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Micro medical devices could transform health care

Posted: 24 Jun 2002 ?? ?Print Version ?Bookmark and Share

Keywords:wireless microsystems? MEMS? microelectromechanical systems? wafer level packaging? RF circuit?

Wireless microsystems promise "spectacular advances in health care" for the blind, deaf and epileptic and those suffering with Parkinson's and heart disease, a celebrated pioneer in the field of microelectromechanical systems (MEMS) maintains.

Calling advances over the last 30 years slow but real, University of Michigan professor Kensall Wise predicted "significant progress" for medical electronics in creating systems that form "a microelectronic bridge to the cellular world."

Wise, director of the university's Engineering Research Center for Wireless Integrated MicroSystems, said in a talk at the 2002 VLSI Circuits Symposium here last week that the demands placed on such tiny, low-power systems have slowed progress. In particular, he said, more work must be done on the wafer-level packaging needed to withstand "the aggressive saltwater environment" presented by living tissue.

Wise described several advances that are now under way.

Arteries cleared

In this decade, persons with clogged arteries could routinely receive stents with molded submillimeter flowmeters, pressure transducers and RF circuits. A doctor, checking whether a stented artery had reclogged over time, would pass a wand over a patient's chest and wirelessly receive blood flow information.

But the implanted device must measure less than 0.5 mm in diameter, a challenge "complicated by the fact that these things like to dissolve in saltwater," Wise said. Thick dielectrics and metal shields can protect a wireless integrated microsystems device?a WIMS?but keeping such well-protected packages small enough will take more work.

In a video, Wise showed a man with Parkinson's disease, his right arm extended and hand shaking violently. When an electrical stimulus was applied to two to four sites in the brain via a "very, very crude" implanted electrode, the shaking temporarily stopped, results that "border on the miraculous," Wise said.

Doctors are about three to five years away from getting high-density implants with 128 electrodes "that are very, very promising" for people with severe Parkinson's, Wise said.

Another serious neurological disorder, epilepsy, may be controlled by a microsystem?now also in trials?that can "detect an epileptic seizure and deliver electric stimulus and drugs to a patient who may not even know" that an attack is under way, Wise said.

Such intelligent implants, he said, require control and digital signal processors that run on a milliwatt or less, much less than the controllers used in pacemakers today.

Ears opened

An audio clip demonstrated what life is like for a profoundly deaf person, someone with no hearing at all. The demonstration of a first-generation cochlear implant created sounds akin to being deep below the ocean?barely distinguishable words. A second-generation implant was much improved, but Wise's audience still had to crane forward to try to pick up the meaning of the words.

"I have a friend who is an R&D manager at General Motors. He is profoundly deaf and told me how his life was changed by his implant, and how he can't wait for the next generation to come along," Wise said, estimating that 30,000 people worldwide have received the implants.

Other remedies will take more time and effort. Blindness requires implants?"retinal prostheses"?that are so tiny that meaningful improvement is probably years away. And paralysis is challenging engineers to "capture motor control signals in the cortex and wire around breaks in the spinal cord."

Progress has come, in part, because funding has doubled for the National Institutes for Health, and due to the more recently established National Institute for Biomedical Imaging and Bioengineering.

Wise described electrode arrays being developed at the University of Michigan that link directly with the central nervous system at 1,024 sites. These probe arrays can monitor neural activity and deliver electrical stimulation and, in some cases, drugs.

Drug-delivery channels can be incorporated with the probes. With an additional mask layer, a standard CMOS process can cut ribs in a silicon chip.

The channels are typically 20 to 30 microns wide and can inject doses of 100 to 200 picoliters a second. With a second additional mask layer, an on-chip flowmeter can be created that operates in pulsed mode to avoid burning the surrounding tissue.

These WIMS "operate at milliwatt levels, with site spacings of 100 to 400 microns, communicating wirelessly at up to 100 kbits/second," Wise said.

The implanted sensors modulate a signal on an implanted antenna, which loads an external coil that can be detected outside the body. The wireless function, or "passive telemetry link," is based on an amplitude-modulated 4-MHz RF carrier, which both powers the implant and detects its output.

"Any microsystem intended for use in the nervous system must be capable of working unattended for decades and must be biocompatible in terms of its interface with living tissue," Wise said.

WIMS developers face a number of engineering challenges. Wise said typical neural signals range from about 20 to 400 microvolts, at frequencies in the range of 10 kHz. The devices must be able to record bandwidth from 10 Hz to 10 kHz, with an overall gain of about 60 dB.

Lifetime reliability

Packaging, which closely relates to reliability and safety, has been a tough problem over the three decades Wise has worked in the field. Exposed silicon etches slowly in the body, though boron doping serves to retard the etching, as it does in normal silicon devices.

Diamond and polymeric coatings are being tried, and metal barrier films can help extend the life of implanted devices. "Glass-silicon packages, which can be formed at wafer level, can provide hermetic encapsulation for times estimated at more than 100 years," he said.

Polymer-based devices have been used in pliant cortical implants, which can move within the brain. But, Wise said, "very significant safety aspects" of polymer microdevices still must be tackled.

Wise became involved in biomedical electronics early on. At the 1971 International Solid-State Circuits Conference?two years after earning his doctorate from Stanford University?he organized a panel discussion about medical electronics applications. He also received a best paper award at that 1971 ISSCC.

? David Lammers

EE Times





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