A team of engineers out of Stanford is introducing a truly tiny wireless cardiac device to demonstrate that, thanks to a little ingenuity and impressive math, all medical implants may soon be powered wirelessly.
Which means that devices such as pacemakers, which owe the majority of their bulk to the battery, are about to get a whole lot smaller.
Head researcher Ada Poon, who earlier this year showed off a proof-of-concept, wirelessly powered device, says the main achievement with the cardiac device is that it can be implanted on the surface of the heart -- a full 5 centimeters inside the chest -- and still be within reach of wireless power transmission.
The researchers describe the implant -- contained in a cube 0.8 millimeters on a side that could fit on the head of a pin -- in the journal Applied Physics Letters. In addition to making the cardiac device significantly smaller than a pacemaker, using radio-wave power would also eliminate the surgeries currently required to replace batteries.
"Wireless power solves both challenges," Poon, assistant professor of electrical engineering, said in a school news release.
Her device relies on both inductive and radiative transmission of power, using a transmitter to send radio waves to a coil of wire inside the body. The radio waves produce sufficient current in the coil to power small devices.
As discussed when she introduced her self-propelled device small enough to swim through the bloodstream, Poon and her team needed to overcome a pretty major hurdle.
To use small coils, they needed higher frequencies (the relationship between wave frequency and antenna size being inverted), but mathematical models hold that high frequency radio waves don't penetrate deep enough into human tissue to power implants. Crunching the numbers, Poon and her team found that high frequency signals actually travel further than previously thought.
"In fact, to achieve greater power efficiency, it is actually advantageous that human tissue is a very poor electrical conductor," said her lab assistant, Sanghoek Kim. "If it were a good conductor, it would absorb energy, create heating, and prevent sufficient power from reaching the implant."
After revising the models, they found that power can transfer through human tissue at up to 1.7 billion cycles per second, which they say is far higher than previously thought, and which allows them to increase power transfer 10 times beyond earlier devices -- which also means they can shrink the receiving antenna by a factor of 10 as well.
They say that at the optimal frequency, a millimeter-radius coil can generate more than 50 microwatts of power. To put this into context, one recently demonstrated pacemaker requires just 8 microwatts.
While Poon's tech is admittedly futuristic, she has applied for a patent on the antenna structure, and we'll continue following her progress.