Hand-sized gecko-inspired adhesives with reversible force capacities as high as 2950 N (29.5 N cm(-2) ) are designed without the use of fibrillar features through a simple scaling theory. The scaling theory describes both natural and synthetic gecko-inspired adhesives, over 14 orders of magnitude in adhesive force capacity, from nanoscopic to macroscopic length scales.

Hydrogels have promising applications in diverse areas, especially wet environments including tissue engineering, wound dressing, biomedical devices, and underwater soft robotics. Despite strong demands in such applications and great progress in irreversible bonding of robust hydrogels to diverse synthetic and biological surfaces, tough hydrogels with fast, strong, and reversible underwater adhesion are still not available. Herein, a strategy to develop hydrogels demonstrating such characteristics by combining macroscale surface engineering and nanoscale dynamic bonds is proposed. Based on this strategy, excellent underwater adhesion performance of tough hydrogels with dynamic ionic and hydrogen bonds, on diverse substrates, including hard glasses, soft hydrogels, and biological tissues is obtained. The proposed strategy can be generalized to develop other soft materials with underwater adhesion.

(1 of 10) 1605350 materials exhibiting both excellent loadbearing capacity and fracture resistance, as can be observed from both hard tissues [3][4][5][6][7] (e.g., nacre and bone) and soft tissues (e.g., ligament and tendon), [8][9][10] by combining rigid, brittle components (either inorganic or organic) and soft, organic components into composite materials. Most of these natural materials have highly complex hierarchical architectures existing over multiple length scales, which results in composite properties that far exceed what could be expected from a simple combination of the individual components. [3] Many researchers have attempted to mimic the unique natural structures of tough, hard hybrid materials, but only a few studies have achieved remarkable success comparable to that of nature. [11][12][13][14] For example, a bioinspired alumina hybrid material with specific strength and toughness comparable to aluminum alloys was synthesized by combining a hard yet brittle ceramic with relatively soft poly(methyl methacrylate), where nacre-like multiple toughening mechanisms at multiple scales resulted in exceptional fracture resistance. [12] As a vital class of soft materials, tough hydrogels have shown strong potential as structural biomaterials. [15][16][17][18][19][20] These hydrogels alone, however, still possess limited mechanical properties (low modulus) when compared to some load-bearing tissues, e.g., ligaments and tendons. To reproduce the exceptional strength and toughness seen in soft load-bearing tissues, one strategy is to combine an energy-dissipative tough hydrogel with rigid yet flexible fibers to create a composite, similar to fibrous tissues. [21][22][23] In this composite concept, the rigid fiberbased component increases the specific strength while the gel matrix dissipates energy. Based on this concept, some attempts have been made to fabricate fiber reinforced hydrogel composites. [24][25][26][27][28][29] Utilizing this technique, researchers have been able to increase and tune the stiffness and toughness achievable with hydrogel-based systems. However, developing soft composites with synergistically improved mechanical properties, such as those seen in hard bioinspired composites, is still a challenge.Recently, our group has developed a new class of tough hydrogels, polyampholyte (PA) gels (fracture energy, T = 3000 J m −2 ), based on multiple ionic bonds acting as reversible sacrificial bonds in the gel network. [18,30] Interestingly, PA gels also demonstrate unique interfacial bonding to charged surfaces, either positive or negative, due to the self-adjustable Coulombic interaction of the dynamic ionic bonds of the PA. [31] The PA gels are synthesized from radical polymerization of oppositely Energy-Dissipative Matrices Enable Synergistic Toughening in Fiber Reinforced Soft CompositesYiwan Huang, Daniel R. King, Tao Lin Sun, Takayuki Nonoyama, Takayuki Kurokawa, Tasuku Nakajima, and Jian Ping Gong* Tough hydrogels have shown strong potential as structural biomaterials. These hydrogels a...

Ligaments are unique wet biological tissues with high tensile modulus and fracture stress, combined with high bending flexibility. Developing synthetic materials with these properties is a significant challenge. Hydrogel composites made from high stiffness fabrics is a strategy to develop such unique materials; however, the ability to produce these materials has proven difficult, since common hydrogels swell in water and interact poorly with solid components, limiting the transfer of force from the fabric to the hydrogel matrix. In this work, for the first time, we successfully produce extraordinarily tough hydrogel composites by strategically selecting a recently developed tough hydrogel that de-swells in water. The new composites, consisting of polyampholyte hydrogels and glass fiber woven fabrics, exhibit extremely high effective toughness (250 000 J m(-2)), high tear strength (similar to 65 N mm(-1)), high tensile modulus (606 MPa), and low bending modulus (4.7 MPa). Even though these composites are composed of water-containing, biocompatible materials, their mechanical properties are comparable to high toughness Kevlar/polyurethane blends and fiber-reinforced polymers. Importantly, the mechanical properties of these composites greatly outperform the properties of either individual component. A mechanism is proposed based on established fabric tearing theory, which will enable the development of a new generation of mechanically robust composites based on fabrics. These results will be important towards developing soft biological prosthetics, and more generally for commercial applications such as tear-resistant gloves and bulletproof vests