Jun 05 2018

Powering Implanted Devices

One of the cutting edge medical technologies that promises to be a game-changer in terms of our ability to affect biological function is the interaction between machines and biology. Of course we already have many medical devices, from cardiac pacemakers to artificial joints. Increasingly sick and aging humans are becoming cyborgs, as we augment and replace broken body parts with machines.

We have only scratched the surface of this potential, however, and the technology is advancing quickly. There are definitely technological hurdles that limit such technology, however, and perhaps chief among them is the need for power.

MIT researchers have recently presented a new method for powering implanted devices that may open the door to a further proliferation of implantable medical devices. They use radio waves as an external power source, which eliminated the need for cumbersome batteries.

Right now power is a major limiting factor for implantable medical devices. We can make small batteries, but they still become the largest part of many devices. Especially as solid state digital technology improves, we can make very tiny electronic devices, and then we attach a relatively large battery to them. Even these “large” batteries also have a limited life span.

There are several possible approaches to this problem. One is to make better batteries, ones that can hold more energy for longer in a smaller package. This is happening, incrementally, but is still a major limiting factor.

Another approach is to harvest ambient energy. This approach has tremendous promise – using material that can convert body heat or the mechanical energy of breathing or moving into electricity to power devices. Since small devices usually only need a tiny amount of energy to operate, the “extra” energy generated by biological processes would be more than enough to power many such devices.

This seems to me as having the best potential. A device that can efficiently harvest ambient energy from the body in which it is implanted could last indefinitely, and requires no connection to anything outside. It would therefore be entirely independent – set it and forget it.

Energy harvesting devices, however, still take up some size. As we try to shrink implantable devices further, to increase their potential applications, anything that increases the size of devices is a drawback.

Finally, we can power devices from the outside. Some devices, for example, might have a battery under the skin, and then a wire connects to the smaller device implanted deeper and provide it with power. The battery is therefore also accessible for recharging or replacement. This is still cumbersome, however, and the requirement for wires has its own limitations.

This brings us to the current technology – external wireless powering. The big advantage here is that devices can potentially be very small, they do not need their own batteries or to be connected by wires, and multiple devices can be powered at once. The MIT researchers’ advance is in using radio waves that are shifted a little in frequency, so that there will be peaks and troughs of constructive and destructive interference. When the peaks of the waves line up, the total radio wave energy is increased and can peak above that needed to power the device.

Currently they have researched their device in pigs. They were able to power a device implanted 10 cm beneath the skin from one meter away. The deeper the implant, the closer the radio wave source has to be. They are hoping to increase this range to allow deeper implants.

This approach would not really work for a device that needs to be operating 100% of the time, like a pacemaker. It would be ideal, however, for a device that only needs to operate intermittently, such as a diagnostic device. A patient, for example, could have a device that monitors some physiological parameter (with video, chemical analysis, electrical analysis, etc.) and then when the patient is in their doctor’s office the device can be interrogated from the outside, powered by external radio waves. At other times the device would be dormant.

We can also envision more sophisticated devices that can be used for surgery. Imagine implanting a small device through the skin or into a blood vessel, or even swallowed, and then that device could be operated from the outside through radio waves, performing microscopic delicate surgery inside the body. Once the surgery is done the device, or army of devices, can be retrieved, or just rendered dormant. Perhaps they will need to be permanently disabled so they cannot be later hacked for nefarious purposes.

This new technology is not a perfect solution, nor would it work for all implantable devices, but it does create new possibilities that are interesting to contemplate.

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