Single-use capacitive pressure sensor employing radial expansion of a silicone tube

This paper reports on a single-use pressure sensor for medical applications. The sensor principle makes use of the radial expansion of a silicone tube which occurs when there is a pressure difference between the inside and the outside of the tube. The change in outer diameter is detected by a capacitive measurement method, whereas the amplification electronics can be separated from the tube being in contact with the fluid. This enables a cross-contamination-free measurement system for patient near application as e.g. within infusion systems. The sensor includes a novel highly sensitive and fast analog amplification circuit in combination with gold electrodes fabricated in PCB technology. This circuit enables to amplify even highly dynamic pressure changes, in the range of a few milliseconds causing capacitive changes on the electrodes in the fF range to a readable voltage level. A functional model is developed based on flex board technology which allows for studying the influence of the electrode geometry as well as the material properties of the silicone tube with respect to the sensitivity of the sensor. The best performing electrode geometry determined by experiments features a sensitivity of 0.195 fF/kPa at a mean coefficient of variation (CV) of 6.4%, considering three individual sensor assemblies. A sensor prototype is designed and fabricated based on the results of the investigated functional model. The performance of the prototype is investigated experimentally in a pressure range from 0 to 40 kPa. As a result a sensitivity of 0.135 V/kPa for DI water as measurement liquid could be achieved and a good linearity of the signal (R2 = 0.996) was observed up to 35 kPa.