From Fiction to Fact – Ultra Low-Power Design for Medical Devices
By Sahil Bansal, Zarlink Semiconductor
One of the recent announcements from network executives introducing new fall television viewing was the return of The Bionic Woman http://www.nbc.com/fall_preview/bionic_woman/. For readers old enough to remember, the original Bionic Woman http://imdc.com/title/tt0073965/ was “classic” 1970s television. Injured in a freak skydiving accident, Jaime Sommers is rebuilt with bionic legs and arm, exceptional hearing and more to give her superhuman capabilities.
Sahil Bansal
By Sahil Bansal, Zarlink Semiconductor
One of the recent announcements from network executives introducing new fall television viewing was the return of The Bionic Woman. For readers old enough to remember, the original Bionic Woman was “classic” 1970s television. Injured in a freak skydiving accident, Jaime Sommers is rebuilt with bionic legs and arm, exceptional hearing and more to give her superhuman capabilities.
Jaime dies and is brought back to life, becomes a secretive operative and fembot-fighting tennis pro. Along the way she becomes a possible dream date for a future engineer, though a few years later the Dukes of Hazzard appear with a cool red car and Daisy Duke in tow.
When this TV show first aired, a lot of these “bionic” capabilities were for the most part the work of creative, possibly very forward-thinking, television writers. Today, however, with The Bionic Woman set to return to the small screen, many of these once fictional technologies are a reality.
A stroke patient can use a network of on-body sensors and in-body implants to stimulate nerves to promote limb movement. Noise reduction and signal processing techniques have dramatically improved hearing aid performance, and devices can be tuned to improve a particular hearing weakness. A pacemaker using a high-speed, wireless link transmits patient health and device performance data to an external monitoring device. Devices implanted into the brain can be used for a range of therapies – from helping to manage tremor for a Parkison’s patient, to possibly controlling addiction.
In particular, the combination of wireless technology and medical devices is set to change healthcare by supporting new levels of programmability and flexibility for each patient. Key to these new implanted devices and therapies is ultra low-power RF IC and MAC (media access controller) technology.
Despite the performance afforded by modern IC technology, electronic systems associated with implanted medical applications present formidable low-power design challenges. For example, most implanted pacemakers have lifetime requirements of greater than seven years with maximum current drains in the order of 10-20 uA. The communication systems are budgeted at total currents averaged over the device lifetime of no more than about 15% of the total power budget or 2-3 microA due to the current consumption demands of supporting pacing therapy.
To conserve power, receivers in implanted medical systems must operate in a very low power “sleep mode” and periodically “sniff” or monitor for an external communication device. To save power, the time between sniffs should be as long as possible, but this is typically limited to 1-10 seconds due to application considerations such as the need for delivering therapy.
Ultra low-power IC design is key – as you add incorporate wireless capability into an implanted medical device, it’s imperative that there’s minimal impact on the battery operating life. There are some unique challenges in designing such a device:
• Low power during communications is required. Implant battery power is limited and the impedance of implant batteries is relatively high. This limits peak currents that may be drained from the supply. During communication sessions, current should be limited to less than 6 mA for most implantable devices;
• Low power when asleep and periodically “sniffing” or looking for a wake-up signal;
•Minimum external component count and minimum physical size. An RF module for a pacemaker should not be more than 3x5x10 mm3 in order to fit within typical pacemaker cans. Furthermore, implant-grade components are expensive and high levels of integration may reduce costs;
• Reasonable data rates are demanded. Pacemaker applications are currently demanding >20 kbps with much higher data rates projected for the future;
•Typically greater than two-meter range since wireless implant telemetry bands (such as the MICS band) are designed to improve upon the very short-range inductive link. Longer ranges imply good sensitivity is needed since small antennas and body loss affect link budget and allowable range.
Low-power IC design was likely not part of the creative process as a backroom of Hollywood writers “designed” the Bionic Woman. It will be interesting to see what technologies today’s writers envision, and consider the possibility that what is fiction today could become a medical reality in 20 years.
Sahil Bansal is the product line marketing manager with Zarlink Semiconductor’s ultra low-power communications group. Zarlink recently introduced the ZL70101 MICS transceiver for medical telemetry applications. Learn more at: http://ulp.zarlink.com/.
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