Ultrasound Stimulation of a Piezoelectric Composite with Compliant Layers on Power Output for Bone Healing in Spinal Fusion Applications
University of Kansas
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Spinal fusion devices have up to 50% failure rates for patients who smoke or are diabetic 1. Bone healing has been accomplished through direct current (DC) electrical stimulation to improve bone healing rates 2. Current DC electrical stimulation can accomplish bone healing, but batteries are used as a source of power. Replacing a battery after fusion would require a second surgery. Many microgenerators use piezoelectric material to convert mechanical energy into electrical potential 3. In a pilot ovine study, composite spinal interbody implants made with stacked layers of piezoelectric fibers provided DC electrical stimulation at the fusion site and substantially enhanced fusion under normal loading after six weeks as compared to a control 2,4,5. To improve manufacturability while maintaining material toughness of the interbody implants, alternative manufacturing methods were explored in which compliant layers of epoxy (EPO-TEK® 301, Billerica, MA) were inserted between stacked discs of piezoelectric material, PZT-SM111 (STEMiNC-0.4mm thickness, 10mm diameter, Part No. SMD10T04F5000S111, Doral, FL). For patients with limited weight bearing abilities that cannot provide enough mechanical energy from human body movement, another method of mechanically stimulating the PZT discs would be beneficial. Many methods of wirelessly powering medical devices have been investigated including electromagnetic waves, thermoelectric devices, and ultrasound 3. Transmitted ultrasound waves can be used to mechanically activate a medical device implanted into human tissue with PZT elements 3. Using ultrasound as the loading source, the effect of varying compliant layer thicknesses on generated electrical potential and power output of the PZT compliant layer adaptive composite stacks (CLACS) in different media (water, tissue, and tissue plus bone), media thicknesses (20mm and 40mm), and loading orientations of the PZT composites with respect to the ultrasound wave front (perpendicular and parallel) were investigated. An Acuson 128xp ultrasound medical imaging machine (Mountain View, CA) was used to mechanically stimulate the PZT elements in the compliant layer thickness composites and voltage measurements were made with a Tektronix (Beaverton, OR) DPO 3034 Digital Phosphor Oscilloscope (300 MHz). The PZT composite was manufactured with six through thickness pre-poled PZT-SM511 discs (STEMiNC-0.4mm thickness, 10mm diameter, Part No. SMD10T04F5000S111, Doral, FL) that were connected electrically in parallel (EPO-TEK® H20E, Billerica, MA) with copper strips, varying compliant layer thickness (0, 0.2, 0.4, and 0.8 mm ± 0.02 mm) of medical grade epoxy slices (EPO-TEK® 301, Billerica, MA) interposed between the PZT discs, and encapsulated in medical grade epoxy (EPO-TEK® 301, Billerica, MA). After assembly, the CLACS were tested at each combination of media type, media thickness, and loading orientations. The CLACS were subject to 4MHz ultrasound waves (1 W) in a Gaussian shape pattern with a V4 vector 128 PZT element array probe (Mountain View, CA). Voltage measurements before rectification (alternating current - AC) and after rectification (direct current - DC) were recorded. Power was computed after rectification by applying an average voltage measured across the lumped internal Oscilloscope channel impedance (1 MΩ) with the influence of a variable load resistance (Ω) and capacitor (c) connected in parallel and then using P=V^2/R by combining Joule’s and Ohm’s law. By adding a compliant layer thickness in between the PZT discs of the CLACS, generated electrical potential and power output increased. As the compliant layer thickness increased there were significant increases in power output due to more deformation that occurred along the entire face and edges of the PZT discs. As media thickness increased, DC power output decreased. The CLACS produced the most power with water as the media. Significantly less power was produced in tissue media, and even less in the tissue plus bone medium (p<0.05 BoxCox Transformation). When the stacked PZT layers were being loaded perpendicular to the wave front of the ultrasound waves, there was significantly more power output than when the stacked PZT layers were being loaded parallel to the ultrasound wave front (p<0.05 BoxCox Transformation). Future work could build off this foundational study to further characterize the CLACS power output behavior. These future studies would include focused vs. unfocused ultrasound loading sources to increase the amount of mechanical energy that influences the CLACS, differing operating frequencies to match and mismatch the resonant frequency of the PZT discs and to determine optimum frequencies for certain media depths, testing more varied loading orientations, differing ultrasound intensity levels, using a combination of radially and through-thickness poled PZT discs, designing circuit components to improve electrical energy transfer (impedance matching) efficiency and implementing pulsed DC circuit logic after rectification to approach a more realistic application of DC electrical stimulation in bone healing for spinal fusion.
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