Hydrogels, three-dimensional polymeric networks capable of absorbing large amounts of water, have garnered significant attention for their potential applications in diverse fields, including tissue engineering, drug delivery, soft robotics, and artificial joints [1, 2, 5]. However, achieving a combination of high mechanical strength, toughness, and low friction, particularly under load-bearing conditions, remains a significant challenge for conventional single-network hydrogels. Triple network (TN) hydrogels, which combine three interpenetrating polymer networks with different properties, have emerged as a promising strategy to overcome these limitations by incorporating multiple energy dissipation mechanisms and synergistic interactions between the networks [35, 42, 43]. This article reviews the design strategies and underlying synergistic enhancement mechanisms responsible for the robust mechanical properties and low friction of triple network hydrogels. We discuss the role of different network components (e.g., brittle, ductile, and reinforcing networks), crosslinking methods (chemical and physical), and the interplay between network structure and macroscopic properties. The analysis highlights how the synergistic effects of multiple networks contribute to enhanced toughness, fatigue resistance, and lubrication, making TN hydrogels promising candidates for applications requiring high mechanical performance and low friction, such as artificial cartilage.

Robustness, Low friction, Synergistic enhancement

Zhang Y S, Khademhosseini A. Advances in engineering hydrogels. Science 356(6337): eaaf3627 (2017)

Jiang Z, Song P. Strong and fast hydrogel actuators. Science 376(6590): 245 (2022)

Fu L L, Li L, Bian Q Y, Xue B, Jin J, Li J Y, Cao Y, Jiang Q, Li H B. Cartilage-like protein hydrogels engineered via entanglement. Nature 618(7966): 740–747 (2023)

Yang J, Li K, Tang C, Liu Z Z, Fan J H, Qin G, Cui W, Zhu L, Chen Q. Recent progress in double network elastomers: One plus one is greater than two. Adv Funct Mater 32(19): 2110244 (2022)

Gao W W, Zhang Y, Zhang Q Z, Zhang L F. Nanoparticle-hydrogel: A hybrid biomaterial system for localized drug delivery. Ann Biomed Eng 44(6): 2049–2061 (2016)

Lin X, Zhao X W, Xu C Z, Wang L L, Xia Y Z. Progress in the mechanical enhancement of hydrogels: Fabrication strategies and underlying mechanisms. J Polym Sci 60(17): 2525–2542 (2022)

Sun T L, Cui K P. Tough and self-healing hydrogels from polyampholytes. In: Self-Healing and Self-Recovering Hydrogels. Creton C, Okay O, Eds. Switzerland: Springer International Publishing, 1997: 295–317.

Sun J Y, Zhao X H, Illeperuma W R K, Chaudhuri O, Oh K H, Mooney D J, Vlassak J J, Suo Z G. Highly stretchable and tough hydrogels. Nature 489(7414): 133–136 (2012)

Ritchie R O. The conflicts between strength and toughness. Nat Mater 10: 817–822 (2011)

Gong J P. Why are double network hydrogels so tough? Soft Matter 6(12): 2583–2590 (2010)

Chen J, Ao Y Y, Lin T R, Yang X, Peng J, Huang W, Li J Q, Zhai M L. High-toughness polyacrylamide gel containing hydrophobic crosslinking and its double network gel. Polymer 87: 73–80 (2016)