A 3-D-Printed Wearable Sensor Based on Fiber Bragg Gratings for Shoulder Motion Monitoring

Rotator cuff (RC) injury is a common pathology that involves the tearing and weakening of the muscles and tendons that make up RC. Rehabilitation after surgery is crucial for recovery and restoring range of motion (ROM). Currently, the assessment of a patient's improvement relies on subjective evaluations by physicians. To objectively evaluate the patient recovery, motion capture (MOCAP) systems can be used in this scenario. However, MOCAP requires extremely structured environments and specialists. To overcome this issue, the combined use of flexible sensors with 3-D printing techniques has been proposed. This solution will allow developing highly performant wearables with a reduced encumbrance, easy to use, and compliant with the human skin. In this study, we exploited the advantages of fiber Bragg gratings (FBGs) combined with those of 3-D printing to propose a transformative approach in the field of wearables for shoulder monitoring. First, the 3-D-printed sensor was designed and fabricated using thermoplastic polyurethane as printing filament to make the system flexible. One of the main novelties relies on the direct integration of the anchorage mechanism (two extensible elements) during printing. To the best of our knowledge, this is one of the first works in which this approach is proposed. Then, the sensor was metrologically assessed to investigate its sensitivity to strain and temperature and the hysteresis error. Finally, a preliminary assessment on a healthy volunteer was proposed to assess the system capability of monitoring shoulder flexion-extension movements with different ROMs and movement rates in the sagittal plane showing promising results.