Power amplification via compliant layer interdigitation and dielectrophoretic structuring of PZT particle composites

Abstract

Nonunion occurs in up to 10% of all fractures, with about 8% of all femoral fractures ending in nonunion, or failed healing with current fixation methods1,2. These failure rates can be caused by factors such as diabetes, osteoporosis, tobacco use, and severe tissue damage3,4. According to the FDA, it takes a minimum of nine moths to declare nonunion after trauma, with no progress in healing for three months5,6. Some adjunct therapy methods are being used to combat these failure rates such as the OsteoGen¬TM direct current bone growth stimulator. However, these devices require an implanted battery that will eventually need removed. The evolution of portable electronics has led to recent popularity of piezoelectric materials for energy harvesting, especially for devices deployed remotely or in vivo. Intramedullary nails could utilize the energy harvesting capabilities of piezoelectric materials to provide electrical stimulation at the fracture site without an implanted battery. However, the efficiency of piezoelectric generators harvesting energy from the human body is lacking due to off-resonance loading7. In addition, piezoelectric ceramics are expensive to manufacture, dense, brittle, and difficult to use in high strain environments. Piezoelectric composites composed of ferroelectric particles distributed in a polymer matrix are desirable due to low cost and tunable properties. In this study, Compliant Layer Adaptive Composite Stacks (CLACS) made with thin piezoelectric composite layers structured by dielectrophoresis (DEP) were investigated to increase the energy harvesting efficiency at low frequencies. To predict power generation capabilities, a theoretical model was developed by using established particle composite models in conjunction with a shear lag structural mechanics model for CLACS. Granular composite discs of lead zirconate titanate particles in an epoxy matrix were manufactured at a 50% volume fraction and structured by DEP, if applicable. CLACS were manufactured using ten composite discs and two compliant layer thicknesses. The stacks were electromechanically tested by varying load, frequency, and resistance. Experimental results showed an increase in power amplification with DEP structured discs and compliant layers. In addition, the theoretical model accurately predicts power production for both 0-3 and 1-3 CLACS at low frequencies. DEP structured particle composite CLACS can provide a method of energy harvesting for devices in remote locations, especially in low frequency high strain environments. Future work could continue the development of piezoelectric particle composite CLACS for use in intramedullary nails. Such studies would evaluate the performance of ring shaped piezoelectric composites, develop theoretical understanding for ring shaped CLACS, investigate fatigue strength of piezoelectric particle composites, and evaluate impact strength of particle composite CLACS as compared to ceramic CLACS. Lastly, overall improvements to particle composite manufacturing methods to reduce variability could be investigated.