MICA: Development of Boron Doped Diamond Based Transcutaneous Blood Gas Sensors for Improved Patient Ventilation Status Monitoring and Control

Dissolved oxygen and carbon dioxide blood gas levels in the body are key indicators of respiratory health status and thus represent an important diagnostic test for illness severity. This information is essential to enabling correct diagnosis and treatment of the patients' needs. However, if only infrequent monitoring of the respiratory gases is possible then life threatening changes go unnoticed which can lead to the patient suffering severe complications such as organ impairment and brain damage. The ability to continuously monitor thus provides considerable clinical advantages to the patient in both the efficacy of their treatment and long term prognosis for recovery. Continuous monitoring is even more important for the health-outcomes of the 300,000 patients treated each year in critical care, including the 70,000 critical care babies. The Covid-19 pandemic, which resulted in patients with severe respiratory distress has also further highlighted the need for continuous respiratory gas monitoring.
The current standard approach on hospital wards is to use the blood gas analyser. This is an stand-alone instrument which requires blood removal from the patient and placement of the blood in the instrument. The number of blood gas analysers per hospital is limited (due to cost), use requires trained staff and patient infection is possible when removing blood samples as a result of breaking the skin barrier. The number available and its mode of operation precludes real time measurement of patient ventilation status. This method whilst being non-ideal is particularly problematic in critical care or neonatal settings where the rapidly changing physiology of the patient, difficult to access blood vessels, small accessible blood volumes, and heightened distress caused by pain and blood loss complicate measurements. The infrequent nature of the measurement also precludes fast reactive treatment.
The aim of this work is to move away from periodic sampling and move towards on-skin transcutaneous sensors, to provide real time and continuous blood gas monitoring of the respiratory gases, without the need to withdraw blood from the patient. The sensors aim to offer responsive and continuous measurement of patient ventilation status, importantly with minimal input required from clinical staff for long term operation, non-invasively. Whilst transcutaneous sensors for carbon dioxide and oxygen exist their uptake into hospitals has been limited due to the below-expectation performance properties of the sensors. Current sensors use two different electrode materials which sense the two different respiratory gases by different measurements methods. The sensor responses, in practise, drift with time, this necessitates frequent removal of the sensors from the body, taking apart and reconstructing the sensor, recalibrating by flowing gas over the sensor, and then replacing on the patient. This results in many periods of no measurement, it is time consuming for clinical staff and requires the staff to have undergone the appropriate training.
We aim to address these issues by using a sensor material and measurement protocols which allow us to tackle the problems which currently hamper current transcutaneous sensors. The project builds upon the world leading achievements of the research team in the field of electrochemical sensing and associated measurement methodologies. The electrode material is essential to sensor stability, reproducibility and robustness. For this reason we will use functionalised boron doped diamond, which can be produced at a competitive cost and can detect both respiratory gases in one measurement. The measurement method adopted, also provides a solution to the sensor drift problem. Using only one measurement electrode we also aim to reduce the spatial footprint of the sensor. Beyond hospital care, the sensors offer benefits in, for example, sport and sleep science, and condition management and diagnostics in the community.

Dissolved oxygen and carbon dioxide blood gas levels are key indicators of respiratory health and represent an important diagnostic test for illness severity. Current procedures nearly all involve a direct skin puncture, removal of the blood and analysis in a blood gas analyser. Low numbers of analysers in hospitals means access is limited and sampling frequency for the patient low. Limited sampling means that potentially life altering complications resulting from excessive or insufficient ventilation can be missed. To significantly improve medical outcomes, this project will develop transcutaneous sensors, which sit on the skin and enable continuous monitoring of respiratory gases. Whilst there are limited transcutaneous devices available, take-up is lacking, primarily due to accuracy and sensor drift which necessitates removal, recalibration and reattachment, often many times during use, an activity requiring trained personnel and takes up valuable time. Research in our group has focused extensively on the use of an inherently stable (thermal, chemical and electrochemical) material, boron doped diamond (BDD), for electrochemical sensing. We have demonstrated through surface engineering, controlled incorporation of very robust forms of sp2 carbon in microspot patterns, which activate the sensor towards dissolved oxygen and carbon dioxide over the range required for transcutaneous monitoring. We use voltammetry as the measurement method, where both gases can be detected in one scan. Having two signals in one scan also enables us to introduce methods of internal referencing to help mitigate against possible drift issues. Our method of blood gas analysis when combined with the robustness and stability of the BDD, helps moves the sensor technology towards higher accuracy and single calibration use with a smaller spatial footprint. All lead to a sensor which offers greater patient comfort, no risk of patient infection and significantly improved ease-of-use.