Implantable sensor to measure liquor pressure of a ventricular drainage system

Introduction The human brain is filled with cerebrospinal fluid (liquor), which is produced in the ventricles. Normally the production and resorption are balanced. Patients suffering from normal pressure hydrocephalus produce more liquor as they resorb[1]. This leads to an increase of the intracranial pressure. This article describes a sensor transponder system to measure pressure in a ventricular drainage system (see Figure 1)which is connected to the human brain. It focuses on the development of an application specific integrated circuit (ASIC) with a micro machined pressure sensor as well as the principle of a wireless readout system using a handheld reader device. Methods As shown in Figure 2 the shunt sensor ASIC consists of six main building blocks: HF-front-end, digital part, EEPROM, biasing & voltage regulation, pressure sensor and readout with A/D-Converter. The pressure sensor was realized by surface micro-machined capacitive sensor elements. If sensors do not require continuous activity it is a good choice to use a transponder based approach. For the shunt sensor a low frequency transponder system was developed (Figure 3). Results The metal of the housing has a significant influence on the wireless transmission.To overcome the damping caused by the metal encapsulation, the handheld reader was designed with a high quality factor of the transmitter coil, to optimize the performance regarding reading distance and lifetime of the reader battery. The calibration of the nonlinear and temperature dependent output of the shunt sensor is performed in a temperature controlled pressure chamber. The typical calibration error is better than 0.8 mmHg as depicted in Figure 4. Conclusion In corporation with the Christoph Miethke GmbH & Co KG a system was developed for wireless measurements of the cerebrospinal fluid pressure in the shunt. The system is approved for use as a long-term implant in humans.