Pre-clinical bench testing
spasticity, fatigue, perspiration, skin irritation, muscular fatigue, linear actuator failure
Accelerated life testing revealed that the linear actuator was the mechanical component most likely to fail. During testing in both test scenarios (with and without tension springs), the linear actuator failed before any of the 3D-printed components failed or exhibited any signs of wear and mechanical fatigue. The linear actuator failed after 86 h of continuous operation (totaling 26,361 actuations) in the first accelerated life test that simulated grasping an object and after only 47 h (totaling 14,269 actuations) in the second test under constant load with the additional 13.3 N spring force. As expected, the test with the springs caused the linear actuator to fail almost half of the time due to the increased torque requirements. In both tests, the linear actuator stopped working due to the failure of the internal gearbox, which resulted in a higher than nominal current draw of the direct current (DC) motor and eventually triggered the linear actuator’s onboard safety power cut-off circuitry. Despite the linear actuator failure preventing the NuroSleeve from being functional, such a failure mode poses no risk to the user and can therefore be categorized as a safe failure. In both accelerated life tests, the linear actuator duty cycle (ratio between the actuation and stationary time durations) was 66% (4 s on, 2 s off), which is much greater than the 10–20% nominal duty cycle specified for the PA-07 linear actuator [The NuroSleeve’s battery life was tested with continuous hand opening and closing movements lasting for 4 s with a 2-s rest in between. In case of the battery life, with no external loading, the battery life was 13 h; with the stated variable load of maximum 13.3 N, the battery life was 11 h. These results are better than the anticipated (calculated) battery life because the linear actuator in these tests was not operating at maximum torque. Initial clinical trial findings indicate that the NuroSleeve’s battery life in real-life usage is substantially longer, with participants only needing to recharge the battery once a week. This discrepancy is likely because the linear actuator’s “real-life” duty cycle is much lower than the one used for bench-testing and depends upon the wearer’s flexion and extension rate, body size, and level of spasticity or tone in their hand.In order to increase user satisfaction and reduce physical problems such as pressure point formation, muscular fatigue, perspiration, and skin irritation [Illustration of the NuroSleeve system in operation. The MCU and battery are worn on the belt, while an IMU sensor attached to the shoe controls the splint motor in continuous modeThe cost estimate of a NuroSleeve