Table of Contents
Team Vision for Subsystem-Level Design PhaseAccomplished this phase:
- Drew prototype
- Determined material cost and purchasing information
- Met with subject matter experts
- Updated risk assessment
- Confirmed usage of foil balloon
- Finalized casing size
- Tested IPS filler designs
- Developed rib movement strategies
- Donation of ABS pellets
- Scale of rib cage
- Decided foil balloon size
- Confirmed physiologically correct pressure differentials for the IPS and lungs during normal breathing
Feasibility: Prototyping, Analysis, Simulation
Casing and IPSDue to a lack of commercially available polycarbonate cylinders, we have decided to use an 8"W by 8"L acrylic cylinder. There will be 3 outlets in the top of the cylinder; one for pressure sensor access to the lungs, one for pressure sensor access to the intrapleural space (IPS), and one to allow for an initial precharge of the intapleural space. Students will be able to make the intrapleural pressure 300Pa lower than atmospheric pressure. To address modeling the special breathing case of asthma, a clamp will be included in the model in order to pinch off the tubing coming from the lungs to represent an inflamed, or blocked, airway.
In order to determine which mechanism was best to decrease IPS, an experiment was conducted. One syringe was filled with a solid insert and it's ability to reach a range of pressure was tested. The second syringe contained an insert with vertical "pores" in it to allow airflow through the insert. Both plunger of each syringe (representing a piston) were pushed approximately the same distance and the total pressure in the models was noted as well as how much pressure was released passively. The solid insert model was determined to be favorable to the porous model since it is easier to fabricate.
There is approximately 0.26ml/kg of pleural fluid in the lungs of a normal, non-smoking human. Using the average weight of a person in North America (80.7kg), we calculated that there is about 20.98mL of fluid. This equates to 21mL in volume. Under the assumption that we used 2-4inch diameter fool balloons in our model, we calculated that an insert would have to be just 0.215mm smaller than our acrylic cylinder to accurately represent intrapleural space.
After discussion amongst the team it was decided that, at least for now, the use of a larger balloon in our smaller cavity (as compared to the current model) will be an adequate decrease in IPS. We expect to revisit filling the cavity with either an insert or filler beads if time and budget allow.
We have begun to design a stand for our model to rest in during use. It will be built out of plywood and contain a ledge so the model can easily slide in and out while remaining securely in place during use.
LungsBy assuming a spherical shape, different volumes were calculated to decide on the size of foil balloon needed. The engineering requirement for the maximum lung volume is 6L. By using the volumetric equation for a sphere, the values for a 4, 5, and 9 inch balloon resulted in .55, 1.07, and 6.26L respectively. As a result of these calculations, it was decided that a single 9 inch balloon would provide the most physiologically accurate values. An upside heart balloon provides a shape similar to the looks of the two lungs.
To maintain residual volume, styrofoam filler beads will be used. Pictured below, styrofoam filler beads are small and light weight. As stated in the engineering requirements, the residual volume should equal 1.5L. With 6.26L available in the 9 inch balloon, the 1.5L of filler beads will easily fit. An airplane headrest, filled with filler beads, has been donated to the team.
After obtaining a foil balloon sample (4 inch hearts), a validation test was run. The current model was utilized with the foil balloon instead of the latex balloon. By doing this, we were able to confirm the foil balloon would react to the pressure change in the same fashion as the latex balloon. The video of the inflating/deflating foil balloon is available below.
Instead of purchasing the 9 inch balloon in a heart shape, it is possible to make your own foil balloon. By making our own balloon, we would have more control over the shape and the two lungs could be better controlled.
DiaphragmFrom benchmarking, we obtained knowledge of a similar lung model designed and fabricated by the University of Wisconsin. This model utilizes a piston mechanism to inflate the lungs in the sealed chamber, simulating a physiologically accurate respiratory action.
Similar ideas have also been implemented in basic automobile design of internal combustion engines (figure below).
Based on the differing size and cost of our model, the following calculations were performed to determine the size and capacity of our diaphragm piston:
Specific designs for rib movement have been drafted and are pictured in the following section. All of these designs are similar in that they source their mechanical movement from the piston. Attachment strategies between the ribs and spine have also been discussed. Preliminary ideas include butterfly hinges, ball and socket joints and a flexible polymer to act as a ligament.
Due to budget constraints, alternatives to buying Makerbot ABS filament ($48/kg) are being explored. Bulk ABS pellets can be purchased for $6.67/kg making it an enticing option. A meeting with Dr. Chris Lewis from the MET department in the plastics lab (82-1165) on March 17th yielded a constructive conversation about the feasibility of manufacturing our own ABS filament. Dr. Lewis graciously donated 0.5 kg of ABS pellets to experiment with and use in the extruder. An active line of communication is in place to schedule a follow up meeting and be trained on the extruder. Students Luiza Mol de Freitas Andrade, Gabriel Mojica and Drew Walters have also donated their time and offered to help with the extrusion process.
Scale of Ribs
With the anticipated size of the casing, the scale of the 3D rib files could be determined. An overlay of the acrylic cylinder and the rib cage can be is pictured below. The multiple used to attain this size of the rib cage was 5x. This created a geometry of 478x368x254mm. This was the largest size tested in a previous feasibility analysis. An array of other cylinder sizes is pictured to show other alternatives. The cylinders pictured are 8x8, 7x10 and 6x14. Altering connection points of the ribs near the top of the sternum can make the rib cage more “barrel chested” to allow for the cylinder to rest further inside the chest cavity.
Drawings, Schematics, Flow Charts, etc.
Bill of Materials (BOM)
The image above is of our Materials and Costs document. The live document can be found here.
The image above is a snapshot of our Bill of Materials. The live document can be found here.
The current risk assessment can be found here.
Plans for next phase
- Obtain pressures under the following conditions utilizing the current model: foil balloon, foil balloon with filler beads
- Decide on final tubing for lung connection size
- Figure out how to utilize filler beads so that they are not moving around in the model
- Figure out if mylar sheets can be utilized to manufacture own foil balloon (desired shape)
- Manufacture ABS filament
- Test ABS filament
- Construct rib movement models
- Machine PVC pipe for rib model prototype
- Confirm piston design and review with SME
- Collaborate with Dakota to confirm rib design
- Re-check volume calculations for piston chamber height
- Assist rib model prototype construction
- Seek SME input for piston volume and lubrication, if necessary
- Design and create preliminary sketch of stand for system
- Determine size and type of outlet fitting between inside and outside of case
- Finalize precharge/plug mechanism
Subject Matter ExpertsSpecial thanks to:
- Dr. Gaborski
- Dr. Mix
- Dr. Lewis
- Luiza Mol de Freitas Andrade
- Beverly Strohm