Needle-type blood gas sensors
The determination of blood gas levels (i.e., partial pressure of oxygen and carbon dioxide (pO2 and pCO2) and pH) is required in cases where a significant change in respiratory activity might be brought about. The levels have been determined using a blood gas analyzer with blood samples collected just prior to the measurement. However, the conventional method provides only fragmentary information about these indicators. Because the changes in blood gas levels are very rapid in abnormal cases, the method might overlook considerable changes. Therefore, in vivo continuous monitoring is more preferable to accurately know the changing status in a human body.
In order to facilitate in vivo continuous monitoring of blood gas levels, very thin disposable pO2, pCO2, and pH sensors which can be incorporated in a syringe needle or a catheter were microfabricated. The most critical part of the sensor was a Ag/AgCl reference electrode with a pinhole structure. Because of the structure and a novel operational mode, reproducible responses were realized for the respective sensors. Performance characterization revealed clear responses and linear calibration plots in the ranges of partial pressure or pH of physiological levels. The sensors were tolerant to ethylene oxide sterilization. The pO2 sensor was tested by inserting it in the femoral artery of an anesthetized dog. The output of the sensor faithfully followed the changes in the pO2 level.
Fig. 1 The needle-type pCO2 sensor. The completed sensor (above) and the sensor incorporated in a syringe needle (21G).
Fig. 2 Structure of the needle-type sensors. (a): pO2 sensor, (b): pCO2 and pH sensors. (c): cross section of the pO2 sensor. The cross section of the pCO2 sensor is almost the same except that iridium oxide is used instead of the platinum cathode. (d): cross section of the pH sensor.
Fig. 3 Calibration plot for the pH sensor.