Wearable Conductive Fiber Sensors for Multi-Axis Human Joint Angle Measurements

Abstract

BACKGROUND: The practice of continuous, long-term monitoring of human joint motion is one that finds many applications, especially in the medical and rehabilitation fields. There is a lack of acceptable devices available to perform such measurements in the field in a reliable and non-intrusive way over a long period of time. The purpose of this study was therefore to develop such a wearable joint monitoring sensor capable of continuous, day-to-day monitoring. METHODS: A novel technique of incorporating conductive fibers into flexible, skin-tight fabrics surrounding a joint is developed. Resistance changes across these conductive fibers are measured, and directly related to specific single or multi-axis joint angles through the use of a non-linear predictor after an initial, one-time calibration. Because these sensors are intended for multiple uses, an automated registration algorithm has been devised using a sensitivity template matched to an array of sensors spanning the joints of interest. In this way, a sensor array can be taken off and put back on an individual for multiple uses, with the sensors automatically calibrating themselves each time. RESULTS: The wearable sensors designed are comfortable, and acceptable for long-term wear in everyday settings. Results have shown the feasibility of this type of sensor, with accurate measurements of joint motion for both a single-axis knee joint and a double axis hip joint when compared to a standard goniometer used to measure joint angles. Self-registration of the sensors was found to be possible with only a few simple motions by the patient. CONCLUSION: After preliminary experiments involving a pants sensing garment for lower body monitoring, it has been seen that this methodology is effective for monitoring joint motion of the hip and knee. This design therefore produces a robust, comfortable, truly wearable joint monitoring device.

Figure 1

Sensor Design Schematic. This particular sensor arrangement shows one sensor thread running lengthwise across a single-axis knee joint.

Figure 2

Lower Body Sensors. Schematic of three sensors positioned to measure three lower body joint angles.

Figure 3

Sensor Output Curve. Preliminary data showing sensor output vs. knee flexion angle.

Figure 4

Sensor Arrays. (a) Array of equidistantly spaced sensors over knee joint. (b) Array shifted by an unknown distance, α.

Figure 5

Sensitivity Shifts. (a) Array of equidistantly spaced sensors over knee joint, with each sensor having unique sensitivity in this calibration position. (b) Shifting of array by an unknown distance, α, will lead to a shift in sensitivities.

Figure 6

Registration Procedure for Hip Sensor Array. For registration of individual sensor arrays, the patient moves only one axis at a time (a) flexion/extension, and (b) abduction/adduction.

Figure 7

Prototype Sensing Garment. Spandex pants with conductive fiber sensors for lower body monitoring.

Figure 8

Preliminary experiments set-up. Measurements were taken from a standard electrogoniometer at Position 1 (0°) and Position 2 (50°)

Figure 9

Sensor outputs – Comparison of goniometer measured knee joint angle and estimated angles from wearable conductive fiber sensor.

Figure 11

Self-Registration Results. Joint angle measurements with sensing garment taken off and put back on before each test.

Figure 12

Multi-Axis Sensor Outputs. Hip sensor outputs for two distinct leg motions.

Figure 13

Hip Joint Measurement Results. Comparison of goniometer measured hip joint angles and estimated angles from wearable conductive fiber sensors: (a) Hip flexion/extension, (b) Hip abduction/adduction.

Figure 10

Frequency Variation Results. Joint angle estimations for various frequencies of joint motion.