Circuit-Free Smart Patch Uses Smartphone Camera to Monitor Pulse and Vessel Stiffness

Optical wrist-worn sensors pose another challenge, as their performance may vary across skin types. To support more accessible monitoring, researchers have developed a circuit-free smart patch that uses a standard smartphone camera to measure pulse and blood vessel stiffness.

The smart patch was designed, manufactured, and tested entirely at Norwegian University of Science and Technology (NTNU; Trondheim, Norway). It is made from silicone rubber and does not require electronics, cables, an external power source, a spectrometer, or a laser. When placed on the skin, the transparent patch works with an ordinary mobile phone camera to assess cardiovascular function.

The technology uses a thin film of elastic polydimethylsiloxane (PDMS), a silicone rubber material, with two engineered surfaces. One side contains a nanoscale pattern that changes color when the material stretches. The other side has a three-dimensional adhesive structure inspired by octopus suction cups, allowing the patch to attach firmly to the skin.

Pulse waves from blood vessels cause small movements in the skin, which stretch the patch. That stretching produces measurable color changes on the nanostructured surface. Because the system relies on mechanical skin movement rather than skin light absorption, it differs from optical monitoring used in smartwatch sensors.

Testing showed that stretching the film by two percent produced a nine percent change in color intensity. The response remained stable through at least 1,000 stress cycles. The biomimetic adhesive structure improved skin adhesion by 65 percent compared with a flat PDMS surface.

In trials involving 13 volunteers, handheld mobile phones recorded detailed pulse waveforms that closely matched clinical reference measurements. The patch outperformed traditional photoplethysmography, image-based photoplethysmography, and piezoelectric pulse sensors. The findings were published in Advanced Materials under the title “Circuit-Free Cardiovascular Monitoring via Smartphone-Readable Skin-Interfaced Nanophotonic Films.”

“We wanted to develop a flexible, low-cost device. What makes it unique is that we can clearly see the signal from the heart on the patch with our own eyes,” said Angelos Xomalis, an associate professor of electronic systems at NTNU.

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