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Glass-based hermetic micro-packaging platform for untethered, implantable, wireless neural systems
Keywords: Neural interface, Low-profile glass encapsulation, Hermeticity evaluation
Passive microelectrode arrays (MEAs) sense neural signals and transfer them via cables to external data acquisition systems [1][2]. However, to scale such systems, for example to acquire an order of magnitude more neural signals from many regions of the brain, actively powered circuits must be implemented, enabling on-board signal processing, multiplexing, and telemetry. However, active circuitry that maintains a constant electric field established between the inside of the device and its surroundings (e.g. the body) presents a major packaging challenge to implantable devices. The proximity and quantity of charged ions initiates chemical and material reactions to the biological environment that could degrade the assembly. Additionally, the packaging should be low- profile and electromagnetically-transparent to enable effective wireless power and data telemetry. Further, final assemblies should be biocompatible and are expected to function reliably and safely for many years. Here we describe a highly manufacturable and hermetic micro-packaging platform that enables both long-term protection of active electronics and the integration of neural recording technology. The proof-of-concept Wireless Test Vehicle (WTV) integrates active electronic components on a Kapton substrate. The WTV is battery-less and receives wireless power through an inductive link at the 13.56MHz ISM band. A 3-axis microelectromechanical system-based (MEMS) accelerometer and a humidity sensor are integrated into the micro- packaging to measure motion changes and water ingress within the cavity. An application specific integrated circuit (ASIC) acquires and multiplexes output data from two sensors before modulating an 850-nm Vertical-Cavity Surface-Emitting Laser (VCSEL) to transmit the recorded data through the saline bath with a typical data rate of 13.56Mbps (clock recovered from power signal, expandable to 1Gbps). After functionality verification, six circuit substrates are mounted in premade, individual fused silica cavities with adhesives. A premade fused silica lid is then boned onto the six cavities at the same time using proprietary low temperature laser bonding process to form a hermetic seal. After bonding, six fused silica cavities were singulated by laser and the fully-populated and sealed WTV assemblies (21mm x 21mm x 2.1mm) were immersed in saline within a test environment. We designed and built a wireless Encapsulation Test System (wETS) to perform accelerated degradation tests on 6 independent WTVs simultaneously. Printed circuit boards were designed to quantitatively measure and regulate wireless power delivery, as well as to collect and convert photons emitted from WTVs infrared laser diode (VCSEL) into equivalent voltage patterns for acquisition. Data from the 6 WTVs was buffered in an FPGA and transferred to external data storage for analysis. Each WTV was anchored within an individual fixture to maintain VCSEL alignment. All WTVs were kept in a temperature-regulated water bath filled with Phosphate Buffered Saline (1 X, pH 7.4). We present preliminary test data collected from 6 WTVs at elevated temperature of 37C and compare the data stability across time. Wireless data link quality and power transfer efficiency were evaluated during the test. Our work to date has included the design, development, and evaluation of a hermetic micro-packaging platform that is well-suited for active neural implants. This platform is extendable to more generalized implantable electronics, for example inertial and electromyography sensors.
David A. Borton, Assistant Professor of Engineering
School of Engineering, Brown University, Providence, RI; Brown Institute for Brain Science (BIBS), Brown University, Providence, RI; Department of Veterans Affairs, Center for Neurorestoration and Neurotechnology, VA Medical Center, Providence, RI
Providence, RI
USA


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