Design and validation of a 3D printed curved hydraulic cylinder for a prosthetic wrist joint

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Introduction: Amputees still experience problems with the use of their upper limb prostheses, mainly in the lack of comfort and functionality. This results in rejection of the prosthesis or overuse injuries due to compensatory movements. The comfort and functionality should be improved by reducing weight and increasing the degrees of freedom. The wrist is an important joint for increasing the mobility. And for reducing the weight, another actuation method than the most available and heavy electrically driven device should be used. Methods: This study designs a hydraulic curved cylinder for the application of a wrist joint. Because of the complex and curved shape the manufacturing method used is 3D printing. The functionality of three 3D printing methods (FDM, MJF and SLA) and three sealing techniques (O-ring, X-ring, and Conus) will be evaluated by testing the amount of friction and leakage to find which configuration performs best and whether it results in an efficient actuator. Results: The SLA-cylinders and the Conus allowed the least leakage. The combination of these two even resulted in zero leakage. For friction the SLA and FDM-cylinders performed equally, only the MJF-cylinder caused more friction. The SLAcylinder is the only cylinder that can hold a constant pressure without leakage for 5 out of 8 pistons, when a 4 kg weight is attached. Finally, only piston-cylinder combinations with piston O-25 or Cylinder X.MJF do not match the requirements for the maximum allowed leakage and adjustment torque. Conclusions: The piston-cylinder combination of SLA-parts and Conus perform the best. For the application of a wrist joint in prostheses more research needed for the repeatability, scalability and long-term performance.