Allophanate formation is a common side reaction in polyurethane synthesis, where the unreacted isocyanate groups bond with an existing urethane linkage. This results in branching or cross-linking. Depending on the level of cross-linking, allophanates affect the properties of the polyurethane to varying degrees; the melt viscosity, molecular weight and polymer solubility can be significantly affected. Optim™ insulation is a siloxane based polyurethane that is extremely biologically stable and hence used as insulation on permanently implanted cardiac leads. Due to the differences in the polarity between the segments of Optim™, allophanate linkages may occur. Attempts to characterize allophanates using traditional analytical techniques have not been successful. The allophanate linkages break down over time when Optim™ is molten, giving Optim™ rheology a time dependent dimension. Concurrent measurements of the molecular weight and allophanate concentration confirm the dissociation of these linkages over time in the molten state. We used rheometry and gel permeation chromatography (GPC) to to confirm the presence of allophanate linkages in our material, and used a titration based technique to quantify the allophanate linkages present in Optim™. We also show that an observed decrease in molecular weight during hydrolytic treatment of Optim™ can be attributed to allophanate hydrolysis, with little to no urethane hydrolysis. Removal of the allophanate linkages from the polymer did not affect the other chemical and mechanical properties of Optim™. This analysis suggests Optim™ insulation has good characteristics for long term biostability in a cardiac lead implementation.

Molecular weight analysis of implanted polyurethane insulated cardiac leads: Role of allophanate linkages

251st National Meeting (2016)

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