MESA+ Institute for Nanotechnology

In this thesis the development of a new type of CO2 sensor is described. The main application of the sensor is measuring the partial pressure of CO2 in the stomach to diagnose gastrointestinal ischemia, which occurs when blood flow is insufficient to deliver oxygen to the stomach and intestines.

The sensor exploits a pH-sensitive hydrogel as sensing material and a micro pressure sensor as transducer. A pH-sensitive hydrogel swells and shrinks in response to pH changes. In this case, however, it is enclosed between the pressure sensor and a porous, silicon cover. The cover contains a reservoir being completely filled with bicarbonate electrolyte and is covered by a gas permeable membrane. The operating principle is as follows: CO2 gas diffuses through the membrane into the electrolyte and starts a reaction resulting in a pH decrease. Consequently, the enclosed pH-sensitive hydrogel generates pressure, which is measured by the pressure sensor. The described process is fully reversible.

The overall dimensions of the final sensor are 2.92x0.95x0.70 mm3 and the hydrogel thickness is 5 μm. The best qualities of the sensor are the lack of a reference electrode and the use of an existing pressure sensor that is easily accepted in the medical field.

The sensor responds well to carbon dioxide with a response time between 2 and 4 minutes per CO2 step showing little hysteresis. A maximum pressure of 0.29x105 Pa is generated at 20 kPa CO2. The sensor enables detection of steps smaller than 0.5 kPa CO2 (limited by the used measurement setup). The sensor shows temperature sensitivity due to the applied construction manner, whereas the kinetics of the hydration of carbon dioxide is the response time limiting factor. However, both effects can relatively easily be compensated for respectively be improved. The sensor is considered to fulfill most of the medical requirements.

A major improvement will be to manufacture the complete sensor with standard cleanroom techniques to increase the yield and reproducibility, and to eliminate the temperature sensitivity. Furthermore, clinical trials should be performed with the sensor mounted in a catheter to verify its working as diagnostic tool.