Current Design Trends in Medical Electronics

Electromedical manufacturers and component suppliers are taking steps to improve both performance and cost-effectiveness of increasingly miniaturized, high-precision, portable devices.

Fernando Lynch

One of the staple concepts of futuristic and science-fiction literature and films is the notion that the boundary between humans and machines is dissolving. These scenarios depict us advancing toward an age in which people are partly or mostly robots—or controlled by computers. Though it is unlikely that this will occur during our lifetimes, it is entirely possible that many of us may find ourselves with one or more sensitive electronic devices implanted into our bodies. In fact, products derived from nanotechnology and microelectromechanical systems (MEMS)—with machines fabricated at the millimeter or molecular level—have already become a reality.

For example, the idea of an electronic device stimulating the heart to beat was considered lunacy in 1930, when the first external pacemakers required ac power and penetration of the chest cavity with a probe. By 1958, battery-powered pacemakers were being implanted. Implantable pacers are now considered a mature product (approximately 150,000 annually are installed), with implantable cardioverter defibrillators (ICD) not far behind.

To date, more than one million people on the planet have been implanted with some kind of electronic device. Implantable electronics are now competing with or complementing pharmaceutical and other treatments for such ailments as brachycardia, tachycardia, Lou Gehrig's disease, Huntington's disease, Parkinson's disease, intractable and chronic pain, muscle spasticity, irregular breathing, urge incontinence, diabetes, and deafness. Implantable electronic products include drug pumps, monitors and delivery systems, cochlear implants, and neurostimulators. Experimentation is already under way on electronic retinal implants that could lead to at least a partial cure of blindness.

Although today the average age of a pacemaker recipient is 70, demographic statistics indicate that an enormous number of now-middle-aged baby boomers will be prime candidates for implantable products in the not-too-distant future. The pressure on manufacturers is to produce devices that are smaller and lighter, with lower total system costs. In addition to these ongoing developments in implantable devices, the trend toward portability and delivery of care at the bedside is accelerating the development of a range of next-generation monitoring, display, and testing equipment designed to be more compact, accurate, and versatile. Products that can meet these objectives while providing lower power consumption, superior functionality, or ease of manufacture will fill a profitable niche in this burgeoning industry. This article outlines several areas of electromedical product design in which the limits of conventional semiconductor technology are being extended through the use of existing circuits in new ways or through the combination of several cell or block functions into a single electronic system (see Table I).

Table I. A sampling of electromedical design application areas and selected available components.

SYSTEM SOLUTIONS FOR IMPLANTABLES

Devices such as implantable pacers or defibrillators are really miniature computers that employ sensitive, low-voltage, low-power, application-specific integrated circuits (ASICs) to monitor, regulate, and control the delivery of electrical impulses to the heart. Implantable cardioverter defibrillators (ICDs) have been in common use for a number of years. When it detects a potentially life-threatening cardiac fibrillation, the ICD applies a high-voltage pulse between two electrodes connected to the heart. The pulse can be as high as 800 V, with the resulting current (during a few milliseconds) reaching several tens of amperes.

The high voltage is generated and stored on a large capacitor through the use of a charge pump. Normally, the shock is delivered to the heart via a two-phase pulse. Figure 1 shows a principal block diagram of a two-phase defibrillator system that features a typical high-voltage bridge required to generate the biphasic pulse. The application consists of two identical half bridges, each having two switches—one to ground and the other to the high voltage. Insulated-gate bipolar transistors (IGBTs) are very often used as the switch element, since they offer minimum on-resistance relative to silicon area. The high-side IGBT requires a gate voltage that is approximately 10 to 15 volts higher than the voltage to be switched. Normally, a transformer is used for level shifting between the high-voltage controller and the switch. Figure 2 shows a principal block diagram of the components required for one half bridge.