[Editorís note: Co-authors are from Cactus Semiconductor Inc. -- Andy Kelly, System and IC Architect, and James McDonald, President.]
Implantable medical devices (IMDs) have an approximate size of ~15 to 50 cc. Such devices are common for use in the chest or abdomen. They can be implanted there using long leads or catheters. IMDs often require invasive surgery to implant. Such IMDs include pacemakers, defibrillators, spinal cord stimulators, drug infusion pumps, and more. These devices are widely popular. However, today miniature implantable medical devices (MIMDs) are capturing much more attention as they continue to usher in a new era of treatment capabilities. The enabling chip technologies behind MIMDs will be critical to their continued adoption.
Todayís MIMDs consist of <4 cc volumes and are applied to areas of the body that include the head, neck, and limbs. They can often be implanted with minimally invasive surgery and sometimes even without surgery at all. A common non-surgical implant method is to use small leads or catheters. MIMDs include ECG/EEG monitors, peripheral nerve stimulators, micro infusion pumps, and other similar devices. A more specific example of an MIMD would be a nerve stimulator with a volume of <1 cc. This device would be small enough to implant at the point of therapy with minimally invasive surgery. With its small footprint compared to an IMD, it would not need to be implanted in the chest nor require long leads routed to the neck. Naturally, there are a lot of miniaturization technologies necessary to make it all happen. So, what are some of these MIMD enabling technologies?
Chip-scale packaging (CSP) is critical. The use of DIP, SOIC, and QFP packaging common with IMDs is replaced by QFN or WLP packaging in MIMDs. As a comparison, these new packages are 250 times smaller than, for example, a DIP package. In addition, stacked chip-scale packaging (SCSP) allows for multiple chips in one package. Itís possible to have stacked on a substrate an application-specific IC (ASIC), radio frequency IC (RFIC), microcontroller IC (MCU), and non-volatile memory (NVM) IC. This can provide complete system functionality in a single package. Packaging advances will continue to play a key role in enabling more and more MIMD applications.
Of course power capabilities are essential, too. With MIMDs, solid-state batteries (SSBs) are critical and have compelling key characteristics for their adoption. They are fabricated on silicon wafers with standard semiconductor processes and equipment. So thereís no big retooling required at any level. They also function much like rechargeable lithium-ion batteries but at a fraction of the size. These commonalities are important to keep design risks and cost low. As SSB capacities (uA-h) increase, application options for these batteries will continue to rise.
Micro-electromechanical systems (MEMS) are also key enablers. They include microscopic sensors, actuators, and machines manufactured with IC processes. Some examples include: pressure sensors for blood pressure and respiration, accelerometers for tracking position and activity, chemical sensors to track such things as glucose or pH, and fluid pumps for drug delivery. The features brought by MEMS are essential to many key MIMD applications.
Last but not least, ASIC chips provide very important features and capabilities. Itís easily arguable that most MIMD applications might be unattainable without the use of an ASIC. With a properly designed ASIC, one can eliminate unnecessary features and functions that wouldnít be possible with off-the-shelf ICs. ASICs allow the best opportunity for optimization of performance for single applications. For example, ASICs allow best-in-class optimization of size and power for MEMS interfaces. Total power consumption is also best optimized using an ASIC. These optimizations can easily lead to reduced battery sizes and even enable the use of solid state batteries previously discussed. The synergistic impact of these enabling technologies are obviously important when developing an MIMD.
One research firm recently had the IMD market pegged at just a bit more than $43 billion in 2011 and reports it will grow to nearly $74 billion by 2018. And this is just the US market. As a component of this market, MIMDs are poised for more adoption. They are achievable by capitalizing on enabling technologies that provide design opportunities that address specific application needs. The companies that can leverage technology enablers to such an end will stand apart as clear successes.