Insulating and jacketing material options for wire and cable are innumerable, even if the field is narrowed to those with some qualification for use in medical electronics. Material selection for medical electronics is a complicated decision that begins with defining “qualification” and “medical”. Device manufacturers rely on a combination of inhouse experiences, cable suppliers, testing laboratories, consulting services, standards, guidance documents, and other publications. Their requirements for new devices may be defined strictly by the FDA, or may further incorporate application, market, or manufacturer preferences. For example, there are a great number of materials that will meet FDA requirements for surface contact patient monitoring cable materials, but a flexible and highly durable, silky-textured, cost competitive material may be preferred by the user. A specification may also call for a higher level of biocompatibility than is strictly required by the FDA. This may be the result of existing qualifications obtained with those materials, or it may be an over-specification that should be explored.

Fig. 1 – Custom cable designs can be parylenecoated for improved surface behavior in contact with skin or fabric.
Factors that contribute to materials selection decisions include aesthetics, flexibility, durability, size range capability, biocompatibility, disinfection and sterilization compatibility, cost, revision control assurance, and environmental regulatory compliance. Subtle differences in priority may result in significant differences in product design. Suppose, for example, an electrosurgical cable requires autoclavability, biocompatibility, and durability. The jacket material selection may be a thermoplastic elastomer. However, if maximum flexibility is required, or if a smooth texture that allows the cable to easily slip along contact surfaces is a priority, then the jacket material selection may be parylene-coated silicone rubber. (See Figure 1)

Biocompatibility and Defining “Medical” Grade

Biocompatibility is ultimately a function of a completed device including all of its components, assembly processes, and overall design. However, thoughtful material selection will help contribute to an overall product assurance. Material suppliers may describe their products as suitable for food contact, or having passed testing for ISO 10993-5 (cytotoxicity), or select other sections. Some maintain drug master files at the FDA with confidential details about formulations and processes that can be used in support of a device application. Some suppliers offer parallel product lines: one for industrial products, and one for medical devices. Chemistry and performance of the two material families may be essentially the same, though the medical suite of materials will have specific revision controls in place, documentation and certifications will accompany each shipment, and price will reflect the additional services. Each level of qualification is intended for different applications defined by device type (surface, external communicating, implant) and contact duration (limited, prolonged, or permanent). It is essential to define your application requirements at the design phase to avoid material changes later. Likewise, it is important to avoid over specifying biocompatibility requirements, as each level of certification, lot testing, or specialty material has an associated cost.

USP VI and ISO 10993

Prior to 1995, the United States Pharmacopoeia referred US device manufacturers to the Tripartite Biocomp - atibility Guidance Document for classification of plastics based on biological response in mice and rabbits to various injection and implantations of extractions and material samples. “USP Class VI” included the full range of injection and implantation studies, and it was not uncommon for material suppliers pursuing medical markets to have their materials tested against these criteria. This enabled device design teams to use the USP classification as a materials selection reference. It did not, however, guarantee that a completed device would pass the same tests or those device manufacturers could avoid testing. Sub sequent processes, adhesives, printing, packaging, sterilization, and other final preparations all contribute to final de vice performance and, thus, must be considered in specific tests to determine the suitability of a material for its intended use.

In May 1995, the FDA began referring to ISO 10993-1 for new device submissions. The ISO standard includes a table that defines devices by contact type and duration. Tests for biological response are then specified according to the resulting category. Although the ISO standard was intended to harmonize device testing re quirements, the FDA and Japanese Ministry of Health, Labor, and Welfare have made certain modifications and may require testing above and beyond what is indicated by the test selection guidance table. In all cases, the standard is intended to offer guidance rather than an exclusive checklist for qualification.

While USP and ISO categories, classes, and test protocols do not specifically match, it continued to be common for material suppliers and design engineers to use USP as a reference and selection criteria until relatively recently. Current design efforts focus on ISO 10993. The 20 sections of the ISO standard cover topics such as test selection, sample preparation, specific methods, animal welfare requirements, degradation products testing, and others. As before, it is not uncommon to see material suppliers reference ISO 10993 in their product literature, but it is essential to know what they are claiming, and whether that will be helpful or necessary in your device design and material selection process.

Selecting Materials Using Biocompatibility Claims

Fig. 2 – Your supplier should maintain an extensive inventory of materials for medical electronics and offer manufacturing capabilities, regulatory, and design expertise.
Cytotoxicity testing (ISO 10993-5) is a comparatively quick and inexpensive test commonly used for materials screening or comparisons. Cyto toxicity, Sensi tization and Irritation (ISO 10993-10) tests are required for all device categories defined by ISO 10993-1. The field of “innumerable material options” begins to narrow as one specifies additional testing and test frequency, but there are still many high performance insulation and jacket materials that have passed ISO 10993-5 and -10 at least on a one-time basis for material selection purposes.

Bracket testing is another practice that materials suppliers use (and actually, ISO suggests) to minimize animal testing and duplication of effort. Compoun ders for example, will conduct one-time tests of representative formulations that bracket a material family (for example, using the same ingredients in different proportions to achieve a range of hardness or flexibility). Device designers may then select materials with optimum physical performance from the entire range of materials based on the representative tests and knowledge of their own application and intended use. (See Table 1)

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