Multi-Material 3D Printing of a Flexible Sensor Based on TPU and CB-TPU for Tennis Player Joint Movement: A Preliminary Study

Monitoring joint biomechanics plays a critical role in both performance optimization and injury prevention in sports such as tennis, where the elbow undergoes high-speed, repetitive flexion-extension movements. Accurate tracking of joint angles can support personalized training, improve stroke efficiency, and facilitate rehabilitation. Conventional systems, including optical motion capture and inertial measurement units (IMUs), offer valuable insights but are limited by rigid structures, complex calibration procedures, and poor skin compliance. To address these limitations, additive manufacturing-particularly 3D Printing Fused Deposition Modeling (FDM) technique-offers a promising route for developing lightweight and flexible wearable systems, tailored to the human body. This study presents the first fully 3D-printed bending sensor integrating a CB-TPU conductive track directly into a flexible TPU 65A substrate during fabrication. The selected material exhibits high softness and elasticity, enhancing conformability to the elbow and overall user comfort. The device was characterized through both static and dynamic tests. Under controlled bending at known angles (0°, 30°, 60°, 90°, 120°), the sensor exhibited a monotonic increase in resistance, with a maximum relative variation (Δ R/R0) of ∼ 10% at 120°. When applied to the elbow, the system reliably followed the joint's flexion-extension and successfully captured the three distinct phases of a simulated tennis forehand: backswing, acceleration, and follow-through. These results highlight the potential of the developed sensing platform as a flexible, comfortable, and low-cost solution for accurate joint monitoring, supporting the integration of 3D-printed smart materials into next-generation wearable technologies for human motion analysis.