Preliminary Detailed Design
Table of Contents
Team Vision for Preliminary Detailed Design Phase
As a result of the previous review, we went back to the drawing board in order to address concerns and recommendations. Therefore, the plans for this phase were altered.
- Made a physical model of the lung
- Confirmed lung dimensions
- Researched and tested new lung material
- Researched alternatives to 3D printing ribs
- Polled current 5th year BME students about the physical/manual diaphragm
- Researched how to allow for interaction between lungs and ribs
- Contacted multiple RIT staff about obtaining a skeleton to use rib cage
- Assembly of 1:5 rib model
- Researched flexible materials for ligaments (Ninjaflex)
- Ninjaflex printing process development
- Alteration of BC2 and BC3 files to help fit cylinder
- Research of rib movement profile
- More robust rib attachment strategy
Prototyping, Engineering Analysis, Simulation
CasingAn alternative casing will be used if, as a result of this design review, we decide to forgo physical ribs. It would be an 8"x12" rectangular prism. The same fittings will be applied to the model, and a piston will still be used. In an effort to remain compliant with our customer requirements, we would then move to an elastic rib membrane.
LungsAlternatives to the foil balloon were considered at the beginning of this phase. Even though the foil balloon provided a physiologically accurate volume, the shape was not preferred. Therefore, the main consideration for the new lung material would be the ability to mold/design the shape.
We decided on a design which would glue two pieces of rubber together. The idea came from the whoopee cushion. In a sense, whoopee cushions are similar to a very sturdy balloon. A feasibility test was run and it was confirmed that the whoopee cushion reacts to pressure changes as did the latex and foil balloons.
Next, a model of the lung was made. Since each lung hold approximately 3L, or 12.68 cups, a model was designed within accurate dimensions. As pictured below, 12.5 cups of flour were packed into 5 sandwich bags. The bags were sealed in a plastic shopping bag and wrapped in tin foil. This design allows for molding so that an accurate shape can be made. Through online research, it was found that the approximate dimensions of the lungs are 21cm in length and 12.3cm in width for a male and 19.0cm and 11.1cm respectively for a female.
After consulting with an employee at Home Depot, it was decided that Gorilla Glue would be the best adhesive to bond rubber. Whoopee cushions were first invented by gluing two sheets of rubber together. The same method was tested during this phase. A dollar store whoopee cushion was cut into two parts. The two sheets were then glued together using Gorilla Glue. After a drying period of 24 hours, the model was tested to ensure proper sealing. The model was filled with water so that the leaks would be visible. The image is posted below.
There was a small break in the seal the first time the model was tested. The break was re-sealed and another 24 hour drying period occurred. The model was re-tested and there was no evidence of leakage. This proves that the proposed rubber-on-rubber design for the lung should be successful.
Below is a drawing which depicts the lungs, intrapleural space (IPS), and a cut image of the ribs. This image provides reasoning why the ribs are designed to be on the outside of the cylinder. Since the the data from the cylinder is supposed to represent the pressure differentials of the IPS, the ribs should not be contained within the space designated for the IPS.
Small Scale Rib Model:
This small scale rib model is 1:5 of the full size rib cage we plan to make. The paper cylinder is a representation of the cylinder used for a pressure vessel. Do to the compression of the cylinder to make it fit, the BC 2 and BC 3 file have been altered by removing a non-functional region of the spine. This saves ¾ of an inch when scaled to 5x. A representation of the altered files can be seen below. Increasing the final size to 5.5x will be evaluated so no post processing is need to fit the cylinder.
Flexible “Ligament Like” Material:
A commercially available, 3D printable, flexible material has been identified to be used for the ligaments attached to the ribs. Ninjaflex has a bulk young’s modulus of ~3 MPa but after printing and cross-patterning of layers is expected to increase this value. This will be evaluated with a tensile testing method. Normal human costal cartilage is ~20 MPa in young adults. Addition resistance can be added with a spring in series with the rib movement cable system.
Rib Attachment Strategy:
Interlocking connections will be added to the ends of the ribs at each connection point. This will add a mechanical anchoring method to the chemical bonding method with glue or acetone.
Rib Movement Profile:
The greatest rib deflection in dogs is seen at the 10th rib. Assuming the same for humans, this makes the 10th rib the most influential in inflating the lungs. This is where the rib-lung inflation system will be located. This system will include an attachment to the 10th rib with a magnet on a track that will interact with a free-hanging piece of metal inside each lung.
Design and Flowcharts
Bill of Material (BOM)Our live Bill of Materials document can be found here.
CasingThe casing will be tested after the shell of the piston is built. All outlets will be closed off and water will be sprayed around any fittings. The casing will then be pumped with air to test for any air leaks. Additionally, a weigh boat of dry ice will be sprayed with water and placed inside a fully sealed model. It will subsequently be visually assessed for vapor leakage. The piston will be pressure tested in the same way.
LungsThe lungs will be made and tested for any leakage. Once the model is built, the pressures can be tested. The current diaphragm does not allow for as much control as the proposed design. Therefore, pressure calculations will need to be a form of testing for the next phase. Also, since both lungs will be made before summer, the storage ability can be tested. With permission from Dr. Bailey, the lung will be left in the cage where it will normally be stored in the BME lab. Upon coming back for fall classes, the lung can be tested in the current model and checked for any sort of damage resulting from storage.
RibsDifferent infill percentages will be tested for rib strength.
The true Young’s Modulus for 3D printed Ninjaflex.
Tolerances for rib attachment strategy.
Risk AssessmentThe current risk assessment can be found here.
Plans for next phase
- Order rubber from the supplier (dependednt)
- Looks at Gum Rubber as a cheaper alternative (supplier)
- Make the rubber lungs for the final product
- Confer with Dr. Linte about possible equations to determine pressure differentials of the proposed design
- Begin storage design
- Finalize fitting sizes
- Determine if we will move forward with original rib idea
- Build rib movement models
- Rib strength analysis
- Ninjaflex analysis
- Provide detailed drawings for chest cavity
- Begin piston model and construction