Design of tough hydrogels has made great progress in the past two decades. However, the synthetic tough gels are usually much softer than some biotissues (e.g., skins with modulus up to 100 MPa). Here we report a new class of ultrastiff and tough supramolecular hydrogels facilely prepared by copolymerization of methacrylic acid and methacrylamide. The gels with water content of approximately 50–70 wt % possessed remarkable mechanical properties, with Young’s modulus of 2.3–217.3 MPa, tensile breaking stress of 1.2–8.3 MPa, breaking strain of 200–620%, and tearing fracture energy of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of modulus. Typical yielding and crazing were observed in the gel under tensile loading, indicating the forced elastic deformation of these hydrogels in a glassy state, as confirmed by dynamic mechanical analysis. The ultrahigh stiffness was attributed to the dense cross-linking and reduced segmental mobility caused by the robust intra- and interchain hydrogen bonds. Because of the dynamic nature of noncovalent bonds, these supramolecular gels also showed rate-dependent mechanical performances along with good shape memory and recyclability. This strategy should be applicable for other systems toward robust mechanical properties, versatile functionalities, and promising applications of hydrogel materials as structural elements.

Hydrogels are usually recognized as soft and weak materials, the poor mechanical properties of which greatly limit their applications as structural elements. Designing of hydrogels with high strength and high modulus has both fundamental and practical significances. Herein we report a series of tough, stiff, and transparent hydrogels facilely prepared by copolymerization of 1-vinylimidazole and methacrylic acid in dimethyl sulfoxide followed by solvent exchange to water. The equilibrated hydrogels with water content of 50–60 wt % possessed excellent mechanical properties, with tensile breaking stress, breaking strain, Young’s modulus, and tearing fracture energy of 1.3–5.4 MPa, 40–330%, 20–170 MPa, and 600–4500 J/m2, respectively. These tough hydrogels were also stable over a wide pH range (2 ≤ pH ≤ 10), resulting from the formation of dense and robust hydrogen bonds between imidazole and carboxylic acid groups. Moreover, the water content and mechanical properties of one gel can be adjusted over a wide range by controlling the dissociation and re-formation of hydrogen bonds during the solvent exchange and heating process; the treated hydrogel with specific characters was stable in water at room temperature. This is because the density of hydrogen bonds can be modulated at high temperature yet immediately fixed at room temperature due to the high stiffness and glassy state of the hydrogel. This strategy to prepare tough and stiff hydrogels should be applicable to other systems as structural materials with promising applications in diverse fields.

Forming robust associative interactions has been an effective strategy for the design of tough hydrogels. However, the role of associative interactions in the dynamics of hydrogels still remains elusive. Here, we report a series of poly­(acrylamide-co-methacrylic acid) hydrogels with moderate water contents and excellent mechanical properties that are facilely synthesized by free-radical copolymerization. The mechanical properties of these hydrogels vary with the feeding molar fraction of acrylamide (f am). The gels with f am of 0.2–0.35 exhibit high toughness and good stability in water, which is related to the dense hydrogen bonds and relatively high segment rigidity of the matrix. Dynamic modulus spectra extended by time-temperature superposition and relaxation measurements indicate that the gels undergo glassy-to-rubbery transition with decreased frequency, and the robust hydrogen bonds, whose density is 1–3 times that of entanglements, retard chain disentanglement and contribute to the plateau modulus of the gels at low frequencies. The activation energy for the dissociation of the robust hydrogen bonds is ∼46 kJ mol–1. Furthermore, a decrease in water content results in the shift of dynamic modulus spectra to low frequencies and an increase in transition temperature due to the reduced segment relaxation. To further examine the structure of gel networks, the tensile behaviors of the gels are analyzed using a viscoelastic model. It is found that each partial chain includes 20–30 Kuhn segments, which are stretched after the fracture of intrachain hydrogen bonds to release the hidden length, dissipate energy, and thus toughen the gels. This understanding of the dynamics of the network at different timescales and the contribution of associative interactions to the mechanical properties should be informative for the design of other tough hydrogels.

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Recent years have witnessed the rapid development of sustainable materials. Along this line, developing biodegradable or recyclable soft electronics is challenging yet important due to their versatile applications in biomedical devices, soft robots, and wearables. Although some degradable bulk hydrogels are directly used as the soft electronics, the sensing performances are usually limited due to the absence of distributed conducting circuits. Here, sustainable hydrogel‐based soft electronics (HSE) are reported that integrate sensing elements and patterned liquid metal (LM) in the gelatin–alginate hybrid hydrogel. The biopolymer hydrogel is transparent, robust, resilient, and recyclable. The HSE is multifunctional; it can sense strain, temperature, heart rate (electrocardiogram), and pH. The strain sensing is sufficiently sensitive to detect a human pulse. In addition, the device serves as a model system for iontophoretic drug delivery by using patterned LM as the soft conductor and electrode. Noncontact detection of nearby objects is also achieved based on electrostatic‐field‐induced voltage. The LM and biopolymer hydrogel are healable, recyclable, and degradable, favoring sustainable applications and reconstruction of the device with new functions. Such HSE with multiple functions and favorable attributes should open opportunities in next‐generation electronic skins and hydrogel machines.