Demonstration that a new flow sensor can operate in the clinical range for cerebrospinal fluid flow

A flow sensor has been fabricated and tested that is capable of measuring the slow flow characteristic of the cerebrospinal fluid in the range from less than 4 mL/h to above 100 mL/h. This sensor is suitable for long-term implantation because it uses a wireless external spectrometer to measure passive subcutaneous components. The sensors are pressure-sensitive capacitors, in the range of 5 pF with an air gap at atmospheric pressure. Each capacitor is in series with an inductor to provide a resonant frequency that varies with flow rate. At constant flow, the system is steady with drift <0.3 mL/h over a month. At variable flow rate, V, the resonant frequency, f₀, which is in the 200-400 MHz range, follows a second order polynomial with respect to V. For this sensor system the uncertainty in measuring f₀ is 30 kHz which corresponds to a sensitivity in measuring flow of ΔV= 0.6 mL/hr. Pressures up to 20 cm H₂O relative to ambient pressure were also measured. An implantable twin capacitor system is proposed that can measure flow, which is fully compensated for all hydrostatic pressures. For twin capacitors, other sources of systematic variation within clinical range, such as temperature and ambient pressure, are smaller than our sensitivity and we delineate a calibration method that should maintain clinically useful accuracy over long times.