Universal Lateral Pressure Percussion Injury Instrument

Transcription

1 Universal Lateral Pressure Percussion Injury Instrument Design Team David Cleary, Brian Morton, Lee Panecki, Eugene Zeleny, Matthew Zucosky Design Advisor Prof. Sinan Müftü Sponsor Children s Hospital Boston Dr. Alexander Rotenberg Abstract An LFPI device is used for inducing Traumatic Brain Injury (TBI) in rats. Standard LFPI devices are mechanically actuated and give inconsistent results. The limitations of the standard device hinder potential progress in the TBI research field. This project was created to design and build a device that addresses the limitations of the standard device for Dr. Alexander Rotenberg of Children s Hospital in Boston. The updated device is required to create reproducible saline pressure impulses ranging from psi, have a simple user interface, robust data logging, interchangeable outlet apertures, and functionality to vary impulse frequency, duration, and amplitude. Extensive analysis of fluid column impacts was performed for both open and closed systems in order to theoretically model the new design and optimize performance. This analysis has involved a numerical characterization of the required impact force, as well as a CFD analysis correlating actuator performance with dimensions of the pressure chamber. The final design includes a custom hydraulic cylinder, a diaphragm-sealed impacting assembly, and is actuated by an industrial solenoid. An improved air bleeding system, interchangeable aperture nozzle, graphical data logging, and LabVIEW user interface have also been included in the updated device. It is capable of delivering the full range of required pressures, can apply multiple impacts, and is easy to use. The new LFPI device has potential to contribute to new areas of TBI research. For additional information, contact Professor Sinan Muftu: s.muftu@neu.edu 49

2 The Need for Project Dr. Alexander Rotenberg and the Center of Life Sciences in Boston require an updated LFPI device to further quantify the effects of traumatic brain injuries (TBI). Standard LFPI Device Dr. Alexander Rotenberg of the Center of Life Sciences in Boston conducts traumatic brain injury (TBI) research using the standard Lateral Fluid Percussion Injury (LFPI) device. The standard device is cumbersome and produces inconsistent results. Major disadvantages of the standard fluid percussion device include user set-up variability, inconsistent saline pressures due to expansion/contraction of O-rings, unknown boundary layer effects, an ineffective air bleeding mechanism, non-intuitive user interface, inconsistent outlet diameters, and an inability to produce consecutive impacts with controlled frequency and duration. The restricted functionality of the standard device limits potential progress in the TBI research field. This project is needed to address all of the shortcomings of the standard device and expand its functionality to facilitate research advancements. The Design Project Objectives and Requirements The objective is to build an updated Lateral Fluid Percussion Injury (LFPI) device. The device will deliver a controlled (and repeatable) external pressure to the dural surface of a rat brain in order to quantify the effects of traumatic brain injury. Design Objectives The objective of this project is to build a novel LFPI device that can deliver a controlled and repeatable pressure impulse to the dural surface of a rat s brain. The operator will be able to vary the duration and amplitude of the pressure impulse, along with the device s nozzle aperture diameter. To increase the design s usability, the device is easily calibrated, has a LabVIEW user interface, and a space saving design. Design Requirements To successfully administer a TBI, the peak pressure of the applied impulse must reach 30 to 65 psi. The total impulse pressure wave must also occur within a period of 20 milliseconds. Additionally, athletic concussions are of interest to the research, which have total durations as low as 6 50

3 milliseconds. This design must deliver pressure impulses within the range of 6 to 20 milliseconds. Further, the device s nozzle aperture diameter must be variable between 2 and 6 millimeters. Design Concepts Considered The overall design of the updated LFPI device was constrained by historical test procedures, but several concepts were considered for its subsystems. The updated LFPI device requires saline as the pressure delivery media in order to compare results with existing research. Actuator Precision, repeatability, and impact frequency requirements necessitate the use of an electronically controlled impacting mechanism. Impacting actuators considered include pneumatic cylinders, voice coils, hydraulic cylinders, spring-loaded pistons, and solenoids. Impacting Subassembly Concepts Assembly Overview In order to accurately transfer force between an actuator and saline fluid column, an intermediate impacting surface is necessary. In addition to efficiently transferring force, this component must maintain a robust hydraulic seal at the top of the fluid column. Potential design concepts included air cylinders, dynamic O-ring seals, and diaphragms. Air Bleeding Concepts Removing air from the saline column is critical to obtain consistent impact pressures. Proposed air bleeding methods included both circulatory and a sealed fluid systems. The positioning of the air bleeding ports was found to be very important, and was tested extensively in the initial prototype. 51

4 Rat Adjustment Platform Rat Positioning Platform Three-axis positioning of the rat model in relation to the device s nozzle is required to obtain satisfactory test results. Rail extrusions, rods, and linear bearings were considered for the x and y-axes, and beam deflection equations were conducted to select appropriate materials with minimal bending. Device Control MatLab and LabVIEW were the primary control options considered for the updated LFPI device. Recommended Design Concept A solenoid actuator was chosen as the most appropriate impacting device. Other device specifications were determined by CFD simulations and testing. The updated device will use 75% less lab space and be significantly easier to operate. Pressure Test Parameters The solenoid actuator design was chosen due to its exceptional response time and wide force range. The fast response time allows the solenoid to create multiple impacts at a precise frequency. Additionally, the force generated by the solenoid is directly proportional to the input voltage, making it easily controlled using LabVIEW. Design Description When the solenoid actuator impacts the intermediate assembly, it sends a pressure wave down a straight path to a rat model s dural surface. The duration of the impact as well as the pressure magnitude can be adjusted from the LabVIEW interface. Upon impact, the LabVIEW interface automatically displays the pressure wave and max pressure, while exporting a corresponding text data file. Voltage Scale Real Time Results LabVIEW Interface The solenoid actuator is powered by a bench-top power supply, which was chosen for its ability to provide the required voltage. To modify the magnitude of the pressure wave, the operator adjusts the power supply voltage. LabVIEW correlates the required voltage to the desired pressure entered by the operator. Triggering the solenoid is accomplished by 52

5 activating a solid state relay controlled by LabVIEW. The time the relay is active controls the duration of the impact. A 3-amp fuse is in-line with the relay to protect it from damaging current, and an arc suppressing diode is in place across the solenoid to protect the relay from reverse voltage while switching. Air Bleeding Path The animal positioning system allows the operator to move the animal with one hand by interacting with a single sterile knob. Sterilizing the knob is accomplished by detachment and autoclaving. The air bleeding process is streamlined by a combination of the internal cone method and a reduced saline volume. The operator also has the option to easily interchange the exit nozzles, depending on the research needs. Analytical Investigations Nozzle CFD Nozzle Simulation To find the actuator s force requirement, numerical models of the system were constructed using Bernoulli s Equation, Pascal s Principle, and dynamic analysis of a blunt body impacting a free liquid surface. Inconclusive results from these studies led to the use of Computational Fluid Dynamics software (CFD). CFD simulations were performed to determine the input pressure required to achieve the required outlet pressure. The pressure could then be correlated to the actuator s required force. The cylinder geometry was iterated to reduce the pressure losses in the design, and the final geometry is a direct result of this analysis. Additionally, the classical system was modeled using CFD to establish a comparison baseline. Experimental Investigations Pressure Head Sealing In parallel to the CFD analysis, prototype cylinders and sealing assemblies were machined to experimentally verify their functionality. Testing showed that static O-ring seals 53

6 work best for the exit assembly. Testing also led to choosing the diaphragm option for the impact assembly due to its low inherent friction. After the seals were manufactured, the prototype was used to experimentally verify the actuator s force requirement. To do this, calibrated weights were dropped from incremental heights to observe the system s pressure response. The height and weight were then used to back-calculate the impact force. Key Advantages of Recommended Concept Impacting Diaphragm Assembly The solenoid-actuated LFPI design is a versatile apparatus that has potential to expand the TBI research field. It has the ability to reliably generate a wide range of novel testing conditions that are unexplored by current research, while also remaining easy to use. Adjusting the test conditions is as simple as entering a few parameters into the computer, and the device will output quality data to show that the intended condition was created. This streamlined process enables researchers to perform the procedure quickly and independently, saving time and resulting in a better user experience. Further, the footprint of the updated device uses 75% less space than the standard device, saving valuable lab space. Financial Issues The original actuator and power supply selections were cost prohibitive, so alternate solutions were implemented. The main financial concerns involved the selection of the device s actuator and the power supply. A voice-coil actuator was originally selected for its precision displacement and force control. A computer-controllable power supply was also selected for integration with the user interface. However, both of these components proved to be cost prohibitive, and less expensive alternatives were selected. 54

7 Recommended Improvements Direct communication between the power supply and LabVIEW interface would reduce component quantity and increase usability. The solenoid actuator design could be improved by integrating the power supply and relay with the LabVIEW interface. From a functionality perspective, this would further increase the device s ability to tailor the pressure pulse, and also streamline the device by combining two components. It should be noted that before the device is used for any live animal testing, it should be put through a formal validation process. Design of the validation testing will be a joint effort between the Center for Life Sciences and the Capstone Team. 55