Researchers have combined an oxygen-sensing film and machine learning to create a wearable sensor capable of measuring tissue oxygenation through a person’s skin. Developed by researchers at the Wellman Center for Photomedicine at Massachusetts General Hospital and Harvard Medical School, the sensor works by detecting the phosphorescence lifetime and intensity of the acrylic oxygen-sensing film that adheres to the skin.

The researchers said that the wireless sensor can monitor oxygen levels on a continuous basis and is easy to operate, making it suitable for remotely monitoring oxygen levels outside of health care settings.

The sensor is worn like a wristwatch, though midway up the forearm, and consists of a 3D-printed casing, a small sensor head, and the film. Electronic components process data from the sensor and allow the device to send recordings via Bluetooth or Wi-Fi.

Two LEDs in the sensor head excite the film with ultraviolet light, and a photodiode detects the phase of the light that is emitted by the film in response to excitation. Comparison of the phase of the light emitted by the LEDs with the phase of light emitted by the oxygen-sensing provides a measure of the oxygen level in the tissue underneath the film.

The researchers used machine learning to train the sensor to accurately measure oxygen levels under different conditions. This approach also allowed the researchers to train the device to account for photobleaching — the tendency for light-excited materials to gradually lose their ability to emit light, which is a common limitation of devices based on the measure of light intensity.

To calibrate the sensor, the researchers exposed it to a variety of temperatures inside a sealed calibration chamber and made adjustments until the phases aligned with those of a commercial sensor.

The researchers tested the sensor on an in vivo pig model. When they attached the device to the front limb of a Yorkshire pig and applied a tourniquet over the pig’s elbow joint, the sensor detected a drop in oxygen level, reflecting the reduction in blood flow. The researchers’ measurements were aligned with those from a commercial reference sensor and were not affected by variations in temperature, humidity, or other environmental factors, indicating that the new sensor could be appropriate for use outside the lab.

“This is the first truly wearable noninvasive transcutaneous oxygen monitor,” researcher Juan Pedro Cascales said. “The simplicity, accuracy, small size, and ease-of-use of the device means it can go just about anywhere and be used by doctors, nurses, paramedics, and patients in their own home.”

Professor Conor Evans, principal investigator on the project, said that the sensor will be especially useful for medical situations where the traditional blood oxygen saturation tools fail to provide adequate information. “The device is intended for any scenario where there is a risk of compromised blood flow and a lack of oxygen to limbs and tissues,” he said. “The applications of this wearable wireless oxygen device range from traumatic injuries such as car accidents and battlefield injuries to post-surgical monitoring and wound care.”

The research team is currently carrying out its first in-human clinical trials. “We are also building smaller, more ergonomic and optimized versions of the device that can communicate with any smartwatch, smartphone, tablet, or computer,” Cascales said.

The research will be presented at the OSA Imaging and Applied Optics Congress and Optical Sensors and Sensing Congress, to be held virtually July 19-23, 2021.