Optical fibre CO2 sensing for monitoring the effects of tissue loading

Afroze, Nadia (2023) Optical fibre CO2 sensing for monitoring the effects of tissue loading. PhD thesis, University of Nottingham.

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The ultimate objective of this PhD thesis is to develop and characterise optical fibre sensors for carbon dioxide (CO2) monitoring during tissue loading. CO2 is a reliable marker for the early detection of pressure ulcers (PUs). In this thesis, a reflection mode optical fibre CO2 sensor (OFCS) was developed for early prediction of PUs formation.

Transcutaneous CO2 probes based on electrochemical sensing are commercially available for measurement of arterial partial pressure. It is therefore important to consider the advantages of OFCS in terms of device form and performance. The most important potential advantage is that an OFCS is very thin (core diameter, Ø 62.5 µm, cladding diameter, Ø 125 µm) and flexible and so could be used for continuous monitoring during patient treatment or during everyday life. Furthermore, electrochemical sensors require skin heating, temperature calibration and use of contact gel; OFCS can potentially monitor without. Therefore, if comparable performance to electrochemical sensing could be achieved, then there is an opportunity for OFCS to be widely adopted for monitoring tissue breakdown.

Skin damage is quite a slow process (1-2 hrs to develop a grade 3 or 4 PUs); a commercially available transcutaneous CO2 probe (tC Sensor 92) response time is around 78 seconds, which is acceptable to clinicians and requires no further improvement. Therefore, a response time of approximately 3 minutes would be acceptable for monitoring PUs. The tC Sensor 92 probe measures the partial pressure of CO2 (PCO2) in units of mmHg or kPa. The measurement range varies according to the different loading amounts on the skin, such as the PCO2 value of 36-44 mmHg (for 40 mmHg or 5.3 kPa loading), 50-60 mmHg (for 80 mmHg or 10.7 kPa loading), and > 80 mmHg (for 120 mmHg or 16 kPa loading). Therefore, an OFCS would need to be able to discriminate these ranges.

The optical fibre tip was coated with thymol blue (TB) using a sol-gel coating process. The highest absorption peak of the OFCS was achieved at a wavelength of around 600 nm. The OFCS had response and recovery times of 60 seconds and 413 seconds, respectively, in the range of 0 ppm (0% CO2) to 50000 ppm (5% CO2). Light from a halogen light source illuminated the optical fibre tip, and the reflected light (proportional to CO2) was detected.

The OFCS has been calibrated in a gas chamber alongside the tC Sensor 92 probe and a commercial CO2 datalogger. The humidity effects on the OFCS were nullified using a polytetrafluoroethylene (PTFE) film as a barrier between the sensor and external environment.

Further characterisation of the sensing film has also been conducted. The dye characteristics such as transmission and absorption spectra of TB were observed by varying pH values and CO2 flow. The highest absorption peak of TB (red), TB (yellow), and TB (blue) were achieved at a wavelength of 549, 452, and 606 nm, respectively. The different shapes of the spectrum of TB according to pH variation and CO2 flow were illustrated using skewness and kurtosis as an alternative to attenuation measurements at a single detection wavelength.

The TB concentration was calculated from the film thickness of the dried film on the OFCS tip. This film thickness was measured using scanning electron microscope (SEM) images. These values were used as input parameters in a simulation of the fibre tip as a Fabry-Perot interferometer (FPI). The highest absorption peak of the FPI was achieved at a wavelength of around 600 nm, and the spectrum of TB contains interference fringes. The fringe visibility was calculated from the simulated reflection spectrum of the FPI. The relationship between fringe visibility and film thickness helped to choose 7.234 µm as the optimum film thickness.

All three CO2 sensors have been used to measure the skin CO2 with and without weight loading. The PCO2 emitted from the human forearm has been measured using the tC Sensor 92 probe without weight loading at sensor temperatures of 37 °C and 42 °C for 30 minutes. The developed 10 OFCSs (OFCS 1-10) and a commercial CO2 sensor were validated on 10 volunteers’ (Volunteer 1-10) forearms and measured the changes in CO2 concentration during loading at a range of 32 °C and 43.4 °C for 20 minutes. The gas flow was stopped when the skin was loaded, which allowed CO2 to build up in the tissue. The OFCS 1-6 measured the CO2 changes on the volunteers’ (Volunteer 1-6) skin and in the air successfully, but unfortunately, the OFCS 7-10 couldn’t measure the CO2 changes on the volunteers’ (Volunteer 7-10) skin and in the air. The commercial CO2 sensor also successfully measured the CO2 changes on the volunteers’ (Volunteer 1-10) skin and in the air. The minimum and maximum achieved skin CO2 concentration was 149 ppm and 429 ppm, respectively, over 3 trials.

In conclusion, promising results from the human forearm using the OFCS and commercial CO2 sensor were achieved. Key improvements of the OFCS are identified which, with future work, will lead to improved performance for eventual deployment in the clinic.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Morgan, Steve
Korposh, Serhiy
Hayes-Gill, Barrie
Correia, Ricardo
Keywords: CO2, PUs, TB, OFCS, volunteer, intensity, absorbance
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA1501 Applied optics. Phonics
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Electrical and Electronic Engineering
Item ID: 76300
Depositing User: Afroze, Nadia
Date Deposited: 14 Dec 2023 04:40
Last Modified: 14 Dec 2023 04:40
URI: https://eprints.nottingham.ac.uk/id/eprint/76300

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