Optimization and Long-Term Stability of Micro Flow Sensors for Smart VP Shunts

This paper reports on a systematic study of the flow sensitivity and resolution of micro-flow sensors intended for use in implantable ventricular-peritoneal shunts. The flow sensors utilize ferromagnetic flaps (transducers) whose deflection is detected by ultra-sensitive MTJ sensors (20mV/V/Oe). A working range of 0-40 ml/h was demonstrated at a maximum uncertainty of 4% RMS and a resolution of 0.4 ml/h. Earlier studies on this sensor unveiled significant low-frequency noise (drift) limiting the sensitivity to 1.4 ml/hr. The present study identifies thermal noise as the main source of low-frequency drift. Using thermal compensation it was found that the drift can be reduced below 2 ml per 24-hr. Combining an array of four transducers operating in series, it has been demonstrated that a sensitivity can be increased 10.9 fold. Furthermore, the report examines the long-term structural stability of the sensors and produces a corrosion report suggesting a lifespan of 15 to 55 years.