Ultralow-Power Carbon Dioxide Sensor for Real-Time Breath Monitoring

Nanoscience Electrical Engineering Chemistry Materials Science Engineering Life Sciences Bioengineering Medicine Respiratory System
carbon dioxide sensor breath monitoring optochemical sensor low power consumption real-time monitoring

Research on an Ultra-Low Power Carbon Dioxide Sensor for Real-Time Breath Monitoring

Academic Background

Carbon dioxide (CO₂) is a critical gas produced during human respiration, and its real-time monitoring is essential for diagnosing and treating respiratory diseases (e.g., asthma, dyspnea, sleep apnea) as well as metabolic disorders. Traditional CO₂ monitoring methods, such as arterial blood gas analysis, are invasive and unsuitable for long-term continuous monitoring. Although non-dispersive infrared (NDIR) sensors are widely used, their large size and high power consumption limit their application in portable devices.

In recent years, optochemical sensors have emerged as potential candidates for non-invasive CO₂ detection due to their compact size and high sensitivity. However, the short lifespan and dye photobleaching issues of optochemical sensors have hindered their application in long-term breath monitoring. To address these challenges, Kim et al. developed a novel optochemical CO₂ sensor based on a fluorescent pH indicator, featuring ultra-low power consumption and enhanced stability, paving the way for real-time breath monitoring.

Source of the Paper

This paper was co-authored by Minjae Kim, Dongho Choi, Chan-Hwi Kang, and Seunghyup Yoo from the Korea Advanced Institute of Science and Technology (KAIST). Published on May 16, 2025, in the journal Device, the paper is titled Ultralow-power carbon dioxide sensor for real-time breath monitoring.

Research Process and Results

1. Sensor Design and Material Development

The researchers designed a sensor based on a flexible circuit, with the core component being the fluorescent pH indicator 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) as the CO₂-sensitive material. HPTS is excited at specific wavelengths, and its fluorescence intensity changes with CO₂ concentration. To enhance the sensor’s stability and light utilization, the researchers used poly(propyl methacrylate) (PPMA) as the polymer matrix and introduced a gas-permeable polyvinylidene fluoride (PVDF) scattering layer.

2. Sensor Fabrication and Integration

The sensor consists of an organic photodiode (OPD), LEDs, an HPTS film, and a PVDF scattering layer. The LEDs emit light at wavelengths of 400 nm and 470 nm, which are used to excite the reference and sensing signals of HPTS, respectively. By optimizing the operating voltage and duty cycle of the LEDs, the researchers reduced the power consumption of the light-emitting components to 171 mW. Additionally, the sensor uses a flexible printed circuit board (FPCB) for electrical connections, ensuring excellent bending performance.

3. Sensor Performance Testing

The researchers conducted comprehensive tests on the sensor’s response time, stability, and accuracy. During CO₂ concentration changes, the sensor demonstrated rapid response (rise time of 9.2 seconds) and recovery (18.6 seconds), outperforming commercial NDIR sensors. Longevity tests showed that the sensor could operate continuously for over 9 hours with an error of less than 5%. Furthermore, while the sensor exhibited significant measurement errors in dry environments, it quickly recovered upon re-exposure to humid air.

4. Real-Time Breath Monitoring Demonstration

To validate the sensor’s practical application potential, the researchers integrated it into a face mask for monitoring volunteers’ breathing patterns. The sensor successfully captured CO₂ concentration changes during inhalation and exhalation, generating a waveform similar to a capnogram. This high temporal resolution enables the sensor to monitor respiratory rates and perform breath waveform analysis.

Conclusion and Significance

This study developed an ultra-low power, highly stable optochemical CO₂ sensor, offering a new solution for real-time, continuous breath monitoring. The sensor’s rapid response, low power consumption, and long lifespan make it suitable for various applications in medical and industrial fields. Additionally, the study elucidated the origin of nonlinear responses and proposed optimization strategies for dye photobleaching, providing important theoretical guidance for the design of future optochemical sensors.

Research Highlights

  1. Ultra-Low Power Consumption: The power consumption of the sensor’s light-emitting components is only 171 mW, significantly lower than traditional sensors.
  2. High Stability: Through material and structural optimization, the sensor achieves continuous operation for over 9 hours with an error of less than 5%.
  3. Rapid Response: The sensor’s response time surpasses that of commercial NDIR sensors, making it suitable for real-time monitoring applications.
  4. Portability: The sensor’s compact size and lightweight design allow for easy integration into portable devices such as face masks.
  5. Theoretical Foundation: The study revealed the origin of HPTS’s nonlinear response and proposed optimization solutions for dye photobleaching.

Additional Valuable Information

The research team also plans to integrate a humidity sensor to further reduce measurement errors in dry environments, enhancing the sensor’s practicality. This multifunctional sensor combination holds great promise in areas such as personal protective equipment and industrial safety monitoring.