Rational Design of Self-Healing Tough Hydrogels: A Mini Review

Abstract

Hydrogels are three-dimensional cross-linked polymer networks which can absorb and retain large amount of water. As representative soft materials with tunable chemical, physical and biological properties, hydrogels with different functions have been developed and utilized in a broad range of applications, from tissue engineering to soft robotics. However, conventional hydrogels usually suffer from weak mechanical properties and they are easily deformed or damaged when they are subjected to mechanical forces. The accumulation of the damage may lead to the permanent structural change and the loss of the functional properties of the hydrogels. Therefore, it is important to develop mechanically robust hydrogels with autonomous self-healing property in order to extend their lifespan for various applications. In this mini review, we focus on the discussion about the appropriate molecular design of the hydrogel network for achieving self-healing and excellent mechanical properties, respectively as well as the corresponding self-healing and toughening mechanisms. We conclude with perspectives on the remaining challenges in the field as well as the recommendations for future development.

Physical Organohydrogels With Extreme Strength and Temperature Tolerance

“…So far, versatile strategies to achieve tough hydrogels have been emerged, including double-network hydrogels (Gong et al, 2003;Gong, 2014;Liang et al, 2016;Chen et al, 2018;Jing et al, 2019), nanocomposite hydrogels (Haraguchi and Takehisa, 2002;Chen et al, 2015;GhavamiNejad et al, 2016;Zhu et al, 2017), topological hydrogels (Okumura and Ito, 2001;Li et al, 2018), macromolecular microsphere composite hydrogels (Huang et al, 2007;Gu et al, 2016;Zhang and Khademhosseini, 2017;Wang Z. et al, 2018), hydrophobic association hydrogels (Li et al, 2012;Mihajlovic et al, 2017;Han et al, 2018), hydrogen bonding/dipole-dipole reinforced hydrogels (Han et al, 2012;Zhang et al, 2015;Qin et al, 2018), and many others (Gong et al, 2016;Liu J. et al, 2017;Zhao et al, 2019). However, almost all of the hydrogels swollen a large amount of water in polymer networks cannot resist a cold or hot environment (Wei et al, 2014(Wei et al, , 2015Wang W. et al, 2018), hindering the application of tough hydrogels in harsh conditions. Subzero temperature results in freezing of hydrogels, while high temperature lead to drying (Rong et al, 2017;Zhang et al, 2018).…”

Specialty Tough Hydrogels and Their Biomedical Applications

“…Tough hydrogels offer an additional advantage given that their superior mechanical properties (tensile strength of 0.1-1 MPa and fracture energy of 10 2 -10 3 Jm −2 ) better match those of native tissues, such as cartilage and tendons, that possess high toughness (1,000 J) and strength (30 MPa). [3,14] In the following section, tough hydrogels for potential applications in cartilage, cardiovascular, and corneal tissue engineering applications are reviewed.…”

“…These limitations have drawn attention to designing materials with enhanced stretchable and toughness properties. [5][6][7][8][9][10][11][12][13][14] However, formulating mechanically robust hydrogels that are also morphologically and chemically stable for clinical use remains a challenge. For example, disruption of covalent bonds in tough interpenetrating network hydrogels can lead to irreparable network damage, employing hydrophobic associations is hampered by poor solubility of hydrophobes, competition from water for binding sites limits the association strength of hydrogen bonds, and ionic crosslinks are vulnerable to mobile ions typically encountered in physiological conditions.…”

Advances in Synthesis and Applications of Self-Healing Hydrogels

“…Recently, mussel-inspired chemistry has been extensively studied. The mussel-inspired hydrogels typically consist of polydopamine (PDA), based on which multiple interactions can happen, including hydrogen bonds, π-π stacking, and, mainly, metal-ligand coordination between metals and catechol groups from 3,4-dihydroxyphenyl- L-alanine, an amino acid ( Li et al, 2015 ; Wang W. et al, 2018 ). Additionally, PDA has abundant functional groups and thus can be easily modified ( Fan et al, 2019 ).…”