Design of Ceramic Packages for Acoustically Coupled Implantable Medical Devices

Abstract

Objective: Ultrasonic acoustic power transfer is an efficient mechanism for coupling energy to millimeter and submillimeter implants in the body. To date, published ultrasonically powered implants have been encapsulated with thin film polymers that are susceptible to well-documented failure modes in vivo, including water penetration and attack by the body. As with all medical implants, packaging with ceramic or metallic materials can reduce water vapor transmission and improve biostability to provide decadal device lifetime.In this paper, we evaluate methods of coupling acoustic energy to the interior of ceramic packages. Methods: The classic wave approach and modal expansion are used to obtain analytical expressions for acoustic transmission through two different package designs and these approaches are validated experimentally. A candidate package design is demonstrated using alumina packages and titanium lids, designed to be acoustically transparent. Results: Bulk modes are shown to be more effective at coupling acoustic energy to a piezoelectric receiver than flexural modes. Using bulk modes, packaged motes have an overall link efficiency of roughly 10%, compared to 25% for unpackaged motes. Packaging does not have a significant effect on translational misalignment penalties, but does increase angular misalignment penalties. Passive amplitudemodulated backscatter communication is demonstrated. Conclusion: Thin lids enable the use of acoustically coupled devices even with package materials of very different acoustic impedance. Significance: This work provides an analysis and method for designing packages that enable acoustic coupling with implantable medical devices, which could facilitate clinical translation. Fig. 1. System overview for acoustically coupled implantable medical devices (IMDs) utilizing ultrasonic backscatter communication. Information from many different energy domains can be encoded in the amplitude of the backscatter (green/blue waves). For these devices to be clinically useful, packaging must be designed that both protects the devices and also allows for acoustic coupling.