Inspired by the compliance found in many organisms, soft robots are made almost entirely out of flexible, soft material, making them suitable for applications in uncertain, dynamic task-environments, including safe human-robot interactions. Their intrinsic compliance absorbs shocks and protects them against mechanical impacts. However, the soft materials used for their construction are highly susceptible to damage, like cuts and perforations caused by sharp objects present in the uncontrolled and unpredictable environments they operate in. In this research we propose to construct soft robotics entirely out of self-healing elastomers. Based on healing capacities found in nature, these polymers are given the ability to heal microscopic and macroscopic damage. Diels-Alder polymers, being thermoreversible covalent networks, were used to develop three applications of self-healing soft pneumatic actuators; a soft gripper, a soft hand, and artificial muscles. Soft pneumatic actuators commonly experience perforations and leaks due to excessive pressures or wear during operation. All three prototypes were designed, using finite element modelling, and mechanically characterized. The manufacturing method of the actuators exploits the self-healing behaviour of the materials, which can be recycled. Realistic macroscopic damage could be healed entirely using a mild heat treatment. At the location of the scar, no weak spots were created and the full performance of the actuators was nearly completely recovered after healing.

No abstract

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.

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.

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.

Healable soft robotic systems have been developed by constructing their flexible membranes out of Diels-Alder polymer networks. In these components, relatively large damages in the centimetre scale can be healed, provided that the temperature is increased to 80-90 °C. This paper presents a new Diels-Alder polymer network that can heal at room temperature by smart design of the network, increasing the molecular mobility in the material at room temperature. This new material is used to develop the first healable soft robotic prototype that can recover its performance from large, realistic damages entirely autonomously. The soft pneumatic hand can heal various damages, including being cut completely in half, without the need for a temperature increase. After healing, the performance of the soft robotic prototype is recovered.

Inspired by the intrinsic softness and the corresponding embodied intelligence principles, soft pneumatic actuators (SPA) have been developed, which ensure safe interaction in unstructured, unknown environments. Due to their intrinsic softness, these actuators have the ability to resist large mechanical impacts. However, the soft materials used in these structures are in general susceptible to damage caused by sharp objects found in the unstructured environments. This paper proposes to integrate a self-healing (SH-) mechanism in SPAs, such that cuts, tears and perforations in the actuator can be self-healed. Diels-Alder (DA-) polymers, covalent polymer network systems based on the thermoreversible DA-reaction, were selected and their mechanical, as well as SH-properties, are described. To evaluate the feasibility of developing an SPA constructed out of SH-material, a single cell prototype, a SH-soft pneumatic cell (SH-SPC), was constructed entirely out of DA-polymers. Exploiting the SH-property of the DA-polymers, a completely new shaping process is presented in this paper, referred to as 'shaping through folding and self-healing'. 3D polygon structures, like the cubic SH-SPC, can be constructed by folding SH-polymer sheet. The sides of the structures can be sealed and made airtight using a SH-procedure at relatively low temperatures (<90 °C). Both the (thermo) mechanical and SH-properties of the SH-SPC prototype were experimentally validated and showed excellent performances. Macroscopic incisions in the prototype were completely healed using a SH-procedure (<70 °C). Starting from this single-cell prototype, it is straight-forward to develop a multi-cell prototype, the first SPA ever built completely out of SH-polymers.

Soft robotics aims at creating systems with improved performance of movement and adaptability in unknown, challenging, environments and with higher level of safety during interactions with humans. This Roadmap on Soft Robotics covers selected aspects for the design of soft robots significantly linked to the area of multifunctional materials, as these are considered a fundamental component in the design of soft robots for an improvement of their peculiar abilities, such as morphing, adaptivity and growth. The roadmap includes different approaches for components and systems design, bioinspired materials, methodologies for building soft robots, strategies for the implementation and control of their functionalities and behaviour, and examples of soft-bodied systems showing abilities across different environments. For each covered topic, the author(s) describe the current status and research directions, current and future challenges, and perspective advances in science and technology to meet the challenges.