Design and prototyping of a granular foot orthosis (FootGO)

Abstract

Granular media was utilized in combination with negative pressure to create a variable stiffness granular foot orthosis (FootGO). To develop the FootGO insole, a complete design process was employed, from requirements evaluation and morphological analyses to iterative mockup fabrication. Three types of granular media (rice, poppy seeds, and polystyrene beads) were used in different prototype configurations. The compositions were varied based on volume fill, particulate size, and granular media type. I conducted a series of uniaxial compression tests on a selection of commercial orthotic foams as well as the FootGO prototypes, and obtained stress-strain curves from these data. The stress-strain curves were examined for basic material behavior trends that corresponded to increasing strain rate and stiffness. Most of the prototypes complied with these trends; those that did not were deemed unsuitable for application in a foot orthosis. Then, the energy absorption for each specimen, both foam and prototype, was determined by numerical integration. The foams reached their failure points within the testing period while the FootGO did not, which indicates that the FootGO has superior durability with respect to strain. The range of energy absorption performance for each FootGO prototype was compared to the range for commercial foams. The various negative pressure settings for the FootGO exhibited energy absorption values that spanned the foam performance range, some of them even extending well beyond the upper limit of that range. This supports my hypothesis that varying the negative pressure applied to the granular media within the FootGO produces corresponding adjustable stiffness values that mimic the impact absorption characteristics of a wide range of orthotic materials.