Smart Ring

Overview

A smart ring compresses the sensing core of a fitness wristband into a band of jewellery weighing four to six grams. It measures heart rate, heart-rate variability, blood-oxygen saturation, skin temperature, movement, and sleep, syncing to a phone over Bluetooth Low Energy. The finger is a better measurement site than the wrist, the digital arteries on the palmar side sit close to the skin and perfuse strongly, but the form factor is brutal: the entire electronics stack must fit in an annular cavity roughly 2.5 mm deep, survive permanent immersion-grade sealing, and run for days on a battery smaller than 25 mAh.

The construction answers those constraints directly. A Titanium Shell of machined titanium forms the structure, a Main Flex Circuit wraps the electronics around the inside, the Sensor Array faces the skin, and an Epoxy Potting Fill pots everything solid, there are no gaskets, screws, or serviceable parts.

Sensing from a finger

The core measurement is photoplethysmography. The Green PPG LED emitters pulse 525 nm light into the finger; with each heartbeat the arteries distend, absorb slightly more light, and the PPG Photodiode sees the returned intensity dip. The Optical Analog Front End sequences the LED flashes, subtracts ambient light, and digitises the waveform, from which firmware extracts pulse rate and beat-to-beat intervals for heart-rate-variability analysis. Green light is used for rate because its shallow penetration makes the signal robust to motion; the Red / Infrared LED pair at 660 and 940 nm penetrates deeper, and the ratio of absorption at the two wavelengths tracks haemoglobin oxygenation for SpO2 estimates during sleep.

The Skin Temperature Sensor reads skin temperature against the finger every minute overnight, resolving trends of a few hundredths of a degree, fine enough to show menstrual-cycle phase shifts and the temperature rise that often precedes illness. The Accelerometer IMU counts activity, detects sleep-stage movement patterns, and gates the optical sensors so PPG reads are taken when the hand is still. The Sensor Contact Domes press the optics lightly into the skin, and opaque barriers in the Optical Sensor Windows stop LED light short-circuiting straight into the detector.

Living on 20 mAh

Power defines the firmware design. The BLE SoC spends almost all of its life asleep at single-digit microamps, waking on a schedule to fire a sensing burst, crunch it locally, and append results to the Buffer Flash Memory buffer, which holds about a week of data when the phone is out of range. Radio time is minimised by syncing summaries rather than raw waveforms. The Power Management IC gates rails so unused sensors draw nothing, and the Fuel Gauge IC coulomb-counts the Curved LiPo Cell, an arc-shaped lithium-polymer cell moulded to the ring's curvature. The budget works out to 4–7 days per charge; larger ring sizes fit physically larger cells and run longer.

Getting 2.4 GHz radio out of a metal torus is the other hard problem. A continuous titanium band would form a shorted turn around the antenna, so the RF Window Slot interrupts the metal with a resin-filled slot and the BLE Antenna is tuned to radiate through it.

Charging and build

Exposed charging contacts corrode in sweat, so charging is inductive: the dock's Dock Transmitter Coil couples power through the resin into the Charging Receiver Coil, the Rectifier / Charge IC applies a CC/CV profile, and the Alignment Magnet snaps the ring into rotational alignment on the Dock Housing post. A full charge takes about 80 minutes from the dock's USB-C input.

The Titanium Outer Band is CNC-machined from Ti-6Al-4V and finished with a PVD Surface Coating for colour and scratch resistance; titanium is chosen for its strength-to-weight ratio and skin biocompatibility. Assembly ends with potting: the flex, cell, and sensors are fixtured in the shell and flooded with biocompatible epoxy, which is why smart rings are waterproof to 10 ATM and equally why the battery cannot be replaced, when the cell fades after two to three years, the ring is done. Sizing is also permanent, so vendors ship plastic fitting kits before the real ring, and users are advised to size for the finger's larger daily state since fingers swell measurably overnight and in heat.