The influence of stacked generators on source impedance of a piezoelectric spinal fusion implant

The failure rate for difficult-to-fuse patients undergoing spinal fusion surgeries can be as high as 29-46%. The primary adjunct treatments which currently address this problem are bone morphogenetic proteins (BMPs) and DC electrical stimulation. BMPs have had clinical problems with off-label use and ectopic bone growth. DC electrical stimulation has had much clinical success, but it requires longer procedures (compared to surgeries with only spinal fusion cages and instrumentation) and it can require revision surgeries to remove the battery pack. To overcome these problems with current adjunct therapies, a piezoelectric spinal fusion implant has been developed which produces electrical stimulation without an external battery pack. One of the primary obstacles with creating a piezoelectric spinal fusion implant was the extremely high source impedance due to the capacitive nature of piezoelectric materials. Stacked actuators have been used with monolithic piezoelectric elements to lower source impedance, so this approach was utilized with a 3-1 composite piezoelectric material made from PZT 5A1 fibers with EPO-TEK 301 epoxy used as the matrix material. Specimens were made with 1 layer, 3 layers, 6 layers, or 9 layers and the effects of the number of layers on average maximum power and the load resistance at which average maximum power occurred (optimal load resistance) were studied. The effects of mechanical preload, mechanical load frequency, mechanical load amplitude, and poling electric field strength were also studied to further characterize the composite material. As the number of layers was varied from 1 to 9 with mechanical load amplitude of 1000 N and mechanical load frequency of 2 Hz, average maximum power was not significantly changed (p