Angelina Schwartz and Dr. Eric Homer, Department of Mechanical Engineering

The aim of 2ft Prosthetics is to design, test, and distribute low-cost prostheses to below-the-knee amputees in developing countries. To do this, a research and development team focuses on the design and construction of the prosthesis. It is then tested on local amputees by the clinical trials team. During this interaction we gain many insights regarding improvements that can be made to the prosthesis to make it more comfortable, reliable, and versatile. We learned that many high- level prostheses commonly used here in the United States are not designed to be used in wet environments. Amputees must remove it to do certain activities such as swimming, or, even daily activities such as bathing or showering. For this reason, we began investigating the functional requirements of a prosthesis that would not be adversely affected by water.

We immediately recognized that our current prosthesis made of PVC components (see Figure 1) was well suited for wet environments. However, in order to provide needed safety for the amputee, the prosthesis would need to offer good traction on wet surfaces in order to prevent slipping. We began searching for materials with low-slip or no-slip properties. We also needed to determine the best way to incorporate that material onto the current model. Six materials were selected for testing—two were different types of soft rubber, one was vinyl, one material was made of rubberized foam and two others were adhesive tapes used in aquatic environments such as a pool side or the deck of a boat. For those materials that were not self-adhesive, we used Gorilla glue to fasten them to the bottom of the PVC sole.

Testing

To determine the material with the best no-slip properties, we designed an experiment to measure the coefficient of friction for each material, as well as bare PVC as a control. We began researching ways that this is commonly tested. One simple experiment that can be used is a static pull test in which the material to be tested is placed in contact with another material and the friction force required to move the materials relative to one another is measured. Because most showers and tubs are either of acrylic or have an acrylic coating, we used an acrylic sheet as the contact surface in our friction tests. Each material was tested a total of 12 times—six times each being both pushed and pulled across a dry surface, and six more times each being pushed and pulled across a wet surface. Figure 2 illustrates the test setup. A spring gauge was usedto measure the maximum force required to move the weight with the test material attached to the bottom. The static coefficient of friction, µ_s, was calculated using

where F is the force measured by the spring gauge and w is the weight of the block. The results are summarized in Table 1.

Conclusion

From the results gathered, the decided best material to incorporate into the current 2ft Prosthetics model was material #6 (clear vinyl made by Zenith). This material exhibited the highest coefficient of static friction on a wet surface. Also, the use of Gorilla Glue was very effective, giving a very strong bond to the PVC sole. Once we completed our tests, we presented our findings to the rest of the volunteers working with 2ft Prosthetics.

With the materials researched and decided, we are now investigating the safety requirements that may exist, as well as working on a patent and how best to market the product. We plan to do this by receiving help from those within the 2ft Prosthetics group that were involved with safety requirements and patents in the original 2ft Prosthetics model. We hope to use this product as a method to generate funds for the group to continue working with amputees in developing countries.