Originally Published MDDI October 2005
A prototype of the cooler is shown here, with the warm surface glued to a copper bar in the back.
Incorporating a heat spreader into an implantable thermoelectric cooler could help detect and stop seizures. As the device cools a small area of the brain, a pipe transfers the heat without damaging tissue.
Although the pipe and the thermoelectric cooler have been built, integrating them is still a challenge. The finished device would have four major components—a heat spreader, thermoelectric cooler, control system, and battery. Design challenges that remain include reducing the size, making the power supply more efficient, and developing appropriate computer software.
Researchers at Washington University School of Medicine (St. Louis) developed the thermoelectric device for use in a small area of the brain where seizures occur in patients with focal epilepsy. Implanted in the neocortex, the device detects unusual electrical activity that is associated with certain seizures. As a current runs through the device, one side becomes cold to reduce brain temperature. The device touches, but is not actually in, the cortex. This placement limits the chance of damaging brain tissue.
During development, researchers were concerned that the other side of the plate would get too warm and would cause internal harm. To safely use the device in humans, researchers needed to diffuse the heat away from the thermoelectric cooler. That's when they turned to the team at Rensselaer Polytechnic Institute (RPI), located in Troy, NY.
"You can cool brain tissue quite a bit, maybe 20ÞC, without doing any damage, but can only heat it a very small amount," says G. P. "Bud" Peterson, PhD, professor of mechanical, aerospace, and nuclear engineering at RPI. "You have to take heat out of a small region and dump it elsewhere in the brain, without raising the temperature."
Peterson and his colleagues created a heat spreader that uses a concept similar to perspiration in the human body. "When you perspire, liquid comes to the surface and vaporizes," says Peterson. "As it evaporates, it cools the skin and cools the body down. Vaporization is a very efficient heat-transfer process."
The device from Rensselaer is a pipe that contains water, which produces evaporation in the heated areas. It disperses and releases heat without raising the temperature too much, like a thermal conductor. As the heat pushes through the pipe, the water changes to vapor. High pressure forces the vapor to condense in the cooler areas.
The pipe is about the size of a business card, with about the same thickness and flexibility. The complete implantable device would be about the same size but a little thicker, says Peterson. Researchers are working on making it thinner and a bit more flexible.
The thermoelectric cooler will need to be miniaturized to meet this requirement. "In rats, we've used a device that varies from 4 × 4 to 4 × 7 mm in length and width," says Steven Rothman, professor of developmental neurology at Washington. "For a human, we imagine that a device would be 1 × 2 cm in length and width, and maybe 2 mm deep."
Rothman envisions that the thermoelectric device would be glued or attached to the heat spreader. The heat generated in the smaller device would go into the larger device and move from there to the bloodstream or bone.
The cooler on the left has been filled with epoxy to prevent body fluids from causing electrical short circuits. The first row of semiconductors is visible in the unfilled device on the right.
Taking the device to the next level in humans means improving the power consumption. Since it isn't implantable yet, researchers are using a large dc power supply. The thermoelectric cooler uses an amp of current, which pushes today's battery technology, says Rothman. "These would require batteries that are probably larger than ones used for electronic devices on the market, like defibrillators." Rothman expects the device would only need to be turned on for a very small fraction of each day, so the power drainage shouldn't be overwhelming.
The cooler should also generate less internal heat. "The efficiency of the thermoelectric device isn't high," says Peterson. "If we take 1 mW [of heat] out of the brain, we have to reject 4 mW out of the brain, because the device generates about 3 mW."
Other issues include having a computer program and amplifier that can support turning on the device only when a seizure is detected or predicted. Rothman believes that although this capability is on the cutting edge of neurology technology, it may be achievable in the near future.
A final concern is biocompatibility. More tests are required before the team will know whether the device can be implanted without causing infection or other serious problems.
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