原创 Why wireless charging suits medical implants

Wireless charging of personal electronics 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. While this might seem a nice but not critical feature at home or office where you have your usual charger and connector already set up, it is a different story in a public place where you need the appropriate charger connector (USB, iPhone, and others), and the cord/connector are subject to extensive use, abuse, and even vandalism. Anytime you can seal the interface between unit A and unit B, that's good in terms of public access, reliability, and options for mischief.

Wireless charging still faces some challenges, but there has been lots of progress in standardisation (admittedly with several competing standards), availability of needed ICs and other components, defining physical form factors, and other aspects. It's hard to say if it will catch on and have long-term success, as there are associated costs with respect to components, size, and charging time. If it doesn't work out, the traditional technique of connecting a charger via a cable assembly is still a viable and well-established fall-back alternative.

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 racier 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, it's not viable for pacemakers and other deeper implanted devices.

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