Plastics Used in Medical Devices Can Undergo Breakdown

A new study has discovered a previously unrecognized pathway of degradation that can occur in silicone-urethane plastics, often used in implantable medical devices.

Researchers at Medtronic (Minneapolis, MN, USA) and the University of Minnesota (Minneapolis, USA) wished to evaluate the molecular and mechanical stability of thermoplastic elastomers over time, focusing on two commercially available segmented polyurethane multiblock polymers containing polydimethylsiloxane and polyether soft segments, named Elast-Eon E2A (E2A) and PurSil 35 (P35), which are used in some implanted biomedical devices, such as pacemakers and defibrillators. While these materials have been studied for failure due to interaction with oxygen, interaction with water as a potential failure mechanism has not yet been studied.

The researchers therefore evaluated the molecular and mechanical stability of the two materials after immersion in buffered water for up to 52 weeks at temperatures ranging from 37–85 °C. Dynamic mechanical spectroscopy experiments, performed in tension and shear, size exclusion chromatography (SEC), and tensile testing were used to characterize the linear viscoelastic properties of compression-molded (dry) specimens, and small-angle X-ray scattering was measured to examine microphase-separated morphology for all test conditions.

The results showed that aging at 85 °C and 52 weeks in phosphate buffered saline lead to a 67% and 50% reduction in molar mass from the original values for E2A and P35, respectively; the researchers attributed the reduction in molar mass to hydrolysis of the polymer backbone. They also found that concomitant with the reduction in molar mass, E2A and P35 transformed with aging from strain hardening to strain-softening materials, characterized by substantially reduced tensile strength and ultimate elongation relative to the original properties. The study was published on November 18, 2012, in the American Chemical Society (ACS) journal Macromolecules.

“The study could have implications for device manufacturers considering use of these plastics in the design of some implantable devices, including cardiac defibrillation leads,” concluded lead author Kimberley Chaffin, PhD, of Medtronic. “By making the conclusions of this novel, scientific research public in a respected peer-reviewed journal, device manufacturers may now consider these important findings in their device designs.”

Related Links:
Medtronic
University of Minnesota