Dipole–Dipole and H‐Bonding Interactions Significantly Enhance the Multifaceted Mechanical Properties of Thermoresponsive Shape Memory Hydrogels

Abstract

applications such as cell scaffolds, soft tissue substitutes and bioactuators. [ 1,3 ] However, their load-bearing applications are often limited by the poor mechanical performances. [ 4,5 ] Conventional hydrogels do not exhibit high mechanical properties because of uneven crosslinking and weak interaction among the chains. Recently, several high mechanical hydrogels have been developed and investigated as potential soft tissue replacements, [ 6,7 ] but few of them exhibit a combination of high mechanical properties including stiffness, toughness, tensile and compressive strengths, anti-fatigue as well as mechanical recoverability. [ 8,9 ] The well-known slide-ring [ 10 ] and tetra-PEG [ 11 ] hydrogels are designed to have ideally homogeneous networks to eliminate the intrinsic defects to a maximum extent, eventually leading to the enhanced mechanical properties. Other strategies are advanced to increase the functionalities [ 12 ] between two crosslink points to toughen the hydrogels, such as inorganic nanocomposite hydrogels, [13][14][15] macromolecular microsphere composite hydrogels, [ 16 ] graphene [ 17,18 ] or its oxide [ 19 ] composite hydrogels and micro- [ 20 ] or nano-structure [ 21 ] hydrogels. All of these gels have high compressive strength and large elongation, but their tensile strength (ranging from 190 kPa to 600 kPa) and modulus are not satisfi ed enough; furthermore, their high elastic properties show no hysteresis behavior, thus lacking a mechanism for mechanical energy dissipating. As a result, the mechanical properties are observably reduced when the gels contain a defect. [ 9 ] In order to solve these problems, a variety of mechanical energy dissipation mechanisms are implemented into the network to toughen the hydrogels. For instance, the PAMPS/PAAm double network (DN) gels can sacrifi ce the rigid chemical bond of the fi rst network to dissipate energy to achieve MPa magnitude order of tensile and compressive stress but the fatigue resistance is low, and the preparation process is relatively complicated. [22][23][24][25] Suo and his co-workers [ 9 ] reported synthesis of a hydrogel with high stretchability and astonishing fracture energies of 9,000 J/m 2 by forming interpenetrating polymer networks of covalently crosslinked PAAm and ionically crosslinked zipper-like High strength hydrogels were previously constructed based on dipole-dipole and hydrogen bonding reinforcement. In spite of the high tensile and compressive strengths achieved, the fracture energy of the hydrogels strengthened with sole noncovalent bondings was rather low due to the lack in energy dissipating mechanism. In this study, combined dipole-dipole and hydrogen bonding interactions reinforced (DHIR) hydrogels are synthesized by onestep copolymerization of three feature monomers, namely acrylonitrile (AN, dipole monomer), acrylamide (AAm, H-bonding monomer), and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, anionic monomer) in the presence of PEGDA575, a hydrophilic crosslinker. The electrostatic repulsion from PAM...

Self-Healable, Tough, and Ultrastretchable Nanocomposite Hydrogels Based on Reversible Polyacrylamide/Montmorillonite Adsorption

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Thermal/Light Dual-Activated Shape Memory Hydrogels Composed of an Agarose/Poly(acrylamide-co-acrylic acid) Interpenetrating Network

“…A high level of long alkyl/fluorocarbon chains allowed the hydrophobic associations to occur and form a secondary structure accompanied by a dramatic change in mechanical strength in response to temperature. Thermal‐activated SME was also observed in a dipole–dipole and hydrogen bonding reinforced hydrogel recently reported by Liu and co‐workers . Although a variety of novel strategies have been proposed to develop thermal‐activated SMHs, agarose as a typical thermal‐responsive gelling biopolymer has rarely been taken into consideration …”