Abstract
The emergence of soft machines and electronics creates new opportunities to engineer robotic systems that are mechanically compliant, deformable, and safe for physical interaction with the human body. Progress, however, depends on new classes of soft multifunctional materials that can operate outside of a hard exterior and withstand the same real-world conditions that human skin and other soft biological materials are typically subjected to. As with their natural counterparts, these materials must be capable of self-repair and healing when damaged to maintain the longevity of the host system and prevent sudden or permanent failure. Here, we provide a perspective on current trends and future opportunities in self-healing soft systems that enhance the durability, mechanical robustness, and longevity of soft-matter machines and electronics.

Introduction
Engineering machines and electronics to be smaller, lighter, and less rigid enables them to better integrate into the environment and interact with humans. Such progress has the potential to revolutionize fields such as personal computing and health care by introducing new classes of textiles with digital circuit functionality, epidermal stickers for wireless biomonitoring, and soft robotic systems capable of human motor assistance or therapy. However, removing the hard exterior and making machines and electronics soft and lightweight introduces challenges in design since the materials are required to be simultaneously durable, tough, and load bearing as well as mechanically compliant and deformable. This requirement introduces a potential dilemma in materials engineering that arises from the typical tradeoff between strength and compliance1, which is also common in natural materials and organisms. In nature, organisms address this apparent tradeoff with passive architectures that combine soft and rigid materials with functionally graded interfaces and active mechanisms, such as reversible rigidity tuning and self healing. However, while there has been significant recent attention paid to the rigidity tuning2 and self-healing ability of naturally stiff polymers and composites3, there has been considerably less attention paid to how self-healing materials can also be used to make soft material systems more mechanically robust.

From this perspective, we present an overview of the current progress in self-healing materials that can enable soft-matter machines and electronics to support more extreme mechanical loading without catastrophic failure. This overview includes a working definition of “self-healing” within the context of soft material systems and examples related to structural materials, electronics, and robotic systems, as distinguished in Fig. 1. More broadly, these material architectures fall under an increasingly growing family of soft bioinspired systems for sensing, circuit wiring, and actuation that exhibit many of the same robust mechanical properties as their natural tissue counterparts. These systems are fabricated using a wide variety of materials and manufacturing methods, from deterministically patterned thin-film wavy electronics produced using cleanroom fabrication methods to soft microfluidics and biohybrid systems created using wet-lab soft lithography techniques. As these soft-matter technologies leave the lab and enter real-world environments, they must be further refined or re-engineered so that they can withstand not only stretching, bending, and twisting but also scrapes, cuts, and punctures.