Change in volume elastic deformation of hydrogels
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Smart hydrogels, or stimuli-responsive hydrogels, are three-dimensional networks composed of crosslinked hydrophilic polymer chains that are able to dramatically change their volume and other properties in response to environmental stimuli such as temperature, pH and certain chemicals. Rapid and significant response to environmental stimuli and high elasticity are critical for the versatility of such smart hydrogels. Here we report the synthesis of smart hydrogels which are rapidly responsive, highly swellable and stretchable, by constructing a nano-structured architecture with activated nanogels as nano-crosslinkers. The nano-structured smart hydrogels show very significant and rapid stimuli-responsive characteristics, as well as highly elastic properties to sustain high compressions, resist slicing and withstand high level of deformation, such as bending, twisting and extensive stretching. Because of the concurrent rapid and significant stimuli-response and high elasticity, these nano-structured smart hydrogels may expand the scope of hydrogel applications, and provide enhanced performance in their applications.
Smart hydrogels, which can dramatically change their volume and other properties in response to environmental stimuli such as temperature, pH and certain chemicals, are having an increasingly important role in myriad applications, including smart actuators for chemical valves1, optical systems2, artificial ‘muscles’3,4, soft biomimetic machines5 and ‘on/off’ switches for chemical reactions6,7, as well as scaffolds for tissue engineering8,9, vehicles for drug delivery8,9,10 and matrices for bioseparation9,11. Rapid and significant response to environmental stimuli and high elasticity are critical for the versatility of such smart hydrogels, because these attributes ensure instantaneous and remarkable feedback after receiving environmental signals and high extensibility and deformability to tolerate external forces. However, conventional hydrogels chemically crosslinked by small molecules always present slow response rate and poor elasticity, which severely limit the scope of hydrogel applications. For example, as a kind of typical thermo-responsive smart hydrogels that can undergo a reversible volume phase transition near the lower critical solution temperature (LCST, ~32 °C), normal poly(N-isopropylacrylamide) (PNIPAM) hydrogels crosslinked by chemical crosslinkers such as N,N-methylenebisacrylamide (MBA) are always elastically poor and shrink/swell slowly upon heating/cooling across the LCST. To overcome these limitations, intense efforts have been devoted to synthesizing hydrogels with either improved response properties12,13,14,15,16,17 or improved elastic properties18,19,20,21,22. To improve the response rate of PNIPAM-based thermo-responsive hydrogels, several strategies have been developed by grafting side chains onto the networks to generate comb-type grafted hydrogels12,13, generating porous structures inside hydrogels14,15 or introducing micellar structures for water pathways in hydrogels16,17. All these strategies can significantly improve the response rate of hydrogels, but they cannot help to improve the elastic property. Several other strategies have been developed to improve the elasticity of hydrogels18, including generating double-network hydrogels consist of two interpenetrating networks19,20, or using exfoliated clay nanoparticles as crosslinkers for synthesizing nanocomposite hydrogels21,22,23. The double-network hydrogels could be highly stretchable, but the stimuli-sensitivities for each polymer in the interpenetrating networks become seriously impaired because of interactions between the polymers. The nanocomposite hydrogels with clay crosslinkers are very elastic and have much better stimuli-responsive property than normal hydrogels; however, their thermo-responsive equilibrium swelling ratio is still limited, for example, the thermo-responsive equilibrium swelling ratio in responding to temper
Smart hydrogels, which can dramatically change their volume and other properties in response to environmental stimuli such as temperature, pH and certain chemicals, are having an increasingly important role in myriad applications, including smart actuators for chemical valves1, optical systems2, artificial ‘muscles’3,4, soft biomimetic machines5 and ‘on/off’ switches for chemical reactions6,7, as well as scaffolds for tissue engineering8,9, vehicles for drug delivery8,9,10 and matrices for bioseparation9,11. Rapid and significant response to environmental stimuli and high elasticity are critical for the versatility of such smart hydrogels, because these attributes ensure instantaneous and remarkable feedback after receiving environmental signals and high extensibility and deformability to tolerate external forces. However, conventional hydrogels chemically crosslinked by small molecules always present slow response rate and poor elasticity, which severely limit the scope of hydrogel applications. For example, as a kind of typical thermo-responsive smart hydrogels that can undergo a reversible volume phase transition near the lower critical solution temperature (LCST, ~32 °C), normal poly(N-isopropylacrylamide) (PNIPAM) hydrogels crosslinked by chemical crosslinkers such as N,N-methylenebisacrylamide (MBA) are always elastically poor and shrink/swell slowly upon heating/cooling across the LCST. To overcome these limitations, intense efforts have been devoted to synthesizing hydrogels with either improved response properties12,13,14,15,16,17 or improved elastic properties18,19,20,21,22. To improve the response rate of PNIPAM-based thermo-responsive hydrogels, several strategies have been developed by grafting side chains onto the networks to generate comb-type grafted hydrogels12,13, generating porous structures inside hydrogels14,15 or introducing micellar structures for water pathways in hydrogels16,17. All these strategies can significantly improve the response rate of hydrogels, but they cannot help to improve the elastic property. Several other strategies have been developed to improve the elasticity of hydrogels18, including generating double-network hydrogels consist of two interpenetrating networks19,20, or using exfoliated clay nanoparticles as crosslinkers for synthesizing nanocomposite hydrogels21,22,23. The double-network hydrogels could be highly stretchable, but the stimuli-sensitivities for each polymer in the interpenetrating networks become seriously impaired because of interactions between the polymers. The nanocomposite hydrogels with clay crosslinkers are very elastic and have much better stimuli-responsive property than normal hydrogels; however, their thermo-responsive equilibrium swelling ratio is still limited, for example, the thermo-responsive equilibrium swelling ratio in responding to temper
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