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INTRACRANIAL PRESSURE MONITOR Lacey Halfen, Jessica Hause, Erin Main, and Peter Strohm Client: Dr. Josh Medow Advisor: Willis Tompkins Department of Biomedical Engineering University of Wisconsin-Madison An intracranial pressure (ICP) monitor is used to detect changes in cerebrospinal fluid (CSF) pressure caused by shunt malfunction. To address the concern of a finite lifespan, an ICP monitor that could be inductively powered through the use of an external power supply was designed. An LC circuit with a MEMS variable capacitor detects changes in pressure and transmits the pressure reading externally through changes in resonance frequency. A biocompatible casing for the internal component was created using silicone (PDMS) and polyimide. Casing demonstrated the ability to transmit pressure changes across a membrane to the internal fluid filled chamber. Previous Work Problem Statement Design a biocompatible casing for an LC circuit with a MEMS variable capacitor. Casing must incorporate a flexible membrane to transmit intracranial pressure changes to a fluid filled chamber which then alters the MEMS capacitor plate distance. Two inductor coils located on opposing sides of the MEMS circuit allow the device to be inductively powered, requiring no exposure through the skin. Motivation Shunt Purpose and Function • Regulation of intracranial pressure • Hydrocephalus • Increased ICP • Drain excess cerebrospinal fluid Shunt Malfunction • 50% failure in the first 2 -3 years • Diagnosis • Invasive: surgery and shunt tap • Non-invasive: physical exam, MRI, and CT AC power supply variable capacitor inductor Figure 1. Parallel LC circuit Figure 2. Capacitance changes correspond to changes in resonance frequency of LC circuit Internal Circuitry L MEMS L Casing Dimensions Design Requirements Accuracy & Reliability • Minimal electronic drift • Lifespan ≈ 20 years Materials • Biocompatible • MRI – no ferrous materials Pressure Ranges • Average: 10 – 15 mm. Hg • Gauge Range: -30 – 100 mm. Hg • Generate pressure waveform Design Components • Internal – pressure gauge • External – power supply and signal receiver Final Casing Design 2. 5 cm 6 mm 1 cm SKULL 1. 5 cm Figure 3. Final casing design Materials and Methods Materials • Silicone (PDMS) – membrane and housing • Polyimide - tube Membrane construction • Spun PDMS at 800 RPMs for 30 sec • Placed polyimide tube end on PDMS layer • Heated at 95 °C for 2 -3 min to polymerize Housing construction • Filled two metal molds with layer of PDMS • Heated at 95 °C for 5 -10 min to polymerize • Attached two halves via oxygen radicals Membrane testing • Air pressure exposure • Seal testing • Movement transfer Future Work 2 mm References Brian, Marshal and Bryant, Charles W. How Capacitors Work. http: //electronics. howstuffworks. com/capacitor. htm. Accessed 22 Oct. 2007. Camino® 110 -4 B. Integra. 2006. 19 October 2007.