1. INTRACRANIAL PRESSURE MONITOR 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 Problem Statement Materials and Methods 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 Design Requirements Future Work 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. Casing Dimensions References Internal Circuitry Previous Work Accuracy & Reliability Minimal electronic drift Lifespan ≈ 20 years Materials Biocompatible MRI – no ferrous materials Pressure Ranges Average: 10 – 15 mmHg Gauge Range: -30 – 100 mmHg Generate pressure waveform Design Components Internal – pressure gauge External – power supply and signal receiver Final Casing Design 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 SKULL 2 mm 1 cm 1.5 cm 2.5 cm 6 mm MEMS LL Test effectiveness of pressure transmission over range of -30 - 100 mmHg Assess long term durability of membrane and casing Incorporate LC circuit with MEMS Examine relationship between CSF pressure and resonant frequency of the MEMS circuit variable capacitor inductor AC power supply Figure 1. Parallel LC circuit Figure 2. Capacitance changes correspond to changes in resonance frequency of LC circuit Figure 3. Final casing design Brian, Marshal and Bryant, Charles W. How Capacitors Work. http://electronics.howstuffworks.com/capacitor.htm. Accessed 22 Oct. 2007. Camino® 110-4B. Integra. 2006. 19 October 2007. Medow, Dr. Josh, M.D. Department of Neurosurgery, UW-Hospital The New ICP-Monitor. Spiegelberg. 2004. 19 October 2007. http://www.spiegelberg.de/products/monitors/icp_monitor_hdm291.html