An Implantable Intracranial Volume System for Hydrocephalus Therapy

The past few decades have seen an acceleration of implantable sensors and systems to improve disease monitoring and treatment. Unfortunately in hydrocephalus, a disease where cerebrospinal fluid (CSF) accumulates within the ventricular system of the brain, diagnostic and treatment options remain limited. The therapy consists of a catheter implanted into the CSF-filled region to drain excess fluid. In 1952 a passive pressure-regulated valve system was invented that drains fluid when intracranial pressure exceeded a set value. Surprisingly this technology is still implemented in patients worldwide even though it has limitations. This thesis proposes and validates a volume sensor to improve diagnostic options available. The measurement principle is based on the high electrical conductivity ratio between brain tissue and CSF. Changes in the fluid volume correlate to changes in the distribution of an induced, local electric field. The outcome of this thesis is a device for monitoring CSF accumulation as well as the design for an implantable system. The volume measurement concept is an adaptation of the impedance technique and is first explored via in-silico brain geometry using finite element analysis. Simulations are performed to obtain design decisions such as catheter diameter and electrode placement. The sensors are designed with an internal catheter as a pathway for volume control. In a scaled down rat-size sensor a minimum outer diameter of 500 µm outer diameter with platinum-iridium cylinders is fabricated. Instrumentation for continuous monitoring as well as a long-term battery-operated wireless circuit using a microcontroller is demonstrated. The sensor and electronics are characterized in bench-top models for calibration of voltage-volume relationships prior to animal experimentation. The feasibility of CSF volume monitoring in an animal model of hydrocephalus is a specific contribution of this thesis. Hydrocephalus was induced in 3 week-old rats by kaolin injection. 28 days post-induction the sensor was implanted into the lateral ventricles and a CSF shunting/infusion protocol was performed. A high correlation between sensor measurements and pump-controlled volume change was observed. While future research of long-term volume monitoring is necessary, these results indicate that volume monitoring is feasible for clinical cases of hydrocephalus.