A smartphone app has been developed that monitors blood pressure by using a simple and cheap 3D-printed plastic attachment that clips in front of the camera flash.

High blood pressure is an underlying trigger for many of the world's deadliest medical conditions, and the new device, which could cost as little as 10 cents, could help make regular checks more easy, affordable, and accessible.

A key advantage is it avoids the problems of blood pressure cuffs. Using one that is too large or small, or using one incorrectly, can give inaccurate readings and thus a false sense of security, or of alarm.

A paper describing the invention was recently published in Scientific Reports, and the next steps would include making the technology more user-friendly, especially for older adults, testing its accuracy across different skin tones, and creating a more universal design.

"Because of their low cost, these clips could be handed out to anyone who needs them but cannot go to a clinic regularly," said senior author Professor Edward Wang, director of the UC San Diego's Digital Health Lab.

"A blood pressure monitoring clip could be given to you at your checkup, much like how you get a pack of floss and toothbrush at your dental visit."

To measure blood pressure, the user simply presses the clip. A customized app guides them on how hard and long to push during the measurement.

The clip features an optical design similar to that of a pinhole camera. The smartphone's flash lights up the user's fingertip. It's then projected as an image of a red circle.

By looking at the size of the circle, the app can measure the amount of pressure the fingertip applies, while the brightness of the circle depicts the volume of blood going in and out, and an algorithm converts this information into systolic and diastolic blood pressure numbers.

"Using a standard blood pressure cuff can be awkward to put on correctly, and this solution has the potential to make it easier for older adults to self-monitor blood pressure," said co-author Dr. Alison Moore, a geriatrician at the UC San Diego School of Medicine.

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