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Miniaturizing Implantable Medical Devices Through Custom Integrated Circuit Design
Keywords: Medical, Custom, Design
Introduction: A new generation of Miniature Implantable Medical Devices (MIMDs) has arrived. Thanks to advances in Micro-Electro-Mechanical Systems (MEMS), electronics packaging, passive devices, and battery technologies, custom Integrated Circuit (IC) design can be employed to help produce new devices that are a fraction of the size of traditional Implantable Medical Devices (IMDs). Traditional IMDs: A traditional IMD consists of a metal case located in the chest or abdomen, and electrical leads connecting the electronics in the case to the point of therapy in the heart, spine, neck, head, or limbs. Examples of these devices include; cardiac pacemakers and defibrillators, neuro-stimulators, and drug infusion pumps. Despite dramatic technological advances in these devices through the years, most of the devices are still relatively large, with volumes ranging from about 15cc to 50cc. These large devices still typically require invasive surgical implant procedures, and have inherent reliability challenges related to their long leads or catheters. MIMDs: The new generation of MIMDs can be implanted at the point of therapy, thus eliminating the need for long leads, and enabling minimally invasive surgical procedures. Examples of such devices include; Insertable Cardiac Monitors (ICMs), miniature drug infusion pumps, and miniature peripheral nerve stimulators. To enable remote placement, the new MIMDs usually require device volumes less than about 5cc. Enabling Technologies: The latest MEMS, electronics packaging, passive devices, and battery technologies are the primary enablers for most of the MIMD miniaturization. MEMS devices, such as accelerometers and pressure sensors, are very small electro- mechanical devices built to scales comparable to ICs. Chip scale package (CSP) technology enables IC Packages that are virtually the same size as the ICs contained within them, and stacked chip scale package (SCSP) technology enables the placement of multiple ICs in a single package. Integrated Passive Device (IPD) technologies enable the integration of multiple inductors, capacitors, and resistors that are impractical for traditional IC integration. Finally, chip-scale Solid State Batteries (SSBs) are available in sizes about 10 times smaller than traditional IMD batteries. Custom Electronics To maximize the benefits of these enabling technologies, the electronic circuits in MIMDs must be designed specifically to their unique characteristics and requirements. For example, a MEMS sensor is likely to require a specific supply voltage, provide a specific range of output voltages, and require specific signal filtering and processing. Customized circuits contained in Application Specific Integrated Circuits (ASICs) are often required to manage these diverging requirements without wasting size or power. Power Reduction Power reduction is one of the major challenges for any IMD development because the power consumption of the implanted devices is directly related to the size of the battery required, and the battery is often the largest component in the device. MIMD size reduction almost always requires significant power reduction. Design Opportunities The extreme size reduction required by MIMDs presents many design challenges. One of the keys to a successful design is to identify opportunities and use them to our advantage. These opportunities arise from specific conditions and characteristics unique to many MIMD applications. For example, while most industrial ICs require an operating range from -40C to 85C, the critical operating temperature of implantable devices is usually limited to a small range around body temperature (37C). Signal frequencies required for MIMD sensors and drivers are typically less than 200Hz, which can be supported by system clocks of 100KHz or less, while many industrial applications require system clocks of 10MHz or more. Biological systems typically do not require extreme precision either. For example, many cardiac pacemakers and neuro-stimulators can make use of digital to analog converters (DACs) with only 8 bits of precision. In contrast, audio systems routinely require 16-bit DACs. MIMD applications present a variety of other opportunities for system designers to exploit, such as Non- Volatile Memory (NVM), wireless battery recharge, and wireless communication. The key to a successful MIMD development is to understand what opportunities are available, and exploit those opportunities to help meet the challenges that the development presents. Conclusions The dramatic miniaturization required to make practical MIMDs presents a wide range of design challenges, but innovative technologies are available to enable that miniaturization. The most effective use of the enabling technologies typically requires a level of integration and customization only achievable with Custom IC design. The requirements of MIMDs vary greatly depending on the application, and there is no single approach or solution that will work for all cases. However, this paper presents several ideas to capitalize on enabling technologies and application-specific design opportunities - to produce practical solutions for a small MIMDs with volume of less than 5cc.
Andrew Kelly, IC / Systems Architect
Cactus Semiconductor
Chandler, Arizona
United States

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