Wireless power transfer deep inside the body runs medical devices and sensors
The discoveries made by Ada Poon, assistant professor of electrical engineering at Stanford University, are reported in the Proceedings of the National Academy of Sciences and focus on Poon’s years of work to eliminate the bulky batteries and clumsy recharging systems that often stop medical devices from being more widely used.
The power technology could provide a path toward a new type of medicine that allows physicians to treat diseases with electronics rather than drugs.
"We need to make these devices as small as possible to more easily implant them deep in the body and create new ways to treat illness and alleviate pain," explained Poon.
Poon’s team built an electronic device smaller than a grain of rice that acts as a pacemaker. The device can be powered or recharged wirelessly by holding a power source about the size of a credit card above the device, outside the body.
The central discovery involves the creation of a new type of wireless power transfer – using roughly the same power as a cell phone – that can safely penetrate deep inside the body. An independent laboratory that tests cell phones found that Poon’s system fell well below the danger exposure levels for human safety.
Poon’s lab has tested the wireless charging system in a pig and used it to power a tiny pacemaker in a rabbit. She is currently preparing the system for testing in humans.
Poon believes the discovery could create a new generation of programmable micro-implants – sensors to monitor vital functions deep inside the body; electrostimulators to change neural signals in the brain; and drug delivery systems to apply medicines directly to affected areas.
William Newsome, director of the Stanford Neurosciences Institute, said Poon’s work created the potential to develop ‘electroceutical’ treatments as alternatives to drug therapies.
Newsome, who was not involved in Poon’s experiments but is familiar with her work, said such treatments could be more effective than drugs for some disorders because electroceutical approaches would use implantable devices to directly modulate activity in specific brain circuits. Drugs, by comparison, act globally throughout the brain.
"To make electroceuticals practical, devices must be miniaturized, and ways must be found to power them wirelessly, deep in the brain, many centimeters from the surface," said Newsome, the Harman Family Provostial Professor and professor of neurobiology at Stanford. "The Poon lab has solved a significant piece of the puzzle for safely powering implantable microdevices, paving the way for new innovation in this field."
The crux of the discovery involves a new way to control electromagnetic waves inside the body.
Before Poon’s discovery, there was a clear divide between the two main types of electromagnetic waves in everyday use, called far-field and near-field waves.
Far-field waves, like those broadcast from radio towers, can travel over long distances. But when they encounter biological tissue, they either reflect off the body harmlessly or get absorbed by the skin as heat. Either way, far-field electromagnetic waves have been ignored as a potential wireless power source for medical devices.
Near-field waves can be safely used in wireless power systems. Some current medical devices like hearing implants use near-field technology. But their limitation is implied by the name: They can transfer power only over short distances, limiting their usefulness deep inside the body.
Poon’s team blended the safety of near-field waves with the reach of far-field waves. Poon accomplished the breakthrough by taking advantage of a simple fact – waves travel differently when they come into contact with different materials such as air, water or biological tissue.
With this principle in mind, Poon designed a power source that generated a special type of near-field wave. When this special wave moved from air to skin, it changed its characteristics in a way that enabled it to propagate – just like the sound waves through the train track. Poon called the new method ‘mid-field wireless transfer’.
In the experiment, Poon used the mid-field transfer system to send power directly to medical implants. It is possible to build tiny batteries into microimplants, and then recharge these batteries wirelessly using the mid-field system. This is not possible with today’s technologies.
Co-author John Ho noted: "With this method, we can safely transmit power to tiny implants in organs like the heart or brain, well beyond the range of current near-field systems."
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