Bioinspired Strategy to Reinforce Hydrogels via Cooperative Effect of Dual Physical Cross‐Linkers

Abstract

including ionic bond, [4][5][6][7][8][9] hydrogen bond, [10][11][12][13][14][15] coordination, [16][17][18] microcrystalline, [19][20][21] and hydrophobic interaction. [22][23][24][25] Although these non-covalent bonds can significantly improve the extensibility and self-recovery of hydrogels, they usually dramatically reduce the toughness. Covalent cross-linkers are always required for reaching high toughness, whereas the permanent fracture of covalent cross-links will lead to irreversible structure damage and significant decline of toughness and mechanical strength. Without the help from covalent cross-linkers, what kind of multiple physical cross-linker system can endow mono-network hydrogels with both rapid self-recovery and high toughness still remains a challenge.Our strategy is inspired by the nacre of abalone shell, which is constituted by alternating layers of CaCO 3 and biopolymers. The hard CaCO 3 layer provides strength and hardness and the soft biopolymer layer provides adhesion and ductility. The cooperative effect of two components leads to eminent strength and toughness. [26,27] This naturally suggests a new strategy to construct high toughness materials via combining both weak and strong non-covalent cross-linkers in one polymeric network. Polyacrylic acid will be an appropriate candidate for the versatile functions of carboxyl group to realize two different interactions in one polymeric network. First, carboxyl groups are able to form ionic bond with alkaline group, such as primary amine and pyridine. [28] Second, carboxyl groups can coordinate with metal ions, such as Al 3+ and Fe 3+ . [29] In this work, we present a new class of polyacrylic acid hydrogels cross-linked by coordinate bonds with Al 3+ and ionic bonds with triamine (diethylenetriamine, DETA). Compared to Al 3+ or DETA cross-linked hydrogels, this dual physically cross-linked hydrogel become super tough and the toughness of hydrogels can be recovered rapidly. In addition, the mechanical properties can be tuned over a wide range by using different molar ratios of cross-linkers. Experimental Section MaterialsDETA, acrylic acid (AAc), ammonium persulfate (APS), and aluminum chloride (AlCl 3 ) were purchased from Aladdin Many biological tissues including cartilage and tendons are composed of polymeric hydrogels, which exhibit high toughness and rapid self-recovery. However, developing hydrogels with both high toughness and rapid recovery remains a challenge. Inspired by the nacre of abalone shell, two non-covalent cross-linkers (Al 3+ and diethylenetriamine [DETA]) with different relaxation times are introduced into a polyacrylic acid network. Compared with mono cross-linked hydrogels, the dual physical cross-linked hydrogel exhibits both high toughness (work of extension at fracture up to 8.0 MJ m −3 ) and rapid self-recovery ability without loss of extensibility. Strong carboxylate-DETA ionic cross-links with longer relaxation time endow the hydrogels with high strength and help to localize the reformation of carboxylate-Al 3+ coordi...

Shape memory hydrogels with remodelable permanent shapes and programmable cold-induced shape recovery behavior

“…As the concentration of DETA increased from 0 to 1.2 M, the band at 1700 cm −1 gradually decreased and the band at 1560 cm −1 gradually increased, corresponding to the formation of charge-assisted hydrogen bonding. 30–32 Such a structural change is also reflected in the mechanical properties (Fig. 2b).…”

Strong and Tough Antifreezing Hydrogel Sensor via the Synergy of Coordination and Hydrogen Bonds

“…This was mainly due to the stronger coordination ability of Al 3+ with carboxyl groups, in comparison to other metal ions under same experimental conditions. 30,31 This conductive hydrogel had excellent mechanical performance, frost resistance, water retention, high electrical conductivity and transparency, and especially, adaptation to multienvironmental conditions (Figure 1c). Importantly, this ionic composite hydrogel could convert changes in stress into electrical signals while monitoring various deformations at different temperatures and in water environments, 33,34 which would have great potential for applications in flexible electronic devices (Figure 1d).…”

“…However, it is hard to achieve a balance between the mechanical and electrical properties by this method, because it destroys the energy dissipation of the hydrogel, rendering it brittle and prone to fracture. , Winter barley is cold-tolerant due to the presence of betaine in the leaves, which provides the inspiration for the design of freeze-resistant hydrogels. , Betaine is a zwitterion, whose positive and negative charges can combine with the water molecules in the hydrogel and reduce the free water in the hydrogel network, thus achieving excellent freeze resistance. , However, this strategy contributes less to the conductivity of hydrogels. As compared to low valence salts, high valence inorganic salts effectively increase the network density of hydrogels by coordinating with metal ions, which improves the antiswelling property and conductivity of hydrogels, especially their mechanical properties . However, there are only a few reports in the literature on the mechanism of enhancement due to the synergetic dynamic metal coordination of high valence metal ions and hydrogen bonding of zwitterion for boosting the mechanical properties and frost resistance of hydrogels.…”

“…28,29 However, this strategy contributes less to the conductivity of hydrogels. As compared to low valence salts, high valence inorganic salts effectively increase the network density of hydrogels by coordinating with metal ions, which improves the antiswelling property and conductivity of hydrogels, 30 especially their mechanical properties. 31 However, there are only a few reports in the literature on the mechanism of enhancement due to the synergetic dynamic metal coordination of high valence metal ions and hydrogen bonding of zwitterion for boosting the mechanical properties and frost resistance of hydrogels.…”

Synthesis of a PNIPAM-Based Composite Hydrogel and Its Multipurpose Applications in Piezoresistive and Temperature Sensing

“…Chemical agents have a fast polymerization speed and high cross-linking strength, and the as-obtained hydrogel exhibits a high mechanical strength but insufficient toughness, easily breaking during the strain test. Physical agents form polymer networks by introducing noncovalent interactions, such as electrostatic, hydrogen binding, and van der Waals forces, into the gel system . Since noncovalent interactions can be continuously reconstructed during the deformation process, the as-prepared hydrogel can withstand great deformation and maintain good flexibility, making it more adaptable to human body motion detection demanding large strains.…”