Water-induced Shape Memory Effect Research Articles

Thermo-aqueous coupling behaviors for shape memory hydrogels: A statistical–mechanical model and simulations faced with experiments

The shape memory behavior of hydrogels endows themselves with the capacities of shape programming and shape recovering, which have great potential in many fields, such as drug release and soft robotics. In thermally responsive hydrogel, particularly, there are two critical factors, temperature and water, that can induce the shape memory effect(SME). Temperature-induced SME has been extensively studied, while water-induced SME has been rarely investigated. In our work, the combined effect of water and temperature on shape memory behavior of hydrogels is experimentally studied and deeply analyzed from a physical perspective. We found that the shape fixed ratio of shape memory hydrogels (SMHs) increases monotonically with temperature, but first increases and then decreases with water content. Moreover, it exits a nonlinear coupling between the water and temperature effects on the SME. On the basis of these findings, a constitutive model is developed to capture this thermo-aqueous coupling using the statistical theory of transient networks, from which we obtain an explicit evolution equation of deformation gradient. In the proposed theory, the temperature-induced SME is investigated through the temperature-water responsive association and dissociation of dynamic bonds while the water-induced SME is explained by the variation of the glass transition temperature according to the Fox–Flory model. Finally, and to provide a final test of the theoretical work, it is implemented into a user element (UEL) subroutine which provide simulations in agreement with our experiments. Our work is expected to provide guidance for future application of SMHs.

PVA–Polystyrene-Based Polymer Films with Water-Induced Shape-Memory Effect

In this work, a series of novel water-induced shape-memory composite films are prepared on the basis of poly(vinyl alcohol)-polystyrene (PVA–PSX) through one pot in situ polymerization method. The neat PVA film is modified to PVA–PSX film by loading calibrated amount of styrene, and the resulted film exhibits high mechanical strength (tensile strength ∼40 MPa, tensile strain ∼315%, Young’s modulus ∼0.25 MPa and toughness ∼5 MJ/m3). More importantly, water-induced shape-memory behavior is observed for the PVA–PSX composite films and among various proportions, PVA–PS3 film (3 v/v% of styrene content) shows the fastest shape recovery rate (160 s). The relative low value of storage modulus and glass transition temperature of the PVA–PS3 films is attributed to water-induced shape recovery. The desirable and multifaceted properties of the PVA–PSX composite film, such as optimum transparency, improved water resistance, enhanced shape recovery, diminished UV-light transmission, and effectual antimicrobial activity, propound that the composite film is a suitable candidate for preparing packaging materials in food and cosmetic industries.

Mechanically Strong, Heat-Resistant, Water-Induced Shape Memory Poly(vinyl alcohol)/Regenerated Cellulose Biocomposites via a Facile Co-precipitation Method.

In this work, poly(vinyl alcohol) (PVA) biocomposites with regenerated cellulose-softwood pulp (RC-SP) as a green reinforcement were prepared via co-precipitation method. Simultaneous precipitation of the two components promotes uniform dispersion of the RC-SP and constructs strong molecular chain entanglements and hydrogen bonding network inside the composites. This physical cross-linking network reduces the water absorption and improves the water resistance of the composites. The incorporation of RC-SP not only improves the thermal decomposition properties of the composites, but also enhances the mechanical properties and dynamic mechanical properties, attributed to the strong interaction between the filler and the matrix. Moreover, the fabricated PVA/RC-SP composites exhibit good water-induced shape memory effect, and shape recovery rate of 10% RC-SP reinforced composite reaches 95.3% after immersing for 35 min. This work provides useful information for the implementation of co-precipitation method and the application of renewable cellulose resources.

Thermal insulation management of biopolymer hairs through water-stimulated shape memory effect of crispness

Animals living in the extremely cold plateau area have shown amazing ability to maintain warm, benefiting from their hair micro-crimps. Investigation of such micro-crimped hairs using water-induced shape memory effect would control the hair thermal insulation through fixing the hair temporary micro-crimps by a self-developed hydration process. The temporary hair micro-crimps were achieved through switching on and off hydrogen bonds between macromolecules through penetrating into and removal of aqueous molecules. The controllable thermal insulation was realized by managing the hair micro-crimps, thus, managing the multiple reflectance of infrared light by hair hierarchical micro-crimps from root to head.

Bioinspired poly(vinyl alcohol)-silk hybrids: Two-way water-sensitive shape-memory materials

Water-sensitive shape memory polymers have drawn significant attention because water is the safest stimulus for living organisms which is much easier to be performed in biological applications. Inspired by the water-stimulated shape memory behavior of supercontracted spider silks, silk fibroin with β-sheet structure is introduced into poly(vinyl alcohol) (PVA) to develop a novel shape memory material. Bending test indicated that the addition of the silk fibroin favors good water-induced shape-memory effect (WSME) of the PVA/silk hybrids. More importantly, the PVA/silk hybrids also display two-way WSME, which could mimic the behavior of pine cones to close their scales in a wet state and open them again in a dry state. It is demonstrated that the asymmetric laminated structure of the hybrids could attribute to the two-way shape-memory effect, whereas the competitive hydrogen bonds between silk fibroin and PVA from water as “switch”, the β-sheet structure as “netpoint” and its swelling plasticizing effect are main factors for the water-sensitive shape-memory behavior. These PVA/silk hybrids not only possess biocompatibility and nontoxicity, but also have ease to be fabricated, which are promising in various potential applications such as biological, medical, textile, and toys.

Water-Induced shape memory effect of nanocellulose papers from sisal cellulose nanofibers with graphene oxide

A novel water-induced shape memory nanocomposites were prepared by introducing graphene oxide (GO), which was based on microcrystalline cellulose nanofibers (MSF-g-COOH) extracting from sisal fibers. The results showed that the water-induced shape memory properties of MSF-g-COOH were significantly improved by the strong hydrogen bonding interaction between MSF-g-COOH and GO, It leads to some additional physically cross-linked points in MSF-g-COOH. On the other hand, at 0.5wt% GO loading, tensile strength and Young modulus of the nanocomposite increased from 139 to 184MPa, and from 5.77 to 8.54GPa, respectively, compared to those of pure MSF-g-COOH. Furthermore, a water-induced model was proposed to discuss the water-induced shape memory behaviors of the MSF-g-COOH/GO nanocomposites. This study provides a framework for developing a cellulose based shape memory polymers (CSMPs) and better understanding the shape recovery mechanism in water-induced CSMPs.

Bioabsorbable radiopaque water-responsive shape memory embolization plug for temporary vascular occlusion.

We describe the preparation, characterization and evaluation of a biodegradable radiopaque water-triggered shape memory embolization plug for temporary vascular occlusion. The shape memory occluding device consists of a composite of a radio-opaque filler and a poly (dl-lactide-co-glycolide) (PLGA) blend, which was coated with a crosslinked poly (ethylene glycol) diacrylate (PEGDA) hydrogel. The mechanical properties, the degradation timeframe, the effect of programming conditions on the shape memory behaviour and the extent of radio-opacity for imaging were evaluated. Based on the tests, the mechanism responsible for the water-induced shape memory effect in such an embolization plug was elucidated. Suitable materials were optimized to fabricate an embolic plug prototype and its invitro performance was evaluated as an occlusion rate (using a custom-built set up) and its biocompatibility. Finally, a feasibility study was conducted invivo in a rabbit model to investigate the ease of device deployment, device migration and extent of vessel occlusion. The invivo results demonstrated that the prototypes were visible under fluoroscopy and complete vascular occlusion occurred within 2min of deployment of the prototypes invivo. In conclusion, the developed embolization plug enables controlled and temporary vascular embolization, and is ready for safety studies.

Shape memory/change effect in a double network nanocomposite tough hydrogel

In this paper, we present a systematic investigation on the shape memory/change effect in a double network nanocomposite tough hydrogel. Water-content dependency of the response of this hydrogel to heating and wetting by water is confirmed. Since this hydrogel is tough (even after being fully wetted in water) and has a relatively lower swelling ratio, apart from conventional shape memory/change effect as in ordinary hydrogels, additional features have been realized. These features include heating induced shape memory effect utilizing the absorbed water as the transition component, mechano-responsive shape change effect after water wetting and water-induced shape memory effect.

Sodium dodecyl sulfate/epoxy composite: water-induced shape memory effect and its mechanism

With the aim of integrating a family of functional composites, possessing a conspicuous water-induced shape memory effect (SME), a novel sodium dodecyl sulfate–epoxy shape memory composite was constructed. The original fabrication strategy of the composite was designed based on the chemical interaction constructed 3D microvoid on the shape memory composite surface. Compared with the pure epoxy shape memory polymer, the composites display a gratifying water-induced shape memory effect. The results indicate that the water-induced shape recovery rate of the composite is accelerated through increasing the temperature or decreasing the specimen thickness. An immersion test in water suggests that the chemical interaction and physical swelling effect have a significant influence on the water-induced shape memory process. This research advocates the design concept and presents some experimental results of the water-driven smart composite. The potential application range is expected to expand more widely, including a humidity sensor, temperature/humidity switch, underwater deployable structure and a power source transforming chemical energy into mechanical energy for an ultralow-power device.

Water-induced shape memory effect of graphene oxide reinforced polyvinyl alcohol nanocomposites

A novel water-induced shape memory polymer based on polyvinyl alcohol (PVA) was prepared by introducing graphene oxide (GO). Due to the strong hydrogen bonding interaction between PVA and GO, some additional physically cross-linked points could be formed in PVA, which largely improved shape memory properties of PVA. Solvent-induced shape memory behavior was observed by immersing PVA/GO nanocomposites in water. The water-induced shape recovery was due to the decrease of glass transition temperature and storage modulus. This could be explained by the swelling plasticizing effect of water on PVA, as indicated by the obvious expansion in volume of PVA. On the other hand, the weakened hydrogen bonding between PVA and GO was also observed after immersing the PVA/GO nanocomposites in water. Thus both the plasticizing effect and the competitive hydrogen bonding were the two main reasons for the shape recovery of PVA/GO nanocomposites. This study provides a framework for developing new shape memory polymers (SMPs) and for better understanding the shape recovery mechanism in solvent-induced SMPs.

Multi-stimuli sensitive shape memory poly(vinyl alcohol)-graft-polyurethane

A multi-stimuli sensitive shape memory poly(vinyl alcohol)-graft-polyurethane was synthesized. Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (1H-NMR) confirmed the chemical structure of the material. The material exhibits good water-induced shape memory behavior and a thermally induced dual shape memory effect. Further study indicated that the material exhibits good triple shape memory behavior. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) demonstrated that the polymer possesses a wide transition temperature, which resulted in excellent dual- and triple-shape memory effects. DSC and swelling experiments were carried out to find that the mechanism of the water-induced shape memory effect involves the decrease of the transition temperature when it swelled in water. Alamar Blue assay showed that the material possesses good biocompatibility and can be potentially used in the field of biomaterials.