Xinxing Liu

Redox-responsive gel-sol/sol-gel transition in poly(acrylic acid) aqueous solution containing Fe(III) ions switched by light.

Redox-responsive gel-sol/sol-gel transition in aqueous PAA system containing Fe(III)-citrate complex was realized by switching the redox states of Fe(III)/F(II) ions conjugated with photoreduction and oxidation. This reversible transition can be indicated chromatically by the Fe(III) ions and repeated many times as long as there is sufficient citric acid.

Magnetic hydrogels with supracolloidal structures prepared by suspension polymerization stabilized by Fe2O3 nanoparticles

Magnetic hydrogels with supracolloidal structures were fabricated by suspension polymerization of N-isopropylacrylamide (NIPAm) and/or acrylamide (Am) stabilized by Fe(2)O(3) nanoparticles. Fe(2)O(3) nanoparticles can self-assemble at liquid-liquid interfaces to form stable water in oil Pickering emulsion droplets. Monomers dissolved in suspended aqueous droplets were subsequently polymerized at 60 degrees C. When NIPAm was homopolymerized the PNIPAm produced deposited from the interior water phase onto the interface to form Fe(2)O(3)/PNIPAm nanocomposite shells because of its hydrophobicity at the reaction temperature. Magnetic and thermosensitive hollow microcapsules were obtained. When Am was homopolymerized magnetic core-shell microcapsules with PAm hydrogel cores and Fe(2)O(3) nanoparticle shells were obtained. When NIPAm and Am were co-polymerized, magnetic hydrogel microcapsules with two kinds of supracolloidal structures were obtained varying with the NIPAm/Am ratio. These microcapsule beads may find applications as delivery vehicles for biomolecules, drugs, cosmetics, food supplements and living cells. Suspension polymerization based on Pickering emulsion droplets opens up a new route to synthesize a variety of hybrid hydrogels with supracolloidal structures.

Multilayer nanocapsules of polysaccharide chitosan and alginate through layer-by-layer assembly directly on PS nanoparticles for release.

Polysaccharide multilayer nanocapsules have been fabricated in aqueous media by the layer-by-layer self-assembly of chitosan (CHI) and sodium alginate (ALG) on monodisperse polystyrene (PS) nanoparticles with a diameter of 180 nm as template, followed by removal of the templates through dissolving in THF. The pH and added salt concentration of the polyelectrolyte deposition solutions were optimized to ensure the alternating deposition. Consequently, the most suitable pH values were found to be 6.0–8.0 for ALG and 3.5 for CHI and were used in the deposition. The concentration of added NaCl used in the adsorption solutions was 0.5 M, which led to an average thickness of about 13 nm for 5 bilayers of CHI/ALG shell-wall.ζ-potential indicated the stepwise and alternating adsorption of CHI and ALG to form multilayer film on the PS nanoparticles. The characteristic bands of PS residue almost disappeared in the IR spectrum of the nanocapsule after dipped in THF, confirming thorough removal of PS templates from the core-shell particles. TEM, SEM and AFM were utilized to observe the nanocapsules of about 225 nm in diameter (by TEM). A hydrophilic drug model, acridine hydrochloride (AH), was chosen to investigate the loading and release properties of the nanocapsules. The positively charged AH spontaneously deposited into the capsule due to the electrostatic interaction with the negatively charged styrene sulfonate residues from the PS template inside the capsule. The rate of AH release became slightly slower when the capsule wall was cross-linked with glutaraldehyde, but the accumulative released amount for the cross-linked capsule was obviously reduced. These nanocapsules made from nature polysaccharides have a potential application in controlled drug release.

Large deformation behavior and effective network chain density of swollen poly(N-isopropylacrylamide)–Laponite nanocomposite hydrogels

Hydrogels were often used in aqueous and/or physiological saline media, thus, the mechanical properties of the swollen gels were of particular importance. Until now, these data were lacking due to the most swollen hydrogels being too weak and brittle to undergo the deformation test at all. The mechanical behavior of swollen poly(N-isopropylacrylamide) (PNIPAm)–Laponite nanocomposite hydrogels (NC gels) were studied with compression and elongation under large strains causing deformation. The swollen NC gels were intact after compression and sustained elongation of up to 500%. Strain hardening was observed in the swollen NC gels for the first time, which was considered to be the extensional limitation of the polymer network chains due to the orientation of the Laponite platelets. Therefore, the strain hardening was enhanced by increasing the Laponite content in the NC gels. When the deformation was low, the compression and elongation stress-deformation curves of the as-prepared and swollen NC gels were described quantitatively with the Mooney–Rivlin model. Whilst for high deformation, Cretons model considering the finite extensibility enabled the prediction of these curves almost quantitatively. Stress-deformation hysteresis was found in the as-prepared and swollen NC gels, which was the result of the deorientation of the clay platelets and relaxation of the polymer chains. The orientation process of the clay platelets dissipated energy during deformation, resulting in a high toughness of the NC gels even in the swollen state. The effective network chain density in the swollen NC gels manifested that the swelling in water only expanded the gel volume and did not damage the cross-linking points. The number of effective network chains attached to one Laponite XLS platelet was almost independent of the clay content, so the addition of clay platelets in the NC gel formed new cross-linking junctions.

Silver nanoparticles directly formed on natural macroporous matrix and their anti-microbial activities

In this study, silver nanoparticles were formed on a natural macroporous matrix, the stem of rice-paper plant, by reducing Ag+ in aqueous solution through in situ processing without using any other stabilizers. The pores of the matrix, with their size of about 100 µm, were thought to act as reaction compartments for the nucleation and growth of silver nanoparticles, and the control of nucleation of silver crystal during the reduction reaction was found to be important to the successful formation of nanosized silver particles onto the matrix. The diameter and amount of resultant silver particles can be controlled by changing the reaction conditions. Under optimized conditions, the content of silver particles in the matrix can reach as high as 1.8 wt% with the particle diameters being kept below 100 nm. The anti-microbial activities in terms of minimum inhibitory concentration (MIC) for the silver nanoparticle composites against Escherichia coli and Candida albicans were assayed in agar gel, and the results show that MIC values for silver nanoparticle composites are 14.1 mg(Ag) l−1 and 28.1 mg(Ag) l−1 for Escherichia coli and Candida albicans respectively, which are comparable to the value for colloidal nanosilver.

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).

Facile fabrication of well-defined hydrogel beads with magnetic nanocomposite shells.

Well-defined magnetic nanocomposite beads with alginate gel cores and shells of iron oxide (gamma-Fe(2)O(3)) nanoparticles were prepared by self-assembly of colloidal particles at liquid-liquid interfaces and subsequent in situ gelation. Fe(2)O(3) nanoparticles could spontaneously adsorb onto the water droplet surfaces to stabilize water-in-hexane emulsions. Water droplets containing sodium alginate were in situ gelled by calcium cations, which were released from calcium-ethylenediamine tetraacetic acid (Ca-EDTA) chelate by decreasing pH value through slow hydrolysis of d-glucono-delta-lactone (GDL). The resulting hybrid beads with a core-shell structure were easily collected by removing hexane. This facile and high efficient fabrication had a 100% yield and could be carried out at room temperature. Insulin microcrystal was encapsulated into the hybrid beads by dispersing them in the aqueous solution of alginate sodium in the fabrication process. The sustained release could be obtained due to the dual barriers of the hydrogel core and the close-packed inorganic shell. The release curves were nicely fitted by the Weibull equation and the release followed Fickian diffusion. The hybrid beads may find applications as delivery vehicles for biomolecules, drugs, cosmetics, food supplements and living cells.

Preferential Adsorption of Poly(ethylene glycol) on Hectorite Clay and Effects on Poly(N-isopropylacrylamide)/Hectorite Nanocomposite Hydrogels

Poly(N-isopropylacrylamide)/Laponite nanocomposite hydrogel (NC gel) was synthesized via in situ polymerization in the Laponite suspension containing PEG. The adsorption of PEG on Laponite platelets was characterized by zeta-potential, which decreased with the PEG adsorption. The tensile strength decreased and elongation at break increased with increasing PEG concentration. The effective network chain density of PNIPAm/Laponite NC gels determined from the equilibrium modulus G(e) decreased upon adsorption of PEG on the Laponite. All of these results revealed the preferential adsorption of PEG on the Laponite platelets occupying the active sites for the PNIPAm chain anchoring, which hindered their cross-linking effect in the NC gels. However, the temperature sensitive swelling behavior still remained in the PNIPAm/Laponite NC gels containing PEG with higher swelling volume below the LCST due to the lower cross-linker density. By adjusting the amount of added PEG, we can easily control the properties of the PNIPAm/Laponite NC gels.