Blinking lights shining on our skin to make biometric measurements have become commonplace. Photoplethysmography (PPG) is used in fitness trackers, smart rings, and even some earbuds to measure heart rate, blood oxygen levels, and more. But like beauty, the sensitivity of these devices is only skin deep. Their sensors cannot penetrate deeply into the body.

Researchers at the University of California San Diego (UCSD) have come up with a possible solution that relies on not just light, but sound as well. Their flexible photoacoustic sensor is designed to diagnose and monitor a range of conditions. One of the key missions is to monitor hemoglobin levels in deep tissues in the body. Tracking blood perfusion or accumulation in a target location can be essential in some circumstances.

The patch uses a matrix of laser diodes along with piezoelectric transducers embedded in a silicone plastic layer. It works by sending pulses of light that can penetrate deep into the body. The target biomolecules in those tissues respond to the light by vibrating, which causes acoustic waves to radiate out from that spot. The piezoelectric transducers convert those sound waves into electrical energy that can be measured and analyzed. The result is a 3D spatial map of the location of the target biomolecules, without the need for potentially harmful X-rays or complex and expensive MRI equipment.

By choosing lasers with different wavelengths, different biomolecules can be targeted by this approach. This technology could be used to detect and monitor conditions such as hemorrhages, blood flow restriction in heart attacks or strokes, or the presence of malignant tumors deep in the body. And the wearable design makes it suitable for long-term monitoring, rather than the snapshots offered by traditional 3D medical imaging technologies.