April 7, 2009
Design Lab Presentation
Update on Status of the Solar Autoclave and Sterilization Indicator
On March 13th the new design team currently working on the solar autoclave system presented the status of the project to their sponsors. Among the attendees of the presentation were Executive Director Lori Hanna, myself (Director of Product Research, Daniel Hensel), longtime sponsor Dr. Margaret Pinnell, and the ETHOS field team accompanying me to Sabana Grande, Nicaragua this summer. The presentation provided a brief overview of the project, the status of what they are working on now, and their plans for the immediate future. The members of the design team, those who presented, are Staci Grey, Christopher McGuinness, Ryan Smolik, Corey Vossler and Kyle Zeller.
The team is on task for the most part. There have been some issues with the pressure release valve that releases pressure if built up beyond 16 or 17psig. Using a proprietary valve with weights, rocks or sand was investigated, but found to be too insensitive. In other words, using these methods, the weighted valve will not release air/steam at the required pressure. The team feels that a precisely machined weight will be necessary, such as those used on traditional pressure cooking pots. This setback has pushed back final design testing by several class periods.
The team displayed the current progress of the sterilization indicator, which is made up of a small plastic container about the size and depth of five nickels stacked together. Inside the plastic container is polywax2000 and a washer. The idea is that the washer starts on top of the solid wax, but as the autoclave heats up and the wax melts, the washer moves to the bottom. There is a calculated amount of wax inside such that when the washer lies on the bottom (i.e. all of the wax has melted), at least 15 minutes have passed, the minimum for sterilizing surgical instruments at 121 deg C and 15psig. There have been problems with these initial prototypes, such as the plastic melting inside the ovens, or pressure being created inside the capsules (due to expansion during the phase change) that causes them to burst. The design team is currently looking into correcting these faults by trying a vented hourglass design instead. This design will be very similar to the capsules previously discussed, except they will be larger, vented and in an hourglass shape and will not contain a washer. Their contingency plan, should all else fail, is to go with a bimetallic-based timer. Currently, the price of on sterilization indicator stands at $2.00, which complies with the team’s goal of under $10 if reusable and about $0.01 if non-reusable.
For the solar autoclave pressure vessel, the team has replaced one of the wooden end caps with an aluminum end cap, shortened the bolts that connect the two end caps and used lock nuts on one end cap. Upon testing, they successfully brought the pressure up to 32psig – well above the 15psig required for sterilization but not quite up to our Factor of Safety of 45psig - before it started to leak at the seal again. They are looking into new sealants, such as silicone pie pans and baking mats. Nevertheless, once again the stresses were well below the yield strength for the can. Then the team discussed how the instruments themselves will lie in the pressure vessel. It was ideal that the instruments would be situated at mid-height in the can, such that they are not lying directly in the water. It was suggested that chicken wire be used to hold the instruments up, but a disadvantage is that only a few instruments could be sterilized at a time due to the size of the can. The team is now developing a test plan for use for the rest of the semester as well as in Nicaragua this summer. Their next task, once the design is completed, will be to determine how much water is needed in the can for sterilization.
The semester is quickly coming to a close and the design team is pooling resources to have testing and prototyping done before the summer. They will present their last findings at their final presentation on the 24th of April at 8am in the UD Design Clinic. Lori and I, as well as the ETHOS field team accompanying me this summer, are anxious to see the final results before I leave for Nicaragua on May 8th. As always, please feel free to comment with questions or suggestions if the urge strikes you!
Gracias, La Paz y Salud,
Daniel J Hensel
Director of Product Research
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