Flexible Graphene Photodetectors Accurately Measure Health Data in Wearables

Flexible Graphene Photodetectors
Photo credit: Alina Hirschmann, ICFO–Institut de Ciencies Fotoniques (via Science Advances)

Wearable health and wellness devices such as smartwatches and fitness trackers use optical detection to monitor vital health data. However, this rigid technology hinders the performance and form factor of the device. Now, in a study published in Science Advances, researchers demonstrated a new class of flexible and transparent wearables based on graphene sensitized with semiconducting quantum dots (GQD). The researchers showed several prototype wearable devices that are able to monitor vital health signs noninvasively, including heart rate, arterial oxygen saturation (SpO2), and respiratory rate.

Our skin is a unique interface for electronic devices to assess the current wellness and health status of our body.

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Optical vital sign monitoring is typically performed through photoplethysmography (PPG). PPG-based wearables are made of discrete, rigid silicon photodiodes that cannot interface conformably with the skin and thereby reduce the accuracy of the data. At the same time, bulky readout electronics and rigid sensors make the wearable uncomfortable to the user, reports Science Advances.

Because of its flexibility and transparency, graphene is a viable material for wearable sensors. Especially, GQD (graphene sensitized with semiconducting quantum dots) photodetectors offer key benefits such as broadband wavelength sensitivity and high responsivity due to a built-in photoconductive gain.

Flexible Graphene Photodetectors
Photo credit: Emre O. Polat, ICFO–Institut de Ciencies Fotoniques (via Science Advances)

For their study, the researchers integrated flexible photodetector in a flexible bracelet and on a mobile phone screen, allowing users to track heart rate and respiration rate from a range of body locations. They used reflectance mode PPG to extract the heart rate of the user from the wrist. On the other hand, they used transmission mode PPG to monitor the heart rate of the user from a finger placed on the health patch.

In this case, the health patch uses the ambient light passing through the tissue, which then reaches the. Because of the broadband wavelength sensitivity of the GQD PD, the health patch can operate in transmission mode using solely ambient light because light of higher wavelength penetrates further into the skin. “This allows a wearable device to detect vital signs that require continuous tracking over a long time. Because of the absence of an external light source, the power consumption of the integrated wearables is very low and limited to the dissipation in the PD and the readout electronics,” the researchers wrote. “Mass-produced and low-cost devices are within reach for this flexible wearable platform, as it is fully compatible with scalable CVD growth and roll-to-roll transfer processes,” they concluded.

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The research was carried out by scientists from ICFO–Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Institució Catalana de Recerça i Estudis Avançats, and Lluis Companys, in Barcelona, Spain.