Tech Briefs

Implanted sensors could be used to measure intracranial pressure in TBI patients.

The objective of this work was to use miniaturized, state-of-the-art pressure/temperature sensors engineered at Lawrence Livermore National Laboratory (LLNL) to measure the immediate increases in intracranial pressure (ICP) combined with longer-term measurements of biological ICP and intracranial temperature. The experience gathered from this work provided valuable data on sensor placement, long-term brain tissue responses to implanted sensors, and sensor capability of dual measurement of biologic ICP and impact pressure transients.

Close-up of the sensor showing the diaphragm (A) and four connector contact strips (B).

Test-ready sensors were produced with a range of diaphragm diameters (200 - 1000 μm). Diaphragm diameter should affect sensitivity of the sensors. These wafer-scale, absolute pressure sensors have an ultra-thin form factor (thickness 90 μm prior to packaging, and 130 μm after final packaging). The new sensors were designed to measure absolute pressure by modifying the original contact stress sensor design to create a reference cavity – a trapped volume of gas that is hermetically sealed within the device. The new sensors predefine the pressure sensor’s reference cavity within the silicon-on-insulator (SOI) wafer.

Five new sensors with diaphragm diameters of 200, 400, 600, 800, and 1000 μm (see figure) underwent testing and calibration to determine the optimum diaphragm diameter for subsequent inanimal testing. The new sensors were packaged with an encapsulating layer of Kapton to protect the sensor diaphragm and electrical connections from body fluids.

The first closed-diaphragm wafer sensors were received from LLNL and immediately tested. Static calibration produced a linear relationship between 5 and 25 PSI with an R2 = 0.986. The closed-diaphragm sensor is 90 microns thick, and the final product with Kapton packaging is 130 microns thick. Dynamic comparison of the new sensor with the existing fluid percussion pressure transducer provided close tracking of pressure events. Initial testing revealed the need for shielding of cables and power supply for subsequent applications.

This work was done by Bruce Lyeth, Ph.D., of the Regents of the University of California, Davis, for the Army Medical Research and Materiel Command. ARL-0190

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