Biomedical Engineering Reference
In-Depth Information
drawn filled tube (DFT) was used. In the DFT wire, the core of the MP35N wire
was filled with silver, which dramatically reduced resistance. Corrosion-
resistant MP35N on the outside of the wire protected the silver from corroding.
Cables composed of many strands of pure MP35N or silver cored MP35N
wires were implemented to further reduce electrical resistance in defibrillator
leads. However, both coils and cables are still susceptible to fatigue fracture
within the body under certain conditions. Moreover, by-products from MP35N
corrosion, such as cobalt ions, have been implicated in the degradation of
polyether polyurethanes' insulation materials, as seen in the complex metal ion
induced oxidation process discussed below.
Insulation
Teflon
Õ
(polytetrafluoroethylene) and polyethylene were used as insulation in
early leads. Both materials had a demonstrated history of use in tissue-
contacting applications. However, Teflon bonded poorly to other parts of the
lead, making production difficult. The use of polyethylene insulation also ended
because it made stiff leads which can increase the possibility of perforating the
heart. Its biostability has also come into question due to oxidation degradation
associated with the inflammatory response. Polyester polyurethanes were tried
because of their excellent mechanical properties but were ultimately rejected
because they are subject to rapid hydrolytic degradation in water. Silicone
rubber became the material of choice for insulation; it was nontoxic, chemically
inert and biostable. Because of low tear strength, however, silicone rubber also
needs thicker walls to minimize mechanical damage. It also has a high
coefficient of friction in blood which made passing two leads in the same vein
difficult. As a result, dual chamber pacing did not realize its full potential until
after the 1970s.
Use of polyether polyurethane as a lead insulation began when it was found
to be an almost ideal material for lead insulation. Polyether polyurethane
showed acceptable biocompatibility for the application; it demonstrated hydro-
lytic stability; it showed high mechanical strength and lubricious properties
when wetted with blood; it could be bonded to other materials; and it was much
more flexible. The increased mechanical properties allowed the insulation
thickness to be downsized. Smaller size, combined with lubricious surface in
blood, made placement of two leads in one vein easier, which made dual
chamber pacing a practical therapy. This evolution of materials applications
demonstrates how a material itself can make complicated therapies possible.
Polyether polyurethanes are copolymers made through condensation
reactions among polytetramethyleneoxide (PTMO), methylene diphenyl
diisocyanate (MDI), and 1,4-butanediol (BDO) (Fig. 4.6). The percentage of
PTMO and its molecular weights determine the mechanical properties of the end
product. Polyurethanes with high PTMO contents and/or high PTMO molecular
