Telemetric Implantable Pressure Sensor for Short- and Long-Term Monitoring of Intracranial Pressure

Patients with hydrocephalus, idiopathic intracranial hypertension and head injury frequently require monitoring of intracranial pressure (ICP) and may need repeated episodes of monitoring months or years apart. The gold standard for measurement of ICP remains the external ventricular catheter. This is a fluid-filled catheter transducer system that allows regular recalibration and correction of zero drift by its position relative to a fixed anatomical reference. It also allows drainage of cerebrospinal fluid (CSF), providing a means of lowering the ICP. Several catheter tip transducer systems are currently in clinical use, including using strain gauges or fiber-optical pressure sensing techniques. In these devices, zero drift and calibration cannot be checked in vivo. All the ICP monitoring devices in current clinical use require a physical connection between the brain and the external environment. This is a source of infection and limits the duration of monitoring. A number of telemetric monitoring devices, in which data is in some way transmitted transcutaneously, have been developed over the last twenty years, but significant technical problems have precluded their use in routine clinical practice. All current ICP monitors are temporary percutaneous implanted devices. Placement of these devices carries significant morbidity, particularly infection. Patients undergoing repeated monitoring require multiple surgical procedures. Apart from decreasing the risk of infection in patients with severe head injury, the clinical value of an accurate telemetric ICP monitoring system which maintains its reliability over a long period of implantation is high. Several neurosurgical conditions require close control of ICP over several years to ensure a satisfactory outcome, such as hydrocephalus treated by a VP shunt or endoscopic third ventriculostomy as well as idiopathic intracranial hypertension. Currently, in these situations, ICP is measured by tapping a ventricular access device or by lumbar puncture; these invasive procedures, often necessary in children, carry a risk of infection. Current ICP monitors show a significant zero drift of the sensor over time which excludes them from being used in long-term implants where recalibration of the sensor is not possible. An implantable telemetric device would overcome many of the problems currently associated with ICP monitoring. Telemetry would allow the ICP to be measured non-invasively at home; symptoms could then be easily evaluated and the necessary adjustments to a programmable shunt, when present, could be performed and confirmed. Utilizing micro- and nanotechnologies as well as advanced packaging technologies, we have developed a fully implantable ICP sensor for long-term use. The sensor implant comprises an absolute capacitive pressure sensor and readout integrated circuit with ultra low power consumption and hence does not require an implanted battery as a power supply, which would otherwise limit the implant's lifetime. The sensor transmits pressure data via inductive coupling to an external device transcutaneously while being powered from outside via radio frequency. The external reader device compensates the absolute pressure data by measuring and subtracting the barometric pressure and displays data representative of intracranial fluid pressure. The implant is encapsulated by several functional layers, ensuring a hermetical sealing while at the same time providing high flexibility. A final biomaterial nano-layer to reduce cell growth and impairment of the mechanical functionality of the sensor is in development. The use of capacitive sensors and the encapsulation scheme guarantees a drift-free performance over time. The implant is designed in a manner so that it can be inserted according to the proven minimally invasive procedures known from current ICP monitors. The measurement tip can be placed either in the ventricle or brain parenchyma while the telemetry unit is located outside the cranial bone directly under the skin for optimal linking to the external unit. Pressure readings can be taken continuously at any time either at home or in hospital. The external read-out device communicates to the clinical periphery or home base station via radio-frequency link. Our device prototypes have successfully passed extensive laboratory tests and are currently tested in vivo in animal trials.