In a previous post, The Enabling Chip Technologies Behind Miniature Implantable Medical Devices, we discussed miniaturization technologies necessary for enabling miniature implantable medical devices (MIMDs) that are gaining fast and widespread market acceptance and use. These devices have truly ushered in a new era of treatment capabilities. The proper combination of these technologies can produce compelling design opportunities.
Enabling chip technologies include chip-scale packaging (CSP), solid-state batteries (SSBs), micro-electromechanical systems (MEMS), and application-specific integrated circuits (ASICs). By taking full advantage of these technologies, new MIMD designs can enjoy significant power and size reduction benefits. They can be well-suited to fitting specific requirements of medical applications.
Medical design opportunity considerations
Military applications can have significant temperature ranges from -55° to 125° Celsius. Industrial applications can range from -40° to 85° Celsius, while commercial applications can range from 0° to 70° Celsius. However, medical applications require far more specific temperature ranges. Pre-implant temperature ranges are from 10° to 50° Celsius, and post-implant temperature ranges can be from just 35° to 40° Celsius. The significantly more narrow temperature range in medical implantable applications can greatly reduce circuit design complexity, leading to both system power and size reductions.
In addition, designers must consider the specific frequency requirements of MIMDs. For example, the bandwidth for EEG/ECG is expected to be ~200Hz. Meanwhile, blood pressure monitors typically operate at ~100Hz. And, accelerometers operate around 1 kHz, as do stimulation therapy devices. Most off-the-shelf commercial components are designed to operate at much higher frequencies and thus consume more power. Designing specifically for the lower-frequency operation of most implantable medical devices can result in significant power savings.
Other MIMD specifications include moderate precision requirements. For example, the DAC amplitude of a stimulator needs to run at ~8 bits, while the resolution of an ECG/EEG ADC requires ~12 to 16 bits. Pressure sensor ADC resolutions are often ~10 bits, and the ADC resolution of accelerometers is commonly ~8 bits. The moderate precision requirements, coupled with the lower frequency or sampling rates mentioned in the preceding paragraph, allow for smaller and lower-power data converter designs.
Proper implementation of non-volatile memory (NVM) is also important. It’s often included in most microcontroller units (MCUs), and, by definition, non-volatile memory retains the memory content when power is removed. Therefore, the MCU’s non-volatile memory can be used to calibrate analog circuits. An inline calibration step during production can help reduce analog performance requirements, simplifying design and reducing power.
Of course, battery recharging is arguably as critical as anything else — in fact, it’s a required design-in for most MIMDs. The most efficient designs will use a recharge session for communication with the MIMD and to also calibrate circuits as necessary.
An MIMD IC-based platform
Leveraging today’s technologies and taking advantage of the above medical design opportunities can lead us to the following IC-based MIMD platform.
The platform will require a wireless link that can be used for both battery recharging and bi-directional communications with the implantable device. A rechargeable solid state battery serves as a space-efficient source of power for the implantable device. Supply monitors and switches provide low battery alerts and control the flow of current among the power interface, solid state battery, and ASIC circuitry. Ultra-low-power timekeeping is extremely important, since this is typically the one circuit that is always on. Control/calibration registers are loaded via a serial peripheral interface (SPI) from the MCU.
The “application-specific circuitry,” along with the sensors/transducers/electrodes, shown in dotted outlines in the below diagram, provides platform customization. The content of these customizable blocks will be based on the targeted medical application and the customer’s unique design/therapy requirements.
By taking advantage of the unique medical implantable requirements, the above IC-based platform seeks to minimize both size and power of the implanted device. An implantable device based on the above platform can achieve a desired volume of less than 1 cc. Thus these devices can be implanted at the point of therapy, eliminating the need for long leads.
Designers must continually capitalize on enabling technologies to turn design opportunities into realities. Obviously, the design of the IC platform is at the heart of this realization for implantable medical devices. Designers must also understand that a platform approach is the best way to reduce time, cost, and risk. Meeting these design challenges is critical to seizing design opportunities that keep you relevant in a $50+ billion implantable medical devices market.
— James McDonald, Cactus Semiconductor President, co-authored this article .