> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— In this paper, we report on the development of an implantable pressure sensing system that is powered by mechanical vibrations in the audible acoustic frequency range. This technique significantly enhances interrogation range, alleviates the misalignment issues commonly encountered with inductive powering, and simplifies the external receiver circuitry. The interrogation scheme consists of two phases: a mechanical vibration phase and an electrical radiation phase. During the first phase, a piezoelectric cantilever acts as an acoustic receiver and charges a capacitor by converting sound vibration harmonics occurring at its resonant frequency into electrical power. In subsequent electrical phase, when the cantilever is not vibrating, the stored electric charge causes an LC tank, in which one of the elements is pressure sensitive, to oscillate at its natural resonance frequency and radiate a high frequency signal that is detectable using an external receiver. The pressure sensor is composed of a planar coil (single loop of wire) with a ferrite core whose distance to the coil varies with applied pressure. A prototype of the implantable pressure sensor is fabricated and tested, both in-vitro and in-vivo (swine bladder). A pressure sensitivity of 1 kHz/cmH 2 O is achieved with minimal misalignment sensitivity between the implanted device, acoustic source, and the receiver coil. Index Terms— Implantable pressure sensor, bladder pressure, acoustic powering, mechanical vibration. I. INTRODUCTION mplantable wireless devices for monitoring physiological parameters have been the focus of considerable investigations since the 1950s following the pioneering works of MacKay using passive (LC) and single transistor relaxation oscillators [1]. Subsequent decades have witnessed a plethora of devices and systems leveraging integrated circuit miniaturization and microelectromechanical system (MEMS) technology [2-8]. Batteries [9] and inductive powering [10-12] Albert Kim is with School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906, USA. (email: kim126@purdue.edu. Teimour Maleki is with the Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47906, USA (email: tmalekij@purdue.edu, phone: 765-496-7491 fax: 765-496-6443). Charles R. Powell is with Department of Urology, Indiana University School of Medicine, Indianapolis, IN. Babak Ziaie is with School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47906, USA. (email: bziaie@purdue.edu, phone 765-494-0725, fax: 765-496-6443) have been the two mainstay energy sources for most implantable wireless systems with the former being more suitable for low-power or short lifetime applications and the latter targeting implants with higher energy budget and longer implantation times. A recent example of battery operated systems is the PillCam [13], the latest generation of earlier radio-pills, which features an imaging system (miniature video camera and light source) interfaced with low-power electronics and an RF radio transmitter. Cochlear implants are an archetype of inductively powered systems that provide auditory sensation for many profoundly deaf patients [14]. Another group of implantable wireless sensors are based on passive transponders in which the resonant frequency of a simple LC circuit, indicative of the measured parameter (e.g., a pressure signal coupled to a variable capacitor or inductor through a movable membrane), is remotely monitored with an external coil [15-17]. The standard interrogation scheme for implantable passive LC transponders is the “phase-dip” method, Figure 1. This technique requires an external coil to be in close proximity and parallel to that of the passive transponder in order to increase the coupling between the two coils. The impedance of the external coil in the vicinity of the resonant frequency of the transponder is monitored by an impedance analyzer. The phase of the impedance drops at the resonant frequency of the implanted sensor, offering a remote measurement technique to interrogate the transponder. Although conceptually simple and easy to fabricate, such passive systems have a limited interrogation range (1-2 cm depending of the diameter of the coils), are alignment sensitive, and require sophisticated receiver circuitry. Despite numerous efforts in this area, very few such devices have been commercialized thus far (an exception being CardioMEMS cardiovascular transponders [18, 19]). In this paper, we present the design, implementation, and characterization of a novel electromechanical interrogation An Implantable Pressure Sensing System with Electromechanical Interrogation Scheme A. Kim, T. Maleki, Member, IEEE, C. R. Powell, and B. Ziaie, Senior Member, IEEE I Figure 1. Schematic of a common interrogation scheme (phase- dip method) for passive transponders. Frequency Phase angle Impedance Analyzer Implant Antennas