Wearable skin-like sensor maps blood-oxygen levels anywhere
Their paper “A flexible organic reflectance oximeter array” published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) details an array composed of four red and four NIR printed organic light-emitting diodes (OLEDs) and eight organic photodiodes, all addressed sequentially using analogue switches to drive the OLEDs and read out the OPD signal.
infrared light into the skin and detects the ratio of
light that is reflected back. (UC Berkeley photo by
Yasser Khan, Arias Research Group).
For their device, the researchers chose red (612nm) and NIR (725nm) OLEDs for which molar absorptivities of HbO2HbO2 and HbHb are significantly different, yielding good reading results for blood-oxygen levels. Taking reflectance oximetry to a large area with pixel-driven arrays means localized readings can be achieved over large skin areas for the 2D mapping of blood-oxygen levels across entire parts of the body, for example to monitor in real time the oxygenation (and healing) of tissues, wounds, skin grafts, or transplanted organs.
Another benefit of this flexible sensor approach over conventional and localized transmission-mode pulse oximetry (usually only performed at a fingertip or the ear lobe), is that oxygenation mapping can be performed even when there is poor blood circulation or no pulse.
In the case of a medical shock, low blood perfusion, or organ injury, the authors highlight, the pulsatile arterial blood signal of becomes too weak to be used for pulse oximetry. The researchers proved their setup on a forearm by occluding blood supply to the arm using a pressure cuff (as those used for measuring arterial tension).
of organic photo detectors (top) and organic red and
infrared LEDs (bottom). (UC Berkeley photo by
Yasser Khan, Arias Research Group).
The OPD and OLED arrays were fabricated on separate substrates and then assembled together to form a system flexible-enough to be comfortable to wear, yielding a high-fidelity sensor–skin interface with an excellent signal-to-noise ratio. Unlike fingertip oximeters, it can detect blood-oxygen levels at nine points in a grid and can be placed anywhere on the skin.
“All medical applications that use oxygen monitoring could benefit from a wearable sensor,” explained Ana Claudia Arias, a professor of electrical engineering and computer sciences at UC Berkeley. “Patients with diabetes, respiration diseases and even sleep apnea could use a sensor that could be worn anywhere to monitor blood-oxygen levels 24/7.”
Taking commercial transmission-mode pulse oximeters as a benchmark, the authors reported a mean error of only 1.1% when taking measurements with their reflectance oximeter on the forehead. For this experiment, the complete ROA was just 43x43mm wide with discrete OLEDs and OPDs active areas of 7x7mm at a 5mm pitch but such sensors could be made on larger PEN substrates and at a higher resolution.
2 pixels in each row. (right) The Red and NIR
OLED arrays in operation.
The printed ROA was interfaced with the control electronics via flexible flat cable connectors. Each pixel of the ROA was composed of one red and one NIR OLED and two OPDs, enabling the signals from the red and NIR channels to be read out sequentially using the two OPDs.
This work was supported in part by Cambridge Display Technology Limited and by Intel Corporation via Semiconductor Research Corporation Grant No. 2014-IN-2571.
University of California Berkeley – www.berkeley.edu
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