Limb Perfusion Device

Transcription

1 1 Limb Perfusion Device Design Team Peter Colby, Eric Greenberg, Nicholas Soo Hoo, Keith Travis Design Advisor Prof. Jeffrey Ruberti Abstract Every year thousands of people lose part or all of their upper extremities after having them severed in a traumatic incident. The problem arises as time elapses and the limb becomes ischemic as distal tissues begin to die. Currently the standard procedure for preserving the limb is to put it on ice and transport it to the nearest facility. This practice is not only inefficient, but limits tissue life to four hours. One way to keep the tissue alive is to constantly supply the tissue with oxygen, nutrients and medication via a perfusate. The goal of this project is to design a portable perfusion device that will keep a severed limb viable for up to 12 hours to allow for transplantation and replantation. Oxygenation of the tissue can be accomplished using an oxygenator which removes CO 2 from and adds O 2 to the perfusion solution. The oxygen is then transferred to the limb in a perfusion loop where O 2 is exchanged for CO 2 before exiting the arm. Calculations were performed to find the perfusate and oxygen flow rates required to sustain all metabolic functions. A peristaltic pump will be used to move the fluid throughout the system and a system of sensors will monitor the system for temperature, pressure, flow rate and gas concentrations. The temperature of the system will be kept between 4-10 o C to decrease metabolic requirements. The system will also include a bubble trap, filters, oxygen tank and cooling loop to ensure maximum performance and safety. A container and tray were designed to hold all of the components and the arm. The packaging and insulation were optimized to fit everything into the smallest possible space. Once these designs have been finalized and the parts assembled testing will be necessary to assure the device works.

2 2 The Need for Project A device that will extend the life The standard practice for keeping a severed limb alive is to of a severed limb to allow for a place it in a cooler of ice en route to the hospital. This technique only higher likelihood of reattachment preserves the limb for four hours by lowering its metabolic needs, but onto a patient. does not oxygenate or prevent ischemia. This short time frame lowers the chances for successful replantation or transplant. Creating a device that can prolong the life of the limb would improve the quality of life for many upper extremity amputee victims. Making the device portable and lightweight would allow it to be used in the field by medics, emergency staff and military personnel. The Design Project Objectives and Requirements The device must be able to keep Design Objectives a severed limb viable for up to The goal is to extend the life of the limb to 12 hours to allow 12 hours. It must also maintain for viable reattachment. The device needs to be portable and oxygenate portability standards and operate a severed limb by means of machine perfusion. The portability will be on battery power. covered by military standard MIL-HDBK-759C which states the size cannot exceed 485mmX 460mm X255mm and must weigh less than 11.2 kg. The size constraints created by the size of an arm make this a difficult objective to meet. The device will closely mimic the biologic functions of the human body by operating at rates calculated to meet the metabolic requirements of the arm. Design Requirements There are several requirements that must be met to ensure the limb is oxygenated and protected for the duration of the trip. Temperature of the fluid and arm will range from 4 to 37 o C but will optimally run at lower temperatures in order to decrease metabolic requirements. The flow rate of the perfusate must be between ml/min and required oxygen consumption rates are 2-21 ml/min dependent on temperature. These specifications ensure the arm will experience a maximum pressure of 200mmHg which is the limit before damage will occur to the tissue. Design Concepts Considered Several concepts were The overall schematic and order of components of the considered with different perfusate system were determined by physiological requirements of the packaging and interface designs. arm. With this in mind it was crucial to find a way to optimize the packaging and minimize the overall dimensions of the container. The

3 3 container is a purchased part where all of the individual parts will be held and connected to the limb. Backpack Concept (see Figure left) The backpack concept uses a bag style container to hold all of the individual components of the perfusate, oxygen and cooling loops. They are affixed to the bag with straps and stitching. The bag can be opened and closed using a zipper and can also be insulated to reduce thermal leak. An additional case is needed for the arm as well as a way to connect the two compartments. Upright Box Concept For this concept all of the components are placed inside of an aluminum box with side handles and a hinged lid. The particular design allows all of the parts to be placed on the bottom of the box with room for insulation and a tray where the arm rests. Because the arm can fit into the box it reduces the need for an additional container. The components are secured to the box with standoffs and are packaged so that all of the parts are easily accessible. Insulated Concept (see Figure left) The insulated concept is an insulated container similar to a cooler that would house all of the system components with the exception of the arm. Similarly to the backpack concept, the limb would have to be placed in a separate container with an interface to attach the arm to the device. The particular container found for this concept is a box that is taller than it is wide and deep. Because of this the components have to be stacked one on top of the other in order to fit everything. Recommended Design Concept The recommended design is an Three different concepts were explored for packaging and aluminum box that can hold all of limb interface designs. All of these designs were feasible, but one the device components and a tray solution was thought to be the most practical and intuitive for use in a to hold the arm in place. medical emergency setting. Design Descriptions The overall dimensions of the aluminum container are 21 x 21 x 15. The box has two side handles and a hinged lid that can be latched closed. There is sufficient interior space to hold all of the components and arrange them on the bottom of the box. Various

4 4 standoffs and fasteners are used to secure the parts to ensure no hardware is damaged. Because the container is not insulated, closed cell foam will be used to thermally isolate the battery and pump from the arm and remaining parts. The arm is secured on an autoclavable tray, shown to the left, which is fitted to the top of the box with brackets. It can be removed to access the other components of the system and the lid can still be closed while the device is running. The tray has a built-in drain where the returning venous fluids are collected and returned to the perfusate loop with the aid of gravity. Having the limb and device components in one container makes the arm easier to attach to the device. Analytical Investigation The average metabolic rates for flow and oxygen consumption were found to be 350 ml/min and 21 ml/min respectively. A temperature curve was created for maximum flow rate and required oxygen delivery at lower temperatures. Using this curve and Henry s Law, a correlation was determined for partial pressure of oxygen in Perfadex at low temperatures. At 10 o C the required p0 2 in Perfadex delivered to the arm is approximately 10 kpa. In order to optimize the device and choose the most logical components, the individual systems and parts were researched. Trade studies were performed for pumps, oxygenators, filters and sensors to help decide which parts to order. Before purchasing any of the components it was important to verify that all of the parts would work correctly together and not fail when subjected to a load. The first step was to create SolidWorks models of the container and all of the components to show the parts could fit inside of the box. Once this was confirmed finite element analysis was applied to the selected standoffs and attaching hardware to make sure none of the components would fail during operation. Key Advantages of Recommended Concept The upright box container concept has several advantages over the other design considerations. Unlike the backpack and insulated container, the upright box allows all of the components to be placed on one layer of the box. This provides easy access to all of the parts if one needs to be replaced or repaired. It is also beneficial

5 5 because the arm is able to fit on a tray at the top of the box. This eliminates the need for multiple containers and greatly reduces the design work required for the arm interface. Financial Issues The device is built for medical testing and not mass production, so keeping costs down was not a primary objective. This device is currently being built for the Plastic Surgery team at Brigham & Women s Hospital located in Boston, MA. Because this will be used for medical testing, it is not necessary to design for massproduction. The hospital is responsible for the cost of all purchased parts. There are few parts that require machining and most of the parts can be purchased from various medical vendors. Many of the components come in contact with the perfusate and those that cannot be autoclaved or sterilized must be thrown away. This increases the overall cost of the device because multiple quantities must be ordered. The overall cost of one assembled device is approximately $5,100. Recommended Improvements With more time and resources Because a lot of time was spent on research and further development would allow physiological design, some of the components were over engineered for a better optimized system. or not optimized. For instance, some of the packaging hardware and fasteners were chosen based on their availability and ease of installment. With more time, these parts would be designed and machined to provide the greatest performance in the smallest space. Ultimately, if there was more time and resources available, the device would be made more custom. In terms of the container, packaging and sensors, everything would be a lot more streamlined and optimized. Another improvement that could be made is to incorporate the sensors and digital readouts into part of the container packaging. Currently, the information received by the sensors is displayed on a computer using LabView software. By creating a separate interface for these sensors it would eliminate the need for a computer to run the device.