Polyurethane-insulated lead

Certain polyether polyurethanes possess ideal properties to make relatively small, maneuverable, and slippery (in blood) leads. Of those available at the time, only the Pellethane 2363 series met our needs. In 1975, the first polyurethane-insulated lead was implanted in a human as part of a spinal stimulator. The softer Shore 80A and stiffer Shore 55D polymers were tested extensively in vivo over the ensuing 5 years for cardiac use. This may leave the impression that we implanted neuro leads in humans first, then tested the material. In fact, the neuro implants were based on the... 

State of the art testing at the time, but we wanted more long term in vivo data for cardiovascular. In vitro and ex vivo testing demonstrated that the materials were biocompatible and stable in the intended environment (as it was understood at the time) [6]. Just to be sure, however, we implanted the materials in the subcutis of rabbits for 2 years, conducting extensive characterization tests as a function of time. No untoward biocompatibility or biostability issues were revealed. Device tests were conducted in canines for 12 weeks, which had been shown to be sufficient time to characterize acute and chronic performance. Human clinical evaluations over 1-2 years (depending on the models) demonstrated that the devices had superior performance compared to their predecessors [7, 8]. Four polyurethane-insulated lead models were market released in the United States in April 1980 with FDA approval. 

Overlays for plywood deserves mentioning. Overlays for plywood may be anything that conceivably can stick to the panel and have end use utility. Overlays include polyester or phenolic impregnated paper (either medium or high density types), fi berg I ass-re in forced plastic, fabric, high pressure laminates, aluminum, lead, polyurethane insulation and pebbles. The uses for plywood are extensive and marriage with other overlay materials expand these uses tremendously. 

Fig. 1.16 Metal ion oxidation is an oxidative degradation of polyurethane insulation, true for both kinds of polyurethane (80A and 55D) (a). Environmental stress cracking is an oxidative condition that takes place on the surface of polyurethane leads (true for 80A) (b)...

Pacemaker (implant) 1958 200 000 Polyurethane, silicone (lead insulation)... 

Fig. 1.24 Two designs of pacing leads using cabled conductors. Above A traditional multifilar coiled conductor for stylet delivery lies parallel to a cabled conductor. The lead is slightly oval (Medtronic 5044. Never marketed). Below. Coaxial design (Medtronic 3830 SelectSecure ). The cathode cable lies at the center of the lead and is then covered with a protective cover (ETFE) and over that a conventional silicone insulator. It is then surrounded with a polyurethane insulated multifilar coiled anode conductor. There is no lumen for a stylet. (Permission for use Medtronic.)...

Another kind of corrosion sustained by medical implants is the metal-ion oxidation sustained by polyurethane pacemaker leads. This failure mode is highly complex and involves water transport across the insulation of the lead, allowing contact with the Co-Cr alloy, which then corrodes by a fretting mechanism. The metal ions, in particular Co +, then penetrate the polyurethane and oxidize it in a catalytic fashion. This leads ultimately to an electrical breach in the lead and failure of the pacemaker, often resulting in the death of the patient. 

Wiggins MJ, WiUcoff B, Anderson JM, Hiltner A. Biodegradation of polyether polyurethane inner insulation in bipolar pacemaker leads. J Biomed Mater Res 2001 58 302-7. Stokes K. Polyurethane pacemaker leads. In Becker K, Whyte J, editors. Clinical evaluation of medical devices. Humana Press 2006. p. 285-304. http //dx.doi.org/ 10.1007/978-l-59745-004-l 17. 

A contemporary ventricular defibrillator lead consists of multiple internally separated metal wires that are externally encased in silicone rubber or polyurethane insulation. This allows the lead to function similarly to a standard pacemaker lead, i.e. transmission of electrical pacing/sensing signals between the heart and generator, but also structurally provides a separate pathway that participates in the delivery of shocks. This pathway includes what are commonly referred to as the lead "coil(s)." Shocks are delivered across the heart between the coil(s) and potentially the ICD can. Each element of the lead has its own pin that may connect to the ICD header. 

Stokes K, Anderson J, McVenes R, et al. (1995) The encapsulation of transvenous polyurethane insulated cardiac pacemaker leads. Cardiovasc Path 4(3) 163-172. 

Pacemaker Interfaces and Leads. Problems of existing pacemaker interfaces and pacemaker lead materials made from siUcones and standard polyurethanes are environmental stress cracking, rigidity, insulation properties, and size. 

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

The tank is typically about sixteen inches in diameter and about four to five feet tall. The top of the tank is domed upward and the bottom of the tank is also domed upward in a concave manner. The outside of the tank is insulated with a polyurethane foam insulation that is squirted into the gap between the tank and a thinner sheet metal jacket. The polyurethane is made of two different components that react and harden when mixed. Included in the mixture is a blowing agent that causes the polyurethane to expand in a foam-like manner. Prior to about 1980, water heaters were insulated with fiberglass insulation. The foam insulation process was developed to allow automation and increased manufacturing speed and reduced costs. A side benefit was improved insulating ability leading to a slight increase in efficiency. 

The electrical properties of the polyurethanes are not adequate for use as primary insulations, but their general toughness leads to their use as cable jacketing materials. 

There are several other commercial products containing segmented polyurethane (SPU) such as pellethane or cardiothane. These SPUs are widely recognized to possess notable biomedical properties as materials for artificial heart and intra-aortic balloon pumping (IABP), and also as coating materials for pacemaker-lead insulators (See Sect. 4.1). 

Coagulation is not the only problem with materials intended for implantation, however. Cardiac pacemakers are intended to correct arrhythmias. Insulating materials for a pacemaker lead must be tough and long lasting. The first leads were insulated with polyethylene or silicone rubber. Neither material was considered ideal because of endocardial reactions (polyethylene) and limited durability (silicone rubber). The strength and flexibility of polyurethanes led to their introduction in 1978 as lead insulators. 

Sucrose reacts with diisocyanates leading to polyurethanes, which are used as thermal insulating foams, notably in cars. Partially protected sucrose esters can be used for the synthesis of better-defined polymers (Scheme 46).265 A first step of hydroxypropylation is sometimes necessary to obtain sufficient miscibility with the diisocyanate derivative, as well as for tuning the physicochemical properties of the polyurethane foams.78,305,420... 

Polyurethane/polyisocyanurate products have higher insulation value and good flammability ratings and are expected to continue to be the leading products in plastic foam market as sheets and slabs. 

Table 1.1 Lead insulation comparison of silicone rubber and polyurethane...

Because silicone has less tear strength, some types of leads with this insulation tend to be larger in diameter than polyurethane leads... 

Some currently used bipolar coaxial leads use both silicone (inner insulation) and polyurethane (outer insulation) coating, incorporating the durability of silicone with the ease of handling of polyurethane while maintaining a thinner external lead diameter. 

In practice, manufacturers have tried to combine the best characteristics of all insulating materials by using silicone for the lead body, with each cable and coil conductor coated with a fluoropoly-mer and a thin polyurethane layer to cover the entire lead. In this way, is possible to improve abrasion resistance and handling performance. Recently developed copolymer obtained by mixing silicone rubber and polyurethane can also be used for the outer layer. 

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