How Medtronic Ensures Medical Plastic Implants Go the Distance

The pace of technology today moves at warp speed, and in a highly regulated sector such as medical technology, these advances can outpace the guidance documents established to outline the required testing to ensure the life-saving devices perform as expected. FDA developed the Medical Device Development Tools (MDDT) program to accelerate the integration of new and proven strategies for evaluating new technology without diluting regulatory oversight or patient safety. It was a welcome development that medical device giant Medtronic has used effectively in an accelerated test methodology to address the performance of new materials at long human implant times.

“Prior to this program, guidelines could take years to develop,” explains Dr. Kimberly Chaffin, Vice President, corporate technologist and Fellow, at Medtronic. “In many cases, the technological advances were outpacing the guidance document updates, giving a false sense of confidence to the medical device developer when the guidance was successfully applied to today’s devices.”

Previous guidance pegged implant longevity at less than five years

MDDTs have been qualified for a range of device types since the program was established in 2017, and it is a testament to its rigor that only 17 MDDT tools have been qualified by the agency thus far. Chaffin discussed with PlasticsToday the Accelerated Testing to Prove Long-term Materials Biostability MDDT that Medtronic uses to collect evidence to support device biostability over modern-day longevity expectations, which often exceed 15 years. “This tool serves to update previous FDA guidance, which is based upon assumptions established in an era when longevity expectations were less than five years,” she notes.

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In previous guidance for proving long-term material biostability, the focus was on oxidation, explained Chaffin. “There are two dominant in vivo reactions — oxidation and hydrolysis. In the predicate guidance, it was assumed that hydrolysis was slow compared to oxidation and expected implant times. This assumption was supported by the historical observation that most material failures were oxidative in nature, said Chaffin. New materials, thus, were developed for oxidative resistance, pushing out longevities and resulting in a transition to hydrolysis-based failure as the most observed failure mode. “The MDDT addresses the gap that exists in the predicate guidance for modern materials and device longevities, where a new material’s susceptibility to failure by hydrolysis is addressed. The advantage of having this MDDT to augment the guidance is that the combination of the two testing strategies results in a more comprehensive assessment of a new material’s susceptibility to in vivo reaction, where the assessment can be performed in vitro and be relevant at decade-plus implant timeframes.”

Context of use

FDA qualifies MDDT tools within a specified context of use, and the context for this particular MDDT limits the tool’s application to polyurethane materials used as insulation on cardiac and/or neuromodulation leads, where the dominant reaction is hydrolysis, explained Chaffin. The tool does not replace the predicate guidance that is focused on a new material’s ability to resist oxidation, she added. In theory, however, this MDDT could be expanded to all types of plastics, elastomers, and adhesives. “In fact, a similar quantitative acceleration protocol is used in other industries,” said Chaffin, noting that she was first exposed to the strategy early in her career when she was working in the automotive industry.

In the medical arena, the careful application of accelerated testing can reduce the use of preclinical models involving animals. Conventional thinking has been that the biological environment is too complex to simulate through in vitro lab experiments. “This thinking suggests we must always utilize animals to assess the biostability of a new material, and because it is difficult to find an animal model that is capable of exposure times consistent with modern-day implants, we must always extrapolate statically noisy data to predict a material’s ability to perform over the service life expected for today’s implants,” said Chaffin. “Through the publication of this MDDT, we have demonstrated that we can accurately predict the long-term in vivo performance of a material, disproving the thesis that the biological environment is too complex to simulate.” Chaffin cites recent research on the reaction rate of a PDMS-urethane material used in the primary insulation on some cardiac leads, where human implant times out to 13 years.

MDDT process revealed material shortcomings

At Medtronic, the MDDT process is used to assess all new materials used in chronic implants. Even before the tool was qualified by FDA, the medical device manufacturer used it to cancel major programs that incorporated new materials that did not perform well at long times, said Chaffin. “In addition, we have used it to introduce new materials to our long-term implant development programs. Through the incorporation of this testing, we have the added confidence that we will not face a material degradation challenge 10 years after implanting our patients. We have a comprehensive publication in process demonstrating its applicability to a wide range of new material chemistries,” said Chaffin. Thus far, three papers have been published showing how the MDDT tool was used to assess materials, as follows:

“Influence of Water on the Structure and Properties of PDMS-Containing Multiblock Polyurethanes” in Macromolecules.

“Polyether Urethane Hydrolytic Stability after Exposure to Deoxygenated Water” in Macromolecules.

“Longevity Expectations for Polymers in Medical Devices Demand New Approaches to Evaluating Their Biostability” in ACS Macro Letters.

Take a deeper dive at Advanced Manufacturing Minneapolis

Chaffin will lead a session on material biostability for chronically implanted medical devices at Advanced Manufacturing Minneapolis during the Medical Polymers track. She will discuss the use of predictive accelerated testing of commonly implanted medical device polymers on Oct. 16 at 3:45 p.m. Advanced Manufacturing Minneapolis brings together Medical Design & Manufacturing (MD&M), Plastec, ATX (automation technology), Design & Manufacturing, and MinnPack under one roof at the Minneapolis Convention Center on Oct. 16 and 17, 2024.