Implantable Sensors Three complicated technologies at work in the body

by David Orenstein free-lance writer

The dream of tiny, implantable sensors that can monitor health is one of historic technological convergence, enthusiastic speculation, but—so far—little reality.

Not long ago, people began to realize that three rapidly advancing technologies—wireless networking, microelectronics, and protein and DNA synthesis—could be combined to produce smart, biologically sensitive sensors capable of transmitting information to a remote observer. The idea of such a glucose monitor cheered researchers who sympathized with the plight of oft-perforated diabetics. More recently the birth of “nanotechnology” (building machines on the scale of billionths of a meter) has inspired fantasies about tiny machines patrolling our bloodstreams to perform maintenance on our cells.

Of the two ideas, glucose monitoring obviously has received more earnest attention. There has always been a desire for a noninvasive (or perpetually invasive, depending on your interpretation) implantable glucose sensor. In the 1990s it seemed that technology no longer stood in the way. All one needed was to get an enzyme or other biological compound that was sensitive to glucose, interface that with some microelectronics, and combine that with low-power wireless circuitry. Other sensors would work in a similar way. Depending on the application, an enzyme or DNA strand specifically sensitive to what you are looking for could be either borrowed from nature or synthesized and integrated with a chip.

Monitoring is not the only application of interfacing electronics with biology. Researchers also have made great strides in using electronics to replace or compensate for physical deficiencies in patients. In the journal Nature earlier this year (2002 Mar 14;416:141), neuroscience researchers at Brown University reported that monkeys with implants in the motor cortex of their brains could manipulate a cursor on a computer screen without using their hands. The hope for this research is to empower paralyzed people to use computers and robotic prosthetics to live fuller lives.

One problem has been finding inexpensive biological molecules that give sensors their sensitivity. Sure, scientists know that specific enzymes or DNA strands will match with glucose or E. coli or anthrax or whatever the sensor is designed to detect, but acquiring them is difficult. They are expensive to make and often are too unstable to store well, as reported by Turner (Science. 2000 Nov 17;290:1315-1317).

While scientists have been tackling that problem, they have been hindered by another: the immune system, which will try to kill the sensor, or, failing that, build a wall around it. Sure enough, today’s glucose monitor implants have only a few days’ life span.

To make sensors last longer, researchers must find a way around this problem. Medtronic has licensed technology for a sensor that can be implanted in a major heart vein. The constant rush of blood ensures that the body will not fully wall-off a sensor. But, as Service writes in Science (2002 Aug 9;297:962-963), a clot could form around the sensor and, if dislodged, cause an embolism in the lungs.

Other efforts to circumvent the problem include doping the sensors or nearby cells with natural body chemicals that encourage blood vessel growth (so the sensors retain access to the bloodstream) or that suppress the immune system’s response. The approaches are in trials.

Despite hurdles, many scientists remain optimistic that medically useful implantable sensors are on the way.

David Orenstein is a technology and business writer in Silicon Valley. To learn more about a technology topic in Computing Care, e-mail him at davealli@attbi.com.