Stanford researchers create tiny, wirelessly powered cardiac device
Stanford electrical engineers overturn existing models to demonstrate the feasibility of a millimeter-sized, wirelessly powered
The findings, say the researchers, could dramatically alter the scale of medical devices implanted in the human body.
Ada Poon, assistant professor of electrical engineering, led the research.
September 4, 2012
By Andrew Myers
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 a small power device on the surface of the body.
The implanted device is contained in a cube just 0.8 millimeter on a side. 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 5 centimeters inside the chest on the surface of the heart – a depth once thought
out of reach for wireless power transmission.
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
Implantable medical devices in the human body have revolutionized medicine. Hundreds of thousands if not millions of pacemakers,
cochlear implants and drug pumps are today helping patients live relatively normal lives, but these devices are not without
First, 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. Second, batteries have finite lives. New surgery is needed when they wane.
"Wireless power solves both challenges," said
assistant professor of electrical engineering, who headed up the research.
She was assisted by Sanghoek Kim and John Ho, both doctoral candidates in her lab.
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 latest device works by a combination of 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 receiving antenna.
That is, to deliver a desired level of power, lower frequency waves require bigger coils. Higher frequency waves can work with
"For implantable medical devices, therefore, the goal is a high-frequency transmitter and a small receiver, but there is one
big hurdle," Kim said.
Existing mathematical models have held that high-frequency radio waves do not penetrate far enough into human tissue,
necessitating the use of low-frequency transmitters and large antennas – too large to be practical for implantable devices.
Ignoring the consensus, Poon proved the models wrong. Human tissues dissipate 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
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, much higher than previously thought.
"In this high-frequency range, we can increase power transfer by about 10 times over earlier devices," said Ho, who honed the
The discovery meant that the team could shrink the receiving antenna by a factor of 10 as well, to a scale that makes wireless
implantable devices feasible. At 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 8-microwatt pacemaker.
With the dimensional challenges solved, the team found itself bound by other engineering constraints. First, electronic medical
devices must meet stringent health standards established by IEEE (Institute of Electrical and Electronics Engineers), particularly
with regard to tissue heating. Second, the team found that the receiving and transmiting antennas had to be optimally oriented to
achieve maximum efficiency. Differences in alignment of just a few degrees could produce troubling drops in power.
"This can't happen 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 transmitting antenna structure.
It delivers consistent power efficiency regardless of orientation of the two antennas.
The new design serves additionally to focus the radio waves precisely at the point inside the body where the implanted device rests
on the surface of the heart – increasing the electric field where it is needed most, but canceling it elsewhere. This helps reduce
overall tissue heating to levels well within the IEEE standards. Poon has applied for a patent on the antenna structure.
This research was made possible by funding from the C2S2 Focus Center, one of six research centers funded under the Focus
Center Research Program, a Semiconductor Research Corporation entity. Lisa Chen also contributed to this study.
Andrew Myers is associate director of communications at the School of Engineering.