How to 3D-Print Compliant Joints with a Selected Stiffness for Cooperative Underactuated Soft Grippers

Soft robotics is an expanding area of research which exploits compliance and adaptability of soft structures to design highly adaptive robots for contact interactions. The ability to 3D-print materials with softer, more elastic materials properties is a recent development and a key enabling technology for the rapid development of soft robots. Among the possible applications, soft cooperative grippers and manipulators have demonstrated a high potential impact. However, retrieving data about mechanical properties of such novel soft materials and information about the parameters to select in the 3D-printers is often not straightforward. The aim of this chapter is to systematically investigate the mechanical properties of 3D-printed specimens from one of the most used soft filament, the Ninjaflex (Lulzbot, USA). In particular, we focus on the characterization of bending stiffness with reference to the selection of different infill density and printing patterns. We also report on how the collected data can be used in the design phase to obtain a desired behaviour of a soft gripper and on repeatability of the 3D-printing process. Finally, we tested with a finger of a cooperative gripper how the different approaches to obtain a given stiffness value affect the flexion/extension trajectory. This work is intended to be a useful tool for researchers who use rapid-prototyping techniques to develop soft grippers and it also serves as a possible guideline to the characterization of the mechanical properties of novel soft filaments.