Polymer Coats Leads on Implantable Medical Device

Langley Research Center’s Soluble Imide (LaRC-SI) was discovered by accident. While researching resins and adhesives for advanced composites for high-speed aircraft, Robert Bryant, a Langley engineer, noticed that one of the polymers he was working with did not behave as predicted. After putting the compound through a two-stage controlled chemical reaction, expecting it to precipitate as a powder after the second stage, he was surprised to see that the compound remained soluble. This novel characteristic ended up making this polymer a very significant finding, eventually leading Bryant and his team to win several NASA technology awards, and an “R&D 100” award.

Medtronic’s cardiac resynchronization therapy devices use the NASA-developed polymer as insulation on thin metal lead wires.

Bryant’s team was, at the time, heavily involved with the aircraft polymer project and could not afford to further develop the polymer resin. Believing it was worth further exploration, though, he developed a plan for funding development and submitted it to Langley’s chief scientist, who endorsed the experimentation. Bryant then left the high-speed civil transport project to develop LaRC-SI.

How it Works

The resulting LaRC-SI polymer is an extremely tough, lightweight thermoplastic that is not only solvent-resistant, but also has the ability to withstand temperature ranges from cryogenic levels to above 200 °C. The thermoplastic’s unique characteristics lend it to many commercial applications.

The unique feature of this compound is the way that it lends itself to easy processing. Most polyimides (members of a group of remarkably strong and incredibly heat- and chemical-resistant polymers) require complex curing cycles before they are usable. LaRC-SI remains soluble in its final form, so no further chemical processing is required to produce final materials, like thin films and varnishes. Since producing LaRC-SI does not require complex manufacturing techniques, it has been processed into useful forms for a variety of applications, including mechanical parts, magnetic components, ceramics, adhesives, composites, flexible circuits, multilayer printed circuits, and coatings on fiber optics, wires, and metals.

Several NASA centers, including Langley, have explored methods for using LaRC-SI in a number of applications from radiation shielding and as an adhesive to uses involving replacement of conventional rigid circuit boards. In the commercial realm, LaRC-SI can now be found in several commercial products, including the thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuator, another “R&D 100” award winner (Spinoff 2005).

Where it Stands

Working with the Innovative Partnerships Program office at Langley, Medtronic Inc., of Minneapolis, Minnesota, licensed the material. This material has been evaluated for space applications, high-performance composites, and harsh environments; however, this partnership represents the first time that the material has been used in a medical device.

Medtronic is testing the material for use as insulation on thin metal wires connected to its implantable cardiac resynchronization therapy (CRT) devices for patients experiencing heart failure, which resynchronize the contractions of the heart’s ventricles by sending tiny electrical impulses to the heart muscle, helping the heart pump blood throughout the body more efficiently.

In March 2007, Medtronic conducted the first clinical implants in the United States and Canada of the Medtronic over-the-wire lead (Model 4196), a dual-electrode left ventricular (LV) lead for use in heart failure patients with cardiac resynchronization therapy devices.

Placing a lead in the LV is widely recognized by physicians as the most challenging aspect of implanting CRT devices. Anatomic challenges can make it difficult to access and work within the coronary sinus to place a lead in the desired vein of the LV. The lead is specially designed for optimal tracking over a guide wire, which is intended to allow physicians greater ability to deliver the left heart lead in difficult-to-access veins.

Once implanted in the LV, two electrodes located at the tip of the lead provide physicians with options to tailor delivery of stimulation for each patient. When approved by the U.S. Food and Drug Administration, the lead is expected to be the smallest LV lead in the U.S. market.