Additive Manufacturing for Bioinspired Structures: Experimental Study to Improve the Multimaterial Adhesion Between Soft and Stiff Materials

The fabrication of bioinspired structures has recently gained an increasing popularity: mimicking the way in which nature develops structures is a vital prerequisite in soft robotics to achieve multiple benefits. Stiff structures connected by soft joints (recalling, for instance, human bones connected by cartilage) are highly appealing: several prototypes have been manufactured and tested, demonstrating their full potential. In the present research, the material extrusion (MEX) additive manufacturing technology has been used to manufacture stiff-soft bioinspired structures activated by shape memory alloy (SMA) actuators. First, three commercially available stiff composite plastic materials were investigated and linked to different 3D printing infills. Surprisingly, we found that the "gyroid" infill was correlated to the mechanical properties, demonstrating that it produces better results in terms of Young's modulus and ultimate tensile strength (UTS) than the widely studied "lines" infill. The primary focus of the research is an experimental study aimed at improving the adhesion at the interface between stiff and soft materials using an inexpensive method (i.e., MEX). Three different variables that have significant effects on the interface bonding were studied: (1) the interface geometry between stiff and soft parts, (2) the mesh overlapping process parameter, and (3) the annealing post-treatment. By optimizing the three variables, a Young's modulus of 48.8 MPa and a UTS of 3.8 MPa were achieved, when nylon+glass fiber (a stiff material) and thermoplastic polyurethane (a soft material) were 3D printed together. In particular, the 3.8 MPa UTS is 48% higher than the highest adhesion between the soft and stiff material (thermoplastic polyurethane [TPU] and acrylonitrile butadiene styrene) reported in literature. Finally, taking advantage of the improved stiff-soft adhesion, a bioinspired robotic finger has been fabricated and tested using an SMA actuator, showing an enormous potential for the proposed additive manufacturing approach in realizing bioinspired systems.