Design and preliminary bench-level validation of a low-cost 3D-printed hip–knee exoskeleton for lower-limb rehabilitation training

Lower-limb impairment caused by neurological injury, musculoskeletal disorders, or reduced mobility often requires repetitive rehabilitation exercises, but continuous therapist-assisted therapy may be difficult to access in low-resource settings. In this study, we developed and preliminarily evaluated a low-cost, lightweight, portable hip-knee exoskeleton prototype capable of generating controlled hip and knee joint motion for rehabilitation-oriented range-of-motion training. The prototype was constructed mainly of 3D-printed polylactic acid parts and powered by a pair of high-torque servo motors in the hip and knee joints. A Bluetooth-connected Android app was programmed to offer two modes: rehabilitation mode for independent joint control and a gait-inspired sinusoidal mode that generates coordinated hip and knee joint trajectories. The cost of the materials used to construct the prototype is estimated to be USD 155 with a weight of about 2.1 kg. Joint motion performance was assessed through time-angle measurements at three predefined speed settings. The hip joint (approximately 60° movement) was achieved in 2.2, 1.6, and 1.1 s and the knee joint (approximately 140° movement) was achieved in 3.0, 2.0, and 0.6 s for slow, medium and fast modes, respectively. The trial-to-trial variation in commanded servo position remained below ±1° under the tested conditions. The results demonstrate the feasibility of generating repeatable commanded hip and knee joint motion under controlled laboratory conditions. However, further investigations involving quantitative load-bearing evaluation, safety assessment, user studies, and clinical trials are required before the system can be considered for practical rehabilitation applications.