Vital Signals launches the $399 Signal Ring, which provides systolic and diastolic blood pressure readings without the need for the traditional arm cuff (Chris Welch/Bloomberg)

Chris Welch / Bloomberg: Vital Signals launches the $399 Signal Ring, which provides systolic and diastolic blood pressure readings without the need for the traditional arm cuff — A three-year-old startup says it has produced a smart ring that can track blood pressure as accurately as a traditional arm cuff and that …