The Jarvik 2000 heart includes the blood pump inside the natural heart (white arrow), the internal cable (blue arrow), the behind-the-ear connector (green arrow) the belt worn controller (yellow arrow), the belt worn battery (grey arrow), and the external cables (black).
Here we cover the background, basic design elements, and surgical placement of the patented Jarvik 2000 post-auricular cable and connector. Transmission of power into the body has been one of the most difficult problems for developers of mechanical circulatory support devices.
The performance objectives are well defined and include:
- Providing ~10 watts of power from outside the body to the implanted blood pump.
- Avoiding local or systemic infection.
- Functioning reliably for many years.
- Permitting an essentially normal quality of life.
The Jarvik 2000 post-auricular connector was designed to achieve these objectives based on the following information, observations, and materials selection:
- The only biological structures that penetrate the skin are teeth and tusks. Fingernails, claws, and antlers are superficial “cutaneous” structures that do not penetrate the skin.
- Teeth and tusks are imbedded in bone, which immobilizes them in relation to the gums or other tissue through which they pass. This avoids excessive trauma at the skin junction.
- The gingival tissue is specialized to resist infection, including being highly vascular and having non-dense relatively open extracellular matrix tissue through which white cells can easily migrate to reach the site of invading micro-organisms.
- Similar to the gingival tissue, the scalp is highly vascular and resistant to infection, probably because attacks by predators to the head and neck are common and those early primates from which humans evolved had a survival advantage if the scalp healed well.
- Successful bone mounted medical implants that preceded our post-auricular connector included pyrolytic carbon pedestals to pass cochlear implant electrode wires into the body, pedestals to provide electric stimulation for artificial vision for the blind, titanium dental implants, and various types of titanium screws and posts that mount into tissues of the face and scalp to anchor cosmetic replacements for structures lost to trauma or cancer, such as the nose, eye, or ear.
- Pyrolytic carbon skull mounted pedestals, although very infection resistant (with some implanted successfully for more than twenty years) are relatively brittle and subject to cracking if the patient bumps his head. This has been reported as a major failure mode of this material. We recognized the need for a stronger material than pyrolytic carbon, and chose titanium.
- External VAD power cables are subject to damage, such as being crushed in a car door. If this occurs the external cable needs to be changed, which requires a connector. To avoid any length of external cable that could not be replaced, we located a connector within the titanium post that passes through the skin.
- Using a skull mounted percutaneous device, the internal power cables would need to pass from the head, across the neck, and to the VAD in the chest. Especially at the neck, there would be motion and flexing of the cables. A redundant multi-strand cable designed for extremely long flex life was developed.
- Human engineering factors included:
- The need for the patient to be able to safely sleep on the connector.
- The need for the patient to be able to change the external cable without being able to see its attachment point (the connector is towards the back of the head).
- The need for the skin junction to resist infection without any dressing
- The ability of the patient to shower, bath, or swim
- The desirability of the cable to be cosmetically acceptable
- Electrical insulation for the implanted wires needed to be biocompatible and stable in body fluids long term. We chose a polymer that had been successfully used for pacemaker lead insulation.
- Anchoring the skull mounted connector required a relatively large diameter flange to permit room for mounting screws. To avoid tunneling this across the neck to the head, we separated the cable connector from the mounting flange into two mating parts, so that the flange did not need to be tunneled.
- Safe surgical fixation to the skull required avoidance of skull penetration by the screws. A set of surgical instruments, including depth-stopped drills and taps, a sized skin punch, and a screw hole locator template were designed.