Wireless, Optomechanical IntraCranial Pressure (ICP) Monitor

Nikos Chronis

Intracranial pressure (ICP) monitoring has been recognized as an important diagnostic tool for patients suffering from traumatic brain injuries (TBI), hydrocephalus, and other ICP-elevated disorders. The use of ICP monitoring has reduced the number of deaths by 20%, minimized secondary brain damage, and significantly decreased recovery time after surgery.

Since their introduction in the early 1970s, however, ICP monitoring systems have undergone few, if any, design changes and remain cumbersome in placement and use. The basic operation principle has remained the same: a catheter containing a pressure sensor is implanted in the patient’s brain, and the signal is analyzed by a bedside electronic readout unit that is continuously connected to the catheter through a cable. The major drawbacks of these systems have also remained essentially the same over the past 30 years: risk of infection, short-term capability of ICP monitoring, patient immobility, potential for brain damage during catheter implantation, and incompatibility with Magnetic Resonance Imaging (MRI).

We have been active in developing a new class of implantable MEMS (MicroElectroMechanical Systems)-based sensors that can provide accurate, long-term, risk-free ICP monitoring. The developed technology overcomes the limitations of current ICP monitors as it is based on a fully implantable (cable-free) optical microsensor that color codes ICP changes. The ICP is converted to near infrared (NIR) wavelength. Operating the device in the NIR region is critical for minimizing light absorption by the skin and surrounding tissue.

The sensor consists of a microlens that focuses NIR light into a quantum dot (QD) bilayer that sits on top of the thin membrane exposed to ICP. When excited with a NIR laser, each QD layer emits fluorescence of a narrow spectrum. The distance between the QD bilayer/membrane and the microlens changes when the ICP changes, resulting in an alteration of the fluorescence intensity ratio of the two QD layers. A non-implantable, portable optical unit is used to excite the QD bilayer and collect the emitted NIR spectrum.

We completed in vitro and in vivo testing of the ICP sensor in sheep, and the sensor reliably measures pressures over the desired ICP range (1-80 mmHg) with high resolution (<1 mmHg). Our ICP sensor has no electronics, requires no power, and is biocompatible. Its novel architecture enables prolonged ICP monitoring (years), eliminates the risk of infection and brain damage (the microsensor has a footprint of ~2 mm2), allows patient’s comfort and mobility, and is compatible with MRI technology.

We should emphasize that there are no commercial pressure sensors for long-term ICP monitoring. Current sensors (e.g., the Codman microsensor) are not fully implantable; they are hardwired to an external unit at the bedside and can monitor ICP for only few days. The proposed ICP monitoring technology will help in efficiently managing and treating TBIs and ICP-related disorders. This technology will also, we believe, inaugurate a new era in the development of implantable, electronic, and power-free miniaturized devices that can be used in a variety of biomedical pressure monitoring applications, including arterial blood, intraocular, and gastrointestinal pressure monitoring.