Paul van Gerven
21 February 2023

Using a purpose-built ultrasensitive photodiode, researchers from Eindhoven University of Technology and Holst Centre can measure heart and respiration at distances up to 130 centimeters.

Clips on fingers or electrodes on the skin: they’re convenient and effective methods of obtaining vital patient information, such as heart and respiration rate and blood oxygenation. But what if you could do all this without any skin contact? Remote monitoring would be easier to set up, more hygienic and more comfortable for patients.

Researchers from Eindhoven University of Technology (TUE) and TNO/Holst Centre have shown it’s possible. Using near-infrared light reflected from the patient’s skin onto a NIR photosensor, they were able to measure someone’s heart rate by detecting minute blood flow fluctuations in a person’s finger at a distance of up to 130 centimeters. Pointing the device at someone’s chest, they could derive the respiration rate. Importantly, these tests were performed in realistic conditions: indoors on a sunny day with the curtains only partially closed.

Riccardo Ollearo
Riccardo Ollearo shows how the photodiode (on the right) picks up the signal from his finger, allowing us to see how fast his heart is beating on the screen (left). Credit: Bart van Overbeeke

Noisy

The technique used by the Eindhoven team is called photoplethysmography. As complicated as that sounds, most people have actually had a simple photoplethysmogram taken when their doctor put a probe on their finger to measure heart rate and blood oxygenation. This pulse oximeter illuminates the fingertip using an LED and measures transmitted light using a photodiode, picking up changes in blood flow. Family doctors may use one to get a quick read on two basic parameters, more elaborate photoplethysmographs can collect a surprising amount of information about the circulatory system.

Commercial infrared cameras coupled with advanced algorithms have been shown to be capable of doing advanced blood circulation measurements at a distance, but that hardware is expensive and bulky. Additionally, camera systems invoke privacy concerns. Searching for a more cost-effective and practical solution, researchers at TUE and Holst Centre looked at a solution-processed, high-sensitivity photodiode to replace the cameras. Solution-processability ensures that mass production of these large-area devices isn’t cost prohibitive, while the sensitivity is needed to pick up a weak signal from a noisy background.

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Two for one

Joining forces with photodetector specialists from Holst Centre, TUE PhD student Riccardo Ollearo built a photodiode by combining an organic and a perovskite layer. Also seen in solar cells, this stacking strategy results in more electrons getting ‘freed’ by photons hitting. Ollearo’s device demonstrated respectable 70 percent quantum efficiency, meaning 70 electrons are collected for every 100 photons that hit the device.

Taking a cue from previous results in solar cell research, however, the young Italian researcher then shone some additional green light onto the photodiode. “I knew from earlier research that illuminating solar cells with additional light can modify their quantum efficiency, and in some cases enhance it. To my surprise, this worked even better than expected in improving the photodiode sensitivity. We were able to increase the efficiency for near-infrared light to over 200 percent,” Ollearo says. It’s because of this high efficiency that the photodiode can distinguish extremely weak infrared signals from a noisy environment.

TUE Holst photodiode artist impression
Setup showing how heart and respiration rate can be remotely monitored using infrared light. Credit: TUE

The researchers still don’t know for sure how the green light enhances the efficiency, but they do have a theory. “We think that the additional green light leads to a build-up of electrons in the perovskite layer. This acts as a reservoir of charges that’s released when infrared photons are absorbed in the organic layer,” says Ollearo. In other words, it’s two for one: for every electron that gets dislodged by an infrared photon, another one joins it.

Next, the researchers want to improve the device before clinically testing it. That will be someone else’s job, however, as Ollearo has run out of time. He will defend this thesis on 21 April.