William Hyman: Biocompatibility of Breathing Tubes Raises Interesting Questions

The 2014 AAMI/FDA Summit on Ventilator Technology addressed a number of important issues on this technology, which were presented as six “clarion themes” in the subsequent report on the event. (Presentation slides from the summit also are available.)

Five of the six themes are familiar with respect to typical AAMI subject matter since they address the technology as a whole, including standards, user competencies, system integration, human factors, and collaboration among stakeholders.

The remaining theme of biocompatibility is perhaps less familiar in the AAMI domain, but is of course crucial for implants and other devices which make, as I refer to it, intimate bodily contact. In the case of ventilators, this intimate bodily contact is in the form of the breathing circuitry, i.e., the tubes and other structures through which gas passes before it enters the lungs. The discussion of this biocompatibility issue had two poles at the summit and in the report. On one side, you had the FDA appearing to advocate for and perhaps announce its intent of stricter requirements and associated testing. On the other side, industry basically argued against more stringent requirements. This included the FDA suggestion that the “indirect” contact between tissue, gas, and materials must be considered in the same way for devices whose materials make direct tissue contact. (A side issue here is whether the FDA has been or can increase requirements through guidance and statements of expectation as opposed to the more formal and cumbersome rule making. This issue has been raised in the context of a number of guidance documents, especially when they result in a manufacturer of a new device having to meet requirements that predecessor devices, including 510(k) predicates, did not have to meet. For example, the agency’s infusion pump guidance appears to have such new requirements.)

Another interesting issue raised at the summit and in the report is, in effect, what makes one polymer the same or different from another polymer of the same chemical name. This has broad implications and has been seen in the ongoing mesh litigation in which some manufacturers allegedly sourced their material from outside the medical spectrum and allegedly used subterfuge in hiding from the material supplier what they were actually making.

At the summit, an FDA speaker noted that there “are often differences in formulations, additives, and plasticizers as well as post-manufacturing residuals.” The addition of plasticizers is interesting in that it can include substantial amounts of material that are not captured by the chemical name of the base polymer, and which may be problematic with respect to tissue contact. For example, the use of DEHP in various products, especially when used by infants, has been addressed by the FDA as well as by consumer groups and manufacturers.

Formulation differences might include knowing that the chemical name of a polymer does not tell you the molecular weight or more realistically the distributions of molecular weights. One example here is that the wear properties of ultra-high molecular weight polyethylene (UHMWPE) are known to be quite different from those of lower molecular weight polyethylene.

In some cases (e.g., polypropylene), there are different molecular arrangements of the polymeric side groups, but all of these arrangements, as well as mixtures of these arrangements, may be called by the same generic chemical name. In still other cases, a chemical name (e.g., polyurethane) covers a broad family of chemical entities rather than a single repeating one.

Not mentioned in the summit coverage are the possible effects of aging, such that new resin might have different characteristics from an older batch of what is nominally the same material. Moreover, new resin might have differences from old resin in ways that are not revealed even in the specifications, especially if sourced from a non-medical supplier. It was also noted by the FDA that raw material properties may differ from post-processed materials. These differences can bring into question assertions that the material of a proposed device is the “same” as that of some previous or predicate device, and therefore additional biocompatibility testing is unnecessary or can be minimized.

Not quite answered here, and not responded to in an e-mail I sent to one of the FDA speakers, is whether the agency’s concerns about what manufacturers have provided in the past implies that the FDA has been accepting that which they should not have been accepting—and which they will no longer accept. Perhaps this is the answer to the question of creating new requirements via guidance documents: the requirement isn’t new, it’s only applying it properly that is new.

William Hyman, ScD, is professor emeritus of biomedical engineering at Texas A&M University. He now lives in New York where he is adjunct professor of biomedical engineering at The Cooper Union.

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