Meanwhile, a high healing efficiency can be realized, when the reversibility of the H-bond was unlocked from the stacking at higher temperature. This feature endows the PU with high mechanical strength. The quadruple H-bonded UPy dimeric moieties in the backbone induce phase separation to form a hard domain as well as enable further aggregation into microcrystals by virtue of the stacking interactions, which are stable in ambient temperature. To balance the self-healing capacity and mechanical performance, a thermal-triggered switch of H-bonding is introduced. In this study, we report a robust self-healing polyurethane (PU) elastomer polypropylene glycol-2-amino-5-(2- hydroxyethyl)-6-methylpyrimidin-4-ol (PPG-mUPy) by integrating ureidopyrimidone (UPy) motifs with a PPG segment with a well-defined architecture and microphase morphology. However, development of a polymeric material with good mechanical performance as well as self-healing capacity is very challenging. Supramolecular interactions have been extensively considered in the field of self-healing materials due to their excellent reversibility and sensitive responsiveness to environmental stimuli. Self-healing materials have attracted considerable attention because of their improved safety, lifetime, energy efficiency and environmental impact. The opportunities, challenges, and future prospects of hydrogel additive manufacturing technologies are discussed. With this review, we also investigated the stimulus types, as well as the advantages and disadvantages of various stimulus-responsive hydrogels in smart hydrogels non-responsive hydrogels and various applications of additive manufacturing hydrogels, such as neural catheter preparation and drug delivery. This study constitutes a literature review to address hydrogel-based additive manufacturing technologies, introducing the characteristics of commonly used 3D printing hydrogel methods, such as direct ink writing, fused deposition modeling, and stereolithography. Variable-feature rapid prototyping technology, also known as 4D printing, is a combination of materials, mathematics, and additives.

Additive manufacturing, also known as 3D printing, was originally referred to as rapid prototyping manufacturing. Hydrogel has become a commonly used material for 3D and 4D printing due to its favorable biocompatibility and low cost. This review bridges two promising research fields, and guides the reader towards selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability and stress relaxation. 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. Based on defined requirements for soft robots, these techniques are critically compared and validated. This paper reviews the wide variety of processing techniques of self-healing polymers for robotics available in literature, and thoroughly discusses limitations and opportunities.

Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. Self-healing polymers address this vulnerability. However, they are prone to various damage types. Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments.

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