Several companies are experimenting with reducing the size of the wearable device from a wristband to a ring. One example is the Signal Ring, a new finger-worn blood-pressure monitor developed by the San Francisco startup Vital Signals (Figure 1). The ring is usually worn on the index finger and connects by Bluetooth to an iOS or Android phone. Unlike general-purpose wearable rings that emphasize sleep, activity and recovery (QH), Signal is being marketed primarily as a blood-pressure device. Vital Signals nevertheless describes it as a wellness product rather than a medical device, and says that its readings are informational and should not be used by themselves to diagnose or treat hypertension.
The hardware employs reflective PPG, the same optical technique used by many watches and fitness trackers to measure heart rate. Light-emitting diodes shine into the finger, and a nearby photodetector measures how much light returns. With each heartbeat, arteries in the finger fill and relax, changing the local blood volume and therefore the amount of light absorbed and reflected. The resulting waveform contains information not only about pulse rate, but also the pulse amplitude, rise time, contour, and timing. These properties reflect the interaction among cardiac output, the traveling pressure wave, and the elastic properties of the arterial wall. Counterintuitively, the finger is a sensible location for such a monitor because it provides a strong peripheral pulse signal. Vital Signals says that its sensor samples the pulse four to five times faster than the optical sensors in typical wearables, allowing it to detect details that lower-speed sensors miss.
A fundamental challenge is that these devices are measuring a change in blood volume rather than pressure itself. In simplified terms, pressure change equals volume change divided by arterial compliance, the ease with which the artery expands which depends on vascular tone and elasticity. Compliance varies with age, temperature, stress, posture, exercise and vascular disease. A stiffer or narrower artery can alter the speed and shape of the pulse in a way that resembles higher pressure, even when pressure itself has not changed. Vital Signals says that a proprietary physiological model measures these vascular changes directly from the high-resolution pulse waveform, and then separates the changes in pressure from changes in vascular compliance. In this manner, the PPG measurement of blood volume oscillations can be converted into pressure units.
This can be a tricky conversion because of all the variables and uncertainties involved, and as a result most cuffless devices require regular comparison with a conventional cuff for calibration purposes. According to Vital Signals, Signal Ring never requires calibration, not during setup and not later.
In terms of accuracy, the company has claimed the ring has met ISO 81060-2 requirements based on a 451-person, multisite clinical study. To meet the standard, the average difference from the reference method must be no more than 5 mmHg, with a standard deviation no more than 8 mmHg for both systolic and diastolic pressure. However, the company has not yet publicly released the study’s numerical results, participant characteristics, blood-pressure range, clinical sites, reference-measurement procedure or performance in different skin tones, ages, body positions and real-world activities. In addition, there is no peer-reviewed paper, public trial registration or independent replication of the study. As a result, the Signal Ring has not been reviewed or cleared by the FDA.
As a reality check, a published 2024 study of a different PPG blood pressure ring reported 526 systolic and 513 diastolic pairs from 89 people; average differences from sphygmomanometry, were 0.16 ± 5.90 mmHg systolic and −0.07 ± 4.68 mmHg diastolic, well within the ISO 81060-2 standards. However, the measurements were made under controlled conditions and presumably used calibration. This small study needs to be replicated to build confidence in the capabilities of the ring form factor to measure blood pressure.
For those interested, the Signal Ring is available for preorder from the company for $399, with no subscription fee. The price includes the ring, a charging case, and a sizing kit; sizes 5 through 13 are offered. Purchasers receive a sizing kit before the actual ring because proper fit is important for obtaining a reliable measurement. The product is initially being sold only in the United States. Sizing kits are expected to begin shipping at the end of July 2026 and initial rings are expected to ship in October 2026. The titanium-and-resin ring is rated IP68 (complete dust protection and continuous water immersion), requires a nearby phone for readings, and is advertised for up to five days of use with the charging case. The company advertises a one-year limited warranty and a 30-day return period. The company's own release says Signal Ring is investigational and has not been reviewed or cleared by the FDA as described above.
Signal occupies an interesting niche among wearable blood-pressure devices. The Omron HeartGuide (QH) and Huawei Watch D2 put a miniature inflatable cuff in the watch strap; they are bulkier and the measurement is intermittent, but their method remains close to conventional sphygmomanometry. Certain versions of Samsung's Galaxy Watch use wrist PPG to produce numerical readings (after three initial upper-arm cuff calibration measurements), and requires recalibration every 28 days. The optical Aktiia/Hilo bracelet also uses personal baseline and monthly calibration. In a manufacturer-involved one-month study of 86 adults, it reported differences of 0.46 ± 7.75 mmHg systolic and 0.39 ± 6.86 mmHg diastolic in accepted seated measurements (QH). Apple has taken a more conservative route; its FDA-cleared watch feature looks for patterns associated with hypertension over 30 days but does not display blood pressure values, and instead notifies users that they may have signs of chronic hypertension, which should be confirmed by a medical professional.
Signal Ring would be more convenient and informative than all of the above if it can supply reliable numbers and background trends without either an inflatable strap or calibration. It would be able to provide the most "continuous", "real-time" blood pressure monitoring among all the options. The ring takes background readings throughout the day (the time interval has not been specified by the company) which are combined into a full day blood pressure portrait. It also possesses a user-activated Realtime Tracking mode which takes measurements every 10 seconds.
ChatGPT summed up the Signal Ring as "a promising device with a missing results table." Signal Ring addresses a real limitation of blood pressure monitoring. A comfortable ring could collect information at times when few people would bother to put on a cuff, potentially revealing patterns that an occasional snapshot misses. Their technology which includes high-speed finger PPG plus a model that accounts for vascular tone is plausible, and the reported 451-person study could be convincing if more details are disclosed. The details matter in terms of how far individual readings differed from the reference, who was tested, which readings failed, and whether performance persisted across the conditions in which the ring will actually be worn. In other words, the missing results table needs to be filled in, and that would be the first step towards obtaining FDA authorization and moving from curiosity to health-informing device.
Figure 1. The mysterious and intriguing Signal Ring. How accurately can it measure blood pressure? Its ring form factor is very convenient (https://signalring.com/).
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