Next-gen semiconductors enable medical innovation

Biomedical research is making huge strides in unlocking the secrets of human physiology and identifying potential new therapeutic and diagnostic instruments. At the same time, advances in electronics are enabling those new devices to be realized. The result is that medical applications make up one of the fastest-growing segments for semiconductors. The growing convergence of electronics and medicine can also be seen in trends common to both disciplines. While it may seem that the two fields have little in common, they actually share several technological frontiers: Both are driven by the need for small physical size, low power and advanced connectivity.

The relentless drive toward miniaturization in electronics is as old as the transistor radio. The frequently quoted Moore's Law, which expresses the rate of increase in the maximum number of devices that can be fabricated on a single chip, is really a measure of the size of the smallest device that can be manufactured.

As in other fields, some medical devices require the maximum possible processing power. In particular, radiological and magnetic imaging systems consume all the gigaflops they can get, and size is generally not the main concern. However, in lots of other applications the computing requirements are more modest, and there isn't room for large components. The enabling factor in these devices is getting just enough processing power into a given space.

The space allowed for a medical device can be subject to some interesting constraints. For example, most people are familiar with the glucose meters that diabetics use to monitor their blood sugar. Typically, these portable electronic medical devices are about the size and shape of a PDA. The user puts a drop of blood onto a test strip and inserts it into the meter, which uses an electrochemical or optical sensor to determine the concentration of glucose

in the blood. One company has created a meter small enough to be integrated into the cap of the pill bottle-size canister that holds the test strips. The sensor, microcontroller, LCD and battery all fit into a finished product roughly the size of a ladies' wristwatch. Small size has also yielded low cost, as the meter is disposable--after the 50 strips are used, the container is thrown away, meter and all.

In the case of implanted devices, size is also important in determining where they can be placed. Consider an aortic aneurysm, which occurs when the heart's main artery, the aorta, develops a weak spot and bulges from blood pressure. A common surgical treatment is to run an artificial liner, called a stent graft, through the weakened aorta. It's now possible to place a tiny pressure sensor inside the aorta at the same time. The sensor uses a microelectromechanical-systems pressure element to monitor the long-term success of the surgery.

The MEMS sensor is made using the same processing technologies as integrated circuits. To read the pressure in the aorta, the surgeon uses a radio-controlled interrogator to activate the device, which transmits via RF. If the stent graft fails, a follow-up exam will show increased pressure, indicating the need for further intervention. Some other implanted devices that benefit from smaller size are cardiac pacemakers, neurostimulators for treating central nervous system disorders (ranging from chronic pain to Parkinson's disease) and hearing aids, including cochlear implants.

One of the most fascinating applications to exploit the mutual shrinking of medical devices and electronics is the sensor that's designed to be swallowed, like a pill. Using RF, these minute instruments travel the full length of the digestive system during their operating lifetimes. The first such sensor transmitted body temperature, and has been used by astronauts and athletes. Newer versions can report pH levels in the esophagus, to diagnose acid reflux disease and other conditions.

The very latest devices transmit still images, which can be assembled into video--allowing a doctor to examine the small intestine (otherwise inaccessible without surgery). The "camera pills" use a minute CMOS imager coupled to an ASIC transmitter. White LEDs surrounding the lens provide illumination. Early work is being done on the next generation of ingestible devices, incorporating navigation control and self-propulsion to allow more detailed imaging of a particular site.

Hand in hand with miniaturization is the trend toward lower power. In an implanted device, the benefit of minimal current consumption and the resulting impact on battery life are obvious. While some implants can be recharged through the skin with inductive coupling, less battery drain is always a desirable objective.

External devices are also sensitive to power consumption. As electronic components get smaller, medical devices that were once stationary are becoming portable. Ultrasonic imagers used to be "transportable" only in the sense that they were usually mounted on carts that could be rolled from room to room. Now, they are becoming available in a form factor very similar to a laptop.

Defibrillators were once used only in hospitals by trained professionals. Automatic external defibrillators are now commonplace in airports, shopping malls, schools and even on airliners. Portable oxygen concentrators extract oxygen from the air for patients on oxygen therapy, and can be carried over the shoulder like a handbag. All of these devices are enabled by low power, in addition to small size. Their tight power budgets are met by a variety of means.

Ironically, reducing the size of electronic components can actually work against power reduction. As transistors get smaller to allow greater density, effective channel lengths get shorter and leakage current increases. Other mechanisms, such as gate tunneling, have a similar adverse impact on power as geometries shrink. With each geometric reduction, chip makers have countered these negative effects by optimizing silicon-processing parameters--an effort that is expected to continue. In addition, the designers of both digital and analog chips spend a large portion of their design activity minimizing the power consumption of their circuits. Reducing supply voltage, managing capacitance, clock gating and other techniques are used to eliminate unnecessary current.

Chip designers also incorporate features into their parts to allow medical-device designers control over power consumption. For example, the dsPIC33F family from Microchip Technology has idle, sleep and doze modes, each with multiple options, giving designers the flexibility to scale power consumption. In many medical devices, a microcontroller spends most of its time doing nothing.

The inputs to vital-sign monitors, infusion pumps, data recorders and many diagnostic instruments are fairly slow-moving temperatures, pressures and bioelectrical signals. Processors in such devices can remain in a low-power state most of the time, waking up every few milliseconds to execute instructions. In this way, the total average current is a fraction of the processor's normal run current.

Further progress in the area of power conservation will enable the development of new classes of devices. Piezoelectric or thermoelectric power sources may someday replace batteries in some implants. Already, a microsensor has been built into a hip implant to monitor the integration of the implant into the healing bone tissue. The device is kinetically powered, using the patient's movement as its energy source.

The third trend shared by electronic and medical devices is connectivity. In both fields, wireless is the leading technology. In 1999, the FCC allotted bandwidth from 402 to 405 MHz to the Medical Implant Communication Service. This band is used to communicate with implanted devices. For example, a pacemaker's operation can be monitored by a basestation near the patient's bed. On a regular basis, the basestation transmits recorded data to the doctor's office for analysis.

Other external devices use Bluetooth, infrared, the ZigBee protocol, Wi-Fi or proprietary protocols to communicate. Home health care networks connect weight scales, blood pressure cuffs, thermometers, spirometers and other diagnostic instruments to telemedicine terminals. These networks allow effective disease-management care, without frequent trips to a doctor's office.

Of course, not all medical networks are wireless. Some devices are complex enough to make use of their own internal networks. For example, a dialysis machine may contain a dozen or more microcontrollers. A controller-area network bus connects a master device with several other devices that control multiple peristaltic pumps throughout the machine. Other nodes on the bus monitor pressures or control flow valves.

Wired and wireless LANs are getting busier in hospitals, as well. Electronic patient records, prescription ordering and delivery, and imaging data can all be made available online or at the patient's bedside.

With innovation in electronics and medicine aligning around miniaturization, low power and connectivity, the development of new electronic medical devices is expected to accelerate. At the same time, it's appropriate to be mindful of the challenges presented by differences between the two fields. The rate of change in semiconductors is far more rapid than in medical devices. Component suppliers naturally prefer to offer their newest products, built on their newest processes. On the other hand, the designer of a medical device typically prefers to design in a component after it has been on the market for a while and established a track record. Similarly, by the time a medical device has passed FDA review and been released to production, the manufacturer is generally reluctant to accept changes to components.

In the end, an ongoing convergence of medical and electronics technology is inevitable. Biomedical research is continuing to identify new treatments for disease. Electronics research is likely to continue to enable the development of devices to apply those treatments. Greater cooperation between engineers in the two fields will advance health care in ways barely imaginable today. n

Steve Kennelly manages the medical products group at Microchip Technology Inc. (Chandler, Ariz.).

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