Leading the Charge in Bone Healing: Design of Compliant Layer Adaptive Composite Stacks for Electrical Stimulation in Orthopedic Implants

Abstract

The overall aim of this research is to develop a robust, adaptable piezoelectric composite load-bearing biomaterial that when integrated with current implants, can harvest human motion and subsequently deliver electrical stimulation to trigger the natural bone healing and remodeling process. Building on the preclinical success of a stacked piezocomposite spinal fusion implant, compliant layer adaptive composite stacks (CLACS) were designed as a scalable biomaterial to increase efficiency of power generation while maintaining mechanical integrity under fatigue loading seen in orthopedic implants. Energy harvesting with piezoelectric material is challenging at low frequencies due to material properties that limit total power generation at these frequencies and brittle mechanical properties. Stacked generators increase power generation at lower voltage levels and resistances, but are not efficient at low frequencies seen in human motion. CLACS integrates compliant layers between the stiff piezoelectric elements within a stack, capitalizing on the benefits of stacked piezoelectric generators, while decreasing stiffness and increasing strain to amplify power generation. The first study evaluated CLACS under compressive loads, demonstrating the power amplification effect as the thickness of the compliant layer increases. The second study characterized the effect of poling direction of piezoelectric discs within a CLACS structure under multiaxial loads, demonstrating an additional increase in power generation when mixed poling directions are used to create mixed-mode CLACS. The final study compared the fatigue performance and power generation capability of three commercially fabricated piezoelectric stack generators with and without CLACS technology in modified implant assemblies. All configurations produced sufficient power to stimulate bone growth, and maintained mechanical strength throughout a high load, low cycle fatigue analysis, thus validating feasibility for use in orthopedic implants. The presented work in this dissertation provides a robust experimental understanding of CLACS and a characterization of how piezoelectric properties and composite structures can be tailored within the CLACS structure to efficiently generate power in low frequency, low impedance applications. The main motivation of this work was to develop a thorough understanding of CLACS behavior for implementation into medical implants to deliver therapeutic electrical stimulation and accelerate rate of bone growth, helping patients completely heal faster. However, the ability to tune composite stiffness by changing compliant material properties, type of piezoelectric material and poling direction, or volume fractions could benefit the energy harvesting potential in fields ranging from civil infrastructure to wind energy, to wearables and athletic equipment.