Tough Hydrogels for Load‐Bearing Applications

Abstract Tough hydrogels have emerged as a promising class of materials to target load‐bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high‐performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state‐of‐the‐art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high‐performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high‐performance hydrogels are compared with existing materials, and promising future opportunities are discussed.

following the experimental procedure, where we calculated the sum of all solids used in a given protocol and divided it by the sum of the solid contents and total mass of solvent used.This calculation was only done in those cases, where the as-prepared hydrogels were used directly for mechanical testing without further swelling, soaking or immersing in other liquids.For hydrogel samples that were immersed into salt or aqueous solutions as part of the preparation or post-processing, the water content was not calculated, since such immersion processes often results in swelling or deswelling of the hydrogel specimen making a calculation based on the preparation protocol redundant.Again, all calculated values for water content or those that have been extracted from plots via im2graph are labelled as orange text and therefore are no absolute values but rather calculated or read-out values.
Referencing.The numbering of the citations in this document is aligned with the numbering of the citations in the main manuscript.This way, supporting information in this document can be easily read out together with the main text of the manuscript and is tied to a fixed number, which consequently and in the case of this document does not begin with number 1.

Additional plots
Figure S1.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels using metal ligand interactions.Labels refer to citations found in Table S1 (SN: black; DN: blue).Figure S2.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels using metal ligand interactions.Labels refer to citations found in Table S1 (SN: black; DN: blue).Figure S3.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels using ionic interactions.Labels refer to citations found in Table S1 (SN: black; DN: blue).Figure S4.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels using ionic interactions.Labels refer to citations found in Table S1 (SN: black; DN: blue).Figure S5.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels using hydrogen bonding interactions.Labels refer to citations found in Table S1 (SN: black; SIPN: gold).Figure S6.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels using hydrogen bonding interactions.Labels refer to citations found in Table S1 (SN: black; SIPN: gold).

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Figure S7.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels through crystallization/salting out.Labels refer to citations found in Table S1 (SN: black; SIPN: gold; DN: blue).

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Figure S8.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels through crystallization/salting out.Labels refer to citations found in Table S1 (SN: black; SIPN: gold; DN: blue).

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Figure S9.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels through micellization.Labels refer to citations found in Table S1 (SN: black; SIPN: gold; DN: blue).

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Figure S10.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels through micellization.Labels refer to citations found in Table S1 (SN: black; SIPN: gold; DN: blue).

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Figure S11.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels through hydrophobic interactions.Labels refer to citations found in Table S1 (SN: black; DN: blue).

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FigureS12.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels through hydrophobic interactions.Labels refer to citations found in TableS1(SN: black; DN: blue).

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Figure S13.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels based on (nano)composites.Labels refer to citations found in Table S1 (SN: black; DN: blue).

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Figure S14.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels based on (nano)composites.Labels refer to citations found in Table S1 (SN: black; DN: blue).

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Figure S15.A) Fracture energy and B) elastic modulus vs. water content for tough hydrogels through self-assembly.Labels refer to citations found in Table S1 (SN: black; DN: blue).

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Figure S16.A) Fracture stress and B) fracture strain vs. water content for tough hydrogels based on selfassembly.Labels refer to citations found inTable S1 (SN: black; DN: blue)

Table S1 .
Collected and extracted data from the literature for this review on tough hydrogels.Each entry is labeled according to the reference citation number in the main manuscript text.Categories are divided according to the individual mode of energy dissipation.Collected mechanical properties are subdivided into Tensile test (T), Fracture testing, Dissipation (referring to the hysteresis work), cyclic tensile tests (fatigue resistance), and self-recovery (mostly determining the hysteresis recovery).