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 p rosthetics and orthotics and their static coefficients of friction (CO F) with skin and socks were characterized. Materials tested included S pence, Poron, nylon-reinforced silicone, Soft Pelite, Medium Pelite, F irm Plastazote, Regular Plastazote, and Nickelplast. A displacement-co ntrolled testing device was constructed to assess the CS of 11.1 mm di ameter material specimens under cyclic loading (1 Hz) to 220 kPa over 10- and 60-min periods. Results demonstrated local CS ranging: from 68 7 kPa (Poron) to 3,990 kPa (Soft Pelite). To fit the cyclic stress-str ain (S-S) data within an error of 4.0 percent full-scale output, the m inimum order of fit required for Spence, Poron, and nylon-reinforced s ilicone was a third-order polynomial; for Soft Pelite, Medium Pelite, Firm Plastazote, and Regular Plastazote, a second-order polynomial; an d for Nickelplast, a linear fit. For all materials, the nonrecovered s trains were related to loading time using an exponential fit. A biaxia l force-controlled load applicator device was used to assess COF at sk in-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 0.4 8 (+/-0.05) to 0.89 (+/-0.09). COFs at skin-material interfaces were s ignificantly (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 syst ems or body-orthosis systems to characterize effects of material featu res on interface pressure and shear stress distributions.