Literature Review: Biological Safety of Parylene C

Medical Plastics and Biomaterials
Magazine
| MPB Article Index

Originally published March 1996

COATINGS

NANCY STARK

A typical acute biological-safety testing profile, as specified in FDA's General Program Memorandum G95-1, can cost in the neighborhood of $6000 and require 90 days to conduct. Subchronic, chronic, and carcinogenicity testing can add several thousand more to the price tag, and there is no guarantee that a material will pass the tests or that the test results won't raise further questions.

Some manufacturers are turning to biological-safety literature reviews as a way of predicting the outcome of safety tests, and of sparing themselves the effort of rediscovering already established information. The article that follows is an example of such a technical report on the surface coating parylene C.

Biological-safety literature reviews are not a new concept. Memorandum G95-1 states that "some devices are made of materials that have been well characterized chemically and physically in the published literature and have a long history of safe use. For the purposes of demonstrating the substantial equivalence of such devices to other marketed products, it may not be necessary to conduct all the tests suggested in the FDA matrix of this guidance." The implication is that a suitable literature survey may suffice to establish substantial equivalence. Investigational device exempion submissions may also require biological-safety reviews. The report of previous investigations must include all prior animal and laboratory testing, including summaries and bibliographies. A 510(k) summary should also include a review of the biological-safety literature when such a review helps support the argument of substantial equivalence. "The summary of adverse safety and effectiveness data . . . should be based upon a reasonable search of all information known or otherwise available. . . . "

FDA's Procedures for Obtaining FDA Approval to Export Unapproved Medical Devices requires a manufacturer applying for an export permit for an unapproved device to submit (1) a statement certifying that a search of Medlars and Dialog databases has been made to identify adverse safety data for similar devices; (2) a summary of adverse safety data; and (3) a summary of available animal safety data. These requirements are intended to enable FDA to determine that export of the device is not contrary to public health and safety.

The ISO/DIS 10993 Part 1--Guidance on Selection of Tests emphasizes that the decision as to whether or not a certain test is performed should be based on the individual characteristics of the material or device under consideration, and that not all tests are necessary or practical for a given material or device. On the other hand, the document also stresses that additional tests not listed in the guidance may be important.

Clinical investigations of medical devices in Europe, per EN 540, require preparation of an Investigator's Brochure that includes, among other things, "a description of the materials used in the device; a summary of the in vitro, ex vivo, and in vivo data relevant to the device; preclinical biological studies; nonclinical laboratory studies; and any animal studies." The Investigator's Brochure shouldn't simply contain a summary of work conducted by the manufacturer, but a literature survey of all known information. Clinical investigations of medical devices subject to ISO 14155 face a similar requirement, in that the Investigator's Brochure must include "a collection of all relevant information known prior to the commencement of the clinical investigation."

When a company applies to market medicinal products in Europe, the European Commission requires the submission of three Expert Reports in the areas of chemical, biological, and pharmaceutical evaluation; pharmacological and toxicological evaluation; and clinical evaluation. While these rules don't apply to medical devices, they bring to light the extent to which the European Union relies on technical reports.

How do you go about conducting a literature search on a material or a device, and what do you look for once you've got one? You'll need access to a computer and modem and an account with a host information provider such as Dialog (Knight-Ridder Information Services, 415/858-3810) or Medlars (National Library of Medicine, 800/638-8480).

As a first step in Dialog, do a quick search in a database called DialIndex. Enter the name of the material or device of interest in DialIndex and it will provide a summary of the number of "hits" in every other database in the Dialog system. The DialIndex search is fast and inexpensive (a couple of dollars) and will tell you quickly whether or not there is any information available about your material or device.

From the DialIndex summary you can tell which databases to search for further information. Those most likely to contain biological safety information are Medline, Toxline, Diogenes, F-D-C Reports, Health Devices Alerts (a very important database covering all medical device reports submitted to FDA), Biosis, Ei Compendex*Plus, and CA Search--all databases available through Dialog. Next, print out the abstracts for each of the "hits" located through the on-line search.

Now the work begins. Each abstract must be read and processed: did the author perform a biological-safety test, a subchronic-safety test, or a chronic-safety test, and report on the outcome? Every instance of biological safety or biological toxicity should be tabulated for the technical review. Many times, the abstract will indicate that safety work was conducted, but the data are in the body of the article. In these instances, the entire article must be obtained from a supplier, and reviewed. In addition, manufacturers and suppliers of the materials should be contacted for toxicity summaries and technical information. Sometimes, even basic textbooks can be consulted to ensure that chemical formulas or other fundamental items are correct.

Finally, the information is assembled into a biological- safety technical report that follows a logical, user-friendly format. The format of the parylene C report is modeled after the Toxicological Expert Reports required by the European Commission's Rules Governing Medicinal Products. The report consists of an introduction giving general technical, chemical, and processing information about the material; a section reviewing the findings on material degradation; a section summarizing the biological-safety data reported in the literature; a section summarizing the medical device reports sent to FDA; a recommended test profile based on General Program Memorandum G95-1; a conclusion; and, finally, the references and citations for each item of data mentioned.

Apart from its multiple uses, taking the time to generate a biological-safety technical report before actual animal testing is undertaken can give the manufacturer a solid sense of the material under investigation. The material's suitability for the current application or other uses, subtle performance problems, and biological-safety profile can all be evaluated in advance to ensure the responsible use of animals in testing.

The following document is the biological-safety literature review on parylene C.

INTRODUCTION

Parylene C is the polymeric form of the low-molecular-weight dimer of para-chloro-xylylene. Supplied by Specialty Coating Systems (Indianapolis), parylene C can be deposited as a continuous coating on a variety of medical device parts to provide an evenly distributed, transparent insulation. This deposition is accomplished by a process termed vapor deposition polymerization, in which dimeric parylene C is vaporized under vacuum at 150°C, pyrolized at 680°C to form a reactive monomer, then pumped into a chamber containing the component to be coated at 25°C. At the low chamber temperature, the monomeric xylylene is deposited on the part, where it immediately polymerizes via a free-radical process. The polymer coating reaches molecular weights of approximately 500,000.

Deposition of the xylylene monomer takes place in only a moderate vacuum (0.1 torr) and is not line-of-sight. That is, the monomer has the opportunity to surround all sides of the part to be coated, penetrating into crevices or tubes and coating sharp points and edges, creating what is called a "conformal" coating. With proper process control, it is possible to deposit a pinhole-free, insulating coating that will provide very low moisture permeability and high part protection to corrosive biological fluids.1

Primer--A-174. Parylene C adherence to substrate metals is not sufficient to achieve the necessary lifetimes required of many implants. Adherence is a function of the chemical nature of the surface to be coated. Yamagishi reported that tantalum and silicon surfaces could be overcoated with silicon dioxide, then with plasma-polymerized methane, and finally with parylene C to achieve satisfactory adherence.2

Pretreatment with a dilute methanol-water solution of the organic silane gamma-methacryloxypropyltrimethoxysilane (A-174) prior to parylene coating is the recommended surface preparation.3 Clean metal surfaces are inorganic. Parylene monomers are organic, and do not bind readily to the electron-rich metallic surface. The organic silane has both inorganic and organic molecular surfaces. During the priming operation, the inorganic surface of the A-174 binds permanently to the inorganic metal, presenting an organic face to the incoming para-chloro-xylylene monomers.

Applications. Most applications of parylene C coating in the medical device industry are for protecting sensitive components from corrosive body fluids or for providing lubricity to surfaces. Typical anticorrosion applications include blood pressure sensors, cardiac-assist devices, prosthetic components, bone pins, electronic circuits, ultrasonic transducers, bone-growth stimulators, and brain probes. Applications to promote lubricity include mandrels, injection needles, cannulae, and catheters.

MATERIAL DEGRADATION

Parylene C is extremely stable chemically and biologically. It is also electrically stable; that is, it does not degrade in the presence of electrical current and is an effective electrical insulator.4 In addition, it is stable to organic-solvent attack, being insoluble in all organic solvents up to 150°C.5 Finally, it is thermally stable and can be expected to survive continuous exposure to air at 100°C for 10 years.6 No known biological degradation reactions or pathways have been reported.

There have been reports of parylene C undergoing stress cracking after repeated cyclic stressing. Parylene-coated wires were mechanically stressed in a flex stress machine and analyzed for current leakage, which was shown to increase as a result of repeated cyclic stressing.7

Schmidt reported stress cracking and pinholing of parylene C after a period of implantation. He implanted 11 parylene C­coated microelectrodes in the dural matter of 6 monkeys in order to monitor neural responses, and found that the impedance of 8 of the 11 electrodes fell drastically within a few months of implantation.8 Explantation and scanning electron microscopy revealed longitudinal stress cracking in the parylene C coating of some of the electrodes. Others exhibited surface craters (pinholing). The remaining 3 electrodes were still operational upon explantation after 3 years.

BIOLOGICAL SAFETY

Acute Tests. There are numerous references to parylene C as biocompatible, but very few actual data show up in the literature. Negative cytotoxicity results have been reported by Ibnabddjalil9 and Bondemark.10 Cell-growth studies using WI-38 cells have been reported by Burkel.11 In that study, either nylon, polyester, or polypropylene microfibers were coated with parylene C and seeded with WI-38 cells. The cells produced from 54 to 100% coverage of the scaffold, depending on the microfiber composition, indicating good compatibility of parylene C with human cell growth.

Blood-compatibility studies have also been reported. Baskin observed that a parylene C coating on polypropylene/polyurethane fabric rendered the fabric nonthrombogenic.12 Blood coagulation was investigated by Kanda, who found that the clotting time was longer for parylene C­coated substrates than for ceramics, aluminum, glass, or substrates coated with silicone or parylene N.13

Test reports on acute systemic toxicity, intracutaneous toxicity, and 5-day implantation have been disseminated.14 The acute systemic toxicity tests, the intracutaneous toxicity tests, and the macroscopic examination of the 5-day implant test were all negative. (No microscopic examination was conducted.)

Finally, there is one report of a severe inflammatory

reaction with a parylene C­coated substrate, although the authors of the study believe the reaction was probably due to an infection in the test rabbits.15

Subchronic and Chronic Tests. There are four reports of long-term cerebral implantation tests using various substrates coated with parylene C. Yuen reported that parylene C­coated materials exhibited little tissue reaction after 8 and 16 weeks of implantation in the cerebral cortex of the cat.16 Hahn reported moderate reactivity after 8 weeks of meningeal implantation in the rabbit.17 Loeb implanted parylene C­ coated microelectrodes into the cerebral cortex of monkeys for 4 months; the long-term presence of electrically active neurons within microns of the plastic surface provided evidence of parylene's biocompatibility.18 Schmidt reported no untoward reactions following implantation of parylene C­coated substrates after 3 years in the motor cortex of monkeys.8

MEDICAL DEVICE REPORTS

Although parylene C­coated devices have been used for more than a decade, only two reports of device malfunctions mention parylene C. The first dates from 1984 and describes a "void in the parylene coating" of an explanted pulse-generator electrode that had been causing pectoral-muscle stimulation in the recipient patient.19 The second dates from 1986 and describes "slight erosion of the parylene coating on the lower edge" of an explanted cardiac pacemaker lead.20 Neither report mentions any biological incompatibility.

BIOCOMPATIBILITY OF A-174

The manufacturer has provided a toxicology summary of A-174, hav- ing conducted eye-irritation, oral and dermal LD50, vapor-inhalation, Ames mutagenicity, Chinese-hamster ovary-mutation, sister-chromatid-exchange, blastogenicity, and aerosol-exposure studies.21 All test results were negative, except that repeated aerosol exposure resulted in granulomatous laryngitis.

RECOMMENDED TEST PROFILE

Test Article. The quantity of parylene C coated onto a device is often quite small. Rather than test actual--and expensive--final products, it may be more practical to fabricate simulated devices of larger dimensions for test purposes. For example, a substrate representative of the fabricated device could be primed with A-174 and coated with parylene C--a less-costly but satisfactory test article so long as the manufacturing process used in the final product is followed exactly. Both parylene C and A-174 can be evaluated in the same tests.

Microtests. Many biocompatibility tests can be run on a "micro" scale, reducing the weight and surface area of test articles needed to conduct the tests yet still maintaining the recommended ratios of test article to extractant. Microtest protocols are recommended wherever possible to further reduce the amount of test article required.

ISO 10993/Tripartite Biocompatibility Guidance. Parylene C­coated devices are frequently permanent, blood-contacting implants. It is worth noting that the quantity or "dose" of material that is likely to be implanted in any one patient is extremely small, possibly on the order of micrograms. However, because the implants are permanent, the device manufacturer is required to consider the profile of biocompatibility tests described in FDA's General Program Memorandum G95-1 (see Table I).

Additional Testing. There are a few other concerns with regard to the biological safety of A-174/parylene C­coated materials that are not adequately addressed through the ISO/Tripartite standards. These have to do primarily with process control. Final, manufactured product should be examined via appropriate analytical chemistry techniques for the presence of unreacted dimers of para-chloro-xylylene, and the manufacturing process adjusted and controlled so that the dimer is essentially absent. Fourier-transform infrared spectroscopy or gas chromatography/ mass spectroscopy can be useful detection techniques. A sampling of final product should also be examined under an SEM to ensure a lack of pinholes, stress cracks, or other breaks in coating integrity. For microelectrodes, it may be possible to use a bubble test, in which the electrodes are immersed in a saline solution and subjected to a current; any subsequent hydrogen evolution indicates a break in a coil insulation.8 In all such cases, the thickness of the coating must be established and controlled to ensure adequate electrical insulation.

CONCLUSION

There is a general belief, and it is probably true, that parylene C is biocompatible and nontoxic. The notable absence of untoward reactions associated with more than a decade of medical use supports this belief. On the down side, however, there is a substantive lack of published evidence available to support parylene C biocompatibility.

ISO/Tripartite Required Tests Recommendation

Cytotoxicity Conduct -- While there are several examples of cytotoxicity tests in the literature, this fast, inexpensive test provides good baseline information for future process checks.

Sensitization Conduct -- There are no cited examples of sensitization testing. The probability of passing the test is high, however, since there are no suspected incidences of sensitization associated with the material.

Irritation or intracutaneous Omit -- There is no need to repeat an intracutaneous test, since there is little opportunity for variability in the formulation of parylene C and the manufacturer has provided recent data for this test.

Acute systemic toxicity Omit -- There is no need to repeat an acute systemic toxicity test, since there is little opportunity for variability in the formulation of parylene C and the manufacturer has provided recent data for this test.

Subchronic toxicity Omit -- There are four examples of long-term (weeks to years) implantation studies in the literature and a notable lack of associated, untoward reactions.

Genotoxicity Omit -- Genotoxicity depends on a chemical reaction taking place between cellular DNA and leached or degraded components from the implanted material. Assuming there are no contaminating dimers, this test can be omitted, since A-174 is nonmutagenic and there is substantial chemical evidence for parylene C stability and lack of reactivity.

Implantation Conduct 7-day test with microscopic examination -- While there are several examples in the literature of cerebral implantation tests and cellular "health" at the site of implantation, there are no examples of muscle implantation. The manufacturer has supplied a 5-day muscle-implantation test with negative results, but it lacks a histological (microscopic) examination.

Hemocompatibility Omit -- Depending on device application and quantity of parylene C exposure. The literature indicates that the material is nonthrombogenic.

Chronic toxicity Possible -- However, the long-term effects of the device as a whole outweigh the toxicity effects of implanting microgram quantities of A-174 and parylene C. Consider a postmarket surveillance study.

Carcinogenicity Omit -- Carcinogenicity depends on a chemical reaction taking place between cellular DNA and leached or degraded components from the implanted material. Assuming there are no contaminating dimers, this test can be omitted, since A-174 is nonmutagenic and there is substantial chemical evidence for parylene C stability and lack of reactivity.

REFERENCES

1. Parylene Conformal Coatings Specifications and Properties, Indianapolis, Specialty Coating Systems, p 3, 1994.

2. Yamagishi FG, "Investigation of Plasma-Polymerized Films as Primers for Parylene-C Coatings on Neural Prosthesis Materials," Thin Solid Films, 202(1):39­50, 1991.

3. Parylene Conformal Coatings Specifications and Properties, Indianapolis, Specialty Coating Systems, p 10, 1994.

4. Ibid, pp 4­5.

5. Ibid, p 8.

6. Ibid, p 7.

7. Nichols MF, "Flexible and Insulative Plasmalene Wire Coatings for Biomedical Applications," Biomed Sci Instrum, 29:77­86, 1993.

8. Schmidt EM, McIntosh JS, and Bak MJ, "Long-Term Implants of Parylene-C Coated Microelectrodes," Med Biol Eng Comput, 26:96­101, 1988.

9. Ibnabddjalil M, Loh IH, Chu CC, et al., "Effect of Surface Plasma Treatment on Chemical, Physical, Morphological, and Mechanical Properties of Totally Absorbable Bone Internal Fixation Devices," J Biomed Mat Res, 283:289­301, 1994.

10. Bondemark L, Kurol J, and Wennberg A, "Orthodontic Rare Earth Magnets--In Vitro Assessment of Cytotoxicity," Br J Orthod, 21(4):335­341, 1994.

11. Burkel WE, and Kahn RH, "Cell-Lined, Nonwoven Microfiber Scaffolds as a Blood Interface," Annals--NY Acad Sci, 283:419­437, 1977.

12. Baskin SG, Navarro LT, Sybers HD, et al., "Tissue Cultured Cells: Potential Blood Compatible Linings for Cardiovascular Prostheses," Polym Sci Tech, 14:143­151, 1981.

13. Kanda Y, Aoshima R, and Takeda A, "Blood Compatibility of Components and Materials in Silicon Integrated Circuits," Electron Lett, 17(16):558­559, 1981.

14. Reports provided by Union Carbide, the parylene manufacturer at the time these tests were conducted in 1992.

15. Ansbacher L, Nichols MN, and Hahn AW, "The Influence of Encephalitozoon cuniculi on Neural Tissue Responses to Implanted Biomaterials in the Rabbit," Lab Anim Sci, 38(6):689­695, 1988.

16. Yuen TG, Agnew WF, and Bullara LA, "Tissue Response to Potential Neuroprosthetic Materials Implanted Subdurally," Biomat, 8(2):138­141, 1987.

17. Hahn AW, York DH, Nichols MF, et al., "Biocompatibility of Glow-Discharge-Polymerized Films and Vacuum-Deposited Parylene," J Appl Polym Sci, 38:55­64, 1984.

18. Loeb GE, Bak MJ, Salcman M, et al., "Evaluation of a New Biocompatible Dielectric Coating: Parylene Insulated Chronic Microelectrodes," Proc Annu Conf Eng Med Biol, 17:46, 1975.

19. PRP-90108, report date 4/24/84.

20. MDR-11993, report date 2/7/86.

21. Toxicology Summary: Organofunctional Silane A-174, Gamma-Methacryloxypropyltrimethoxysilane, Indianapolis, Specialty Coatings Systems, 1989.

Nancy Stark, PhD, is president of Clinical Design Group (Chicago), an independent firm specializing in safety, efficacy, and performance testing of medical devices. The firm provides consulting in biological safety and clinical research, preparing material safety reports and clinical profiles, conducting public and on-site training, and contracting clinical research investigations.

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