Electronic Design

Implantables Will Challenge Mixed-Signal Designs

As more biomedical equipment devices exploit ultra-low-power mixed-signal design innovations, the variety of medical semiconductor applications continues to expand. The ability to combine analog and digital signals in very close proximity alongside wireless communications has led to the development of new implantable medical devices.

Implantable devices have a number of fundamental design requirements. They must feature a long and relatively maintenance-free life due to the cost and complications of explantation and the serious implications of malfunctions or failure. In addition, they must have ultra-low power consumption, and the battery life must be as long as possible (typically more than 10 years for some devices).

Adaptability is another important consideration. Controlling a medical condition may require different therapeutic modes for each patient, and inputs from a growing number of sensors. Size also is critical. The methods used by clinicians to program and retrieve patient data from an implanted device impact the treatment decisions and patient convenience. Such functionality means more use of low-power, low-data-rate wireless communications.

In 1999, the U.S. Federal Communications Commission (FCC) set aside a frequency band between 402 and 405 MHz specifically for wireless data communications between implanted medical devices and external equipment. Known as the medical implant communications service (MICS), the wireless link promises to replace current magnetic-inductive-coupling techniques with a faster data-transfer rate and a longer link range.

The MICS band also is compatible with international frequency bands for implantable devices. The European Telecommunications Standards Institute (ETSI) recently standardized the frequencies and electromagnetic compliance requirements of ultra-low-power active medical implants (ULP-AMIs). The common standards eventually will allow patients with implantable devices to obtain care in both Europe and the U.S.

The significance of the market for portable and implantable medical devices is reflected in the fact that the worldwide cardiac-rhythm-management (CRM) market (i.e., pacemakers, defibrillators, and congestive heart failure devices) currently tallies in at $11 billion annually. Some segments are growing about 20% per year.

The current range of implanted devices includes glucose meters and continuous glucose monitors, insulin pumps, blood-pressure monitors, and various neuro-stimulators used to treat chronic pain, epilepsy, spasticity, and tremors. A number of companies are developing other neuro-stimulation devices to treat obesity, migraines, incontinence, depression, and hypertension. The true benefit of a MICS-enabled device becomes clear in the automated control and external monitoring of a physiological parameter.

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