No abstract

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self‐healing polymers address this vulnerability. Self‐healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self‐healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self‐healing polymers, have been created. Herein, the wide variety of processing techniques of self‐healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross‐links present in the self‐healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi‐material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self‐healing polymer and the intended soft robotics application.

The field of self-healing soft robots was initiated a few years ago. A healing ability can be integrated in soft robots by manufacturing their soft membranes out of synthetic self-healing polymers, more specifically elastomeric Diels-Alder (DA) networks. As such they can recover completely from macroscopic damage, including scratches, cuts, and ruptures. Before this research, these robots were manufactured using a technique named ''shapingthrough-folding-and-self-healing.'' This technique requires extensive manual labor, is relatively slow, and does not allow for complex shapes. In this article, an additive manufacturing methodology, fused filament fabrication, is developed for the thermoreversible DA polymers, and the approach is validated on a soft robotic gripper. The reversibility of their network permits manufacturing these flexible self-healing polymers through reactive printing into the complex shapes required in soft robotics. The degree of freedom in the design of soft robotics that this new manufacturing technique offers is illustrated through the construction of adaptive DHAS gripper fingers, based on the design by FESTO. Being constructed out of self-healing soft flexible polymer, the fingers can recover entirely from large cuts, tears, and punctures. This is highlighted through various damage-heal cycles.

In new-generation soft robots, the actuation performance can be increased by using multiple materials in the actuator designs. However, the lifetime of these actuators is often limited due to failure that occurs at the weak multi-material interfaces that rely almost entirely on physical interactions and where stress concentration appears during actuation. This paper proposes to develop soft pneumatic actuators out of multiple Diels–Alder polymers that can generate strong covalent bonds at the multi-material interface by means of a heat–cool cycle. Through tensile testing it is proven that high interfacial strength can be obtained between two merged Diels–Alder polymers. This merging principle is exploited in the manufacturing of multi-material bending soft pneumatic actuators in which interfaces are no longer the weakest links. The applicability of the actuators is illustrated by their operation in a soft hand and a soft gripper demonstrator. In addition, the use of Diels–Alder polymers incorporates healability in bending actuators. It is experimentally illustrated that full recovery of severe damage can be obtained by subjecting the multi-material actuators to a healing cycle.