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Overcome challenges in implant design

Posted: 01 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:in-body communications design? design challenges of in-body communications? medical device design? medical device design challenges? Ake Sivard?

Medical device designers are slowly putting science-fiction writers out of work. Just years ago, the concept of in-body communications networks would have strictly been the domain of Star Trek fans. Today, thanks to advanced ultralow-power RF capabilities, your implanted pacemaker may be making a wireless phone call to the doctor's office to report your latest health data.

Almost every aspect of a patient's health can now be monitored or regulated by an implanted device. These devices pose unique power, signal processing and communication challenges for designers.

Cost concerns underscore the need for integration, as component prices in the implanted world differ vastly from those in the commercial world. A crystal normally costing $0.25 can have a price of $10 if it's destined for a pacemaker. A reason for this price difference is that many component companies refuse to supply devices for implantable applications for fear of being sued in the case of failure, resulting in less competition. Moreover, implantable components must pass more testing, qualification and documentation than their industrial equivalents, adding further costs for the end-user.

Medical devices can be categorized into the following: those that use an internal non-rechargeable battery (e.g. pacemakers) and those that couple power inductively (e.g. cochlear implants). The first type uses a duty-cycling OS to conserve power. The transceiver is "off" most of the time, meaning the off-state current and the current required to periodically look for a communicating device must be extremely low (less than 1A). In both cases, low power (less than 6?A) for transmit and receive is also required.

Strike a balance
Power and space-saving goals have a significant impact on every stage of radio design. Designers must always consider the modulation scheme to be used, the required BER and how it can be achieved, interference scenarios to be considered, and the required operating range and data transfer speed. The designer must strike a balance of compromises and trade-offs to meet the performance requirements for in-body communications.

In general, constant envelope modulation schemes offer better power-consumption regimes than higher-level modulations that require a higher SNR in the receiver. Similarly, the use of lower data rates and narrower occupied bandwidths may initially seem viable. However, for minimum overall power consumption defined in terms of Joules/bit, it's recommended that implantable transceivers use the highest possible data rate that satisfies the sensitivity requirements of the application receiver.

Systems that require low data rates (even in the low kilohertz range) should buffer data, operate at the highest data rate possible and exploit duty cycling of the power states to reduce average current consumption. Sending data in short bursts not only conserves power, but also reduces the potential time window for interference and provides more forgiving power-supply decoupling requirements. The last point is important for implant systems, which frequently use batteries with high impedance.

This approach also uses a very high data rate suitable for intermittent telemetry applications, as a large capacitor can have its charge mortgaged for the period of the radio transmission, and then recharged at a lower rate. Another fact that points in favor of a high data rate is that the transmission will occur during a shorter time period, making it possible for more users to share the same radio channel.

Many aspects of health can now be monitored or regulated by an implanted device.

A useful rule of thumb is that the radio channel in these short-range applicationswith all its imperfections and possible interferencescannot be relied upon to provide a BER of better than 1 in 10-3. This means that to provide the BERs required by the application, some form of error correction mechanism is required. The automatic repeat request's disadvantage is considerably slowing the data-transfer rate under poor channel conditions. The relatively small overhead associated with using a Reed Solomon FEC scheme becomes attractive where large amounts of data must be transferred.

Even with these considerations, the receiver architecture remains a challenge. Direct-conversion receivers, long considered the holy grail of receiver design, pose serious disadvantages, especially in terms of DC offset caused by local oscillator leakage. Additionally, the sensitivity can be compromised by phase noise to a greater extent than the superheterodyne approach. However, space considerations push the superheterodyne toward the use of an integrated IF filter and power consumption toward a relatively low IF. Where power availability allows, the use of an image suppression mixer has advantages. The effects of interferers on the implant are helped by the poor antenna efficiency, but any attempt to meet the usual 3V per meter immunity test at more than 15 percent off-tune represents a definite challenge.

Radio design for implants poses many challenges for the engineerfrom power consumption and standards to antenna performance. No ideal solution exists, and the best approach is inevitably a balancing act to meet many requirements. As a general guideline, radio-system design for implantable devices should be as simple as possible, and every microampere of current used must "count."

In-body material
Biocompatibility is an issue for any in-body device, as the implant itself and antenna must be non-toxic and passive to body liquids. Titanium is the ideal material for the implanted device, as it's very compatible with the human body. Any part of the titanium exposed to body fluids or air quickly oxidizes to form a thin layer of non-reactive titanium oxide. For further precaution, a passive coating is usually applied to the implant.

Effective RF performance with a small, low-power antenna requires the use of very-low-resistivity metal, preferably copper, silver or gold. However, biocompatibility restricts the choice to platinum or platinum iridiumboth have relatively high resistivity. There must also be no DC potential between any external metals on the implanted device.

While plenty of published material exists on RF transmission through free space or air, little has been written on transmission through a medium such as the body. The human body is not an ideal medium for transmitting an RF wave, and changes occur as the body ages and the posture changes. Each part of the body has a different dielectric constant and conductivity.

This bidirectional half-duplex design uses an architecture that minimizes current consumption.

While the high r reduces the wavelength within a body, designers have to compensate for gains and losses when transmitting an RF wave through a human. One key thing to bear in mind is that tuning components must operate over a much larger range than for free air application. Unlike in air, it's not possible to set up an antenna and implant for optimum performance outside the body. The antenna tuning must be done frequently within the body, so the use of automatic tuning is a must, if not before every communication session, at least on a regular basis.

- Ake Sivard
Manager, Product Line Marketing

??Peter Bradley
Senior ASIC Design Engineer

??Peter Chadwick
Senior Radio Systems Consultant

??Peter Chadwick
Senior Radio Systems Consultant

??Henry Higgins
Microelectronics Division

??Zarlink Semiconductor Inc.

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