Material properties of commonly-used interface materials and their static coefficients of friction with skin and socks

Compressive stiffness (CS) of different supporting materials used in prosthetics and orthotics and their static coefficients of friction (COF) with skin and socks were characterized. Materials tested included Spence, Poron, nylon-reinforced silicone, Soft Pelite, Medium Pelite, Firm Plastazote, Regular Plastazote, and Nickelplast. A displacement-controlled testing device was constructed to assess the CS of 11.1 mm diameter material specimens under cyclic loading (1 Hz) to 220 kPa over 10- and 60-min periods. Results demonstrated local CS ranging: from 687 kPa (Poron) to 3,990 kPa (Soft Pelite). To fit the cyclic stress-strain (S-S) data within an error of 4.0 percent full-scale output, the minimum order of fit required for Spence, Poron, and nylon-reinforced silicone was a third-order polynomial; for Soft Pelite, Medium Pelite, Firm Plastazote, and Regular Plastazote, a second-order polynomial; and for Nickelplast, a linear fit. For all materials, the nonrecovered strains were related to loading time using an exponential fit. A biaxial force-controlled load applicator device was used to assess COF at skin-material, sock-material, and skin-sock interfaces for shear forces of 1 to 4 N applied to a 10.2x7.8 mm loading pad. COFs ranged from 0.48 (+/-0.05) to 0.89 (+/-0.09). COFs at skin-material interfaces were significantly (p<0.05) higher than those at skin-sock interfaces. There was a trend of a higher COF at sock-material interfaces than at skin-sock interfaces. These data are of potential utility in finite element modeling sensitivity analysis of residual limb-prosthetic socket systems or body-orthosis systems to characterize effects of material features on interface pressure and shear stress distributions.