Integrated communications from different in-body implants and on-body sensors will allow hearing for the deaf, sight for the blind, and mobility for the disabled. In-body communications will also improve therapy and diagnoses. For example, an implanted pacemaker will regularly transmit performance data and the patient's condition to a local clinic.
A key element of an RF-linked implant is the in-body antenna. It must meet biocompatible and size-limit requirements, and thus faces numerous RF challenges. Unlike free-air performance, the human body is often an unpredictable and hostile environment for a wireless signal.
OPERATING IN THE MICS BAND
The Federal Communications Commission (FCC) established the 402-405 MHz medical implant communications service (MICS) band for high-speed, short-range (up to two metres) wireless links between implanted devices and monitoring tools. MICS does not require a license, but is restricted to a maximum effective radiated power (ERP) of 25microwatts.
In applications such as pacemaker monitoring, the transceiver only operates for short, intermittent periods. When not in use, the transceiver can be put into "sleep mode," where it draws only a very tiny current. However, it will sense a "wake-up" signal generated by an external basestation.
The transceiver can also initiate communication by sending a message to a nearby base station. For a pacemaker, this could be useful for providing an early warning that battery life is low. Potentially, an implant could also transmit an emergency signal if it detects a health problem, such as cardiac arrest.
RF AND THE HUMAN BODY
The human body is not an ideal medium for transmitting an RF wave. It is partially conductive and consists of materials of different dielectric constants and characteristic impedance. This means that at the interface of two body materials, such as muscle and fat, the difference can cause a wave to be partly reflected rather than transmitted.
Values of dielectric constant and conductivity for some body tissue are given in Table 1, where it can be seen that there is frequency dependence. Signal penetration into body tissue is also important. A reduction of signal to 36% of the peak is an often-used reference. This is frequency-dependent, as shown in Table 2.
A potential RF performance hurdle is the placement of the implanted device. A surgeon will place an implant where it will be clinically effective, with little concern for RF propagation. Therefore, the antenna must operate effectively from various depths and through layers of fat, muscle, and skin with unpredictable thickness
The 25microwatts limit applies to the signal level outside of the body, which allows for implant power levels to be increased to compensate for body losses. Once the implant is in place, the RF link can be used to adjust the signal level to the highest allowable level.
The impedance radiated by the antenna will also vary as the patient moves or ages and the location of the implanted device shifts. An automatic tuning circuit and firmware routine that operates each time the transceiver is powered up can compensate for impedance change.
There are conflicting requirements when designing an in-body communications system. 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 choices to titanium, platinum, or platinum/iridium—all of which have relatively high resistivity.
One solution is to use gold for the antenna, and coat the implant in a passive material that is impervious to water and other body liquids, has low RF loss, low dielectric constant, and high resistivity. There are also device manufacturers that accept the RF drawbacks of platinum to ensure antenna biocompatibility.
BUILDING A PATCH ANTENNA
Figure 1 shows a patch antenna constructed by thick film printing onto alumina, while the hole was laser drilled and plated through. The patch was attached to the implant case with conductive epoxy. In this case, the connection to the RF circuitry was by a sealed hole. The whole assembly was coated in a passive polymer seal.
The resulting patch was characterised operating in a body phantom − a liquid that mimics the varying electrical performance of the human body − and was found to be capacitive at 403MHz with very low radiation resistance. The antenna was matched as closely as possible with discrete components. Fine-tuning is then done within the transceiver, and needs to be repeated at regular intervals to maintain optimum performance.
An in-body transceiver can be integrated with a range of implants for a variety of different uses, including pacemaker monitoring and Functional Electrical Stimulus (FES). These applications can use the same transceiver with a choice of antennas.
Using FES to replace lost limb function, for example paralysis as a result of a stroke, requires implant(s) to stimulate muscles or nerves in response to movement detected by sensors elsewhere on the body.
The patient wears a strap-on motion sensor that transmits data to a processor pack similar to a mobile phone, which is worn by the patient. The sensor transmits over a short distance (less than two metres) without the problems or limitations of in-body transmission. Since the device is not an implant, a frequency with a higher power level can be used, such as the Industrial Scientific Medical (ISM) bands, while the link to the in-body implant will use the MICS frequency.
Operating in real time, the processor receives signals from the sensors, processes the data, and transmits it to the implants to stimulate a muscle or nerve. The processor may have inputs from several sensors and transmit signals to several stimulus implants.
A patient using FES may have several RF-linked implants. To ensure the basestation is communicating with the correct device, each implant transceiver must have a unique identifier to prevent false wake-up or interrogation.
RF-linked implants can be used for diagnosis, therapy, or to restore lost function. Each application will need to be tailored to the position and frequency of use. A key component of an in-body communications system is the antenna. The antenna must be designed as part of the implant, and not as an add-on or afterthought, since this will lead to performance issues.
The use of RF technology in medical applications promises many benefits for patients, including better diagnoses and tailored therapy, fewer hospital visits, and peace of mind that implant performance is being monitoring. For healthcare providers, improved implant monitoring potentially lowers medical costs by extending the time between hospital visits and surgical procedures. We are only just beginning to realise the potential for RF communications in healthcare.