Development of a magnetic composite material for measurement of residual limb displacements in prosthetic sockets

INTRODUCTION: Wearable limb–socket displacement sensors may help patients and prosthetists identify a deteriorating socket fit and justify the need for repair or replacement. METHODS: A novel sensor using an inductive sensing modality was developed to detect limb-to-socket distances. Key detection elements were a coil antenna placed in the socket wall and a magnetic composite sheath worn over the outside of the prosthesis user's elastomeric liner. The sheath was a nylon or cotton prosthetic stocking coated with a polyurethane composite. The polyurethane composite contained embedded iron particles (75 wt%). RESULTS: Brushing γ-glycidoxypropyltriethoxysilane onto the sheath fabric, coating it first with unfilled polyurethane and then iron-filled polyurethane, enhanced bonding between the sheath and the composite and overcame mechanical degradation problems. A γ-glycidoxypropyltriethoxysilane-rich fumed silica layer applied to the outside of the sheath reduced friction and improved durability. Field testing demonstrated less than a 3% signal degradation from four weeks of field use. CONCLUSIONS: The developed wearable displacement sensor meets durability and performance needs, and is ready for large-scale clinical testing.