Implants to remake medicine

San Francisco -- The new day in medical care heralded by implantable electronics devices is finally coming to pass, a University of Michigan researcher told the International Electron Devices Meeting here last week.

"Only today are integrated microsystems finally beginning to create the revolution in health care that was originally envisioned," Kensall Wise, director of the university's Engineering Research Center for Wireless Integrated MicroSystems, said in an IEDM plenary talk that described 40 years of development of these devices.

Wise said today's achievements are only the beginning. "As part of efforts to engineer a better health care delivery system, wearable devices have been targeted to improve in-hospital patient monitoring, prevent errors in delivering medication, improve outpatient monitoring and provide in-home lifelines for the elderly," he said. "It is widely expected that the next 50 years will see progress in biotechnology and health care akin to what we have seen in microelectronics during the last 50."

Wise's current interest is to harness the nervous system, which is the body system closest to being emulated by microelectronics. The neurons that make it up act as pulse-rate-modulated logic elements, forming complex three-dimensional networks that implement our sensory and other physiological functions. "There is great hope that neural prostheses will help in overcoming problems like deafness, blindness, epilepsy, Parkinson's disease and paralysis," Wise said.

Deep brain stimulation for the relief of Parkinsonian tremors has been nothing short of miraculous in many cases, he said. The technique, in which a four-electrode probe is positioned in the subthalamic nucleus and driven by a device resembling a pacemaker, masks tremor, much as if positive-feedback channels are being blocked. The therapy has been used on tens of thousands of patients worldwide, and when the electrodes are correctly positioned, there are few, if any, side effects, Wise said. Many severely disabled patients have been returned to normal life.

The electrodes must be positioned to better than 1 mm at a depth of several centimeters. Missing the target area can result in speech, balance or gait difficulties. "More-sophisticated systems that can more precisely shape the stimulating current field should result in considerably better performance," said Wise.

Help is also on the way for the sight and hearing impaired. Wireless implantable microsystems for these applications could be realized during the coming decade.

"Such microsystems will demand a tissue interface that is stable over years," said Wise, along with "high-density three-dimensional electrode arrays, micropower signal-processing circuits, bidirectional wireless interfaces, hermetic packaging and suitable power sources."

Wireless operation of implantable systems is key to their successful deployment in clinical applications. Internal power sources can be used for some implantable devices, and energy scavenging is an increasingly active research area for others. However, size and power levels often dictate the wireless transmission of power along with bi- directional data.

The most common example is the cochlear implant, where the main energy source--a battery--and the speech processor are in an external module behind the ear. To avoid tissue damage, power dissipation needs to be limited to a few tens of milliwatts and the unit must not exceed 1°C. RF carrier frequencies of less than 10 MHz are used to avoid tissue losses at higher frequencies.

Packaging is one of the challenges of implantable biomedical microsystems, and depends on the implant configuration used. Glass-silicon and silicon-silicon packages have been explored at wafer level and shown to be suitable for long-term use in vivo.

"There are many challenges ahead, but if past successes are any guide, even the limited performance of initial devices may give the body enough to do far better than we even dare to hope for," said Wise.