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.

Keywords: elastic modulus; energy dissipation; fracture energy; load‐bearing applications; tough hydrogels.

Figure 1

Number of publications N that meet the search criteria of “tough hydrogels” within the last 20 years. Citation Report graphic is derived from Clarivate Web of Science, Copyright Clarivate 2023. All rights reserved.

Figure 2

Uniaxial tensile test experiment of a plastically deforming material (red) and an elastic hydrogel (black).

Figure 3

a) Pure shear test and corresponding force–displacement plots according to b) Griffith and c+d) Rivlin‐Thomas using unnotched samples (black lines) and single notched specimen with crack lengths c (dashed lines). e) Trouser tearing test and f) corresponding force–displacement plot showing stick‐slip (dashed lines) and steady tearing (dotted lines).

Figure 4

Cyclic tensile tests of tough hydrogels showing stress–strain diagrams of a sample with a) high damping capacity, b) quasi‐elastic behavior, c) low hysteresis recovery, and d) good self‐recovery.

Figure 5

Mechanical stress causing energy dissipation in a) conventional hydrogels, b) double networks, and c) non‐covalently cross‐linked networks.

Figure 6

An overview of energy dissipation modes each represented by a specific molecular interactions/architectures, which have served the fabrication of tough hydrogels. Individual modes of energy dissipation are categorized according to eletrostatic interactions, microphase saparation, composites, or self‐assembly.

Figure 7

Tough hydrogels with energy dissipation driven by metal coordination (squares), ionic interactions (diamond), and hydrogen bonding (triangles) showing plots of fracture energy versus elastic modulus (a + c) and fracture stress versus fracture strain (b + d) with each datapoint featuring the corresponding citation number. Color grading corresponds to the water content of the respective hydrogel specimen, hollow symbols represent samples of unknown water content.

Figure 8

Tough hydrogels with energy dissipation driven by crystallization/salting‐out (pentagon), micellization (spheres) and hydrophobic interactions (top‐down triangles) showing plots of fracture energy versus elastic modulus (a + c) and fracture stress versus fracture strain (b + d) with each datapoint featuring the corresponding citation number. Color grading corresponds to the water content of the respective hydrogel specimen, hollow symbols represent samples of unknown water content.

Figure 9

Tough hydrogels with energy dissipation driven through (nano)composite structures (stars) and self‐assembly processes (crosses) showing plots of fracture energy versus elastic modulus (a + c) and fracture stress versus fracture strain (b + d) with each datapoint featuring the corresponding citation number. Color grading corresponds to the water content of the respective hydrogel specimen, hollow symbols represent samples of unknown water content.

Figure 10

Energy‐dissipating tough hydrogels that unite high fracture energies with high elastic moduli. Single and double networks are marked with a single (*) and double asterisk (**), respectively.

Figure 11

Number of publications related to tough hydrogels cited within this review according to their respective areas of applications.

Figure 12

Chosen examples of tough hydrogels applied in the areas of a) tissue engineering, b) soft electronics, c) shape memory materials, d) 3D printing, e) biomedical devices, and f) adhesives. Notably, the majority of the highlighted examples ultimately serve biomedical applications.

Figure 13

Statistic distributions on tough hydrogels properties of the analyzed literature for fracture energy (a), elastic modulus (c), fracture stress (b), and fracture strain (d). e) Ashby plot of chosen natural (brown) and synthetic materials (plastics: blue, thermosets/rubbers: turquoise, TPE: green) together with covalent (grey) and tough hydrogels (pink). Tough hydrogels with most extreme properties from Figure 10 are shaded red. The depicted data is limited exclusively to values in between $\mathrm{\Gamma }=0.1-1000\mathrm{kJ}{\mathrm{m}}^{-2}$ and E = 0.1–1000 and was partially sourced from Ansys GRANTA EduPack software © (the graphic was recreated).