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Design Components
There were six key components that allowed the Sole Survivors to fully resolve the project's primary and secondary objectives. First of all, the primary material that was used to make a Sole Survivor was 3mm thick neoprene. Neoprene is a synthetic rubber best known for its usage in the making of wetsuits. Thanks to this prior knowledge, it became clear that neoprene was the best option to fulfill the secondary qualities. This thickness of neoprene did not perform well at all in the sting simulation pendulum testing, only being able to stop a dull barb hitting it at average force and nothing else.
Figure 1
Therefore, to remedy this problem, rubber soling sheets were sewn in between two layers of neoprene so that it would pass the puncture tests without sacrificing the neoprene's flexibility and comfortability. An illustration of this neoprene "pocket" to hold the rubber can be seen above in Figure 1. The rubber sheet was cut into large separate pieces to prevent it from contorting against the foot's top contour, thereby any chances of motion being hindered or uncomfortable.
Fabrication of the insole for a Sole Survivor was achieved by completely covering a composite sheet of 3D printed PLA hexagons in a thermoplastic adhesive. The rigidity offered by the PLA was necessary in order to completely negate the worst-case scenario of stepping directly on a barb with the user's full body weight. Figure 2 below depicts the process of drawing out a foot shape and cutting it out of the printed sheet.
Figure 2
Without a thermoplastic adhesive applied to it, the insole showed a semi-flexible quality but also was extremely brittle because it was only held together by the linkages between the hexagons. Fully encasing the sheet of PLA hexagons in hot glue eliminated the brittleness while granting the insole more flexibility than before. The case for this is demonstrated in Figure 3.
Figure 3
Figure 3
Lastly, the final design incorporated the insole by way of another neoprene "pocket", similar to Figure 1. The pocket prevented the insole from moving around too much, allowing the user a stable platform with which to walk on. It also kept sand particles from getting stuck on the insole and possibly wearing away the hot glue casing.
Figure 4
The final two components of a Sole Survivor, the zipper and the velcro strap, played a more important role than they would seem at first.
The justification for to use a zipper as the primary putting-on-and-taking-off mechanism was that this would be the easiest to operate in such a manner for any user. The zipper's placement along the side of the foot and up the ankle along with its 22.86 cm length made this operation even more easier.
To prevent the footwear from slipping off of the user's feet, the cinch strap was attached directly above the ankle area. The 50.8 cm length of the strap made it long enough to wrap around the lower leg area very securely without any chance of the velcro being ripped off by the ocean currents. Additionally, the tightening of the strap sealed off water from seeping in to form a bubble between the skin and the neoprene-rubber layer, thereby not inhibiting the user's movements in the water.
Figure 5
Performance Testing
The circumstances surrounding the COVID-19 pandemic made it difficult to access a stingray in order to perform live testing of the Sole Survivors. To approximate as best as possible, performance testing was conducted through two phases: puncture-resistance testing and field testing.
Figure 6
The pendulum, as seen in the diagram to the left in Figure 6, was calibrated to simulate a stingray sting impulse based on data given to us by the sponsors. Materials that were not punctured or showed promise in other aspects such as flexibility would move onto combination puncture testing where a variety of combinations of materials were tested to see if any of these combinations would give the ability to make a water sock with them. Finally, a final series of puncture tests were conducted on the final prototype to assess the validity of the final design. A secondary test was also conducted by putting the entire body weight of one of our team members with an insole sample onto a sharp barb as a proof of concept which also showed great success in validating our design.
Field testing of the Sole Survivors took place at the beach. The categories they were graded on, which can be seen in the results table in Figure 7, were chosen to best represent the functional requirements for a footwear-type item. The grading scale was based on trying to quantitatively pinpoint how the final design fulfilled the secondary objectives.
Figure 8: Demonstration of Zipper's Functionality (not Final Design)
The "Putting On" category received the highest possible score because of the zipper design and location; as seen in Figure 8 above, the wide opening provided ample room to slide a foot in and zip shut.
Grades for "Feel of the Sole" and "Bottom Contour" reflected the insole's ability to fulfill both the primary puncture-resistance quality and the secondary functionality qualities. Preventing the former from scoring higher was the outer neoprene pocket not being sewn tightly around the sole, thereby leading to very minor issues with maintaining footing.
Figure 7
The top contour of the final design graded extremely well against previous designs thanks to the rubber being cut into separate pieces rather than being kept as one sheet. The rubber was able to conform more to the shape of the foot and did not hinder flexibility or movement.
All phases of overall movement using the Sole Survivors were completed without any issues arising. Walking around with them on regular ground, dry sand, and wet sand did not feel as if one was walking with shoes on at all, but rather with socks. The use of neoprene was able to lead to this development. Moving around, even swimming, with them on in the ocean felt almost exactly the same as when just simply strolling on the beach. The drag from the ocean current and crashing waves proved to be a nearly nonexistent factor in moving around. Tightly strapping the velcro down helped to keep any noticeable instances of drag from being bothersome to the user.
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