Noninvasive CO2 Sensor Using Dual Photodetectors
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Solution Overview
Problem
Existing non-invasive sensors for measuring partial pressure of carbon dioxide (CO2) and other blood gases face challenges such as size constraints, sensitivity issues, calibration requirements, drift problems, and the use of costly and bulky laser light sources, which are not suitable for long-term monitoring and are prone to interference and temperature changes.
Innovation Solution
A sensor design featuring a broad band light source, a gas measuring chamber with a reflective inner surface, and dual photodetectors for calibration-free operation, where one photodetector detects light absorbed by CO2 and the other acts as a zero-reference detector, eliminating the need for external lasers and minimizing interference, with a compact design for efficient light transmission and auto-calibration capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If electrochemical sensors are used for non-invasive blood gas measurement, then accurate estimation of partial pressure of blood gases is achieved, but frequent calibration and drift occur requiring remembraning
Solution Approach 1:
The patent replaces electrochemical sensors with an optical sensor system that uses light absorption to detect CO2 concentration. The optical sensor includes a light source emitting at a wavelength absorbed by CO2, a measurement chamber for receiving skin-derived gases, and a photodetector to measure light intensity changes. This substitution eliminates the need for electrolyte solutions and membrane replacements required by electrochemical sensors, providing long-term stable operation without frequent calibration or remembraning.
Solution Approach 2:
The patent changes the detection parameter from electrochemical reactions to optical absorption. By selecting a specific wavelength where CO2 absorbs light strongly and using a photodetector to measure the absorbed light intensity, the system achieves accurate CO2 concentration measurement. The optical properties of CO2 (absorption coefficient at specific wavelengths) are utilized to provide stable, drift-free measurements over extended periods.
2Measurement precision
If laser light sources are used for optical sensing, then sensitivity is improved, but cost and device size increase significantly
Solution Approach 1:
The patent replaces expensive, bulky laser light sources with inexpensive LED light sources that emit at wavelengths absorbed by CO2. While LEDs have shorter lifetimes compared to lasers, they provide sufficient sensitivity for transcutaneous CO2 measurement and dramatically reduce cost and device size. The LED-based optical sensor can be manufactured as a compact, disposable, or long-term wearable device without the complexity of laser systems.
Solution Approach 2:
The patent extracts only the essential function of light emission at CO2-absorbing wavelengths, removing the unnecessary complexity of laser cavities, cooling systems, and precise wavelength control mechanisms. By using LEDs that naturally emit at appropriate infrared wavelengths, the system achieves the required sensitivity with minimal device footprint and low cost.
3Measurement precision
If laser wavelength is tuned for CO2 absorption, then measurement specificity is improved, but temperature drift causes wavelength instability
Solution Approach 1:
The patent uses LED light sources that emit at fixed wavelengths determined by their semiconductor bandgap, which are inherently stable and require no active wavelength control. The LED automatically provides light at the appropriate CO2 absorption wavelength without needing temperature compensation or modulation monitoring systems. This self-stabilizing property eliminates the drift problems associated with laser wavelength tuning.
Solution Approach 2:
The patent accepts the shorter lifetime of LEDs in exchange for their superior wavelength stability and low cost. LEDs maintain consistent emission wavelengths throughout their operational life without the temperature-induced drift that plagues laser systems, eliminating the need for expensive wavelength stabilization mechanisms.
4Use of energy by moving object
If fiber optic light guides are used to transmit laser light to sensor, then light transmission is achieved, but cables break due to stress over time
Solution Approach 1:
The patent merges the light source and sensor into a single integrated unit, eliminating the need for separate fiber optic cables to transmit light between components. The LED and photodetector are positioned in close proximity within the sensor housing, with the measurement chamber directly connecting to the skin surface. This integration removes the fragile fiber optic connection point that is prone to stress-induced failure.
Solution Approach 2:
The patent removes the fiber optic light guide component entirely from the system architecture. By using an integrated LED-based optical sensor, the system eliminates the external light transmission pathway that requires fragile cables, improving overall system reliability for long-term wearable or implantable applications.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The sensor provides reliable, long-term CO2 concentration measurements with short response times, is temperature stable, and avoids complex calibration procedures, offering improved sensitivity and efficiency while being cost-effective and resistant to temperature fluctuations.
Implementation Method 1
a broad band light source transmitting light into the gas measuring chamber, a detector system comprising a first and a second photodetector for detecting light transmitted from the light source through the gas measuring chamber, wherein the first photodetector detects light at a wavelength wherein CO2 absorbs light
Implementation Method 2
a detector system comprising a first and a second photodetector for detecting light transmitted from the light source through the gas measuring chamber
Implementation Method 3
a gas measuring chamber with a reflective inner surface
Data Source
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AI summary
Herein is disclosed a sensor for noninvasive measurement of the partial pressure of CO2 (pCO2) in the skin of a human. The sensor comprises a housing, a gas measuring chamber for measuring gases, at least one chimney for communication of gases diffusing through the skin to the gas measuring chamber, a broad band light source transmitting light into the gas measuring chamber and a detector system comprising a first and a second photodetector. The first photodetector detects light at a wavelength wherein CO2 absorbs light and the second photodetector acts as a zero reference detector by measuring light in a freeband where no gases absorb light.