No Charge for This Smart Ring

The nemesis of wearable device developers everywhere is undoubtedly batteries. You can't live with them, you can't live without them. They add bulk and inconvenience to wearables that might otherwise be able to transparently integrate into our everyday lives. These problems are especially pronounced when it comes to very small wearable devices, like smart rings. In these cases, the batteries have to be so small that frequent recharges are needed, or if not, they make the ring so large as to be awkward and unfashionable.

There are ways around these issues for some applications — like putting onboard microcontrollers into deep sleep states when not in use. But when continuous wireless communication is necessary, as may be the case with an input device, batteries are drained very rapidly. This has led researchers to explore the possibility of communicating wirelessly in alternative ways, such that a battery would not be needed.

One popular approach that has been experimented with involves the use of RF backscatter technology, which allows for battery-free data transmission. However, RF backscatter requires a large antenna for operation, so that does not really solve the problem for compact wearables. Near-field communication is a much more compact option. However, it can only communicate at distances of one to two centimeters, which is just not going to cut it for most use cases.

A team at the University of Tokyo and Meta Research Labs has recently reported on their work in the area of battery-free smart rings. They call their device picoRing, and it solves the problems exhibited by existing technologies, allowing it to wirelessly communicate at reasonably large distances without a big antenna. The unique two-part picoRing system is capable of continuously transmitting actions like button presses, slides, and scrolls without any source of power.

Well, sort of. The ring itself does not require a power source, but it must be paired with a wristband, which contains a rechargeable battery. This is not just a bait-and-switch, however, because a sizable battery can be more comfortably housed in a wristband, allowing the ring itself to remain small and practical.

The system relies on a principle called passive inductive telemetry in which the ring contains a coil connected to a capacitor which can be inserted into, or removed from, the circuit via a switch that is used as an input. This altered capacitance causes the resonance of the coil to change in a unique way. The wristband generates a weak magnetic field and couples with the ring. In this way, the resonance of the ring can be read by the wristband, and that reading can in turn be used to infer the state of the switch.

This arrangement does allow the ring to be very small, but the wristband does still have a fairly bulky battery. Furthermore, the wristband is not self-contained — it is wired to a vector network analyzer (VNA) that monitors the state of the ring. Another limitation of the system is that a ring can only have a single input interface. The team hopes to overcome these issues by experimenting with smaller VNAs and more functional switches, but the path forward is not entirely clear. It may be some time before picoRing makes its way out of the research lab.