Biomedical Engineering Reference
In-Depth Information
4.7 Metal ion oxidation (MIO) breach in inner insulation of lead made of a
polyether polyurethane (courtesy of Medtronic, Inc.).
tissue. Inside the capsule are foreign body giant cells that directly contact the
materials and continuously secrete oxidative agents in the attempt to degrade the
structures.
After years of investigation, these oxidative agents were found to be
responsible for the environmental stress cracking of the lead insulation. Here,
specific polyether components on the outside surface of the lead insulation were
observed to be oxidized by these agents. This combined chain breakage and
residual stress resulted in ductile cracking in the insulation surface (Zhao et al.,
1991; Casas et al., 1999; Lyu and Untereker, 2009). This degradation reaction is
a unique, biological mechanism that is difficult to predict by simple in vitro
tests. Nonetheless, some progress has been made in this area and its importance
is addressed in ISO 10993, Part 13: Identification and Quantification of
Degradation Products from Polymeric Devices (ISO 10993, 1997).
4.8 Biological environmental stress cracking (ESC) breach in outer insulation
of lead made of polyether polyurethane (courtesy of Medtronic, Inc.).
