Non-covalent Tough Hydrogels for Functional Actuators
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Non-covalent Tough Hydrogels for Functional Actuators Jun Fu*, Guorong Gao, and Yuanna Sun Polymers and Composites Division, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo 315201, China ABSTRACT Tough and responsive hydrogels have recently attracted great research interests for potential applications in artifical muscles, soft robotics, and actuators, etc. This paper overviews our recent progresses in the design and synthesis of hydrogels with very high strength and toughness, and actuators based on these hydrogels. Inorganic nanospheres, nanorods, and nanosheets are exploited as multi-functional crosslinkers to adsorb or bond with hydrophilic chains, leading to hydrogels with very high strength, toughness, fatigue resistance, and/or self-healing. Introduction of functional groups including ionic monomers and amino groups results in hydrogels reponsive to pH, ionic strength and electric field. Besides, ionoprinting has been used to change local crosslink density based on reversible chelating/decomposition of metal ions with functional groups. This process is rapid and thus enables reversible and rapid actuation of hydrogel devices. Our studies will further aim to develop sophiscated devices by assembling hydrogel actuators. INTRODUCTION Hydrogels have been widely recognized as soft and wet materials with great potentials applications as tissue engineering scaffolds, artificial muscles, and soft robotcis, etc. The weakness and fragility of conventional hydrogels have seriously limited their applications [1, 2]. Recently, numerous strategies have been proposed to improve the mechanical properties of hydrogels [3]. Representative ones include slide-ring hydrogels [4], double network (DN) hydrogels [5, 6], nanoparticle-crosslinked hydrogels[7-9], and hydrogels comprised of noncovalent or supramolecular polymer networks [10-12]. The reversible non-covalent or supramolecular interactions may serve as synergistic energy dissipation mechanism, and result in high strength and toughness. Moreover, non-covalent interactions, including hydrogen bonding[13], hydrophobic association[14], electrostatic interaction[15], and supramolecular recognition[16], etc, may decompose, weaken, or re-build upon external stimulus. Therefore, many non-covalent hydrogels are responsive , which are also known as smart hydrogels, and have been demonstrated with great success in fabricating hydrogel actuators that are actuated by light, pH, salt, magnetic field, and electric field, etc. Recent studies in Ningbo Institute of Materials Technology and Engineering of Chinese Academy of Sciences have developed a series of new concept tough and responsive hydrogels. Inorganic nanospheres, nanorods, nanosheets, and triblock copolymer micelles are exploited as multi-functional crosslinkers to adsorb or bond with hydrophilic chains, leading to hydrogels with very high strength, toughness, fatigue resistance, and/or self-healing. Such new concept hydrogels are introduced with functional
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