KSEF R&D Excellence: Development of Method for Sealing an Implantable Medical Device

The proposal is to create a novel sealing mechanism for an adjustable systemicpulmonary artery (SPA) shunt. SPA shunts are used to provide pulmonary blood flow for children with complex cardiac malformations who have no other source of blood flow to their lungs. The amount of blood flow is critical for survival. However, there is currently no means to mechanically adjust the amount of blood flow to the lungs. The adjustable SPA shunt is intended to provide a definitive solution to this clinical problem. The adjustable SPA shunt works by a screw-plunger mechanism which surrounds a conventional 4 mm expanded poly1etrafluoroethylene (PTFE) graft. A stepper motor, implanted in the upper abdomen, turns a cable which leads into the resistor mechanism. Within the resistor mechanism, screw threads present on the cable and the resistor housing cause linear displacement of the end cable, which terminates in a plunger on the graft wall. The plunger can be depressed into the graft lumen, or released away from the graft lumen. Graft diameter, and thus flow, can then be adjusted. A wireless communication system and an internal power source allow the device to be fully implantable. Clinical success ofthe device depends on continued function of the mechanical components of the device. Proteinaceous extracellular fluid will congeal in the screw thread mechanism, preventing its function. Proteins may also form a cast around the graft (within the resistor housing), also preventing further changes in graft shape. In order to prevent these technical problems, an appropriate seal is necessary between the graft and the housing. In this way, the potential complications described above are prevented. Adding to the difficulty of sealing the graft/device interface is that the device must be placed after the graft has been sewn in place. The device, if placed on the graft pre-implantation (in the factory) would have sufficient bulk as to interfere with surgical placement of the graft. The device must be attached to the graft after implantation. The sealing mechanism described in this proposal consists of two synthetic O-rings glued onto the PTFE graft with cyanoacrylate glue. The distance between the O-rings corresponds to the length of the resistor. The resistor comes in two parts, and snaps around the graft. Test resistors will be manufactured with grooves corresponding to the O-rings on the graft. When the resistors are attached to the graft, the groove-O-ring interface will fonn a press-fit seal. 5 different materials (polyvinyl chloride. polypropylene, nylon, PTFE, and acetal) will be tested. 2 sizes of O-rings will be made from each material. If seaJing is inadequate, the resistor grooves will be lined with .5 mm thick Silastic@ to enhance sealing. In vitro testing will consist of placing the test resistors into a gelatin bath for 4 hours, and then testing resistor function. The 2 best material/size combinations will then be tested in vivo by placing them in the peritoneal cavities of rats for I month. Resistor function will then be tested. The adjustable SPA shunt is a complex device. offering physiological control previously unattainable. As of this time, no variable resistor exists in any capacity for the circulatory system. Proper function ofthe device depends on the working mechanism being protected biological deposits. Eventual device success may substantially lower mortality for complex congenital cardiac surgery.