Biomedical Lead Body Simulation

Implantable biomedical devices, such as pacemakers and defibrillators, are used to control heart rhythm, or shock the human heart back into rhythm. These devices are generally stainless steel, approximately the size of a silver dollar; contain programmable electronics, and a battery power source. Pacemaker or defibrillators are surgically implanted, usually just below the skin in the pectoral muscle. They are usually designed to function, without failure or replacement for a design life of five to seven years.

The pacemaker or defibrillator transmits electrical signals to the human heart through multiple signal wires contained in a protective sheath body or lead. The signal wires may be bundled multi-stranded, or multiple filars (wires) wound into a helical coil configuration. These signal wires are protected from damage by a multi-lumen (multiple opening) lead body formed from extruded Silicon or other materials. Newer lead body extrusions may be co-extruded; meaning extruded using multiple materials for greater flexibility and toughness.

Lead design, analysis and testing are a very complex, time consuming process for the biomedical engineer. The lead body must be designed to contain multiple lumens, with one or more lumens containing a signal wire. Also, the lead cross-section must be the smallest diameter possible, while providing sufficient axial stiffness and bending flexibility to allow the lead to be implanted by the cardiovascular surgeon.

Proposed multi-lumen cross-section designs are created using CAD technology, and evaluated using highly non-linear, finite element modeling (FEM) methods. These computer simulations replicate the anticipated axial, torsion, bending and crushing conditions which the lead is expected to survive within the body. Computer simulations predicting lead performance is followed by prototyping and extensive physical testing.

Implantation of the lead in the body is performed by the cardiovascular surgeon, with the lead generally passing between the clavicle and collarbone on its way between the device and the human heart. This potentially subjects the lead to crushing loads due to compression between the clavicle and collarbone.

Download / Play an FEM Simulation
(Playable with QuickTime Player)

The FEM simulation presented in this article, allows the design engineer to evaluate stress levels within the lead body, as it is compressed between two rigid surfaces representing the clavicle and collarbone. This model is much more complex than it first appears :

• Multi-lumen lead cross-section is represented by a non-linear, self-contacting, incompressible, elastomer.
• Model contains multiple elastic contact bodies, simulating multi-stranded and helical coil signal wires.
• Solution employs a large displacement solution technique.
• Clavicle is represented by a fixed, rigid contact surface.
• Collarbone is represented by an incrementally displaced, moving rigid contact surface.

As the lead is compressed between the surfaces, the individual contact bodies are allowed to move, contact the lead body lumen surfaces and be compressed. Stress distributions in the lead body are presented as color filled contours. Stresses in the individual components may also be displayed.

This type of FEM simulation may be used as an engineering analysis tool to evaluate the effects on stress distributions by varying lead lumen locations, geometry, signal wire types, diameters and predictions of lead body design life.

To see the resume of the expert associated with this case study, see the link below.