Sticking Bluetooth in a dead pig wearing a T-shirt to improve medical implants, Ars Technica

        

Although pacemakers led the way, I think it’s reasonable to expect that our human bodies are going to end up containing many more active devices. These might mean long-term implants that provide aid to problematic organs, or they could be temporary devices that provide evidence to diagnose a condition or deliver medicine to a specific location. Many of these devices would need to be able to communicate with the outside world, which has proven to be more difficult than expected. The solution, however, may be as simple aschanging your shirt.

I was surprised to learn that internal medical devices consume relatively high levels of power as they communicate with the outside world. It’s not that things like low-energy Bluetooth don’t work, but they use far more power when transmitting from inside the body than they do outside of the body. The problem turns out to be something called “total internal reflection.”

Totally into reflection

You may remember learning about Snell’s law and total internal reflection at school, but if not, don’t worry — I’m a trained explainer. When light travels across an interface between two materials, its direction of travel will change. Going from something like air to glass will result in a light ray traveling closer to perpendicular to the interface. But going from glass to air results in a light ray ending up with an angle closer to parallel. If the angle of the ray in air is already close to parallel to the interface, the light ray in the glass cannot bend far enough; instead, it’s reflected. Since the light wave isinsidethe glass, we refer to this as total internal reflection.

Radio waves and microwaves are just light waves with a much longer wavelength. Snell’s law still applies, and total internal reflection still happens. For normal-use transmitters, total internal reflection almost never occurs.

But for a medical device, the transmitter is inside your body. When we break the radio waves emitted by the transmitter up into rays, they are all pointed outward from the transmitter at an angle to one another. Many of those rays are completely reflected at the skin. And because a body is approximately elliptical, the reflected wave will hit the body at some other location where it will be totally internally reflected again (any reflection more than eight degrees from perpendicular will be reflected). The wave simply cannot leave the body.

To put it more technically: the Bluetooth radiation will only be efficiently transmitted if the curvature of the emitted waves matches the curvature of the skin (ie, all rays hit at right angles to the surface). That happens to be difficult to arrange for a tiny antenna in a device that, by design, moves around.

Opening an evanescent escape route

To solve this problem, a group of researchers returned to what we know about total internal reflection. The physics of the reflection process is not completely confined to the interface. The wave hits the interface, and a small amount of it hangs out on the other side for a bit before it is sucked back into the material and sent on its way. This bit on the wrong side of the interface is called an evanescent wave, and it dies away rapidly as we move away from the surface.

If the evanescent wave hits another interface (another person close by, for example), the wave can start to propagate again (inside the body of the second person). This wave could be detected if it weren’t actually in someone’s body. Putting Wi-Fi antennas in bags of water that are pressed against the skin would work (the human body is mostly water), but as solutions go, it is not very elegant and certainly not comfortable to wear.

So the researchers went with an alternative. Their solution is to create clothing that has patterns printed from conductive ink. These patterns are constructed in such a way that they collectively re-radiate the evanescent wave as if the bag of water was actually there.

Of course, this is not the sort of experiment that you can just immediately perform on people, so the researchers used a dead pig wearing a printed T-shirt instead. They placed a Bluetooth transmitter inside the pig and measured the battery life, signal strength, and latency. The signal strength was even higher compared to a naked pig, while at low transmitter power, latency was reduced by a factor of three. Battery life also increased from (hours to) hours.

In the future, keep an eye out for people wearing strangely patterned metallic T-shirts. They will either be the next generation of hipster, on some serious meds, or both.

Physical Review Applied, 2019, DOI:10. 1103 / PhysRevApplied. 12. 054020(About DOIs)