Wireless charging can have real impact in medical devices: Page 2 of 3

April 22, 2015 // By Bill Schweber, EDN
Wireless charging of personal devices such as smartphones, laptops, and tablets is a hot topic. Several standards are competing to make it possible to just place the device on a table's surface or charging pad, and sit back while energy is transferred without the need to connect to a discrete, physical cable.
But there are situations where practical wireless charging would bring real benefits: implanted medical devices. In many cases, the battery's size is a limiting factor in use, since it needs to power the device for several years. In some cases, such as brain stimulators, the battery is actually sited in the patient's chest, with wires running between the two locations, obviously not a desirable situation, but there is no choice.

Why not use wireless charging for medical implants? It has been done, but with mixed results. There are several obstacles: the distance of between one and as much as five cm, the body tissue and fluids between source and receiver, the small size of the implant (as little as a few millimeters), and movement of the receiver target, to cite a few. The obvious approach using inductive transfer only works adequately when the source and receiver are close and properly oriented, and the receiver coil is relatively large; it is already in use for some types of cochlear implants where these conditions can be met. However, its not viable for pacemakers and other deeper implanted devices.

Yet there is innovation and progress, as shown by an article in Physics Today, "Wireless power for tiny medical implants." This informative and very readable article discussed work done at Stanford University using directed GHz energy to address the issue. The article included explanation of the core problems, the RF modelling they used, and the result achieved thus far.

These results were impressive. The Stanford group reports that "in experiments on a pig cadaver, 0.04% of the source power was transferred to a 2-mm receiver 5 cm beneath the surface" using 1.6 GHz radiation. Yes, you read that right: just 0.04% which would be totally unacceptable for charging a consumer device such as a smartphone. But the report notes that even such a low efficiency is sufficient for many implanted devices (a state-of-the-art cardiac pacemaker uses under 10 W, many orders of magnitude less than a consumer product.

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