Wireless self-propelled chip swims through blood vessels
28 February 2012
A tiny, wirelessly powered, self-propelled medical device
capable of controlled motion through blood vessels has been developed by
electrical engineers at Stanford University.
The device, which looks like a tiny circuit board 3x4mm, is
powered wirelessly by radio waves and can be injected or implanted
in the body. A radio transmitter outside the body sends signals
inside the body to an independent device that picks up the signal
with an antenna of coiled wire.
The transmitter and the antennae are magnetically coupled such that
any change in current flow in the transmitter induces a voltage in
the other wire. The power can be used to run electronics on the
device and propel it through the bloodstream.
The current prototype of the radio-powered self-propelled chip
Developed by Ada Poon, assistant professor of electrical engineering
at Stanford, the device has the potential to to deliver drugs,
perform analyses, remove blood clots or remove plaque from arteries.
Team member Teresa Meng, a professor of electrical engineering and
computer science, said, "While we have gotten very good at shrinking
electronic and mechanical components of implants, energy storage has
lagged in the move to miniaturize. This hinders us in where we can
place implants within the body and also creates the risk of
corrosion or broken wires, not to mention replacing aging
Professor Ada Poon (right) holding the chip in
a dish, with graduate students Daniel Pivonka (left) and Anatoly
The key to creating the battery-less device was rethinking old
assumptions about dissipation of radio waves in the human body.
Professor Poon realized that scientists had been approaching the
problem incorrectly in assuming that human muscle, fat and bone were
generally good conductors of electricity. She modelled tissue as a
dielectric — a type of insulator and also discovered that human
tissue is a "low-loss" dielectric which means radio waves travel much farther in human
tissue than originally thought.
"When we extended things to higher frequencies using a simple
model of tissue, we realized that the optimal frequency for wireless
powering is actually around one gigahertz," said Poon, "about 100
times higher than previously thought."
More significantly, however, her revelation meant that antennae
inside the body could be 100 times smaller and yet deliver the power
needed by the medical device. The antenna on the device is just 2
millimetres square, small enough to travel through the bloodstream.
She has developed two types of self-propelled devices. One drives
electrical current directly through the fluid to create a
directional force that pushes the device forward. This type of
device is capable of moving at just over half-a-centimetre per
second. The second type switches current back-and-forth through a
wire loop to produce a swishing motion similar to the motion a
kayaker makes to paddle upstream.
"There is considerable room for improvement and much work remains
before such devices are ready for medical applications," said Poon.
"But for the first time in decades the possibility seems closer than