Increasing wireless power transfer efficiency on implantable biomedical devices using ferrite based negative permeability metamaterial design

Abstract

Summary form only given. Modern implantable neursotimulation devices have functions that can significantly improve the quality of life of patients by partially restoring lost sensory and motor functions. Because of this, these devices have to be designed with utmost care to run reliably and efficiently for long periods of time, often decades. Due to the likely continuous active role of neurostimulators, power must be supplied to the implant without resorting to wires. One such mechanism is inductive near-field resonant coupling. This technique generally consists of two coils, one mounted on the external power supply and one mounted on the implantable device. These coils operate at resonance in order to wirelessly transfer power between them by virtue of their inductive coupling, thus making it possible to recharge the device. Metamaterials are materials designed to exhibit a desired electromagnetic response at a certain frequency. Metamaterials exhibit properties not present in nature: one such property is a negative refractive index. It has being demonstrated that a left handed metamaterial, one with simultaneous negative permittivity and permeability, can be used to focus and enhance evanescent waves. Because of this latter property, attempts have been made to use metamaterials to enhance the coupling of near-field inductive wireless power transfer systems. In this abstract we propose the use of a negative permeability slab in order to enhance the power transfer efficiency of resonant inductive coils for implantable medical devices. Because of the target application the proposed metamaterial design must work at very low frequencies, on the order of a few megahertz. Further, unit cell have dimensions deep into the subwavelength limit, on the order of centimeters (> λ/1000). Our design uses ferrite materials and planar and nonplanar fabrication techniques in order to achieve the necessary minimization. An inductive resonant wireless power system is built to work in conjunction with the designed metamaterial. Measured wireless power transfer efficiency will be shown in the presence and absence of the metamaterial design, and an increase in efficiency will be demonstrated. Additionally the role of unit cell size for homogenization of metamaterials in the deep subwavelength limit is going to be discussed.