Engineers have found a safe way to wirelessly transfer energy to medical implants in the body. They used it to power a tiny batteryless pacemaker that's about the size of a grain of rice. And so far it works! In rabbits for now.
Bulky batteries and clumsy wiring systems have restricted the use of life-saving pacemakers and pain-relieving nerve stimulators. The world's smallest pacemaker that doesn't require electrodes was recently revealed, but no matter how small they get, their batteries will always need to be replaced eventually.
As an alternative, a team of Stanford researchers led by Ada Poon wanted to transfer power to microimplants without wires or batteries. They developed a new method of controlling electromagnetic waves inside the body.
Electromagnetic waves are used to broadcast signals from radio towers, heat food in microwaves, and power electric toothbrushes that can recharge wirelessly in their plastic cradles. There are two main types of electromagnetic waves -- far-field and near-field -- but neither of them are an ideal power source for medical devices. When far-field waves encounter biological tissue, they're either reflected off or they're absorbed by the skin as heat. Near-field waves are used in current devices (like hearing aids) but their range is limited.
So Poon's team blended the two and created mid-field waves. When this wave moves from the air to skin, it changes its characteristics in a way that allows it to propagate. They call their system "mid-field wireless transfer," and they can use it to focus electromagnetic energy directly on an implant deep within biological tissue. Here's how it works:
- First, they had to design a transmitter: a 6-centimeter-squared metal plate with four trident-shaped cutouts arranged in a circle, Science describes.
- These use roughly the same power as a cellphone, and they just need to be placed on the skin above an implant.
- Then they used that to transmit power to a tiny receiver coil mounted onto a 2-millimeter-long electrostimulator (or pacemaker, pictured) that can be embedded into organs.
- In pig hearts and brains, they delivered 2,000 microwatts of power through 5 centimeters of tissue to reach the vital organ. (Conventional pacemakers require 8 microwatts of power.)
- Then in a rabbit, they implanted the batteryless electrostimulator 4.5 centimeters deep on the surface of the heart. The device regulated the rabbit's cardiac rhythm without burning its skin.
In the future, this kind of pacemaker can be powered or recharged wirelessly by holding a power source the size of a credit card above the device, outside the body. According to Poon, an independent lab that tests cellphones found that their system fell well below the danger exposure levels for human safety.
The team is preparing the system for testing in humans, and they've launched a company, Vivonda Medical, to adapt the technology.
The work was published in Proceedings of the National Academy of Sciences last month.
Images: Austin Yee