Carbon supplants silicon in electronic medical sensors

BOSTON — Going organic isn’t just for produce anymore — it’s all the rage in flexible electronics. Electronic medical devices constructed from carbon-based components — the materials scientist’s definition of organic — are cheap, bendable and friendlier to the body than the old standby silicon.

Researchers discussed several such organic devices December 2 at the Materials Research Society meeting, including a sensor that detects muscle contractions and one that monitors a person’s pulse and blood oxygen levels.

Their versatile qualities make organic electronics ideal for such medical applications, says bioelectrical engineer Annalisa Bonfiglio of the University of Cagliari in Italy. “It’s a radical new world relative to silicon electronics,” she says.

Silicon is a great base material for electronics, but silicon-based devices are generally rigid, brittle and relatively expensive to make — plus they can irritate human tissue. Organic bioelectronics are more compatible with living things, which are also carbon-based. And organic devices are flexible: They often come in the form of thin polymer films that can conform to human skin, says Bonfiglio.

One such device is a sensor that might replace the clunky contraptions that clamp on the end of a finger or earlobe to measure blood oxygen levels. These devices, called oximeters, send two different wavelengths of light into the body and then measure how much light gets absorbed by hemoglobin, the protein in blood that carries oxygen. The difference in the amount of light absorbed by oxygenated hemoglobin, which is bright red, and the deoxygenated form, which is purplish-blue, can be used to calculate blood oxygen levels.

The new sensor, described by Yasser Khan, a graduate student in the lab of Ana Arias at the University of California, Berkeley, uses red and green organic light-emitting diodes that sit within a thin polymer film containing an organic light detector. The film, about the size of a postage stamp, can be wrapped around a finger or slapped on the wrist. The sensor also can detect a person’s pulse by measuring how often a fresh sample of blood has absorbed the device’s emitted light. Tests that compared the new sensor with a standard oximeter revealed it has the same sensitivity.

Another organic sensor might replace the metal electrodes that are implanted beneath the skin of people with prosthetic limbs. The standard sensors are typically made up of many tiny needles that detect the electrical signals of nerve cells. But because the sensors are placed under the skin, they increase the risk of infections. The new organic sensor, developed in the lab of Ifor Samuel at the University of St. Andrews in Scotland, sits on the skin’s surface.

The new sensor exploits the fact that muscle fibers run in a particular direction, and thus scatter light differently depending on whether the shined light is perpendicular or parallel to the fibers. When placed on the upper arm, for example, the sensor can discern how the muscles are contracting and whether the arm is making a turning motion or a lifting motion. It manages this trick by employing light-emitting diodes, two that are perpendicular to the muscle fibers and two that are parallel to the fibers, in a thin film. When Samuel’s team placed the sensor on the upper arm of a person and connected it with wires to a robotic arm, the robotic arm mimicked the movements of the person’s lower arm.

Both of the new sensors have yet to make their way into the medical diagnostics realm; the field of organic bioelectronics is still relatively young, says Bonfiglio. But she says, “We are now starting to see some real products.”