pH- and Thermal-Responsive Multishape Memory Hydrogel

Abstract

A multistimuli sensitive shape memory hydrogel with dual and triple shape memory properties was prepared by grafting dansyl groups into the network of polyacrylamide (PAAM). The hydrophobic aggregation of dansyl groups acted as molecular switches, which showed reversible aggregationdisaggregation transition in aqueous solution in response to the pH or temperature change.KEYWORDS: multishape memory, multistimuli sensitivity, hydrogel, hydrophobic aggregation, dansyl groups S hape memory polymers (SMPs) are an important class of stimuli-responsive materials, which can be deformed and fixed into a temporary shape, and recover to their original shape when exposed to a specific external stimulus. The capacities of remembering two or more shapes under different conditions are called dual-shape memory effect (dual-SME) and mutishape memory effect (multi-SME), which give the SMPs great potential for a wide range of applications, such as smart devices, sensors, and actuators. To realize more complex actuation and to expand the technical potential of SMPs, exploring new triggering mechanisms and multishape memory processes have been the focus of SMP research. 1−4 So far, the most common stimulus for shape memory effect is heating, but other stimuli like pH have also become increasingly attractive in recent years. pH is of important environmental value in typical physiological, biological and/or chemical systems. It can also be utilized in the applications where large temperature swings are undesirable. For example, an SMP using pH-sensitive host−guest complexes as the molecular switches was reported by our group. It could be processed into a temporary shape at pH 11.5 and was found to be able to recover to the original shape at pH 7.0. 5 Subsequently, a kind of pH-sensitive polyurethane using hydrogen bonding as molecular switches, which could recover its initial shape at pH 1.5, was prepared by Zhou et al. 6 These works, however, only focused on the pH-induced shape memory effect. Although a few reports have developed some multi-stimuli-responsive behaviors of materials, 7−9 seldom SMP has been reported in response to multistimuli of pH values and other stimuli up to now; however, it is obvious that the feedback behavior of materials usually occurs because of a combination of multiple factors in nature.To develop multishape memory polymers that are capable of remembering more than one temporary state, therefore more flexible and able to meet more complex demand of applications, is another attractive subject in the field of SMPs. Generally, multishape SMPs need to have two or more distinct thermal phase switches, or a single broad thermal transition, and the multishape memory effect is triggered by heating. 4,10−13 Although Hu et al. 14 and Zhou et al. 15 reported triple SMP being responsive to thermal and water stimuli, respectively, a combination of both multistimuli and multishape memory effect in one material is still a big challenge.In this letter, we synthesized a versatile pH-and thermalresponsive mult...

A Dual‐Crosslinked Strategy to Construct Physical Hydrogels with High Strength, Toughness, Good Mechanical Recoverability, and Shape‐Memory Ability

“…It could be also seen that both recovery rates (including elastic modulus and dissipated energy) decreased as ε max increased and recovery rate of elastic modulus dramatically dropped with the increase of ε max . For viscoelastic systems, which have been reported by Gong et al, there might be a two‐stage recovery process after the first loading, which was governed by the competition between the elasticity of the primary chain and the strength of the temporarily reformed bonds. A fast recovery process occurred during the unloading process because elastic retraction dominated the recovery.…”

“…[1] They have been acquired extensive uses in the field of pharmaceutics and biomedicine such as carriers for drug and gene delivery, [2] scaffolds for tissue engineering, [3,4] and valves in microfluidics [5] due to their unique physicochemical properties. Most other promising applications covalent bond, physical interactions based on supramolecular associations such as ionic bond, [27] hydrogen bonding, [28,29] crystallization, [30] and hydrophobic interaction, [31] is reversible and can respond to various stimuli, which can be used as the dynamic crosslinking points to improve self-recovery and versatility of tough hydrogels. [32] Gong et al designed a tough polyampholyte hydrogel by forming ionic bonds with different strength, wherein a small fraction of strong bonds served as permanent crosslinks to hold the integrity of gel structure and a high fraction of weak bonds served as reversible crosslinks to toughen the gel and enhance the recoverability.…”

Trends in polymeric shape memory hydrogels and hydrogel actuators

“…46 And, what is more, the reversible aggregationdisaggregation transition of the dansyl group can respond to both pH and temperature, thus the hydrogels showed a tripleshape memory behavior by controlling the programming process. 47 2.2.3 Time-controlled multi-shape memory effects. Another simple method to realize a multi-shape memory effect is by adjusting the processing time.…”

In situ addition of graphitic carbon into a NiCo2O4/CoO composite: enhanced catalysis toward the oxygen evolution reaction

“…Thus, the stability of the proposed electrodes herein exceeds that of the benchmarked IrO 2 and RuO 2 based electrodes previously reported within literature. [41][42][43][44] The number of moles of O 2 generated per second per mole by the NiCo 2 O 4 /CoO/graphitic carbon composite at h 10 , is expressed using turn over frequency, calculated using current density obtained from LSV. 45 The value of TOF for the NiCo 2 O 4 catalyst is found to be 1.53 Â 10 À2 s À1 , which is close to the TOF value for NiCo 2 O 4 /NiO (1.4 Â 10 À2 s À1 ) 46 and superior to that of the TOF value reported for NiFe 2 O 4 (5.74 Â 10 À4 s À1 ) and M x -Ni 1Àx Fe 2 O 4 (0.7-1.87 Â 10 À4 s À1 ).…”