Radio waves are poorly suited for water-based environments such as the ocean, or even the body. A relatively large amount of energy is required for RF signal propagation in water because it is efficiently absorbed across a wide frequency range. For implants, significant heat would be generated at the site of the radio transmitter, and batteries would be quickly depleted. Implants that need to talk to each other inside the body face many of the same issues as submarines, which use sonar for communication and enemy detection. The solution for implants then, is to adopt a sound-based protocol — in other words, an ultrasonic body area network.
Tommaso Melodia, a researcher at the University of Buffalo, has just received a National Science Foundation (NSF) grant to the tune of a half a million dollars to develop such a network. The majority of the groundwork will be to conduct detailed simulations of the propagation of ultrasound inside various tissues. Although doctors routinely use ultrasound for diagnostics, the energy is typically delivered from outside the body. When internal emitters are required to image things like the heart, the transducer can isolate inside a probe in the esophagus, or inside a catheter tip in the vasculature. Here, the surrounding blood can, in theory, help to dissipate any heat. Once the safety details for ultrasonic communication are worked out, the challenge of designing network protocols that are ideally suited to the particulars of a sonic environment can be addressed.
It would be convenient if each device added to the body could just be assigned an IP address and be assimilated into the local ethernet. While we may eventually get to that point, the first devices may more likely use much simpler protocols. For example, if an implanted insulin pump needs to poll glucose levels from sensors in a just few problem areas, like maybe an ailing retina, one-way communication would probably suffice. If, however, your diabetes has already won you an Alpha IMS vision implant, your retina may already be full to the hilt with hardware. In that case it would be nice if the vision implant already had a glucose sensor that spoke some common language.
What might the ultrasonic language look like?
Although there are many ways it can be generated, sound energy does not come free. Creatures like bats and dolphins work hard to produce it, and have already figured out ways to get the most bang for their buck. In a crowded environment like a cave, or an undersea hunting pod, the difference between communication pulses and imaging pulses becomes blurred. For ultrasonic implants, communication is the main priority although imaging functions will likely follow. Like a dolphin resonating the swim bladder of its prey, imaging implants can also become assault implants. Sentinel devices might be poised at critical points in the circulation, dispersing gaggles of platelets threatening to form a clot, or busting cancer cells with beams of directed energy. To imagine what shape implant communication pulses might take, we need search no further than the backyard on a spring morning.
Communicating birds face a tough challenge of recognizing friends amidst the overwhelming clamour of many species. Sound is not like radio where there are many megahertz-wide channels. For practical purposes, there really is only one channel — low frequency. For implants, the lower end will probably need to bottom out well above 20 kHz or so, while really high frequencies probably won’t get too far. (Below 20 kHz you are beginning to interfere with the auditory system. While not directly audible at that frequency, outer hair cells in the cochlea can oscillate above 20kHz to fine-tune hearing.) The solution that many birds — and other creatures — use is to rapidly sweep across the frequency range in a short “chirped” pulse. While these chirps are great for sonar-style searches, they also seem to be ideal in many circumstances for communications, and to impart recognizability to emitters and receivers.
There will no doubt be many initial solutions to the problems arising in ultrasonic communications. Piezo-based emitters that can also act as receivers, or even power transducers, may be one technology ripe for exploration. Other unforeseen technologies may soon follow.
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