Russell Dibb, David Williams, and Dr. Mark Colton, Mechanical Engineering Department
The purpose of the prosthetic leg project was to research and design a below-the-knee prosthetic leg that could be used in developing countries. Many prosthetic legs that are currently on the market are very expensive and difficult to manufacture in undeveloped areas of the world. The engineering challenges that the prosthetic leg design addressed include a device that is 1) capable of being manufactured at a low cost ($100-$500), 2) simply and easily reproducible in a developing country, 3) durable enough to hold up to the vigorous conditions that exist in a developing country, and 4) able to effectively modeling the biomechanics of the human leg. Our research focused on the suspension system and structure of below-the-knee prostheses.
A unique design that we considered in our project uses technology similar to current prosthetic limbs that use curved members. Initially, these slightly curved blades, or thin, semi-rectangular bars connected into the foot and socket of the leg to form a structure and allowed for some shock absorption. Using computer-aided design (CAD) tools, we attempted to design the blades to bend under stress and return to their original form once the stress is removed. We then applied different loading scenarios to our analytical prototype and analyzed the resulting maximum stresses.
One obstacle to this process was our inability to find a material that was strong enough and that could be easily found and manufactured in developing nations. No widely available or suitable material could be found to satisfy our safety criteria. Prosthetic limbs must be safe enough to withstand around four times the body weight of an individual – the approximate force generated by jumping. For example, if an amputee were to use our leg design, it should not flex under his/her weight, but if he/she were to jump and land on the prosthetic leg, this system should safely absorb the shock by deforming elastically but not plastically. Once stable, it would return to its fully extended form.
In order to yield a safer product, we designed the leg with thicker curved members. Accordingly, the weight, expense, and rigidity of metals caused us to consider alternative structural materials such as bamboo and various plastics. One such plastic material, polyvinyl chloride (PVC), proved to be an excellent choice for the construction of the prosthesis. It is inexpensive, easily formed and manufactured into many shapes, and durable. Most surprisingly, it allows for energy storage and release when flexed and relaxed. This is essential to effectively modeling the biomechanics of the human leg and foot.
Using commonly available machining tools (a hack saw and drill), we strategically machined notches into a segment of PVC pipe. We then baked the pipe in a conventional kitchen oven until the PVC was soft and moldable. Wearing protective gloves, we formed the PVC with our hands into the desired shape. The key components of this shape are the curved semicircular members that form the toe and heel of the prosthesis and allow for shock absorption.
Several compression tests were performed in order to verify our CAD models and establish that the prosthesis could safely support forces four times great than the body weight of the amputee. Using the Instron machine in the Mechanical Engineering Materials Laboratory, we performed several tests and found that many prototype iterations of the leg supported over 800 lbs. The loading force versus displacement data showing the maximum load of one of the prototypes is shown in Figure 1. In addition to the maximum force test, we performed 1000-cycle fatigue tests on the 3-piece prototype. In these loading scenarios, the concept prototypes typically cracked. We therefore added a bolt to the design to reduce the stresses at the peak of the cut (see Figure 2). Further testing and analysis will be required to determine the exact durability of this concept.
This concept met several of our design goals. The cost of the prosthesis is well below our target range, and the product is easily manufactured. To fulfill our durability goal of supporting four times body weight, we had to modify the design of the prosthesis to use 2-3 PVC pipes of varying sizes that flexed in concert when loaded. Lastly, the foot absorbs some impact; however, future modifications to the design may include cutting a slot in the PVC shaft to create a bowed-out shin that models the human leg by acting as a shock absorber (see Figure 2). Fatigue, load, and deflection testing would need to be performed on the bowed-out design.
Additional research is needed to optimize the design of the leg to maximize shock absorption and strength while minimizing weight and material costs. Research and testing will be required to resolve complications involving the structure’s interface with the socket and the leg stump of the amputee. These complications, however, were outside the scope of this particular ORCA project and were addressed by other members of the prosthetic leg project team. Through the continued collaboration with the different areas of the team, we are working with local humanitarian organizations to set up a clinic in a developing nation and instruct the people there how to build our prosthetic device.