We’ve known for some time that Apple was looking for ways that the Apple Watch could be used to measure blood pressure. Back in June 2016 the company filed a patent application for a blood pressure monitor. That patent was for a wearable blood pressure monitor - a cuff that would be worn on the arm. That patent application is here.
Apple has continued to investigate the blood pressure technology and appears to have come up with a way to actually incorporate it into the Apple Watch - rather than require a separate device. Just a few months later (in September 2016) Apple filed another patent, this time for a “wrist-worn device to measure blood pressure”. US Patent No 10,646,121 (here) describes how blood pressure could be accurately measured without a bulky cuff - instead Apple would incorporate sensors into the watch (although the company doesn’t actually refer to the Watch in the patent, just a wrist worn device).
Apple’s patent application explains the method and discusses the challenges associated with getting an accurate measurement. Apple’s method is based on Applanation Tonometry - which is a test used to measure fluid pressure in the eye (definition here). This process measures how much pressure is required to temporarily flatten the cornea.
Apple describes how its device could apply pressure in a similar way to the radius artery on the wrist (this artery lies on the front of the forearm near the wrist and close to the surface.)
Apple's patent describes two layers of capacitive sensors concealed under the bracelet. These are set up in a criss-cross fashion. The sensors are pressed into the user's skin by a device inside the bracelet. The pressure causes the upper and lower sensors to touch where they cross - creating an electrical signal that is evaluated by the watch. Thereby the blood pressure is measured.
Apple claims to be able to filter out the pulse and measure the median blood pressure accurately.
According to Apple, the pressure in the bracelet can be generated in different ways: by an air pump or directly by a small motor. It seems that a modification of the Taptic Engine is used.
What is striking about this patent is that Apple relies on a bracelet as a dedicated device for measuring blood pressure rather than a bulky cuff. It is conceivable that Apple could not include the technology in a new Apple Watch - instead selling it as a fitness accessory (that could also be used with older models).
As always with patents, caution is required: patented technology does not mean that it will ever make it into production, let alone on the shelves in an Apple Store.
This story includes reporting from Macwelt.