Incorporating a Ferrous Polymer Target into Elastomeric Liners for Socket Fit Sensing in Prosthesis Users

Abstract

Liner-to-socket distance measurement using inductive sensing may be an effective means to continuously monitor socket fit in people using trans-tibial prostheses. A practical limitation, however, is a means to incorporate a thin uniform-thickness layer of conductive or magnetically permeable target material into the wide range of prosthetic liner products that people with limb amputation commonly use. In this paper, a method is presented whereby a 0.50-mm thickness ferrous polymer made from a SEEPS polymer and iron powder that is formed adjacent to a 0.25-mm thick non-ferrous layer of SEEPS polymer is assembled between two sheets of elastic fabric material. Bench testing showed that the fabrication procedure achieved a root-mean-square error in the thickness of this construct of 58 μm, helping to create a consistent calibration result over the entire surface. The original fabric backing of an off-the-shelf prosthetic liner was removed and replaced with the developed construct. When worn in the shoe of an able-bodied participant for 7.5 h per day for 28 days, the sensor well maintained the shape of its calibration curve at the start of wear, but a distance offset (shifting of the y-intercept) was introduced that increased during the initial approximately 12 days of wear. When the distance offset was corrected, for the primary distance range of clinical interest for this application (0.00-5.00 mm), the sensor maintained its calibration within 4.4%. Before being used in clinical application for liner-to-socket distance monitoring, new ferrous liners may need to be pre-worn so as to achieve a consistent distance reference.

Keywords: amputee; distance sensing; inductive sensor; interface mechanics; prosthetics; residual limb; socket fit; trans-tibial; volume management.

Figure 1

Ferrous liner. (a) Cross-section of a ferrous liner immediately after fabrication. (b) Three possible configurations of measurement. Orange arrows indicate distance between the target and socket.

Figure 2

Mold on hotplates. Top lid being positioned in place.

Figure 3

FPPC construct being removed from the mold.

Figure 4

Completed elastomeric liner with an embedded ferrous polymer target. The arrow indicates the proximal edge of the e-FPPC. The circles indicate locations of sensor antennae when the liner is within the user’s socket.

Figure 5

Calibration unit.

Figure 6

Results from the 28-day durability test. (a) Data from all test days corrected for distance offset (left shoe sample (Trial 1)). Results were comparable for both samples. (b) Distance offset over time for the first and second trials for both samples and the mean. S1 = left shoe sample; S2 = right shoe sample; Tr1 = Trial 1; Tr2 = Trial 2.

Figure 7

Exemplary data from a participant with trans-tibial amputation. Data from three steps are shown. Stance phase is unshaded, and swing phase is shaded gray.

Figure 8

Range of limb–socket distance within a step: swing phase maximum and stance phase minimum. Results from steps wearing a three-ply sock and no sock during a trial are shown.