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Automated high-throughput hermetic failure monitoring system for mm-sized wireless implantable devices
Keywords: Wireless LC sensor, Hermetic failure, implantable medical device
Implantable medical devices (IMDs) are often required to be electrically functional, mechanically durable, and chemically stable for more than 10 years in harsh environment in the human body, which is filled with biological fluids containing NaCl, KCl, phosphates, carbonates, enzymes, and various proteins [1]-[3]. A biocompatible and hermetic package prevents the ionic biological fluids penetration into the IMDs, which causes device performance degradation eventually leading to their failure. Also, the package plays the role of a buffer against physical stress from the IMDs and a protection layer against harmful chemicals inside the IMDs from leaking into the surrounding tissue. Since the hermetic package protects both the IMD and its surrounding tissue during long-term implantation, it should be carefully examined not to have any crack or holes in advance with accurate hermeticity testing methods. The common method to test the hermeticity of IMDs is to measure their leak rate using the methods defined in the MIL-STD-883 standard, such as helium leak test, radioisotope fine leak test, optical fine/gross leak test, and so on [2]. These traditional leak tests are not, however, suitable for mm-sized implants. The millimeter-sized IMDs operated through near-field wireless links often have inductor-capacitor (LC) tank circuit for receiving power to eliminate transcutaneous interconnects that elevate the risk of infection and remove bulky batteries. This passive LC-tank can be utilized to wirelessly test hermetic failure of the IMDs without adding any other component to the IMD by monitoring its resonance frequency shift at the minimum reflected power back to an adjacent readout coil. The passive wireless humidity monitoring sensor, implemented within a 7 1.2 1.5 mm3 volume has been proposed in [4], and successfully recorded its resonant frequency shift with respect to relative humidity change inside the package. However, manual data collection for one sample at a time with a network analyzer is labor intensive. The recent work in [5] has replaced the bulky network analyzer with a customized portable readout system to monitor the humidity level, and reduced the overall volume for the system with similar limitation of manual data collection for one sample at a time. In this work, we propose an automated and high throughput hermetic failure monitoring system for mm-sized biomedical implants using an inductive link array. The prototype system includes a cost-effective vector network analyzer (VNA), a customized readout coil array with a single-pole 8 throws (SP8T) RF switch on a 27.6 45.6 mm2 2- layer 1 oz. FR4 printed circuit board (PCB), and an Arduino-based controller. The readout coil array has been geometrically optimized to monitor the hermetic failure of mm-sized passive implants, wrapped with a receiver coil (part of the LC-tank), and coated with 5 um thick parylene-C and polydimethylsiloxane (PDMS), by examining the frequency shift in the phase-dip of the reader coil input impedance or disappearance of the phase- dip in the case of packaging failure. The IMD samples are held in place at the center of the readout coil underneath by a 3D-printed structure to improve the accuracy of angular and horizontal alignment. The locater embedded with the IMD is inserted in a test tube that is filled with 0.9% saline solution. Each test tube is accurately aligned on each readout coil that is selectively activated by a command from the graphical user interface (GUI). The portable VNA records the reflected S- parameter from the LC-tank through an activated readout coil. The GUI software conducts continuous reading of phase-dip for a range of preconfigured frequency values. The scripts simultaneously render two types of plots: the phase impedance versus frequency and its corresponding time-domain plot on the host machine to which the VNA is attached. We also provide a mobile phone user-interface to deliver the same plots in real time for remote usage. The system can be utilized for accelerated hermetic failure testing of 8 IMDs immersed in saline solution in parallel to estimate their lifetime without much manual intervention over a long period with a resolution of 186 kHz up to 85 ℃ (for accelerated lifetime testing). When the moisture penetrates into the package, the parasitic capacitance around of the Rx inductor increases due to increased permittivity of the surrounding environment adjacent to the coil, dropping the resonant frequency of the LC-tank. The hermetic failure of parylene-C underneath the PDMS layer results in the phase-dip disappearance out of the monitored frequency range. Since the PDMS is not a hermetic package but a hydrophilic material that is coated around the implants for mechanical protection and matching for surrounding tissue, the phase-dip frequency shift is not considered to be hermetic failure of the IMD. When the phase-dip is disappeared from the designated frequency range, the test duration time can be converted to a data point in the mean time of failure (MTTF). This work will include the accelerated testing results to estimate the lifespan for 8 samples of 1 1 mm2 sized passive implants in 5 um thick parylene-C and polydimethylsiloxane (PDMS) package by monitoring the phase- dip frequency shift and disappearance over 1 month.
Pyungwoo Yeon, Graduate Research Assistant
Georgia Institute of Technology
Atlanta, Georgia
United States

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