In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness-hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m 2 ), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m 2 ). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human-machine interfaces. elastomer | stretchable materials | toughness | hysteresis | fatigue S tretchable materials such as elastomers and gels enable the fast-moving field of soft (and possibly biocompatible) systems. Examples include stretchable electronics (1-4), soft robots (5, 6), ionotronics (7-9), drug delivery (10, 11), and tissue regeneration (12). Many systems require that the stretchable materials have high toughness (i.e., dissipate much energy to resist the extension of cracks), but have low hysteresis (i.e., dissipate little energy during normal operation of load and unload). These two requirements, however, usually conflict: Toughness and hysteresis are often correlated. Toughness and hysteresis both result from energy dissipation, just under different conditions. A stretchable material of a single polymer network usually has low hysteresis and low toughness-that is, the stress-stretch curves for load and unload almost coincide, and the material ruptures at a much-reduced stretch when containing a crack (13).The toughness-hysteresis correlation has a molecular origin (Fig. 1A). A stretchable material such as an elastomer or a gel has a molecular architecture that mixes strong and weak bonds, enabling the hybrid behavior of solid and liquid. Strong bonds (e.g., covalent bonds) link monomer units into polymer chains, and cross-link the polymer chains into a network. Weak bonds (e.g., hydrogen bonds and van der Waals interaction) aggregate the monomer units of different polymer chains, as well as solvent molecules, into a condensed phase, but allow them to change neighbors constantly, transmit force negligibly, and ac...

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Noncovalent adhesion has long been developed for numerous applications, including pressure-sensitive adhesives, wound closure, and drug delivery. Recent advances highlight an urgent need: a general principle to guide the development of instant, tough, noncovalent adhesion. Here, we show that noncovalent adhesion can be both instant and tough by separately selecting two types of noncovalent bonds for distinct functions: tougheners and interlinks. We demonstrate the principle using a hydrogel with a covalent polymer network and noncovalent tougheners, adhering another material through noncovalent interlinks. The adhesion is instant if the interlinks form fast. When an external force separates the adhesion, the covalent polymer network transmits the force through the bulk of the hydrogel to the front of the separation. The adhesion is tough if the interlinks are strong enough for many tougheners to unzip. Our best result achieves adhesion energy above 750 J/m2 within seconds. The adhesion detaches in response to a cue, such as a change in pH or temperature. We identify several topologies of noncovalent adhesion and demonstrate them in the form of tape, powder, brush, solution, and interpolymer complex. The abundant diversity of noncovalent bonds offers enormous design space to create instant, tough, noncovalent adhesion for engineering and medicine.