Weixiang Sun

Infrared-driving actuation based on bilayer graphene oxide-poly(N-isopropylacrylamide) nanocomposite hydrogels

Stimulus-responsive hydrogels are utilized as smart materials in actuators for transforming external stimuli into actuation movements. Infrared (IR) irradiation is considered to be an ideal driving energy because it can penetrate into biomaterials without direct contact and can be remotely controlled. In the present work, a new IR-driving bilayer hydrogel actuator is prepared by stacking a graphene oxide (GO)-hectorite clay-poly(N-isopropylacrylamide) (PNIPAm) gel layer onto a hectorite clay-PNIPAm gel layer, synthesized through stepwise in situ polymerization. GO in the gel absorbs the IR irradiation and rapidly and efficiently transforms it into thermal energy, resulting in a much faster temperature increase in the GO-containing gel layer than that of the gel layer without GO, and the temperature of the former becomes higher than that of the latter. This bilayer structure with different temperatures changes the isotropic volume contraction into an anisotropic deformation, i.e., bending, which is always toward the GO-containing layer. Moreover, this bending occurs in the atmosphere, owing to the self-supporting capability of the tough gels. The repetition of the bending recovery is realized by turning the IR light on and off. According to these observations, the bilayer gel with GO provides a tough and IR-driving material for new soft actuators.

Fast self-healing of graphene oxide-hectorite clay-poly(N,N-dimethylacrylamide) hybrid hydrogels realized by near-infrared irradiation.

Self-healing hydrogels were proposed to be used as biomaterials, because of the capability of spontaneously healing injury, but most of the reported self-healing hydrogels do not possess high mechanical strength and fast self-healing at the same time. Herein, we prepared graphene oxide (GO)-hectorite clay-poly(N,N-dimethylacrylamide) (PDMAA) hybrid hydrogels with enhanced mechanical properties and fast self-healing capability realized by near-infrared (NIR) irradiation. The physical cross-linking between clay sheets and PDMAA chains provided the hydrogel with mechanical strength to maintain its stability in shape and architecture. GO sheets in the hybrid hydrogels acted as not only a collaborative cross-linking agent but also as a NIR absorber to absorb the NIR irradiation energy and transform it to thermal energy rapidly and efficiently, resulting in a rapid temperature increase of the GO containing gels. The chain mutual diffusion and the reformation of physical cross-linking occurred more quickly at higher temperature; consequently, the damaged hydrogel was almost completely recovered in a few minutes upon irradiation. We also demonstrated a potential application of the hybrid hydrogel as a self-healing surgical dressing.

Notch insensitive and self-healing PNIPAm–PAM–clay nanocomposite hydrogels

In the present work, hydrophilic monomer acrylamide (AM) was copolymerized with N-isopropylacrylamide (NIPAm) in an aqueous hectorite clay suspension to prepare PNIPAm-PAM-clay nanocomposite hydrogels (NC gels). With increasing AM content, the elongation at break of the copolymerized NC gels increased but the strength as well as the hysteresis during the loading-unloading cycle decreased, showing faster relaxation due to the more hydrophilic copolymer chains with the AM segments. The elongation at break of the copolymerized NC gels was independent of the notch length and notch type, while the fracture energy was greatly increased to 3000-5000 J m(-2) from 700 J m(-2) for the pure PNIPAm NC gels. The copolymer chains resulted in this notch insensitivity by easily dispersing the stress concentration at the notch tip through disorientation of the copolymer chains and clay platelets. The copolymerized NC gels also exhibited excellent self-healing capability; the cut surfaces were connected together by simply keeping in contact for a period of time (about 4 days at 20 °C). This self-healing was accelerated by increasing the treatment temperature (about 4 h at 80 °C).

Programmable and Bidirectional Bending of Soft Actuators Based on Janus Structure with Sticky Tough PAA-Clay Hydrogel

Facile preparation, rapid actuating, and versatile actions are great challenges in exploring new kinds of hydrogel actuators. In this paper, we presented a facile sticking method to prepare Janus bilayer and multilayer hydrogel actuators that benefited from a special tough and adhesive PAA-clay hydrogel. Combining physical and chemical cross-linking reagents, we endowed the PAA gel with both toughness and adhesion. This PAA gel was reinforced by further cross-linking with Fe3+. These two hydrogels with different cross-linking densities exhibited different swelling capabilities and moduli in the media manipulated by pH and ionic strength, thus acting as promising candidates for soft actuators. On the basis of these gels, we designed hydrogel actuators of rapid response in several minutes and precisely controlled actuating direction by sticking two hydrogel layers together. Elaborate soft actuators such as bidirectional bending flytrap, gel hand with grasp, open, and gesturing actions as well as word-writing actuator were prepared. This method could be generalized by using other stimuli-responsive hydrogels combined with the adhesive PAA gel, which would open a new way to programmable and versatile soft actuators.

Rapid shape memory and pH-modulated spontaneous actuation of dopamine containing hydrogels

The dopamine containing hydrogels with rapid responsive shape memory capability were synthesized by a one-pot method. The temporary shape of hydrogel was fixed within 20 s in NaOH solution by the tris-complex crosslinking of metal-ligand complex between Fe3+ ions and catechol groups, while the permanent shape was recovered completely in HCl solution within 60 s upon the change from tris-complex to mono-complex. The hydrogel showed unique spontaneous actuation behavior. It could curl spontaneously without further external force deformation when immersed in NaOH solution again after the first shape recovery in HCl solution. This might be attributed to the competitive result of swelling and additional tris-complex crosslinking formation when immersed in NaOH solution. In addition, the hydrogels also had proper modulus, elongation ratio and tensile strength. Such hydrogel provides a new candidate material for designing soft actuators and robots modulated with spontaneous actuating.

Binding Interaction and Gelation in Aqueous Mixtures of Poly(N-isopropylacrylamide) and Hectorite Clay

The binding interaction between poly(N-isopropylacrylamide) (PNIPAm) chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode. A large frequency shift Δf in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer. The relationship between the dissipation shift ΔD and Δf revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets. In dilute aqueous mixtures, the PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. A gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. The Raman spectrum pointed out the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.

Large amplitude oscillatory shear studies on the strain-stiffening behavior of gelatin gels

Linear and nonlinear viscoelasticity of gelatin solutions was investigated by rheology. The dynamic mechanical properties during the sol-gel transition of gelatin followed the time-cure superposition. The fractal dimension df of the critical gel was estimated as 1.76, which indicated a loose network. A high sol fraction ws = 0.61 was evaluated from the plateau modulus by semi-empirical models. Strain-stiffening behavior was observed under large amplitude oscillatory shear (LAOS) for the gelatin gel. The strain and frequency dependence of the minimum strain modulus GM, energy dissipation Ed, and nonlinear viscoelastic parameter NE was illustrated in Pipkin diagrams and explained by the strain induced helix formation reported previously by others. The BST model described the strain-stiffening behavior of gelatin gel quite well, whereas the Gent and worm-like chain network models overestimated the strain-stiffening at large strains.

Scaling of the dynamic response of hectorite clay suspensions containing poly(ethylene glycol) along the universal route of aging

The temporal growth of dynamic response during the aging of hectorite clay suspensions containing NaCl with different poly(ethylene glycol) (PEG) concentrations was measured. For each PEG concentration, the dynamic responses at different waiting times were shifted into a master curve against the rescaled frequency, which indicated that the dynamic spectrum was preserved during the aging process. At each fixed waiting time, the dynamic responses of suspensions with different PEG concentrations were also shifted into a master curve against the rescaled frequency, which revealed that the modification of inter-particle interaction by the PEG adsorption did not change the relaxation during the aging route. These two superposition approaches led to a universal master curve revealing the equivalent effect of PEG concentration and waiting time on the dynamic response. Finally, the relative mean relaxation time extracted from the master curves of waiting time and PEG concentration reflecting the slowing down of the dynamics during aging was superimposed to a master curve, implying the existence of a similar dynamic response. The present results suggested that the universal route of aging was preserved for the clay suspensions and a similar dynamic response was found from the mechanical spectra at different aging stages.

Effect of Salt Concentration on the Motion of Particles near the Substrate in Drying Sessile Colloidal Droplets

The motions of the particles on the substrate of a drying sessile colloidal droplet of water were measured using multiparticle tracking. Droplets with different concentrations (0-250 mM) of sodium chloride (NaCl) were compared. Several statistical quantities were proposed to characterize the heterogeneous behaviors of the particles and distinguish the effects of the flow field and the substrate interaction. For the salt-free droplet, most of the particles were nonadsorbed and mobile without friction. With the presence of salt, the fraction of the adsorbed particles increases with increasing evaporation time and the initial salt concentration, which was explained by Derjaguin-Landau-Verwey-Overbeek interaction. The fraction of mobile particles is mostly frictionless for all samples. At low salt concentrations, the velocity of mobile particles increases with the evaporation time to a peak and then decreases. The velocity is lower for higher salt concentrations. The effect of salt on the nonadsorbed particles was attributed to the electrokinetic effect.

Self-healable tough supramolecular hydrogels crosslinked by poly-cyclodextrin through host-guest interaction

A new supramolecular hydrogel with superior self-healing and shape memory properties was synthesized via in-situ copolymerization of the adamantane-containing monomer N-adamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the polycyclodextrin (PCD) aqueous solution, where PCD served as the polyfunctional physical cross-linker. The PCD-Ad hydrogels showed strength of 40 kPa with breaking strain of 600%, but small tensile loading-unloading hysteresis and residual deformation benefited from the reversible crosslinking structure. Excellent self-healing property was realized at 70 °C and humid condition. The healing percentage reached over 70% after 120 min, which showed enhanced self-healing efficiency compared with the reported supramolecular hydrogels crosslinked by CD and Ad. This may benefit from the polyfunctional PCD crosslinking reagent, which provided more host sites and binding opportunities for the complexation during the self-healing process. Shape memory behavior was also realized through the reversible second crosslinked network by the carboxyl groups of PAAc with Fe3+ ions. The present study provides a new method to improve self-healing ability of the supramolecular hydrogels, which will find the applications in self-healing coating, biomedical materials and soft actuators.