Biomedical Engineering Reference
In-Depth Information
4.6 Chemical structure of an example of polyether polyurethane. The structure
consists of distinct repeating hard and soft segments.
weights are soft and flexible, making them superior for lead insulation.
Polyurethanes with low PTMO contents, however, are rigid and strong, making
them more suitable for use in connector modules or lead connector ends. Com-
modity polyether polyurethanes are readily available (e.g., Pellethane
Õ
from
Lubrizol), while medical-grade materials are made by only a few manufacturers.
Examples of the latter include the brand names Elasthane
Õ
and Tecothane
Õ
.
Though polyether polyurethanes seemed to be an excellent insulating
material early on, the softer grade polyether polyurethanes were found to be
subject to two previously unknown failure mechanisms: metal ion oxidation
(MIO) and environmental stress cracking (ESC). MIO was identified after a
discovery of brittle cracking in lead insulation made of the soft grade polyether
polyurethane. These cracks started from the insulation surface, at the side
contacting the conducting metal wires it protected. This cracking developed
when polyurethane insulation reacted with the metal ions that resulted from the
corrosion reactions of conducting wires. MP35N has a very slow corrosion
reaction and is not usually detected for commodity applications. Despite
extensive prior preclinical and clinical tests, however, this rare reaction caused
significant insulation failure of pacing leads (Fig. 4.7).
Unlike MIO, ESC is a ductile cracking that can appear on the lead external
surface i.e., where direct tissue contact occurs and residual stress resides (Fig.
4.8). This failure was found to occur only in vivo. The term ESC is the same
term as that used for a commonly observed failure mechanism that occurs in
amorphous polymers (such as bisphenol A polycarbonate) that contact small
molecular weight chemicals and are under mechanical stress (Ward and Hadley,
1993). However, these two mechanisms are fundamentally different. In vivo
ESC in polyether polyurethane surfaces results from chemical degradation
reactions of the polyether segments by the oxidative agents produced by
inflammatory cells in the surrounding tissues. When foreign materials are
implanted, the body defends itself in a series of biological reactions that attempt
to destroy and wall-off the implant. Inflammation reactions are part of the
defence action that occurs immediately after implantation. Cells such as neutro-
phils and monocytes migrate to the implant surfaces and produce oxidative
agents in the attempt to degrade the foreign material. If an implant is large and
stable, and does not release toxic chemicals and particulates, the inflammation
reactions will demonstrate the pathway of a common foreign body response.
Fibrous capsules form and separate the implant from the surrounding body
