Dual-responsive shape memory hydrogels with novel thermoplasticity based on a hydrophobically modified polyampholyte

Four-Dimensional Printing for Hydrogel: Theoretical Concept, 4D Materials, Shape-Morphing Way, and Future Perspectives

The limitations and challenges possessed in static 3D materials necessitated a new era of 4D shape-morphing constructs for wide applications in diverse fields of science. Shape-morphing behavior of 3D constructs over time is 4D design. Four-dimensional printing technology overcomes the static nature of 3D, improves substantial mechanical strength, and instills versatility and clinical and nonclinical functionality under set environmental conditions (physiological and artificial). Four-dimensional printing of hydrogel-forming materials possesses remarkable properties compared to other printing techniques and has emerged as the most established technique for drug delivery, disease diagnosis, tissue engineering, and biomedical application using shape-morphing materials (natural, synthetic, semisynthetic, and functionalized) in response to single or multiple stimuli. In this article, we addressed a fundamental concept of 4D-printing evolution, 4D printing of hydrogel, shape-morphing way, classification, and future challenges. Moreover, the study compiled a comparative analysis of 4D techniques, 4D products, and mechanical perspectives for their functionality and shape-morphing dynamics. Eventually, despite several advantages of 4D technology over 3D technique in hydrogel fabrication, there are still various challenges to address with using current advanced and sophisticated technology for rapid, safe, biocompatible, and clinical transformation from small-scale laboratory (lab-to-bed translation) to commercial scale.

A pH controlled temperature response reprogramming hydrogel for monitoring human electrophysiological signals

This study proposes a simple method to prepare a pH-responsive and shape memory hydrogel based on cooperative hydrophobic interaction and hydrogen bonding. Acryloyl 11-aminoundecanoic acid (A11AUA) and acrylamide were selected as hydrophobic monomers and hydrophilic monomers, respectively. The mechanical properties of the prepared hydrogel strongly depend on the pH. Under acidic conditions, the maximum tensile strength of the hydrogel can reach 7.8 MPa, and the tensile modulus of the hydrogel can be increased by more than 10 000 times. The mechanical properties of acidic gels are affected by temperature and exhibit a temperature-controlled shape memory function. The acidic gel is immersed in NaOH and HCl solutions in sequence to achieve the function of reprogramming. Hydrogels under alkaline and neutral conditions exhibit conductivity and adhesion properties controlled by pH. Using the hydrogel as an adhesive electrode, the performance of the hydrogel in monitoring human electrophysiological signals was discussed.

Ultrafast and Programmable Shape Memory Hydrogel of Gelatin Soaked in Tannic Acid Solution

Shape memory hydrogels have been paid plenty of attention as a kind of intelligent soft material. However, complicated preparation and slow and uncontrollable shape change have hindered their applications in smart actuators. In this work, a temperature-responsive strong hydrogel was prepared by a facial soaking method without any chemical reactions, i.e., soaking gelatin hydrogel in aqueous tannic acid solution. The hydrogel was constructed by hydrogen bonding between gelatin and tannic acid beside the triple helix of gelatin chains without any chemical cross-linkers. The hydrogel showed ultrafast shape memory and body-temperature response. The hydrogel can be fixed in temporary shape in only 1 s at 25 °C and recover to the original shape in also 1 s at 37 °C, superior to the reported shape memory hydrogels. Furthermore, the hydrogel shape change can be programmed by fixing the temperature, and the designed shape is achieved stepwise by adjusting the recovery temperature. In addition, the hydrogel is stable in water without further swelling. These excellent features will initiate new prosperity of the shape memory hydrogel in biomedical technology, underwater actuators, and soft robots.

A comprehensive review of the structures and properties of ionic polymeric materials

This review focuses on the mechanistic approach, the structure–property relationship and applications of ionic polymeric materials.

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.

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 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.

Rigid and Strong Thermoresponsive Shape Memory Hydrogels Transformed from Poly(vinylpyrrolidone- co-acryloxy acetophenone) Organogels

Shape memory hydrogels (SMHs) have a wide range of potential practical applications. However, the mechanically weak and soft nature of most SMHs strongly impedes their applications. Here, we report a novel kind of thermal-responsive SMH with high tensile strength and high elastic moduli. Organogels are first prepared by the copolymerization of a hydrophilic monomer N-vinylpyrrolidone (NVP) and a hydrophobic monomer acryloxy acetophenone (AAP) in N, N'-dimethylformamide (DMF) solutions, and then, poly(vinylpyrrolidone- co-acryloxy acetophenone) [poly(NVP- co-AAP)] hydrogels are obtained by solvent exchange with water. Because of the strong and reversible hydrophobic association and π-π stacking of acetophenone groups, the poly(NVP- co-AAP) hydrogels exhibit tensile strengths up to 8.41 ± 0.83 MPa and Young's moduli up to 94.2 ± 1.3 MPa, which are more than 1 or 3 orders of magnitude higher than those of the organogels, respectively. The poly(NVP- co-AAP) hydrogels exhibit good shape memory behaviors, with a complete fixation ratio and a recovery ratio of 74-89%, as well as very fast shape-fixing and recovering rates (in seconds). These rigid and strong hydrogels are demonstrated to be an ideal shape memory material for surgical fixation devices to wrap around and support various shapes of limbs.

Functional Stimuli-Responsive Gels: Hydrogels and Microgels

One strategy that has gained much attention in the last decades is the understanding and further mimicking of structures and behaviours found in nature, as inspiration to develop materials with additional functionalities. This review presents recent advances in stimuli-responsive gels with emphasis on functional hydrogels and microgels. The first part of the review highlights the high impact of stimuli-responsive hydrogels in materials science. From macro to micro scale, the review also collects the most recent studies on the preparation of hybrid polymeric microgels composed of a nanoparticle (able to respond to external stimuli), encapsulated or grown into a stimuli-responsive matrix (microgel). This combination gave rise to interesting multi-responsive functional microgels and paved a new path for the preparation of multi-stimuli "smart" systems. Finally, special attention is focused on a new generation of functional stimuli-responsive polymer hydrogels able to self-shape (shape-memory) and/or self-repair. This last functionality could be considered as the closing loop for smart polymeric gels.

Semicrystalline Hydrophobically Associated Hydrogels with Integrated High Performances

Hydrophobically associated hydrogels (HA gels) are one of most extensively investigated high strength hydrogels. Semicrystalline HA gels, prepared by micellar copolymerization, show high strength and notable functionalities of self-healing and shape-memory. However, the hydrophobic comonomers in these semicrystalline HA gels are usually limited to the long alkyl length monomers (18-alkyl(meth)acrylates). In the present work, N-acryloyl 11-aminoundecanoic acid (A11AUA), consisting of 10 -CH₂ groups and a -COOH group at the end of alkyl chain, was used as hydrophobic comonomer to prepare physical A11AUA-based HA gels in the presence of high concentration cetyltrimethylammonium bromide (CTAB) or sodium dodecyl sulfate. Differential scanning calorimetry, wide-angle X-ray scattering, and small-angle X-ray scattering experiments had identified that the A11AUA-based HA gels possessed crystalline domains and clusters of crystalline domains, while lauryl methacrylate (C12M)-based HA gels were amorphous. As a result, A11AUA-based HA gels displayed much better tensile properties than those of C12M-based HA gels. At the optimal condition, the A11AUA-CTAB HA gel demonstrated integrated high performances, including high stiffness (E of 1016 kPa), high strength (σ_f of 0.75 MPa), high toughness (T of 7540 J/m²), rapid self-recovery (94% recovery after heat treatment at 60 °C for 2 min), outstanding shape memory (fully recovered to the permanent shape only 2-14 s), and excellent self-healing properties (as healed at 60 °C for 2 h; stress and strain healing efficiency reached to 64% and 85%, respectively). We believe this work provides a new insight for HA gels, which is beneficial to design new hydrogels with integrated high performances, such as high strength, high toughness, large extensibility, and shape-memory and self-healing properties.

Multivalent cations-triggered rapid shape memory sodium carboxymethyl cellulose/polyacrylamide hydrogels with tunable mechanical strength

A novel multivalent cations-triggered shape memory hydrogels were synthesized in a one-pot method, and interpenetrating double network was formed by chemically cross-linked polyacrylamide (PAM) network and physically cross-linked sodium carboxymethyl cellulose network. The temporary shape was fixed by complexation between a native biopolymer, sodium carboxymethyl cellulose (CMC), and transition metal ions, specifically Fe³⁺, Ag⁺, Al³⁺, Cu²⁺, Ni²⁺, and Mg²⁺. In particular, CMC-Fe³⁺ hydrogel exhibits excellent shape fixity ratio (95%). Therefore, we chose PAM/CMC_1.0-Fe³⁺ hydrogel as the model material and further investigated its shape recovery process. It was found that a wide range of molecules and anions could be applied to break off the temporary cross-links between CMC and Fe³⁺. The PAM/CMC composite hydrogels also exhibited excellent tunable mechanical properties. The mechanical properties of the composite hydrogel can be adjusted by changing the cross-linking densities. The presented strategy could enrich the construction as well as application of biopolymers based shape memory hydrogels.

A New Type of Photo-Thermo Staged-Responsive Shape-Memory Polyurethanes Network

In this paper, we developed a photo-thermo staged-responsive shape-memory polymer network which has a unique ability of being spontaneously photo-responsive deformable and thermo-responsive shape recovery. This new type of shape-memory polyurethane network (A-SMPUs) was successfully synthesized with 4,4-azodibenzoic acid (Azoa), hexamethylenediisocyanate (HDI) and polycaprolactone (PCL), followed by chemical cross-linking with glycerol (Gl). The structures, morphology, and shape-memory properties of A-SMPUs have been carefully investigated. The results demonstrate that the A-SMPUs form micro-phase separation structures consisting of a semi-crystallized PCL soft phase and an Azoa amorphous hard phase that could influence the crystallinity of PCL soft phases. The chemical cross-linking provided a stable network and good thermal stability to the A-SMPUs. All A-SMPUs exhibited good triple-shape-memory properties with higher than 97% shape fixity ratio and 95% shape recovery ratio. Additionally, the A-SMPUs with higher Azoa content exhibited interesting photo-thermo two-staged responsiveness. A pre-processed film with orientated Azoa structure exhibited spontaneous curling deformation upon exposing to ultraviolet (UV) light, and curling deformation is constant even under Vis light. Finally, the curling deformation can spontaneously recover to the original shape by applying a thermal stimulus. This work demonstrates new synergistically multi-responsive SMPUs that will have many applications in smart science and technology.

Shape Memory Polymers Based on Supramolecular Interactions

Shape memory polymers (SMPs), with the capability to change from one or more temporary shapes to predetermined shapes in response to an external stimulus, have attracted much interest from both academia and industries. When introducing supramolecular interactions that have been featured as dynamic and reversible into the design of novel SMPs, intriguing and unique functionalities have been engendered and thereby broaden the potential applications of the SMPs to new territories. In this review, we summarize recent progress made in SMPs based on supramolecular interactions, provide insight into the material design and shape memory mechanism, elucidate and evaluate their properties and performance, and point out opportunities and applications of SMPs.

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 102 to 6 × 102 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m3. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.

Shape-Memory Hydrogels: Evolution of Structural Principles To Enable Shape Switching of Hydrophilic Polymer Networks

The ability of hydrophilic chain segments in polymer networks to strongly interact with water allows the volumetric expansion of the material and formation of a hydrogel. When polymer chain segments undergo reversible hydration depending on environmental conditions, smart hydrogels can be realized, which are able to shrink/swell and thus alter their volume on demand. In contrast, implementing the capacity of hydrogels to switch their shape rather than volume demands more sophisticated chemical approaches and structural concepts. In this Account, the principles of hydrogel network design, incorporation of molecular switches, and hydrogel microstructures are summarized that enable a spatially directed actuation of hydrogels by a shape-memory effect (SME) without major volume alteration. The SME involves an elastic deformation (programming) of samples, which are temporarily fixed by reversible covalent or physical cross-links resulting in a temporary shape. The material can reverse to the original shape when these molecular switches are affected by application of a suitable stimulus. Hydrophobic shape-memory polymers (SMPs), which are established with complex functions including multiple or reversible shape-switching, may provide inspiration for the molecular architecture of shape-memory hydrogels (SMHs), but cannot be identically copied in the world of hydrophilic soft materials. For instance, fixation of the temporary shape requires cross-links to be formed also in an aqueous environment, which may not be realized, for example, by crystalline domains from the hydrophilic main chains as these may dissolve in presence of water. Accordingly, dual-shape hydrogels have evolved, where, for example, hydrophobic crystallizable side chains have been linked into hydrophilic polymer networks to act as temperature-sensitive temporary cross-links. By incorporating a second type of such side chains, triple-shape hydrogels can be realized. Considering the typically given light permeability of hydrogels and the fully hydrated state with easy permeation by small molecules, other types of stimuli like light, pH, or ions can be employed that may not be easily used in hydrophobic SMPs. In some cases, those molecular switches can respond to more than one stimulus, thus increasing the number of opportunities to induce actuation of these synthetic hydrogels. Beyond this, biopolymer-based hydrogels can be equipped with a shape switching function when facilitating, for example, triple helix formation in proteins or ionic interactions in polysaccharides. Eventually, microstructured SMHs such as hybrid or porous structures can combine the shape-switching function with an improved performance by helping to overcome frequent shortcomings of hydrogels such as low mechanical strength or volume change upon temporary cross-link cleavage. Specifically, shape switching without major volume alteration is possible in porous SMHs by decoupling small volume changes of pore walls on the microscale and the macroscopic sample size. Furthermore, oligomeric rather than short aliphatic side chains as molecular switches allow stabilization of the sample volumes. Based on those structural principles and switching functionalities, SMHs have already entered into applications as soft actuators and are considered, for example, for cell manipulation in biomedicine. In the context of those applications, switching kinetics, switching forces, and reversibility of switching are aspects to be further explored.

A mechanically robust hydrogel with thermally induced plasticity and a shape memory effect

A new class of smart structural hydrogels is prepared by introducing dual cross-linkers into a single-network system. The present hydrogel, on the one hand, exhibits excellent mechanical properties; on the other hand, it exhibits thermally induced plasticity and a shape memory effect without any overlap.

Fe3+-, pH-, Thermoresponsive Supramolecular Hydrogel with Multishape Memory Effect

Poor, nontunable mechanical properties as well as finite shape memory performance pose a barrier to shape memory hydrogels to realize practical applications. Here, a new shape memory hydrogel with tunable mechanical properties and multishape memory effect was presented. Three programmable reversible systems including PBA-diol ester bonds, AAc-Fe³⁺, and coil-helix transition of agar were applied to memorize temporary shapes and endow the hydrogel with outstanding multishape memory functionalities. Moreover, through changing the cross-linking densities, the mechanical properties of the as-prepared hydrogel can be adjusted.

Poly(vinyl alcohol)-Tannic Acid Hydrogels with Excellent Mechanical Properties and Shape Memory Behaviors

Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the "permanent" cross-link and the weaker H-bonding between PVA chains as the "temporary" cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively.

NIR-Triggered Rapid Shape Memory PAM-GO-Gelatin Hydrogels with High Mechanical Strength

Shape memory hydrogels containing over 76 wt % of water were synthesized in a one-pot method, and interpenetrating double network was formed by physically cross-linked gelatin network and chemically cross-linked polyacrylamide (PAM) network with graphene oxide (GO). The temporary shape was quickly fixed by cooling in ice water for 30 s after deformation at 80 °C for 10 s. Shape recovery started in 10 s under near-infrared (NIR) irradiation and almost completed within 60 s depending on the curling angle. A small amount of GO in the hydrogels (≤1.5 mg/mL) played a key role in NIR energy absorption and transformation into thermal energy. The hydrogel without GO showed no response to the NIR irradiation and cannot recover to its permanent shape by NIR irradiation. Temperature sweep was conducted in the cycle of 20 °C → 80 °C → 20 °C, and the structure change in the hydrogels with temperature was investigated according to the storage modulus G' and tangent of the loss angle tan δ as a function of the hydrogel composition. The shape-memory capability was confirmed as the contribution from the triple-helix cross-linking network of gelatin. High mechanical toughness (strength > 400 kPa and broken strain > 500%) was achieved by the double-network with the sacrificial gelatin network and GO bridging to dissipate deformation energy. The optimized composition of the hydrogel was found to be a key point to realize stable temporary shape and rapid recovery to the permanent shape controlled by NIR irradiation with reasonable strength. The facile preparation and noncontact gentle stimulus of the present hydrogel hold great potential to be used in soft actuator materials.

Tough polypseudorotaxane supramolecular hydrogels with dual-responsive shape memory properties

We report a highly compressible polypseudorotaxane supramolecular hydrogel with antifatigue properties that can bear 80% compressive strain without rupture.

UV-controlled shape memory hydrogels triggered by photoacid generator

UV-controlled shape memory hydrogel is designed with PhotoAcid Generator (PAG) as the trigger.