Wireless pacemaker is much smaller than a penny

A wireless system that powers very small medical implants could one day let doctors treat diseases and relieve pain without medications. (Credit: Stanford)

Engineers have built an electronic pacemaker that’s smaller than a grain of rice and 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 team tested the wireless charging system in a pig and used it to power a tiny pacemaker in a rabbit. They’re preparing the system for testing in humans.

Should such tests be approved and prove successful, it would still take several years to satisfy the safety and efficacy requirements for using this wireless charging system in commercial medical devices. But it has the potential to eliminate bulky batteries and clumsy recharging systems and lead to a type of medicine where physicians treat disease and alleviate pain with electronics instead of 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,” says Ada Poon, assistant professor of electrical engineering at Stanford University.

The central discovery is an engineering breakthrough that creates 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 says that the system falls well below the danger exposure levels for human safety, Poon writes.

More effective than drugs

The discovery could spawn a new generation of programmable microimplants—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, Poon says.

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The work creates the potential to develop “electroceutical” treatments as alternatives to drug therapies, says William Newsome, professor of neurobiology and director of the Stanford Neurosciences Institute.

Newsome, who was not involved in Poon’s experiments but is familiar with her work, says 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,” he says. “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.”

Far-field vs. near-field

The crux of the discovery, reported in the Proceedings of the National Academy of Sciences, involves a new way to control electromagnetic waves inside the body.

Electromagnetic waves pervade the universe. We use them every day when we broadcast signals from giant radio towers, cook in microwave ovens, or use an electric toothbrush that recharges wirelessly in a special cradle next to the bathroom sink.

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.

Blended waves

What Poon did was to blend the safety of near-field waves with the reach of far-field waves. She accomplished this 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.

For instance, when you put your ear on a railroad track, you can hear the vibration of the wheels long before the train itself because sound waves travel faster and further through metal than they do through air.

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.

She calls this new method mid-field wireless transfer.

In the experiment, Poon used her mid-field transfer system to send power directly to tiny medical implants. But 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.

“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,” says graduate student John Ho.

Source: Stanford University