Ultralow-power radio-frequency technologies are spurring the development of innovative medical tools, from "camera capsules" that can be swallowed to implanted devices that wirelessly transmit patient data.
Traditional medical monitoring equipment has been dominated by standard components, such as microcontrollers, analog-to-digital and digital-to-analog converters and memories. For example, since most diabetic patients now own a portable monitor, the blood glucometer market has been a high-volume opportunity for microcontrollers and ASICs.
However, medical monitoring machines such as electrocardiograms and ultrasounds are produced in quantities that typically do not exceed 200,000 units per year. This limited commercial potential has represented a significant hurdle for IC manufacturers targeting the medical market.
This is changing today. Our aging population is driving wide-scale demand for more advanced healthcare treatments, including wireless implant devices delivering ongoing and cost-effective monitoring of a patient's condition.
One recent example of the marriage of ultralow-power and wireless expertise for medical applications is the launch of Given Imaging's wireless endoscopy M2A camera capsule.
Traditional endoscopy requires a hospital stay, with a specialist inserting a long, flexible optical cable down the throat of a patient. In comparison, Given Imaging's disposable camera capsule is easily swallowed and travels naturally through the gastrointestinal tract while transmitting two high-definition, full-color images per second to a belt receiver worn by the patient. Images are downloaded from the receiver to a workstation, and a video of the camera's journey is produced that potentially reveals pathologies and diseases of the previously unobservable small intestine.
The capsule consists of a microchip camera, light-emitting diodes that act as a flash, an RF transmitter chip, two silver-oxide batteries and other components, including an antenna. The key challenge for designers was to ensure the imager and RF chip could operate over the course of an eight-hour examination. Design techniques refined in pacemakers were merged with RF expertise, resulting in a design that does not compromise power or performance.
Evolving from the swallowable camera capsule's one-way wireless link, the next technology leap is implanted medical devices, such as pacemakers and defibrillators, that offer better device management via two-way communication.
A 402- to 405-MHz medical implant communications service (MICS) frequency band has been allocated for implanted device communications. Using MICS, a healthcare provider can establish a high-speed, short-range wireless link between an implanted device and monitoring systems to receive patient health and device operating data. The two-way RF link means the doctor can wirelessly adjust the performance of the implanted device, allowing more proactive patient care while avoiding surgery.
In-body communication systems pose very unique challenges for IC designers. Implanted devices typically operate for seven to 10 years, meaning that at every stage of design IC makers must pay strict attention to meeting long-term low-power performance demands. The human body is also not an ideal medium for transmitting a RF wave, with materials such as fat and muscle exhibiting varying resistance to electrical signals.
Another potential hurdle is the placement of the implanted device. A physician will locate the implant where it provides the best patient care and comfort, with little concern for RF propagation. Therefore, the transceiver must be designed to operate effectively from various depths and through unpredictable thicknesses of fat, muscle and skin.
Biocompatibility is another issue for implanted transceivers. Low-resistivity metals that deliver superior RF performance are not necessarily the best choice for an in-body device.
With in-body sensors, today's reactive healthcare approach will be transformed into a more proactive system. In-body transceivers are currently under development, some with higher data rates (500 kbits/second) but a shorter operating range, of 2 to 4 meters. Others are targeting longer-range operation (up to 10 meters) but with much lower data rates, in the range of 10 to 50 kbits/s. All systems must transmit and receive at current well below 10 milliamps at 3 volts.
For RF IC design, optimizing the architecture for a given application is a key requirement when considering ultralow-power consumption. For example, using direct conversion with a high modulation index for low-data-rate applications can reduce power. Where design freedom exists, choosing a modulation scheme that allows constant envelope techniques rather than high-level modulation can reduce power.
Using digital signal processing within complete receiver or transmitter sections can also yield power efficiency benefits, even when considering data conversion requirements. Such circuit techniques as switching power amplifiers, rather than linear or semilinear variants, can also be beneficial for low power consumption.
Traditional RF design alone will not meet the low-power requirements of the medical industry, however. Every aspect of the design must be optimized to reduce power. When a part of the circuit is not required, it must be powered down and/or the clocks gated. At the transistor level, devices should be operated in the optimum inversion regime to ensure the best possible saturation voltage, matching, transconductance and noise trade-off.
Thermal noise vs. flicker noise can also be traded. Lower flicker noise (large-area transistors) can allow higher thermal noise, thereby allowing lower device current. For digital circuits, transistor sizing can yield very high-power savings gains. The ratio of parasitic capacitance to gate input capacitance should be determined so minimum-size cells can be used where possible and drivers are sized only as appropriate between blocks and for off-chip signals. Clock tree buffering can eat up power unless carefully controlled. Again, in digital implementations, gate and cell mapping can be used to reduce switching activity and the probability of switching.
Implementing a deep-sleep mode is crucial for meeting desired power performance. Many medical monitoring devices will have a reasonably low duty cycle, and the RF transmitter must "turn off" and "wake up" on demand. Off-the-shelf transceivers with a sleep mode of 1 mA have been available for many years, but most medical devices will require sleep modes well below 500 nanoamps.
With camera capsules, implanted devices and other disposable sensors requiring an extremely small footprint at a reasonable cost, designers must strive for a high level of integration for key components. New deep-submicron CMOS processes offered by foundry suppliers often include RF extensions. Key components such as inductors are precharacterized and implemented on-chip, which saves cost and space.
Today, full transceivers with fewer than 10 external components have been successfully developed for key medical applications. Semiconductor makers have also taken advantage of new chip assembly technologies such as chip-scale packaging that are providing a die-size-level packaging solution with bumps that are compatible with ball grid array assembly equipment. The end result is a much smaller footprint than conventional packages such as dual-in-line packages and BGAs, but with easier assembly than flip-chip bumped dice.
Current endoscopy techniques do not use RF technology, and they are expensive, cause patient discomfort and offer limited examination results. Emerging RF technologies are allowing the development of entirely new approaches. The wireless camera capsule is not simply about removing a cable but about creating a new device that provides a more comprehensive examination, more information for a doctor to analyze and increased comfort for patients.
In the near future, we will increasingly see the marriage of ultralow-power RF and medical devices implantables with telemetry, fitness monitors on watches, hearing aids with wireless capabilities.
Indeed, the ultralow-power medical wireless revolution is happening right now.
Francois Pelletier (firstname.lastname@example.org) is product line director at Zarlink Semiconductor Inc. (Ottawa).
See related graphic |
sends wireless images to a belt worn by the patient, whose status is monitored via an implantable device.
Source: Analog Devices Inc.