A team of engineers at Stanford has demonstrated the feasibility of a super-small, implantable cardiac device that gets its power not from batteries, but from radio waves transmitted from outside the body. The implanted device is contained in a cube just eight-tenths of a millimeter in radius. It could fit on the head of pin.
The findings were published in the journal Applied Physics Letters. In their paper, the researchers demonstrated wireless power transfer to a millimeter-sized device implanted five centimeters inside the chest on the surface of the heart—a depth once thought out of reach for wireless power transmission.
The paper's senior author was Ada Poon, a professor of electrical engineering at Stanford. Sanghoek Kim and John Ho, both doctoral candidates in Poon's lab, were first authors.
The engineers say the research is a major step toward a day when all implants are driven wirelessly. Beyond the heart, they believe such devices might include swallowable endoscopes—so-called "pillcams" that travel the digestive tract—permanent pacemakers and precision brain stimulators; virtually any medical applications where device size and power matter.A revolution in the body
First off, they require power, which means batteries, and batteries are bulky. In a device like a pacemaker, the battery alone accounts for as much as half the volume of the device it drives. Second, batteries have finite lives. New surgery is needed when they wane.
"Wireless power solves both challenges," said Poon.
Last year, Poon made headlines when she demonstrated a wirelessly powered, self-propelled device capable of swimming through the bloodstream. To get there she needed to overturn some long-held assumptions about delivery of wireless power through the human body.
Her device works by a combination inductive and radiative transmission of power. Both are types of electromagnetic transfer in which a transmitter sends radio waves to a coil of wire inside the body. The radio waves produce an electrical current in the coil sufficient to operate a small device.
There is an indirect relationship between the frequency of the transmitted radio waves and the size of the receive antenna. That is, to deliver a desired level of power, lower frequency waves require bigger coils. Higher frequency waves can work with smaller coils.
"For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one big hurdle," explained Kim.Ignoring consensus
Ignoring the consensus, Poon proved the models wrong. Human tissue dissipates electric fields quickly, it is true, but radio waves can travel in a different way—as alternating waves of electric and magnetic fields. With the correct equations in hand, she discovered that high-frequency signals travel much deeper than anyone suspected.
"In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor," said Kim. "If it were a good conductor, it would absorb energy, create heating and prevent sufficient power from reaching the implant."
According to their revised models, the researchers found that the maximum power transfer through human tissue occurs at about 1.7 billion cycles per second.
"In this high-frequency range, we can increase power transfer by about ten times over earlier devices," said Ho, who honed the mathematical models.
The discovery meant that the team could shrink the receive antenna by a factor of ten as well, to a scale that makes wireless implantable devices feasible. At that the optimal frequency, a millimeter-radius coil is capable of harvesting more than 50 microwatts of power, well in excess of the needs of a recently demonstrated eight-microwatt pacemaker.Additional challenges
"This can't happen with medical devices," said Poon. "As the human heart and body are in constant motion, solving this issue was critical to the success of our research."The team responded by designing an innovative slotted transmit antenna structure. It delivers consistent power efficiency regardless of orientation of the two antennas.
This article was written by Andrew Myers, associate director of communications for the Stanford University School of Engineering.
Andrew Myers | EurekAlert!
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