Capacitive Air-Bubbles Detector for Extracorporeal Blood Circulation

Extracorporeal blood circuits, or ECBC, have been used in hemodialysis, hemofiltration, plasmaphersis, and for assisted blood circulation during open-heart surgery for many years. ECBC devices consist of numerous individual parts which, dependent upon their operating characteristics, are potential generators of embolism or air bubbles in the blood. The purpose of this study is to construct a capacitive device capable of detecting these embolisms in an ECBC, and to investigate the relation between the size of air bubbles and change in capacitance and output voltage at various applied frequencies. Theoretical analysis of the system and simulation using Multisim2001 software were obtained and compared with experimental results. Results showed that the device was capable of detecting air bubbles with diameters 620 µm and over using Dextran70 and 730 µm using human blood. The capacitance of the capacitor was found to decrease as air bubble diameter increased whereas output voltage increased as air bubble diameter increased. The output voltage was found to increase when the frequency decreased, as theoretically predicted. The main finding was that air bubble, with diameters from 620 µm to 4.21 mm in Dextran70 solution and human blood, produced significant changes in the capacitance of the test cell device. In addition, experimental results were in good agreement with simulation and theoretical analysis. The sensitivity of the device for the Dextran70 was found to be 16.8 mV/nF obtained at 40 Hz and for human blood it was 16.4 mV/nF at 30 Hz.