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

Abstract

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.

An Inductive Sensing System to Measure In-Socket Residual Limb Displacements for People Using Lower-Limb Prostheses

“…Laminating sensors into the socket wall and embedding the target material into prosthetic liners overcame limitations of prior designs, allowing many sensors to be used without disturbing the normal limb–socket interface [ 16 , 17 , 18 ]. The quick in-socket calibration procedure that accounted for each sensor’s unique position and curvature within the socket further contributed to the practicality of the design by reducing the error to less than 3% FSO.…”

“…The wearable sensing system consisted of a low-profile sensor, a flexible iron-seeded elastomeric target, and a portable data acquisition unit ( Figure 1 ) [ 16 , 17 , 18 ]. Sensors were embedded into prosthetic sockets and flexible targets were created by incorporating iron particles into elastomeric prosthetic liners so that the sensors measured the distance between the liner and socket wall.…”

“…The dual-stage calibration procedure was conducted to determine the offset between a single, detailed calibration and the unique in-socket response due to variation in sensor configuration and target variability. This dual-stage calibration also minimizes any error associated with inconsistency in the target liner during in-plane movements of the limb, which were not assessed in this study but have been addressed previously [ 18 ].…”

“…However, debris build-up between fibers of the target material caused significant signal degradation [ 16 , 17 ]. To address the degradation challenges associated with conductive fabric targets, a novel iron-seeded polymeric target was developed and incorporated into prosthetic sheaths worn over elastomeric liners [ 18 ]. The ferromagnetic composite sheaths performed better than conductive targets, reducing signal degradation from 50% in two weeks to less than 3% over four weeks [ 16 , 18 ].…”

“…To address the degradation challenges associated with conductive fabric targets, a novel iron-seeded polymeric target was developed and incorporated into prosthetic sheaths worn over elastomeric liners [ 18 ]. The ferromagnetic composite sheaths performed better than conductive targets, reducing signal degradation from 50% in two weeks to less than 3% over four weeks [ 16 , 18 ]. However, the sheath was susceptible to bunching and it added thickness to the liner that some participants found uncomfortable.…”