Implantable Pressure Sensors for Chronic Monitoring

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dc.contributor.advisor Budgett,, D en
dc.contributor.advisor McCormick, D en
dc.contributor.advisor Malpas, S en Leung, Dixon en 2020-03-26T23:20:22Z en 2019 en
dc.identifier.uri en
dc.description.abstract Pressure is a fundamental driving factor for the exchange of oxygen and nutrients within capillaries. Nowhere is this pressure more critical than within the brain. Here, the brain tissue, its vascular network and cerebrospinal fluid (CSF) are encased within a rigid skull, with limited capacity to expand. Pressure in the brain is crucial, yet there is no current device capable of monitoring this pressure long term. One disease where intracranial pressure (ICP) is chronically increased is hydrocephalus. Without treatment, vital exchange is reduced, which can lead to brain damage. Management commonly involves the placement of a drainage catheter, termed a shunt, for the removal of excess CSF. However, shunts have a very high failure rate and the diagnosis of elevated ICP is difficult. Modern diagnosis of shunt failure is by expensive MR imaging, or CT scans which deliver ionizing radiation. Currently, there are no chronically implantable sensors which can provide long-term accurate pressure measurements. The issue is that without an ability to recalibrate a sensor, the measured value will drift with time and thus become clinically irrelevant. The primary aim of this research is to extend the useful lifetime of implantable pressure sensors. This was achieved through adding the ability to re-calibrate a sensor after it is implanted by inclusion of a novel pressure switch. The pressure switch is a coin-shaped device which is connected into the shunt. A diaphragm within the device deflects in response to fluid pressure. This motion becomes impeded once a pressure threshold is reached in the form of a stop for the diaphragm. The resulting change in compliance can be detected by the pressure sensor. This switching response signals that a designated pressure has been reached and allows the device to be re-calibrated without an external reference. Stability of the pressure switch is crucial for effective re-calibration. The implant is constructed from a single material to reduce stresses induced during assembly. Electronic components no longer contribute to drift. Only one stable pressure point is required for re-calibration. These design approaches collectively minimize the sources of drift and extend the useful lifetime. Stability performance was evaluated by performing accelerated tests on bench top prototypes. Pressure cycling was performed over one-million cycles, which supports re-calibration for 10 years of operation. After cycling, the worst-case long-term stability was ±0.852 mmHg. The drift performance was demonstrated to be well within the requirements of ±2 mmHg, as required by standard ANSI/AAMI NS-28:1998 (R2015), Intracranial Pressure Monitoring Devices. Mechanical related drift caused by surface changes were investigated. Observation of contacting surfaces was performed using Scanning Electron Microscopy and measurements via stylus profilometry. Surface remodeling occurred after pressure cycling with a 5 μm material removal at vertically protruded edges. As a chronic implant the size of the pressure switch is important. Device dimensioning was dominated by the ability to maintain a stable pressure reference. Analytical modelling was performed using leak-rate estimation. A device with an enclosure of 0.4 cm3 can remain stable within 2 mmHg for 5 years. This sizing is comparable to most commercial hydrocephalus shunt valves. Dimensions of the pressure sensitive diaphragm defined the re-calibration response. A 10 mm diameter, 25 μm thick diaphragm was informed through analytical modelling using mechanical plate theory. Pressure switches made from Titanium Grade II were prototyped for long-term testing. The fabrication processes involved precision machining and hermetic laser welding. The conventional fabrication techniques utilized and material choice are directly applicable for producing a fully implantable device. It is proposed that the re-calibration procedure can be performed easily by the patient via pressing on tubing adjacent to the pressure switch. Sensor re-calibration was demonstrated on 2-French implantable pressure catheters (Model IPR-2093, Millar Inc.). The variability of the detected re-calibration pressure was within ±0.15 mmHg. The outcome of this PhD research provides a new perspective and approach for reducing pressure sensor drift. A novel approach for re-calibrating implantable pressure sensors to a level of accuracy meeting the needs of monitoring ICP over long durations is demonstrated. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.relation.isreferencedby UoA99265291110302091 en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights.uri en
dc.rights.uri en
dc.title Implantable Pressure Sensors for Chronic Monitoring en
dc.type Thesis en Bioengineering en The University of Auckland en Doctoral en PhD en
dc.rights.holder Copyright: The author en
dc.rights.accessrights en
pubs.elements-id 797040 en Bioengineering Institute en
pubs.record-created-at-source-date 2020-03-27 en

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