It was just last year that we heard about how scientists from Michigan State University had developed a smartphone case/app combo that could measure users' blood pressure. Now, they've created an app that does the job using nothing but an unadorned iPhone.

The prototype app utilizes Apple's 3D Touch feature, which premiered in the iPhone 6s and 6s Plus (similar technology is also now built into some Android phones). Basically, this function utilizes strain gauges beneath the touchscreen to measure how hard the user is pressing their fingertip on the screen.

The first time they use the app, the user is guided through a procedure in which they set the size of an onscreen target area for the pad of their index finger. In subsequent uses, that target appears as a rectangular box located at the edge of the screen, adjacent to the front-facing camera. Holding the phone at heart level, the user places their finger on the screen so it covers that box, along with the camera's lens.

Via an onscreen display, they're then guided in consistently applying finger pressure to the screen. As they do so, the strain gauges measure the pressure caused by blood pulsing through the fingertip's transverse palmer arch artery, while the camera visually detects blood volume oscillations. Using this data, an algorithm determines the user's current blood pressure.

In lab tests, the app has been shown to produce readings that are about as accurate as those from a conventional finger cuff. It is hoped that once it's developed further, the app could be available for consumer use by late 2019.

"Like our original device, the application still needs to be validated in a standard regulatory test," says lead scientist Prof. Ramakrishna Mukkamala. "But because no additional hardware is needed, we believe that the app could reach society faster."

A paper on the research was recently published in the journal Scientific Reports.

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