Shape Memory Hydrogels Research Articles

A novel mechanical robust, self-healing and shape memory hydrogel based on PVA reinforced by cellulose nanocrystal

Fabrication of multifunctional hydrogel with excellent mechanical property, self-healing and shape memory ability is still a challenging issue. In this study, a mechanical robust multifunctional hydrogel based on PVA reinforced by cellulose nanocrystal using nanofiller reinforcement strategy was reported. Owing to the hydrogen bonding and supramolecular interaction between PVA and CNC, the prepared hydrogel exhibited satisfying mechanical property. The obtained multifunctional PVA hydrogel could be potentially applied in biomedical or artificial skins fields.

Editable and bidirectional shape memory chitin hydrogels based on physical/chemical crosslinking

Advances in soft robotics have enabled rapid progress in shape memory hydrogels, including unidirectional, bidirectional and multidirectional shape memory hydrogels. However, shape memory hydrogels with editable feature have hardly been reported. Here, chitin based bilayer hydrogels were prepared based on alkali/urea aqueous system. Due to the different swelling properties of the two layers, namely physical crosslinking layer and chemical crosslinking layer, the hydrogels can bend toward opposite direction in water and ethanol, showing a bidirectional shape memory behavior. Meanwhile, this bidirectional shape memory hydrogels can be programmed to different shapes with external force after immerging in alkali solution as a result of the hydrogen bond breakage and reformation via hydroxyl ion, displaying an editable feature. We believe that the editable and bidirectional shape memory chitin bilayer hydrogels have great potential in soft robotics.

Combinational Hydrogel and Xerogel Actuators Showing NIR Manipulating Complex Actions

Near-infrared (NIR)-driven shape memory hydrogels are synthesized with a one-pot polymerization of N,N-dimethylacrylamide in the inorganic clay and graphene oxide (GO) suspension. The hydrogel consists of only a physically crosslinked network, which is partially thermoreversible. With the efficient photothermal energy transformation of GO in the hydrogels, the shape recovery from the temporal shape is achieved by NIR irradiation. The optimal shape fixing percentage and recovery rate are found at moderate monomer and crosslinker contents. Meanwhile, the xerogel dried from the hydrogel also shows a fast NIR response shape change. The NIR manipulating combinational hydrogel-xerogel actuators are prepared by combining the wet and soft hydrogel and its dry and rigid xerogel together. The actuators achieve complex actions of turning and lifting under sequential NIR irradiation to carry an object up- and downward and around obstacles, or to transfer an object to a target position. This work provides a new idea for designing combinational actuators to fulfil complex actions.

POSS-Induced Dynamic Cross-Links Produced Self-Healing and Shape Memory Physical Hydrogels When Copolymerized with N-Isopropyl acrylamide.

The hydrogel morphology and viscoelasticity and the two-way shape memory behavior of copolymers of N-isopropyl acrylamide-polyhedral oligomeric silsesquioxane (PNIPAm-POSS) were investigated. NIPAm-POSS copolymers are unentangled and exhibit a thermoplastic and thermoresponsive hydrogel behavior ( Romo-Uribe ; ; Albanil , Eur. Polym. J. 2018 , 99 , 350 - 360 ). Here, we demonstrated by high-resolution transmission electron microscopy that POSS segregated into crystals to form physical cross-links. The hydrogels exhibited significant swelling, higher than chemical hydrogels at the same POSS content. The morphology of the hydrogels consisted of a honeycomb structure, and the pore size and swelling were a decreasing function of POSS. Moreover, the hydrogels were self-healing, exhibiting a liquid-like behavior under large amplitude shear and rubber-like behavior immediately after cessation of shear, that is, the POSS aggregates break and re-form, acting as dynamic cross-linkers. Strikingly, the physical networks behaved as perfect networks, thus matching chemically cross-linked networks and holding the scaling |η*| ∼ ω-1. Furthermore, the physical hydrogels exhibited a thermally activated shape memory (SM) behavior, without external stress and the lower critical solution temperature (LCST) being the only stimulus. The SM process involved swelling-deswelling, effectively opening opportunities for drug delivery. Furthermore, the SM was reversible, and no external stress was involved, therefore behaving as the two-way SM. Finally, POSS modulated the shear elastic modulus G' and LCST; therefore, the PNIPAm-POSS copolymers open opportunities for reusable, thermoformable SM hydrogels with tuned mechanical modulus and activation temperature.

Ion responsiveness of polyacrylamide/sodium alginate (PAM/SA) shape memory hydrogel

ABSTRACTPolyacrylamide/sodium alginate (PAM/SA) hydrogel was prepared from sodium alginate (SA) and acrylamide (AM) by covalently cross-linking. Temporary shape double network PAM/SA hydrogels were obtained by ionic cross-linking formed by complexation of SA and six metal cations (Fe3+, Cu2+, Ag+, Ca2+, Ni2+, and Co2+), among the PAM/SA—Fe3+ hydrogels has the best shape fixity ratio (83%). When the hydrogel is placed in different recovery liquids (sodium citrate, EDTA, sulfuric acid, and potassium carbonate), the shape recovery ratio (98.9%) is the best in sodium citrate solution, and the recovery time (182 s) of the hydrogel is the shortest. In general, Fe3+, Cu2+, and Ag+ are suitable for the construction of ionic-shape memory hydrogels. The PAM/SA hydrogels also exhibited excellent adjustable mechanical properties by changing metal cations and soaking time. The strategy could broaden the construction of ion-responsive shape-memory hydrogels.

Optimized Association of Short Alkyl Side Chains Enables Stiff, Self-Recoverable, and Durable Shape-Memory Hydrogel.

This work reports a self-healing and shape-memory hydrogel integrating multiple mechanical properties. The network configuration is featured as entangled networks cross-linked by distributed association of very short alkyl chains (hexyl, six carbons). These cross-linking knots are interconnected by the long hydrophilic polyvinyl alcohol backbone. The optimal aggregation of hexyl side chains leads to the broadened distribution in bonding strength as verified by static and dynamic mechanical characterization. These structural features contribute to high strength, toughness, stiffness, and yet fast recoverability. Furthermore, the hydrophobic and supramolecular nature of aggregated alkyl chains offers high durability and solvent-assistant healing function. Finally, distributed association of hexyl side chains confers a broadened temperature-dependent modulus, allowing for encoding stepwise shape recovery from a temporary shape at different temperatures and/or times.

One-pot fabrication of triple shape memory hydrogel based on coordination bond, the dynamic borate ester bonds, and hydrogen bond

ABSTRACTIn order to achieve the application of shape memory hydrogel (SMH), the hydrogel must be simple in design, adjustable in mechanical properties, and easy to achieve in shape memory properties. In the article, triple SMHs were fabricated by simple method with adjustable mechanical properties. The triple SMH including coordination bond between sodium alginate and Fe3+, the dynamic borate ester bonds, and hydrogen bond of poly (vinyl alcohol) (PVA) were designed to memorize temporary shapes.

Quadruple-shape hydrogels

The capability of directed movements by two subsequent shape changes could be implemented in shape-memory hydrogels by incorporation of two types of crystallizable side chains. While in non-swollen polymer networks even more directed movements could be realized, the creation of multi-shape hydrogels is still a challenge. We hypothesize that a quadruple-shape effect in hydrogels can be realized, when a swelling capacity almost independent of temperature is generated, whereby directed movements could be enabled, which are not related to swelling. In this case, entropy elastic recovery could be realized by hydrophilic segments and the fixation of different macroscopic shapes by means of three semi-crystalline side chains generating temporary crosslinks. Monomethacrylated semi-crystalline oligomers were connected as side chains in a hydrophilic polymer network via radical copolymerization. Computer assisted modelling was utilized to design a demonstrator capable of complex shape shifts by creating a casting mold via 3D printing from polyvinyl alcohol. The demonstrator was obtained after copolymerization of polymer network forming components within the mold, which was subsequently dissolved in water. A thermally-induced quadruple-shape effect was realized after equilibrium swelling of the polymer network in water. Three directed movements were successfully obtained when the temperature was continuously increased from 5 °C to 90 °C with a recovery ratio of the original shape above 90%. Hence, a thermally-induced quadruple-shape effect as new record for hydrogels was realized. Here, the temperature range for the multi-shape effect was limited by water as swelling media (0 °C–100 °C), simultaneously distinctly separated thermal transitions were required, and the overall elasticity indispensable for successive deformations was reduced as result of partially chain segment orientation induced by swelling in water. Conclusively the challenges for penta- or hexa-shape gels are the design of systems enabling higher elastic deformability and covering a larger temperature range by switching to a different solvent.

DNA-Based Hydrogels Loaded with Au Nanoparticles or Au Nanorods: Thermoresponsive Plasmonic Matrices for Shape-Memory, Self-Healing, Controlled Release, and Mechanical Applications.

Gold nanoparticles (AuNPs) or gold nanorods (AuNRs) are loaded in polyacrylamide hydrogels cooperatively cross-linked by bis-acrylamide and nucleic acid duplexes or boronate ester-glucosamine and nucleic acid duplexes. The thermoplasmonic properties of AuNPs and AuNRs are used to control the stiffness of the hydrogels. The irradiation of the AuNP-loaded (λ = 532 nm) or the AuNR-loaded (λ = 808 nm) hydrogels leads to thermoplasmonic heating of the hydrogels, the dehybridization of the DNA duplexes, and the formation of hydrogels with lower stiffness. By ON/OFF irradiation, the hydrogels are switched between low- and high-stiffness states. The reversible control over the stiffness properties of the hydrogels is used to develop shape-memory hydrogels and self-healing soft materials and to tailor thermoplasmonic switchable drug release. In addition, by designing bilayer composites of AuNP- and AuNR-loaded hydrogels, a reversible thermoplasmonic, light-induced bending is demonstrated, where the bending direction is controlled by the stress generated in the respective bilayer composite.

A high strength hydrogel with quadruple-shape memory under the ambient condition

It is worth looking forward to manufacturing a shape memory hydrogel (SMH) with good mechanical properties for practical applications in a simple and rapid manner. In this study, the one-pot method has been proposed to prepare a quadruple shape memory hydrogel (PAA-CS, poly(acrylic acid)-chitosan). The PAA-CS hydrogel obtained by mixing acrylic acid (AA), chitosan (CS), initiator, and cross-linker at 60 °C shows excellent function of shape memory and recovery less than 20 s. Moreover, its maximum deformation can reach to 100%. Scanning electron microscopy (SEM) showed that PAA-CS hydrogels would form new cross-linked networks in NaOH, NaCl, and FeCl3, and these new networks can trigger quadruple shape memory. In addition, the tensile and compressive strength tests showed the PAA-CS hydrogel possesses outstanding mechanical properties. Its excellent property has enormous potential in applications where multi-shape changes responding to different stimuli are required.

Thermal- and salt-activated shape memory hydrogels based on a gelatin/polyacrylamide double network.

Shape memory hydrogels have been extensively studied in the past decades owing to their exceptionally promising potential in a wide range of applications. Here, we present a gelatin/polyacrylamide double network hydrogel with thermal- and salt-activated shape memory effect. The thermally activated behavior is attributed to the reversible triple helix transformation of gelatin, and the salt-activated performance can be ascribed to the formation of hydrophobic interaction domains under the Hofmeister effect. The hydrogel can memorize a temporary shape successfully through soaking with (NH4)2SO4 solution or decreasing temperature, and recovers its permanent shape by extracting ions with deionized water or increasing temperature. In particular, the hydrogel exhibits excellent shape fixity and recovery ratio. The presented strategy may enrich the construction as well as application of biopolymer based shape memory hydrogels.

Photo-adaptable shape memory hydrogels based on orthogonal supramolecular interactions

A novel shape memory hydrogel with photo-adaptable permanent shape has been developed on the basis of alginate–Ca2+ coordination and the host–guest interaction between α-cyclodextrin and azobenzene.

High strength hydrogels with multiple shape-memory ability based on hydrophobic and electrostatic interactions.

Hydrogels with multiple shape-memory ability have aroused great interest due to their promising applications in various fields. Nevertheless, the weak mechanical performance of most shape-memory hydrogels seriously impedes the practical application in more complex environments. Herein, we reported a novel hydrogel with both high mechanical and multi-shape memory properties composed of stearyl methacrylate (SMA), acrylic acid (AA) and quaternary chitosan (QCH). The electrostatic interactions between AA and QCH together with the hydrophobic interactions of alkyl chains in SMA endowed the hydrogel with great strain-stress and fatigue resistance. Furthermore, due to the reversible destruction and construction of physical cross-links, the prepared hydrogels also exhibited the shape-memory ability in response to different stimuli, such as temperature, pH and NaCl solution. Additionally, the multiple shape-memory effect could be accomplished via programmable combination because of the relatively independent physical interactions in the hydrogels.

A pH-induced self-healable shape memory hydrogel with metal-coordination cross-links

A 4-armed PEG–DA hydrogel was fabricated, which showed regulated shape memory and self-healing properties at different pH values.

A body temperature and water-induced shape memory hydrogel with excellent mechanical properties

A body temperature and water-induced shape memory hydrogel with excellent mechanical properties was prepared by crosslinking dopamine-terminated tetra-poly(ethylene glycol) with an oxidation reaction.

Correction: Trends in polymeric shape memory hydrogels and hydrogel actuators

Correction for ‘Trends in polymeric shape memory hydrogels and hydrogel actuators’ by Jiaojiao Shang et al., Polym. Chem., 2019, 10, 1036–1055.

Trends in polymeric shape memory hydrogels and hydrogel actuators

Recently, “smart” hydrogels with either shape memory behavior or reversible actuation have received particular attention and have been further developed into sensors, actuators, or artificial muscles.

A supramolecular hydrogel with monitorable macro/microscopic shape memory performance.

A supramolecular shape memory hydrogel (SMH) with monitorable shape memory performance both in macro and microscopic scales was prepared through a combination of pH-responsive fluorescent carbon nanoparticles, polyvinyl alcohol and cross-linked polyacrylamide. The investigation of our SMH will provide a good understanding of the shape memory mechanism and guide in the structure design of the novel SMH.

Highly stretchable and compressible shape memory hydrogels based on polyurethane network and supramolecular interaction

We synthesized a new type of PU hydrogel via a simple free radical polymerization, where double bond terminated-WPU (waterborne polyurethane) as the first crosslinker, N,N-methylene bisacrylamide (MBA) as the second cross-linker and MAH-β-CD (maleic anhydride modified β-cyclodextrin) as noncovalently crosslinker, which formed hydrogen bonds with GO and host-guest interaction with WPU. The tensile strength and compressive strength are over 500 kPa and 2.5 MPa at the optimal proportion. By changing the ratio of the chemical component, we synthesized WPU hydrogels with high tensile strength and fracture elongation and surprisingly the swollen hydrogel has excellent compressive properties and fatigue resistance performance.

Double network shape memory hydrogels activated by near-infrared with high mechanical toughness, nontoxicity, and 3D printability

In this work, near-infrared (NIR)-responsive double network shape memory hydrogels were formed by chemically cross-linking Pluronic F127 diacrylate macromer (F127DA) and physical blending of poly(lactide-co-glycolide) (PLGA) with graphene oxide (GO, an energy convertor to convert NIR irradiation to thermal energy). The hydrogels were manufactured with 3D-printing technology using ultraviolet light polymerization. The morphologies and crystalline structure of the hydrogels were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The NIR and thermal activated shape memory properties, mechanical toughness, and cytotoxicity were investigated. The shape memory properties were improved by incorporating GO into the hydrogels and the mechanical properties were enhanced by the addition of PLGA, which served as a second network. The cytotoxicity was assessed by the CCK-8 assay, which revealed no cytotoxicity. The nontoxicity, high mechanical toughness (3.45 MPa in the swollen state), and biosafety at the shape recovery temperature (36 ± 1 °C), which was achieved by NIR stimuli, indicates that shape memory hydrogels can be used in biomedical materials safely. The practical potential of the F127DA/PLGA/GO hydrogels was further revealed by their 3D-printing performance, their shape fixity ratio greater than 85%, and their shape recovery time under 300 s. Our results propose that F127DA/PLGA/GO hydrogels will be a promising material in biomedical applications as a drug carrier and an antibacterial scaffold.