The main components of the sensor are a pH-sensitive polymer (hydrogel) and a micro pressure sensor. The polymer contains a large quantity of water and shrinks or swells in response to the changing pH-value. It is sandwiched between the micro pressure sensor and a porous, silicon cover. The cover contains a reservoir with bicarbonate electrolyte, covered by a gas-permeable membrane.
Carbon dioxide flows from the stomach through the gas-permeable membrane into the electrolyte, where it initiates a reaction that lowers the pH-value. The pH-sensitive polymer tries to swell in response to this. However, because it is in a confined space it exerts a pressure which the pressure sensor subsequently measures. Conversely, if the carbon dioxide concentration falls, the pH-value increases and the pressure generated by the polymer decreases.
Due to its small size (2.9 mm x 0.9 mm x 0.7 mm) the sensor easily fits in a catheter tip. The catheter is inserted via the nose into the stomach and remains there until the measurement has been completed. Herber developed his sensor to detect gastrointestinal ischaemia at an early stage. Gastrointestinal ischaemia can cause pain after eating, pain after physical exertion, diarrhoea, nausea and a possibly serious loss in weight.
Future plans
Measurements under laboratory conditions have yielded highly promising results to date. New measurements will shortly be carried out at the Medische Spectrum Twente hospital: a 24-hour measurement and an exertion measurement on a home trainer. The sensor is resistant to hydrochloric acid because in both cases it will remain in the stomach for some time. For the measurements, Herber is developing a prototype catheter in cooperation with Sentron Europe BV. They want to produce the sensor, if the measurements prove to be successful.
Interest has also been expressed in using the sensor to measure carbon dioxide levels in the brains of intensive care patients, as this can provide information about the patient's recovery. Further the sensor can be adapted to measure other substances such as glucose or specific ions.
Sebastiaan Herber's research is being funded by Technology Foundation STW.