The implants are powered by radio frequency waves, which can safely pass through human tissues. In tests in animals, the researchers showed that the waves can power devices located 10cm deep in tissue, from a distance of 1metre. If the sensors are located very close to the skin's surface, the researchers add, they can be powered from up to 38meters away.
"Even though these tiny implantable devices have no batteries, we can now communicate with them from a distance outside the body," says Assistant Professor Fadel Adib from MIT.
Such devices could be used to deliver drugs, monitor conditions inside the body, or treat disease by stimulating the brain with electricity or light, the researchers say.
The researchers tested a prototype about the size of a grain of rice, but anticipate that it could be made even smaller.
"Having the capacity to communicate with these systems without the need for a battery would be a significant advance. These devices could be compatible with sensing conditions as well as aiding in the delivery of a drug," says Assistant Professor Giovanni Traverso at BWH.
In the brain, implantable electrodes that deliver an electrical current are used for a technique known as deep brain stimulation – often used to treat Parkinson's disease or epilepsy. These electrodes are currently controlled by a pacemaker-like device implanted under the skin, which could be eliminated if wireless power is used. Wireless brain implants could also help deliver light to stimulate or inhibit neuron activity through optogenetics, which so far has not been adapted for use in humans but could be useful for treating many neurological disorders.
Assist Prof. Adib who envisions much smaller, battery-free devices, has been exploring the possibility of wirelessly powering implantable devices with radio waves emitted by antennas outside the body.
This has been difficult to achieve because radio waves tend to dissipate as they pass through the body, so they end up being too weak to supply enough power. To overcome that, the researchers devised a system that they call "In Vivo Networking" (IVN). This system relies on an array of antennas that emit radio waves of slightly different frequencies. As the radio waves travel, they overlap and combine in different ways. At certain points, where the high points of the waves overlap, they can provide enough energy to power an implanted sensor.
"We chose frequencies that are slightly different from each other, and in doing so, we know that at some point in time these are going to reach their highs at the same time. When they reach their highs at the same time, they are able to overcome the energy threshold needed to power the device," Assist Prof. Adib says.
With the new system, the researchers explain that they don't need to know the exact location of the sensors in the body, as the power is transmitted over a large area. This also means that they can power multiple devices at once. At the same time that the sensors receive a burst of power, they also receive a signal telling them to relay information back to the antenna. This signal could also be used to stimulate release of a drug, a burst of electricity, or a pulse of light, the researchers add.
The team is now working on making the power delivery more efficient and transferring it over greater distances.
The hope is that this technology also has the potential to improve RFID applications in other areas such as inventory control, retail analytics, and smart environments, allowing for longer-distance object tracking and communication.