if the objective of developing these technologies is to further reduce the sizes of BioMEMS devices, parylene provides the best available conformal coating. It generates exceptionally durable, yet ultra-thin, bio-compatible, pinhole-free coatings, uniformly deposited on HARMS; highly resistant to surface corrosion, parylene prevents bodily fluids or substances from penetrating the medical device, allowing it to function as designed. Parylene's other advantageous properties -- chemical inertness, high dielectric strength, lightweightedness, transparency -- further benefit BioMEMS applications.

In addition, parylene coatings offer pinhole free conformal protection for microfluidic structures with film-thicknesses less than 1µm and aspect ratios exceeding 10. Parylene-C is particularly useful for promoting minimal cell adhesion. The additional benefit of non-cytotoxicity is apparent for MEMS' devices used in cell applications; parylene films diminish blockages within microfluidic channels by limiting the incidence of cell agglomeration around devices. Then too, surface modification (SM/F) using O2 plasma promotes HeLa cell adhesion by at least twofold, suggesting local activation in reaction chambers increases cell-capture, further demonstrating the extent of parylene's bio-compatible properties.

For instance, SM/F procedures support precise management of cell behavior and bio-molecular spatial location. Parylene's versatile, bio-compatible and pinhole-free conformal coatings are useful to instruments studying and mapping multi-component combinations of molecules and receptor-ligand interactions occurring in the body. Parylene allows bio-compatible surface modification of virtually any implant-material. Micropatterning with parylene “peel-off” stencils offers the advantages of patterning biomolecules in hydrated environments, with high uniformity over a large area, and a minimal sub-100nm nanoscale resolution. This approach is particularly useful for patterning chemically and biologically sensitive molecules and helping to preserve their conformation and bioactivity.

Conclusion
Parylene polymers provide conformal, protective coatings for medical instruments. due to such properties as exceptional bio-compatibility, hydrophobicity, reliable mechanical performance, and functional stability in bodily fluids. To this end, parylene demonstrates adequate elastomeric properties, essential to sustaining strains during surgical implantation and long-term usage in the body. Its vapor deposition process promotes application of a reliably conformal and durable film on complex implant shapes, through a wide range of substrate/product materials, including ceramic, composite and metallic substance. In particular parylene C has demonstrated considerable versatility for biomedical applications, although types D and N have also been successfully adapted for numerous uses. IParylene's bio-compatible films homogeneously coat BioMEMS structures designed to stimulate and monitor critical bodily functions, without interfering with the actual processes themselves, generating exceptionally dependable process compatibility.


Best Implantable Device Coating

How Parylene Protects

Implantable devices place a special set of requirements and challenges on their coatings. The moisture and broad mixture of chemicals that are found inside of the body are challenging in and of themselves. However, the body also has needs from the coatings that are placed within it. They need to be non-irritating and inert enough to be harmless. For most applications, the best choice is USP Class VI compliant parylene coatings.

USP Class VI Certification
The most important factor in finding the best implantable coating is whether or not it holds a biocompatibility certification. The industry's go-to standard is the United States Pharmacopeia's Class VI certification which is typically also covered by products that also comply with the European ISO's 10993 standard.

To achieve Class VI certification, a coating material starts by proving itself to be inert when injected into the body of a test animal. First, it gets mixed with four different carriers -- saline solution, alcohol saline, vegetable oil and polyethylene glycol -- and, as appropriate, injected either intravenously or into a body cavity (intraperitoneally). If, after three days, the animal is still alive and shows significant reactions, the material goes to the next phase of certification.

Next, it gets mixed with the same carriers and injected into multiple sites on two animals. To pass (and earn Class I, II, III or IV certification), the animal must show no sign of reaction after three days of daily checks.

Finally, to earn the certification, strips of the material get surgically implanted in two animal's muscle tissue. Assuming that the material causes no significant reaction after five or seven days, it earns its USP Class VI certification.

A coating that has its biocompatibility certification in place has two key benefits. First, once it is tested, you should not have to test it again, saving you from an expensive delay. Second, the certification gives you a sense of assurance that it is an appropriate choice for your implantable device or item.

Parylene Coating for Implantable Devices
Parylene isn't only the gold standard for conformal coatings. It is also the best option for implantable devices. On just about every metric, it perfectly suits the needs of biological applications. Its USP Class VI certification also means that it is safe to use.

More than almost any other conformal coating, parylene dimer is chemically inert. It resists both the acids and bases that are typically encountered when implanted. At the same time it also has excellent resistance to moisture and to corrosive materials -- like salt. While it is unlikely to encounter industrial solvents when implanted, it can also withstand a full range of organic chemicals.

The unique mechanism of parylene adhesion also makes it a good choice in implantable devices. It generally adheres to itself. This means that it forms a tight seal around implantable devices, protecting them from the body and the body from them.

Vapor based deposition means that a parylene coating is more truly conformal than that created by any other compound. Since parylene coats any part of a device that air can touch, it covers exposed surfaces underneath parts and even areas inside of the item that are not otherwise covered. Given that, over time, bodily fluids can also permeate any of those cracks or flaws, parylene is a safe option.

Parylene coatings aren't just truly conformal. They are also unique in their ability to create a truly conformal coating in thicknesses that are measured in microns. While other coatings can approach parylene's performance, they typically are anywhere from 10 to hundreds of times thicker when applied. Given that size is often a concern in implantable device, parylene's thinness is another significant benefit.

Even though it dries, parylene has a high degree of lubricity -- similar to PTFE (teflon). Its dry film lubricity makes it an excellent match for implantable devices since it reduces the risk of irritation or inflammation when the device is positioned. In fact, many needles are coated with parylene to make injections both easier and potentially less painful.


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