Poster Q6 - Multi-Material Study of a Passive Prosthetic Foot Design Using 3D Printing

Introduction: In today’s age, there are several different types and ways of building a prosthetic foot. Most can be classified into three main categories: The energy storage and return (ESR) feet, passive feet, and the bionic feet. Out of the three, the passive feet are superior concerning weight, it is the lightest of all three. The light weight of the passive feet reduces the burden on the remaining muscles of the patient. Two main methods are used to achieve the low weight of the prosthetic feet: the selection of the material and structural optimization of the design of the foot. In this study, we looked at four different materials on an optimized foot model.

Materials and

Methods: For each prototype, we used the optimized design proposed by Tao [1]. For Prototype Mark 1, each prototype consists of one of the following materials: Acrylonitrile Butadiene Styrene (ABS), Polyethylene Terephthalate Glycol (PETG), Polylactic Acid (PLA), & Thermoplastic Polyurethane (TPU). Each prototype was designed on Fusion360 using a 10% infill (ABS, PETG, PLA) or 20% infill (TPU). We printed everything on a CreatorPro2 3D Printer, and its slicer, FlashPrint5, by Flashforge. Table 1 outlines the specific printer settings for each of these prototypes.

For Prototype Mark 2, we printed a sole and base prototype using PLA for the base and TPU for the sole. The sole is removable and was printed in two pieces. The sole was printed on an Ender 3V2 using the Creality software. The model was also scaled up to have an overall length of 246mm. A clearance of .5mm was used for every connection point to ensure a secure fit. Table 2 outlines the specific printer settings for this prototype. For both Prototype Mark 1 and 2, each prototype was weighed using a scale with 1 g precision and measured in millimeters.

Results, Conclusions, and Discussions: For Prototype Mark 1 and the base of Mark 2 the outside trapezoid had a bottom length of 210mm and a top length of 55mm connected by sides with 60 and 30 degree angles. The width of the foot is 43mm. There were sections carved out to minimize weight (Figure 1). For the Prototype Mark 2 sole, the thickness of the TPU was 8mm. Table 1 and Table 2 show the weights for each material.

We chose a 10% infill to learn how to model the interior mesh and determine what orientation is strongest to print since it does make a difference in printing the parts and in the simulation of performance. The TPU prototype was printed with a higher percentage of 20% because of the material’s elasticity.

In conclusion, the model printed in ABS was the lightest out of the four different materials, although it is generally considered to be a more brittle material; for example, PETG had been found to have a higher compressive strength [2]. The TPU model was easily deformed (Ninjaflex was used, which has a Shore Hardness of 85A; 95A would be more rigid while 85A has a lot more give), but it could be useful for a detachable sole to increase grip or allow for interchangeable soles with various grips and weights depending on the user’s activity and terrain.

In the future, a static stress analysis in Fusion 360 can be conducted to further determine which material can withstand the greatest weight. An event simulation could determine the friction or grip of each material while a design optimization simulation could be used to determine infill pattern and percentage usefulness. The cost and ease of printing in each material should also be considered and factored in when choosing a material for a base or choosing which sole material to use for which activity and terrain.

Acknowledgements (Optional): This research was partially funded by the SCIO grant.

References:
[1] Z. Tao, H.-J. Ahn, C. Lian, K.-H. Lee, and C.-H. Lee, “Design and optimization of prosthetic foot by using polylactic acid 3D printing,” J. Mech. Sci. Technol., vol. 31, no. 5, pp. 2393–2398, May 2017, doi: 10.1007/s12206-017-0436-2.
[2] B. Tyagi, A. Raj, A. Sahai, and R. S. Sharma, “Enhancing compressive strength in polymer composites utilized for application of foot prostheses,” J. Polym. Res., vol. 31, no. 2, p. 30, Feb. 2024, doi: 10.1007/s10965-024-03880-1.