In the world of medical science, mobile circuits are quickly evolving into less cumbersome devices. They are providing more information and using less power. At the same time, they're improving quality of life. Although this evolution is only beginning, it's already resulting in smaller process capabilities, longer-lasting batteries, and advancements in medical understanding and circuit designs. Currently, electronic devices monitor blood glucose levels, blood pressure, and many other medical conditions or concerns (FIG. 1). The future isn't just in monitoring medical conditions, however. It's in controlling them.

Thanks to circuitry advancements, many medical devices are becoming mobile, fully implantable, and unnoticeable to the user and the outside world. Clearly, these developments will eventually lead to long-term implantable circuits. Before these devices can be born, however, many challenges and tradeoffs need to be resolved.

First, one must determine the specific needs that will be satisfied by the device. To control most medical conditions, it's vital to have timely information. Real-time data enables a device to quickly adapt to a diet, activity level, or medication. A situation can then be controlled before it becomes critical or life threatening. To provide this kind of information, however, a device must be mobile, easily accessible, and capable of constant monitoring. It also has to be self-adaptable and extremely reliable.

Many of these requirements sound easily achievable. It's only when they're combined that the trouble starts. For instance, envision a device that's mobile and continuously monitoring. Clearly, battery life will be a consideration. Now, add easy accessibility to the picture. Obviously, the device will have to be small in size. Luckily, this combination supports mobility. But it contradicts battery life and memory requirements for self-adaptation. Now, factor in that it must be very accurate and cannot be altered by external forces (heat, light, water, sweat, being bumped or other movements, etc.). The implications of protecting the device simply become impractical.

Think about how the human body works, how it adapts to changes, and how the internal organs are protected. The only conclusion is that the ideal device must be wireless and implantable. Of course, it will put severe constraints on battery life, size, adaptability, and data-transfer capabilities. Even more importantly, a medical procedure will be required to install such a device.

Yet a wireless implantable medical device offers one major advantage: the potential to improve a person's quality of life. Ideally, an implantable device should place minimal restrictions on the physical activities in which a person can participate. The goal is to enable the individual to monitor/control a medical condition without worrying or feeling restricted by the medical device.

For many individuals, such devices could provide real-time information combined with corrections to a given medical circumstance. Subsequently, the medical condition could become a non-issue in everyday activities. For this vision to become a reality, however, a number of medical and design issues must be overcome.

Initially, the success of a wireless implantable device will be based on the installation procedure. It must be low risk. Due to the limitations of sensor or battery life, it also should be able to be done multiple times. These requirements put a great deal of pressure on the medical, electrical, and sensor communities. They must determine how to obtain critical medical information from the body. In addition, these communities need to figure out how to control a medical condition without sacrificing vital information or placing too much risk on the patient during installation.

Sensor and circuit designers face severe limitations when trying to control a device's size. Size plays an important role in determining how and where a medical device can be installed. It also factors into the device's overall cost. Among other cost variables are the medical risk with installation and the device's lifetime. For example, being smaller may make a device easier to install. But a smaller device will usually cost more while limiting battery or device life.

Next to installation, the success of wireless-implantable devices rests on adaptability. These devices must change with the medical condition or the body. Over time, the way that the sensing device interacts with the body will be modified. In addition, the device must adapt to activities like exercise, being physically bumped, or shifting within the body.

Generally, controlling a medical condition requires different algorithms or coefficients for every individual. It may even require additional/other information from the body. If the devices aren't designed with adjustability or adaptability in mind, the advantages of wireless two-way communication can be lost.

The final factor in the success of wireless implantable devices is the length of time that they can remain in the body. "The longer the better" is a good rule of thumb here. Initially, the body will require time to adjust to the implant. The size of the device and the invasiveness of the installation procedure are just some of the factors that determine the body's adjustment time. If the battery or sensor life is too short, the actual sensing time—which takes place after the body has adjusted—may be limited.