Skip to : [Content] [Navigation]
 

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published May1997

BIOCOMPATIBILITY

Regulatory Guidelines For Biocompatibility Safety Testing

Lise S. Bollen and Ove Svendsen

Biomaterials and medical devices constitute an extremely diverse, heterogeneous category of items. Because the use of these products normally entails their direct or indirect contact with patients, there is an obligation on the part of manufacturers to establish the safety of their products before they are marketed. Medical device safety evaluation assesses the risk of adverse health effects due to normal use and likely misuse of a device. Since adverse health effects could result from exposure to the materials from which a device is made, preclinical assessment of the toxic potential of such materials or components is needed to minimize the potential hazard to the patient.

A thorough biocompatibility safety testing program will typically comprise in vivo studies supplemented by select in vitro assays. Photo: Northview Biosciences, Inc.

Until recently, the regulations governing the manufacture and sale of medical devices varied greatly among countries. Since January 1995, medical devices to be marketed in the European Union (EU) have been required to comply with EU Medical Devices Directive 93/42/EEC, which specifies requirements for safety assessment issues. The purpose of the directive is to promote a single European market for trade in medical devices, while ensuring that users and patients are not exposed to unnecessary risks.

At present, safety assessments of medical devices are guided by the toxicological and other studies recommended in the International Organization for Standardization (ISO) 10993-1/EN 30993-1 standard. At present, 17 parts of the standard are either accepted or under preparation. Tests that may be used in an evaluation of medical device biocompatibility include procedures for cytotoxicity, skin sensitization, dermal irritation and intracutaneous reactivity, acute systemic toxicity, subchronic toxicity, mutagenicity, implantation, hemocompatibility, chronic toxicity, and carcinogenicity.

This article is an introduction to a relatively new and rather complicated field in toxicology—the toxicological testing of medical devices. The guidelines for testing such products are discussed and a general description of the various test procedures given. Future developments in the field of biocompatibility regarding international harmonization and the potential for new methodologies are also addressed.

INTERNATIONAL REGULATORY EFFORTS

ISO is in the process of publishing a series of standards on the biological evaluation of medical devices—ISO 10993.1 Many parts of this series have been accepted as international standards, while the rest are under development (see Table I). The subject of the first part, ISO 10993-1, is the categorizing and performance of safety testing. Part two of the standard, ISO 10993-2, is concerned with animal welfare requirements; another section, ISO 10993-12, deals with sample preparation and reference materials. Most of the remaining parts of the standard treat the individual tests.

Table I. Listing of individual parts of ISO 10993, Biological Evaluation of Medical Devices.

The EU has issued a council directive—93/42/EEC, 1993—concerning medical devices.2 All medical devices to be sold on the EU market must comply with this directive after June 14, 1998. The European Committee for Standardization (CEN) is currently in the process of adopting the ISO 10993 standard as the European standard.

In 1986 the responsible authorities in the United Kingdom, United States, and Canada issued the Tripartite document, which was a guidance on the selection of toxicological tests for medical device safety testing.3 This document has now been replaced by ISO 10993-1 as a first step in the process of international harmonization. In 1995 FDA chose to accept the ISO 10993-1 standard, with a modification of the matrix listing (see sidebar below).

Japanese authorities have also issued a guideline for toxicological testing of medical devices. This document is available in an unofficial translation as Guidelines for Basic Biological Tests of Medical Materials and Devices.4 It resembles ISO 10993 in structure and content, but recommends modified tests and sample preparations.

Figure 1. Flowchart illustrating steps in the biological evaluation of medical devices according to the ISO 10993-1 standard.

The procedure for using the ISO 10993-1 standard is illustrated by the flowchart in Figure 1. The standard is applicable only for devices that are directly or indirectly in contact with the body or body fluids. If a device is to be subjected to the standard, the first step is to characterize the material. Such characterization need not always be followed by biological evaluation, because there may be sufficient historical data to verify that the device meets the requirements of the standard. If the material and/or the intended use of the device is different from any historical safe device, biological evaluation has to be performed. By following the standard, a suitable test program can be chosen depending on the type and duration of body contact.

Within the EU, all new medical devices must carry the CE mark from June 14, 1998. This should ensure the availability of relevant documentation regarding biocompatibility and the lack of health problems associated with the use of a device. It is noteworthy that the approval of such documentation is not, as it was previously, accorded by the national health authorities, but rather by the so-called notified bodies, whose experts review the products and production facilities of medical device manufacturers.

DEVICE CATEGORY AND CHOICE OF TEST PROGRAM

The need to evaluate a medical device biologically depends on the material used in the device, the intended body contact, and the duration of that contact. A device designed for surface contact for a limited time is not as likely to be bioincompatible as a permanent-exposure implant device made of the same material. The ISO 10993-1 standard divides medical devices into three main categories: surface devices, externally communicating devices, and implant devices. Each category is further divided into subcategories according to the type of contact to which the patient is exposed (see Table II).



Table II. Device categories and examples according to ISO 10993-1.

The choice of test program for a device in a given category depends on the duration of the contact. Three different time spans are given: limited contact (<24 hours), prolonged contact (24 hours­30 days), and permanent contact (>30 days). ISO 10993-1 lists the tests that must be considered for each category (note: the accompanying table could not be reproduced. Please contact the editors for more info).

The ISO test matrix should not be considered as a checklist for the different tests that have to be performed, but rather as a guide for qualified toxicologists who also take into consideration material information and historical data from similar devices. The certifying authorities in most countries (e.g., notified bodies, FDA, Japanese authorities) are generally cooperative when a company must decide on a test program for a device. It is therefore advisable to maintain close contact with the relevant authorities during the entire process. However, testing should not be performed simply to meet regulatory requirements. This is important not only to lessen the risk of overtesting and excessive use of experimental animals, but also because a strict regulatory approach may mask potential negative health effects that might be identified via optional or nonroutine testing procedures.

As regards CE marking of existing products on the market or safety evaluation of medical devices already in clinical use, appropriate historical or clinical data should be employed whenever possible to avoid unnecessary testing.

PREPARATION OF EXTRACTS

ISO 10993-12 describes how samples for biological evaluation should be selected, prepared, and extracted. Other guidelines provide similar descriptions, which differ slightly in the specifics of the extraction procedures.

The device to be tested (the test article) should be a representative specimen of the mass-produced device. It should also be finished or treated (e.g., coated or sterilized) in the same way as the mass-produced device.

Because the toxic potential of materials and devices depends to a substantial degree on the leachability and toxicity of soluble components, extracts of the device are normally used in the tests. In some tests, however, an evaluation under normal-use conditions is mimicked by using the device or a piece of the device directly. Ideally, extraction media should constitute a series of media with decreasing polarity to ensure the extraction of components of widely different solubility properties. The most commonly used extraction media are physiological saline, vegetable oil, dimethylsulfoxide, and ethanol. Other extraction media such as polyethylene glycol or aqueous dilutions of ethanol may be selected in certain cases. For in vitro cytotoxicity testing, complete cell-culture medium is most often employed.

The various guidelines also differ somewhat with respect to the temperature at which the extraction is conducted. Some leachable compounds may be chemically altered at high temperatures, and it is now generally recommended that extraction be conducted at 37°C—simulating body temperature—for 72 hours. This procedure will probably become increasingly accepted as the most appropriate extraction method. For in vitro cytotoxicity tests, extraction at 37°C for 24 hours is usually recommended, since certain constituents of the media are relatively labile.

The amount of leachable substances released to the extraction media is related to the surface area and thickness of the product to be extracted. Recommendations vary from 1.25 to 6 cm2 of product per milliliter of extraction medium, depending on the size and shape of the product, or from 0.1 to 0.2g of product per milliliter of extraction medium when a surface area cannot readily be estimated (e.g., for powders or granulates). In any case, the specific properties of the product must be taken into account in order to make usable extracts.

For cases in which a medical device comprises several components made from different materials, the ideal procedure from a toxicological point of view would be to test extracts of the components separately. However, in some situations this is not practical, and extracts of the whole device may be used instead.

BIOLOGICAL CONTROL TESTS

Biological control tests are not described in the ISO 10993 standard for biological evaluation of medical devices, since these particular tests are designed primarily for batch-control purposes. Such tests are also used during the product development phase to identify sources of contamination and to establish procedures that ensure the intended quality of the end product.

Microbiological Control Tests. Microbiological control tests are necessary to establish the microbiological status of an end product—factors such as sterility, absence of pathological bacteria, or limits for microbial counts. Furthermore, it is often necessary to monitor the microbiological load of raw materials and intermediary products, or to check the efficiency of production and sterilization processes. The tests are performed by rinsing the materials or products in physiological saline and assessing the rinsing medium for microbes, or by directly incubating the products in growth media. Although the presence of pathological microbes on medical devices can represent a potential health problem, the subject is not within the scope of this article.

Tests for Endotoxins. Even sterile medical devices may contain cell-wall lipopolysaccharides originating from gram-negative bacteria. Such so-called endotoxins or pyrogens can cause an abrupt fever reaction after entering directly into the body from sources such as venous catheters, syringes, or implant components.

Two different biological assays can be used to measure the presence of endotoxins: the rabbit pyrogen test and the Limulus test. In both cases, an eluate is prepared—normally by rinsing the surfaces of the product with water—and then tested for endotoxins. In the rabbit pyrogen test, the eluate is injected intravenously and the rectal temperature of the animal is measured after the injection. In the Limulus test, the eluate is incubated together with lysate from the blood of the horseshoe crab (Limulus polyphemus), which contains a substance that forms a gel in the presence of endotoxins.

Test for Nonspecific Toxicity. This test is designed to assess any nonspecific adverse effect that occurs following intravenous injection of a device eluate in mice. The test is often performed with the same eluate used for the pyrogen test. The mice are inspected regularly for any signs of ill health, which can indicate the presence of toxic substances leaching from the product.

Sidebar: Biocompatibility Testing and the Blue Book Memorandum

In May 1995, FDA issued Blue Book Memorandum G95-1, Use of International Standard ISO-10993, "Biological Evaluation of Medical Devices Part 1: Evaluation and Testing." This memo was a huge step toward international harmonization of device biocompatibility testing, although there are still some significant differences between FDA and European requirements. The Blue Book supersedes the Tripartite Guidance as a guideline for planning biocompatibility testing of medical devices for the U.S. market. The most significant change from the Tripartite is FDA's increased emphasis on a case-by-case testing strategy for individual devices.

The biocompatibility of a device depends on several factors, especially the type of patient tissue that will be exposed to device materials and the duration of the exposure. Neither the Blue Book memo nor ISO 10993 prescribes a specific battery of tests for any particular medical device. Rather, they provide a framework that can be used to design a biocompatibility testing program. As with the Tripartite and the ISO standard, the core of the Blue Book memo is a materials biocompatibility matrix. The matrix categorizes devices based on the type and duration of tissue contact. It also presents a list of potential biological effects. For each device category, certain effects must be considered. In preparing a regulatory submission, manufacturers must address each of the biological effects pertinent to their device. The matrix from the Blue Book memo incorporates several ISO features:

  • Contact-Duration Scheme—This is potentially one of the most important changes for device manufacturers. The upper limit for duration category A ("limited") was raised from 30 minutes to 24 hours. For some types of devices, such as surgical devices and accessories, this has significantly reduced testing requirements.

  • Device Categories—Those in the Blue Book matrix are substantially the same as in the ISO standard.

  • Test Selection—The Blue Book matrix moves in the direction of what is recommended in the ISO standard. However, FDA often goes beyond ISO, especially in tests for systemic toxicity, subchronic toxicity, and implantation. The Office of Device Evaluation (ODE) emphasizes that the matrix is "only a framework for the selection of tests and not a checklist of every required test." In some cases (e.g., subchronic testing of balloon catheters), FDA has already said it will require testing beyond what is suggested in the matrix.

The test-selection matrix applies to most premarket approval submissions. The Blue Book memo also contains a flowchart to assist in selecting toxicity tests for 510(k) devices. If there is no significant change between the new device and the predicate device in materials, manufacturing, sterilization methods, and body contact, no further biocompatibility testing may be necessary. Otherwise, manufacturers are referred to "Device Specific Tox Profiles" (to be issued by FDA at some yet-to-be-determined date) or to the Blue Book matrix.

When designing a biocompatibility testing program, manufacturers should also consult other applicable FDA publications, such as Biocompatibility of Medical Devices (from the Center for Devices and Radiological Health), the Guidelines for the Intraarticular Prosthetic Knee Ligament, or the PTCA Catheter System Testing Guideline. Other useful documents include ASTM F 748, Practice for Selecting Generic Biological Test Methods for Materials and Devices, and the AAMI Standards and Recommended Practices, Volume 4: Biological Evaluation of Medical Devices. (These and other publications are available from CDRH Facts-On-Demand at 800/899-0381.) For most projects, companies should review testing plans with FDA before beginning the actual studies.

Since the Blue Book memo was issued, a few trends have emerged:

  • Test-Method Harmonization— FDA has not adopted the ISO standards for specific test methods. However, these standards are generally very similar to what has been standard industry practice in the United States, and FDA has accepted ISO-compliant test procedures.

  • Increased Emphasis on Analytical Characterization—Most reviewers will now insist on at least basic analytical characterization of device materials, usually physicochemical tests, infrared spectroscopy, and cytotoxicity. Often a more thorough characterization is appropriate, using techniques such as extract studies, chromatography, and/or determination of physical properties of the materials.

  • More-Sophisticated Genotoxicity Testing—Under the Tripartite Guidance, manufacturers often simply used an Ames test to satisfy genotoxicity requirements. The ISO standard on genotoxicity (10993-3) requires at least three assays to look at the different levels of genotoxic effects—gene mutations, chromosomal aberrations, and DNA effects. While FDA has not adopted this test standard, its review of genotox data is becoming more rigorous—moving closer to that of European regulators even as the preferred test batteries continue to differ.

So what's the bottom line for manufacturers planning biocompatibility testing programs? Work with an experienced toxicologist to apply the Blue Book matrix and the ISO standard to your device. Avoid cookie-cutter solutions by carefully evaluating the intended use of the device. And if at all possible, consult your FDA reviewer before beginning your biocompatibility testing program.

RELATION TO U.S. PHARMACOPEIA

To test medical device biocompatibility, manufacturers often use USP procedures such as the USP in vivo biological reactivity tests (Class I–VI plastics tests).

While class plastics tests have some value in a biocompatibility testing program, a full Class VI test is rarely needed for a medical device. As a general rule, the Blue Book memo and ISO documents take a broader and more thorough view of biocompatibility than does the U.S. Pharmacopeia, and they supersede the USP for evaluating which studies to submit to FDA in support of product registrations.

SOURCES OF BIOCOMPATIBILITY TEST DATA

Data to satisfy biocompatibility test requirements may come from any of several sources. Most commonly, companies arrange for their own biocompatibility studies. Material vendors are often willing to share test data they have generated. If vendor data are used, manufacturers should obtain copies of the original study reports, and they should conduct at least some confirmatory testing of their own (e.g., cytotoxicity and hemocompatibility studies).

If available, clinical data can be used to satisfy some biocompatibility biological-effect categories in the matrix. Manufacturers may use analytical data (e.g., extraction studies) to eliminate the need for biological testing in a particular category.

Note on Contributors: Timothy V. Doherty, DVM, is director of in vivo services and Jeffrey Wallace is manager of toxicology services at the Northview Pacific Laboratories (Berkeley, CA) division of Northview Biosciences, Inc. (Northbrook, IL). The information is derived from the company's publication entitled Assessing Biocompatibility: A Guide for Medical Device Manufacturers.

TESTS FOR BIOLOGICAL EVALUATION

The section that follows provides a brief description of the individual tests included in the ISO 10993/EN 30993 standard.

Cytotoxicity. The aim of in vitro cytotoxicity tests is to detect the potential ability of a device to induce sublethal or lethal effects as observed at the cellular level. According to ISO 10993-1, the in vitro cytotoxicity assay is one of two tests--the other is the sensitization test described below--that must be considered in the evaluation of all device categories. For additional information on biocompatibility testing please refer to page 32 of Medical plastics and Biomaterials May/June issue.

Three main types of cell-culture assays have been developed: the elution test, the direct-contact test, and the agar diffusion test. (The ISO 10993-5 standard gives a detailed description of all three methods.) In the elution test, an extract (eluate) of the material is prepared and added in varied concentrations to the cell cultures. Growth inhibition is a widely used parameter, but others may also be used. In the direct-contact test, pieces of test material are placed directly on top of the cell layer, which is covered only by a layer of liquid cell-culture medium. Toxic substances leaching from the test material may depress the growth rate of the cells or damage them in various ways. In the agar diffusion test, a piece of test material is placed on an agar layer covering a confluent monolayer of cells. Toxic substances leaching from the material diffuse through the thin agar layer and kill or disrupt adjacent cells in the monolayer. As always, the physical and chemical properties of the test material should be considered before the choice of the test system is made.

There is usually a good qualitative correlation between results from cell-culture tests and studies performed in vivo with respect to cytotoxicity versus primary tissue effects. It is important to recognize, however, that although cell-culture toxicity is in general a good and sensitive indicator of primary tissue compatibility, exceptions may arise in cases where leaching substances cause tissue damage in vivo through more complex mechanisms. At present, the in vitro cytotoxicity assays should be used as screening tests and considered primarily as supplements to the various in vivo tests.

Sensitization. The sensitization test recognizes a potential sensitization reaction induced by a device, and is required by the ISO 10993-1 standard for all device categories. The sensitization reaction is also known as allergic contact dermatitis, which is an immunologically mediated cutaneous reaction. This is in contrast to irritant contact dermatitis (skin irritation)--a skin reaction caused by the primary and direct effect of a substance on the skin. In animals, the sensitization reactions manifest themselves as redness (erythema) and swelling (edema).

The preferred animal species for sensitization testing is the albino guinea pig. (There is no reliable alternative in vitro test that can predict the sensitizing potential of a substance.) The various available guinea pig methods have certain features in common: an induction (sensitization) phase, when the potential allergen is presented to the organism, followed by a rest period and a subsequent challenge phase to determine whether or not sensitization has occurred.

One of the most recognized and validated assays is the guinea pig maximization test (GPMT). A test design very similar to the GPMT is widely used for assessing the sensitizing potential of medical devices. After a challenge period, the skin reactions are graded on a ranking scale according to the degree of erythema and edema.

Predictive tests in guinea pigs are important tools in identifying the possible hazard to a population repeatedly exposed to a substance. Nevertheless, results from sensitization tests in guinea pigs have to be evaluated carefully. A positive test result in this assay may rate a substance as a stronger sensitizer than it appears to be during actual use. On the other hand, a negative result in such a sensitive assay ensures a considerable safety margin regarding the potential risk to humans.

Skin Irritation. The ISO 10993-10 standard describes skin-irritation tests for both single and cumulative exposure to a device. The preferred animal species is the albino rabbit, whose highly sensitive, light skin makes it possible to detect even very slight skin irritation caused by a substance. Skin-irritation tests of medical devices are performed either with two extracts obtained with polar and nonpolar solvents or with the device itself.

In the single-exposure test, rabbits are treated for several hours only, whereas for the cumulative test the same procedure is repeated for several days. All extracts and extractants are applied to intact skin sites. Skin reaction is seen as redness or swelling and is graded according to a specified classification system.

Dermal irritation is the production of reversible changes in the skin following the application of a substance, whereas dermal corrosion is the production of irreversible tissue damage (scar formation) in the skin. Materials that leak corrosive substances are not likely candidates for medical device production.

Intracutaneous Reactivity. The intracutaneous reactivity test is designed to assess the localized reaction of tissue to leachable substances. The test is required for consideration in nearly all the device categories in ISO 10993-1 (see Table III). Polar and nonpolar solvent extracts are administered as intracutaneous injections to rabbits. Undesirable intracutaneous reactivity includes redness or swelling.

Acute Systemic Toxicity. Acute systemic toxicity is the adverse effect occurring within a short time after administration of a single dose of a substance. ISO 10993-1 requires that the test for acute systemic toxicity be considered for all device categories that indicate blood contact. For this test, extracts of medical devices are usually administered intravenously or intraperitoneally in rabbits or mice.

Determining acute systemic toxicity is usually an initial step in the assessment and evaluation of the toxic characteristics of a substance. By providing information on health hazards likely to arise from short-term exposure, the acute systemic toxicity test can serve as a first step in the establishment of a dosage regimen in subchronic and other studies, and can also supply initial data on the mode of toxic action of a substance. The test is similar to the nonspecific toxicity test. Normally, only one of these two procedures is included in a test battery.

Genotoxicity. Genetic toxicology tests are used to investigate materials for possible mutagenic effects--that is, damage to the body's genes or chromosomes. The tests are performed both in vitro and in vivo. ISO 10993-1 requires the genotoxicity (mutagenicity) test to be considered for all device categories indicating permanent (>30 days) body contact (except for surface devices with skin contact only).

A mutation is a change in the formation content of the genetic material (DNA code) that is propagated through subsequent generations of cells. Mutations can be classified into two general types: gene mutations and chromosomal mutations. Gene mutations are changes in nucleotide sequences at one or several coding segments within a gene; chromosomal mutations are morphological alterations or aberrations in the gross structure of the chromosomes.

The simplest and most sensitive assays for detecting induced gene mutations are those using bacteria. Gene mutations can also be detected in cultured mammalian cells. Current in vivo assays for gene mutations are cumbersome and not widely used. The simplest and most sensitive assays for investigating chromosomal aberrations are those that use cultured mammalian cells. However, two well-established in vivo procedures are also available: chromosomal aberrations can be studied in bone marrow or peripheral blood cells of rodents dosed with a suspect chemical or extract either by counting micronuclei in maturing erythrocytes (micronucleus test) or by analyzing chromosomes in metaphase cells.

In addition to these mutagenicity tests, various assays can measure the induction of an overall genotoxic response--an indirect indicator of potential damage to the genetic material.

Implantation. Implantation tests are designed to assess any localized effects of a device designed to be used inside the human body. Implantation testing methods essentially attempt to imitate the intended use conditions of an implanted material. Although different tests use various animal species, the rabbit has become the species of choice, with implantation performed in the paravertebral muscle. Implantation can be either surgical or nonsurgical: the surgical method involves the creation of a pouch in the muscle into which the implant is placed, while the nonsurgical method uses a cannula and stylet to insert a cylinder-shaped implant. Through a macroscopic examination (which may be supplemented with microscopic analysis), the degree of tissue reaction in the paravertebral muscle is evaluated as a measure of biocompatibility.

Hemocompatibility. The purpose of hemocompatibility testing is to look for possible undesirable changes in the blood caused directly by a medical device or by chemicals leaching from a device. Undesirable effects of device materials on the blood may include hemolysis, thrombus formation, alterations in coagulation parameters, and immunological changes. According to the ISO 10993-4 (EN 30993-4) standard, devices that only come into very brief contact with circulating blood--for example, lancets, hypodermic needles, or capillary tubes--generally do not require blood/device interaction testing.

ISO 10993-4 describes hemocompatibility tests in five different categories--thrombosis, coagulation, platelets, hematology, and immunology. Most of the individual tests are not discussed in detail, but they may be performed either in vivo or, preferably, in vitro. There is still some uncertainty with respect to what is actually required by the regulatory authorities for the hemocompatibility test.

Subchronic and Chronic Toxicity. Subchronic toxicity is the potentially adverse effect that can occur as a result of the repeated daily dosing of a substance to experimental animals over a portion of their life span. In the assessment and evaluation of the toxic characteristics of a chemical, the determination of subchronic toxicity is carried out after initial information on toxicity has been obtained by acute testing, and provides data on possible health hazards likely to arise from repeated exposures over a limited time. Such testing can furnish information on target organs and the possibilities of toxin accumulation, and provide an estimate of a no-effect exposure level that can be used to select dose levels for chronic studies and establish safety criteria for human exposure.

In subchronic or chronic toxicity studies, one or two animal species are dosed daily, usually for a period of 3 to 6 months; the rat is the standard animal species of choice. The animals are given the test substance in increasing doses. The dose level of the low-dose group should be at the level of human exposure. When extracts of medical devices are employed, one dose level (the highest practically applicable volume) is often sufficient, since strong toxicity is generally not expected.

Carcinogenicity. The objective of long-term carcinogenicity studies is to observe test animals over a major portion of their life span to detect any development of neoplastic lesions (tumor induction) during or after exposure to various doses of a test substance.

Carcinogenicity testing is normally conducted with oral dosing. For implants and medical devices, however, only extracts can be tested and they must be administered intravenously, necessitating certain modifications of the standard procedure. There are only a very few products for which this comprehensive test can be justified.

In carcinogenicity studies, mice or rats are dosed every day for 18 to 24 months. For medical device extracts, one dose level (again the highest practically applicable volume) is usually sufficient. At the completion of the dosing period, all surviving animals are sacrificed and their organs and tissues examined microscopically for the presence of tumors. An increased incidence of one or more category of tumors in the dosed group would indicate that the product tested has the potential to induce tumors and could be considered a possible carcinogen in humans.

ALTERNATIVE TEST METHODS

As mentioned previously, a major goal in international toxicological testing is to reduce not only the use of in vivo studies but also the number of animals employed in these tests. A few of the in vivo procedures used today for testing medical devices may be of questionable worth for safety evaluation. However, the availability of accepted and validated in vitro assays is still limited. Substantial resources have been made available for validation of alternative in vitro assays in toxicology as replacements for animal tests, but it may take years before validated methods can be implemented, and any goal of replacing all in vivo studies with in vitro assays will probably never be met.

Recently, a working group under the auspices of the European Center for Validation of Alternative Methods (ECVAM) has recommended a few alternative methods that can be used for safer testing of medical devices.5 These include two in vitro tests as potential substitutes for the in vivo assays for skin and eye irritation. However, the implementation of validated protocols and internationally accepted guidelines for these tests is likely to be delayed into the next century.

CONCLUSION

In recent years, the biological evaluation of medical devices has become more globally harmonized, concurrently with the publication of the ISO 10993 standard for medical device testing. Some countries still require their national guidelines to be met, but most accept ISO 10993 as a parallel alternative to their own regulations. European harmonization, represented by EN 30993, has made it easier to obtain the required CE marking for a product that will be accepted throughout the EU member countries. It must be emphasized that all guidelines, whether accepted or under preparation, should be regarded as more of a dynamic process than a rigid structure, since the various standards are subjected to continuous revision and evaluation. Impact on this work will come not only from authorities and notified bodies but also from national and international expert and working groups similar to ECVAM.

It is important that neither the ISO nor the EN guidelines be used as a prescriptive "cookbook" in the assessment of safety to humans. Recommended tests must be conducted with consideration for the information available from other sources, with knowledge of the type of material a device is made from, and with awareness of its planned end use.

Modern materials and process technology have opened up innumerable possibilities for the creation of new or improved medical devices. Current developments range from the formulation of novel polymers to innovative applications of metals, ceramics, composites, and tissue-engineered products. With a comprehensive, harmonized biological evaluation program rapidly gaining acceptance, the combined advances in materials technology, biotechnology, and medicine are likely to revolutionize the future of the industry.

REFERENCES

1. Biological Evaluation of Medical Devices, ISO 10993 Standard Series, Geneva, International Organization for Standardization, ongoing.

2. Council Directive 93/42/EEC of 14 June 1993 Concerning Medical Devices, Official Journal of the European Communities, vol. 36, July, 1993.

3. Toxicology Subgroup, Tripartite Subcommittee on Medical Devices, "Tripartite Biocompatibility Guidance for Medical Devices," Rockville, MD, FDA, Center for Devices and Radiological Health (CDRH), 1986.

4. Guidelines for Basic Biological Tests of Medical Materials and Devices. Unofficial translation of Japanese guideline ISBN 4-8408-0392-7.

5. Svendsen O, Garthoff B, Spielmann H, et al., Alternatives to the Animal Testing of Medical Devices (the Report and Recommendations of ECVAM Workshop 17 to be published in ATLA, in press, 1996.)

Lise S. Bollen is head of the medical devices department at Scantox A/S (Lille Skensved, Denmark).

Ove Svendsen, PhD, DVM, is chief scientific officer at Scantox, and is currently at The Royal Veterinary and Agricultural University in Copenhagen. Founded in 1977, Scantox is an independent contract research laboratory specializing in toxicology, pharmacology, and bioassays. The company conducts biocompatiblity testing in compliance with EN 45001 and GLP regulations, and can perform all tests required by ISO 10993-1.

Copyright ©1997 Medical Plastics and Biomaterials