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.

Keywords: Amputees; biomechanical testing/analysis; limb prosthetics; rehabilitation; sensor design.

Figure 1.

Sensor system. (a) Block diagram showing arrangement of sensor components. (b) Antenna and instrumented socket. Left: A schematic of the antenna. The black rectangle is the tank capacitor. Right: The regions of the composite sheath embedded with magnetic particles are shown as red stripes. The gold areas on the socket represent the antennas embedded within the socket. The blue layer is the elastomeric prosthetic liner.

Figure 2.

Test setup with digital scale and test sample. This device was used to evaluate sensor sensitivity for different composite designs.

Figure 3.

Sensitivity testing results from evaluation of puck samples. Sensitivity increased with iron concentration. A polynomial curve fit for the 75 wt% is shown.

Figure 4.

Resolution results from evaluation of puck samples. Measurement resolution at high antenna-to-target distances depended upon iron filler concentration.

Figure 5.

Results from composite-to-sheath bonding evaluations. Left: Nylon sample showing delamination. Right: Edge of cotton sheath showing composite sitting on top of the cotton rather than encapsulating the cotton fibers.

Figure 6.

Magnetic composite sheath formation. A coated sheath on a foam positive about to be inserted into the negative mold.

Figure 7.

Results from testing target antenna misalignment: decay in signal amplitude due to edge proximity. The antenna diameter was 3.2 cm. Based on these results, during clinical use the center of the 32.0-mm diameter antenna was required to be positioned at least 5.1 cm from the target edge. This meant that the target positioned on a liner was required to overlap at least 3.5 cm with the edge of the antenna.

Figure 8.

Custom test fixture for characterizing effect of compression on the magnetic composite material. The antenna was placed within a cavity in the base so that only the composite target was compressed.

Figure 9.

Compression testing results from a magnetic composite sheath with a nylon substrate. Percent signal shift increased with stress and with distance from the target. However, the increases were small.

Figure 10.

Signal loss from field use of the magnetic composite sheath. One participant used the sheath for two weeks and the other for four weeks.