Biomedical sensor for transcutaneous oxygen measurements using thick film technology
The measurement of the partial pressure of oxygen in arterial blood is essential for the analysis of a patient's respiratory condition. There are several commercially available methods and systems to measure this parameter transcutaneously. However, they tend to be cumbersome and costly. To overcome the disadvantages presented, a new type of sensor for transcutaneous blood gas measurement was investigated, employing thick film technology, which is an excellent technique to produce sensors in bulk, as it is cost effective and easy for reproducible fabrication. This thesis describes the application of thick film technology to the investigation and production of these sensors, which are small in size and readily disposable. Such advantages are greatly welcomed from the medical point of view. The devices under investigation were based on amperometry. Gold electrodes were printed on an alumina substrate and covered with a layer of electrolyte gel and then finally, with a membrane. An external silver/silver chloride reference electrode was also employed in this electrochemical cell set-up. The project also involved several electronic circuits to support the transcutaneous oxygen measurement. The main studies were concentrated on the materials employed as the electrolyte and membrane. Investigations were carried out to evaluate the performance of these devices in atmospheric and hydrated environment as well as under the influences of different temperatures. Detailed discussions of the results were presented and future work for the project is identified. The novel contributions towards this research work were categorized into two major modules. Firstly, in the heating module, a single element taking both the roles of a heating element and a feedback temperature sensor was employed for the wheatstone bridge circuit configuration to regulate the transcutaneous temperature. In addition, the oxygen sensing module included studies on the effectiveness of using Nafion polyelectrolytes to achieve amperometric measurements for transcutaneous oxygen monitoring. From the experimental results, the most promising choice for the thick film transcutaneous oxygen sensor was the prototype with Nafion as the electrolyte and PTFE as the membrane. The disposable prototype produced results achieving low manufacturing cost of approximately £1 and was able to make continuous measurement of up to 46 hours. It proved to be compact, non-biohazardous and portable with a good degree of user-friendliness. It also provided accurate and reproducible measurements of not more than 3% error. Thorough intensive research activities were carried out to overcome all existing problems in order to achieve the objectives of the research project. With more than 70% of the specifications being met, the positive results had presented a successful design for the fundamental version of the disposable transcutaneous oxygen sensor employing thick film fabrication processes.