Additive Manufactured Biomimicking Actuator with Shape Memory Polymer Composite for Prosthetic Actuators

In this article we present a biomimicking skeletal muscle actuator for prosthetics to overcome the issues of high operating temperatures, low contractile strain, complexity, high weight and cost, and linear output as seen with current and academic prosthetic actuators. The actuator proposed is a fuse filament fabrication (FFF) printed shape memory polymer (SMP) actuator that has nonlinear contractile and passive forces, contractile forces and strains comparable with mammalian skeletal muscle, reaction time under 1 s, low operating temperature of 70°C, and has a low mass (74.0 mg), volume (46.74 mm³), and material costs of $0.0098 per actuator. The FFF actuator is a SMP melt blend composite of polylactic acid and thermoplastic polyurethane with a mixture ratio of 7:3. The FFF actuator here has nonlinear contractile properties that have peak contractile stresses ranging from 0.58 to 0.17 MPa for 100-60% applied strains, respectively, where mammalian contraction values range from 0.1 MPa (typical/average) to 0.45 MPa (maximum). In addition, the optimal strain of 60% has a strain recovery of 29.2%, which is comparable with the 20% for typical mammalian skeletal muscle strains. The actuator material was first characterized with Fourier transform infrared spectroscopy, scanning electron microscope (SEM), and tensile testing to determine the quality of extruded filament followed by force generation and strain recovery. Lastly, the FFF actuator was tested for thermal degradation and layer adhesion with SEM and differential scanning calorimetry (DSC) testing. SEM imaging revealed that there are layer separations throughout the actuator and DSC showed thermal degradation caused by oxidation and random chain scission due to thermal cycling.