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Wearable Pulse Sensor Developed for Health Monitoring

Professor Ji-woong Yang from the Department of Energy Science and Engineering at the Daegu Gyeongbuk Institute of Science and Technology has achieved a breakthrough: creating the most powerful eco-friendly quantum dot photosensor globally, operating without an external power supply.

Wearable Pulse Sensor Developed for Health Monitoring

Image Credit: ACS Nano (2023).

The eco-friendly quantum dot photonic sensor, a result of collaborative efforts between Professor Moon-kee Choi's team at the Department of New Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), and Professor Dae-hyeong Kim's team at the Department of Chemical and Biomolecular Engineering, Seoul National University (under President Hong-lim Ryu), has been confirmed to reliably measure light signals without relying on any external power source, thanks to its utilization of the photovoltaic effect.

The collaborative research team has engineered a skin-attachable, remarkably thin pulse sensor using this photosensor technology. They have introduced a wearable pulse sensor capable of consistently capturing pulse signals even amidst various deformations. This groundbreaking work has been published in the journal ACS Nano.

The COVID-19 epidemic and the aging population have increased demand in recent years for healthcare monitoring devices that may be worn on the body for extended periods of time to collect biometric data.

Silicon-based photosensors, despite their functionality, are not widely adopted in practical applications due to their weight and inflexibility, leading to discomfort during prolonged wear. Moreover, their inability to establish close skin contact hampers the accurate acquisition of biometric signals.

Three scientists who discovered and developed quantum dots, commonly referred to as the seeds of nanoscience, were given this year’s Nobel Prize in Chemistry. With superior optical and electrical characteristics over conventional semiconductor materials, quantum dots - ultra-fine semiconductor particles that are only a few nanometers (nm, or one billionth of a meter) in size - can rapidly separate electrons and electron holes.

Quantum dots have been the subject of extensive photosensor research due to their quick response time when utilized as a photosensor. Nonetheless, most research employs quantum dots, such as lead sulfide (PbS), which includes hazardous heavy metals, and the majority of existing quantum dot photosensors are thicker than a few micrometers. Therefore, in practice, the dots cannot be used as a wearable photosensor.

Now, the research team has effectively created an ultra-high-performance quantum dot photosensor based on the environmentally benign, heavy metal-free copper-indium-selenide (Cu-In-Se) quantum dots. It was widely acknowledged that eco-friendly quantum dot-based photosensors performed poorly.

Despite these challenges, the research team successfully enhanced the electrical characteristics of eco-friendly quantum dots by meticulously managing their size and composition. They also devised a novel organic-inorganic hybrid charge transfer layer specifically tailored for these dots, creating an eco-friendly quantum dot photosensor that outperforms existing toxic quantum dot counterparts in terms of performance.

Even with a quantum dot absorption layer as thin as 40 nanometers (nm), the study team’s environmentally friendly quantum dot photosensor exhibits excellent gadget performance. It also performs well in terms of light-sensing without the need for an additional power supply. These two characteristics could be very helpful for the applications and usage of wearable photosensors.

The photosensor made on a flexible polymer substrate is combined with a light source by the research team to create a wearable pulse sensor. Once affixed to the human body, the sensor has the flexibility to function steadily within a 0.5 mm radius of curvature. It is also capable of measuring the pulse steadily in a variety of motion-filled scenarios, including walking and sprinting.

Professor Ji-woong Yang says, “By controlling the structure of eco-friendly quantum dots and developing a charge transfer layer optimized for the dots, we were able to make a high-performance eco-friendly quantum dot photosensor.”

We were able to create an ultra-thin pulse sensor with high flexibility based on the eco-friendly quantum dot photosensor that requires no external power source. It could be used for various next-generation photosensor applications, such as lidar and infrared cameras, as well as wearable health care monitoring systems.

Moon-kee Choi, Professor, Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology

Journal Reference:

Li, S., et al. (2023). Ultrathin Self-Powered Heavy-Metal-Free Cu–In–Se Quantum Dot Photodetectors for Wearable Health Monitoring. ACS Publications. /doi/10.1021/acsnano.3c05178

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